DR. SOWGAT MUZAHID

Molecular hydrogen (H2) is the most abundant molecule in the universe. It plays a crucial role in cooling and molecular chemistry of the interstellar medium (ISM). The Lyman- and Warner-bands absorption of H2 are detected in 10 - 20% of the high redshift Damped Lyman-alpha absorbers (DLAs). Due to the lack of far-UV (FUV) sensitive space based spectrograph in the past H2 has never been studied in low-z DLAs/sub-DLAs systematically. However, the unprecedented FUV sensitivity of the Cosmic Origins Spectrograph (COS) aboard the HST has changed this scenario dramatically. Recently, we have conducted a survey of molecular hydrogen in low-z DLAs/sub-DLAs in the public HST/COS archive and found that ~50% of them show H2 absorption. Simple photoionization models favour a radiation field much weaker than the mean Galactic ISM field with particle density in the range 10 - 100 cm^-3. The large impact parameters of the identified host-galaxies suggest that the H2 bearing gas is not related to star forming disks but stem from self-shielded, tidally stripped or ejected disk-material in the extended halo. Results of our survey (see 2015MNRAS.448.2840M for details) will be summarized in the talk.