The Nearest Ultra Diffuse Galaxy: UGC 2162

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) × 10⁷ {M}_⊙ and is embedded within a cloud of HI gas ∼10 times more massive: ∼1.9(±0.6) × 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) × 10⁹ {M}_⊙ (I.e., M/L ∼ 200). The estimated virial mass from the cumulative mass curve is ∼8(±2)×10¹⁰ 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}_\star ∼ 10⁷ {M}_⊙ “dark galaxies” in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.