CO observations of the interacting galaxy pair NGC 5394/95

Abstract

BIMA 12CO J = 1 → 0 observations are presented of the spiral galaxies NGC 5394 and NGC 5395 that have undergone a recent, grazing encounter. In NGC 5394 approximately 80% of the CO emission detected by BIMA is concentrated in the central 800 pc (FWHM) starburst region, and the rest is from a portion of the inner disk south and west of the central starburst. In an encounter simulation that reproduces some of the main features of this galaxy pair, a considerable amount of gas in NGC 5394 falls into the central region early in the collision. The observed total gas distribution in the disk of NGC 5394 is lopsided, with more H I, CO, and Hα emission coming from the western or southwestern side. The innermost western arm of NGC 5394 is seen in CO and Hα emission, but the eastern inner-disk arm, which is very bright in the optical continuum, is not detected in CO or Hα emission. The NGC 5394 starburst region is similar in radio continuum luminosity and size to the M82 starburst and has a CO luminosity ∼4 times greater. A CO position-velocity diagram of the NGC 5394 nucleus reveals two separate velocity features very close to the center. This may indicate a nuclear ring or the "twin peaks" of an ILR or some depletion of 12CO J = 1 → 0 at the nucleus. From a comparison of the radio continuum, Hα, 60 μm, and CO luminosities, we estimate that the average extinction A v of the starburst nucleus is 3-4 mag, the star formation rate is ∼6 M⊙ yr-1, and the conversion factor N(H 2)/ICO in the starburst is a factor of 3-4 below the standard value. Comparison of NGC 5394 with two other systems previously studied suggests that in prograde grazing encounters a central starburst may not develop until near the end of the ocular phase. Very little of the CO emission from NGC 5395 found in previous single-dish observations is detected in the BIMA data; thus molecular gas in NGC 5395 does not appear to be strongly concentrated in compressed ridges.