LOPSIDED SPIRAL GALAXIES AND A LIMIT ON THE GALAXY ACCRETION RATE

We present a measurement of lopsidedness for the stellar disks of 60 f ield spiral galaxies in terms of the azimuthal m = 1 Fourier amplitude , A(1), of the stellar light. We confirm the previous result (Rix & Za ritsky) that similar to 30% of field spiral galaxies in a magnitude-li mited sample exhibit significant lopsidedness ([A(1)/A(0)] greater tha n or equal to 0.2) at large radii (R > 1.5 disk scalelengths). We conj ecture that this lopsidedness is caused by tidal interactions and calc ulate an upper limit on the accretion rate of small galaxies. We explo it the correlation between lopsidedness and photometric measures of re cent star formation (Zaritsky) to obtain two independent estimates of the lifetime of these m = 1 distortions. First, we show that lopsided galaxies have an excess of blue luminosity relative to that of symmetr ic galaxies with the same H I linewidth, which we attribute to a recen t star formation episode that was triggered by an interaction between the galaxy and a companion. We use stellar population models (Bruzual & Chariot) to estimate the time since that interaction. Second, we use the N-body simulation of an infalling satellite by Walker, Mihos, & H ernquist to estimate how fast tidally induced m = 1 distortions are er ased through phase mixing. Both approaches indicate that the observati ons are consistent with a hypothesized tidal interaction that occurred about 1 Gyr ago for galaxies that are lopsided at the 20% level. By c ombining this lifetime estimate for lopsidedness, the observed frequen cy of such distortions, and a correction to the survey volume that dep ends on the increase in luminosity during an interaction, we derive an upper limit on the current companion accretion rate of field spiral g alaxies (for companion masses similar to 10% parent galaxy mass) that lies in the range 0.07-0.25 Gyr(-1). The principal uncertainty in this limit arises from ambiguities in the interpretation of the correlatio n between lopsidedness and M(B).