NGC 5846-UDG1: A Galaxy Formed Mostly by Star Formation in Massive, Extremely Dense Clumps of Gas

Abstract

It has been shown that ultra-diffuse galaxies (UDGs) have higher specific frequencies of globular clusters, on average, than other dwarf galaxies with similar luminosities. The UDG NGC 5846-UDG1 is among the most extreme examples of globular cluster–rich galaxies found so far. Here we present new Hubble Space Telescope observations and analysis of this galaxy and its globular cluster system. We find that NGC 5846-UDG1 hosts 54 ± 9 globular clusters, three to four times more than any previously known galaxy with a similar luminosity and higher than reported in previous studies. With a galaxy luminosity of L V,gal ≈ 6 × 107 L ⊙ (M ⋆ ≈ 1.2 × 108 M ⊙) and a total globular cluster luminosity of L V,GCs ≈ 7.6 × 106 L ⊙, we find that the clusters currently comprise ∼13% of the total light. Taking into account the effects of mass loss from clusters during their formation and throughout their lifetime, we infer that most of the stars in the galaxy likely formed in globular clusters, and very little to no “normal” low-density star formation occurred. This result implies that the most extreme conditions during early galaxy formation promoted star formation in massive and dense clumps, in contrast to the dispersed star formation observed in galaxies today.

Dynamical Friction in Globular Cluster-rich Ultra-diffuse Galaxies: The Case of NGC5846-UDG1

“…As briefly reviewed in Section 2.2, GCs are expected to lose ( )  1 of their mass over their life. With this in mind, Danieli et al (2022) pointed out that the GC population may have initially comprised an ( )  1 fraction of the stellar mass in UDG1. This scenario could make two-body relaxation surprisingly efficient, if, in addition, the total halo mass of (and therefore velocity dispersion in) UDG1 is small.…”

“…This is timely in part due to recent studies of ultra-diffuse galaxies (UDGs), which often times host large numbers of GCs (van Dokkum et al 2017, 2018Lim et al 2018;Shen et al 2021). In this paper we consider NGC5846-UDG1 (UDG1 for short; Forbes et al 2019Forbes et al , 2021Müller et al 2020Müller et al , 2021Danieli et al 2022), that recently attracted considerable attention. 5 At a distance of ∼25 Mpc, stellar luminosity of ∼6 × 10 7 L e , and half-light radius r eff ∼ 2 kpc, UDG1 harbors some ∼50 GC candidates, representing ∼10% of the stellar mass at the preset day (Danieli et al 2022).…”

“…The left panel of Figure 1 shows the V-band Hubble Space Telescope (HST) WFC3/UVIS image of UDG1 and its nearby field, adapted from Danieli et al (2022). The right panel shows all compact sources that were selected as GC candidates based on the photometric selection criteria in Danieli et al (2022).…”

“…The left panel of Figure 1 shows the V-band Hubble Space Telescope (HST) WFC3/UVIS image of UDG1 and its nearby field, adapted from Danieli et al (2022). The right panel shows all compact sources that were selected as GC candidates based on the photometric selection criteria in Danieli et al (2022). In this work we focus on a low contamination sample of GC candidates, consisting of the 33 m V < 25.0 mag objects contained within 2r eff (twice the Sérsic half-light radius of the stellar body; inner circle in Figure 1), which has a background contamination of about one object, estimated by comparison to the nearby field (Danieli et al 2022).…”

“…The right panel shows all compact sources that were selected as GC candidates based on the photometric selection criteria in Danieli et al (2022). In this work we focus on a low contamination sample of GC candidates, consisting of the 33 m V < 25.0 mag objects contained within 2r eff (twice the Sérsic half-light radius of the stellar body; inner circle in Figure 1), which has a background contamination of about one object, estimated by comparison to the nearby field (Danieli et al 2022). Spectroscopic information is available for 11 of these bright GCs (Müller et al 2020).…”