Denis Erkal

Dr Denis Erkal

Senior Lecturer of Astrophysics
+44 (0)1483 686990
12 BC 03

Academic and research departments

Astrophysics Research Group.


Areas of specialism

Galactic archaeology; Near field cosmology


Research interests

Globular cluster stream around a Milky Way-like halo
Sagittarius in Gaia DR2
Tuc III stream in the presence of the LMC


W. Cerny, Joshua D. Simon, TIANRU LI, A Drlica-Wagner, A.B Pace, C. E Martınez-Vazquez, A. H. Riley, B. Mutlu-Pakdil, S Mau, P. S. Ferguson, DENIS ERKAL, Ricardo R. Muñoz, C. R Bom, J. L. Carlin, D. Carollo, Y. Choi, A. P. Ji, D Martınez-Delgado, V Manwadkar, A. E Miller, NOELIA ESTELLA DONATA NOEL, Joanna D Sakowska, D. J. Sand, G. S. Stringfellow, E. J. Tollerud, AK Vivas, Julio A. Carballo-Bello, D. Hernandez-Lang, DEBORAH JAMES, J. L. Nilo Castellon, K. A. G Olsen, A. Zenteno (2022)Pegasus IV: Discovery and Spectroscopic Confirmation of an Ultra-Faint Dwarf Galaxy in the Constellation Pegasus

We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a compact, ultra-faint stellar system ($r_{1/2} = 41^{+8}_{-6}$ pc; $M_V = -4.25 \pm 0.2$ mag) located at a heliocentric distance of $90^{+4}_{-6}$ kpc. Based on spectra of seven non-variable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV's velocity dispersion, measuring $\sigma_{v} = 3.3^{+1.7}_{-1.1} \text{ km s}^{-1}$ (after excluding three velocity outliers); this implies a mass-to-light ratio of $M_{1/2}/L_{V,1/2} = 167^{+224}_{-99} M_{\odot}/L_{\odot}$ for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of $\rm [Fe/H] = -2.67^{+0.25}_{-0.29}$ dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a non-zero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV's proper motion using data from Gaia Early Data Release 3, finding ($\mu_{\alpha*}, \mu_{\delta}) = (0.33\pm 0.07, -0.21 \pm 0.08) \text{ mas yr}^{-1}$. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than ten half-light radii away from the system's centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.

S. Mau, W. Cerny, A.B Pace, Y. Choi, A. Drlica-Wagner, L. Santana-Silver, A.H Riley, Denis Erkal, S. Stringfellow, M. Adamow, J.L Carlin, R.A Gruendal, D. Hernandez-Lang, N. Kuropatkin, T.S Li, C.E Martinez-Vasquez, E. Morganson, B. Mutlu-Pakdil, E.H Neilsen, D.L Nidever, K.A.G Olsen, D.J Sand, E.J Tollerud, D.L Tucker, B. Yanny, A. Zenteno, S. Allam, W.A Barkhouse, K. Bechtol, E.F Bell, P. Balaji, D. Crnojevic, J. Esteves, P.S Ferguson, C. Gallart, A.K Hughes, D.J James, P. Jethwa, L.C Johnson, K. Kuehn, S. Majewski, Y-Y. Mao, P. Massana, M. McNanna, A. Monachesi, E.O Nadler, Noelia Noel, A. Palmese, F. Paz-Chinchon, A. Pieres, J. Sanchez, N. Shipp, J.D Simon, M. Soares-Santos, K. Tavangar, R.P van der Marel, A.K Vivas, A.R Walker, R.H Wechsler (2020)Two Ultra-Faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey, In: Astrophysical Journal American Astronomical Society

We report the discovery of two ultra-faint stellar systems found in early data from the DECam Local Volume Exploration survey (DELVE). The rst system, Centaurus I (DELVE J1238

Alexander P. Ji, Sergey E. Koposov, Ting S Li, DENIS ERKAL, Andrew B. Pace, Joshua D. Simon, Vasily Belokurov, Lara Cullinane, Gary S. Da Costa, Kyler Kuehn, Geraint F. Lewis, Dougal Mackey, Nora Shipp, Jeffrey D. Simpson, Daniel B. Zucker, Terese T. Hansen, Joss Bland-Hawthorn (2021)Kinematics of Antlia 2 and Crater 2 from The Southern Stellar Stream Spectroscopic Survey (S5), In: The Astrophysical Journal IOP Publishing

We present new spectroscopic observations of the diffuse Milky Way satellite galaxies Antlia 2 and Crater 2, taken as part of the Southern Stellar Stream Spectroscopic Survey (S5). The new observations approximately double the number of confirmed member stars in each galaxy and more than double the spatial extent of spectroscopic observations in Antlia 2. A full kinematic analysis, including Gaia EDR3 proper motions, detects a clear velocity gradient in Antlia 2 and a tentative velocity gradient in Crater 2. The velocity gradient magnitudes and directions are consistent with particle stream simulations of tidal disruption. Furthermore, the orbit and kinematics of Antlia 2 require a model that includes the reflex motion of the Milky Way induced by the Large Magellanic Cloud. We also find that Antlia 2's metallicity was previously overestimated, so it lies on the empirical luminosity-metallicity relation and is likely only now experiencing substantial stellar mass loss. This low stellar mass loss contrasts with current dynamical models of Antlia 2's size and velocity dispersion, which require it to have lost more than 90% of its stars to tides. Overall, the new kinematic measurements support a tidal disruption scenario for the origin of these large and extended dwarf spheroidal galaxies.

Sergey E Koposov, Douglas Boubert, Ting S Li, Denis Erkal, Gary S Da Costa, Daniel B Zucker, Alexander P Ji, Kyler Kuehn, Geraint F Lewis, Dougal Mackey, Jeffrey D Simpson, Nora Shipp, Zhen Wan, Vasily Belokurov, Joss Bland-Hawthorn, Sarah L Martell, Thomas Nordlander, Andrew B Pace, Gayandhi M De Silva, Mei-Yu Wang (2020)Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A*, In: Monthly Notices of the Royal Astronomical Society491(2)pp. 2465-2480 Oxford University Press

We present the serendipitous discovery of the fastest main-sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a ∼2.35 M⊙ A-type star located at a distance of ∼9 kpc from the Sun and has a heliocentric radial velocity of 1017 ± 2.7 kms−1 without any signature of velocity variability. The current 3D velocity of the star in the Galactic frame is 1755 ± 50 kms−1⁠. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of ∼1800 kms−1 and travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion Vy,⊙ = 246.1 ± 5.3 kms−1 or position R0 = 8.12 ± 0.23 kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disc of young stars at the Galactic Centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.

D Erkal, V Belokurov, C F P Laporte, S E Koposov, T S Li, C J Grillmair, N Kallivayalil, A M Price-Whelan, N W Evans, K Hawkins, D Hendel, C Mateu, J F Navarro, A del Pino, C T Slater, S T Sohn (2019)The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

In a companion paper by Koposov et al., RR Lyrae from Gaia Data Release 2 are used to demonstrate that stars in the Orphan stream have velocity vectors significantly misaligned with the stream track, suggesting that it has received a large gravitational perturbation from a satellite of the Milky Way. We argue that such a mismatch cannot arise due to any realistic static Milky Way potential and then explore the perturbative effects of the Large Magellanic Cloud (LMC). We find that the LMC can produce precisely the observed motion-track mismatch and we therefore use the Orphan stream to measure the mass of the Cloud. We simultaneously fit the Milky Way and LMC potentials and infer that a total LMC mass of 1.38+0.27−0.24×1011M⊙ is required to bend the Orphan Stream, showing for the first time that the LMC has a large and measurable effect on structures orbiting the Milky Way. This has far-reaching consequences for any technique which assumes that tracers are orbiting a static Milky Way. Furthermore, we measure the Milky Way mass within 50 kpc to be 3.80+0.14−0.11×1011M⊙⁠. Finally, we use these results to predict that, due to the reflex motion of the Milky Way in response to the LMC, the outskirts of the Milky Way’s stellar halo should exhibit a bulk, upwards motion.

