%0 Generic %1 muzahid2014hstcos %A Muzahid, Sowgat %A Srianand, Raghunathan %A Charlton, Jane %D 2014 %K dla gas low-z molecular %T A HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature %U http://arxiv.org/abs/1410.3828 %X 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.

@misc{muzahid2014hstcos, 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.}, added-at = {2014-10-16T10:06:24.000+0200}, author = {Muzahid, Sowgat and Srianand, Raghunathan and Charlton, Jane}, biburl = {https://www.bibsonomy.org/bibtex/2aa76bffd526ea9a9ac1bb15e8d46ccf2/miki}, description = {[1410.3828] A HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z &lt; 1: Molecular fraction and excitation temperature}, interhash = {82088bf87c9af1b1e97547acf6da6b03}, intrahash = {aa76bffd526ea9a9ac1bb15e8d46ccf2}, keywords = {dla gas low-z molecular}, note = {cite arxiv:1410.3828Comment: 14 pages, 9 figures, 4 tables (+additional 21 pages of appendices), Submitted to the MNRAS, comments welcome!}, timestamp = {2014-10-16T10:06:24.000+0200}, title = {A HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature}, url = {http://arxiv.org/abs/1410.3828}, year = 2014 }