Eridanus II: A Fossil from Reionization with an Off-center Star Cluster

Abstract

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M V = −7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z ∼ 6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 ± 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II’s cluster is offset from the center of the galaxy by a projected distance of 23 ± 3 pc. This offset could be an indication of a small (∼50–75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of and is aligned with the orientation of Eri II within 3° ± 6°, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.

The First Billion Years project: Finding infant globular clusters at z = 6

“…see Harris 1996;Larsen et al 2002;Masters et al 2010) and for UFDs ∼30 ∼ 100 pc (e.g. see Bechtol et al 2015;Koposov et al 2015;Simon et al 2015). Thus the ratio of sizes we find between our potential infant GCs and proto-UFDs are similar to the ratio of sizes of these objects at z = 0.…”

The Star Formation History of Eridanus II: On the Role of Supernova Feedback in the Quenching of Ultrafaint Dwarf Galaxies*

“…The remaining candidates, tidal interactions or ram pressure stripping, would result in quenching times that depend on the accretion time of each dwarf galaxy into the MW. In a paper that was accepted and posted in ArXiv after this one was submitted, Simon et al (2021) analyzed the same Eri II (F606W −F814W, F814W) CMD with the methodology used in Brown et al (2014). They reach a conclusion on the age and duration of the star formation in Eri II that is totally compatible with our scenario.…”

EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf

“…Discovered by the Dark Energy Survey (DES, Bechtol et al 2015;Koposov et al 2015), EriII is an ultra-faint dwarf with a stellar mass of 8.3 +1.7 −1.4 × 10 4 M and a dynamical mass estimate of 1.2 +0.4 −0.3 × 10 7 M within its half-light radius (Li et al 2017). Its low metallicity implies an old stellar population and it is a good candidate for being a 'fossil' galaxy that was quenched by reionisation (Simon et al 2021). Deep imaging of EriII has revealed a lone SC offset in projection from EriII's photometric centre (Koposov et al 2015;Crnojević et al 2016;Simon et al 2021), making it the faintest known galaxy to host a SC.…”

“…Its low metallicity implies an old stellar population and it is a good candidate for being a 'fossil' galaxy that was quenched by reionisation (Simon et al 2021). Deep imaging of EriII has revealed a lone SC offset in projection from EriII's photometric centre (Koposov et al 2015;Crnojević et al 2016;Simon et al 2021), making it the faintest known galaxy to host a SC. The large projected half-light radius of this SC (15 ± 1 pc), and its large offset from EriII's photometric centre (23 ± 3 pc; Simon et al 2021 3 ), can be naturally explained if it is orbiting within a central dark matter core of size ∼ 70 pc and density 2 × 10 8 M kpc −3 (Amorisco 2017; Contenta et al 2018).…”

“…Deep imaging of EriII has revealed a lone SC offset in projection from EriII's photometric centre (Koposov et al 2015;Crnojević et al 2016;Simon et al 2021), making it the faintest known galaxy to host a SC. The large projected half-light radius of this SC (15 ± 1 pc), and its large offset from EriII's photometric centre (23 ± 3 pc; Simon et al 2021 3 ), can be naturally explained if it is orbiting within a central dark matter core of size ∼ 70 pc and density 2 × 10 8 M kpc −3 (Amorisco 2017; Contenta et al 2018). By contrast, a 'pristine cuspy' inner dark matter profile -as expected in pure dark matter structure formation simulations in ΛCDM -rapidly destroys the SC, making such a scenario unlikely.…”