The CO-to-H${}_2$ Conversion Factor in the Central Molecular Zone of the Milky Way using CO isotopologues

Abstract

We performed correlation analyses between the ${}^{12}$CO and ${}^{13}$CO $J=$1-0 line intensities in order to derive the variability of the CO-to-H${}_2$ conversion factor ($X_{\mathrm{CO},\mathrm{iso}}$) in the central molecular zone (CMZ) of our Galaxy. New high-resolution $X_{\mathrm{CO},\mathrm{iso}}$ maps at a resolution of $\sim 30$" and the longitude-velocity diagram (LVD) at resolution $\sim 30$" $\times 2$ km s${}^{-1}$ are presented using the ${}^{12}$CO and ${}^{13}$CO archival survey data obtained by the Nobeyama 45 m telescope. We revealed the variation of $X_{\mathrm{CO},\mathrm{iso}}$ in the CMZ within the range of $X_{\mathrm{CO},\mathrm{iso}}\sim (0.2-1.3)\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$, if we assume the normalization value of $0.59\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$. The mean value is obtained as $X_{\mathrm{CO},\mathrm{iso}}=(0.48\pm 0.15)\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ in the CMZ of our Galaxy.

15 pages, 14 figures, 3 tables, accepted for Publications of the Astronomical Society of Japan (PASJ)