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 th ) of the diffuse, neutral gas. By combining [C II] 158 µm, 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 ∼1 kiloparsec. We find a distribution of thermal pressures in the P th /k ∼ 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 th /k ≈ 3600 K cm −3 . These results are consistent with thermal pressure measurements using other observational methods. We find that P 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 ∼ 30% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.

Testing Feedback-regulated Star Formation in Gas-rich, Turbulent Disk Galaxies

“…This is to say that our data suggests that local Universe spirals are consistent with theoretical predictions of Σ SFR − P from feedback regulated star formation models. This is similar to the results of Herrera-Camus et al (2017) who find that KINGFISH galaxies, with P/k ∼ 10 3 .5 − 10 4 .5 are consistent with predictions from Kim et al (2013).…”

Cloud-scale Molecular Gas Properties in 15 Nearby Galaxies

“…Given that the σ and Σ values we measure resemble those typically found for HI, the internal pressure in the molecular gas is likely comparable to the ambient medium pressure in these galaxies. Reinforcing this view, the pressures implied by our measurements approach the thermal pressures found by Herrera-Camus et al (2017) in the HI dominated parts of KINGFISH galaxies. They showed that these thermal pressures are typically a factor of ∼ 3 lower than the ambient gas pressure.…”

Physical conditions in the gas phases of the giant H II region LMC-N 11

“…C). The resulting pressure is then about 10 2.5−5 K cm −3 , which is similar to what is found in predominantly atomic gas within Herschel KINGFISH galaxies (Herrera-Camus et al 2017) T H2 : fixed 4) n H2 : pressure eq. / overdense → N(H 2 |C + ) I([CII]) (mol.)…”

“…In the neutral gas, C + may exist in the H 0 phase but also in the H 2 phase, in regions where CO is photodissociated while H 2 is self-shielded and shielded by dust, the so-called CO-dark molecular gas (e.g., Madden et al 1997;Grenier et al 2005;Wolfire et al 2010). The [C ii] line has been used to trace the star-formation rate (SFR; e.g., De Looze et al 2014;Pineda et al 2014), to infer physical conditions in photodissociation regions (PDRs), and to calculate the CO-to-H 2 conversion factor (X CO ; e.g., Jameson et al 2018;Herrera-Camus et al 2017;Pineda et al 2017). The growing number of observations in both Galactic and extragalac-tic environments (with routine detections at z > 6; e.g., Aravena et al 2016) has renewed the interest in understanding [C ii] as a diagnostic tool.…”