TY - JOUR
T1 - An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1
T2 - Molecular fraction and excitation temperature
AU - Muzahid, S.
AU - Srianand, R.
AU - Charlton, J.
N1 - Funding Information:
This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. We thank Patrick Petitjean and the anonymous reviewer for constructive comments. SM thankfully acknowledge Dr Eric Feigelson for useful discussions on various statistical tests, survival analysis in particular.
Publisher Copyright:
© 2015 The Authors.
PY - 2015/4/11
Y1 - 2015/4/11
N2 - We present the results of a systematic search for molecular hydrogen (H2) in low-redshift (0.05 ≲ z ≲ 0.7) Damped Lyα absorbers (DLAs) and sub-DLAs with N(H I) ≳ 1019.0 cm-2, in the archival Hubble Space Telescope/CosmicOrigins Spectrograph spectra.Our core sample is comprised of 27 systems with a median logN(H I) = 19.6. On the average, our survey is sensitive down to logN(H2)=14.4 corresponding to a molecular fraction of log fH2 = -4.9 at the median N(H I). H2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this fH2 limit. The H2 detection rate of 50-12 +25 per cent seen in our sample, is a factor of ≳2 higher than that of the high-z sample of Noterdaeme et al., for systems with N(H2) > 1014.4 cm-2. In spite of having N(H I) values typically lower by a factor of 10, low-z H2 systems show molecular fractions (log fH2 = -1.93 ± 0.63) that are comparable to the high-z sample. The rotational excitation temperatures (T01 = 133 ± 55 K), as measured in our low-z sample, are typically consistent with high-z measurements. Simple photoionization models favour a radiation field much weaker than the mean Galactic interstellar medium field for a particle density in the range 10-100 cm-3. The impact parameters of the identified host-galaxy candidates are in the range 10 ≲ ρ (kpc) ≲ 80. We, therefore, conjecture that the low-z H2 bearing gas is not related to star-forming discs but stems from self-shielded, tidally stripped or ejected disc-material in the extended halo.
AB - We present the results of a systematic search for molecular hydrogen (H2) in low-redshift (0.05 ≲ z ≲ 0.7) Damped Lyα absorbers (DLAs) and sub-DLAs with N(H I) ≳ 1019.0 cm-2, in the archival Hubble Space Telescope/CosmicOrigins Spectrograph spectra.Our core sample is comprised of 27 systems with a median logN(H I) = 19.6. On the average, our survey is sensitive down to logN(H2)=14.4 corresponding to a molecular fraction of log fH2 = -4.9 at the median N(H I). H2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this fH2 limit. The H2 detection rate of 50-12 +25 per cent seen in our sample, is a factor of ≳2 higher than that of the high-z sample of Noterdaeme et al., for systems with N(H2) > 1014.4 cm-2. In spite of having N(H I) values typically lower by a factor of 10, low-z H2 systems show molecular fractions (log fH2 = -1.93 ± 0.63) that are comparable to the high-z sample. The rotational excitation temperatures (T01 = 133 ± 55 K), as measured in our low-z sample, are typically consistent with high-z measurements. Simple photoionization models favour a radiation field much weaker than the mean Galactic interstellar medium field for a particle density in the range 10-100 cm-3. The impact parameters of the identified host-galaxy candidates are in the range 10 ≲ ρ (kpc) ≲ 80. We, therefore, conjecture that the low-z H2 bearing gas is not related to star-forming discs but stems from self-shielded, tidally stripped or ejected disc-material in the extended halo.
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U2 - 10.1093/mnras/stv133
DO - 10.1093/mnras/stv133
M3 - Article
AN - SCOPUS:84928566974
VL - 448
SP - 2840
EP - 2853
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 3
ER -