D Boubert, V Belokurov, Denis Erkal, G Iorio (2018)A Magellanic origin for the Virgo substructure, In: Monthly Notices of the Royal Astronomical Societysty3014 Oxford University Press (

The Milky Way halo has been mapped out in recent work using a sample of RR Lyrae stars drawn from a cross-match of Gaia with 2MASS. We investigate the significant residual in this map which we constrain to lie at Galactocentric radii 12 < R < 27 kpc and extend over 2600 deg2 of the sky. A counterpart of this structure exists in both the Catalina Real Time Survey and the sample of RR Lyrae variables identified in Pan-STARRS, demonstrating that this structure is not caused by the spatial inhomogeneity of Gaia. The structure is likely the Virgo Stellar Stream and/or Virgo Over-Density. We show the structure is aligned with the Magellanic Stream and suggest that it is either debris from a disrupted dwarf galaxy that was a member of the Vast Polar Structure or that it is SMC debris from a tidal interaction of the SMC and LMC 3 Gyr ago. If the latter then the sub-structure in Virgo may have a Magellanic origin.

T S Li, S E Koposov, D B Zucker, G F Lewis, K Kuehn, J D Simpson, A P Ji, N Shipp, Y-Y Mao, M Geha, A B Pace, A D Mackey, S Allam, D L Tucker, G S Da Costa, Denis Erkal, J D Simon, J R Mould, S L Martell, Z Wan, G M De Silva, K Bechtol, E Balbinot, V Belokurov, J Bland-Hawthorn, A R Casey, L Cullinane, A Drlica-Wagner, S Sharma, A K Vivas, R H Wechsler, B Yanny (2019)The Southern Stellar Stream Spectroscopic Survey (S⁵): Overview, Target Selection, Data Reduction, Validation, and Early Science, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We introduce the Southern Stellar Stream Spectroscopy Survey (S⁵), an on-going program to map the kinematics and chemistry of stellar streams in the Southern Hemisphere. The initial focus of S⁵ has been spectroscopic observations of recently identified streams within the footprint of the Dark Energy Survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9-m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S⁵ has mapped 9 DES streams and 3 streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S⁵ program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S⁵, including future public data releases, can be found at

D. Boubert, D. Erkal, A. Gualandris (2020)Deflection of the hypervelocity stars by the pull of the Large Magellanic Cloud on the Milky Way, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

Stars slingshotted by the supermassive black hole at the Galactic centre escape from the Milky Way so quickly that their trajectories are almost straight lines. Previous works have shown how these `hypervelocity stars' (stars moving faster than the local Galactic escape speed) are subsequently de ected by the gravitational field of the Milky Way and the Large Magellanic Cloud (LMC), but have neglected to account for the reflex motion of the Milky Way in response to the y-by of the LMC. A consequence of this motion is that the hypervelocity stars we see in the outskirts of the Milky Way today were ejected from where the Milky Way centre was hundreds of millions of years ago. This change in perspective causes large apparent de ections of several degrees in the trajectories of the hypervelocity stars. We quantify these deflections by simulating the ejection of hypervelocity stars from an isolated Milky Way (with a spherical or flattened dark matter halo), from a fixed-in-place Milky Way with a passing LMC, and from a Milky Way which responds to the passage of the LMC, finding that LMC passage causes larger de ections than can be caused by a attened Galactic dark matter halo in CDM. The 10 as yr

J. I. Read, D. Erkal (2019)Abundance matching with the mean star formation rate: there is no missing satellites problem in the Milky Way above M200∼109M, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We introduce a novel abundance matching technique that produces a more accurate estimate of the pre-infall halo mass, M200, for satellite galaxies. To achieve this, we abundance match with the mean star formation rate, averaged over the time when a galaxy was forming stars, ⟨SFR⟩, instead of the stellar mass, M∗. Using data from the Sloan Digital Sky Survey, the GAMA survey and the Bolshoi simulation, we obtain a statistical ⟨SFR⟩−M200 relation in ΛCDM. We then compare the pre-infall halo mass, Mabund200, derived from this relation with the pre-infall dynamical mass, Mdyn200, for 21 nearby dSph and dIrr galaxies, finding a good agreement between the two. As a first application, we use our new ⟨SFR⟩−M200 relation to empirically measure the cumulative mass function of a volume-complete sample of bright Milky Way satellites within 280 kpc of the Galactic centre. Comparing this with a suite of cosmological 'zoom' simulations of Milky Way-mass halos that account for subhalo depletion by the Milky Way disc, we find no missing satellites problem above M200∼09M⊙ in the Milky Way. We discuss how this empirical method can be applied to a larger sample of nearby spiral galaxies.

Andrew R. Casey, Alexander P. Ji, Terese T. Hansen, Ting S Li, Sergey E. Koposov, Gary Da Costa, Joss Bland-Hawthorn, Lara Cullinane, DENIS ERKAL, Geraint F. Lewis, Kyler Kuehn, Dougal Mackey, Sarah L. Martell, Andrew B. Pace, Jeffrey D. Simpson, Daniel B. Zucker (2021)Signature of a massive rotating metal-poor star imprinted in the Phoenix stellar stream *, In: The Astrophysical journal IOP Publishing

The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low mass globular cluster. In this work we use Magellan/MIKE high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity (σ([Fe II/H]) = 0.04 +0.11 −0.03) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr II/Fe] spanning ∼1 dex, while [Ba II/Fe] shows nearly no intrinsic spread (σ([Ba II/H]) = 0.03 +0.10 −0.02). This abundance signature is best interpreted as slow neutron capture element production from a massive fast-rotating metal-poor star (15−20 M , v ini /v crit = 0.4, [Fe/H] = −3.8). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant (A(Li) = 3.1 ± 0.1). At [Fe/H] = −2.93 it is among the most metal-poor lithium-rich giants known.

W. Cerny, A.B Pace, A Drlica-Wagner, S. E. Koposov, A. K. Vivas, S Mau, A. H. Riley, C. R Bom, J. L. Carlin, Y. Choi, DENIS ERKAL, P. S. Ferguson, D. J. James, T. S. Li, D Martinez-Delgado, C. E. Martínez-Vázquez, Ricardo R. Muñoz, B. Mutlu-Pakdil, K. A. G. Olsen, A Pieres, Joanna D Sakowska, D. J. Sand, J. D. Simon, A Smercina, G. S. Stringfellow, E. J. Tollerud, M. Adamów, D. Hernandez-Lang, N Kuropatkin, L. Santana-Silva, D. L. Tucker, A. Zenteno, (2021)Eridanus IV: an Ultra-Faint Dwarf Galaxy Candidate Discovered in the DECam Local Volume Exploration Survey, In: The Astrophysical Journal Letters920(2) IOP

We present the discovery of a candidate ultra-faint Milky Way satellite, Eridanus IV (DELVE J0505$-$0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). Eridanus IV is a faint ($M_V = -4.7 \pm 0.2$), extended ($r_{1/2} = 75^{+16}_{-13}$ pc), and elliptical ($\epsilon = 0.54 \pm 0.1$) system at a heliocentric distance of $76.7^{+4.0}_{-6.1}$ kpc, with a stellar population that is well-described by an old, metal-poor isochrone (age of $\tau \sim 13.0$ Gyr and metallicity of ${\rm [Fe/H] \lesssim -2.1}$ dex). These properties are consistent with the known population of ultra-faint Milky Way satellite galaxies. Eridanus IV is also prominently detected using proper motion measurements from Gaia Early Data Release 3, with a systemic proper motion of $(\mu_{\alpha} \cos \delta, \mu_{\delta}) = (+0.25 \pm 0.06, -0.10 \pm 0.05)$ mas yr$^{-1}$ measured from its horizontal branch and red giant branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.

