Three papers today. First, the environment dependence of SF in the center of IC342 looks at Xco and what causes it to vary. The abstract indicates that Xco varies with sigma H2, and that it agrees with theoretical predictions, so they must be claiming a decrease in 12CO optical depth do to the of molecular gas.
The second a paper is on JD’s favorite topic, Cosmic Rays. How Cosmic Rays couple to a multiphase ISM is evaluated, with a focus on where the energy is deposited. They end up finding that the CR energy injection rate (a function of density) ends up being fairly consistent with the average density of the multiphase ISM, as long as you don’t get too dense or to close to a GMC, or have very tangled B fields. At that point it should CR energy injection rates should increase significant, but I’ll have to read the paper to see by how much.
Environmental Dependence of Star Formation Law in the Disk and Center of IC 342 Authors: Hsi-An Pan (1 and 2), Nario Kuno (1 and 2), Akihiko Hirota (2) ((1) Department of Astronomical Science, The Graduate University for Advanced Studies, Kanagawa, Japan, (2) Nobeyama Radio Observatory of NAOJ, Nagano, Japan)
(Submitted on 1 Nov 2013)
Abstract: The Kennicutt-Schmidt (K–S) law in IC 342 is examined using the 12CO-to-H2 conversion factor (Xco,v), which depends on the metallicity and CO intensity. Additionally, an optically thin 13CO (1-0) is also independently used to analyze the K–S law. Xco,v is two to three times lower than the Galactic standard Xco in the galactic center and approximately two times higher than Xco at the disk. The surface densities of molecular gas (Sigma_H2) derived from 12CO and 13CO are consistent at the environment in a high-Sigma_H2 region. By comparing the K-S law in the disk and the central regions of IC 342, we found that the power law index of K-S law (N) increases toward the central region. Furthermore, the dependence of N on Sigma_H2 is observed. Specifically, N increases with Sigma_H2. The derived N in this work and previous observations are consistent with the implication that star formation is likely triggered by gravitational instability in the disk (low-Sigma_H2 region) of IC 342 and both gravitational instability and cloud-cloud collisions in the central region (high-Sigma_H2 regime). In addition, the increasing N toward the high-Sigma_H2 domain also matches the theoretical prediction regarding the properties of giant molecular clouds. The results of IC 342 are supported by the same analysis of other nearby galaxies.
Abstract: How cosmic rays sample the multi-phase interstellar medium (ISM) in starburst galaxies has important implications for many science goals, including evaluating the cosmic ray calorimeter model for these systems, predicting their neutrino fluxes, and modeling their winds. Here, we use Monte Carlo simulations to study cosmic ray sampling of a simple, two-phase ISM under conditions similar to those of the prototypical starburst galaxy M82. The assumption that cosmic rays sample the mean density of the ISM in the starburst region is assessed over a multi-dimensional parameter space where we vary the number of molecular clouds, the galactic wind speed, the extent to which the magnetic field is tangled, and the cosmic ray injection mechanism. We evaluate the ratio of the emissivity from pion production in molecular clouds to the emissivity that would be observed if the cosmic rays sampled the mean density, and seek areas of parameter space where this ratio differs significantly from unity. The assumption that cosmic rays sample the mean density holds over much of parameter space; however, this assumption begins to break down for high cloud density, injection close to the clouds, and a very tangled magnetic field. We conclude by evaluating the extent to which our simulated starburst region behaves as a proton calorimeter and constructing the time-dependent spectrum of a burst of cosmic rays.