Astron. Astrophys. 343, 367-372 (1999)

1. The central regions of NGC 1365

NGC 1365 is a prominent barred spiral galaxy in the Fornax cluster with a heliocentric velocity of +1632 km s^-1 (see Lindblad 1999for an extensive review). The galaxy displays a wide range of phenomena indicating activity - including a Seyfert 1.5 type nucleus with strong, broad and narrow H lines and ejection of hot gas from the nucleus (Véron et al. 1980; Jörsäter et al. 1984; Jörsäter & Lindblad 1989). The nucleus is a moderately strong infrared source as seen by IRAS (Lonsdale et al. 1985; Ghosh et al. 1993). The nuclear region contains an extended X-ray source. In addition, there are several discrete X-ray sources within the optical image of the galaxy, one of which is highly variable (Turner et al. 1993; Iyomoto et al. 1997; Komossa & Schulz 1998). At an assumed distance of 18.6 Mpc (Madore et al. 1998), corresponds to 90 pc.

At Stockholm Observatory, we have been studying the central region of NGC 1365 in great detail also in the radio region using the VLA, the NRAO 12-m millimeter wave telescope and the 15-m SEST (Sandqvist et al. 1982, 1988, 1995; Jörsäter & van Moorsel 1995). Strong CO emission peaks near the inner edges of the bar dust lanes, close to the central regions of starburst activity, and a number of radio sources form a ring-like structure around the edge of a rotating nuclear disk which coincides with the general region of optical hot spots. Our VLA continuum observations show that some of the radio sources are smaller than 0:001 and we have suggested that they may be radio supernovae. At the core of the galaxy there is a weak, steeply non-thermal radio source from which emanates a -long radio jet, which is also steeply non-thermal (Sandqvist et al. 1995). The jet is aligned along the symmetry axis of a conical shell of hot ionized [O III ] gas and both are projected along the minor axis of the galaxy (Hjelm & Lindblad 1996). Kristen et al. (1997) have used the Hubble Space Telescope to study the Seyfert nucleus and nuclear hot spots in NGC 1365 and find these hot spots to be resolved into a number of bright compact condensations. One of these condensations (SSC:10) coincides exactly with a radio source (A). Kristen et al. interpret these condensations as super star clusters (SSC) and the radio source as a radio supernova in one of these clusters.

Molecular gas has previously been mapped in the bar and central region of the Seyfert galaxy NGC 1365 with the SEST using the and 2-1 CO lines with resolutions of and , respectively (Sandqvist et al. 1995). The CO molecular gas is strongly concentrated to the nucleus, where the CO integrated line intensity has a maximum, and the global CO distribution falls off roughly exponentially with the distance from the centre of the galaxy. There is some CO alignmnent with the dust lanes in the bar and some weak emission has been detected in the western spiral arm near the end of the bar at the position of a major H I concentration observed by Jörsäter & van Moorsel (1995). The central CO luminosity corresponds to a molecular hydrogen mass of within a projected radius of 2.0 kpc. The global molecular hydrogen gas mass is , which is similar to the total amount of neutral atomic hydrogen, , found by Jörsäter & van Moorsel (1995) using the VLA. (Here we have rescaled the published values due to the change in adopted distance of NGC 1365 from 20 to 18.6 Mpc). The distribution of the H I is, however, radically different from that of the CO. Whereas the molecular mass is concentrated to the nucleus and bar region, the H I is predominantly located in the spiral arm regions. In particular, the H I distribution shows a hole in the central region which coincidies with the CO emission. This indicates that the gas is predominantly molecular in the centre and the inner bar regions.

We have carried out new observations of the central region of NGC 1365, predominantly in the CO line, but also in other molecular line transitions in the millimetre wave region (for preliminary reports, see Sandqvist 1996and Kristen et al. 1998). Since the CO line is excited in regions of higher excitation and density than the and 1-0 lines, it is a good probe of the molecular gas as it passes through the shocks in the inner bar. The higher resolution offered by the line observations also enables a better comparison with the Inner Lindblad Resonances and with predictions of molecular gas kinematic transport inward along the bar to the central star burst region, expected from numerical simulations of gas flow in this galaxy (Lindblad et al. 1996).

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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