We have discovered a third quasar absorption system which is consistent with a low deuterium to hydrogen abundance ratio , D/H = 3.4 \times 10 ^ { -5 } .
The z _ { abs } \sim 2.8 partial Lyman limit system towards Q0130-4021 provides the strongest evidence to date against large D/H ratios because the H I absorption , which consists of a single high column density component with unsaturated high order Lyman series lines , is readily modeled – a task which is more complex in other D/H systems .
We have obtained twenty-two hours of spectra from the HIRES spectrograph on the W.M .
Keck telescope , which allow a detailed description of the Hydrogen .
We see excess absorption on the blue wing of the H I Ly \alpha line , near the expected position of deuterium .
However , we find that Deuterium can not explain all of the excess absorption , and hence there must be contamination by additional absorption , probably H I .
This extra H I can account for most or all of the absorption at the D position , and hence D/H = 0 is allowed .
We find an upper limit of D/H \leq 6.7 \times 10 ^ { -5 } in this system , consistent with the value of D/H \simeq 3.4 \times 10 ^ { -5 } deduced towards QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler ( 1998a , 1998b ) .
This absorption system shows only weak metal line absorption , and we estimate [ Si/H ] \leq - 2.6 – indicating that the D/H ratio of the system is likely primordial .
All four of the known high redshift absorption line systems simple enough to provide useful limits on D are consistent with D/H = 3.4 \pm 0.25 \times 10 ^ { -5 } .
Conversely , this QSO provides the third case which is inconsistent with much larger values .