Galaxy group at z = 0.3 associated with the damped Lyman α system towards quasar Q1127-145

Abstract

We performed a spectroscopic galaxy survey, complete to m_F814W <= 20.3 (L_B > 0.15L^*_B at z = 0.3), within 100 × 100 arcsec² of the quasar Q1127-145 (z_em = 1.18). The Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (UVES) quasar spectrum contains three z_abs < 0.33 MgII absorption systems. We obtained eight new galaxy redshifts, adding to the four previously known, and galaxy star formation rates (SFRs) and metallicities were computed where possible. A strong MgII system [W_r(2796) = 1.8 Å], which is a known damped Lyman α absorber (DLA), had three previously identified galaxies; we found two additional galaxies associated with this system. These five galaxies form a group with diverse properties, such as a luminosity range of 0.04 <= L_B <= 0.63 L^*_B, an impact parameter range of 17 <= D <= 241 kpc and a velocity dispersion of σ = 115 kms^-1. The DLA group galaxy redshifts span beyond the 350 kms^-1 velocity spread of the metallic absorption lines of the DLA itself. The two brightest group galaxies have SFRs of ~fewM_solar yr^-1 and should not have strong winds. We have sufficient spectroscopic information to directly compare three of the five group galaxies' (emission-line) metallicities with the DLA (absorption) metallicity: the DLA metallicity is one-tenth solar, substantially lower than the three galaxies' metallicity which ranges between less than 1/2 solar to solar. Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC-2) imaging shows perturbed morphologies for the three brightest group galaxies, with tidal tails extending ~25 kpc. We favour a scenario where the DLA absorption originates from tidal debris in the group environment.

Another absorber exhibits weak MgII absorption [W_r(2796) = 0.03 Å] and had a previously identified galaxy at a similar redshift. We have identified a second galaxy associated with this system. Both galaxies have solar metallicities and unperturbed morphologies in the HST/WFPC-2 image. The SFR of one galaxy is much lower than that expected for strong outflows. Finally, we have also identified five galaxies at large impact parameters with no associated MgII absorption [W_r(2796) <~ 5.7 mÅ, 3σ] in the spectrum of Q1127-145.