Dr Michelle Collins


Phone: Work: 01483 68 3572
Room no: 24 BC 03

Further information


Michelle Collins was awarded a PhD in Astronomy by the University of Cambridge in 2011, where she worked under the supervision of Scott Chapman and Mike Irwin. After receiving her degree, she continued her research for 3 years as a Galaxies and Cosmology fellow at the Max-Planck Institute for Astronomy in Heidelberg. In October 2014, she took up a Hubble Fellowship at Yale University, working alongside Marla Geha and her research group. She officially joined the University of Surrey as a lecturer at the same time, and will be permanently based at Surrey from October 2015. Her research focuses on the observations of Local Group galaxies, particularly on performing resolved stellar spectroscopy in Andromeda and its dwarf galaxies in order to constrain dark matter models, and the processes governing galaxy evolution.

Research Interests

  • Galactic archaeology
  • Galaxy formation and evolution
  • Local Group galaxies
  • Dark matter


Journal articles

  • Contenta F, Balbinot E, Petts J, Read J, Gieles M, Collins M, Peñarrubia J, Delorme M, Gualandris A. (2018) 'Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II'. Monthly Notices of the Royal Astronomical Society,
    [ Status: Accepted ]


    We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results imply that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.

  • Collins MLM, Tollerud E, Sand D, Bonaca A, Willman B, Strader J. (2017) 'Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V'. Monthly Notices of the Royal Astronomical Society, 467 (1), pp. 573-585.


    We present a chemodynamical analysis of the Leo V dwarf galaxy, based on Keck II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system of hvr i = 170.9 +2.1 −1.9 km s−1 , and barely resolve a velocity dispersion for the system, with σvr = 2.3 +3.2 −1.6 km s−1 , consistent with previous studies of Leo V. The poorly resolved dispersion means we are unable to adequately constrain the dark matter content of Leo V. We find an average metallicity for the dwarf of [Fe/H]= −2.48 ± 0.21, and measure a significant spread in the iron abundance of its member stars, with −3.1 ≤[Fe/H]≤ −1.9 dex, which cleanly identifies Leo V as a dwarf galaxy that has been able to self-enrich its stellar population through extended star formation. Owing to the tentative photometric evidence for tidal substructure around Leo V, we also investigate whether there is any evidence for tidal stripping or shocking of the system within its dynamics. We measure a significant velocity gradient across the system, of dv dχ = −4.1 +2.8 −2.6 km s−1 per arcmin (or dv dχ = −71.9 +50.8 −45.6 km s−1 kpc−1 ), which points almost directly toward the Galactic centre. We argue that Leo V is likely a dwarf on the brink of dissolution, having just barely survived a past encounter with the centre of the Milky Way.

  • Küpper A, Johnston K, Mieske S, Collins M, Tollerud E. (2017) 'Exploding Satellites -- The Tidal Debris of the Ultra-Faint Dwarf Galaxy Hercules'. The Astrophysical Journal: an international review of astronomy and astronomical physics, 834 (2)


    The ultra-faint satellite galaxy Hercules has a strongly elongated and irregular morphology with detections of tidal features up to 1.3 deg (3 kpc) from its center. This suggests that Hercules may be dissolving under the Milky Way’s gravitational influence, and hence could be a tidal stream in formation rather than a bound, dark-matter dominated satellite. Using Bayesian inference in combination with N-body simulations, we show that Hercules has to be on a very eccentric orbit ( ≈ 0.95) within the Milky Way in this scenario. On such an orbit, Hercules “explodes” as a consequence of the last tidal shock at pericenter 0.5 Gyr ago. It is currently decelerating towards apocenter of its orbit with a velocity of V = 157 km s−1 – of which 99% is directed radially outwards. Due to differential orbital precession caused by the non-spherical nature of the Galactic potential, its debris fans out nearly perpendicular to its orbit. This explains why Hercules has an elongated shape without showing a distance gradient along its main body: it is in fact a stream that is significantly broader than it is long. In other words, it is moving perpendicular to its apparent major axis. In this scenario, there is a spike in the radial velocity profile created by the dominant debris component that formed through the last pericenter passage. This is similar to kinematic substructure that is observed in the real Hercules. Modeling a satellite on such a highly eccentric orbit is strongly dependent on the form of the Galactic potential. We therefore propose that detailed kinematic investigation of Hercules and other exploding satellite candidates can yield strong constraints on the potential of the Milky Way.

