J/A+A/549/A17ivo://CDS.VizieR/J/A+A/549/A17doi:10.26093/cds/vizier.35490017CDSBuchbender C.Kramer C.Gonzalez-Garcia M.Israel F.P.,Garcia-Burillo S.van der Werf P.Braine J.Rosolowsky E.Mookerjea B.,Aalto S.Boquien M.Gratier P.Henkel C.Quintana-Lacaci G.Verley S.,der Tak F.van2023-07-24T15:17:08Z2012-12-06T10:46:58ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgalaxiesradio-galaxiesinterstellar-mediumWe 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. Using the IRAM 30m telescope, we detected HCN, HCO^+^, ^12^CO, and ^13^CO in six GMCs along the major axis of M33 at a resolution of ~114pc and out to a radial distance of 3.4kpc. 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 M33-type subsolar metallicity. 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 M33. 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 3 and 10mag and of moderate densities of n=3x10^3^-3x10^4^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.2013A&A...549A..17Bhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/549/A17CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/ApJS/149/343 : Giant molecular clouds in M33 (Engargiola+, 2003)J/A+A/426/119 : 1.2mm mapping of RCW 106 GMC (Mookerjea+, 2004)J/MNRAS/353/1025 : JHK photometry of 4 W51 star clusters (Nanda+, 2004)J/MNRAS/367/1609 : RCW 106 Giant Molecular Cloud 13CO mapping (Bains+, 2006)J/A+A/471/103 : SIMBA observations of SMC (Bot+, 2007)J/ApJ/654/240 : Giant molecular clouds in M31 (Rosolowsky+, 2007)J/ApJ/654/338 : Spitzer photometry in W3 molecular cloud (Ruch+, 2007)J/ApJ/661/830 : Giant molecular clouds of M33 (Rosolowsky+, 2007)J/MNRAS/379/663 : W3 giant molecular cloud 850um observations (Moore+, 2007)J/A+A/489/533 : M83 giant molecular clouds volume densities (Heiner+, 2008)J/ApJ/686/948 : CO in extragalactic giant molecular clouds (Bolatto+, 2008)J/ApJS/178/56 : CO observations of LMC Giant Molecular clouds (Fukui+, 2008)J/ApJ/699/1092 : Giant molecular clouds (SRBY) (Heyer+, 2009)J/AJ/137/4072 : Infrared photometry of G216-2.5 YSOs (Megeath+, 2009)J/A+A/534/A131 : Clumps in giant molecular cloud G345.5+1.0 (Lopez+, 2011)J/A+A/542/A108 : Giant molecular clouds in M33 (Gratier+, 2012)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/549/A17https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/549/A17http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/549/A17https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/549/A17https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/549/A17http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/549/A17GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/549/A17/list (List of sources and FITS files)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/549/A17/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/549/A17/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/549/A17/list?GETtext/xml+votable180.050000true23.538333330.81833330.0055555555555555566/2155https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/549/A17?format=asciiRadiodefaultJ/A+A/549/A17/listList of sources and FITS filesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainNameGiant molecular cloud name (in M33)meta.id;meta.maincharsp12lplot of the ^12^CO (2MHz resolution) line spectrummeta.ref.urlchar12CO-lName of the FITS file with ^12^CO 115.271GHz line (2MHz resolution) in subdirectory spmeta.idcharsp12hplot of the ^12^CO (0.33MHz resolution) line spectrummeta.ref.urlchar12CO-hName of the FITS file with ^12^CO 115.271GHz line (333kHz resolution) in subdirectory spmeta.idcharsp13lplot of the ^13^CO (2MHz resolution) line spectrummeta.ref.urlchar13CO-lName of the FITS file with ^13^CO 110.201GHz line (2MHz resolution) in subdirectory spmeta.idcharsp13hplot of the ^13^CO (0.33MHz resolution) line spectrummeta.ref.urlchar13CO-hName of the FITS file with ^13^CO 110.201GHz line (333kHz resolution) in subdirectory spmeta.idcharspHCNplot of the HCN line spectrummeta.ref.urlcharHCOpName of the FITS file with HCOp 89.188GHz line (2MHz resolution) in subdirectory spmeta.id;meta.filecharHCNName of the FITS file with HCN 88.631GHz line (2MHz resolution) in subdirectory spmeta.id;meta.filecharspHCOplot of the HCOp line spectrummeta.ref.urlchar