CO observations in active galactic nuclei and starbursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena, such as outflows and high... Show moreCO observations in active galactic nuclei and starbursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena, such as outflows and high supernova rates. We investigate the effect of mechanical heating on atomic fine-structure and molecular lines and on their ratios. We try to use those ratios as a diagnostic to constrain the amount of mechanical heating in an object and also study its significance on estimating the H2 mass. Equilibrium photodissociation models (PDRs hereafter) were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity-gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to mechanical heating (γmech hereafter). These emission becomes at least one order of magnitude brighter in clouds with n ~ 105 cm-3 and a star formation rate of 1M⊙ yr-1 (corresponding to γmech = 2 × 10-19 erg cm-3 s-1). The emission of low-J CO lines is not as sensitive to γmech, but they do become brighter in response to γmech. Generally, for all of the lines we considered, γmech increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also affected, strongly in some cases. Ratios involving HCN are a good diagnostic for γmech, where the HCN(1–0)/CO(1–0) increases from 0.06 to 0.25, and the HCN(1–0)/HCO+(1–0) increase from 0.15 to 0.5 for amounts of γmech that are equivalent to 5% of the surface heating rate. Both ratios increase to more than 1 for higher γmech, as opposed to being much less than unity in pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the mechanical heating rate (as opposed to only low-J ratios). The second one is that the mechanical heating rate should be taken into account when determing AV or, equivalently, NH, and consequently the cloud mass. Ignoring γmech will also lead to large errors in density and radiation field estimates. Key words: galaxies: ISM / photon-dominated region / turbulence / ISM: molecules / ISM: clouds Show less
Boquien, M.; Calzetti, D.; Aalto, S.; Boselli, A.; Braine, J.; Buat, V.; ... ; Xilouris, M. 2015
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
Relaño, M.; Verley, S.; Pérez, I.; Kramer, C.; Calzetti, D.; Xilouris, E.; ... ; Werf, P.P. van der 2013
Aims: Within the framework of the Herschel M 33 extended survey HerM33es and in combination with multi-wavelength data we study the spectral energy distribution (SED) of a set of H ii regions in... Show moreAims: Within the framework of the Herschel M 33 extended survey HerM33es and in combination with multi-wavelength data we study the spectral energy distribution (SED) of a set of H ii regions in the Local Group galaxy M 33 as a function of the morphology. We analyse the emission distribution in regions with different morphologies and present models to infer the H{$α$} emission measure observed for H ii regions with well defined morphology. Methods: We present a catalogue of 119 H ii regions morphologically classified: 9 filled, 47 mixed, 36 shell, and 27 clear shell H ii regions. For each object we extracted the photometry at twelve available wavelength bands, covering a wide wavelength range from FUV-1516 å (GALEX) to IR-250 {$μ$}m (Herschel), and we obtained the SED for each object. We also obtained emission line profiles in vertical and horizontal directions across the regions to study the location of the stellar, ionised gas, and dust components. We constructed a simple geometrical model for the clear shell regions, whose properties allowed us to infer the electron density of these regions. Results: We find trends for the SEDs related to the morphology of the regions, showing that the star and gas-dust configuration affects the ratios of the emission in different bands. The mixed and filled regions show higher emission at 24 {$μ$}m, corresponding to warm dust, than the shells and clear shells. This could be due to the proximity of the dust to the stellar clusters in the case of filled and mixed regions. The far-IR peak for shells and clear shells seems to be located towards longer wavelengths, indicating that the dust is colder for this type of object. The logarithmic 100 {$μ$}m/70 {$μ$}m ratio for filled and mixed regions remains constant over one order of magnitude in H{$α$} and FUV surface brightness, while the shells and clear shells exhibit a wider range of values of almost two orders of magnitude. We derive dust masses and dust temperatures for each H ii region by fitting the individual SEDs with dust models proposed in the literature. The derived dust mass range is between 10$^{2}$-10$^{4}$ M$_{⊙}$ and the cold dust temperature spans T$_{cold}$ ~{} 12-27 K. The spherical geometrical model proposed for the H{$α$} clear shells is confirmed by the emission profile obtained from the observations and is used to infer the electron density within the envelope: the typical electron density is 0.7 {plusmn} 0.3 cm$^{-3}$, while filled regions can reach values that are two to five times higher. Appendices are available in electronic form at http://www.aanda.orgShow less
Herrera, C.; Rubio, M.; Bolatto, A.; Boulanger, F.; Israel, F.P.; Rantakyrö, F. 2013
