Dr Michelle Collins

Reader (Associate Professor)
+44 (0)1483 683572
24 BC 03

Academic and research departments

School of Mathematics and Physics.



Research interests



Michelle Louise L.M. Collins, Noushin Karim, David Martinez-Delgado, Matteo Monelli, Erik J Tollerud, Giuseppe Donatiello, Mahdieh Navabi, Emily Charles, Walter Boschin (2024)Pisces VII/Triangulum III – M33’s second dwarf satellite galaxy, In: Monthly notices of the Royal Astronomical Society528(2)pp. 2614-2620 Oxford University Press

Pisces VII/Triangulum III (Pisc VII) was discovered in the DESI Legacy Imaging Survey and was shown to be a Local Group dwarf galaxy with follow-up imaging from the 4-m Telescopio Nazionale Galileo. However, this imaging was unable to reach the horizontal branch of Pisc VII, preventing a precision distance measurement. The distance bound from the red giant branch population placed Pisc VII as either an isolated ultra-faint dwarf galaxy or the second known satellite galaxy of Triangulum (M33). Using deep imaging from Gemini GMOS-N, we have resolved the horizontal branch of Pisc VII, and measure a distance of $D=916^{+65}_{-53}$ kpc, making Pisc VII a likely satellite of M33. We also remeasure its size and luminosity from this deeper data, finding $r_{\rm half}=186^{+58}_{-32}$ pc, MV = −6.0 ± 0.3, and $L=2.2^{+0.7}_{-0.5}\times 10^4\, {\rm L}_\odot$. Given its position in the M33 halo, we argue that Pisc VII could support the theory that M33 is on its first infall to the Andromeda system. We also discuss the presence of blue plume and helium burning stars in the colour–magnitude diagram of Pisc VII that are consistent with ages of ∼1.5 Gyr. If these are truly members of the galaxy, it would transform our understanding of how reionization affects the faintest galaxies. Future deep imaging and dynamics could allow significant insight into both the stellar populations of Pisc VII and the evolution of M33.

M. McNanna, K. Bechtol, S. Mau, E. O. Nadler, J. Medoff, A. Drlica-Wagner, W Cerny, D. Crnojevic, B. Mutlu-Pakdıl, A. K. Vivas, A. B. Pace, J. L. Carlin, M. L. M. Collins, P. S. Ferguson, D. Martinez-Delgado, C. E. Martinez-Vazquez, N. E. D. Noel, A. H. Riley, D. J. Sand, A. Smercina, E Tollerud, R Wechsler, T. M. C. Abbott, M. Aguena, O. Alves, D. Bacon, C. R. Bom, D. Brooks, D Burke, J Carballo-Bello, A. Carnero Rosell, J. Carretero, L. N. Da Costa, T. M. Davis, J De Vicente, H. T. Diehl, P. Doel, I Ferrero, J Frieman, G. Giannini, D. Gruen, G. Gutierrez, R. A. Gruendl, S. R. Hinton, D. L. Hollowood, K Honscheid, D. J. James, K Kuehn, J. L. Marshall, J. Mena-Fernandez, R. Miquel, M. E. S. Pereira, A. Pieres, A. A. Plazas Malagon, J Sakowska, E. Sanchez, D. Sanchez Cid, B. Santiago, I Sevilla-Noarbe, M. Smith, G. S. Stringfellow, E. Suchyta, M. E. C. Swanson, G. Tarle, N. Weaverdyck, P. Wiseman (2024)A Search for Faint Resolved Galaxies Beyond the Milky Way in DES Year 6: A New Faint, Diffuse Dwarf Satellite of NGC 55, In: The Astrophysical Journal961(1) IOP Publishing

We report results from a systematic wide-area search for faint dwarf galaxies at heliocentric distances from 0.3 to 2 Mpc using the full 6 yr of data from the Dark Energy Survey (DES). Unlike previous searches over the DES data, this search specifically targeted a field population of faint galaxies located beyond the Milky Way virial radius. We derive our detection efficiency for faint, resolved dwarf galaxies in the Local Volume with a set of synthetic galaxies and expect our search to be complete to MV ∼ (−7, −10) mag for galaxies at D = (0.3, 2.0) Mpc. We find no new field dwarfs in the DES footprint, but we report the discovery of one high-significance candidate dwarf galaxy at a distance of 2.2−0.12+0.05Mpc, a potential satellite of the Local Volume galaxy NGC 55, separated by 47′ (physical separation as small as 30 kpc). We estimate this dwarf galaxy to have an absolute V-band magnitude of −8.0−0.3+0.5mag and an azimuthally averaged physical half-light radius of 2.2−0.4+0.5kpc, making this one of the lowest surface brightness galaxies ever found with μ=32.3magarcsec−2. This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.

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

Burçin Mutlu-Pakdil, David J. Sand, Denija Crnojevi, Edward W. Olszewski, Dennis Zaritsky, Jay Strader, Michelle L. Collins, Anil C. Seth, Beth Willman (2020)The Elusive Distance Gradient in the Ultrafaint Dwarf Galaxy Hercules: A Combined Hubble Space Telescope and Gaia View, In: The Astrophysical journal902(2)106 The American Astronomical Society

The ultrafaint dwarf galaxy Hercules has an extremely elongated morphology with both photometric overdensities and kinematic members at large radii, suggesting that it may be tidally disrupting due to a previous close encounter with the Milky Way. To explain its observational peculiarities, we present a deep Hubble Space Telescope (HST) imaging study of Hercules and its surrounding regions and investigate its tidal history through a careful search for a distance gradient along its stretched body. Our off-center HST data clearly resolve a main sequence, showing that the stellar extension seen along the major axis of Hercules is genuine, not a clump of background galaxies. Utilizing Gaia DR2 data, we clean the region around the Hercules of field contamination, and find four new plausible member stars, all of which are located on the outskirts of the dwarf galaxy. We update the distance to Hercules, and find 130.6 6.1 kpc ( ) for the main body, which is consistent with earlier estimates in the literature. While we find no conclusive evidence for a distance gradient, our work demonstrates that constraining a distance gradient in such a faint system is not trivial, and the possible thickness of the dwarf along the line of sight and field contamination make it harder to make decisive conclusions even with these high-precision data. Future studies coupled with tailored theoretical models are needed to understand the true nature of Hercules and of tidal distortion observables in ultrafaint galaxies in general.

