GR 8: CO-to-H 2 Conversion Factor at Extremely Low Metallicity

Verter, Frances; Hodge, P. W.

doi:10.1086/175821

Cited by 52 publications

(49 citation statements)

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“…This X CO is estimated to be accurate to a factor of 2 in the Milky Way disk (Solomon et al 1987 and references therein). X CO appears to underpredict N H 2 in low-metallicity galaxies (Verter & Hodge 1995;Arimoto, Sofue, & Tsujimoto 1996;Wilson 1995) and may overpredict N H 2 in galactic centers (Dahmen et al 1998;Meier & Turner 2001). Our derived N H 2 values within the disk of M83 are estimated to be accurate to within a factor of 2 in the disk, with higher internal relative accuracy.…”

Section: M Co Observationsmentioning

confidence: 68%

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Crosthwaite¹,

Turner²,

Buchholz³

et al. 2002

The Astronomical Journal

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We present '' on-the-fly '' maps of CO (1-0) and CO (2-1) in the barred spiral galaxy M83. The entire optical disk is filled with CO emission in both lines, including interarm regions. For a '' standard '' conversion factor the total molecular mass is 2:5 Â 10 9 M , 4% of the total dynamical mass. Molecular gas constitutes 80% of total gas mass within the inner 10 0 and 30% overall. CO (1-0) emission and 21 cm continuum emission are extremely well correlated spatially in regions where H 2 dominates the gas surface density. The spatial correlation between CO (1-0) and 100 lm emission is not as good. While the kinematics of the outer H i disk is severely nonaxisymmetrically perturbed, the kinematics of the CO disk and inner H i disk is well behaved and circular. The molecular disk appears to have a distinct hard edge at 5 0 . The hard edge is accompanied by a steep decline in the total gas surface density at that radius, although the H i does continue outward at a lower level. This edge is coincident with the onset of strong warping of the outer disk. The mean ratio of CO (2-1)/ CO (1-0) emission is $1, high by Galactic standards. Peaks in this ratio in interarm regions suggest that the CO emission is optically thin there, among other possibilities. Thus, M83 is unusual in that it has a relatively axisymmetric, almost entirely molecular, inner 10 0 gas disk surrounded by a warped, entirely atomic, outer gas disk. A sharp discontinuity separates the inner and outer gas disks. This kind of gas distribution is not seen in other late-type, gas-rich spirals.

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Section: M Co Observationsmentioning

confidence: 68%

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Crosthwaite¹,

Turner²,

Buchholz³

et al. 2002

The Astronomical Journal

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“…Clearly more observations, especially of galaxies with metallicities in the range 7.8 ≤ 12 + log(O/H) ≤ 8.0, are needed before the behavior of CO emission at low metallicities can be understood. Verter & Hodge (1995) obtained very low upper limits on CO emission in the nearby dwarf galaxy GR 8, and compared these limits to the H 2 column density (N H2 ) inferred from the presence of star formation in the form of HII regions. We will apply their analysis to WLM, using the Hα data of Hodge & Miller (1995):…”

Section: Discussionmentioning

confidence: 99%

A search for CO in the Local Group dwarf irregular galaxy WLM

Taylor

Klein

2001

A&A

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Abstract. We present 12 CO J = 1 → 0 and J = 2 → 1 observations of the low metallicity (12 + log(O/H) = 7.74) Local Group dwarf irregular galaxy WLM made with the 15 m SEST and 14 m FCRAO telescopes. Despite the presence a number of HII regions, we find no CO emission. We obtain low upper limits on the integrated intensity (ICO ≤ 0.18 K km s −1 for CO (1 → 0)). The non-detection is consistent with the result of Taylor, Kobulnicky & Skillman (1998), that dwarf galaxies below a metallicity of ∼7.9 are not detected in CO emission. WLM shows that this trend continues for low metallicity galaxies even as their metallicities approach 7.9. These results are consistent with the models of the metal poor ISM by Norman & Spaans (1997). By comparing our CO data with observations of star formation in WLM, we find evidence for a high CO to H2 conversion factor.

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“…The uncertainties in the estimation of the molecular cloud masses, which are obtained from CO intensities through a calibration parameter χ, are large because the dependence of χ on metallicity or galactocentric distance (Verter & Hodge 1995;Wilson 1995) in each galaxy is not completely well determinated.…”

Section: The Present-day Radial Distributionsmentioning

confidence: 99%

Oxygen and nitrogen abundances in Virgo and field spirals

Pilyugin

Mollá

Ferrini

et al. 2002

A&A

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Abstract. The oxygen and nitrogen abundances in the H ii regions of the nine Virgo spirals of the sample from Skillman et al. (1996) and in nine field spiral galaxies are re-determined with the recently suggested P -method. We confirm that there is an abundance segregation in the sample of Virgo spirals in the sense that the H i deficient Virgo spirals near the core of the cluster have higher oxygen abundances in comparison to the spirals at the periphery of the Virgo cluster. At the same time both the Virgo periphery and core spirals have counterparts among field spirals. Some field spirals have H i to optical radius ratios, similar to that in H i deficient Virgo core spirals. We conclude that if there is a difference in the abundance properties of the Virgo and field spirals, this difference appears to be small and masked by the observational errors.

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GR 8: CO-to-H 2 Conversion Factor at Extremely Low Metallicity

Cited by 52 publications

References 0 publications

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

A search for CO in the Local Group dwarf irregular galaxy WLM

Oxygen and nitrogen abundances in Virgo and field spirals

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