Abstract
Knowledge about the distribution of CO emission in the Milky Way is essential to understanding the impact of the Galactic environment on the formation and evolution of structures in the interstellar medium. However, our current insight as to the fraction of CO in the spiral arm and interarm regions is still limited by large uncertainties in assumed rotation curve models or distance determination techniques. In this work we use the Bayesian approach from Reid et al. (2016, ApJ, 823, 77; 2019, ApJ, 885, 131), which is based on our most precise knowledge at present about the structure and kinematics of the Milky Way, to obtain the current best assessment of the Galactic distribution of 13CO from the Galactic Ring Survey. We performed two different distance estimates that either included (Run A) or excluded (Run B) a model for Galactic features, such as spiral arms or spurs. We also included a prior for the solution of the kinematic distance ambiguity that was determined from a compilation of literature distances and an assumed size-linewidth relationship. Even though the two distance runs show strong differences due to the prior for Galactic features for Run A and larger uncertainties due to kinematic distances in Run B, the majority of their distance results are consistent with each other within the uncertainties. We find that the fraction of 13CO emission associated with spiral arm features ranges from 76 to 84% between the two distance runs. The vertical distribution of the gas is concentrated around the Galactic midplane, showing full-width at half-maximum values of ~75 pc. We do not find any significant difference between gas emission properties associated with spiral arm and interarm features. In particular, the distribution of velocity dispersion values of gas emission in spurs and spiral arms is very similar. We detect a trend of higher velocity dispersion values with increasing heliocentric distance, which we, however, attribute to beam averaging effects caused by differences in spatial resolution. We argue that the true distribution of the gas emission is likely more similar to a combination of the two distance results discussed, and we highlight the importance of using complementary distance estimations to safeguard against the pitfalls of any single approach. We conclude that the methodology presented in this work is a promising way to determine distances to gas emission features in Galactic plane surveys.
Highlights
A long-standing problem in astrophysics is how molecular gas, in particular the isotopologues of carbon monoxide (CO), is distributed in the Milky Way
We calculate the kinematic distances using methods contained in the Bayesian distance calculator (BDC) v2.4 and solve for the kinematic distance ambiguity (KDA) by using the Monte Carlo approach outlined in Sect. 3.1 of Roman-Duval et al (2016), assuming a Gaussian vertical density profile of the molecular gas with a full-width at halfmaximum (FWHM) of 110 pc as was done in that study. For these pure kinematic distance solutions we find that ∼58% of the integrated 13CO emission and ∼52% of the fit components overlap with the positions of spiral arms from our assumed model
These results demonstrate that compared to pure kinematic distances both our BDC runs contain a significant enhancement of 13CO emission at the position of spiral arm features
Summary
A long-standing problem in astrophysics is how molecular gas, in particular the isotopologues of carbon monoxide (CO), is distributed in the Milky Way (for reviews see e.g. Combes 1991; Heyer & Dame 2015). Knowledge about the location of the molecular gas in our Galaxy is essential to answer important open questions in interstellar medium (ISM) research, such as the impact and importance of different Galactic environments (e.g. spiral arm and interarm regions) on star formation and the origin and evolution of ISM structures. Rice et al 2016; MivilleDeschênes et al 2017) Notwithstanding all these issues in establishing reliable distances, many studies of molecular clouds extracted from 12CO (1–0) or 13CO (1–0) surveys tried to identify their position within the Galaxy (Combes 1991; Heyer & Dame 2015; Rice et al 2016; Miville-Deschênes et al 2017) and found large variations in how well the clouds trace the gaseous spiral arm structure and the fraction of clouds located in interarm regions. This approach allows us to determine lower and upper limits for the fraction of emission within spiral arm and interarm locations, and enables us to discuss the robustness of our results in terms of how much the gas emission varies with Galactocentric distance and Galactic features
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