| Description |
Observations of interstellar molecules in distant galaxies are essential for investigating the chemical evolution of galaxies. Absorption line systems in quasar spectra, especially the damped Lyman alpha (DLA) absorbers, provide excellent venues for directly studying the interstellar matter (ISM) in distant galaxies, selected independently of the galaxy luminosities. A few cold, dusty absorption systems have been discovered using radio surveys and the Sloan Digital Sky Survey. These absorbers, far richer in dust/molecules than the general DLA population, give us rare opportunities to probe molecular clouds at high redshift. Here we propose Herschel observations of 4 quasars with such DLAs at z < 1.5 to further observe their molecular content. These DLAs exhibit H I 21-cm absorption and other signatures of cold ISM such as the 2175 A bump from carbonaceous dust or low spin temperature. Furthermore, we have detected 9.7 micron silicate absorption toward two of these DLAs with Spitzer IRS. We request HIFI spectroscopy to observe absorption lines of CO, CN, NH, H2O, and SPIRE photometry of the 4 quasars to determine the continuum accurately. In addition, we request SPIRE photometry of 10 other quasars with strong absorbers rich in dust/metals, in order to identify bright targets for future Herschel spectroscopy. Our overall science goals are: (1) to estimate molecular abundances and kinematics of the absorbing gas; (2) to constrain isotopic ratios such as 12C/13C from 12CO/13CO or 12CN/13CN; (3) to estimate the cosmic microwave background temperature at the absorber redshifts. Finally, (4) the proposed program will help to further constrain the cosmic variation of fundamental constants such as the electron-to-proton mass ratio. |