Alessandro Savino, James Bullock, Michelle Louise Miller Collins, M. C. Cooper, AJ Deason, Aaron Dotter, MA Fardal, A. M. N Ferguson, Tobias K Fritz, Marla Geha, Karoline M Gilbert, Puragra Guhathakurta, RA Ibata, M. Irwin, Myoungwon Jeon, Evan Kirby, G. F. Lewis, D Mackey, S. Majewski, AW McConnachie, N Martin, Ekta Patel, R Michael Rich, J. D. Simon, S T Sohn, Erik J. Tollerud, Roeland P. van der Marel (2023)The Hubble Space Telescope Survey of M31 Satellite Galaxies II. The Star Formation Histories of Ultra-Faint Dwarf Galaxies, In: The Astrophysical journal IOP

We present the lifetime star formation histories (SFHs) for six ultra-faint dwarf (UFD; $M_V>-7.0$, $ 4.9

Janet Preston, Michelle Collins, R Michael Rich, Rodrigo Ibata, Nicolas F Martin, Mark Fardal (2021)The nature of the Eastern Extent in the outer halo of M31, In: Monthly Notices of the Royal Astronomical Society504(2)pp. 3098-3110 Oxford University Press

We present the first comprehensive spectroscopic study of the Andromeda galaxy’s Eastern Extent. This ∼4° long filamentary structure, located 70–90 kpc from the centre of M31, lies perpendicular to Andromeda’s minor axis and the Giant Stellar Stream and overlaps Stream C. In this work, we explore the properties of the Eastern Extent to look for possible connections between it, the Giant Stellar Stream and Stream C. We present the kinematics and photometry for ∼50 red giant branch stars in seven fields along the Eastern Extent. We measure the systemic velocities for these fields and find them to be −368  km s−1 ≲ vv ≲ −331  km s−1, with a slight velocity gradient of −0.51 ± 0.21  km s−1 kpc−1 towards the Giant Stellar Stream. We derive the photometric metallicities for stars in the Eastern Extent, finding them to be metal-poor with values of −1.0 ≲ [Fe/H]phot ≲ −0.7 with an 〈[Fe/H]phot〉 ∼ −0.9. We find consistent properties for the Eastern Extent, Stream B and one of the substructures in Stream C, Stream Cr, plausibly linking these features. Stream Cp and its associated globular cluster, EC4, have distinctly different properties indicative of a separate structure. When we compare the properties of the Eastern Extent to those of the Giant Stellar Stream, we find them to be consistent, albeit slightly more metal-poor, such that the Eastern Extent could plausibly comprise stars stripped from the progenitor of the Giant Stellar Stream.

Janet Preston, Michelle L M Collins, Rodrigo A Ibata, Erik J Tollerud, R Michael Rich, Ana Bonaca, Alan W McConnachie, Dougal Mackey, Geraint F Lewis, Nicolas F Martin, Jorge Peñarrubia, Scott C Chapman, Maxime Delorme (2019)A Dwarf Disrupting - Andromeda XXVII and the North West Stream, In: Monthly Notices of the Royal Astronomical Society

We present a kinematic and spectroscopic analysis of 38 red giant branch stars, in 7 fields, spanning the dwarf spheroidal galaxy Andromeda XXVII and the upper segment of the North West Stream. Both features are located in the outer halo of the Andromeda galaxy at a projected radius of 50-80 kpc, with the stream extending for ∼3° on the sky. Our data is obtained as part of the PAndAS survey and enables us to confirm that Andromeda XXVII’s heliocentric distance is 827 ± 47 kpc and spectroscopic metallicity is -2.1+0.4−0.5⁠. We also re-derive Andromeda XXVII’s kinematic properties, measuring a systemic velocity = -526.1+10.0−11.0 kms−1 and a velocity dispersion that we find to be non-Gaussian but for which we derive a formal value of 27.0+2.2−3.9 kms−1. In the upper segment of the North West Stream we measure mean values for the metallicity = -1.8±0.4, systemic velocity = -519.4 ±4.0 kms−1 and velocity dispersion = 10.0±4.0 kms−1. We also detect a velocity gradient of 1.7±0.3 kms−1 kpc−1 on an infall trajectory towards M31. With a similar gradient, acting in the same direction, in the lower segment we suggest that the North West Stream is not a single structure. As the properties of the upper segment of the North West Stream and Andromeda XXVII are consistent within 90% confidence limits, it is likely that the two are related and plausible that Andromeda XXVII is the progenitor of this stream.

Michelle L M Collins, Erik J Tollerud, R Michael Rich, Rodrigo A Ibata, Nicolas F Martin, Scott C Chapman, Karoline M Gilbert, Janet Preston (2019)A detailed study of Andromeda XIX, an extreme local analogue of ultra diffuse galaxies, In: Monthly Notices of the Royal Astronomical Societystz3252 Oxford University Press (OUP)

With a central surface brightness of μ0 = 29.3 mag. per sq. arcsec, and half-light radius of rhalf=3.1+0.9−1.1kpc, Andromeda XIX (And XIX) is an extremely diffuse satellite of Andromeda. We present spectra for ∼100 red giant branch stars in this galaxy, plus 16 stars in a nearby stellar stream. With this exquisite dataset, we re-derive the properties of And XIX, measuring a systemic velocity of ˂vr˃ = − 109.0 ± 1.6 kms−1 and a velocity dispersion of σvr=7.8+1.7−1.5σvr=7.8+1.7−1.5 kms−1(higher than derived in our previous work). We marginally detect a velocity gradient along the major axis of dvdχ=−2.1±1.8 kms−1dvdχ=−2.1±1.8 kms−1kpc−1. We find its mass-to-light ratio is higher than galaxies of comparable stellar mass (⁠[M/L]half=278+146−198M⊙/L⊙), but its dynamics place it in a halo with a similar total mass to these galaxies. This could suggest that And XIX is a “puffed up” dwarf galaxy, whose properties have been altered by tidal processes, similar to its Milky Way counterpart, Antlia II. For the nearby stream, we measure vr = −279.2 ± 3.7 kms−1, and σv=13.8+3.5−2.6 kms−1. We measure its metallicity, and find it to be more metal rich than And XIX, implying that the two features are unrelated. Finally, And XIX’s dynamical and structural properties imply it is a local analogue to ultra diffuse galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted.

