Speaker
Van Hiep Nguyen
Description
Many studies have proved the existence of the “dark interstellar
medium” (dark ISM) which is not detected by traditional radio
emissions from atomic hydrogen (HI) and carbon monoxide (CO)
molecules. In recent years, OH has emerged as a powerful indicator of
dark-ISM. In this study, we use HI and OH data from the Arecibo
Millennium survey (Heiles and Troland 2003) which observed
absorption and emission pairs towards 79 extragalatic radio continuum
sources. The Λ-doubling transitions of ground-state OH at 1665.402
and 1667.359 MHz were observed along with HI towards 48 of the 79
survey positions. By newly reducing this unpublished data, OH
absorption was detected in 23 lines- of-sight, we find that the OH 1665
and 1667 lines satisfy the optically thin assumption with the optical
depth τ less than 0.25 and they are in general not in Local Thermal
Equilibrium.
By comparing the thermal dust data from Planck satellite (Release 1.2)
and the Sloan Digital Sky Survey (Schlafly et al. 2011) with HI data
from Millennium survey, we confirm the tight linear correlations
between optical depth τ353, dust radiance R, reddening E(B-V) and the
total proton column density N(H). We estimate the molecular hydrogen
column densities N(H2)=1⁄2[N(H)-N(HI)] from these linear
relationships and hence the OH abundance ratio XOH=N(OH)/N(H2),
for which few literature measurement exist. The XOH ratios derived
from the three N(H) proxies are consistent and appear to be constant
around 5.0×10-6. Since these results are obtained in a wide ranges of
longitude l and latitude b with some sightlines through the Galactic
plane, it suggests that OH main lines are excellent tracers of molecular
gas in the interstellar medium including regimes where the usefulness
of CO is compromised.
