Deep ALMA imaging of the merger NGC 1614

Abstract

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. 12 CO, 13 CO, and C 18 O 1−0 high-sensitivity ALMA observations (∼4 × 2) were used to assess the properties of the largescale 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 12 CO 1−0 and 13 CO 1−0 emission show significant differences. 12 CO 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. 13 CO emission is for the first time detected in the large-scale dust lane. In contrast to 12 CO, its line emission peaks between the dust lane and the circumnuclear molecular ring. A 12 CO-to-13 CO 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 C 18 O emission in NGC 1614, but we do observe gas emitting the high-density tracers CN and CS. Conclusions. We find that the 12 CO-to-13 CO 1−0 line ratio in NGC 1614 changes from >45 in the 2 kpc 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 12 CO luminosity. In the inner 1.5 kpc, 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 16 O-to-18 O 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 18 O relative to 16 O. We suggest that the massive inflow of gas may be partially responsible for the low 18 O/ 16 O abundance since it will dilute the starburst enrichment with unprocessed gas from greater radial distances. The 12 CO-to-13 CO 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.

No AGN evidence in NGC 1614 from deep radio VLBI observations

“…This is in agreement with the SF-powered molecular outflow found in NGC 1614 by García-Burillo et al (2015). It is also possible that a massive inflow of gas caused by the final phase of the merging event has not yet been fully funneled to the central region, which will eventually cause and enhancement in star formation, and an active phase of the AGN (König et al 2016). In any case, our finding supports the model where mechanical heating from SN shocks and superwinds can heat the surrounding medium, inhibiting the accretion of material towards the central AGN and powering the warm gas in the starburst ring of NCG1614 (Saito et al 2017).…”

“…Several recent millimeter studies have been performed to determine the properties of the fuel of the star formation, i.e., the molecular gas, and its inflows and outflows (Olsson et al 2010;Imanishi & Nakanishi 2013;Xu et al 2015; García-Burillo et al 2015; König et al 2016;Saito et al 2016Saito et al , 2017. However, while the bulk of observational evidence favors a pure starburst scenario for NGC 1614, the existence of an intrinsically weak AGN at its very center has not yet been completely ruled out (Risaliti et al 2000;Wilson et al 2008;Pereira-Santaella et al 2015b).…”

Spatially Resolved CO SLED of the Luminous Merger Remnant NGC 1614 with ALMA

“…The CO(1-0) observations toward NGC1614 were also carried out during Cycle2 (ID: 2013.1.00991.S; König et al 2016) using both ten 7m antennas of ACA (Iguchi et al 2009) and thirty-one 12m antennas of the 12m array. These ancillary data are used to increase the sensitivity and recover extended emission that we ignore due to the uv-clipping (Section 2.1).…”

“…We retrieved those data from the ALMA archive, and used one SPW which was tuned to the CO(1-0) line. Detailed description of this project is found in König et al (2016)and we list some parameters in Table 2 We performed calibration and imaging using CASA (version 3.4, 4.1.0, and 4.2.2 for Band 7, 9, and 3/6, respectively; McMullin et al 2007). The detailed procedure for data processing is summarized in S16a.…”

“…High-resolution (∼0 25∼83 pc) ALMA Band9 observations revealed that the CO(6-5) distribution coincides with a starburst ring (r<350 pc) detected in Pa α ( Figure 1(c); Alonso-Herrero et al 2001) and 8.4GHz radio continuum emission (Xu et al 2015), while the CO(2-1) and CO(3-2) emission are more extended (Wilson et al 2008;König et al 2013;Sliwa et al 2014). Recent high-sensitivity CO(2-1) observations revealed the presence of a high-velocity wing, which might be associated with a nuclear molecular outflow (García-Burillo et al 2015), whereas deep CO(1-0) imaging found that extended molecular gas components associated with the southern tidal tail and a dust lane (König et al 2016). Continuum emission from 4.81 to 691 GHz is confined within the central 1 7 radius (=560 pc) and the radioto-FIR spectral energy distribution (SED) can be explained by star-forming activities (Saito et al 2016a, hereafter S16a).…”