摘要：Context. Neither Hi nor CO emission can reveal a significant
quantity of so-called dark gas in the interstellar medium (ISM). It is considered that
CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark
gas. Determination of physical properties of DMG is critical for understanding ISM
evolution. Previous studies of DMG in the Galactic plane are based on assumptions of
excitation temperature and volume density. Independent measurements of temperature and
volume density are necessary.
Aims. We intend to characterize physical properties of DMG in the
Galactic plane based on C+ data from the Herschel open time key
program, namely Galactic Observations of Terahertz C+ (GOT C+) and Hi narrow
self-absorption (HINSA) data from international Hi 21 cm Galactic plane
surveys.
Methods. We identified DMG clouds with HINSA features by comparing
Hi, C+,
and CO spectra. We derived the Hi excitation temperature and Hi column
density through spectral analysis of HINSA features. The Hi volume density was
determined by utilizing the on-the-sky dimension of the cold foreground Hi cloud
under the assumption of axial symmetry. The column and volume density of H2 were derived through excitation
analysis of C+
emission. The derived parameters were then compared with a chemical evolutionary
model.
Results. We identified 36 DMG clouds with HINSA features. Based on
uncertainty analysis, optical depth of HiτHi of
1 is a reasonable value for most clouds. With the assumption of τHi =
1, these clouds were characterized by excitation temperatures in a range
of 20 K to 92 K with a median value of 55 K and volume densities in the range of
6.2 × 101
cm-3 to
1.2 × 103
cm-3 with a
median value of 2.3 ×
102 cm-3. The fraction of DMG column density in the cloud
(fDMG) decreases with increasing excitation
temperature following an empirical relation fDMG =−2.1 ×
10-3Tex,(τHi
= 1) + 1.0. The relation between fDMG and total
hydrogen column density NH is given by fDMG = 1.0−3.7 ×
1020/NH. We divided the clouds
into a high extinction group and low extinction group with the dividing threshold being
total hydrogen column density NH of 5.0 × 1021 cm-2 (AV = 2.7 mag).
The values of fDMG in the low extinction group
(AV ≤
2.7 mag) are consistent with the results of the time-dependent,
chemical evolutionary model at the age of ~10 Myr. Our empirical relation cannot be explained by the
chemical evolutionary model for clouds in the high extinction group (AV > 2.7 mag).
Compared to clouds in the low extinction group (AV ≤ 2.7 mag), clouds in the high
extinction group (AV > 2.7 mag) have comparable volume
densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in
clouds of the high extinction group (AV > 2.7 mag) are 6.6 × 102 times smaller than the
canonical value in the Milky Way.
Conclusions. The molecular gas seems to be the dominate component in
these clouds. The high percentage of DMG in clouds of the high extinction group
(AV >
2.7 mag) may support the idea that molecular clouds are forming from
pre-existing molecular gas, i.e., a cold gas with a high H2 content but that contains a
little or no CO content.
