Molecular Hydrogen Absorption from the Halo of a z ∼ 0.4 Galaxy

Lyman- and Werner-band absorption of molecular hydrogen ({{{H}}}₂) is detected in ∼50% of low-redshift (z\lt 1) DLAs/sub-DLAs with N({{{H}}}₂) \gt {10}^14.4 cm^-2. However, the true origin(s) of the {{{H}}}₂-bearing gas remain elusive. Here we report a new detection of an {{{H}}}₂ absorber at {z}_{{abs}} = 0.4298 in the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph spectra of quasar PKS 2128-123. The total N({{H}} {{I}}) of {10}^{19.50+/- 0.15} cm^-2 classifies the absorber as a sub-DLA. {{{H}}}₂ absorption is detected up to the J = 3 rotational level with a total {log}N({{{H}}}₂) = 16.36 ± 0.08, corresponding to a molecular fraction of {log}{f}_{{{H}}₂} = -2.84 ± 0.17. The excitation temperature of {T}_{{ex}} = 206 ± 6 K 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}κ ∼ -0.45 for the {{{H}}}₂-absorbing gas. The host galaxy of the sub-DLA is detected at an impact parameter of ρ ∼ 48 kpc with an inclination angle of I ∼ 48° and an azimuthal angle of Φ ∼ 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., 2θ \gt 150^\circ ), 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 {{{H}}}₂-bearing gas is stemming from a dwarf-satellite 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.