2017ApJ...836..191T

We describe the structural, stellar population and gas properties of the nearest ultra diffuse galaxy discovered so far: UGC 2162 (z = 0.00392; R_e,g=1.7(±0.2) kpc; µ_g(0) = 24.4 ± 0.1 mag arcsec^–2; g-i = 0.33 ± 0.02). This galaxy, located at a distance of 12.3(±1.7) Mpc, is a member of the M77 group. UGC 2162 has a stellar mass of ∼2(_–1⁺²) x 10⁷ M_☉ and is embedded within a cloud of HI gas ∼10 times more massive: ∼1.9(±0.6) x 10⁸ M_☉. Using the width of its HI line as a dynamical proxy, the enclosed mass within the inner R ∼ 5 kpc is ∼4.6(±0.8) x 10⁹ M_☉ (i.e., M/L ∼ 200). The estimated virial mass from the cumulative mass curve is ∼8(±2)x10¹⁰ M_☉. 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 is ∼0.01 M_☉ yr^–1. This SFR would double the stellar mass of the object in ∼2 Gyr. If the object were to stop forming stars at this moment, after a passive evolution, its surface brightness would become extremely faint: µ_g(0) ∼27 mag arcsec^–2 and its size would remain large R_e,g ∼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_* ∼10⁷ M_☉ "dark galaxies" in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.