Counting clusters to probe ancient star formation

Title: NGC5846-UDG1: A galaxy formed mostly by star formation in massive, extremely dense clumps of gas

Authors: Shany Danieli, Pieter van Dokkum, Sebastian Trujillo-Gomez, J. M. Diederik Kruijssen, Aaron J. Romanowsky, Scott Carlsten, Zili Shen, Jiaxuan Li, Roberto Abraham, Jean Brodie, Charlie Conroy, Jonah S. Gannon, Johnny Greco

First Author’s Institution: Department of Astrophysical Sciences, Princeton University

Status: Submitted to The Astrophysical Journal Letters

Stars can find a home in many different places: some reside in galaxies, whereas others can reside in tightly-bound star clusters, which themselves orbit galaxies. The stars that exist in the densest, oldest star clusters—globular clustersformed in extreme conditions, at very early times in regions of space with extremely high gas pressures. Globular clusters (GCs) are then ancient relics encoding information on conditions for star formation in the early Universe. 

Today’s authors use Hubble Space Telescope data to investigate the GC population of the ultra-diffuse galaxy NGC5846-UDG1 (UDG1 for short; image of galaxy and its GCs shown in Figure 1). Ultra-diffuse galaxies (UDGs) are a type of low-surface brightness dwarf galaxy that can be approximately the size of the Milky Way galaxy, but up to a factor of 100 less luminous (see here and here for previous bites on UDGs). Exactly how these odd galaxies form is an open question in astronomy. So, what can UDG1’s globular cluster population tell us about galaxy formation?

Figure 1: V-band image of UDG1 (diffuse grey points) and its globular cluster candidates (black points). Adapted from Figure 1 in the paper.

A GC-rich galaxy!

A key result of today’s paper is that UGD1 has a significantly higher number of globular clusters than would be expected for a galaxy of its mass, with the tally coming in at 54 (+/- 9)! In addition to the number of GCs, the authors also estimate the total light from the GCs relative to the total light in the galaxy itself. The authors find that the GCs currently make up 13% of the total light of the galaxy – this is the highest fraction of stars in globular clusters known for any galaxy to date, and is 100 times larger than the GC fraction of the Milky Way! 

While the current fraction of light in GCs for UDG1 is 13%, it is expected that this figure is likely to have been much higher at earlier times due to the dynamical evolution of GCs. GCs are expected to interact with their environment and lose mass (or be completely destroyed) via tidal stripping over time. The stars that are lost from the GCs may end up contributing significantly to the stellar content of the galaxy itself (see this astrobite for more).

These mass loss processes can lead to GC systems losing up to 80–90% of their stellar mass content over their lifetime. With a current GC fraction of 13%, the authors use an analytical model to estimate that the original GC fraction (before any mass loss processes occur) is likely to have been 65%, indicating that the majority of the stars in this galaxy at present day originally formed in bound clusters, from extremely dense gas clumps.

These results are summarised in Figure 2, which displays the cumulative proportion of mass in GCs (currently observed GCs in black; the initial GC model in purple (before mass loss effects); and Milky Way values in orange).

Figure 2: The cumulative fraction of galaxy mass in clusters for NGC5846-UDG1 in comparison to the Milky Way. The black line is the current observed values, the purple line is the modelled values expected at the time of formation, and the orange line is the Milky Way values. Mcl refers to the total stellar mass of the globular clusters. Adapted from Figure 5 in the paper.

Implications for star formation

The large difference between the Milky Way and UDG1 GC fractions highlight the rare conditions under which UDG1 originally formed. Globular cluster formation (and their subsequent destruction) is very likely to have been the dominant star formation mode for UDG1, with these stars originally forming from extremely dense, high pressure gas clumps. This idea is supported by the fact that the spatial distribution and ages of the GC stars versus the galaxy stars in UDG1 are very similar, indicating that the galaxy stars have likely formed from disrupted GCs. 

It is unlikely that UDG1 is the only galaxy of its kind. The authors note that there are hints some UDGs in the Coma cluster have similarly high GC fractions to UDG1. While the uncertainties on these measurements are high due to the galaxies in the Coma cluster being located farther away, it is still a promising indicator that star formation through extremely dense clumps at early times may be a viable way to build a galaxy!

Astrobite edited by Gloria Fonesca Alvarez

Featured image credit: NASA 

About Katy Proctor

I am a first-year PhD student at the International Centre for Radio Astronomy Research at the University of Western Australia. My research is focused on using cosmological simulations to study the build up of stellar halos. Outside of research, I can usually be found climbing up walls or playing guitar.

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