W Cerny, C. E. Martínez-Vázquez, Alex Drlica-Wagner, Andrew B Pace, B. Mutlu-Pakdil, T. S. Li, A. H. Riley, D. Crnojević, C. R Bom, J. A. Carballo-Bello, J. L. Carlin, Anirudh Chiti, Y. Choi, M.L.M Collins, E Darragh-Ford, P. S. Ferguson, Marla Geha, D. Martínez-Delgado, S Mau, Pol Massana, Gustavo E. Medina, Rr Munoz, E. O. Nadler, N. E. D. Noël, K Olsen, A Pieres, Joanna Diana Sakowska, J. D. Simon, Guy S Stringfellow, Erik J. Tollerud, A. K. Vivas, A. R. Walker, R. H. Wechsler, DELVE Collaborat (2023)Six More Ultra-faint Milky Way Companions Discovered in the DECam Local Volume Exploration Survey, In: The Astrophysical journal953(1)1 IOP Publishing Ltd

We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (M ( V ) & GE; -3.2 mag) and old, metal-poor stellar populations (& tau; > 10 Gyr, [Fe/H] < -1.4 dex). Three of these systems are more extended (r (1/2) > 15 pc), while the other three are compact (r (1/2) < 10 pc). From these properties, we infer that the former three systems (Bootes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Bootes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.

Zhen Wan, William H Oliver, Geraint F Lewis, Justin Read, Michelle Collins (2019)On the origin of the asymmetric dwarf galaxy distribution around andromeda, In: Monthly Notices of the Royal Astronomical Society492(1)pp. 456-467 Oxford University Press

The dwarf galaxy distribution surrounding M31 is significantly anisotropic in nature. Of the 30 dwarf galaxies in this distribution, 15 form a disc-like structure and 23 are contained within the hemisphere facing the Milky Way. Using a realistic local potential, we analyse the conditions required to produce and maintain these asymmetries. We find that some dwarf galaxies are required to have highly eccentric orbits in order to preserve the presence of the hemispherical asymmetry with an appropriately large radial dispersion. Under the assumption that the dwarf galaxies originate from a single association or accretion event, we find that the initial size and specific energy of that association must both be relatively large in order to produce the observed hemispherical asymmetry. However if the association was large in physical size, the very high-energy required would enable several dwarf galaxies to escape from the M31 and be captured by the Milky Way. Furthermore, we find that associations that result in this structure have total specific energies concentrated around E = V_esc2 - V_init2 ̃ 200^2 - 300^2 {km^2 s^{-2}}, implying that the initial velocity and initial position needed to produce the structure are strongly correlated. The overlap of initial conditions required to produce the radial dispersion, angular dispersion, and the planar structure is small and suggests that either they did not originate from a single accretion event, or that these asymmetric structures are short-lived.

Daniel R. Weisz, Nicolas F. Martin, Andrew E. Dolphin, Saundra M. Albers, Michelle L. M. Collins, Annette M. N. Ferguson, Geraint F. Lewis, Dougal Mackey, Alan McConnachie, R. Michael Rich, Evan D. Skillman (2019)Comparing the Quenching Times of Faint M31 and Milky Way Satellite Galaxies, In: The Astrophysical Journal885(1) IOP Publishing

We present the star formation histories (SFHs) of 20 faint M31 satellites (−12 ≲ M V ≲ −6) that were measured by modeling sub-horizontal branch depth color–magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration 3–6 Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent (

Burçin Mutlu-Pakdil, David J. Sand, Matthew G. Walker, Nelson Caldwell, Jeffrey L. Carlin, Michelle L. Collins, Denija Crnojević, Mario Mateo, Edward W. Olszewski, Anil C. Seth, Jay Strader, Beth Willman, Dennis Zaritsky (2019)Signatures of Tidal Disruption in Ultra-Faint Dwarf Galaxies: A Combined HST, Gaia, and MMT/Hectochelle Study of Leo V, In: The Astrophysical Journal IOP Publishing

The ultra-faint dwarf galaxy Leo V has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of i) high-precision photometry obtained with the Hubble Space Telescope (HST), ii) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and iii) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across Leo V might be due to small number statistics. We update the systemic proper motion of Leo V, finding (μαcosδ,μδ)=(0.009±0.560, −0.777±0.314) mas yr−1, which is consistent with its reported orbit that did not put Leo V at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested Leo V was disrupting, however, we also find new plausible member stars, two of which are located ˃5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of Leo V still remains an open question.

Nicolas F. Martin, Michelle L. M. Collins, Nicolas Longeard, Erik Tollerud (2018)Current velocity data on dwarf galaxy NGC1052-DF2 do not constrain it to lack dark matter, In: Astrophysical Journal Letters859(L5)pp. 1-5 American Astronomical Society

It was recently proposed that the globular cluster system of the very low surface-brightness galaxy NGC1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the velocity dispersion of the tracer globular clusters implies a mundane velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius (M/LV < 6.7 at the 90-percent confidence level) or within the radius of the furthest tracer (M/LV < 8.1 at the 90-percent confidence level). This limit may imply a mass-to-light ratio on the low end for a dwarf galaxy but many Local Group dwarf galaxies fall well within this contraint. With this study, we emphasize the need to reliably account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers.

