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Astron. Astrophys. 347, 37-46 (1999)

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1. Introduction

Except for the 12CO(1-0) line, few detailed maps of the molecular gas distribution in galaxies have been published so far. 13CO(1-0) has been rarely observed in galaxies, and 12CO(2-1) even more rarely. The weakness of the 13CO(1-0) line in galaxies and the technical problems of observing at the 230 GHz frequency of 12CO(2-1) account for this rarity of observations. However, for a better knowledge of the interstellar medium in galaxies, more tracers than 12CO(1-0) must be observed. A crucial issue that can be studied through these observations is the validity of the conversion factor [FORMULA] [FORMULA] (K [FORMULA] pc2)-1 that is valid for self-gravitating molecular clouds (see Sage & Isbell 1991).

13CO(1-0) has been observed with interferometers in only a few galaxies. In IC 342, a galaxy similar to the Milky Way, Wright et al. (1993) showed a variation of the intensity ratio [FORMULA]CO(1-0)/13CO(1-0). Those authors interpret 13CO peaks as molecular clouds while 12CO would trace a more diffuse medium. The variations of [FORMULA] can be interpreted in different ways (Sage & Isbell 1991, Sakamoto et al. 1997), including variation of gas density, kinetic temperature, and relative abundance of isotopes. Downes et al. (1992) explained these ratio variations by filling factors of clouds varying across the central part of IC 342.

Up to now 12CO(2-1) has been observed mainly in the GMCs of our Galaxy, where the ratio [FORMULA]CO(2-1)/12CO(1-0) has a typical value of 1.0 (Plambeck & Williams 1979), indicating gas at low kinetic temperature ([FORMULA] K). Radford et al. (1991) observed a low ratio [FORMULA] in infrared-luminous galaxies, indicating the presence of subthermally excited CO. Braine et al. (1993) surveyed of 81 galaxies in the CO transitions, and found a average ratio [FORMULA], indicating cold, optically thick gas. Similarly in the central region of IC 342, Eckart et al. (1990) found a ratio [FORMULA] everywhere, with a slightly higher value in the center, possibly indicating an increase in the gas kinetic temperature toward the center. A ratio [FORMULA] was found close to a CO arm, indicating warm gas heated by star formation. The starburst galaxy M82 shows a different behaviour, with an unusually high ratio [FORMULA] (Knapp et al. 1980, Loiseau et al. 1990), indicating hot gas ([FORMULA]K) heated by star formation.

We have observed in the 13CO(1-0) and 12CO(1-0) lines the central region ([FORMULA]kpc) of the barred spiral galaxy NGC 1530. This galaxy contains large amount of molecular gas in its central kiloparsec, due to the accreting action of its bar (cf models by Athanassoula 1992, Friedli & Benz 1993, Piner, Stone, & Teuben 1995). The bar has driven a high fraction ([FORMULA]) of the total gas of the galaxy into the center (Downes et al. 1996, DRSR hereafter). CO(1-0) has been extensively studied in this galaxy (Regan et al. 1995, DRSR, Reynaud & Downes 1997 (RD97), Reynaud & Downes 1998). RD97 also mapped HCN(1-0), showing that the dense gas as traced by HCN is mainly concentrated in a nuclear ring or unresolved spiral at galactic radius [FORMULA] kpc. This concentration of dense gas is connected with the presence of an inner Lindblad resonance at radius [FORMULA] kpc.

The ionized gas was mapped in H[FORMULA] by Regan et al. (1996). Greve et al. (1999) compared the distribution and kinematics of ionized and molecular gas in the bar of NGC 1530.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999