{
  "id": "1410.3828",
  "version": "v1",
  "published": "2014-10-14T20:00:04.000Z",
  "updated": "2014-10-14T20:00:04.000Z",
  "title": "A HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature",
  "authors": [
    "Sowgat Muzahid",
    "Raghunathan Srianand",
    "Jane Charlton"
  ],
  "comment": "14 pages, 9 figures, 4 tables (+additional 21 pages of appendices), Submitted to the MNRAS, comments welcome!",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We present the results of a systematic search for molecular hydrogen (H2) in low redshift (0.05 < z < 0.7) damped and sub-damped Lya absorption systems (DLAs/sub-DLAs) with N(HI) > 10^19.0 cm^{-2}, in the archival Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) spectra. Our core sample is comprised of 27 systems with a median log N(HI) = 19.6. This is a factor of ~ 10 lower than that of the high-z (z > 2) sample of Noterdaeme et al. H2 absorption is detected in 10 cases (3/5 in DLAs and 7/22 in sub-DLAs). On the average, our survey is sensitive down to log N(H2) = 14.4 corresponding to a molecular mass fraction of log f_H2 = -4.9 at the median N(HI). The H2 incidence rate of 50^{+25}_{-12} percent, is a factor of > 2 higher than that of the high-z sample, for systems with N(H2) > 10^14.4 cm^{-2}. The enhanced incidence rate could result from the increase of the cosmic mean metallicities of DLAs and sub-DLAs with cosmic time. In spite of having systematically lower N(HI) values, low-z H2 systems show molecular mass fractions (log f_H2 = -1.93 $\\pm$ 0.63) that are comparable to the high-z measurements. The rotational excitation temperatures (T_01 = 133 $\\pm$ 55 K), as measured in our low-z sample, are typically consistent with high-z measurements. Simple photoionization models demonstrate that the radiation field prevailing in the low-z H2 systems cannot have an appreciable stellar contribution for densities of 10-100 cm^{-3}. The impact parameters of the nearest possible host-galaxy candidates are typically large (e.g. 10 < $\\rho$ (kpc) < 80). We, therefore, conjecture that the low-z H2 bearing gas is not related to star-forming disks but stems from self-shielded, tidally stripped or ejected disk-material in the extended halo.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2014-10-14T20:00:04.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "molecular hydrogen",
      "excitation temperature",
      "molecular fraction",
      "hst/cos survey",
      "molecular mass fraction"
    ],
    "publication": {
      "doi": "10.1093/mnras/stv133",
      "journal": "Monthly Notices of the Royal Astronomical Society",
      "year": 2015,
      "month": "Apr",
      "volume": 448,
      "number": 3,
      "pages": 2840
    },
    "note": {
      "typesetting": "TeX",
      "pages": 14,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "adsabs": "2015MNRAS.448.2840M"
    }
  }
}