Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-02-23
Astron.J.133:2132-2155,2007
Astronomy and Astrophysics
Astrophysics
26 pages, 18 figures, 8 tables (emulateapj). Accepted for publication in The Astronomical Journal, Vol. 133. Figs. 1, 2, 3, &
Scientific paper
10.1086/513317
This paper presents new images and spectroscopy 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 HST archival V image. 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 tidal tails indicative of two former disk galaxies. 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 0.1-1.0 Gyr, photometric masses between 2x10^6 and 2x10^7 Msun, and are gravitationally bound young globulars. (2) The blue central disk appears to be young. It is exponential, can be traced to >10 kpc radius, and has a smooth structure and colors suggest- ing a dominant, ~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 mean outflow velocity of this gas is -620 km/s, while the maximum velocity reaches -1050 km/s. We suggest that NGC 34 stems from two recently merged gas-rich disk galaxies with an estimated mass ratio between 1/3 and 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. (Abridged)
Schweizer Francois
Seitzer Patrick
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