Discovery of a Small Central Disk of CO and H I in the Merger Remnant NGC 34

Abstract

We present CO(1-0) and H I (21 cm) observations of the central region of the wet merger remnant NGC 34. The Combined Array for Research in Millimeter-wave Astronomy observations detect a regularly rotating disk in CO with a diameter of 2.1 kpc and a total molecular hydrogen mass of (2.1 ± 0.2) × 10^9 M_☉. The rotation curve of this gas disk rises steeply, reaching maximum velocities at 1'' (410 pc) from the center. Interestingly, H I observations performed with the Karl G. Jansky Very Large Array show that the absorption against the central continuum has the exact same velocity range as the CO in emission. This strongly suggests that the absorbing H I also lies within 1'' from the center, is mixed in, and corotates with the molecular gas. A comparison of H I absorption profiles taken at different resolutions (5''-45'') shows that the spectra at lower resolutions are less deep at the systemic velocity. This provides evidence for H I emission in the larger beams, covering the region from 1 kpc to 9 kpc from the center. The central rapidly rotating disk was likely formed either during the merger or from fall-back material. Lastly, the radio continuum flux of the central source at mm wavelengths (5.4 ± 1.8 mJy) is significantly higher than expected from an extrapolation of the synchrotron spectrum, indicating the contribution of thermal free-free emission from the central starburst.