We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 x 10 super(5) M sub(middot in circle) in the inner disks of 5 large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC ...4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and super(12)CO (J = 1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H sub(2) mass (or X sub(CO)) for each galaxy is 1-2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1), all within a factor of two of the Milky Way disk value (~2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1)). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-X sub(CO) trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
RESOLVED MEASUREMENTS OF X CO IN NGC 6946 Donovan Meyer, Jennifer; Koda, Jin; Momose, Rieko ...
The Astrophysical journal,
01/2012, Letnik:
744, Številka:
1
Journal Article
Recenzirano
We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image ...fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the 12CO (J = 1-0) transition. By combining observations from the Nobeyama Radio Observatory 45 m single dish telescope and the Combined Array for Research in Millimeter Astronomy interferometer, we are able to obtain high image fidelity and accurate measurements of L CO compared with previous purely interferometric studies. We resolve individual GMCs, measure their luminosities and virial masses, and derive X CO--the conversion factor from CO measurements to H2 masses--within individual clouds. On average, we find that X CO = 1.2 X 1020 cm--2 (K km s--1)--1, which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within ~400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.
We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image ...fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the {sup 12}CO (J = 1-0) transition. By combining observations from the Nobeyama Radio Observatory 45 m single dish telescope and the Combined Array for Research in Millimeter Astronomy interferometer, we are able to obtain high image fidelity and accurate measurements of L{sub CO} compared with previous purely interferometric studies. We resolve individual GMCs, measure their luminosities and virial masses, and derive X{sub CO}-the conversion factor from CO measurements to H{sub 2} masses-within individual clouds. On average, we find that X{sub CO} = 1.2 Multiplication-Sign 10{sup 20} cm{sup -2} (K km s{sup -1}){sup -1}, which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within {approx}400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.
We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 \(\times\) 10\(^{5}\) \Msun in the inner disks of five large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, ...and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and \(^{12}\)CO (J=1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H\(_{2}\) mass (or \xcons) for each galaxy is 1-2 \xcounits, all within a factor of two of the Milky Way disk value (\(\sim\)2 \xcounits). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-\xco trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image ...fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the 12CO (J=1-0) transition. By combining observations from the Nobeyama Radio Observatory 45-meter single dish telescope and the Combined Array for Research in Millimeter Astronomy (CARMA) interferometer, we are able to obtain high image fidelity and accurate measurements of LCO compared with previous purely interferometric studies. We resolve individual giant molecular clouds (GMCs), measure their luminosities and virial masses, and derive Xco - the conversion factor from CO measurements to H2 masses - within individual clouds. On average, we find that Xco = 1.2 \times 10^20 cm-2 / (K km s-1), which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within \sim400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.
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