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تسجيل مستخدم جديدThe Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of CO-Dark Gas in the Perseus Arm
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We report the results from a new, highly sensitive ($Delta T_{mb} sim 3 $mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9-pixel grid covering a $1deg$ x $1deg$ patch of sky in the direction of $l = 105deg, b = +2.50deg$ towards the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope (GBT) 1667 MHz OH results with archival CO J=1-0 observations from the Five College Radio Astronomy Observatory (FCRAO) Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically-independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an A$_{rm v}$ of 0.2, but the CO emission does not appear until the volume density exceeds 100-200 cm$^{-3}$. These results demonstrate that a combination of low column density A$_{rm v}$ and low volume density $n_{H}$ can explain the lack of CO emission along sight lines exhibiting OH emission. The 18-cm OH main lines, with their low critical density of $n^{*}$ $ sim 1 $ cm$^{-3}$, are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically-thin radio tracer for diffuse H$_2$.
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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. We identified 3
6 DMG clouds with C$^+$ emission (data from Galactic Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on uncertainty analysis, optical depth of HI $taurm_{HI}$ of 1 is a reasonable value for most clouds. With the assumption of $taurm_{HI}=1$, 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 $6.2times10^1$ cm$^{-3}$ to $1.2times 10^3$ cm$^{-3}$ with a median value of $2.3times 10^2$ cm$^{-3}$. The fraction of DMG column density in the cloud ($frm_{DMG}$) decreases with increasing excitation temperature following an empirical relation $frm_{DMG}=-2.1times 10^{-3}T_(ex,tau_{HI}=1)$+1.0. The relation between $frm_{DMG}$ and total hydrogen column density $N_H$ is given by $frm_{DMG}$=$1.0-3.7times 10^{20}/N_H$. The values of $frm_{DMG}$ in the clouds of low extinction group ($Arm_V le 2.7$ 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 ($Arm_V > 2.7$ mag). Compared to clouds in the low extinction group ($Arm_V le 2.7$ mag), clouds in the high extinction group ($Arm_V > 2.7$ mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group ($Arm_V > 2.7$ mag) are $6.6times 10^2$ times smaller than the canonical value in the Milky Way. #[Full version of abstract is shown in the text.]#
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The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a significant H_2 mas
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