Alan W. McConnachie, Rodrigo Ibata, Nicolas Martin, Annette M. N. Ferguson, Michelle Collins, Stephen Gwyn, Mike Irwin, Geraint F. Lewis, A. Dougal Mackey, Tim Davidge, Veronica Arias, Anthony Conn, Patrick Côté, Denija Crnojevic, Avon Huxor, Jorge Penarrubia, Chelsea Spengler, Nial Tanvir, David Valls-Gabaud, Arif Babul, Pauline Barmby, Nicholas F. Bate, Edouard Bernard, Scott Chapman, Aaron Dotter, William Harris, Brendan McMonigal, Julio Navarro, Thomas H. Puzia, R. Michael Rich, Guillaume Thomas, Lawrence M. Widrow (2018)The Large-scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey, In: The Astrophysical Journal868(1) The American Astronomical Society

The Pan-Andromeda Archaeological Survey is a survey of >400 square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of an L͙ galaxy group to large projected galactocentric radii. Here, we collate and summarize the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyr. We suggest that a few of the substructures farthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured data set; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31ʼs stellar halo and identify three new faint structures. M31ʼs halo, in projection, appears to be dominated by two “mega-structures,” which can be considered as the two most significant branches of a merger tree produced by breaking M31ʼs stellar halo into increasingly smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.

Nicolas Longeard, Nicolas Martin, Rodrigo A. Ibata, Michelle L. M. Collins, Benjamin P. M. Laevens, Eric Bell, Dougal Mackey (2019)Detailed study of the Milky Way globular cluster Laevens 3, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We present a photometric and spectroscopic study of the Milky Way satellite Laevens 3. Using MegaCam/CFHT g and i photometry and Keck II/DEIMOS multi-object spectroscopy, we refine the structural and stellar properties of the system. The Laevens 3 colour-magnitude diagram shows that it is quite metal-poor, old (13.0 ± 1.0 Gyr), and at a distance of 61.4 ± 1.0 kpc, partly based on two RR Lyrae stars. The system is faint (⁠MV=−2.8+0.2−0.3 mag) and compact (rh = 11.4 ± 1.0 pc). From the spectroscopy, we constrain the systemic metallicity ([Fe/H]spectro = −1.8 ± 0.1 dex) but the metallicity and velocity dispersions are both unresolved. Using Gaia DR2, we infer a mean proper motion of (μ∗α,μδ)=(0.51±0.28,−0.83±0.27) mas yr−1, which, combined with the system’s radial velocity (˂vr˃ = −70.2 ± 0.5 km s−1), translates into a halo orbit with a pericenter and apocenter of 40.7+5.6−14.7 and 85.6+17.2−5.9 kpc, respectively. Overall, Laevens 3 shares the typical properties of the Milky Way's outer halo globular clusters. Furthermore, we find that this system shows signs of mass-segregation which strengthens our conclusion that Laevens 3 is a globular cluster.

R Michael Rich, Aleksandr Mosenkov, Henry Lee-Saunders, Andreas Koch, John Kormendy, Julia Kennefick, Noah Brosch, Laura Sales, James Bullock, Andreas Burkert, Michelle Collins, Michael Cooper, Michael Fusco, David Reitzel, David Thilker, Dave G Milewski, Lydia Elias, M L Saade, Laura De Groot (2019)The Halos and Environments of Nearby Galaxies (HERON) I: Imaging, Sample Characteristics, and Envelope Diameters, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

We use a dedicated 0.7-m telescope to image the halos of 119 galaxies in the Local Volume to μr ∼ 28 − 30 mag/arcsec2. The sample is primarily from the 2MASS Large Galaxy Atlas and extended to include nearby dwarf galaxies and more distant giant ellipticals, and spans fully the galaxy colour{magnitude diagram including the blue cloud and red sequence. We present an initial overview, including deep images of our galaxies. Our observations reproduce previously reported low surface brightness structures, including extended plumes in M51, and a newly discovered tidally extended dwarf galaxy in NGC7331. Low surface brightness structures, or "envelopes", exceeding 50 kpc in diameter are found mostly in galaxies with MV < -20:5, and classic interaction signatures are infrequent. Defining a halo diameter at the surface brightness 28 mag/arcsec2, we find that halo diameter is correlated with total galaxy luminosity. Extended signatures of interaction are found throughout the galaxy colour{magnitude diagram without preference for the red or blue sequences, or the green valley. Large envelopes may be found throughout the colour{magnitude diagram with some preference for the bright end of the red sequence. Spiral and S0 galaxies have broadly similar sizes, but ellipticals extend to notably greater diameters, reaching 150 kpc. We propose that the extended envelopes of disk galaxies are dominated by an extension of the disk population rather than by a classical population II halo.

R. Michael Rich, Christian L. Johnson, Michael Young, Iulia Simion, William L. Clarkson, Catherine Pilachowski, Scott Michael, Andrea Kunder, A. Katherina Vivas, Andreas Koch, Tommaso Marchetti, Rodrigo Ibata, Nicolas Martin, Annie C. Robin, Nadege Lagarde, Michelle Collins, Zeljko Ivezic, Roberto de Propris, Juntai Shen (2020)The Blanco DECam Bulge Survey. I. The Survey Description and Early Results, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

The Blanco Dark Energy Camera (DECam) Bulge survey is a Vera Rubin Observatory (LSST) pathfinder imaging survey, spanning 200 sq. deg. of the Southern Galactic bulge,

Daniel R Weisz, Andrew E Dolphin, Nicolas F Martin, Saundra M Albers, Michelle L. M. Collins, Annette M N Ferguson, Geraint F Lewis, A Dougal Mackey, Alan McConnachie, R Michael Rich, Evan D Skillman (2019)A rogues gallery of Andromeda’s dwarf galaxies – II. Precise distances to 17 faint satellites, In: Monthly Notices of the Royal Astronomical Society489(1)pp. 763-770 Oxford University Press

