Dense gas in M 33 (HerM33es)

Buchbender, C.; Kramer, C.; Gonzalez-Garcia, M.; Israel, F.P.; García-Burillo, S.; Werf, P.P. van der; Braine, J.; Rosolowsky, E.; Mookerjea, B.; Aalto, S.; Boquien, M.; Gratier, P.; Henkel, C.; Quintana-Lacaci, G.; Verley, S.; Tak, F. van der

doi:doi:10.1051/0004-6361/201219436

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 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...

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 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/A17

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Dense gas in M 33 (HerM33es)