* Astronomy

Title: The nearest ultra diffuse galaxy: UGC2162
Author: Ignacio Trujillo, Javier Román, Mercedes Filho, Jorge Sánchez Almeida

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(±0.2) kpc; _g(0)=24.4±0.1 mag/arcsec²; 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. UGC2162 has a stellar mass of ~2^{+2}_{-1} x 10^7 solar masses and is embedded within a cloud of HI gas ~10 times more massive: ~1.9(±0.6) x 10^8 solar masses. 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 10^9 solar masses (i.e. M/L~200). The estimated virial mass from the cumulative mass curve is ~8(±2) x 10^10 solar masses. 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 1/3 the solar value. Its estimated Star Formation Rate (SFR) is ~0.01 solar masses/yr. 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: g(0)~27 mag/arcsec² 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_*~10^7 solar masses "dark galaxies" in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.

Read more (1569kb, PDF)