Origins of ultradiffuse galaxies in the Coma cluster – II. Constraints from their stellar populations

Abstract

In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar populations of seven spectroscopically confirmed ultra-diffuse galaxies (UDGs) in the Coma cluster. We find intermediate to old ages (∼ 7 Gyr), low metallicities ([Z/H]∼ -0.7 dex) and mostly super-solar abundance patterns ([Mg/Fe] ∼ 0.13 dex). These properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same area in the cluster and are mostly consistent with being the continuity of the stellar mass scaling relations of more massive galaxies. These UDGs' star formation histories imply a relatively recent infall into the Coma cluster, consistent with the theoretical predictions for a dwarf-like origin. However, considering the scatter in the resulting properties and including other UDGs in Coma, together with the results from the velocity phase-space study of the Paper I in this series, a mixed-bag of origins is needed to explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs are field dwarfs that become quenched through their later infall onto cluster environments, whereas some UDGs could be be genuine primordial galaxies that failed to develop due to an early quenching phase. The unknown proportion of dwarf-like to primordial-like UDGs leaves the enigma of the nature of UDGs still open.

One Hundred SMUDGes in S-PLUS: Ultra-diffuse Galaxies Flourish in the Field

“…The reported ages of UDGs have usually been large, > 4 Gyrs, but again almost all published results are for Coma galaxies (Gu et al 2018;Ruiz-Lara et al 2018;Ferré-Mateu et al 2018). In the limited available examples of field UDGs, however, the reported luminosity-weighted ages have consistently been younger, with ages ranging from 1 to 3 Gyr (Martínez-Delgado et al 2016;Greco et al 2018;Martín-Navarro et al 2019).…”

“…In Figure 8 we compare the luminosityweighted age distribution of galaxies in our sample to that of UDGs observed in the Coma (Gu et al 2018;Ruiz-Lara et al 2018;Ferré-Mateu et al 2018) and Virgo (Pandya et al 2018) clus- Considering that we only have luminosityweighted ages, the results from the our analysis are expected to be biased toward the youngest populations within a galaxy. Therefore, a few different, non-exclusive scenarios can explain the additional fraction of UDGs with young ages.…”

“…Prole et al 2019), are available for many UDGs but are of limited value in determining the properties of the stellar populations, while spectroscopy, which can provide the necessary information, is only available for a small number of galaxies (e.g. Martínez-Delgado et al 2016;Kadowaki et al 2017;Ruiz-Lara et al 2018;Ferré-Mateu et al 2018;Gu et al 2018).…”

“…Comparison of the luminosity-weighted ages of our UDGs, which are primarily in the field, and those of UDGs in the literature(Pandya et al 2018;Gu et al 2018;Ruiz-Lara et al 2018;Ferré- Mateu et al 2018), which are primarily in clusters.…”

The environment of H i-bearing ultra-diffuse galaxies in the ALFALFA survey

“…Three main formation pathways for UDGs have been discussed in the literature: first, that UDGs are "failed" L * galaxies that lost their gas at early times (e.g., Beasley & Trujillo 2016;Peng & Lim 2016; van Dokkum et al 2016); second, that UDGs occupy the high spin tail of the galaxy angular momentum distribution (e.g., Amorisco & Loeb 2016); third, that UDGs are dwarf galaxies that have been made diffuse by stellar feedback and outflows (e.g., Collins et al 2013). More recent observations and simulations have increasingly favored the latter two explanations for the majority of the UDG population Beasley & Trujillo 2016;Di Cintio et al 2017;Papastergis et al 2017;Amorisco et al 2018;Chan et al 2018;Ferré-Mateu et al 2018), suggesting that feedback and internal processes are important to UDG formation; however, Bennet et al (2018) suggests that some UDGs may have a tidal origin and the high-resolution simulations Jiang et al (2018a,b) find that UDGs live in halos with ordinary spin parameters.…”