Thermal Pressure in the Cold Neutral Medium of Nearby Galaxies

Abstract

Dynamic and thermal processes regulate the structure of the multi-phase interstellar medium (ISM), and ultimately establish how galaxies evolve through star formation. Thus, to constrain ISM models and better understand the interplay of these processes, it is of great interest to measure the thermal pressure ($P_{\mathrm{th}}$) of the diffuse, neutral gas. By combining [C II] 158 $\mu$m, HI, and CO data from 31 galaxies selected from the Herschel KINGFISH sample, we have measured thermal pressures in 534 predominantly atomic regions with typical sizes of $\sim$1 kiloparsec. We find a distribution of thermal pressures in the $P_{\mathrm{th}}/k\sim 10^3-10^5$ K cm${}^{-3}$ range. For a sub-sample of regions with conditions similar to those of the diffuse, neutral gas in the Galactic plane, we find thermal pressures that follow a log-normal distribution with a median value of $P_{\mathrm{th}}/k\approx 3600$ K cm${}^{-3}$. These results are consistent with thermal pressure measurements using other observational methods. We find that $P_{\mathrm{th}}$ increases with radiation field strength and star formation activity, as expected from the close link between the heating of the gas and the star formation rate. Our thermal pressure measurements fall in the regime where a two-phase ISM with cold and warm neutral medium could exist in pressure equilibrium. Finally, we find that the midplane thermal pressure of the diffuse gas is about $\sim 30$% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.

Accepted for publication in ApJ on Dec. 20th, 2016; 17 pages; 7 figures and 1 table

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• EDGE-CALIFA survey: Self-regulation of Star formation at kpc scales
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• Characterizing the Multi-Phase Origin of [CII] Emission in M101 and NGC 6946 with Velocity Resolved Spectroscopy
• Spatially Resolved Chandra Spectroscopy of the Large Magellanic Cloud Supernova Remnant N132D
• Ultra-Diffuse Galaxies as Extreme Star-forming Environments II: Star Formation and Pressure Balance in HI-Rich UDGs
• The EDGE-CALIFA survey: The resolved star formation efficiency and local physical conditions
• ISM conditions in z~0.2 Lyman-Break Analogs
• The Case for Thermalization as a Contributor to the [CII] Deficit
• Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages
• ALPINE: A Large Survey to Understand Teenage Galaxies
• Modeling the Mass Distribution and Gravitational Potential of Nearby Disk Galaxies: Implications for the ISM Dynamical Equilibrium
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