The central regions of galaxies, including our own, are characterized by a unique population of molecular clouds, both denser and warmer than typical disk clouds. In galaxies undergoing a nuclear starburst, these clouds are subject to turbulence and intense UV radiation, as well as shocks, due to both a steep gravitational potential and the formation and death of young massive stars. They form a distinct type of interstellar molecular environment, different from Galactic dark clouds and the surroundings of H II regions. Galactic center clouds are a rich source of molecular emission; 25 molecular species have been discovered in the nuclear regions of external galaxies.
Formaldehyde and methanol are both useful tracers of the dense interstellar gas phase. Their molecular structure is more complicated than that of HCN or CS, the "classical" tracers of dense gas. This leads to a greater wealth of transitions which can be observed in the millimeter wavelength range, providing a powerful tool to understand the physical and chemical conditions of the gas associated with starbursts.
The first formaldehyde transitions detected in external galaxies were K -doublet ortho transitions in the centimeter wavelength range ( at 6 cm and at 2 cm; Gardner & Whiteoak 1974, 1976, 1979, Whiteoak & Gardner 1976, Cohen et al. 1979, Graham et al. 1978, Seaquist & Bell 1990, Baan & Goss 1992). Galaxies where these lines have been detected include NGC 253, NGC 4945, M 82, Centaurus A, NGC 3628, M 31 and the LMC. Recently, the transition has been detected toward the Einstein Ring gravitational lens system B0218+357 at a redshift of (Menten & Reid 1996). In all the above sources the lines are observed in absorption; with 2.7 K, they can absorb even the microwave background. The line emission was first observed toward Arp 220 (Baan et al. 1986). Detections in nine other galaxies and an interfer- ometric map of Arp 220 (Baan et al. 1993, Baan & Haschick 1995) followed. These lines are interpreted as megamasers.
There are less observations of the rotational transitions at millimeter wavelengths, even though these transitions are sensitive tracers of cloud temperature and density. The para transition was discovered in M 82 (Baan et al. 1990), the para line was observed in NGC 253 (Petuchowski & Bennett 1992), and the ortho transition was detected in the LMC by Johansson et al. (1994) and tentatively in the southern galaxy NGC 4945 (Mauersberger et al. 1996b).
The first successful observation of extragalactic methanol () was reported in 1987 by Henkel et al. . They detected a superposition of four transitions at 96 GHz in NGC 253, IC 342 and (tentatively) NGC 6946. This group of transitions requires only low excitation and all four components are among the strongest methanol lines observed in Galactic sources. The same group of transitions was detected in the southern starburst galaxy NGC 4945 (Henkel et al. 1990). The transition was seen toward NGC 253 by Henkel et al. (1993). Methanol masers, very common in the Galaxy, are also detected toward the Large Magellanic Cloud at 6.6 GHz (Sinclair et al. 1992, Ellingsen et al. 1994). Here, we report observations of six and seven rotational transitions toward five galaxies, thus considerably expanding the existing database.
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998