# Dr Denis Erkal

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

Astrophysics Research Group.

### Areas of specialism

Galactic archaeology; Near field cosmology

### Publications

Erkal Denis, Belokurov Vasily A, Parkin Daniel L (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.
Sanders Jason L, Lilley Edward J, Vasiliev Eugene, Evans N Wyn, Erkal Denis (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.
Read J. I., Erkal D. (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.
Li T S, Koposov S E, Zucker D B, Lewis G F, Kuehn K, Simpson J D, Ji A P, Shipp N, Mao Y-Y, Geha M, Pace A B, Mackey A D, Allam S, Tucker D L, Da Costa G S, Erkal Denis, Simon J D, De Silva G M, Mould J R, Martell S L, Wan Z, Bechtol K, Balbinot E, Belokurov V, Bland-Hawthorn J, Sharma S, Casey A R, Cullinane L, Drlica-Wagner A, Vivas A K, Wechsler R H, Yanny B (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 http://s5collab.github.io.
Jethwa P, Torrealba G, Navarrete C, Carballo-Bello J.A., de Boer Thomas, Erkal Denis, Koposov S.E., Duffau S, Geisler D, Catelan M, Belokurov V (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.
De Leo Michele, Carrera Ricardo, Noel Noelia E.D, Read Justin I., Erkal Denis, Gallart Carme 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.
Boubert D, Belokurov V, Erkal Denis, Iorio G (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.
Belokurov Vasily A., Erkal Denis (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.
Boubert D., Erkal D., Evans N. W., Izzard Robert (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.
Koposov Sergey E, Walker Matthew G, Belokurov Vasily, Casey Andrew R, Geringer-Sameth Alex, Mackey Dougal, Da Costa Gary, Erkal Denis, Jethwa Prashin, Mateo Mario, Olszewski Edward W, Bailey III John I (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.
Li T. S., Simon J. D., Kuehn K., Pace A. B., Erkal D., Bechtol K., Yanny B., Drlica-Wagner A., Marshall J. L., Lidman C., Balbinot E., Shipp N., Carollo D., Jenkins S., Martínez-Vázquez C. E., Stringer K. M., Vivas A. K., Walker A. R., Wechsler R. H., Avila S., Abdalla F. B., Allam S., Annis J., Bertin E., Brooks D., Buckley-Geer E., Burke D. L., Rosell A. Carnero, D’Andrea C. B., Kind M. Carrasco, Carretero J., Cunha C. E., Costa L. N. da, Davis C., Vicente J. De, Doel P., Eifler T. F., García-Bellido J., Evrard A. E., Flaugher B., Frieman J., Gaztanaga E., Gerdes D. W., Gruen D., Gruendl R. A., Gschwend J., Gutierrez G., James D. J., Hartley W. G., Hollowood D. L., Honscheid K., Krause E., Maia M. A. G., March M., Menanteau F., Miquel R., Plazas A. A., Schindler R., Sanchez E., Santiago B., Scarpine V., Schubnell M., Sevilla-Noarbe I., Swanson M. E. C., Smith M., Smith R. C., Soares-Santos M., Sobreira F., Suchyta E., Tarle G., Tucker D. L. (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.
Sedda Manuel Arca, Gualandris Alessia, Do Tuan, Feldmeier-Krause Anja, Neumayer Nadine, Erkal Denis 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
Erkal Denis, Boubert Douglas, Gualandris Alessia, Evans N. Wyn, Antonini Fabio (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 ⊙ .
Sand David J., Strader Jay, Willman Beth, Gregory Alexandra L., Collins Michelle L. M., Erkal Denis, Tollerud Erik, Delorme Maxime, Hill Lewis 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
Simon J.D, Li T.S, Erkal D., Pace A.B, Drlica-Wagner A., James D.J, Marshall J.L, Bechtol K., Hansen T., Kuehn K., Lidman C., Allam S., Annis J., Avila S., Bertin E., Brooks D., Carretero J., Burke D.L, Rosell A. Carnero, Kind M. Carrosco, da Costa L.N, De Vicente J., Desai S., Doel P., Eifler T.F, Garcia-Bellido J., Everett S., Fosalba P., Frieman J., Gaztanaga E., Gerdes D.W, Gruen D., Gruendi R.A, Gschwend J., Gutierrez G., Kuropatkin N., Hollowood D.L, Honscheid K., Krause E., MacCrann N., Maia M.A.G, March M., Miquel R., Palmese A., Paz-Chinchon F., Sanchez E., Plazas A.A, Reil K., Roodman A., Santiago B., Scarpine V., Schubnell M., Serrano S., Smith M., Suchyta E., Tarle G., Walker A.R 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.
Boubert D., Erkal D., Gualandris A. 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