Denis Erkal, Vasily A Belokurov, Daniel L Parkin (2020)Equilibrium models of the Milky Way mass are biased high by the LMC, In: Monthly Notices of the Royal Astronomical Society498(4)pp. 5574-5580 Oxford University Press (OUP)

Recent measurements suggest that the Large Magellanic Cloud (LMC) may weigh as much as 25 per cent of the Milky Way (MW). In this work, we explore how such a large satellite affects mass estimates of the MW based on equilibrium modelling of the stellar halo or other tracers. In particular, we show that if the LMC is ignored, the MW mass within 200 kpc is overestimated by as much as 50 per cent. This bias is due to the bulk motion in the outskirts of the Galaxy’s halo and can be, at least in part, accounted for with a simple modification to the equilibrium modelling. Finally, we show that the LMC has a substantial effect on the orbit Leo I which acts to increase its present-day speed relative to the MW. We estimate that accounting for a 1.5×1011M⊙ LMC would lower the inferred MW mass to ∼1012M⊙⁠.

Denis Erkal, T S Li, S E Koposov, V Belokurov, E Balbinot, K Bechtol, B Buncher, A Drlica-Wagner, K Kuehn, J L Marshall, C E Martínez-Vázquez, A B Pace, N Shipp, J D Simon, K M Stringer, A K Vivas, R H Wechsler, B Yanny, F B Abdalla, S Allam, J Annis, S Avila, E Bertin, D Brooks, E Buckley-Geer, D L Burke, A Carnero Rosell, M Carrasco Kind, J Carretero, C B D’Andrea, L N da Costa, C Davis, J De Vicente, P Doel, T F Eifler, A E Evrard, B Flaugher, J Frieman, J García-Bellido, E Gaztanaga, D W Gerdes, D Gruen, R A Gruendl, J Gschwend, G Gutierrez, W G Hartley, D L Hollowood, K Honscheid, D J James, E Krause, M A G Maia, M March, F Menanteau, R Miquel, R L C Ogando, A A Plazas, E Sanchez, B Santiago, V Scarpine, R Schindler, I Sevilla-Noarbe, M Smith, R C Smith, M Soares-Santos, F Sobreira, E Suchyta, M E C Swanson, G Tarle, D L Tucker, A R Walker (2018)Modelling the Tucana III stream - a close passage with the LMC, In: Monthly Notices of the Royal Astronomical Society481(3)pp. 3148-3159 Oxford University Press (OUP)

We present results of the first dynamical stream fits to the recently discovered Tucana III stream. These fits assume a fixed Milky Way potential and give proper motion predictions, which can be tested with the upcoming Gaia Data Release 2. These fits reveal that Tucana III is on an eccentric orbit around the Milky Way and, more interestingly, that Tucana III passed within 15 kpc of the Large Magellanic Cloud (LMC) approximately 75 Myr ago. Given this close passage, we fit the Tucana III stream in the combined presence of the Milky Way and the LMC. We find that the predicted proper motions depend on the assumed mass of the LMC and that the LMC can induce a substantial proper motion perpendicular to the stream track. A detection of this misalignment will directly probe the extent of the LMC’s influence on our Galaxy, and has implications for nearly all methods which attempt to constraint the Milky Way potential. Such a measurement will be possible with the upcoming Gaia DR2, allowing for a measurement of the LMC’s mass.

Jason L Sanders, Edward J Lilley, Eugene Vasiliev, N Wyn Evans, Denis Erkal (2020)Models of distorted and evolving dark matter haloes, In: Monthly Notices of the Royal Astronomical Society499(4)pp. 4793-4813 Oxford University Press

We investigate the ability of basis function expansions to reproduce the evolution of a Milky Way-like dark matter halo, extracted from a cosmological zoom-in simulation. For each snapshot, the density of the halo is reduced to a basis function expansion, with interpolation used to recreate the evolution between snapshots. The angular variation of the halo density is described by spherical harmonics, and the radial variation either by biorthonormal basis functions adapted to handle truncated haloes or by splines. High fidelity orbit reconstructions are attainable using either method with similar computational expense. We quantify how the error in the reconstructed orbits varies with expansion order and snapshot spacing. Despite the many possible biorthonormal expansions, it is hard to beat a conventional Hernquist–Ostriker expansion with a moderate number of terms (≳15 radial and ≳6 angular). As two applications of the developed machinery, we assess the impact of the time-dependence of the potential on (i) the orbits of Milky Way satellites and (ii) planes of satellites as observed in the Milky Way and other nearby galaxies. Time evolution over the last 5 Gyr introduces an uncertainty in the Milky Way satellites’ orbital parameters of $\sim 15 \, \mathrm{per\, cent}$, comparable to that induced by the observational errors or the uncertainty in the present-day Milky Way potential. On average, planes of satellites grow at similar rates in evolving and time-independent potentials. There can be more, or less, growth in the plane’s thickness, if the plane becomes less, or more, aligned with the major or minor axis of the evolving halo.

Sergey E Koposov, Matthew G Walker, Vasily Belokurov, Andrew R Casey, Alex Geringer-Sameth, Dougal Mackey, Gary Da Costa, Denis Erkal, Prashin Jethwa, Mario Mateo, Edward W Olszewski, John I Bailey III (2018)Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

We report the discovery of a nearby dwarf galaxy in the constellation of Hydrus, between the Large and the Small Magellanic Clouds. Hydrus 1 is a mildy elliptical ultra-faint system with luminosity MV ∼ −4.7 and size 53 ± 3 pc, located 28 kpc from the Sun and 24 kpc from the LMC. From spectroscopy of ∼ 30 member stars, we measure a velocity dispersion of 2.7±0.5 km s−1 and find tentative evidence for a radial velocity gradient consistent with 3 km s−1 rotation. Hydrus 1’s velocity dispersion indicates that the system is dark matter dominated, but its dynamical mass-to-light ratio M/L=66+29 −20 is significantly smaller than typical for ultra-faint dwarfs at similar luminosity. The kinematics and spatial position of Hydrus 1 make it a very plausible member of the family of satellites brought into the Milky Way by the Magellanic Clouds. While Hydrus 1’s proximity and well-measured kinematics make it a promising target for dark matter annihilation searches, we find no evidence for significant gamma-ray emission from Hydrus 1. The new dwarf is a metal-poor galaxy with a mean metallicity [Fe/H]=−2.5 and [Fe/H] standard deviation of 0.4 dex, similar to other systems of similar luminosity. Alpha-abundances of Hyi 1 members indicate that star-formation was extended, lasting between 0.1 and 1 Gyr, with self-enrichment dominated by SN Ia. The dwarf also hosts a highly carbon-enhanced extremely metal-poor star with [Fe/H]∼ −3.2 and [C/Fe] ∼ +3.0.

