We aim to better understand the heating of the gas by observing the prominent gas cooling line [CII] at 158um in the low-metallicity environment of the Local Group spiral galaxy M33 at scales of 280pc. In particular, we aim at describing the variatio
n of the photoelectric heating efficiency with galactic environment. In this unbiased study, we used ISO/LWS [CII] observations along the major axis of M33, in combination with Herschel PACS and SPIRE continuum maps, IRAM 30m CO 2-1 and VLA HI data to study the variation of velocity integrated intensities. The ratio of [CII] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, FUV radiation fields, and average column densities of the molecular clouds. The heating efficiency stays constant at 0.8% in the inner 4.5kpc radius of the galaxy where it starts to increase to reach values of ~3% in the outskirts at about 6kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields, for optical extinctions of only a few magnitudes at constant metallicity. In view of the significant fraction of HI emission stemming from PDRs, and for typical pressures found in the Galactic cold neutral medium (CNM) traced by HI emission, the CNM contributes ~15% to the observed [CII] emission in the inner 2kpc radius of M33. The CNM contribution remains largely undetermined in the south, while positions between 2 and 7.3kpc radial distance in the north of M33 show a contribution of ~40%+-20%.
Within the framework of the HERM33ES key project, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. We use PACS and SPIRE maps at 100, 160, 25
0, 350, and 500 micron wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 and 500 micron using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 micron). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60K plus/minus 10K. The temperature of the cold component drops significantly from about 24K in the inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.
Studying molecular gas in the central regions of the star burst galaxies NGC4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. We estimate temperature, molecular density and col
umn densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Using the new NANTEN2 4m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4-3 and [CI] 3P1-3 P0 at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of 12CO and 13CO using radiative transfer models and estimate the physical conditions of CO and [CI]. The centers of NGC4945 and Circinus are very [CI] bright objects, exhibiting [CI] 3P1 - 3 P0 luminosities of 91 and 67Kkms-1kpc2, respectively. The [CI] 3P1-3 P0/CO 4-3 ratio of integrated intensities are large at 1.2 in NGC4945 and 2.8 in Circinus. Combining previous CO J= 1-0, 2-1 and 3-2 and 13CO J= 1-0, 2-1 studies with our new observations, the radiative transfer calculations give a range of densities, n(H2) = 10^3-3*104^cm-3, and a wide range of kinetic temperatures, Tkin = 20 - 100K, depending on the density. Future CO J= 7-6 and [CI] 2-1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.
