Alpha Enhancement In Strong And Weak MgII Absorbers

Details Alpha Enhancement In Strong And Weak MgII Absorbers Alpha Enhancement In Strong And Weak MgII Absorbers

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21348206w&link_type=abstract

American Astronomical Society, AAS Meeting #213, #482.06; Bulletin of the American Astronomical Society, Vol. 41, p.451

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In order to study the evolution of MgII absorbers, we consider the ratio of FeII to MgII as an indicator of alpha-enhancement. Our sample consisted of 81 strong and 100 weak MgII absorbers drawn from the VLT/UVES archives of high resolution (R=45,000) spectra. The redshifts range from z=0.24 to z=2.5, corresponding to a large fraction of cosmic history, 2.3-8.7 Gyrs. We find an evolution in the distribution of the ratio of the equivalent widths of FeII 2374 to MgII 2796 for strong MgII absorbers (those with Wr(2796)>0.3A). At lower redshifts, there is a significant absence of absorbers with small ratios of FeII to MgII. A Kolmogorov-Smirnov (KS) test indicates that there is only a 3.1% chance that the high redshift (1.2 < z < 2.5) and low redshift (0.24 < z 1.0A are considered, the significance is much greater, with a KS probability of P(KS) = 6E-4. FeII 2374 was chosen for this comparison as it was nearly always detected for these strong MgII systems, but was rarely saturated. For weak MgII absorbers (those with Wr(2796)>0.3A), Narayanan (2008) found a different evolutionary trend, with an absence of systems with large FeII to MgII ratios at higher redshifts. Low values of FeII could indicate high ionization conditions, but we consider it more likely that they indicate alpha-enhanced gas. Thus the evolutionary trend for strong MgII systems implies that alpha-enhancement is rare at lower redshifts, just as most weak MgII absorbers at higher redshifts are alpha-enhanced. This can be explained by the idea that superwinds make a large contribution to strong MgII absorption at high redshift, particularly for the strongest absorbers. Similarly, weak MgII absorbers, which are generally associated with lower luminosity galaxies, have not yet had time for enrichment by Type Ia supernova until z<1.2.

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