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

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 $(0.6-2.6)\times 10^4$ cm${}^{-3}$ and $(2.9-12)\times 10^4$ 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.

12 pages, 6 figures, accepted for publication in ApJ

References in corpus (16)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• A 2 mm spectral line survey of the starburst galaxy NGC 253
• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Molecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
• Molecular clouds have power-law probability distribution functions
• A Portrait of Cold Gas in Galaxies at 60pc Resolution and a Simple Method to Test Hypotheses That Link Small-Scale ISM Structure to Galaxy-Scale Processes
• Lambda = 3 mm line survey of nearby active galaxies
• Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
• Optical Depth Estimates and Effective Critical Densities of Dense Gas Tracers in the Inner Parts of Nearby Galaxy Discs
• Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands
• Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
• Molecular-Cloud-Scale Chemical Composition II: Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band
• Kinetic temperature of massive star forming molecular clumps measured with formaldehyde. II. The Large Magellanic Cloud
• A 3 mm Spectral Line Survey toward the Barred Spiral Galaxy NGC 3627

Cited by in corpus (1)

• Searching for shocks in high-mass starless clump candidates