Complex Radio Structure in the Peculiar Pair of Interacting Starburst Galaxies NGC 3395 and NGC 3396

Abstract

We have observed the peculiar pair of galaxies NGC 3395 (Sc) and NGC 3396 (Irr) with the VLA in several configurations at four frequencies, the X (8414.9 MHz), C (4885.1 MHz), L (1464.9 MHz), and P (327.5 MHz) bands. The integrated nonthermal radio spectra of the two galaxies appear to be similar, but their radio morphologies are very different. NGC 3396 has one weak and two strong radio peaks with relatively flat spectral indices for the two strong peaks, and nearly all its radio emission is confined within the optical image. The radio emission of NGC 3395 is more extended and has no strong peaks. <P />The two galaxies have a common striking feature: the radio emission is not centered on the optical but is much stronger in the half of each galaxy that is nearer its companion. This contradicts models in which tidal interactions cause gas to flow into the center of the galaxy and produce starbursts only there. In NGC 3395 the tidal interaction is more likely to produce shocks and star formation over much of the galaxy, although radio emission is weaker in the northwestern part of the galaxy. There may be several very weak radio spots following a distorted optical spiral arm. Also, in NGC 3395 the diffuse radio emission extends beyond the optical image. A radio bridge is found connecting the two galaxies and following the intergalactic optical emission fairly closely. <P />We do not find any evidence for a compact source ≤140 pc radiating more than 10<SUP>27</SUP> ergs s<SUP>-1</SUP> in the L band in either galaxy. The continued existence of these galaxies, subject to disruption by external tidal forces and internal starbursts, raises several interesting questions, and we discuss these and other implications of the observations.