1996A&A...310..725N

Using the IRAM 30-m telescope, we have mapped the λ1.2mm continuum emission and the ¹²CO J=1-0 and 2-1 line emissions in the edge-on spiral galaxy NGC4565. We selected NGC4565 because it is strong in the HI line, but weak in CO, contrary to the galaxies mapped so far at 1.2mm. The CO emission in NGC4565 resembles that in the Milky Way. It shows a compact central source surrounded by a `molecular ring'. The molecular ring peaks between 1' and 1.5' from the center (3-5kpc) and has an outer radius of ≃3'. It is twice smaller than the broad `plateau' observed in HI and shows narrow structures which could be spiral arms. The λ 1.2mm continuum follows CO near the center and HI at the periphery. It shows, like CO, a central peak and an inner ring and, like HI, a weaker, extended plateau. This is the first time that dust emission is unambiguously detected in extragalactic HI clouds. Like HI, the 1.2mm contours are warped near the NW edge of the galaxy. The warp, already apparent at half the optical radius, reaches a height of 50"(2.4kpc) at the edge of the optical disk. The average dust temperature is 18 K near the center and 15K in the HI plateau. From the 1.2mm continuum intensity and the HI line integrated intensity, we derive a dust absorption cross section per H atom σ_1.2mm^H=τ/N_H=5x10^–27cm² in the plateau. This value is very close to that predicted by Draine & Lee (1984) for the local diffuse clouds. The velocity field derived from CO can be described by rotation (solid body rotation between 0 and 4 kpc, constant velocity further out) plus non-circular motions. The non-circular motions are observed mostly near the `arms' and near the nucleus. They are probably the signature of a spiral density wave and/or of a central bar. The presence of a bar could explain the boxy shape of the central bulge on optical photographs.