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Cold dust and molecular line emission in NGC 4565

N. Neininger, M. Guélin , S. García-Burillo, R. Zylka, R. Wielebinski
Institut de Radioastronomie Millimétrique, 300, rue de la Piscine F-38406 St.Martin d'Hères, France
Centro Astronómico de Yebes, IGN, E-19080 Guadalajara, Spain
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Abstract: Using the IRAM 30-m telescope, we have mapped the 1.2 mm continuum emission and the J= 10 and 21 line emissions in the edge-on spiral galaxy NGC 4565. We selected NGC 4565 because it is strong in the HI line, but weak in CO, contrary to the galaxies mapped so far at 1.2 mm.

Figure 9: The 1.2mmcontinuum emission, smoothed to an angular resolution of 20 and superimposed on a optical photograph of NGC4565 (Lick Observatory). The contour levels are 3.0 (), 6.0, 9.0, 21 mJy/ beam. The noise level increases by about towards the outer parts of the map. Note the warp of the continuum contours to the NW.

The CO emission in NGC 4565 resembles that in the Milky Way. It shows a compact central source surrounded by a `molecular ring'. The molecular ring peaks between and from the center (3-5 kpc) and has an outer radius of . It is twice smaller than the broad `plateau' observed in HI and shows narrow structures which could be spiral arms.

The 1.2 mm 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 1.2 mm contours are warped near the NW edge of the galaxy. The warp, already apparent at half the optical radius, reaches a height of 50 (2.4 kpc) at the edge of the optical disk (Fig. 9).

The average dust temperature is 18 K near the center and 15 K in the HI plateau. From the 1.2 mm continuum intensity and the HI line integrated intensity, we derive a dust absorption cross section per H atom cm 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 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.

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Next: CO in the Up: Scientific Results Previous: Dust and Gas