P. S. Ferguson, J. D. Sakowska, N. Shipp, A Drlica-Wagner, T. S. Li, W. Cerny, K. Tavangar, A. B. Pace, J. L. Marshall, A. H. Riley, M. Adamów, J. L. Carlin, Y. Choi, D. Erkal, D. J. James, Sergey E. Koposov, N Kuropatkin, C. E. Martínez-Vázquez, S Mau, B. Mutlu-Pakdil, K. A. G. Olsen, G. S. Stringfellow, B Yanny (2021)DELVE-ing into the Jet: a thin stellar stream on a retrograde orbit at 30 kpc, In: Astrophysical Journal American Astronomical Society

We perform a detailed photometric and astrometric analysis of stars in the Jet stream using data from the first data release of the DECam Local Volume Exploration Survey (DELVE) DR1 and Gaia EDR3. We discover that the stream extends over ∼ 29 • on the sky (increasing the known length by 18 •), which is comparable to the kinematically cold Phoenix, ATLAS, and GD-1 streams. Using blue horizontal branch stars, we resolve a distance gradient along the Jet stream of 0.2 kpc/deg, with distances ranging from D ∼ 27−34 kpc. We use natural splines to simultaneously fit the stream track, width, and intensity to quantitatively characterize density variations in the Jet stream, including a large gap, and identify substructure off the main track of the stream. Furthermore, we report the first measurement of the proper motion of the Jet stream and find that it is well-aligned with the stream track suggesting the stream has likely not been significantly perturbed perpendicular to the line of sight. Corresponding author: Peter Ferguson 2 DELVE Collaboration Finally, we fit the stream with a dynamical model and find that the stream is on a retrograde orbit, and is well fit by a gravitational potential including the Milky Way and Large Magellanic Cloud. These results indicate the Jet stream is an excellent candidate for future studies with deeper photometry, astrometry, and spectroscopy to study the potential of the Milky Way and probe perturbations from baryonic and dark matter substructure.

P Jethwa, G Torrealba, C Navarrete, J.A. Carballo-Bello, Thomas de Boer, Denis Erkal, S.E. Koposov, S Duffau, D Geisler, M Catelan, V Belokurov (2018)Discovery of a thin stellar stream in the SLAMS survey, In: Monthly Notices of the Royal Astronomical Society480(4)pp. 5342-5351 Oxford University Press

We report the discovery of a thin stellar stream - which we name the Jet stream - cross- ing the constellations of Hydra and Pyxis. The discovery was made in data from the SLAMS survey, which comprises deep g and r imaging for a 650 square degree region above the Galactic disc performed by the CTIO Blanco + DECam. SLAMS photomet- ric catalogues have been made publicly available. The stream is approximately 0.18 degrees wide and 10 degrees long, though it is truncated by the survey footprint. Its colour-magnitude diagram is consistent with an old, metal-poor stellar population at a heliocentric distance of approximately 29 kpc. We corroborate this measurement by identifying a spatially coincident overdensity of likely blue horizontal branch stars at the same distance. There is no obvious candidate for a surviving stream progenitor.

Nilanjan Banik, Jo Bovy, Gianfranco Bertone, Denis Erkal, T. J. L de Boer (2021)Novel constraints on the particle nature of dark matter from stellar streams, In: Journal of Cosmology and Astroparticle Physics

We analyze the distribution of stars along the GD-1 stream with a combination of data from the ${\it Gaia}$ satellite and the Pan-STARRS survey, and we show that the population of subhalos predicted by the cold dark matter paradigm are necessary and sufficient to explain the perturbations observed in the linear density of stars. This allows us to set novel constraints on alternative dark matter scenarios that predict a suppression of the subhalo mass function on scales smaller than the mass of dwarf galaxies. A combined analysis of the density perturbations in the GD-1 and Pal 5 streams leads to a $95\%$ lower limit on the mass of warm dark matter thermal relics $m_{\rm WDM}>4.6$ keV; adding dwarf satellite counts strengthens this to $m_{\rm WDM}>6.3$ keV.

Eugene Vasiliev, Vasily Belokurov, Denis Erkal (2020)Tango for three: Sagittarius, LMC, and the Milky Way, In: Monthly Notices of the Royal Astronomical Society501(2)pp. 2279-2304 Oxford University Press

We assemble a catalogue of candidate Sagittarius stream members with 5D and 6D phase-space information, using astrometric data from Gaia DR2, distances estimated from RR Lyrae stars, and line-of-sight velocities from various spectroscopic surveys. We find a clear misalignment between the stream track and the direction of the reflex-corrected proper motions in the leading arm of the stream, which we interpret as a signature of a time-dependent perturbation of the gravitational potential. A likely cause of this perturbation is the recent passage of the most massive Milky Way satellite – the Large Magellanic Cloud (LMC). We develop novel methods for simulating the Sagittarius stream in the presence of the LMC, using specially tailored N-body simulations and a flexible parametrization of the Milky Way halo density profile. We find that while models without the LMC can fit most stream features rather well, they fail to reproduce the misalignment and overestimate the distance to the leading arm apocentre. On the other hand, models with an LMC mass in the range (1.3±0.3)×1011M⊙ rectify these deficiencies. We demonstrate that the stream can not be modelled adequately in a static Milky Way. Instead, our Galaxy is required to lurch toward the massive in-falling Cloud, giving the Sgr stream its peculiar shape and kinematics. By exploring the parameter space of Milky Way potentials, we determine the enclosed mass within 100 kpc to be (5.6±0.4)×1011M⊙⁠, and the virial mass to be (9.0±1.3)×1011M⊙⁠, and find tentative evidence for a radially-varying shape and orientation of the Galactic halo.

A D Mackey, L R Cullinane, G S Da Costa, D Erkal, S E Koposov, V Belokurov (2022)The Magellanic Edges Survey III. Kinematics of the disturbed LMC outskirts, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

We explore the structural and kinematic properties of the outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. Even at large galactocentric radii (8○ < R < 11○), we find the north-eastern LMC disk is relatively unperturbed: its kinematics are consistent with a disk of inclination ∼36.5○ and line-of-nodes position angle ∼145○ east of north. In contrast, fields at similar radii in the southern and western disk are significantly perturbed from equilibrium, with non-zero radial and vertical velocities, and distances significantly in front of the disk plane implied by our north-eastern fields. We compare our observations to simple dynamical models of the Magellanic/Milky Way system which describe the LMC as a collection of tracer particles within a rigid potential, and the Small Magellanic Cloud (SMC) as a rigid Hernquist potential. A possible SMC crossing of the LMC disk plane ∼400 Myr ago, in combination with the LMC’s infall to the Milky Way potential, can qualitatively explain many of the perturbations in the outer disk. Additionally, we find the claw-like and arm-like structures south of the LMC have similar metallicities to the outer LMC disk ([Fe/H] ∼ −1), and are likely comprised of perturbed LMC disk material. The claw-like substructure is particularly disturbed, with out-of-plane velocities >60 km s−1 and apparent counter-rotation relative to the LMC’s disk motion. More detailed N-body models are necessary to elucidate the origin of these southern features, potentially requiring repeated interactions with the SMC prior to ∼1 Gyr ago.

S. Mau, W. Cerny, A. B. Pace, Y. Choi, A. Drlica-Wagner, L. Santana-Silva, A. H. Riley, D. Erkal, G. S. Stringfellow, M. Adamów, J. L. Carlin, R. A. Gruendl, D. Hernandez-Lang, N. Kuropatkin, T. S. Li, C. E. Martínez-Vázquez, E. Morganson, B. Mutlu-Pakdil, E. H. Neilsen, D. L. Nidever, K. A. G. Olsen, D. J. Sand, E. J. Tollerud, D. L. Tucker, B. Yanny, A. Zenteno, S. Allam, W. A. Barkhouse, K. Bechtol, E. F. Bell, P. Balaji, D. Crnojević, J. Esteves, P. S. Ferguson, C. Gallart, A. K. Hughes, D. J. James, P. Jethwa, L. C. Johnson, K. Kuehn, S. Majewski, Y.-Y. Mao, P. Massana, M. McNanna, A. Monachesi, E. O. Nadler, N.E.D Noel, A. Palmese, F. Paz-Chinchon, A. Pieres, J. Sanchez, N. Shipp, J. D. Simon, M. Soares-Santos, K. Tavangar, R. P. van der Marel, A. K. Vivas, A. R. Walker, R. H. Wechsler (2020)Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey, In: The Astrophysical Journal890(2)136 IOP Publishing

