The CO luminosity and CO-H<sub>2</sub>conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Liszt, H. S.; Pety, J.; Lucas, R.

doi:10.1051/0004-6361/201014510

Cited by 129 publications

(187 citation statements)

References 45 publications

(114 reference statements)

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“…With dN(HCO + )/dV = 1.5 × 10 12 cm −2 (km s −1 ) −1 and X(HCO + ) = 3 × 10 −9 (Fig. 7 here or see Liszt et al 2010) the implied mean column density of clouds with linewidth dV (in units of km s −1 ) is N cld (H) ≈ 2N(H 2 ) = 10 21 cm −2 dV 4 . For dV = 2−4 km s −1 these would be classic diffuse-translucent clouds with A V ≈ 1−2 mag.…”

Section: H I and H 2 Volume Filling Factorsmentioning

confidence: 93%

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Liszt

Gérin

2015

A&A

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Context. The sightline to W31C G10.62-0.38 was extensively observed in absorption under the PRISMAS program on Herschel. Aims. We relate absorbing material to the older view of Galactic molecules gained from CO emission. Methods. We used the Arizona Radio Observatory Kitt Peak 12m antenna to observe emission from the J = 1−0 lines of carbon monoxide, HCO + , and HNC, and the J = 2−1 line of CS toward and around the continuum peak used for absorption studies and we compare them with CH, HNC, C + , and other absorption spectra from PRISMAS. We develop a kinematic analysis that allows a continuous description of the spectral properties and relates them to viewing geometry in the Galaxy. Results. As it is for CH, HF, C + , HCO + , and other species observed in absorption, mm-wave emission in CO, HCO + , HNC, and CS is continuous over the full velocity range expected for material between the Sun and W31 4.95 kpc away. CO emission is much stronger than average in the Galactic molecular ring and the mean H 2 density derived from CH, 4 cm −3 < ∼ 2 n(H 2 ) < ∼ 10 cm −3 at 4 < ∼ R < ∼ 6.4 kpc, is similarly elevated. The CO-H 2 conversion factor falls in a narrow range X CO = 1−2 × 10 20 H 2 cm −2 (K − km s −1 ) −1 if the emitting gas is mostly on the near side of the subcentral point, as we suggest. The brightnesses of HCO + , HNC, and CS are comparable (0.83%, 0.51%, and 1.1%, respectively, relative to CO) and have no variation in galactocentric radius with respect to CO. Comparison of the profile-averaged HCO + emission brightness and optical depth implies local densities n(H) ≈ 135 ± 25 cm −3 with most of the excitation of HCO + from electrons. At this level of density, a consistent picture of the H 2 -bearing gas, which also accounts for the CO emission, has a volume filling factor of 3% and a 5 pc clump or cloud size.

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Section: H I and H 2 Volume Filling Factorsmentioning

confidence: 93%

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Liszt

Gérin

2015

A&A

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“…This result is expected given the tight relation between stellar mass, stellar mass surface density, colour, specific SFR, metallicity, and the H-band luminosity (Gavazzi et al 1996;Boselli et al 2001), the parameter used to derive the luminosity-dependent X CO for each target galaxy. The debate over any possible systematic variation in the CO-to-H 2 conversion factor on the physical properties of the ISM, such as the metallicity or the intensity of the interstellar radiation field, is still far from being settled both observationally and theoretically Bell et al 2007;Bolatto et al 2008;Liszt et al 2010;Leroy et al 2011;Shetty et al 2011a,b;Narayanan et al 2012;Kennicutt & Evans 2012;Bolatto et al 2013;Sandstrom et al 2013). We thus cannot state from the present analysis which is the most reliable between the two sets of relations.…”

