A mapping survey of the (CO)-C-13 and (CO)-C-12 emission in galaxies

Tad. Paglione et al., A mapping survey of the (CO)-C-13 and (CO)-C-12 emission in galaxies, ASTROPH J S, 135(2), 2001, pp. 183-200
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0067-0049 → ACNP
Year of publication
183 - 200
SICI code
We present spectra of the extended (CO)-C-12 and (CO)-C-13 J=1 -->0 emissio n along the major axes of 17 nearby galaxies. Spatial variations in the rat io of CO and (CO)-C-13 integrated intensities, R, are found in nearly every galaxy observed. There is an overall variation in R of 20%-40% from the in ner 2 kpc to the disk. Roughly one-third of the survey galaxies have such g radients in R detected above the 2 sigma confidence level. Though some gala xies show a lower central value of R, on average R inside 2 kpc is 10%-30% higher than R outside of 2 kpc. The average CO/(CO)-C-13 intensity ratio wi thin the central 2 kpc of the survey sources is 11.60 +/-0.4 (based on the noise) +/-1.5 (based on systematic uncertainties estimated from daily varia tions in CO and (CO)-C-13 intensities). The 1 sigma dispersion in R between galactic nuclei of 4.2 is also quite large. The average value of R outside 2 kpc is 9.8 +/-0.6 +/-1.2 with a standard deviation of 4.5. An increase i n the CO/(CO)-C-13 intensity ratio from disk to nucleus may imply that the conversion factor between CO intensity and H-2 column density, X, is lower in galactic nuclei. In addition, variations in physical conditions, most no tably the gas kinetic temperature, affect both R and X. Abundance variation s probably do not cause the gradient in R, though we do not rule out a decr ease in effective cloud column densities in galactic nuclei possibly caused by destructive starburst superwinds. A modest rise in temperature (less th an a factor of 2 or 3) from outside a 2 kpc radius toward the nucleus can e asily account for the observed gradient. These results support previous wor k implying that X is lower in the center of the Milky Way and probably most galactic nuclei. Therefore, calculating H-2 masses using the standard Gala ctic X-factor, especially within the central few kiloparsecs of galaxies, o verestimates the true mass by factors of a few. The standard X-factor still appears to be appropriate for galactic disks.