We report the discovery of two ultra-faint stellar systems found in early data from the DECam Local Volume Exploration survey (DELVE). The first system, Centaurus I (DELVE J1238–4054), is identified as a resolved overdensity of old and metal-poor stars with a heliocentric distance of ${____text{}}{D}_{____odot }={116.3}_{-0.6}^{+0.6}____,____mathrm{kpc}$, a half-light radius of ${r}_{h}={2.3}_{-0.3}^{+0.4}____,____mathrm{arcmin}$, an age of $____tau ____gt 12.85____,____mathrm{Gyr}$, a metallicity of $Z={0.0002}_{-0.0002}^{+0.0001}$, and an absolute magnitude of ${M}_{V}=-{5.55}_{-0.11}^{+0.11}____,____mathrm{mag}$. This characterization is consistent with the population of ultra-faint satellites and confirmation of this system would make Centaurus I one of the brightest recently discovered ultra-faint dwarf galaxies. Centaurus I is detected in Gaia DR2 with a clear and distinct proper motion signal, confirming that it is a real association of stars distinct from the Milky Way foreground; this is further supported by the clustering of blue horizontal branch stars near the centroid of the system. The second system, DELVE 1 (DELVE J1630–0058), is identified as a resolved overdensity of stars with a heliocentric distance of ${____text{}}{D}_{____odot }={19.0}_{-0.6}^{+0.5}____,____mathrm{kpc}$, a half-light radius of ${r}_{h}={0.97}_{-0.17}^{+0.24}____,____mathrm{arcmin}$, an age of $____tau ={12.5}_{-0.7}^{+1.0}____,____mathrm{Gyr}$, a metallicity of $Z={0.0005}_{-0.0001}^{+0.0002}$, and an absolute magnitude of ${M}_{V}=-{0.2}_{-0.6}^{+0.8}____,____mathrm{mag}$, consistent with the known population of faint halo star clusters. Given the low number of probable member stars at magnitudes accessible with Gaia DR2, a proper motion signal for DELVE 1 is only marginally detected. We compare the spatial position and proper motion of both Centaurus I and DELVE 1 with simulations of the accreted satellite population of the Large Magellanic Cloud (LMC) and find that neither is likely to be associated with the LMC.

Mark Gieles, Denis Erkal, Fabio Antonini, Eduardo Balbinot, Jorge Peñarrubia (2021)A supra-massive population of stellar-mass black holes in the globular cluster Palomar 5, In: Nature Astronomy Nature Research

Palomar 5 is one of the sparsest star clusters in the Galactic halo and is best-known for its spectacular tidal tails, spanning over 20 degrees across the sky. With N-body simulations we show that both distinguishing features can result from a stellar-mass black hole population, comprising ~20% of the present-day cluster mass. In this scenario, Palomar 5 formed with a `normal' black hole mass fraction of a few per cent, but stars were lost at a higher rate than black holes, such that the black hole fraction gradually increased. This inflated the cluster, enhancing tidal stripping and tail formation. A gigayear from now, the cluster will dissolve as a 100% black hole cluster. Initially denser clusters end up with lower black hole fractions, smaller sizes, and no observable tails. Black hole-dominated, extended star clusters are therefore the likely progenitors of the recently discovered thin stellar streams in the Galactic halo.

Michele De Leo, Ricardo Carrera, Noelia E.D Noel, Justin I. Read, Denis Erkal, Carme Gallart (2020)Revealing the tidal scars of the Small Magellanic Cloud, In: Monthly Notices of the Royal Astronomical Society495(1)pp. 98-113 Oxford University Press (OUP)

Due to their close proximity, the Large and Small Magellanic Clouds (LMC/SMC) provide natural laboratories for understanding how galaxies form and evolve. With the goal of determining the structure and dynamical state of the SMC, we present new spectroscopic data for ∼3000 SMC red giant branch stars observed using the AAOmega spectrograph at the Anglo-Australian Telescope. We complement our data with further spectroscopic measurements from previous studies that used the same instrumental configuration as well as proper motions from the Gaia Data Release 2 catalogue. Analysing the photometric and stellar kinematic data, we find that the SMC centre of mass presents a conspicuous offset from the velocity centre of its associated H i gas, suggesting that the SMC gas is likely to be far from dynamical equilibrium. Furthermore, we find evidence that the SMC is currently undergoing tidal disruption by the LMC within 2 kpc of the centre of the SMC, and possibly all the way into the very core. This is revealed by a net outward motion of stars from the SMC centre along the direction towards the LMC and an apparent tangential anisotropy at all radii. The latter is expected if the SMC is undergoing significant tidal stripping, as we demonstrate using a suite of N-body simulations of the SMC/LMC system disrupting around the Milky Way. Our results suggest that dynamical models for the SMC that assume a steady state will need to be revisited.

A Drlica-Wagner, Jeffrey L Carlin, David L Nidever, P. S. Ferguson, N Kuropatkin, M. Adamów, W. Cerny, Yumi Choi, J Esteves, C. E. Martínez-Vázquez, S Mau, A. E Miller, B. Mutlu-Pakdil, E Neilsen, K Olsen, A.B Pace, A. H. Riley, Joanna D Sakowska, DJ Sand, L. Santana-Silva, EJ Tollerud, D Tucker, AK Vivas, E Zaborowski, A. Zenteno, T Abbott, S Allam, K Bechtol, C Bell, Eric F. Bell, P Bilaji, C. R Bom, Julio A. Carballo-Bello, D. Crnojević, Maria-Rosa L. Cioni, A Diaz-Ocampo, T. L de Boer, DENIS ERKAL, RA Gruendl, D. Hernandez-Lang, ASHLEY KATE HUGHES, DEBORAH JAMES, L Johnson, TIANRU LI, Y.-Y. Mao, D Martinez-Delgado, POL MASSANA ZAPATA, M. McNanna, R Morgan, E. O. Nadler, NED Noël, A. Palmese, AHG Peter, ES Rykoff, JLG Sanchez, N. Shipp, Joshua D. Simon, A Smercina, M Soares-Santos, G Stringfellow, K. Tavangar, Roeland van der Marel, Alistair Walker, RH Wechsler, J. Wu, B Yanny, MF Fitzpatrick, Luan Huang, A Jacques, R Nikutta, AMY SCOTT (2021)The DECam Local Volume Exploration Survey: Overview and First Data Release, In: The Astrophysical journal. Supplement series256(2) IOP Publishing

The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ~15,000 deg2 of high Galactic latitude (|b| > 10°) southern sky to a 5σ depth of g, r, i, z ~ 23.5 mag. In addition, DELVE will cover a region of ~2200 deg2 around the Magellanic Clouds to a depth of g, r, i ~ 24.5 mag and an area of ~135 deg2 around four Magellanic analogs to a depth of g, i ~ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. Furthermore, we also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ~520 million astronomical sources covering ~5000 deg2 of the southern sky to a 5σ point-source depth of g = 24.3 mag, r = 23.9 mag, i = 23.3 mag, and z = 22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.

Vasily A. Belokurov, Denis Erkal (2018)Clouds in Arms, In: Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

We use astrometry and broad-band photometry from Data Release 2 of the ESA’s Gaia mission to map out low surface-brightness features in the stellar density distribution around the Large and Small Magellanic Clouds. The LMC appears to have grown two thin and long stellar streams in its Northern and Southern regions, highly reminiscent of spiral arms. We use computer simulations of the Magellanic Clouds’ in-fall to demonstrate that these arms were likely pulled out of the LMC’s disc due to the combined influence of the SMC’s most recent fly-by and the tidal field of the Milky Way.

