The globular clusters and star formation history of the isolated, quiescent ultra-diffuse galaxy DGSAT I

Abstract

We investigate the isolated, quiescent ultra-diffuse galaxy (UDG) DGSAT I and its globular cluster (GC) system using two orbits of Hubble Space Telescope Advanced Camera for Surveys imaging in the F606W and F814W filters. This is the first study of GCs around a UDG in a low-density environment. DGSAT I was previously found to host an irregular blue low surface brightness clump, that we confirm as very likely belonging to the galaxy rather than being a chance projection, and represents a recent episode of star formation (∼500 Myr) that challenges some UDG formation scenarios. We select GC candidates based on colours and magnitudes, and construct a self consistent model of the GC radial surface density profile along with the background. We find a half-number radius of RGC = 2.7 ± 0.1 kpc (more compact than the diffuse starlight) and a total of 12 ± 2 GCs. The total mass fraction in GCs is relatively high, supporting an overmassive dark matter halo as also implied by the high velocity dispersion previously measured. The GCs extend to higher luminosities than expected, and have colours that are unusually similar to their host galaxy colour, with a very narrow spread–all of which suggest an early, intense burst of cluster formation. The nature and origin of this galaxy remain puzzling, but the most likely scenario is a ‘failed galaxy’ that formed relatively few stars for its halo mass, and could be related to cluster UDGs whose size and quiescence pre-date their infall.

The Next Generation Virgo Cluster Survey (NGVS). XXXV. First Kinematical Clues of Overly Massive Dark Matter Halos in Several Ultradiffuse Galaxies in the Virgo Cluster

“…The UDGs are extremely low surface brightness galaxies that have luminosities and stellar masses consistent with those of dwarf galaxies but sizes more typical of massive galaxies (e.g., Binggeli et al 1985;van Dokkum et al 2015). Possible formation mechanisms for UDGs either place them in dwarf-mass halos, in which case their baryonic efficiency will be in agreement with what is known for dwarf galaxies (e.g., Amorisco & Loeb 2016;Di Cintio et al 2017;Carleton et al 2019;Tremmel et al 2020;Wright et al 2021), or, alternatively, propose that they are "failed galaxies," meaning that they inhabit more massive halos that were destined to form a brighter galaxy, perhaps comparable to the MW, but their star formation process was truncated at an earlier epoch, freezing their stellar content to that of a dwarf (e.g., van Dokkum et al 2015van Dokkum et al , 2016Janssens et al 2022). The extreme cases of the "dark matter-free" UDGs (so far, only two of these have been discovered: NGC1052-DF2 and NGC1052-DF4) are better explained by a highvelocity collision of gas-rich galaxies (van Dokkum et al 2022).…”

“…The UDGs with available velocity dispersion estimates are scarce due to their intrinsic low surface brightness (which limits the availability of velocity dispersion for field stars) and finite number of GCs (which negatively impact the accuracy of velocity dispersion estimates). To date, when velocity dispersion has been available, it has supported low dark matter contents consistent with dwarf-mass halos for UDGs (van Dokkum et al 2017(van Dokkum et al , 2019Danieli et al 2019;Chilingarian et al 2019;Collins et al 2020;Gannon et al 2021) with perhaps a handful of exceptions, such as DGSAT-1, a relatively isolated UDG (Martínez-Delgado et al 2016;Janssens et al 2022), or VLSB-B in the Virgo cluster (Toloba et al 2018). The need for more observed systems with available kinematical data is indisputable.…”

“…This includes some UDGs with two to four times more GCs than dwarfs of similar luminosities (Lim et al 2018(Lim et al , 2020Danieli et al 2022), even though they do not quite reach the level of an MW-like object. Other cases, such as DGSAT-1, NGC 1052-DF2, or NGC 5846-UDG1 show odd distributions of their GC luminosities, colors, or both (Fensch et al 2019;Shen et al 2021;Danieli et al 2022;Janssens et al 2022;van Dokkum et al 2022), also hinting at the need for more exotic origins in these galaxies. The trail of UDGs that might have larger halos than expected is one worth pursuing, as their unusual properties could reveal new physics or mechanisms heavily impacting the efficiency with which halos form stars, a pillar of galaxy formation theories.…”

Keck Spectroscopy of the Coma Cluster Ultra-Diffuse Galaxy Y358: Dynamical Mass in a Wider Context

“…𝑐 = Circularised using literature 𝑏/𝑎. 𝑑 = Note that some of these GC's may be overluminous (Janssens et al 2022)…”

“…The observational expectation is for the galaxy to have quenched early and catastrophically. In doing so it fails to form a large portion of its expected stellar mass (van Dokkum et al 2016;Peng & Lim 2016;Villaume et al 2022;Danieli et al 2022;Janssens et al 2022;Buzzo et al 2022). These massive halo UDGs are not reproduced by leading cosmological simulations of galaxy formation.…”