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Lyman- and Werner-band absorption of molecular hydrogen (H$_2$) is detectedin $\sim$50% of low redshift ($z<1$) DLAs/sub-DLAs with $N$(H$_2$) >10$^{14.4}$ cm$^{-2}$. However the true origin(s) of the H$_2$ bearing gasremain elusive. Here we report a new detection of an H$_{2}$ absorber at $z=$0.4298 in the HST/COS spectra of quasar PKS 2128-123. The total $N$(HI) of10$^{19.50\pm0.15}$ cm$^{-2}$ classifies the absorber as a sub-DLA. H$_{2}$absorption is detected up to the $J=3$ rotational level with a total $\logN$(H$_{2}$) = 16.36$\pm$0.08 corresponding to a molecular fraction of log$f$(H$_{2}$) = $-$2.84$\pm$0.17. The excitation temperature of $T_{ex}$ =206$\pm$6K indicates the presence of cold gas. Using detailed ionizationmodelling we obtain a near-solar metallicity (i.e., [O/H]= $-$0.26$\pm$0.19)and a dust-to-gas ratio of $\log \kappa \sim -0.45$ for the H$_{2}$ absorbinggas. The host-galaxy of the sub-DLA is detected at an impact parameter of $\rho\sim$ 48 kpc with an inclination angle of $i \sim$ 48 degree and an azimuthalangle of $\Phi \sim$ 15 degree with respect to the QSO sightline. We show thatco-rotating gas in an extended disk cannot explain the observed kinematics ofMg II absorption. Moreover, the inferred high metallicity is not consistentwith the scenario of gas accretion. An outflow from the central region of thehost-galaxy, on the other hand, would require a large opening angle (i.e.,2$\theta>$150 degree), much larger than the observed outflow opening angles inSeyfert galaxies, in order to intercept the QSO sightline. We thus favor ascenario in which the H$_2$ bearing gas is stemming from a dwarf-satellitegalaxy, presumably via tidal and/or ram-pressure stripping. Detection of adwarf galaxy candidate in the HST/WFPC2 image at an impact parameter of$\sim$12 kpc reinforces such an idea.

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