We present new horizontal branch (HB) distance measurements to 17 of the faintest known M31 satellites (−6 ≲ MV ≲ −13) based on deep Hubble Space Telescope (HST) imaging. The colour–magnitude diagrams extend ∼1–2 mag below the HB, which provides for well-defined HBs, even for faint galaxies in which the tip of the red giant branch (TRGB) is sparsely populated. We determine distances across the sample to an average precision of 4 per cent (∼30 kpc at 800 kpc).We find that the majority of these galaxies are in good agreement, though slightly farther (0.1–0.2 mag) when compared to recent ground-based TRGB distances. Two galaxies (And IX and And XVII) have discrepant HST and ground-based distances by ∼0.3 mag (∼150 kpc), which may be due to contamination from Milky Way foreground stars and/or M31 halo stars in sparsely populated TRGB regions. We use the new distances to update the luminosities and structural parameters for these 17 M31 satellites. The new distances do not substantially change the spatial configuration of the M31 satellite system. We comment on future prospects for precise and accurate HB distances for faint galaxies in the Local Group and beyond.

AHW Küpper, KV Johnston, S Mieske, Michelle Collins, EJ Tollerud (2017)Exploding Satellites -- The Tidal Debris of the Ultra-Faint Dwarf Galaxy Hercules, In: The Astrophysical Journal: an international review of astronomy and astronomical physics834(2) American Astronomical Society

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.

Alexandra L Gregory, Michelle L M Collins, Justin Read, Michael J Irwin, Rodrigo A Ibata, Nicolas F Martin, Alan W McConnachie, Daniel R Weisz (2019)Kinematics of the Tucana Dwarf Galaxy: an unusually dense dwarf in the Local Group, In: Monthly Notices of the Royal Astronomical Society485(2)pp. 2010-2025 Oxford University Press (OUP)

We present new FLAMES+GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of vTuc=216.7+2.9−2.8vTuc=216.7−2.8+2.9 km s−1, and a velocity dispersion of σv,Tuc=14.4+2.8−2.3σv,Tuc=14.4−2.3+2.8 km s−1. We also detect a rotation gradient of dvrdχ=7.6+4.2−4.3dvrdχ=7.6−4.3+4.2 km s−1 kpc−1, which reduces the systemic velocity to vTuc=215.2+2.8−2.7vTuc=215.2−2.7+2.8 km s−1 and the velocity dispersion to σv,Tuc=13.3+2.7−2.3σv,Tuc=13.3−2.3+2.7 km s−1. We perform Jeans modelling of the density profile of Tucana, using the line-of-sight velocities of the member stars. We find that it favours a high central density consistent with ‘pristine’ subhaloes in Λ cold dark matter, and a massive dark matter halo (∼1010 M⊙) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models.

Nicolas Longeard, Nicolas Martin, Else Starkenburg, Rodrigo A Ibata, Michelle L. M. Collins, Marla Geha, Benjamin P M Laevens, R Michael Rich, David S Aguado, Anke Arentsen, Raymond G Carlberg, Patrick Côté, Vanessa Hill, Pascale Jablonka, Jonay I González Hernández, Julio F Navarro, Rubén Sánchez-Janssen, Eline Tolstoy, Kim A Venn, Kris Youakim (2018)Pristine dwarf galaxy survey – I. A detailed photometric and spectroscopic study of the very metal-poor Draco II satellite, In: Monthly Notices of the Royal Astronomical Society480(2)pp. 2609-2627 Oxford University Press

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour–magnitude space implies Dra II is old (13.5±0.5 Gyr), very metal-poor, very faint (LV = 180+124 −72 L), and at a distance d = 21.5 ± 0.4 kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H] = −2.7 ± 0.1 dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved ((σvr < 5.9km s−1 at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity (vr = −342.5+1.1 −1.2 km s−1). We further show that the spectroscopically confirmed members of Dra II have a mean proper motion of (μ ∗ α,μδ) = (1.26 ± 0.27, 0.94 ± 0.28) mas/yr in the Gaia DR2 data, which translates to an orbit with a pericentre and an apocentre of 21.3+0.7 −1.0 and 153.8+56.7 −34.7 kpc, respectively. Taken altogether, these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.

A D Mackey, A M N Ferguson, A P Huxor, J Veljanoski, G F Lewis, A W McConnachie, N F Martin, R A Ibata, M J Irwin, P Côté, M L M Collins, N R Tanvir, N F Bate (2019)The outer halo globular cluster system of M31 – III. Relationship to the stellar halo, In: Monthly Notices of the Royal Astronomical Society484(2)pp. 1756-1789 Oxford University Press

We utilize the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo inM31 at projected radii Rproj =25–150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index Γ=−2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < −1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities at 99.95 per cent significance. We use our stellar density measurements and previous kinematic data to demonstrate that ≈35–60 per cent of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least ∼40 per cent of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside Rproj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth.

