Astron. Astrophys. 356, L49-L52 (2000)

1. Introduction

In recent years there has been a growing interest for the vertical structure of the HI disks of spiral galaxies and for the disk-halo connection. Evidence for HI gas flows between disk and halo comes from the detection of large vertical motions in several galaxies viewed close to face-on (Dickey et al. 1990; Kamphuis 1993; Schulman & Bregman 1994) and the connection of this high velocity gas with HI holes and star formation activity in the disk (Kamphuis & Sancisi 1993). More evidence comes from the study of edge-on galaxies like NGC 891 in which the HI has been found to extend up to at least 5 kpc into the halo, where it seems to rotate about 25 km more slowly than in the plane (Swaters et al. 1997). The combination of the results from face-on and edge-on galaxies leads to the picture of effervescent galaxies (Sancisi et al. 1996), consistent with galactic fountains models (Bregman 1980; Spitzer 1990).

The galaxy studied here, NGC 2403, has an intermediate inclination (i=). The consequence is that the measured line-of-sight velocities are a combination of rotational, radial and vertical motions and the column densities are integrated along an oblique line-of-sight. Therefore, the interpretation is less straightforward than in face-on or edge-on galaxies. However, there is the advantage that information is obtained on both the vertical density structure and the vertical kinematics of the HI for the same object.

For this study we have used the HI observations obtained by Sicking (1997) with the Westerbork Radio Telescope, which have about a factor of 2 better sensitivity and a higher velocity resolution than those of Wevers et al. (1986). Fig. 1 shows the optical image, the total HI density distribution, the velocity field and the HI position-velocity map along the major axis. The latter shows that the HI line profiles at any position along the major axis are not symmetrical with respect to the rotation velocities, as they would be if they were determined by random motions only. Instead, they are systematically more extended towards the systemic velocity. This striking asymmetry is particularly obvious in the lowest contour and extends systematically over almost the whole major axis. It was already noticeable in the maps produced by Wevers et al. (1986) and by Begeman (1987) and the puzzle presented by those early observations has motivated this study. The presence of such a `beard' is remarkable considering the size of the beam (see lower left in Fig. 1) with respect to the size of the galaxy. Usually such an asymmetry is seen in edge-on galaxies or in galaxies which are not well resolved. In such cases the telescope beam `sees' not only the emission from a small area on the major axis but also larger areas away from it which have lower line-of-sight velocities and therefore cause the observed asymmetry. NGC 2403 is neither highly inclined nor poorly resolved.

Fig. 1. Optical image (top left; from the Palomar Digitized Sky Survey), HI column density map (top right; the column densities range from to atoms/cm2) and HI velocity field (bottom left; the contours run from 14 to 254 km in steps of 15 km .). The beam () is shown in the lower left corner. The bottom right panel shows the HI position-velocity map along the major axis (PA=). Contours are -2.5 (dashed), 2.5 (1.8), 10, 20, 40, 60 and 100 mJy/beam. The black dots indicate the rotation curve derived by Sicking (1997). The angular and velocity resolutions ( km ) are indicated by the cross in the lower left corner.

What is the origin of the `beard' ? It is clear that it cannot simply be explained by gas moving perpendicularly away from or towards the disk, because that would produce extensions symmetric with respect to the rotation velocity. Neither can it be the result of deviations from axial symmetry or circular motion. These would affect the kinematics of the disk and produce visible effects in the velocity field, but not a low density asymmetry as observed. A likely explanation for the observed asymmetry is that not all of the HI is concentrated in a thin disk, but that part of it is in a vertically extended component. In this case, given the inclination of 61 degrees, a line-of-sight to a point on the major axis will also intercept the HI located above and below the plane which has line-of-sight velocities lower than those in the plane. This will produce a systematic broadening of the HI profiles towards the systemic velocity of the kind seen in Fig. 1. It is this effect that we study here with 3-D models of the density distribution and kinematics of the HI in NGC 2403.

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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