The nearest ultra diffuse galaxy: UGC2162

Ignacio Trujillo; Javier Román; Mercedes Filho; Jorge Sánchez Almeida

doi:10.3847/1538-4357/aa5cbb

paper

The nearest ultra diffuse galaxy: UGC2162

arXiv:1701.03804 · doi:10.3847/1538-4357/aa5cbb

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; $μ_{g} (0)$ =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 $_{⊙}$ and is embedded within a cloud of HI gas $\sim$ 10 times more massive: $\sim$ 1.9( $\pm$ 0.6) $\times$ 10 $^{8}$ M $_{⊙}$ . 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 $_{⊙}$ (i.e. M/L $\sim$ 200). The estimated virial mass from the cumulative mass curve is $\sim$ 8( $\pm$ 2) $\times$ 10 $^{10}$ 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 (SFR) is $\sim$ 0.01 M $_{⊙}$ /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: $μ_{g} (0)$ $\sim$ 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 $_{⋆}$ $\sim$ 10 $^{7}$ M $_{⊙}$ "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

The nearest ultra diffuse galaxy: UGC2162

Abstract

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