2016A&A...594A..70K

Aims. Observations of the molecular gas over scales of ∼0.5 to several kpc provide crucial information on how molecular gas moves through galaxies, especially in mergers and interacting systems, where it ultimately reaches the galaxy center, accumulates, and feeds nuclear activity. Studying the processes involved in the gas transport is one of the important steps forward to understand galaxy evolution.
Methods. ¹²CO, ¹³CO, and C¹⁸O 1-0 high-sensitivity ALMA observations (∼4''x2'') were used to assess the properties of the large-scale molecular gas reservoir and its connection to the circumnuclear molecular ring in the merger NGC 1614. Specifically, the role of excitation and abundances were studied in this context. We also observed the molecular gas high-density tracers CN and CS.
Results. The spatial distributions of the detected ¹²CO 1-0 and ¹³CO 1-0 emission show significant differences. ¹²CO traces the large-scale molecular gas reservoir, which is associated with a dust lane that harbors infalling gas, and extends into the southern tidal tails. ¹³CO emission is for the first time detected in the large-scale dust lane. In contrast to ¹²CO, its line emission peaks between the dust lane and the circumnuclear molecular ring. A ¹²CO-to-¹³CO 1-0 intensity ratio map shows high values in the ring region (∼30) that are typical for the centers of luminous galaxy mergers and even more extreme values in the dust lane (>45). Surprisingly, we do not detect C¹⁸O emission in NGC 1614, but we do observe gas emitting the high-density tracers CN and CS.
Conclusions. We find that the ¹²CO-to-¹³CO 1-0 line ratio in NGC 1614 changes from >45 in the 2kpc dust lane to ∼30 in the starburst nucleus. This drop in ratio with decreasing radius is consistent with the molecular gas in the dust lane being kept in a diffuse, unbound state while it is being funneled toward the nucleus. This also explains why there are no (or very faint) signs of star formation in the dust lane, despite its high ¹²CO luminosity. In the inner 1.5kpc, the gas is compressed into denser and most likely self-gravitating clouds (traced by CN and CS emission), allowing it to power the intense central starburst. We find a high ¹⁶O-to-¹⁸O abundance ratio in the starburst region (≥900), typical of quiescent disk gas. This is surprising because by now, the starburst is expected to have enriched the nuclear interstellar medium in ¹⁸O relative to ¹⁶O. We suggest that the massive inflow of gas may be partially responsible for the low ¹⁸O/¹⁶O abundance since it will dilute the starburst enrichment with unprocessed gas from greater radial distances. The ¹²CO-to-¹³CO abundance of >90 we infer from the line ratio is consistent with this scenario. It suggests that the nucleus of NGC 1614 is in a transient phase of its evolution where the starburst and the nuclear growth is still being fuelled by returning gas from the minor merger event.