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Dr Denis Erkal

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

My publications


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, 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 from the Sun and 24 kpc from the LMC. From spectroscopy of 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
?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
Erkal Denis, Li T S, Koposov S E, Belokurov V, Balbinot E, Bechtol K, Buncher B, Drlica-Wagner A, Kuehn K, Marshall J L, Martínez-Vázquez C E, Pace A B, Shipp N, Simon J D, Stringer K M, Vivas A K, Wechsler R H, Yanny B, Abdalla F B, Allam S, Annis J, Avila S, Bertin E, Brooks D, Buckley-Geer E, Burke D L, Rosell A Carnero, Kind M Carrasco, Carretero J, D?Andrea C B, da Costa L N, Davis C, De Vicente J, Doel P, Eifler T F, Evrard A E, Flaugher B, Frieman J, García-Bellido J, Gaztanaga E, Gerdes D W, Gruen D, Gruendl R A, Gschwend J, Gutierrez G, 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, Swanson M E C, Tarle G, Tucker D L, Walker A R (2018) Modelling the Tucana III stream - a close passage with the LMC, Monthly Notices of the Royal Astronomical Society 481 (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.
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, Monthly Notices of the Royal Astronomical Society 480 (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.
Belokurov V, Erkal Denis, Evans NW, Koposov SE, Deason AJ (2018) Co-formation of the disc and the stellar halo, Monthly Notices of the Royal Astronomical Society 478 (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 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 for stars with [Fe/H]> ?1.7 we measure extreme values of ² range considered, i.e. ?3 20 ) 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
Erkal Denis, Boubert Douglas, Gualandris Alessia, Evans N. Wyn, Antonini Fabio (2018) A hypervelocity star with a Magellanic origin, 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 ™ .
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., Carollo D., Jenkins S., Martínez-Vázquez C. E., Shipp N., Stringer K. M., Vivas A. K., Walker A. R., Wechsler R. H., Abdalla F. B., Allam S., Annis J., Avila S., Bertin E., Brooks D., Buckley-Geer E., Burke D. L., Rosell A. Carnero, Kind M. Carrasco, Carretero J., Cunha C. E., D?Andrea C. B., Costa L. N. da, Davis C., Vicente J. De, Doel P., Eifler T. F., Evrard A. E., Flaugher B., Frieman J., García-Bellido J., Gaztanaga E., Gerdes D. W., Gruen D., Gruendl R. A., Gschwend J., Gutierrez G., Hartley W. G., Hollowood D. L., Honscheid K., James D. J., Krause E., Maia M. A. G., March M., Menanteau F., Miquel R., Plazas A. A., Sanchez E., Santiago B., Scarpine V., Schindler R., Schubnell M., Sevilla-Noarbe I., Smith M., Smith R. C., Soares-Santos M., Sobreira F., Suchyta E., Swanson M. E. C., Tarle G., Tucker D. L. (2018) The First Tidally Disrupted Ultra-faint Dwarf Galaxy?: A Spectroscopic Analysis of the Tucana III Stream, The Astrophysical Journal 866 (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.
Belokurov Vasily A., Erkal Denis (2018) Clouds in Arms, 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, Belokurov V, Erkal Denis, Iorio G (2018) A Magellanic origin for the Virgo substructure, Monthly Notices of the Royal Astronomical Society sty3014 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
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 M2009M™, 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, èSFRé, 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 èSFRé?M200 relation in ›CDM. 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 èSFRé?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 M2009M™ in the Milky Way. We discuss how this empirical method can be applied to a larger sample of nearby spiral galaxies.
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, 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.