Remnant of a wet merger: NGC 34 and its young massive clusters, young stellar disk, and strong gaseous outflow

This paper presents new images and spectroscopic observations of NGC 34 (Mrk 938) obtained with the du Pont 2.5 m and Baade 6.5 m telescopes at Las Campanas, plus photometry of an archival V image obtained with Hubble Space Telescope. This MV = - 21.6 galaxy has often been classified as a Seyfert 2, yet recently published infrared spectra suggest a dominant central starburst. We find that the galaxy features a single nucleus, a main spheroid containing a blue central disk and much outer fine structure, and tidal tails indicative of two former disk galaxies. At present these galaxies appear to have completed merging. The remnant shows three clear optical signs that the merger was gas-rich ('' wet '') and accompanied by a starburst: (1) It sports a rich system of young star clusters, of which 87 have absolute magnitudes - 10: 0 - MV >= - 15.4. Five clusters with available spectra have ages in the range 0.1-1.0Gyr and photometric masses of 2 x 10(6) M circle dot less than or similar to M less than or similar to 2 x 10(7) M circle dot; they are gravitationally bound young globular clusters. ( 2) The blue central disk appears to be young. It is exponential, can be traced to greater than or similar to 10 kpc radius, and has a smooth structure and colors suggesting that its optical light is dominated by a similar to 400 Myr old poststarburst population. And ( 3), the center of NGC 34 drives a strong outflow of cool, neutral gas, as revealed by broad blueshifted Na I D lines. The center-of-line velocity of this gas is -620 km s (-1), while the maximum detected outflow velocity reaches - 1050 km s (-1). Assessing all available evidence, we suggest that NGC 34 stems from two recently merged gas-rich disk galaxies with an estimated mass ratio of 1/3 less than or similar to m/M less than or similar to 2/ 3. The remnant seems to have first experienced a galaxy-wide starburst that then shrank to its current central and obscured state. The strong gaseous outflow came last.