A PORTRAIT OF COLD GAS IN GALAXIES AT 60 pc RESOLUTION AND A SIMPLE METHOD TO TEST HYPOTHESES THAT LINK SMALL-SCALE ISM STRUCTURE TO GALAXY-SCALE PROCESSES

Abstract

The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key roles in the ability of gas to form stars. New high fidelity, high resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional "cloud properties" treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12 CO data from recent surveys, we characterize the molecular ISM at 60 pc resolution in the Antennae, the Large Magellanic Cloud, M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60 pc scales and show galaxy-to-galaxy and intra-galaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1σ width of ∼0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60 pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.

Cloud-scale ISM Structure and Star Formation in M51

“…Leroy et al (2016) demonstrated that the results match those from mass-weighted averages of cloud catalogs well, but with far fewer assumptions. We write the resulting measurements as, e.g., Σ 40pc .…”

“…The rms line width of CO, σ 40pc , measured from the "equivalent width" and corrected for channelization and channel-to-channel correlation following Leroy et al (2016). For a given temperature and when the line width is purely turbulent in nature, this corresponds to the turbulent Mach number, M. σ 40pc may also contain a contribution from bulk motions unresolved at the 40 pc resolution of PAWS (Colombo et al 2014b;Meidt et al 2013, and S. Meidt et al, submitted).…”

“…At 10 ′′ , using only the 70µm data, the rms scatter is higher, ∼8.5 × 10 6 L ⊙ kpc −2 . Cloud Scale Properties: We measure the intensityweighted cloud-scale properties of the gas in each beam following Leroy et al (2016). In brief, we begin with the native 40 pc resolution PAWS cube.…”

“…The recent star formation rate (SFR) correlates better with tracers of molecular gas than tracers of atomic gas (Schruba et al 2011;Blanc et al 2009;Bigiel et al 2008;Leroy et al 2008), even though atomic gas represents the dominant reservoir by mass of the interstellar medium (ISM) in galaxies at z = 0 (e.g., Saintonge et al 2011). But even within the molecular ISM of a galaxy, only a small fraction of the gas participates in star formation at any given time (e.g., Heiderman et al 2010;Lada et al 2010), and the properties of molecular gas vary among galaxies and among regions within galaxies (e.g., Hughes et al 2013b; Leroy et al 2016). The SFR per unit molecular gas mass should depend on these properties: e.g., the density, turbulence, and balance of potential and kinetic energy.…”

“…In particular, we follow Meidt et al (2013) who also compared PAWS to infrared (IR) data, focusing on the impact of dynamics on the ability of gas to form stars. We use the methodology described by Leroy et al (2016). In this approach, we calculate the molecular gas depletion time, τ mol Dep ≡ M mol /SFR, averaged over a moderate-sized area, θ = 370−1100 pc, and compare this to the mass-weighted 40 pc surface density, line width, and self-gravity (virial parameter) with in the larger beam.…”