[Paper Review] The Dense Gas Mass Fraction and the Relationship to Star Formation in M51

M. H. Heyer, Benjamin Gregg+16 more|arXiv (Cornell University)|Apr 20, 2022

Astrophysics and Star Formation Studies75 references9 citations

TL;DR

This study uses high-resolution 12CO and HCN J=1-0 observations from the Large Millimeter Telescope to investigate dense gas mass fractions and star formation efficiency in M51. It finds that dense gas fraction is nearly constant in spiral arms and interarm regions but rises sharply in the central bulge due to elevated pressure, which shifts molecular clouds from self-gravitating to diffuse states, reducing dense gas star formation efficiency in high-pressure environments.

ABSTRACT

Observations of 12CO J=1-0 and HCN J=1-0 emission from NGC 5194 (M51) made with the 50~meter Large Millimeter Telescope and the SEQUOIA focal plane array are presented. Using the HCN to CO ratio, we examine the dense gas mass fraction over a range of environmental conditions within the galaxy. Within the disk, the dense gas mass fraction varies along spiral arms but the average value over all spiral arms is comparable to the mean value of interarm regions. We suggest that the near constant dense gas mass fraction throughout the disk arises from a population of density stratified, self gravitating molecular clouds and the required density threshold to detect each spectral line. The measured dense gas fraction significantly increases in the central bulge in response to the effective pressure, P_e, from the weight from the stellar and gas components. This pressure modifies the dynamical state of the molecular cloud population and possibly, the HCN emitting regions, in the central bulge from self-gravitating to diffuse configurations in which P_e is greater than the gravitational energy density of individual clouds. Diffuse molecular clouds comprise a significant fraction of the molecular gas mass in the central bulge, which may account for the measured sublinear relationships between the surface densities of the star formation rate and molecular and dense gas.

Motivation & Objective

• To investigate how environmental conditions such as pressure and spiral structure modulate dense gas mass fraction and star formation efficiency in M51.
• To determine whether the dense gas mass fraction remains constant across different galactic environments or varies systematically.
• To examine the role of external pressure in transforming molecular cloud dynamics from self-gravitating to diffuse configurations.
• To assess the implications of cloud state transitions for the observed sublinear scaling between star formation rate and dense gas surface density.
• To test whether the HCN J=1-0 line traces the true dense gas responsible for star formation in high-pressure regions like the central bulge.

Proposed method

• Acquired 12CO J=1-0 and HCN J=1-0 emission data with the 50 m Large Millimeter Telescope using the SEQUOIA focal plane array at 582 pc resolution.
• Processed data using LMT spectral line software with baseline subtraction and coaddition of multiple maps into spectral line cubes.
• Resampled CO and HCN data to match HCN resolution (λ/D = 14′′) and spectrally smoothed to 5 km s−1 for direct comparison.
• Calculated dense gas mass fraction using the HCN-to-CO luminosity ratio, corrected for beam efficiency and main beam temperature.
• Derived star formation rate surface density from GALEX, Spitzer, and SDSS photometry, and stellar mass surface density from Galfit modeling of 3.6 µm images.
• Classified regions into spiral arms, interarm, and central bulge based on stellar mass surface density and morphological features, using Galfit models with bulge, spiral, and disk components.

Experimental results

Research questions

• RQ1 How does the dense gas mass fraction vary across different galactic environments in M51, particularly between spiral arms and interarm regions?
• RQ2 What is the role of external pressure (Pe) in modifying the dynamical state of molecular clouds and the detectability of dense gas via HCN J=1-0 emission?
• RQ3 Why is the star formation efficiency in dense gas sublinear with respect to dense gas surface density in high-pressure regions like the central bulge?
• RQ4 Does the transition from self-gravitating to diffuse cloud states in the central bulge explain the observed suppression of dense gas star formation efficiency?
• RQ5 Can the HCN J=1-0 line still be a reliable tracer of dense gas in high-pressure environments where cloud structures are dynamically altered?

Key findings

• The dense gas mass fraction is nearly constant across spiral arms and interarm regions in M51’s disk, with no significant average difference between the two environments.
• In the central bulge, where stellar mass surface density exceeds 500 M⊙/pc², the dense gas mass fraction increases steeply due to elevated external pressure.
• The transition from self-gravitating to diffuse molecular clouds occurs at stellar mass surface densities between 400 and 1000 M⊙/pc², marked by a drop in dense gas star formation efficiency.
• In the central bulge, star formation is increasingly confined to compact, high-density, self-gravitating cores that do not significantly contribute to HCN J=1-0 luminosity, explaining the reduced SFEdense.
• The sublinear scaling between star formation rate and dense gas surface density in high-pressure regions is attributed to the dominance of diffuse molecular gas that contributes to HCN emission but not to star formation.
• The self-gravitational energy density (UG) falls below the external pressure (Pe) in the central bulge, indicating a shift from self-gravitating to pressure-confined cloud configurations, which suppresses detectable dense gas emission relative to star formation.