2020ApJ...893...84B

We present a Hubble Space Telescope Cosmic Origins Spectrograph spectrum of the QSO SDSS J095109.12+330745.8 (z_em=0.645) whose sightline passes through the SMC-like dwarf galaxy UGC 5282 (M_B=-16.0, cz = 1577 km s^–1), 1.2 kpc in projection from the central H II region of the galaxy. Damped Lyα (DLA) absorption is detected at the redshift of UGC 5282 with log [N(H I) cm^–2]=20.89_–0.21^+0.12. Analysis of the accompanying S II, P II, and O I metal lines yields a neutral gas metallicity, Z_HI, of [S/H] ≃ [P/H] = -0.80 ± 0.24. The metallicity of ionized gas from the central H II region Z_HII measured from its emission lines is [O/H] = -0.37 ± 0.10, a difference of +0.43 ± 0.26 from Z_HI. This difference δ is consistent with that seen toward H II regions in other star-forming galaxies and supports the idea that ionized gas near star-forming regions shows systematically higher metallicities than exist in the rest of a galaxy's neutral interstellar medium (ISM). The positive values of δ found in UGC 5282 (and the other star-forming galaxies) is likely due to infalling low-metallicity gas from the intergalactic medium that mixes with the galaxy's ISM on kiloparsec scales. This model is also consistent with broad Lyα emission detected at the bottom of the DLA absorption, offset by ∼125 km s^–1 from the absorption velocity. Models of galaxy evolution that attempt to replicate population characteristics, such as the mass-metallicity relation, may need to start with a galaxy metallicity represented by Z_HI rather than that measured traditionally from Z_HII.