  • Contenta F, Gieles M, Balbinot E, Collins MLM. (2016) 'The contribution of dissolving star clusters to the population of ultra faint objects in the outer halo of the Milky Way'. Monthly Notices of the Royal Astronomical Society, 466 (2), pp. 1741-1756.


    In the last decade, several ultra faint objects (UFOs, MV −3.5) have been discovered in the outer halo of the Milky Way. For some of these objects, it is not clear whether they are star clusters or (ultra faint) dwarf galaxies. In this work, we quantify the contribution of star clusters to the population of UFOs. We extrapolated the mass and Galactocentric radius distribution of the globular clusters using a population model, finding that the Milky Way contains about 3.3+7.3 −1.6 star clusters with MV −3.5 and Galactocentric radius ≥20 kpc. To understand whether dissolving clusters can appear as UFOs, we run a suite of direct N-body models, varying the orbit, the Galactic potential, the binary fraction and the black hole (BH) natal kick velocities. In the analyses, we consider observational biases such as luminosity limit, field stars and line-of-sight projection. We find that star clusters contribute to both the compact and the extended population of UFOs: clusters without BHs appear compact with radii ∼5 pc, while clusters that retain their BHs after formation have radii 20 pc. The properties of the extended clusters are remarkably similar to those of dwarf galaxies: high-inferred mass-to-light ratios due to binaries, binary properties mildly affected by dynamical evolution, no observable mass segregation and flattened stellar mass function. We conclude that the slope of the stellar mass function as a function of Galactocentric radius and the presence/absence of cold streams can discriminate between dark matter-free and dark matter-dominated UFOs.

  • Voggel K, Hilker M, Baumgardt H, Collins MLM, Grebel E, Husemann B, Richtler T, Frank M. (2016) 'Probing the boundary between star clusters and dwarf galaxies: A MUSE view on the dynamics of Crater/Laevens I'. Monthly Notices of the Royal Astronomical Society, 460 (3), pp. 3384-3397.
    [ Status: Accepted ]


    We present MUSE observations of the debated ultra faint stellar system Crater. We spectroscopically confirm 26 member stars of this system via radial velocity measure-ments. We derive the systematic instrumental velocity uncertainty of MUSE spectra to be 2.27kms−1. This new dataset increases the confirmed member stars of Crater by a factor of 3. One out of three bright blue stars and a fainter blue star just above the main-sequence-turn-off are also found to be likely members of the system. The ob-servations reveal that Crater has a systemic radial velocity of vsys = 148.18+1.08−1.15 kms−1, whereas the most likely velocity dispersion of this system is σv = 2.04+2.19−1.06 kms−1. The total dynamical mass of the system, assuming dynamical equilibrium is thenMtot = 1.50+4.9−1.2 · 105M implying a mass-to-light ratio of M/LV=8.52+28.0−6.5 M/L, which is consistent with a purely baryonic stellar population within its errors and no sig-nificant evidence for the presence dark matter was found. We also find evidence for a velocity gradient in the radial velocity distribution. We conclude that our findings strongly support that Crater is a faint intermediate-age outer halo globular cluster and not a dwarf galaxy.

