12CO( J =1–0) and 13CO( J =1–0) Mapping of the Starburst Galaxy M82

• Published in: Publications of the Astronomical Society of Japan 1998-06, Vol.50 (3), p.309-315
• Main Authors: Kikumoto, Takahiro, Taniguchi, Yoshiaki, Nakai, Naomasa, Ishizuki, Sumio, Matsushita, Satoki, Kawabe, Ryohei
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/50.3.309

We have mapped the central 1 kpc region of the nearby starburst galaxy M82 in both the 12CO(J=1–0) and 13CO(J=1–0) emission lines under the same angular resolution $(\sim 7^{\prime \prime} \times 6^{\prime \prime})$. The intensity ratio (R) of 12CO(J=1–0) to 13CO(J=1–0) emission shows the significant spatial variation: $(\sim 7^{\prime \prime} \times 6^{\prime \prime})$ (the NE lobe), $R\simeq 20-25$ (the SW lobe), and R >70 (the 2.2 μm secondary peak). These values are significantly higher than those of giant molecular clouds in our Galaxy and molecular gas in other normal spiral galaxies. We also find that a very high ratio (R > 90) is achieved around the brightest radio continuum source, 41.9+58, which is considered to be a supernova remnant (or remnants). Based on the LTE model, we show that the molecular gas in M82 tends to be optically thin, even for the 12CO emission; its optical depth is a few or less. The smaller optical depths may naturally explain the higher R values observed in M82. We discuss the possibility that a superwind causes the formation of such clouds.