Yanny B., Zenteno A., Allam S., Barkhouse W. A., Bechtol K., Bell E. F., Balaji P., Crnojević D., Esteves J., Ferguson P. S., Gallart C., Hughes A. K., James D. J., Jethwa P., Mao Y.-Y., Johnson L. C., Kuehn K., Majewski S., Massana P., McNanna M., Monachesi A., Nadler E. O., Noel N.E.D, Sanchez J., Palmese A., Paz-Chinchon F., Pieres A., Shipp N., Simon J. D., Soares-Santos M., Tavangar K., van der Marel R. P., Vivas A. K., Pace A. B., Walker A. R., Wechsler R. H., Mau S., Cerny W., Choi Y., Erkal D., Drlica-Wagner A., Santana-Silva L., Riley A. H., Stringfellow G. S., Adamów M., Carlin J. L., Gruendl R. A., Hernandez-Lang D., Kuropatkin N., Mutlu-Pakdil B., Li T. S., Martínez-Vázquez C. E., Morganson E., Neilsen E. H., Nidever D. L., Olsen K. A. G., Sand D. J., Tollerud E. J., Tucker D. L. (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.
Noel Noelia, Palmese A., Paz-Chinchon F., Pieres A., Sanchez J., Shipp N., Simon J.D, Soares-Santos M., Tavangar K., van der Marel R.P, Vivas A.K, Pace A.B, Walker A.R, Wechsler R.H, Mau S., Cerny W., Choi Y., Drlica-Wagner A., Santana-Silver L., Riley A.H, Erkal Denis, Stringfellow S., Adamow M., Kuropatkin N., Carlin J.L, Gruendal R.A, Hernandez-Lang D., Li T.S, Martinez-Vasquez C.E, Morganson E., Mutlu-Pakdil B., Neilsen E.H, Nidever D.L, Tucker D.L, Olsen K.A.G, Sand D.J, Tollerud E.J, Yanny B., Zenteno A., Allam S., Barkhouse W.A, Bechtol K., Bell E.F, Balaji P., Crnojevic D., Esteves J., Ferguson P.S, Gallart C., Hughes A.K, James D.J, Jethwa P., Mao Y-Y., Johnson L.C, Kuehn K., Majewski S., Massana P., McNanna M., Monachesi A., Nadler E.O 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
Shipp N., Li T. S., Pace A. B., Erkal D., Drlica-Wagner A., Yanny B., Belokurov V., Wester W., Koposov S. E., Kuehn K., Lewis G. F., Simpson J. D., Wan Z., Zucker D. B., Martell S. L., Wang M. Y. (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.
de Boer T.J.L, Erkal Denis, Gieles M. 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<15 deg, show-ing an even further extent to GD-1 beyond the earlier extension of Price-Whelan & Bonaca(2018). We constrain the stream track and density using a Boolean matched filter approachand find three large under densities and find significant residuals in the stream track lining upwith these gaps. In particular, a gap is visible atφ1=-3 deg, surrounded by a clear sinusoidal wiggle. We argue that this wiggle is due to a perturbation since it has the wrong orientationto come from a progenitor. We compute a total initial stellar mass of the stream segment of 1.58±0.07×104M. With the extended view of the spur in this work, we argue that the spurmay be unrelated to the adjacent gap in the stream. Finally, we show that an interaction withthe Sagittarius dwarf can create features similar to the spur.
Lewis Geraint F, Mackey Dougal, Simpson Jeffrey D, Shipp Nora, Wan Zhen, Belokurov Vasily, Bland-Hawthorn Joss, Martell Sarah L, Nordlander Thomas, Pace Andrew B, De Silva Gayandhi M, Wang Mei-Yu, Erkal Denis, Koposov Sergey E, Boubert Douglas, Li Ting S, Da Costa Gary S, Kuehn Kyler, Zucker Daniel B, Ji Alexander P (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.
Erkal D, Belokurov V, Laporte C F P, Koposov S E, Li T S, Grillmair C J, Kallivayalil N, Price-Whelan A M, Evans N W, Hawkins K, Hendel D, Mateu C, Navarro J F, Pino A del, Slater C T, Sohn S T (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.
Belokurov Vasily A, Erkal Denis (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.
Hartley W G, Hollowood D L, Honscheid K, James D J, Krause E, Maia M A G, March M, Menanteau F, Miquel R, Ogando R L C, Plazas A A, Sanchez E, Santiago B, Scarpine V, Schindler R, Sevilla-Noarbe I, Smith M, Smith R C, Soares-Santos M, Sobreira F, Suchyta E, Walker A R, Swanson M E C, Tarle G, Tucker D L, Bechtol K, Erkal Denis, Li T S, Koposov S E, Belokurov V, Balbinot E, Buncher B, Drlica-Wagner A, Kuehn K, Shipp N, Marshall J L, Martínez-Vázquez C E, Pace A B, Simon J D, Stringer K M, Vivas A K, Wechsler R H, Annis J, Yanny B, Abdalla F B, Allam S, Avila S, Bertin E, Brooks D, Buckley-Geer E, Burke D L, D’Andrea C B, Rosell A Carnero, Kind M Carrasco, Carretero J, da Costa L N, Davis C, De Vicente J, Doel P, Eifler T F, García-Bellido J, Evrard A E, Flaugher B, Frieman J, Gaztanaga E, Gerdes D W, Gruen D, Gruendl R A, Gschwend J, Gutierrez G (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.
Belokurov V, Erkal Denis, Evans NW, Koposov SE, Deason AJ (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 <[Fe/H]−1, the stellar halo’s spin is minimal, at the level of 20 < v¯θ(kms−1 ) < 30. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo were deposited in a major accretion event by a satellite with Mvir > 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.