  • Martin NF, Ibata RA, Collins MLM, Rich RM, Bell EF, Ferguson AMN, Laevens BPM, Rix H-W, Chapman SC, Koch A. (2016) 'TRIANGULUM II: A VERY METAL-POOR AND DYNAMICALLY HOT STELLAR SYSTEM'. ASTROPHYSICAL JOURNAL, 818 (1) Article number ARTN 40
  • Collins MLM, Martin NF, Rich RM, Ibata RA, Chapman SC, McConnachie AW, Ferguson AM, Irwin MJ, Lewis GF. (2015) 'COMPARING THE OBSERVABLE PROPERTIES OF DWARF GALAXIES ON AND OFF THE ANDROMEDA PLANE'. The Astrophysical Journal Letters, 799 Article number L13


    The thin, extended planes of satellite galaxies detected around both the Milky Way and Andromeda are not a natural prediction of the Λ-cold dark matter paradigm. Galaxies in these distinct planes may have formed and evolved in a different way (e.g., tidally) from their off-plane neighbors. If this were the case, one would expect the onand off-plane dwarf galaxies in Andromeda to have experienced different evolutionary histories, which should be reflected by the chemistries, dynamics, and star formation histories of the two populations. In this work, we present new, robust kinematic observations for two on-plane M31 dwarf spheroidal galaxies (And XVI and XVII) and compile and compare all available observational metrics for the on- and off-plane dwarfs to search for a signal that would corroborate such a hypothesis. We find that, barring their spatial alignment, the on- and off-plane Andromeda dwarf galaxies are indistinguishable from one another, arguing against vastly different formative and evolutionary histories for these two populations.

  • Collins MLM, Chapman SC, Rich RM, Ibata RA, Martin NF, Irwin MJ, Bate NF, Lewis GF, Penarrubia J, Arimoto N, Casey CM, Ferguson AMN, Koch A, McConnachie AW, Tanvir N. (2014) 'THE MASSES OF LOCAL GROUP DWARF SPHEROIDAL GALAXIES: THE DEATH OF THE UNIVERSAL MASS PROFILE'. The Astrophysical Journal, 783 (7)


    We investigate the claim that all dwarf spheroidal galaxies (dSphs) reside within halos that share a common, universal mass profile as has been derived for dSphs of the galaxy. By folding in kinematic information for 25 Andromeda dSphs, more than doubling the previous sample size, we find that a singular mass profile cannot be found to fit all of the observations well. Further, the best-fit dark matter density profile measured solely for the Milky Way dSphs is marginally discrepant with that of the Andromeda dSphs (at just beyond the 1σ level), where a profile with lower maximum circular velocity, and hence mass, is preferred. The agreement is significantly better when three extreme Andromeda outliers, And XIX, XXI, and XXV, all of which have large half-light radii (600 pc) and low-velocity dispersions (σv < 5 km s−1), are omitted from the sample. We argue that the unusual properties of these outliers are likely caused by tidal interactions with the host galaxy.

Theses and dissertations

  • Contenta F. (2017) Probing dark matter with star clusters..
    [ Status: Approved ]


    Star clusters are collisional and dark matter (DM) free stellar systems, where their evolution is ruled by two-body interactions and the galactic potential. Using direct summation N-body simulations, I study how the observational properties of star clusters can be used to: (i) distinguish between DM free and DM dominated objects. From observations, the nature of several faint stellar systems in the Milky Way halo is not clear, therefore, I quantify the contribution of star clusters to the faint stellar systems population. (ii) Probe the underlying DM density of their host galaxy. I apply a new method to the recently discovered Eridanus~II ultra-faint dwarf galaxy that hosts a star cluster in its centre. I find that a cored DM density profile naturally reproduces the observed properties of Eridanus II’s star cluster. (iii) Infer their progenitor properties if they are accreted star clusters, such as Crater. From its properties I find that Crater is likely to be tidally stripped from a dwarf galaxy, and it must have formed extended and with a low concentration. Throughout this thesis, the comparison of simulations and data took into consideration observational biases and uncertainties. I show that the initial conditions of star clusters can heavily influence its present-day properties, and that the stellar evolution prescriptions can also impact the final star cluster properties, such as the neutron stars natal kick distribution. I conclude, through a series of test cases, that N-body simulations can be used to reproduce the observed properties of star clusters, and these can ultimately probe their host galaxy DM distribution.

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