Probability distribution functions of gas in M31 and M51

Abstract

We present probability distribution functions (PDFs) of the surface densities of ionized and neutral gas in the nearby spiral galaxies M31 and M51, as well as of dust emission and extinction Av in M31. The PDFs are close to lognormal and those for HI and Av in M31 are nearly identical. However, the PDFs for H2 are wider than the HI PDFs and the M51 PDFs have larger dispersions than those for M31. We use a simple model to determine how the PDFs are changed by variations in the line-of-sight (LOS) pathlength L through the gas, telescope resolution and the volume filling factor of the gas, f_v. In each of these cases the dispersion sigma of the lognormal PDF depends on the variable with a negative power law. We also derive PDFs of mean LOS volume densities of gas components in M31 and M51. Combining these with the volume density PDFs for different components of the ISM in the Milky Way (MW), we find that sigma decreases with increasing length L with an exponent of -0.76 +/- 0.06, which is steeper than expected. We show that the difference is due to variations in f_v. As f_v is similar in M31, M51 and the MW, the density structure in the gas in these galaxies must be similar. Finally, we demonstrate that an increase in f_v with increasing distance to the Galactic plane explains the decrease in sigma with latitude of the PDFs of emission measure and FUV emission observed for the MW.

15 pages, 7 figures, 7 tables, accepted for publication in Monthly Notices of the Royal Astronomical Society

References in corpus (17)

• Theory of Star Formation
• The Statistics of Supersonic Isothermal Turbulence
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• The Vertical Structure of Warm Ionised Gas in the Milky Way
• Molecular Hydrogen and Global Star Formation Relations in Galaxies
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Unfolding the Laws of Star Formation: The Density Distribution of Molecular Clouds
• A complete 12CO 2-1 map of M51 with HERA: I. Radial averages of CO, HI, and radio continuum
• The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way
• The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
• The supernova-regulated ISM. I. The multi-phase structure
• High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
• Density probability distribution functions of diffuse gas in the Milky Way
• Densities and filling factors of the DIG in the Solar neighbourhood
• The Star Formation Law in a Multifractal ISM

Cited by in corpus (5)

• The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016
• Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
• The Resolved Distributions of Dust Mass and Temperature in Local Group Galaxies
• Diffuse Ionized Gas in Simulations of Multiphase, Star-Forming Galactic Disks
• Escaping the maze: a statistical sub-grid model for cloud-scale density structures in the interstellar medium