Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM

Nguyen, Hiep; Dawson, J. R.; Miville-Deschênes, M.-A.; Tang, Ningyu; Li, Di; Heiles, Carl; Murray, Claire E.; Stanimirović, Snežana; Gibson, Steven J.; McClure-Griffiths, N. M.; Troland, T. H.; Bronfman, L.; Finger, Ricardo

doi:10.3847/1538-4357/aac82b

Cited by 69 publications

(96 citation statements)

References 95 publications

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“…Liszt (2014a,b) compared N H I and E(B − V), resulting in N H (cm −2 )/E(B − V) = 8.3 × 10 21 cm −2 mag −1 for |b| ≥ 20 • and 0.015 ≤ E(B − V) ≤ 0.075. Similar results have also been found by Lenz et al (2017) in low N H I (< 4 × 10 20 cm −2 ) regions, and by Nguyen et al (2018) along purely atomic sightlines (8.8 × 10 21 and (9.4 ± 1.6) × 10 21 cm −2 mag −1 , respectively). γ-ray observations from Fermi show the ratio decreases from atomic clouds to dense molecular clouds by a factor of ∼0.6 (Remy et al 2018).…”

Section: Hco + As a Molecular Gas Tracersupporting

confidence: 87%

“…We adopted the corrected H I column density derived from ON-OFF observations in the millennium survey for 3C454.3 and 3C120 (Heiles & Troland 2003;Nguyen et al 2018). These H I spectra have a velocity resolution of 0.161 km s −1 and an RMS noise of 56 mK per channel.…”

Section: Archival H I Datamentioning

confidence: 99%

“…The relation between the abundance of CO, HCO + and NH 2 (circles from our work, squares fromLucas & Liszt 1996;Liszt & Lucas 1998). Red and black points denote sightlines using NH/E(B − V) conversion factor ofNguyen et al 2018 and Bohlin et al 1978, respectively. Empty red circles denote sightlines with H I opacity correction.…”

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confidence: 99%

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Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Luo

Tang³

et al. 2020

ApJL

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Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to τ CO rms ∼ 0.002 and τ HCO + rms ∼ 0.0008) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A v < 0.32 mag and HCO + was down to A v < 0.2 mag, where atomic gas and dark molecular gas (DMG) starts to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures T ex of 4.1 K and 2.7 K, for CO and for HCO + , respectively, which are close to the cosmic microwave background (CMB) and provide explanation for their being undercounted in emission surveys. A stronger linear correlation was found between N HCO + and N H2 (Pearson correlation coefficient P ∼ 0.93) than that of N CO and N H2 (P ∼ 0.33), suggesting HCO + being a better tracer of H 2 than CO in diffuse gas. The derived CO-to-H 2 conversion factor (the CO X-factor) of (14 ± 3) × 10 20 cm −2 (K km s −1 ) −1 is approximately 6 times larger than the average value found in the Milky Way.

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Section: Hco + As a Molecular Gas Tracersupporting

confidence: 87%

Section: Archival H I Datamentioning

confidence: 99%

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Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Luo

Tang³

et al. 2020

ApJL

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“…Weselak et al (2010) report an abundance ratio of 1.05 ± 0.14 × 10 −7 determined using a combination of archival and new measurements of UV absorption from OH and H 2 in front of several O and B stars, which we adopt in this paper. This result is also consistent with the value of 1.0 ± 0.2 × 10 −7 reported by Liszt & Lucas (2002), ∼ 1.3 × 10 −7 , from Rugel et al (2018), and ∼ 1.0 × 10 −7 from Nguyen et al (2018). A thorough summary of the OH to H 2 ratio is provided by Nguyen et al (2018), who also report that molecular gas content derived from dust reddening from Green et al (2018) provides a more accurate result than that derived from infra-red data (Planck Collaboration et al 2014).…”

Section: Total Molecular Gas Column Densitiessupporting

confidence: 92%

OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5

Engelke

Allen

2019

ApJ

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We report column densities of molecular gas in the W5 star-forming region as traced with OH 18-cm emission in a grid survey using the Green Bank Telescope. OH appears to trace a greater column density than does CO in 8 out of 15 cases containing OH emission detections; the two molecules trace the same column densities for the other 7 cases. OH and CO trace a similar morphology of molecular gas with a nearly one-to-one correspondence. The mass of molecular gas traced by OH in the portion of the survey containing OH emission is 1.7 (+ 0.6 or -0.2) ×10 4 M , whereas the corresponding CO detections trace 9.9×10 3 M (±0.7)×10 3 . We find that for lines observed in absorption, calculations assuming uniform gas and continuum distributions underestimate column density values by 1 to 2 orders of magnitude, making them unreliable for our purposes. Modeling of this behavior in terms of OH cloud structure on a scale smaller than telescopic resolution leads us to estimate that the filling factor of OH gas is a few to 10 percent. Consideration of filling factor effects also results in a method of constraining the excitation temperature values. The total molecular gas content of W5 may be approximately two to three times what we report from direct measurement, because we excluded absorption line detections from the mass estimate.

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“…This makes calculating a column density of OH relatively straightforward as long as a good estimate for the excitation temperatures can be found. With an OH column density known from emission observations, the H 2 column density can be directly estimated using the N(H 2 )/N(OH) ratio of approximately 10 −7 measured from UV absorption data (Weselak et al 2010;Nguyen et al 2018;Engelke & Allen 2018).…”

Section: Introductionmentioning

confidence: 99%

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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Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM

Cited by 69 publications

References 95 publications

Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5

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

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