I have used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect, for the first time, molecular emission from a galaxy selected to be near a projected background quasar. The galaxy, associated with an absorber (log NHI/cm-2 = 19.63 ± 0.08 at z=0.101) towards quasar PKS 0439-433, was found in a study by Petitjean et al. (1996, A&A, 313, L25). Above is a B-Band image of the quasar field (courtesy of H.-W. Chen). The CO(1-0) emission (shown here with contours) that was detected with ALMA, is co-spatial with the stellar disk, and yields a molecular mass of 4.2 x 109 M⊙ (for a Galactic CO-H2 conversion factor. This is significantly larger than the upper limit on the atomic gas mass that is found from 21cm emission studies done by Kanekar et al. (2001, A&A, 367, 46).
The ALMA observations resolve the CO velocity field. I have modeled this emission with a simple exponential disk using a custom MCMC fitting routine. From this, we obtain a rotational velocity of 134 ± 11 km/s and a resultant dynamical mass of >4 x 1010 M⊙. Despite the high metallicity and large molecular mass, the z=0.101 galaxy has a low star formation rate, implying a large gas consumption timescale, larger than that typical of late-type galaxies. Most gas is likely to be in a diffuse extended phase, rather than in dense molecular clouds.
By combining the results of emission and absorption studies, we find that the strongest molecular absorption component toward the quasar cannot arise from the molecular disk, but is likely to arise from diffuse gas in the galaxy’s circumgalactic medium.
This study is the first to highlight the potential of combining molecular and stellar emission line studies with optical absorption line studies to achieve a more complete picture of the gas within and surrounding high-redshift galaxies. The press release is here and the full paper is here