Deep Imaging of Eridanus Ii and Its Lone Star Cluster*

Abstract

We present deep imaging of the most distant dwarf discovered by the Dark Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry reaches ∼3 mag deeper than previous work and allows us to confirm the presence of a stellar cluster whose position is consistent with Eri II's center. This makes Eri II, at = -M 7.1 V , the least luminous galaxy known to host a (possibly central) cluster. The cluster is partially resolved, and at = -M 3.5V it accounts for ∼4% of Eri II's luminosity. We derive updated structural parameters for Eri II, which has a half-light radius of ∼280 pc and is elongated (ò ∼ 0.48) at a measured distance of D ∼ 370 kpc. The colormagnitude diagram displays a blue, extended horizontal branch, as well as a less populated red horizontal branch. A central concentration of stars brighter than the old main-sequence turnoff hints at a possible intermediate-age (∼3 Gyr) population; alternatively, these sources could be blue straggler stars. A deep Green Bank Telescope observation of Eri II reveals no associated atomic gas.

Dwarf Galaxy Discoveries from the KMTNet Supernova Program. I. The NGC 2784 Galaxy Group*

“…-Cumulative luminosity function of galaxies in several galaxy groups. The crosses, diamonds, circles, pluses, triangles, and boxes represent the galaxies in the Local Group (McConnachie 2012;Martin et al 2013a,b), NGC 5128 group (Tully et al 2015;Crnojević et al 2016), NGC 2784 group (this study), M81 group (Chiboucas et al 2009), M83 group (Müller et al 2015), and M106 group (Kim et al 2011), respectively. The solid lines indicate the best fitting cumulative Schechter function on each data.…”

“…The same group also reported ∼ 40 new dwarf galaxy candidates in the Centaurus group region (Müller et al 2017). Other studies have explored the NGC 5128 group (Crnojević et al 2016), the M 106 group (Kim et al 2011), the NGC 1023 group (Trentham & Tully 2009), and the M 101 group (Merritt et al 2014). These studies all discovered new dwarf galaxies, reaching as faint as M V ∼ −10 mag.…”

Farthest Neighbor: The Distant Milky Way Satellite Eridanus II*

“…However, Westmeier et al (2015) measured the H I gas content using HIPASS data (Barnes et al 2001) and did not detect any H I gas associated with EriII. Crnojević et al (2016) also obtained H I observations from the Green Bank Telescope and found a more stringent upper limit on the H I mass (M HI <2800  M ), indicating that EriII is an extremely gas-poor system. With deep imaging from Magellan/Megacam, Crnojević et al (2016) found a possible intermediate-age (∼3 Gyr) population in EriII.…”

“…Crnojević et al (2016) also obtained H I observations from the Green Bank Telescope and found a more stringent upper limit on the H I mass (M HI <2800  M ), indicating that EriII is an extremely gas-poor system. With deep imaging from Magellan/Megacam, Crnojević et al (2016) found a possible intermediate-age (∼3 Gyr) population in EriII. Moreover, they confirmed that there is a star cluster whose projected position is very close to the center of EriII, making it the least luminous galaxy known to host a (possibly central) star cluster.…”

“…One of the largest most luminous and most distant newly discovered satellites is EridanusII (Eri II), which has an absolute magnitude of M V ∼−7, a half-light radius of r h ∼280 pc, and a Galactocentric distance of D∼370 kpc (Bechtol et al 2015;Koposov et al 2015;Crnojević et al 2016; see Table 1). …”