Nicolas Longeard, Nicolas Martin, Else Starkenburg, Rodrigo A Ibata, Michelle L M Collins, Benjamin P M Laevens, Dougal Mackey, R Michael Rich, David S Aguado, Anke Arentsen, Pascale Jablonka, Jonay I González Hernández, Julio F Navarro, Rubén Sánchez-Janssen (2019)The Pristine Dwarf-Galaxy survey – II. In-depth observational study of the faint Milky Way satellite Sagittarius II, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive Calcium H&K doublet photometry, augmented with Keck II/DEIMOS multi-object spectroscopy. We are able to derive and refine the Sgr II structural and stellar properties: the colour-magnitude diagram implies Sgr II is old (12.0 +- 0.5 Gyr) and metal-poor. The CaHK photometry confirms the metal-poor nature of the satellite ([Fe/H]CaHK = -2.32 +- 0.04 dex) and suggests that Sgr II hosts more than one single stellar population ({____sigma} CaHK = 0.11+0.05-0.03 dex). From the deep spectroscopic data, the velocity dispersion of the system is found to be {____sigma}vr = 2.7+1.3-1.0 km s-1 after excluding two potential binary stars. Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we are able to confirm the metallicity and dispersion inferred from the Pristine photometric metallicities: ([Fe/H] = -2.23 +- 0.05 dex, {____sigma}spectro = 0.10+0.06-0.04 dex). Sgr II's metallicity and absolute magnitude (MV = -5.7 +- 0.1 mag) place the system on the luminosity-metallicity relation of the Milky Way dwarf galaxies despite its small size. The low, but resolved metallicity and velocity dispersions paint the picture of a slightly dark matter-dominated satellite. Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocenter of 118.4+28.4-23.7 kpc and a pericenter of 54.8+3.3-6.1 kpc. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf's influence.

Duncan A Forbes, Justin I Read, Mark Gieles, Michelle L M Collins (2018)Extending the globular cluster system–halo mass relation to the lowest galaxy masses, In: Monthly Notices of the Royal Astronomical Society481(4)pp. 5592-5605

High-mass galaxies, with halo masses M200 ≥ 1010M⊙ , reveal a remarkable near-linear relation between their globular cluster (GC) system mass and their host galaxy halo mass. Extending this relation to the mass range of dwarf galaxies has been problematic due to the difficulty in measuring independent halo masses. Here we derive new halo masses based on stellar and H i gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find that the GC system mass–halo mass relation for galaxies populated by GCs holds from halo masses of M200 ∼ 1014 M ⊙ ⊙ down to below M200 ∼109 M ⊙ ⊙ , although there is a substantial increase in scatter towards low masses. In particular, three well-studied ultradiffuse galaxies, with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass ratios. We compare our GC system–halo mass relation to the recent model of El Badry et al., finding that their fiducial model does not reproduce our data in the low-mass regime. This may suggest that GC formation needs to be more efficient than assumed in their model, or it may be due to the onset of stochastic GC occupation in low-mass haloes. Finally, we briefly discuss the stellar mass–halo mass relation for our low-mass galaxies with GCs, and we suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.

Ekta Patel, Jeffrey L Carlin, Erik J Tollerud, Michelle L M Collins, Gregory A Dooley (2018)ΛCDM predictions for the satellite population of M33, In: Monthly Notices of the Royal Astronomical Society480(2)pp. 1883-1897 Oxford University Press

Triangulum (M33) is the most massive satellite galaxy of Andromeda (M31), with a stellar mass of about 3 × 109  M⊙. Based on abundance matching techniques, M33’s total mass at infall is estimated to be of order 1011M⊙. ΛCDM simulations predict that M33-mass halos host several of their own low-mass satellite companions, yet only one candidate M33 satellite has been detected in deep photometric surveys to date. This ‘satellites of satellites’ hierarchy has recently been explored in the context of the dwarf galaxies discovered around the Milky Way’s Magellanic Clouds in the Dark Energy Survey. Here, we discuss the number of satellite galaxies predicted to reside within the virial radius (∼160 kpc) of M33 based on ΛCDM simulations. We also calculate the expected number of satellite detections in N fields of data using various ground-based imagers. Finally, we discuss how these satellite population predictions change as a function of M33’s past orbital history. If M33 is on its first infall into M31’s halo, its proposed satellites are expected to remain bound to M33 today. However, if M33 experienced a recent tidal interaction with M31, the number of surviving satellites depends strongly on the distance achieved at pericenter due to the effects of tidal stripping. We conclude that a survey extending to ∼100 kpc around M33 would be sufficient to constrain its orbital history and a majority of its satellite population. In the era of WFIRST, surveys of this size will be considered small observing programs.

Filippo Contenta, Eduardo Balbinot, James Petts, Justin Read, Mark Gieles, Michelle Collins, Jorge Peñarrubia, Maxime Delorme, Alessia Gualandris (2018)Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

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.

K Voggel, M Hilker, H Baumgardt, Michelle Collins, EK Grebel, B Husemann, T Richtler, MJ Frank (2016)Probing the boundary between star clusters and dwarf galaxies: A MUSE view on the dynamics of Crater/Laevens I, In: Monthly Notices of the Royal Astronomical Society460(3)pp. 3384-3397 Oxford University Press

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.

Dougal Mackey, Geraint F. Lewis, Brendon J. Brewer, Annette M. N. Ferguson, Jovan Veljanoski, Avon P. Huxor, Michelle L. M. Collins, Patrick Côté, Rodrigo A. Ibata, Mike J. Irwin, Nicolas Martin, Alan W. McConnachie, Jorge Peñarrubia, Nial Tanvir, Zhen Wan (2019)Two major accretion epochs in M31 from two distinct populations of globular clusters, In: Nature Nature Research

Large galaxies grow through the accumulation of dwarf galaxies. In principle it is possible to trace this growth history through the properties of a galaxy’s stellar halo. Previous investigations of M31 (Andromeda) showed that outside a radius of 25 kpc the population of halo globular clusters were rotating aligned with the stellar disk, as were more centrally located clusters. The M31 stellar halo also contains coherent substructures, along with a smooth component. Many of the globular clusters outside 25 kpc are associated with the most prominent substructures, while some are part of the smooth halo. Here we report on a new analysis of the kinematics of these globular clusters. We find two distinct populations rotating with perpendicular orientations. The rotation axis for the population associated with the smooth halo is aligned with that for the plane of dwarf galaxies that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, likely separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain.

Michelle Collins, EJ Tollerud, DJ Sand, A Bonaca, B Willman, J Strader (2017)Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V, In: Monthly Notices of the Royal Astronomical Society467(1)pp. 573-585 Oxford University Press

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.

