Kavli Affiliate: Donald G. York
| First 5 Authors: David V. Bowen, Todd M. Tripp, Edward B. Jenkins, Max Pettini, Doron Chelouche
| Summary:
We present an HST Cosmic Origins Spectrograph (COS) 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 HII region of the galaxy. Damped Ly$alpha$
(DLA) absorption is detected at the redshift of UGC 5282 with $log$[$N$(HI)
cm$^{-2}:$] = 20.89$^{+0.12}_{-0.21}::$. Analysis of the accompanying SII,
PII and OI metal lines yields a neutral gas metallicity, $Z_{HI}$, of [S/H]
$simeq$ [P/H] $= -0.80pm0.24$. The metallicity of ionized gas from the
central HII region, $Z_{HII}:$, measured from its emission lines is [O/H]$=
-0.37pm0.10$, a difference of $+0.43pm 0.26$ from $Z_{HI}:$. This difference
$delta$ is consistent with that seen towards HII 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 $delta$ 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 kpc scales. This model is also consistent with broad Ly$alpha$ emission
detected at the bottom of the DLA absorption, offset by $sim 125$ 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 $Z_{HII}:$.
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