We present the results of a systematic search for molecular hydrogen ( H _ { 2 } ) in low redshift ( 0.05 \lesssim z \lesssim 0.7 ) DLAs and sub-DLAs with N ( \mbox { H { \sc i } } ) \gtrsim 10 ^ { 19.0 } cm ^ { -2 } , in the archival HST / COS spectra .
Our core sample is comprised of 27 systems with a median \log N ( \mbox { H { \sc i } } ) = 19.6 .
On the average , our survey is sensitive down to \log~ { } N ( \mbox { H { $ { } _ { 2 } $ } } ) = 14.4 corresponding to a molecular fraction of \log f _ { \mbox { \tiny H { $ { } _ { 2 } $ } } } = -4.9 at the median N ( \mbox { H { \sc i } } ) .
H _ { 2 } is detected in 10 cases ( 3/5 DLAs and 7/22 sub-DLAs ) down to this f _ { \mbox { \tiny H { $ { } _ { 2 } $ } } } limit .
The H _ { 2 } detection rate of 50 ^ { +25 } _ { -12 } percent seen in our sample , is a factor of \gtrsim 2 higher than that of the high- z sample of , for systems with N ( \mbox { H { $ { } _ { 2 } $ } } ) > 10 ^ { 14.4 } cm ^ { -2 } .
In spite of having N ( \mbox { H { \sc i } } ) values typically lower by a factor of 10 , low- z H _ { 2 } systems show molecular fractions ( \log f _ { \mbox { \tiny H { $ { } _ { 2 } $ } } } = -1.93 \pm 0.63 ) that are comparable to the high- z sample .
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 favor a radiation field much weaker than the mean Galactic ISM 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 \lesssim \rho ( kpc ) \lesssim 80 .
We , therefore , conjecture that the low- z H _ { 2 } bearing gas is not related to star-forming disks but stems from self-shielded , tidally stripped or ejected disk-material in the extended halo .