MOLECULAR HYDROGEN ABSORPTION from the HALO of A z ∼ 0.4 GALAXY

Lyman- and Werner-band absorption of molecular hydrogen (H2) is detected in ∼50% of low-redshift (z < 1) DLAs/sub-DLAs with N(H2) > 1014.4 cm-2. However, the true origin(s) of the H2-bearing gas remain elusive. Here we report a new detection of an H2 absorber at zabs = 0.4298 in the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph spectra of quasar PKS 2128123. The total N (H I) of 1019.50±0.15 cm-2 classifies the absorber as a sub-DLA. H2 absorption is detected up to the J = 3 rotational level with a total logN (H2) = 16.36 ± 0.08, corresponding to a molecular fraction of log fH2 = -2.84 ± 0.17. The excitation temperature of Tex = 206 ± 6K indicates the presence of cold gas. Using detailed ionization modeling, we obtain a near-solar metallicity (i.e., [O/ H] = -0.26 ± 0.19) and a dust-to-gas ratio of log k ∼ -0.45 for the H2-absorbing gas. The host galaxy of the sub- DLA is detected at an impact parameter of p∼ 48 kpc with an inclination angle of i ∼ 48° and an azimuthal angle of π ∼ 15° with respect to the QSO sightline. We show that corotating gas in an extended disk cannot explain the observed kinematics of Mg II absorption. Moreover, the inferred high metallicity is not consistent with the scenario of gas accretion. An outflow from the central region of the host galaxy, on the other hand, would require a large opening angle (i.e., 2q > 150°), much larger than the observed outflow opening angles in Seyfert galaxies, in order to intercept the QSO sightline. We thus favor a scenario in which the H2-bearing gas is stemming from a dwarfsatellite galaxy, presumably via tidal and/or ram pressure stripping. Detection of a dwarf galaxy candidate in the HST/WFPC2 image at an impact parameter of ∼12 kpc reinforces such an idea.