Metals and a search for molecules in the distant Universe: Magellan mike observations of sub-DLAs at 2 < z < 3

Abstract

We present abundance measurements of the elements Zn, S, O, C, Si and Fe for four sub-DLAs at redshifts ranging from z=2.173 to z=2.635 using observations from the MIKE spectrograph on the Magellan telescope to constrain the chemical enrichment and star formation of gas-rich galaxies. Using weakly depleted elements O, S, and or Zn, we find the metallicities after the photoionization corrections to be [S/H]=-0.50\\pm0.11, [O/H]>-0.84, [O/H]=-1.27\\pm0.12, and [Zn/H]=+0.40\\pm0.12 for the absorbers at z=2.173, 2.236, 2.539, and 2.635, respectively. Moreover, we are able to put constraints on the electron densities using the fine structure lines of C II* and Si II* for two of the sub-DLAs. We find that these values are much higher than the median values found in DLAs in the literature. Furthermore, we estimate the cooling rate lc=1.20\\times10-26 erg s-1 per H atom for an absorber at z=2.173, suggesting higher star formation rate density in this sub-DLA than the typical star formation rate density for DLAs at similar redshifts. We also study the metallicity versus velocity dispersion relation for our absorbers. Most of the absorbers follow the trend one can expect from the mass versus metallicity relation for sub-DLAs in the literature. Finally, we are able to put limits on the molecular column density from the non detections of various strong lines of CO molecules. We estimate 3\\sigma upper limits of log N(CO,J=0)<13.87, log N(CO,J=0)<13.17, and log N(CO,J=0)<13.08, respectively, from the non-detections of absorption from the J=0 level in the CO AX 0-0, 1-0, and 2-0 bands near 1544, 1510, and 1478\\AA.