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

Abstract

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.

Tracing the Formation of Molecular Clouds in a Low-metallicity Galaxy: An H i Narrow Self-absorption Survey of the Large Magellanic Cloud

“…Various molecular tracers have been used to trace H 2 , eg. [C I] (Pineda et al 2017;Valentino et al 2018;Okada et al 2019), [C II] (Tang et al 2016;Zanella et al 2018;Rybak et al 2019), OH (Donate et al 2019;Engelke & Allen 2019;Tang et al 2017), the main content of molecular clouds, with CO being one of the most widely used (e.g. Heyer & Dame 2015;Genzel et al 2015;Su et al 2019).…”

SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data Description & Release

“…It is generally not appropriate for the SPLASH dataset, where T * C ∼ 10-20 K over much of the survey field. We note that unsuitable use of this expression by Engelke & Allen (2019) appears to be largely responsible for the factor of 10-100 underestimates they find for column density as measured from OH absorption lines.…”

Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy

“…As an illustration, the volume density at this location is 1.8 × 10 −9 OH molecules cm −3 per mK of 1667 MHz main-beam brightness temperature, assuming we are far enough in the outskirts of the Galaxy to set T c (s ≥ S) = 3K (i.e. slightly above CMB alone), and choosing T 67 ex = 4.1K (a reasonable value between the ones found in Engelke & Allen (2018) and Li et al (2018)). Using the abundance ratio N(OH)/N(H 2 ) = 2 × 10 −6 , the corresponding H 2 volume density is ∼ 7 × 10 −3 H 2 molecules cm −3 at this location.…”

“…Since 2012, we have demonstrated in a series of papers (Allen et al 2012(Allen et al , 2013(Allen et al , 2015Busch et al 2019) that OH successfully functions as an alternate tracer for molecular gas in observations towards a quiescent region of the Outer Galaxy; further observations in the vicinity of the W5 starforming region extend the use of OH as an alternate tracer to star-forming regions and further develop the techniques of the field (Engelke & Allen 2018Engelke et al 2020). These projects confirmed the viability of OH as a tracer for molecular gas and studied the structure, column density, volume density, and kinematics of molecular gas in these regions.…”

“…Additionally, these tracers cannot provide radial velocity measurements and hence we lack structure information about the distribution of the dark gas in the plane of the Galaxy. Using a molecule, and particularly for chemical reasons, an abundant hydride (Neufeld & Wolfire 2016) such as CH (We-selak et al 2010;Jacob et al 2019Jacob et al , 2020 or OH (Weinreb et al 1963(Weinreb et al , 1965Heiles 1968;Dickey et al 1981;Wannier et al 1993;Allen et al 2012;Dawson et al 2014;Allen et al 2015;Engelke & Allen 2018;Nguyen et al 2018;Rugel et al 2018;Engelke & Allen 2019;Busch et al 2019) can provide both the unambiguous conclusion that the dark gas is indeed diffuse molecular gas, and the radial velocity information from a spectral line to describe the Galactic distribution of the gas by utilizing the velocity field of the Galaxy (Brand & Blitz 1993) Studies of the large-scale distribution of gas have been completed mainly using HI (Kalberla et al 2005;Peek et al 2011;Winkel et al 2015) or CO (Heithausen et al 1993;Heyer et al 1998;Dame et al 2001;Heyer & Dame 2015), and thus the diffuse (or "dark") molecular gas, and its mass content and Galactic structure, are inherently excluded by these studies (Blitz et al 1990;Goldsmith et al 2010;Li et al 2015;Li et al 2018).…”