The H I observations presented in the previous section have provided new evidence bearing on the dynamics of NGC 3310 and on the origin of the starburst. The main results are the two conspicuous H I tails connected to the northern and southern sides of the disk and the unusually large velocity dispersion of the H I in the disk.
The velocity dispersion of the H I in the disk, of up to
40 kms-1, quite large compared to the values of 7 to 12 kms-1
usually found in spiral galaxies, can be attributed for only
a small part to profile broadening caused by differential rotation
inside the instrumental beam. Most of it must come from an
intrinsically high gas turbulence or, more likely, to deviations from
circular motion, like the streamings seen in H
.
Such deviations
would cause, because of the relatively large beam (
kpc),
broad velocity profiles. The H I data confirm the results of
optical spectroscopy and indicate that the gaseous disk of NGC
3310 is highly disturbed. Large deviations from circular rotation
must be present all over the disk and it is not clear whether all
non-circular motions are in the plane of the disk.
The two extended tails on opposite sides of the NGC 3310 disk have similar H I masses but different morphologies and kinematics. The one to the south is more extended, reaching out to 51 kpc, broader on the sky and more patchy and has almost constant radial velocity (close to the systemic velocity). It has lower H I surface density and no optical counterpart. Its velocity dispersion is around 13 kms-1. The tail on the northern side is less extended, half the length, narrower and covers a larger velocity range. Its velocity dispersion is very similar to that of the southern tail.
The northern tail has been constructed with the assumption that the H I emission with anomalous velocities 804 to 854 kms-1on the northern side of NGC 3310 and the H I coinciding with the "arrow'' form one continuous structure. This is clearly in contrast with the interpretation of the arrow as a one-sided jet violently emitted from a compact central source as proposed by Bertola & Sharp (1984). According to the interpretation favoured here, the chain of compact H II regions visible to the north of the bright optical disk would be part of a curved structure continuing to the west with the "arrow'' and coinciding with the H I tail. On Plate 2 of Bertola & Sharp (1984) the continuity between the arrow and the chain of northern optical knots seems obvious. This would imply, however, similar radial velocities of H I and H II, whereas the measurements by Mulder & van Driel (1996) seem to indicate that the H II velocities are about 100 kms-1 higher. It is possible that there is confusion with H II emission from the disk. At any rate, it is clear that this point is important for the interpretation proposed here and should be verified by accurately determining the optical radial velocities.
In conclusion, it seems that in all their properties the two H I tails found associated with NGC 3310 resemble the tidal tails seen in gravitationally interacting systems and mergers. It should be noted, however, that in this respect the relative orientation of the two tails may pose a problem (see below).
All the evidence discussed above - the optical ripples, the disturbed kinematics of the gaseous disk and the two H I tails - seems to point to a recent merger event. A merger or a major accretion event for NGC 3310 has been suggested before (Balick & Heckman 1981; Schweizer & Seitzer 1988). One possibility is the type of encounter in which an Irr I galaxy is being cannibalized by NGC 3310 as proposed by Balick & Heckman (1981). Any such explanation should, however, also account for the presence of the two gaseous tails revealed by the present study. The two equal mass, extended tails, the H I gas content and the small total mass seem to point to a merger between two galaxies of small and comparable masses, of which at least one gas-rich. Indeed, Schweizer's (1978) five characteristics to be expected for a recently merged pair of galaxies - a pair of long tails, an isolated merger-remnant candidate, a single nucleus, chaotic motions and the tails moving in opposite directions - seem to be all present for NGC 3310. For the fifth condition - the two tails moving in opposite directions -, there may be a problem concerning the orientation of the northern tail. It is possible, however, that the tail is not in one plane and that the optical and HI pictures are the projection of a more complex structure. The cluster of northern HII regions does suggest a possible turn of direction from north-north-east to the south-west (see Plate 2 of Bertola & Sharp 1984). The actual merger process may have been more complex and has, perhaps, involved a third small object as the "bow'' and the other ripples seem to suggest. It is remarkable, at any rate, that after such an apparently "major'' merger event there should still be a disk. This is clearly different in many respects (in spite of the remarkable R1/4 photometric profile) from the well studied merger case NGC 7252, the "Atoms-for-Peace'' (Hibbard et al. 1994). The unsettled disk we observe now in NGC 3310 could then be either a newly formed disk with spiral arms and star formation going on, or the disturbed disk of one of the progenitors which has survived the merger and is now undergoing new star formation. This would argue in favour of the robustness of disks.
The present observations have added a few more pieces to the interesting puzzle of NGC 3310. It is clear, however, that for a better understanding of the origin of its peculiar morphology and kinematics detailed numerical simulations as done for example by Hibbard & Mihos (1995) for NGC 7252 are needed.
Acknowledgements
We wish to thank Rob Swaters and Marc Balcells for providing the R-band image of NGC 3310, and Tjeerd van Albada and Piet van der Kruit for helpfull comments. We have used the GIPSY package, developed at the Kapteyn Astronomical Institute, for the data reduction and analysis. The Westerbork Synthesis Radio Telescope is operated by the Netherlands Foundation for Research in Astronomy (NFRA/ASTRON), with financial support by the Netherlands Organization for Scientific Research (N.W.O.).
Copyright ESO 2001