N. Shipp, T. S. Li, A. B. Pace, D. Erkal, A. Drlica-Wagner, B. Yanny, V. Belokurov, W. Wester, S. E. Koposov, K. Kuehn, G. F. Lewis, J. D. Simpson, Z. Wan, D. B. Zucker, S. L. Martell, M. Y. Wang (2019)Proper Motions of Stellar Streams Discovered in the Dark Energy Survey, In: The Astrophysical Journal885(3) The American Astronomical Society

We cross-match high-precision astrometric data from Gaia DR2 with accurate multi-band photometry from the Dark Energy Survey (DES) DR1 to confidently measure proper motions for nine stellar streams in the DES footprint: Aliqa Uma, ATLAS, Chenab, Elqui, Indus, Jhelum, Phoenix, Tucana III, and Turranburra. We determine low-confidence proper motion measurements for four additional stellar streams: Ravi, Wambelong, Willka Yaku, and Turbio. We find evidence for a misalignment between stream tracks and the systemic proper motion of streams that may suggest a systematic gravitational in uence from the Large Magellanic Cloud. These proper motions, when combined with radial velocity measurements, will allow for detailed orbit modeling which can be used to constrain properties of the LMC and its on nearby streams, as well as global properties of the Milky Way's gravitational potential.

D. Boubert, D. Erkal, N. W. Evans, Robert Izzard (2017)Hypervelocity runaways from the Large Magellanic Cloud, In: Monthly Notices of the Royal Astronomical Society469(2)pp. 2151-2162

We explore the possibility that the observed population of Galactic hypervelocity stars (HVSs) originate as runaway stars from the Large Magellanic Cloud (LMC). Pairing a binary evolution code with an N-body simulation of the interaction of the LMC with the Milky Way, we predict the spatial distribution and kinematics of an LMC runaway population. We find that runaway stars from the LMC can contribute Galactic HVSs at a rate of 3 × 10−6 yr−1. This is composed of stars at different points of stellar evolution, ranging from the main sequence to those at the tip of the asymptotic giant branch. We find that the known B-type HVSs have kinematics that are consistent with an LMC origin. There is an additional population of hypervelocity white dwarfs whose progenitors were massive runaway stars. Runaways that are even more massive will themselves go supernova, producing a remnant whose velocity will be modulated by a supernova kick. This latter scenario has some exotic consequences, such as pulsars and supernovae far from star-forming regions, and a small rate of microlensing from compact sources around the halo of the LMC.

V Belokurov, Denis Erkal, NW Evans, SE Koposov, AJ Deason (2018)Co-formation of the disc and the stellar halo, In: Monthly Notices of the Royal Astronomical Society478(1)pp. 611-619 Oxford University Press

Using a large sample of Main Sequence stars with 7-D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stellar halo. We demonstrate that the halo’s velocity ellipsoid evolves strongly with metallicity. At the low [Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared to the tangential one) is mildly radial with 0.2 < β < 0.4. However, for stars with [Fe/H]> −1.7 we measure extreme values of β ∼ 0.9. Across the metallicity range considered, i.e. −3 1010M⊙ around the epoch of the Galactic disc formation, i.e. between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialisation of the massive progenitor’s orbit, amplified by the action of the growing disc.

Manuel Arca Sedda, Alessia Gualandris, Tuan Do, Anja Feldmeier-Krause, Nadine Neumayer, Denis Erkal (2020)On the origin of a rotating metal-poor stellar population in the Milky Way Nuclear Cluster, In: Astrophysical Journal Letters IOP Publishing

We explore the origin of a population of stars recently detected in the inner parsec of the Milky Way Nuclear Cluster (NC), which exhibit sub-solar metallicity and a higher rotation compared to the dominant population. Using state-of-the-art N-body simulations, we model the infall of massive stellar systems into the Galactic center, both of Galactic and extra-galactic origin. We show that the newly discovered population can either be the remnant of a massive star cluster formed a few kpc away from the Galactic center (Galactic scenario) or be accreted from a dwarf galaxy originally located at 10-100 kpc (extragalactic scenario) and that reached the Galactic center 3

T. S. Li, J. D. Simon, K. Kuehn, A. B. Pace, D. Erkal, K. Bechtol, B. Yanny, A. Drlica-Wagner, J. L. Marshall, C. Lidman, E. Balbinot, D. Carollo, S. Jenkins, C. E. Martínez-Vázquez, N. Shipp, K. M. Stringer, A. K. Vivas, A. R. Walker, R. H. Wechsler, F. B. Abdalla, S. Allam, J. Annis, S. Avila, E. Bertin, D. Brooks, E. Buckley-Geer, D. L. Burke, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, C. E. Cunha, C. B. D’Andrea, L. N. da Costa, C. Davis, J. De Vicente, P. Doel, T. F. Eifler, A. E. Evrard, B. Flaugher, J. Frieman, J. García-Bellido, E. Gaztanaga, D. W. Gerdes, D. Gruen, R. A. Gruendl, J. Gschwend, G. Gutierrez, W. G. Hartley, D. L. Hollowood, K. Honscheid, D. J. James, E. Krause, M. A. G. Maia, M. March, F. Menanteau, R. Miquel, A. A. Plazas, E. Sanchez, B. Santiago, V. Scarpine, R. Schindler, M. Schubnell, I. Sevilla-Noarbe, M. Smith, R. C. Smith, M. Soares-Santos, F. Sobreira, E. Suchyta, M. E. C. Swanson, G. Tarle, D. L. Tucker (2018)The First Tidally Disrupted Ultra-faint Dwarf Galaxy?: A Spectroscopic Analysis of the Tucana III Stream, In: The Astrophysical Journal866(1) IOP Publishing / The American Astronomical Society

We present a spectroscopic study of the tidal tails and core of the Milky Way satellite Tucana III, collectively referred to as the Tucana III stream, using the 2dF+AAOmega spectrograph on the Anglo-Australian Telescope and the IMACS spectrograph on the Magellan Baade Telescope. In addition to recovering the brightest nine previously known member stars in the Tucana III core, we identify 22 members in the tidal tails. We observe strong evidence for a velocity gradient of 8.0 ± 0.4 km/s-1 deg-1 over at least 3° on the sky. Based on the continuity in velocity, we confirm that the Tucana III tails are real tidal extensions of Tucana III. The large velocity gradient of the stream implies that Tucana III is likely on a radial orbit. We successfully obtain metallicities for four members in the core and 12 members in the tails. We find that members close to the ends of the stream tend to be more metal-poor than members in the core, indicating a possible metallicity gradient between the center of the progenitor halo and its edge. The spread in metallicity suggests that the progenitor of the Tucana III stream is likely a dwarf galaxy rather than a star cluster. Furthermore, we find that with the precise photometry of the Dark Energy Survey data, there is a discernible color offset between metal-rich disk stars and metal-poor stream members. This metallicity-dependent color offers a more efficient method to recognize metal-poor targets and will increase the selection efficiency of stream members for future spectroscopic follow-up programs on stellar streams.

Alis J Deason, Denis Erkal, Vasily Belokurov, Azadeh Fattahi, Facundo A Gómez, Robert J. J Grand, Rüdiger Pakmor, Xiang-Xiang Xue, Chao Liu, Chengqun Yang, Lan Zhang, Gang Zhao (2020)The mass of the Milky Way out to 100 kpc using halo stars, In: Monthly Notices of the Royal Astronomical Society501(4)pp. 5964-5972 Royal Astronomical Society

We use a distribution function analysis to estimate the mass of the Milky Way out to 100 kpc using a large sample of halo stars. These stars are compiled from the literature, and the vast majority (~98%) have 6D phase-space information. We pay particular attention to systematic effects, such as the dynamical influence of the Large Magellanic Cloud (LMC), and the effect of unrelaxed substructure. The LMC biases the (pre-LMC infall) halo mass estimates towards higher values, while realistic stellar halos from cosmological simulations tend to underestimate the true halo mass. After applying our method to the Milky Way data we find a mass within 100 kpc of M(< 100 kpc) = 6.07 +/- 0.29 (stat.) +/- 1.21 (sys.) x 10^11 M_Sun. For this estimate, we have approximately corrected for the reflex motion induced by the LMC using the Erkal et al. model, which assumes a rigid potential for the LMC and MW. Furthermore, stars that likely belong to the Sagittarius stream are removed, and we include a 5% systematic bias, and a 20% systematic uncertainty based on our tests with cosmological simulations. Assuming the mass-concentration relation for Navarro-Frenk-White haloes, our mass estimate favours a total (pre-LMC infall) Milky Way mass of M_200c = 1.01 +/- 0.24 x 10^12 M_Sun, or (post-LMC infall) mass of M_200c = 1.16 +/- 0.24 x 10^12 M_Sun when a 1.5 x 10^11 M_Sun mass of a rigid LMC is included.

