The Dawes Review: A Decade of Ultra-Diffuse Galaxies

Abstract

It has been 10 years since the initial discovery of Ultra-Diffuse Galaxies (UDGs) in the Coma cluster and the revelation that large, low surface brightness galaxies may constitute a greater fraction of galaxies than first thought. This left an open question: Are UDGs something special, or just an extension of the previously known dwarf galaxy population? Seeking to answer this question, in the decade following, dedicated simulations have studied and proposed a myriad of formation pathways to create UDGs. Observations have then pushed the limits of world-class observatories to perform detailed studies of these galaxies in large numbers across the full range of environments in the local Universe. These observations stress test simulations and challenge previous galaxy formation wisdom, with UDGs posing many open puzzles beyond just their unknown formation mechanism. To provide a few pertinent examples: there is observational evidence that not all UDGs follow the standard stellar mass -- halo mass relationship; there is evidence for UDGs with extraordinarily high levels of alpha enhancement; and there is evidence that some UDGs are much more globular cluster rich than other dwarfs of similar stellar mass. In this Dawes review, we undertake the task of summarising the decade of science since the discovery of UDGs. We focus on the quiescent population of UDGs and review their general properties, their proposed formation scenarios, their internal properties and their globular cluster systems. We also provide a brief conjecture on some future directions for the next decade of UDG research.

Figures (20)

• Figure 1: Half-light radius vs. V-band magnitude for a range of (primarily elliptical) systems. Blue points are from Brodie2011, grey points represent galaxies in SDSS with $0.01<z<0.05$ from Simard2011 and red points are the UDGs first detected in the Coma Cluster by vanDokkum2015. The regions approximately corresponding to globular clusters (GC), dwarf spheroidals (dSph), ultra-compact dwarfs (UCD), dwarf ellipticals (dE), normal ellipticals (E) and giant ellipticals (gE) are as labelled. The region meeting the original UDG definition is shaded red and labelled appropriately. UDGs reside in a region of parameter space previously sparsely populated by surveys such as SDSS (highlighted in red).
• Figure 2: Dark Energy Camera Legacy Survey (DECaLS) images of three galaxies chosen for their similar distance as indicated in the top left of each cutout. The brightness of the images has been increased by a factor of 2 to aid the visibility of the UDGs. Left: A prototypical dwarf elliptical, MATLAS-49. Centre: An HI-bearing field UDG, AGC242019. Right: A quiescent, GC-rich UDG, NGC5846_UDG1 (MATLAS-2019). UDGs are noticeably larger than their dwarf elliptical (and dwarf irregular, although not shown here) counterparts. The UDG definition takes in a range of morphologies from blue, star-forming field UDGs to red, quenched group/cluster UDGs.
• Figure 3: Number of UDGs vs. total environmental halo mass, reproduced from Carleton2023. See also Karunakaran2023 and Goto2023 alternative plots of this relationship published at a similar time, but focusing on lower density environments. Data are included from the studies of Impey1988Munoz2015vanderBurg2016vanderBurg2017Yagi2016Trujillo2017Roman2017bManceraPina2018Janssens2019Bachmann2021 per the legend. Carleton2023's work on El Gordo UDGs is indicated as "this work" in the legend. The dashed line represents the relationship derived in vanderBurg2017 of $N_{\rm UDG} \propto M_{200}^{0.93\pm0.16}$. The near linearity of this relationship indicates that the environment may not play a strong role in the net creation/destruction of UDGs.
• Figure 4: Summary table of predicted UDG properties for each formation scenario as described in section \ref{['sec:formation_scenarios']}. We either write '-' if the prediction is totally unclear or add a '?' next to a likely property that is yet not well-established. 'Int.' refers to intermediate ages in first column. We provide this table as a basic guide for future UDG studies.
• Figure 5: Galaxy velocity dispersion vs galaxy stellar mass. The non-UDGs that establish the normal relation are from the works of Chilingarian2009mcconnachie2012Cappellari2013Harris2013. UDGs are plotted from the catalogue of Gannon2024b with dynamical masses coming from their stellar velocity dispersions (red triangles) and from their GC system velocity dispersions (blue squares). In general, UDGs with secure velocity dispersion measurements do not appear to be dynamically hotter or colder than dwarf galaxies of a similar stellar mass.