Section: The Scaling Relationsmentioning

confidence: 99%

Cold gas properties of theHerschelReference Survey

Boselli

Cortese

Boquien

et al. 2014

A&A

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We study the properties of the cold gas component of the interstellar medium of the Herschel Reference Survey, a complete volumelimited (15 < ∼ D < ∼ 25 Mpc), K-band-selected sample of galaxies spanning a wide range in morphological type (from ellipticals to dwarf irregulars) and stellar mass (10 9 < ∼ M star < ∼ 10 11 M ). The multifrequency data in our hands are used to trace the molecular gas mass distribution and the main scaling relations of the sample, which put strong constraints on galaxy formation simulations. We extend the main scaling relations concerning the total and the molecular gas component determined for massive galaxies (M star > ∼ 10 10 M ) from the COLD GASS survey down to stellar masses M star 10 9 M . As scaling variables we use the total stellar mass M star , the stellar surface density μ star , the specific star formation rate SSFR, and the metallicity of the target galaxies. By comparing molecular gas masses determined using a constant or a luminosity dependent X CO conversion factor, we estimate the robustness of these scaling relations on the very uncertain assumptions used to transform CO line intensities into molecular gas masses. The molecular gas distribution of a K-band-selected sample is significantly different from that of a far-infrared-selected sample since it includes a significantly smaller number of objects with M(H 2 ) < ∼ 6 × 10 9 M . In spiral galaxies the molecular gas phase is only 25-30% of the atomic gas. The analysis also indicates that the slope of the main scaling relations depends on the adopted conversion factor. Among the sampled relations, all those concerning M(gas)/M star are statistically significant and show little variation with X CO . We observe a significant correlation between M(H 2 )/M star and SSFR, M(H 2 )/M(Hi) and μ star , M(H 2 )/M(Hi) and 12 + log (O/H), regardless of the adopted X CO . The total and molecular gas consumption timescales are anticorrelated with the specific star formation rate. The comparison of HRS and COLD GASS data indicates that some of the observed scaling relations are nonlinear.

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“…According to Liszt et al (2010), the specific brightness W CO /N CO is larger in warm and sub-thermally excited gas. Such environments correspond to kinetic temperatures that are much higher than the CO(1 → 0) excitation temperature, in agreement with our findings.…”

Section: Excitation Conditions Of Comentioning

confidence: 99%

High-resolution HI and CO observations of high-latitude intermediate-velocity clouds

Röhser

Kerp

Bekhti

et al. 2016

A&A

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Context. Intermediate-velocity clouds (IVCs) are HI halo clouds that are likely related to a Galactic fountain process. In-falling IVCs are candidates for the re-accretion of matter onto the Milky Way. Aims. We study the evolution of IVCs at the disk-halo interface, focussing on the transition from atomic to molecular IVCs. We compare an atomic IVC to a molecular IVC and characterise their structural differences in order to investigate how molecular IVCs form high above the Galactic plane. Methods. With high-resolution HI observations of the Westerbork Synthesis Radio Telescope and 12 CO(1 → 0) and 13 CO(1 → 0) observations with the IRAM 30 m telescope, we analyse the small-scale structures within the two clouds. By correlating HI and far-infrared (FIR) dust continuum emission from the Planck satellite, the distribution of molecular hydrogen (H 2 ) is estimated. We conduct a detailed comparison of the HI, FIR, and CO data and study variations of the X CO conversion factor. Results. The atomic IVC does not disclose detectable CO emission. The atomic small-scale structure, as revealed by the highresolution HI data, shows low peak HI column densities and low HI fluxes as compared to the molecular IVC. The molecular IVC exhibits a rich molecular structure and most of the CO emission is observed at the eastern edge of the cloud. There is observational evidence that the molecular IVC is in a transient and, thus, non-equilibrium phase. The average X CO factor is close to the canonical value of the Milky Way disk. Conclusions. We propose that the two IVCs represent different states in a gradual transition from atomic to molecular clouds. The molecular IVC appears to be more condensed allowing the formation of H 2 and CO in shielded regions all over the cloud. Ram pressure may accumulate gas and thus facilitate the formation of H 2 . We show evidence that the atomic IVC will evolve also into a molecular IVC in a few Myr.

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The CO luminosity and CO-H₂conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Cited by 129 publications

References 45 publications

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Cold gas properties of theHerschelReference Survey

High-resolution HI and CO observations of high-latitude intermediate-velocity clouds

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The CO luminosity and CO-H2conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Cited by 129 publications

References 45 publications

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

Cold gas properties of theHerschelReference Survey

High-resolution HI and CO observations of high-latitude intermediate-velocity clouds

Contact Info

Product

Resources

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The CO luminosity and CO-H₂conversion factor of diffuse ISM: does CO emission trace dense molecular gas?