Filippo Contenta, Mark Gieles, Eduardo Balbinot, Michelle Collins (2016)The contribution of dissolving star clusters to the population of ultra faint objects in the outer halo of the Milky Way, In: Monthly Notices of the Royal Astronomical Society466(2)pp. 1741-1756 Oxford Univercity Press

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.

Emily J. E Charles, Michelle L. M Collins, R. Michael Rich, Justin I Read, Stacy Y Kim, Rodrigo A Ibata, Nicolas F Martin, Scott C Chapman, Eduardo Balbinot, Daniel R Weisz (2023)Andromeda XXV - a dwarf galaxy with a low central dark matter density, In: Monthly Notices of the Royal Astronomical Society521(3)pp. 3527-3539 Oxford University Press

Andromeda (And) XXV has previously been reported as a dwarf spheroidal galaxy (dSph) with little-to-no dark matter. However, the uncertainties on this result were significant. In this study, we double the number of member stars and re-derive the kinematics and mass of And XXV. We find that And XXV has a systemic velocity of $\nu_\mathrm{r}=-107.7\pm1.0 \mathrm{~km s}^{-1}$ and a velocity dispersion of $\sigma_\nu=4.5\pm1.0\mathrm{~km s}^{-1}$. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of $M(r< r_\mathrm{h})=6.9^{+3.2}_{-2.8}\times10^6\mathrm{~M}_\odot$. This mass corresponds to a mass-to-light ratio of $\mathrm{[M/L]}_\mathrm{r_\mathrm{h}}=37^{+17}_{-15}\mathrm{~M}_\odot/\mathrm{L}_\odot$, demonstrating, for the first time, that And XXV has an unambiguous dark matter component. We also measure the metallicity of And XXV to be $\mathrm{[Fe/H]}=-1.9\pm0.1$$\mathrm{~}$dex, which is in agreement with previous results. Finally, we extend the analysis of And XXV to include mass modelling using GravSphere. We find that And XXV has a low central dark matter density, $\rho_\mathrm{DM}(150\mathrm{pc})= 2.7^{+1.8}_{-1.6}\times10^7\mathrm{~M}_\odot\mathrm{kpc}^{-3}$, making And XXV a clear outlier when compared to other Local Group (LG) dSphs of the similar stellar mass. In a companion paper, we will explore whether some combination of dark matter cusp-core transformations and/or tides can explain And XXV's low density.

Michelle L. M. Collins, Benjamin F. Williams, Erik J. Tollerud, Eduardo Balbinot, Karoline M Gilbert, Andrew Dolphin (2022)A detailed star formation history for the extremely diffuse Andromeda XIX dwarf galaxy, In: Monthly Notices of the Royal Astronomical Societystac2794pp. 1-7 Oxford University Press

We present deep imaging of the ultra-diffuse Andromeda XIX dwarf galaxy from the Advance Camera for Surveys on the Hubble Space Telescope which resolves its stellar populations to below the oldest main sequence turn-off. We derive a full star formation history for the galaxy using MATCH, and find no evidence of star formation in the past 8 Gyr. We calculate a quenching time of 𝜏₉₀ = 9.7 ± 0.2 Gyr, suggesting Andromeda XIX ceased forming stars very early on. This early quenching, combined with its extremely large half-light radius, low density dark matter halo and lower than expected metallicity make it a unique galaxy within the Local Group and raises questions about how it formed. The early quenching time allows us to rule out feedback from bursty star formation as a means to explain its diffuse stellar population and low density dark matter halo. We find that the extended stellar population, low density halo and star formation could be explained by either tidal interactions (such as tidal shocking) or by late dry mergers, with the latter also explaining its low metallicity. Proper motions and detailed abundances would allow us to distinguish between these two scenarios.

MICHELLE LOUISE MILLER COLLINS, Justin I Read (2022)Observational constraints on stellar feedback in dwarf galaxies, In: Nature astronomy6pp. 647-658 Nature Research

Feedback to the interstellar medium (ISM) from ionising radiation, stellar winds and supernovae is central to regulating star formation in galaxies. Due to their low mass (M * < 10 9 M), dwarf galaxies are particularly susceptible to such processes, making them ideal sites to study the detailed physics of feedback. In this perspective, we summarise the latest observational evidences for feedback from star forming regions and how this drives the formation of 'superbubbles' and galaxy-wide winds. We discuss the important role of external ionising radiation – 'reionisation' – for the smallest galaxies. And, we discuss the observational evidences that this feedback directly impacts galaxy properties such as their star formation histories, metal content, colours, sizes, morphologies and even their inner dark matter densities. We conclude with a look to the future, summarising the key questions that remain unanswered and listing some of the outstanding challenges for galaxy formation theories.

MICHELLE LOUISE MILLER COLLINS, EMILY JANE CHARLES, David Martínez-Delgado, Matteo Monelli, NOUSHIN KARIM, Giuseppe Donatiello, Erik J. Tollerud, Walter Boschin (2022)Pegasus V/Andromeda XXXIV -a newly discovered ultra-faint dwarf galaxy on the outskirts of Andromeda, In: MNRAS515(1)pp. L72-L77 OUP

We report the discovery of an ultra-faint dwarf in the constellation of Pegasus. Pegasus V/Andromeda XXXIV (Peg V) was initially identified in the public imaging data release of the DESI Legacy Imaging Surveys and confirmed with deep imaging from Gemini/GMOS-N. The colour-magnitude diagram shows a sparse red giant branch (RGB) population and a strong over-density of blue horizontal branch stars. We measure a distance to Peg V of í µí°· = 692 +33 −31 kpc, making it a distant satellite of Andromeda with í µí±€ í µí±‰ = −6.3 ± 0.2 and a half-light radius of í µí±Ÿ half = 89 ± 41 pc. It is located ∼ 260 kpc from Andromeda in the outskirts of its halo. The RGB is well-fit by a metal-poor isochrone with [Fe/H]= −3.2, suggesting it is very metal poor. This, combined with its blue horizontal branch could imply that it is a reionisation fossil. This is the first detection of an ultra-faint dwarf outside the deep Pan-Andromeda Archaeological Survey area, and points to a rich, faint satellite population in the outskirts of our nearest neighbour.

