Molecular Gas Density Measured with H$_2$CO and CS toward a Spiral Arm of M51

Yuri Nishimura; Yoshimasa Watanabe; Nanase Harada; Kotaro Kohno; Satoshi Yamamoto

doi:10.3847/1538-4357/ab24d3

paper

Molecular Gas Density Measured with H $_{2}$ CO and CS toward a Spiral Arm of M51

arXiv:1905.11087 · doi:10.3847/1538-4357/ab24d3

Abstract

Observations of various molecular lines toward a disk region of a nearby galaxy are now feasible, and they are being employed as diagnostic tools to study star-formation activities there. However, the spatial resolution attainable for a nearby galaxy with currently available radio telescopes is $10 - 1000$ pc, which is much larger than the scales of individual star-forming regions and molecular cloud cores. Hence, it is of fundamental importance to elucidate which part of an interstellar medium such spatially-unresolved observations are tracing. Here we present sensitive measurements of the H $_{2}$ CO ( $1_{01} - 0_{00}$ ) line at 72 GHz toward giant molecular clouds (GMCs) in the spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction with the previously observed H $_{2}$ CO ( $2_{02} - 1_{01}$ ) and CS ( $2 - 1$ and $3 - 2$ ) lines, we derive the H $_{2}$ density of the emitting regions to be $(0.6 - 2.6) \times 1 0^{4}$ cm $^{- 3}$ and $(2.9 - 12) \times 1 0^{4}$ cm $^{- 3}$ for H $_{2}$ CO and CS, respectively, by the non-LTE analyses, where we assume the source size of $0.8 - 1$ kpc and the gas kinetic temperature of $10 - 20$ K. The derived H $_{2}$ density indicates that the emission of H $_{2}$ CO and CS is not localized to star-forming cores, but is likely distributed over an entire region of GMCs. Such widespread distributions of H $_{2}$ CO and CS are also supported by models assuming lognormal density distributions over the 1 kpc region. Thus, contributions from the widespread less-dense components should be taken into account for interpretation of these molecular emission observed with a GMC-scale resolution. The different H $_{2}$ densities derived for H $_{2}$ CO and CS imply their different distributions. We discuss this differences in terms of the formation processes of H $_{2}$ CO and CS.

arXiv comment12 pages, 6 figures, accepted for publication in ApJ

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Molecular Gas Density Measured with H2​CO and CS toward a Spiral Arm of M51

Abstract

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Molecular Gas Density Measured with H $_{2}$ CO and CS toward a Spiral Arm of M51