Observations of the CS $J = 1\rightarrow 0$ Line in Galaxies
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Session 23 -- Galaxies I
Oral presentation, Wednesday, January 12, 10:15-11:45, Salon IV Room (Crystal Gateway)

## [23.05] Observations of the CS $J = 1\rightarrow 0$ Line in Galaxies

\def\jon {\ifmmode{{\rm J=1\rightarrow 0}} \else{{$J$=1$\rightarrow$0}} \fi} \def\jtw {\ifmmode{{\rm J=2\rightarrow 1}} \else{{$J$=2$\rightarrow$1}} \fi} T.A.D. Paglione, J.M. Jackson (Boston U.), S. Ishizuki (Tohoku U.)

We report both the first detection of CS \jon\ emission, and the first interferometric observations of CS in galaxies. Because it requires relatively high densities for thermal excitation ($n_{{}_{\rm H_2}} >10^4{\rm cm}^{-3}$), CS effectively traces the denser core regions of molecular clouds. CS \jon\ was observed towards the central regions of the starburst galaxies NGC 253, M82, and NGC 6946, as well as IC 342 with the NRO 45~m. NGC 6946 has the highest CS \jon\ luminosity of the galaxies surveyed. The CS, CO, 60, and 100$\mu$m luminosities appear well correlated. However, the CS \jon\ emission is not as well correlated with HCN \jon\ emission, which is sensitive to very high gas densities ($n_{{}_{\rm H_2}} >10^5{\rm cm}^{-3}$). Multitransition analysis including up to five line ratios indicates widely varying degrees of molecular excitation between galaxies, and that the average gas densities within the central kiloparsec of these galaxies are similar to those of dense cloud cores in our Galaxy. The CS emission from 0.8 to 7~mm is apparently not well represented by only a single component radiative transfer model.

CS emission was also observed using the Nobeyama Millimeter Array. Observations of CS \jon\ from NGC 253 reveal an unresolved ($\theta < 10''$) source at the nucleus. The CS \jtw\ emission in IC 342 is distributed quite differently from the CO, with weaker emission in the ridges and peaking toward the northern edge of the nuclear ring. This conspicuous difference suggests large physical or chemical gradients over small scales ($< 100$ pc).