L R Cullinane, A D Mackey, G S Da Costa, D Erkal, S E Koposov, V Belokurov (2021)The Magellanic Edges Survey II. Formation of the LMC's northern arm, In: Monthly Notices of the Royal Astronomical Societypp. 1-25 Oxford University Press

The highly-substructured outskirts of the Magellanic Clouds provide ideal locations for studying the complex interaction history between both Clouds and the Milky Way (MW). In this paper, we investigate the origin of a >20 • long arm-like feature in the northern outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. We find that the arm has a similar geometry and metallicity to the nearby outer LMC disk, indicating that it is comprised of perturbed disk material. Whilst the azimuthal velocity and velocity dispersions along the arm are consistent with those in the outer LMC, the in-plane radial velocity and out-of-plane vertical velocity are significantly perturbed from equilibrium disk kinematics. We compare these observations to a new suite of dynamical models of the Magellanic/MW system, which describe the LMC as a collection of tracer particles within a rigid potential, and the SMC as a rigid Hernquist potential. Our models indicate the tidal force of the MW during the LMC's infall is likely responsible for the observed increasing out-of-plane velocity along the arm. Our models also suggest close LMC/SMC interactions within the past Gyr, particularly the SMC's pericentric passage ~150 Myr ago and a possible SMC crossing of the LMC disk plane ~400 Myr ago, likely do not perturb stars that today comprise the arm. Historical interactions with the SMC prior to ~1 Gyr ago may be required to explain some of the observed kinematic properties of the arm, in particular its strongly negative in-plane radial velocity.

Vasily A Belokurov, Denis Erkal (2020)Limit on the LMC mass from a census of its satellites, In: Monthly Notices of the Royal Astronomical Society495(3)pp. 2554-2563 Oxford University Press (OUP)

We study the orbits of dwarf galaxies in the combined presence of the Milky Way and Large Magellanic Cloud (LMC) and find six dwarfs that were likely accreted with the LMC (Car 2, Car 3, Hor 1, Hyi 1, Phe 2, and Ret 2), in addition to the Small Magellanic Cloud (SMC), representing strong evidence of dwarf galaxy group infall. This procedure depends on the gravitational pull of the LMC, allowing us to place a lower bound on the Cloud’s mass of MLMC˃1.24×1011M⊙ if we assume that these are LMC satellites. This mass estimate is validated by applying the technique to a cosmological zoom-in simulation of a Milky Way-like galaxy with an LMC analogue where we find that while this lower bound may be overestimated, it will improve in the future with smaller observational errors. We apply this technique to dwarf galaxies lacking radial velocities and find that Eri 3 has a broad range of radial velocities for which it has a significant chance (˃0.4) of having been bound to the Cloud. We study the non-Magellanic classical satellites and find that Fornax has an appreciable probability of being an LMC satellite if the LMC is sufficiently massive (⁠∼2.5×1011M⊙⁠). In addition, we explore how the orbits of Milky Way satellites change in the presence of the LMC and find a significant change for several objects. Finally, we find that the dwarf galaxies likely to be LMC satellites are slightly smaller than Milky Way satellites at a fixed luminosity, possibly due to the different tidal environments they have experienced.

Alexandra L. Gregory, Michelle L. M. Collins, Denis Erkal, Erik Tollerud, Maxime Delorme, Lewis Hill, David J. Sand, Jay Strader, Beth Willman (2020)Uncovering the Orbit of the Hercules Dwarf Galaxy, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We present new chemo{kinematics of the Hercules dwarf galaxy based on Keck II{ DEIMOS spectroscopy. Our 21 conrmed members, including 9 newly con- rmed members, have a systemic velocity of vHerc = 46:4 1:3 kms

Denis Erkal, Douglas Boubert, Alessia Gualandris, N. Wyn Evans, Fabio Antonini (2018)A hypervelocity star with a Magellanic origin, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

Using proper motion measurements from Gaia DR2, we probe the origin of 26 previously known hypervelocity stars (HVSs) around the Milky Way. We find that a significant fraction of these stars have a high probability of originating close to the Milky Way centre, but there is one obvious outlier. HVS3 is highly likely to be coming almost from the centre of the Large Magellanic Cloud (LMC). During its closest approach, 21.1 +6.1 −4.6 Myr ago, it had a relative velocity of 870 +69 −66 kms −1 with respect to the LMC. This large kick velocity is only consistent with the Hills mechanism, requiring a massive black hole at the centre of the LMC. This provides strong direct evidence that the LMC itself harbours a massive black hole of at least 4×10 3 −10 4 M ⊙ .

Anirudh Chiti, Anna Frebel, Joshua D Simon, Denis Erkal, Laura J Chang, Lina Necib, Alexander P Ji, Helmut Jerjen, Dongwon Kim, John E Norris (2021)An extended halo around an ancient dwarf galaxy, In: NATURE ASTRONOMY Springer Nature

The Milky Way is surrounded by dozens of ultra-faint (< $10^5$ solar luminosities) dwarf satellite galaxies. They are the surviving remnants of the earliest galaxies, as confirmed by their ancient (~13 billion years old) and chemically primitive stars. Simulations suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and supernova feedback. However, the signatures of these events would lie outside their core regions (>2 half-light radii), which are spectroscopically unstudied due to the sparseness of their distant stars. Here we identify members of the Tucana II ultra-faint dwarf galaxy in its outer region (up to 9 half-light radii), demonstrating the system to be dramatically more spatially extended and chemically primitive than previously found. These distant stars are extremely metal-poor (=-3.02; less than ~1/1000th of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish, for the first time, an extended dark matter halo surrounding an ultra-faint dwarf galaxy out to one kiloparsec, with a total mass of >$10^7$ solar masses. This measurement is consistent with the expected ~2x$10^7$ solar masses using a generalized NFW density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger. We demonstrate that spatially extended stellar populations, which other ultra-faint dwarfs hint at hosting as well, are observable in principle and open the possibility for detailed studies of the stellar halos of relic galaxies.

Kathy Vivas, Clara Martínez-Vázquez, Alistair Walker, Vasily Belokurov, Ting Li, Denis Erkal (2021)Variable Stars in the giant satellite galaxy Antlia 2, In: The Astrophysical Journal American Astronomical Society

We report 350 pulsating variable stars found in four DECam fields (∼ 12 sq. deg.) covering the Antlia 2 satellite galaxy. The sample of variables includes 318 RR Lyrae stars and eight anomalous Cepheids in the galaxy. Reclassification of several objects designated previously to be RR Lyrae as Anomalous Cepheids gets rid of the satellite's stars intervening along the line of sight. This in turn removes the need for prolific tidal disruption of the dwarf, in agreement with the recently updated proper motion and peri-centre measurements based on Gaia EDR3. There are also several bright foreground RR Lyrae stars in the field, and two distant background variables located ∼ 45 kpc behind Antlia 2. We found RR Lyrae stars over the full search area, suggesting that the galaxy is very large and likely extends beyond our observed area. The mean period of the RRab in Antlia 2 is 0.599 days, while the RRc have a mean period of 0.368 days, indicating the galaxy is an Oosterhoff-intermediate system. The distance to Antlia 2 based on the RR Lyrae stars is 124.1 kpc (µ 0 = 20.47) with a dispersion of 5.4 kpc. We measured a clear distance gradient along the semi-major axis of the galaxy, with the SouthEast side of Antlia 2 being ∼ 13 kpc farther away from the NorthWest side. This elongation along the line of sight is likely due to the ongoing tidal disruption of Ant 2.