David Martínez-Delgado, Noushin Karim, Emily J. E. Charles, Walter Boschin, Matteo Monelli, Michelle L. M. Collins, Giuseppe J. Donatiello, Emilio J. Alfaro (2022)Pisces VII: discovery of a possible satellite of Messier 33 in the DESI legacy imaging surveys, In: Monthly Notices of the Royal Astronomical Society509(1)pp. 16-24 Oxford University Press

We report deep imaging observations with DOLoRes@TNG of an ultra-faint dwarf satellite candidate of the Triangulum galaxy (M33) found by visual inspection of the public imaging data release of the DESI Legacy Imaging Surveys. Pisces VII/Triangulum (Tri) III is found at a projected distance of 72 kpc from M33, and using the tip of the red giant branch method, we estimate a distance of D = 1.0(-0.2)(+0.3)Mpc, meaning the galaxy could either be an isolated ultra-faint or the second known satellite of M33. We estimate an absolute magnitude of M-V = -6.1 +/- 0.2 if Pisces VII/Tri II is at the distance of M33, or as bright as M-V = -6.8 +/- 0.2 if the galaxy is isolated. At the isolated distance, it has a physical half-light radius of r(h) = 131 +/- 61 pc consistent with similarly faint galaxies around the Milky Way. As the tip of the red giant branch is sparsely populated, constraining a precision distance is not possible, but if Pisces VII/Tri III can be confirmed as a true satellite of M33, it is a significant finding. With only one potential satellite detected around M33 previously (Andromeda XXII/Tri I), it lacks a significant satellite population in stark contrast to the similarly massive Large Magellanic Cloud. The detection of more satellites in the outskirts of M33 could help to better illuminate if this discrepancy between expectation and observations is due to a poor understanding of the galaxy formation process, or if it is due to the low luminosity and surface brightness of the M33 satellite population which has thus far fallen below the detection limits of previous surveys. If it is truly isolated, it would be the faintest known field dwarf detected to date.

Michelle L M Collins, Justin I Read, Rodrigo A Ibata, R Michael Rich, Nicolas F Martin, Jorge Peñarrubia, Scott C Chapman, Erik J Tollerud, Daniel R Weisz (2021)Andromeda XXI – a dwarf galaxy in a low-density dark matter halo, In: Monthly notices of the Royal Astronomical Society505(4)pp. 5686-5701 Oxford University Press

Andromeda XXI (And XXI) has been proposed as a dwarf spheroidal galaxy with a central dark matter density that is lower than expected in the standard $\Lambda$ cold dark matter ($\Lambda$CDM) cosmology. In this work, we present dynamical observations for 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low-density outlier. We measure a systemic velocity of $v_r=-363.4\pm 1.0{\rm \, km\, s^{-1}}$ and a velocity dispersion of $\sigma _v=6.1^{+1.0}_{-0.9}{\rm \, km\, s^{-1}}$, consistent with previous work and within $1\sigma$ of predictions made using the modified Newtonian dynamics framework. We also measure the metallicity of our member stars from their spectra, finding a mean value of ${\rm [Fe/H]}=-1.7\pm 0.1$ dex. We model the dark matter density profile of And XXI using an improved version of gravsphere, finding a central density of $\rho _{\rm DM}({\rm 150 pc})=2.6_{-1.5}^{+2.4} \times 10^7 \, {\rm M_\odot \, kpc^{-3}}$ at 68 per cent confidence, and a density at two half-light radii of $\rho _{\rm DM}({\rm 1.75 kpc})=0.9_{-0.2}^{+0.3} \times 10^6 \, {\rm M_\odot \, kpc^{-3}}$ at 68 per cent confidence. These are both a factor of${\sim }3\!-\!5$ lower than the densities expected from abundance matching in $\Lambda$CDM. We show that this cannot be explained by ‘dark matter heating’ since And XXI had too little star formation to significantly lower its inner dark matter density, while dark matter heating only acts on the profile inside the half-light radius. However, And XXI’s low density can be accommodated within $\Lambda$CDM if it experienced extreme tidal stripping (losing ${\gt}95{{\ \rm per\ cent}}$ of its mass), or if it inhabits a low-concentration halo on a plunging orbit that experienced repeated tidal shocks.

Alessandro Savino, Daniel R. Weisz, Evan D. Skillman, Andrew Dolphin, Nitya Kallivayalil, Andrew Wetzel, Jay Anderson, Gurtina Besla, Michael Boylan-Kolchin, James S. Bullock, Andrew A. Cole, Michelle L. M. Collins, M. C. Cooper, Alis J. Deason, Aaron L. Dotter, Mark Fardal, Annette M. N. Ferguson, Tobias K. Fritz, Marla C. Geha, Karoline M. Gilbert, Puragra Guhathakurta, Rodrigo Ibata, Michael J. Irwin, Myoungwon Jeon, Evan Kirby, Geraint F. Lewis, Dougal Mackey, Steven R. Majewski, Nicolas Martin, Alan McConnachie, Ekta Patel, R. Michael Rich, Joshua D. Simon, Sangmo Tony Sohn, Erik J. Tollerud, Roeland P. van der Marel (2022)The Hubble Space Telescope Survey of M31 Satellite Galaxies. I. RR Lyrae-based Distances and Refined 3D Geometric Structure, In: The Astrophysical Journal938(2)101 IOP Publishing

We measure homogeneous distances to M31 and 38 associated stellar systems (-16.8 4700 RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 day and 0.04 mag. Based on period-Wesenheit-metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of similar to 20 kpc (3%) and similar to 10 kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that similar to 80% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms 7-23 kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess the physical proximity of notable associations such as the NGC 147/185 pair and M33/AND xxii; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with M ( V ) > - 9.5, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.

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