Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull's eye at z ≃ 2.46
Abstract
We present the detection and detailed analysis of a diffuse molecular cloud at zabs = 2.4636 towards the quasar SDSS J 1513+0352 (zem ≃ 2.68) observed with the X-shooter spectrograph at the Very Large Telescope. We measured very high column densities of atomic and molecular hydrogen with log N(H I, H2) ≃ 21.8, 21.3. This is the highest H2 column density ever measured in an intervening damped Lyman-α system but we did not detect CO, implying log N(CO)/N(H2) < -7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to be Z ≃ 0.15 Z⊙ and determined the amount of dust by measuring the induced extinction of the background quasar light, AV ≃ 0.4. We simultaneously detected Lyman-α emission at the same redshift with a centroid located at a most probable impact parameter of only ρ ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (Tk ≃ 90 K and n ≃ 250 cm-3) derived through the excitation of H2 rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H I column densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.
Based on observations performed with the Very Large Telescope of the European Southern Observatory under Prog. ID 095.A-0224(A). The final reduced spectrum (FITS file) is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A184- Publication:
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Astronomy and Astrophysics
- Pub Date:
- October 2018
- DOI:
- 10.1051/0004-6361/201833446
- arXiv:
- arXiv:1806.07827
- Bibcode:
- 2018A&A...618A.184R
- Keywords:
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- quasars: absorption lines;
- galaxies: high-redshift;
- galaxies: ISM;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- accepted for publication in A&