J/A+A/675/A145ivo://CDS.VizieR/J/A+A/675/A145doi:10.26093/cds/vizier.36750145CDSLiszt H.Gerin M.2025-04-01T20:00:53Z2023-07-14T14:07:49ZCDS support team

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

cds-question@unistra.frradio-sourcesspectroscopyinterstellar-mediumMore gas is sometimes inferred in molecular cloud complexes than is represented in HI or CO emission, and this is called dark neutral medium (DNM). Our aim is to extend a study of DNM along 13 directions in the outskirts of Chamaeleon by determining the atomic or molecular character of the DNM along a larger sample of sightlines. We acquired ALMA ground rotational state absorption profiles of HCO+ and other molecules toward 33 compact extragalactic continuum background sources seen toward the Galactic anticenter, deriving N(H_2_)=N(HCO+)/3 10^-9^ as before. We observed J=1-0 CO emission with the IRAM 30m telescope in directions where HCO+ was newly detected. HCO+ absorption was detected in 28 of 33 new directions and CO emission along 19 of those 28. The five sightlines lacking detectable HCO+ have three times lower <E_B-V_> and <N(DNM)>. Binned in E_B-V_, N(H_2_) and N(DNM) are strongly correlated and vary by factors of 50=100 over the observed range E_B-V_~0.05-1mag, while N(HI) varies by factors of only 2-3. On average N(DNM) and N(H_2_) are well matched, and detecting HCO- absorption adds little to no H_2_ in excess of the previously inferred DNM. There are five cases where 2N(H_2_)<N(DNM)/2 indicates saturation of the HI emission profile. For sightlines with W_CO_>=1K-km/s, the CO-H_2_ conversion factor N(H_2_)/W_CO_=2-3x10^20^cm^-2^/(1K-km/s) is higher than is derived from studies of resolved clouds in gamma-rays. Our work sampled primarily atomic gas with a mean H_2_ fraction ~1/3, but the DNM is almost entirely molecular. CO fulfills its role as an H_2_ tracer when its emission is strong, but large-scale CO surveys are not sensitive to H_2_ columns associated with typical values N(DNM)=2-6x10^20^cm^-2^. Lower X_CO_ values from gamma-ray studies arise in part from different definitions and usage. Sightlines with W_CO_>=1K-km/s represent 2/3 of the H_2_ detected in HCO+ and detecting 90% of the H_2_ would require detecting CO at levels W_co_~=0.2-0.3K-km/s.2023A&A...675A.145Lhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/675/A145CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/675/A145https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/675/A145http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/675/A145https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/675/A145https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/675/A145http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/675/A145GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/675/A145/list (List of fits spectra)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/675/A145/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/675/A145/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/675/A145/list?GETtext/xml+votable180.050000true30.9433711.580320.0055555555555555566/80 250 438 465 514 532 548 574 583 839 1025 1038 1129 1132 1154 1187 1546 1576 18271 23252 23261 23323 23333 23335 23370 23416 23427 23436 23513 23548 24099 24123https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/675/A145?format=asciiRadiodefaultJ/A+A/675/A145/listList of fits spectrarecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintObs.dateObservation datetime.epoch;obsnullablebVELO-LSRLower value of VELO-LSR intervalm/sphys.veloc;pos.heliocentricdoubleBVELO-LSRUpper value of VELO-LSR intervalm/sphys.veloc;pos.heliocentricdoubledVELO-LSRVELO-LSR resolutionm/sspect.resolutionfloatFreqObserved frequencyHzem.freqdoublesizeSize of FITS fileKibytephys.size;meta.fitsintspShow the spectrummeta.ref.urlcharFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fits