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Keywords:

  • ISM: bubbles;
  • ISM: molecules;
  • ISM: structure;
  • galaxies: haloes;
  • galaxies: individual: NGC 4631;
  • galaxies: ISM

ABSTRACT

We have made the first map of CO(J= 3–2) emission covering the disc of the edge-on galaxy, NGC 4631, which is known for its spectacular gaseous halo. The strongest emission, which we model with a Gaussian ring, occurs within a radius of 5 kpc. Weaker disc emission is detected out to radii of 12 kpc, the most extensive molecular component yet seen in this galaxy. From comparisons with infrared data, we find that CO(J= 3–2) emission more closely follows the hot dust component, rather than the cold dust, consistent with it being a good tracer of star formation. The first maps of R3−2/1−0, H2 mass surface density and star formation efficiency (SFE) have been made for the inner 2.4 kpc radius region. Only 20 per cent of the star formation occurs in this region and excitation conditions are typical of galaxy discs, rather than of central starbursts. The SFE suggests long gas consumption time-scales (>109 yr).

The velocity field is dominated by a steeply rising rotation curve in the region of the central molecular ring followed by a flatter curve in the disc. A very steep gradient in the rotation curve is observed at the nucleus, providing the first evidence for a central concentration of mass: Mdyn= 5 × 107 M within a radius of 282 pc. The velocity field shows anomalous features indicating the presence of molecular outflows; one of them is associated with a previously observed CO(J= 1–0) expanding shell. Consistent with these outflows is the presence of a thick (z up to 1.4 kpc) CO(J= 3–2) disc. We suggest that the interaction between NGC 4631 and its companion(s) has agitated the disc and also initiated star formation which was likely higher in the past than it is now. These may be necessary conditions for seeing prominent haloes.