Pol Massana, Noelia E D Noël, David L Nidever, Denis Erkal, Thomas J L de Boer, Yumi Choi, Steven R Majewski, Knut Olsen, Antonela Monachesi, Carme Gallart, Roeland P van der Marel, Tomás Ruiz-Lara, Dennis Zaritsky, Nicolas F Martin, Ricardo R Muñoz, Maria-Rosa L Cioni, Cameron P M Bell, Eric F Bell, Guy S Stringfellow, Vasily Belokurov, Matteo Monelli, Alistair R Walker, David Martínez-Delgado, A Katherina Vivas, Blair C Conn (2020)SMASHing the low surface brightness SMC, In: Monthly Notices of the Royal Astronomical Society498(1)pp. 1034-1049 Oxford University Press

The periphery of the Small Magellanic Cloud (SMC) can unlock important information regarding galaxy formation and evolution in interacting systems. Here, we present a detailed study of the extended stellar structure of the SMC using deep colour–magnitude diagrams, obtained as part of the Survey of the MAgellanic Stellar History (SMASH). Special care was taken in the decontamination of our data from Milky Way (MW) foreground stars, including from foreground globular clusters NGC 362 and 47 Tuc. We derived the SMC surface brightness using a ‘conservative’ approach from which we calculated the general parameters of the SMC, finding a staggered surface brightness profile. We also traced the fainter outskirts by constructing a stellar density profile. This approach, based on stellar counts of the oldest main-sequence turn-off stars, uncovered a tidally disrupted stellar feature that reaches as far out as 12 deg from the SMC centre. We also serendipitously found a faint feature of unknown origin located at ∼14 deg from the centre of the SMC and that we tentatively associated with a more distant structure. We compared our results to in-house simulations of a 1 × 109 M⊙ SMC, finding that its elliptical shape can be explained by its tidal disruption under the combined presence of the MW and the Large Magellanic Cloud. Finally, we found that the older stellar populations show a smooth profile while the younger component presents a jump in the density followed by a flat profile, confirming the heavily disturbed nature of the SMC.

T.J.L de Boer, Denis Erkal, M. Gieles (2020)A closer look at the spur, blob, wiggle, and gaps in GD-1, In: Monthly Notices of the Royal Astronomical Society Royal Astronomical Society

The GD-1 stream is one of the longest and coldest stellar streams discovered to date, and one of the best objects for constraining the dark matter properties of the Milky Way. Using data from Gaia DR2 we study the proper motions, distance, morphology and density of the streamto uncover small scale perturbations. The proper motion cleaned data shows a clear distance gradient across the stream, ranging from 7 to 12 kpc. However, unlike earlier studies thatfound a continuous gradient, we uncover a distance minimum atφ1≈-50 deg, after which the distance increases again. We can reliably trace the stream between -85< φ1

J.D Simon, T.S Li, D. Erkal, A.B Pace, A. Drlica-Wagner, D.J James, J.L Marshall, K. Bechtol, T. Hansen, K. Kuehn, C. Lidman, S. Allam, J. Annis, S. Avila, E. Bertin, D. Brooks, D.L Burke, A. Carnero Rosell, M. Carrosco Kind, J. Carretero, L.N da Costa, J. De Vicente, S. Desai, P. Doel, T.F Eifler, S. Everett, P. Fosalba, J. Frieman, J. Garcia-Bellido, E. Gaztanaga, D.W Gerdes, D. Gruen, R.A Gruendi, J. Gschwend, G. Gutierrez, D.L Hollowood, K. Honscheid, E. Krause, N. Kuropatkin, N. MacCrann, M.A.G Maia, M. March, R. Miquel, A. Palmese, F. Paz-Chinchon, A.A Plazas, K. Reil, A. Roodman, E. Sanchez, B. Santiago, V. Scarpine, M. Schubnell, S. Serrano, M. Smith, E. Suchyta, G. Tarle, A.R Walker (2020)BIRDS OF A FEATHER? MAGELLAN/IMACS SPECTROSCOPY OF THE ULTRA-FAINT SATELLITES GRUS II, TUCANA IV, AND TUCANA V, In: The Astrophysical Journal IOP Publishing

We present Magellan/IMACS spectroscopy of three recently discovered ultra-faint Milky Way satellites, Grus II, Tucana IV, and Tucana V. We measure systemic velocities of vhel = −110.0±0.5 km s−1, vhel = 15.9+1.8 −1.7 km s−1, and vhel = −36.2+2.5 −2.2 km s−1 for the three objects, respectively. Their large relative velocities demonstrate that the satellites are unrelated despite their close physical proximity. We determine a velocity dispersion for Tuc IV of σ = 4.3+1.7 −1.0 km s−1, but we cannot resolve the velocity dispersions of the other two systems. For Gru II we place an upper limit (90% confidence) on the dispersion of σ < 1.9 km s−1, and for Tuc V we do not obtain any useful limits. All three satellites have metallicities below [Fe/H] = −2.1, but none has a detectable metallicity spread. We determine proper motions for each satellite based on Gaia astrometry and compute their orbits around the Milky Way. Gru II is on a tightly bound orbit with a pericenter of 25+6 −7 kpc and orbital eccentricity of 0.45+0.08 −0.05. Tuc V likely has an apocenter beyond 100 kpc, and could be approaching the Milky Way for the first time. The current orbit of Tuc IV is similar to that of Gru II, with a pericenter of 25+11 −8 kpc and an eccentricity of 0.36+0.13 −0.06. However, a backward integration of the position of Tuc IV demonstrates that it collided with the Large Magellanic Cloud at an impact parameter of 4 kpc ∼ 120 Myr ago, deflecting its trajectory and possibly altering its internal kinematics. Based on their sizes, masses, and metallicities, we classify Gru II and Tuc IV as likely dwarf galaxies, but the nature of Tuc V remains uncertain.

DENIS ERKAL, Alis J. Deason, Vasily Belokurov, Xiang-Xiang Xue, Sergey E. Koposov, Sarah A. Bird, Chao Liu, Iulia T. Simion, Chengqun Yang, Lan Zhang, Gang Zhao (2021)Detection of the LMC-induced sloshing of the Galactic halo, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

A wealth of recent studies have shown that the LMC is likely massive, with a halo mass > 10¹¹MꙨ. One consequence of having such a nearby and massive neighbour is that the inner Milky Way is expected to be accelerated with respect to our Galaxy’s outskirts (beyond ~ 30 kpc). In this work we compile a sample of ~ 500 stars with radial velocities in the distant stellar halo, rGC > 50 kpc, to test this hypothesis. These stars span a large fraction of the sky and thus give a global view of the stellar halo. We find that stars in the Southern hemisphere are on average blueshifted, while stars in the North are redshifted, consistent with the expected, mostly downwards acceleration of the inner halo due to the LMC. We compare these results with simulations and find the signal is consistent with the infall of a 1:5 10¹¹MꙨ LMC. We cross-match our stellar sample with Gaia DR2 and find that the mean proper motions are not yet precise enough to discern the LMC’s effect. Our results show that the Milky Way is significantly out of equilibrium and that the LMC has a substantial effect on our Galaxy.