The nearest ultra diffuse galaxy: UGC2162

Abstract

We describe the structural, stellar population and gas properties of the nearest Ultra Diffuse Galaxy (UDG) discovered so far: UGC2162 (z=0.00392; R${}_{e,g}$=1.7$(\pm$0.2) kpc; $\hat{I}{¼}_g(0)$=24.4$\pm$0.1 mag/arcsec${}^2$; g-i=0.33$\pm$0.02). This galaxy, located at a distance of 12.3($\pm$1.7) Mpc, is a member of the M77 group. UGC2162 has a stellar mass of $\sim$2(${}_{-1}^{+2}$)$\times$10${}^7$ M${}_{\odot }$ and is embedded within a cloud of HI gas $\sim$10 times more massive: $\sim$1.9($\pm$0.6)$\times$10${}^8$ M${}_{\odot }$. Using the width of its HI line as a dynamical proxy, the enclosed mass within the inner R$\sim$5 kpc is $\sim$4.6($\pm$0.8)$\times$10${}^9$ M${}_{\odot }$ (i.e. M/L$\sim$200). The estimated virial mass from the cumulative mass curve is $\sim$8($\pm$2)$\times$10${}^{10}$ M${}_{\odot }$. Ultra-deep imaging from the IAC Stripe82 Legacy Project show that the galaxy is irregular and has many star forming knots, with a gas-phase metallicity around one-third of the solar value. Its estimated Star Formation Rate (SFR) is $\sim$0.01 M${}_{\odot }$/yr. This SFR would double the stellar mass of the object in $\sim$2 Gyr. If the object were to stop forming stars at this moment, after a passive evolution, its surface brightness would become extremely faint: $\hat{I}{¼}_g(0)$$\sim$27 mag/arcsec${}^2$ and its size would remain large R${}_{e,g}$$\sim$ 1.8 kpc. Such faintness would make it almost undetectable to most present-day surveys. This suggests that there could be an important population of M${}_{\star }$$\sim$10${}^7$ M${}_{\odot }$ "dark galaxies" in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.

5 pages; 3 figures, accepted for publication in ApJ; minor changes to match the published version