THE STRUCTURE OF DARK MOLECULAR GAS IN THE GALAXY. I. A PILOT SURVEY FOR 18 cm OH EMISSION TOWARD<i>l</i>≈ 105°,<i>b</i>≈ +1°

Allen, R. J.; Hogg, D. E.; Engelke, Philip D.

doi:10.1088/0004-6256/149/4/123

Cited by 49 publications

(66 citation statements)

References 28 publications

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“…Solomon et al 1987), 13 CO (see, e.g. Heyer et al 2009) and more recently in OH by Allen et al (2015)). Since our model includes the H2 and CO molecule kinetics, we can use CD criteria based on both total gas column density and intensity in CO lines.…”

Section: Clouds Definitionmentioning

confidence: 95%

Giant molecular cloud scaling relations: the role of the cloud definition

Khoperskov

Vasiliev

Ladeyschikov

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We investigate physical properties of molecular clouds in disc galaxies with different morphology: a galaxy without prominent structure, a spiral barred galaxy and a galaxy with flocculent structure. Our N -body/hydrodynamical simulations take into account non-equilibrium H 2 and CO chemical kinetics, self-gravity, star formation and feedback processes. For the simulated galaxies the scaling relations of giant molecular clouds or so called Larson's relations are studied for two types of a cloud definition (or extraction methods): the first one is based on total column density position-position (PP) datasets and the second one is indicated by the CO (1-0) line emission used position-position-velocity (PPV) data. We find that the cloud populations obtained by using both cloud extraction methods generally have similar physical parameters. Except that for the CO data the mass spectrum of clouds has a tail with low-massive objects M ∼ 10 3 − 10 4 M . Varying column density threshold the power-law indices in the scaling relations are significantly changed. In contrast, the relations are invariant to CO brightness temperature threshold. Finally, we find that the mass spectra of clouds for the PPV data are almost insensitive to the galactic morphology, whereas the spectra for the PP data demonstrate significant variations.

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Section: Clouds Definitionmentioning

confidence: 95%

Giant molecular cloud scaling relations: the role of the cloud definition

Khoperskov

Vasiliev

Ladeyschikov

et al. 2015

Monthly Notices of the Royal Astronomical Society

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“…Most of the direct detections of molecular gas were made with CO emission. There are, however, examples of interstellar molecules detected toward lines of sight without corresponding CO emissions (Wannier et al 1993;Magnani & Onello 1995;Allen et al 2015). If the nondetection of CO is taken as a sign of missing molecular gas, the fraction of dark gas varies from 12% to 100% for individual components in Liszt & Pety (2012).…”

Section: Introductionmentioning

confidence: 99%

Physical properties of CO-dark molecular gas traced by C⁺

Tang

Heiles

et al. 2016

A&A

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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 H 2 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 Hi τ Hi of 1 is a reasonable value for most clouds. With the assumption of τ 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 6.2 × 10 1 cm −3 to 1.2 × 10 3 cm −3 with a median value of 2.3 × 10 2 cm −3 . The fraction of DMG column density in the cloud ( f DMG ) decreases with increasing excitation temperature following an empirical relation f DMG = −2.1 × 10 −3 T ex , (τ Hi = 1) + 1.0. The relation between f DMG and total hydrogen column density N H is given by f DMG = 1.0 − 3.7 × 10 20 /N H . We divided the clouds into a high extinction group and low extinction group with the dividing threshold being total hydrogen column density N H of 5.0 × 10 21 cm −2 (A V = 2.7 mag). The values of f DMG in the low extinction group (A V ≤ 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 (A V > 2.7 mag). Compared to clouds in the low extinction group (A V ≤ 2.7 mag), clouds in the high extinction group (A 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 (A V > 2.7 mag) are 6.6 × 10 2 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 (A V > 2.7 mag) may...

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“…This process was repeated for all 81 Perseus Arm features (and lack of features around V LSR ∼ −65 km s −1 ) until all of the data in the OSD was processed. The error analysis is the same procedure as outlined in Allen et al (2015) section 4.2. We also computed the scaled difference between the 1665 and 1667 MHz lines with each sightline and find that most emission is the LTE ratio of 5:9, as is expected if these lines are excited chiefly by collisions.…”

Section: Determining Line Strengthsmentioning

confidence: 99%

“…In the outer Galaxy, we can typically avoid this issue as the integrated background synchrotron emission is much weaker. More recent observations in the outer Galaxy by Allen et al (2015, hereafter Paper 1) using the GBT have shown main-line (1665 and 1667 MHz) OH emission to be present in regions largely devoid of CO emission, strongly suggesting that OH may be a good tracer of the dark gas. The main lines of OH emission in these regions are observed to be in the 5:9 ratio characteristic of optically-thin emission lines with level populations in local thermodynamic equilibrium (LTE).…”

Section: Introductionmentioning

confidence: 99%

“…Here, the column density is large enough so that H 2 has sufficient column to remain selfshielded against the ambient UV flux, but CO is photodissociated and the carbon is the form of C or C + . However, a large fraction of the faint CO gas might also arise in the diffuse ISM (Papadopoulos et al 2002;Liszt et al 2010;Allen et al 2012Allen et al , 2015Xu et al 2016a).…”

Section: Introductionmentioning

confidence: 99%

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The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm

Busch

Allen

Engelke

et al. 2019

ApJ

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We report the results from a new, highly sensitive (∆T mb ∼ 3mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9-pixel grid covering a 1 • × 1 • patch of sky in the direction of l = 105. • 00, b = +2. • 50 towards the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope (GBT) 1667 MHz OH results with archival 12 CO(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 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 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 * ∼ 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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THE STRUCTURE OF DARK MOLECULAR GAS IN THE GALAXY. I. A PILOT SURVEY FOR 18 cm OH EMISSION TOWARDl≈ 105°,b≈ +1°

Cited by 49 publications

References 28 publications

Giant molecular cloud scaling relations: the role of the cloud definition

Giant molecular cloud scaling relations: the role of the cloud definition

Physical properties of CO-dark molecular gas traced by C⁺

The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm

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THE STRUCTURE OF DARK MOLECULAR GAS IN THE GALAXY. I. A PILOT SURVEY FOR 18 cm OH EMISSION TOWARDl≈ 105°,b≈ +1°

Cited by 49 publications

References 28 publications

Giant molecular cloud scaling relations: the role of the cloud definition

Giant molecular cloud scaling relations: the role of the cloud definition

Physical properties of CO-dark molecular gas traced by C+

The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm

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Physical properties of CO-dark molecular gas traced by C⁺