Aims: We aim to better understand the heating of gas by observing the prominent gas cooling line [C ii] at 158 {$μ$}m in the low-metallicity environment of the Local Group spiral galaxy M 33 on... Show moreAims: We aim to better understand the heating of gas by observing the prominent gas cooling line [C ii] at 158 {$μ$}m in the low-metallicity environment of the Local Group spiral galaxy M 33 on scales of 280 pc. In particular, we describe the variation of the photoelectric heating efficiency with the galactic environment. Methods: In this study, we present [C ii] observations along the major axis of M 33 using the Infrared Space Observatory in combination with Herschel continuum maps, IRAM 30 m CO 2-1, and VLA H i data to study the variation in velocity integrated intensities. The ratio of [C ii] 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, far-ultraviolet radiation fields, and average column densities of the molecular clouds. Results: The heating efficiency stays constant at 0.8% in the inner 4.5 kpc radius of the galaxy, where it increases to reach values of ~{}3% in the outskirts at about a 6 kpc 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. For the significant fraction of H i emission stemming from PDRs and for typical pressures found in the Galactic cold neutral medium (CNM) traced by H i emission, the CNM contributes ~{}15% to the observed [C ii] emission in the inner 2 kpc radius of M 33. The CNM contribution remains largely undetermined in the south, while positions between radial distances of 2 and 7.3 kpc in the north of M 33 show a contribution of ~{}40% {plusmn} 20%. Herschel is an ESA space observatory with science instruments provided by European-led PrincipalInvestigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgShow less
Buchbender, C.; Kramer, C.; Gonzalez-Garcia, M.; Israel, F.P.; García-Burillo, S.; Werf, P.P. van der; ... ; Tak, F. van der 2013
Aims: We aim to better understand the emission of molecular tracers of the diffuse and dense gas in giant molecular clouds and the influence that metallicity, optical extinction, density, far-UV... Show moreAims: We aim to better understand the emission of molecular tracers of the diffuse and dense gas in giant molecular clouds and the influence that metallicity, optical extinction, density, far-UV field, and star formation rate have on these tracers. Methods: Using the IRAM 30 m telescope, we detected HCN, HCO$^{+}$, $^{12}$CO, and $^{13}$CO in six GMCs along the major axis of M 33 at a resolution of ~{}114 pc and out to a radial distance of 3.4 kpc. Optical, far-infrared, and submillimeter data from Herschel and other observatories complement these observations. To interpret the observed molecular line emission, we created two grids of models of photon-dominated regions, one for solar and one for M 33-type subsolar metallicity. Results: The observed HCO$^{+}$/HCN line ratios range between 1.1 and 2.5. Similarly high ratios have been observed in the Large Magellanic Cloud. The HCN/CO ratio varies between 0.4% and 2.9% in the disk of M 33. The $^{12}$CO/$^{13}$CO line ratio varies between 9 and 15 similar to variations found in the diffuse gas and the centers of GMCs of the Milky Way. Stacking of all spectra allowed HNC and C$_{2}$H to be detected. The resulting HCO$^{+}$/HNC and HCN/HNC ratios of ~{}8 and 6, respectively, lie at the high end of ratios observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, while HCO$^{+}$ abundances are enhanced. For HCN this effect is more pronounced at low optical extinctions. The observed HCO$^{+}$/HCN and HCN/CO line ratios are naturally explained by subsolar PDR models of low optical extinctions between 4 and 10 mag and of moderate densities of n 3 { imes} 10$^{3}$-3 { imes} 10$^{4}$ cm$^{-3}$, while the FUV field strength only has a small effect on the modeled line ratios. The line ratios are almost equally well reproduced by the solar-metallicity models, indicating that variations in metallicity only play a minor role in influencing these line ratios. Based on observations with the IRAM 30m telescope, Herschel, and other observatories. IRAM is supported by CNRS/INSU (France), the MPG (Germany), and the IGN (Spain). Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgFITS files of the presented spectra of the ground-state transitions of HCN, HCO$^{+}$, $^{12}$CO and $^{13}$CO are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A17Show less
