A self-gravitating simulation of the M51 system

Citations Over Time

Abstract

We simulate the spiral structure of M51 (NGC 5194) by the tidal passage of its companion galaxy (NGC 5195). This work extends the non-self-gravitating numerical studies of M51 by the Toomres. We place constraints-on the possible orbit of the companion by time-reversing the spiral arms of M51 to obtain estimates of both the time since passage of M51's disk (~ 70 million years) and position angle on the sky (300^deg^) toward the companion at this passage. We use this information along with NGC 5195's observed parameters to obtain a family of possible conic orbits for the companion. Other improvements in our simulations include disk self-gravitation and a three component ("stars," "gas clouds," and inert dark halo) model with collisions of the clouds. Our simulations show that the spiral structure of M51 (from 4.4' in to about 1.3') is consistent with a recent close passage of its companion. We can reproduce the radii and azimuths of points along the sharp dust lane spiral arms, the general shape of the stellar disk seen in the IR by Burkhead, and the clumps (seen in H I) north of the companion. All these features are in the correct orientation and size relative to the companion's position. We confine the ratio of halo mass to disk mass for M51 to be between 1.0 and 3.0. We find that the present orbit of the companion can be matched by a small osculating orbit inclined at i = 50^deg^ to the plane of M51 with a semimajor axis, a = 16.6 kpc (assuming a distance of 9.6 Mpc to M51). We find that the mass of NGC 5195 must be roughly 1/10 the mass of M51 to produce a good match. This indicates that the highly disturbed companion's mass/luminosity is somewhat abnormal. The small orbit we obtain indicates that the companion is in a shrinking orbit which will eventually merge with M51. We interpret the extended H I tail recently observed by Rots and collaborators as a remnant of an earlier passage of the companion. We propose three regions of spiral structure in M51: (1) the Rots et al. extended tidal arm; (2) tidal arms from the most recent passage; (3) the inner density wave excited by the tidal arms from the most recent encounter and possibly from previous encounters.

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