On the radial profile of gas-phase Fe/α ratio around distant galaxies

This paper presents a study of the chemical compositions in cool gas around a sample of 27 intermediate-redshift galaxies. The sample comprises 13 massive quiescent galaxies at z = 0.40-0.73 probed by QSO sightlines at projected distances d = 3-400 kpc, and 14 star-forming galaxies at z = 0.10-1.24 probed by QSO sightlines at d = 8-163 kpc. The main goal of this study is to examine the radial profiles of the gas-phase Fe/alpha ratio in galaxy haloes based on the observed Fe II to MgII column density ratios. Because Mg+ and Fe+ share similar ionization potentials, the relative ionization correction is small in moderately ionized gas and the observed ionic abundance ratio N(Fe II)/N(Mg II) places a lower limit to the underlying (Fe/Mg) elemental abundance ratio. For quiescent galaxies, a median and dispersion of log < N(Fe II)/N(Mg II)>(med) < -0.06 +/- 0.15 is found at d less than or similar to 60 kpc, which declines to log < N(Fe II)/N(Mg II)>(med) < -0.3 at d greater than or similar to 100 kpc. On the other hand, star-forming galaxies exhibit log < N(Fe II)/N(Mg II)> = -0.25 +/- 0.21 at d less than or similar to 60 kpc and log < N(Fe II)/N(Mg II)> = -0.9 +/- 0.4 at larger distances. Including possible differential dust depletion or ionization correction would only increase the inferred (Fe/Mg) ratio. The observed N(Fe II)/N(Mg II) implies supersolar Fe/alpha ratios in the inner halo of quiescent galaxies. An enhanced Fe abundance indicates a substantial contribution by Type Ia supernovae in the chemical enrichment, which is at least comparable to what is observed in the solar neighbourhood or in intracluster media but differs from young star-forming regions. In the outer haloes of quiescent galaxies and in haloes around star-forming galaxy, however, the observed N(Fe II)/N(Mg II) is consistent with an alpha-element enhanced enrichment pattern, suggesting a core-collapse supernovae dominated enrichment history.