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

Abstract

We present high-resolution (1 0) Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO(1-0) and CO(2-1) rotational transitions toward the nearby IR-luminous merger NGC1614 supplemented with ALMA archival data of CO(3-2)and CO(6-5) transitions. The CO(6-5) emission arises from the starburst ring (central 590 pc in radius), while the lower-J CO lines are distributed over the outer disk (∼3.3 kpc in radius). Radiative transfer and photon-dominated region (PDR) modeling reveals that the starburst ring has a single warmer gas component with more a intense far-ultraviolet radiation field (ñ 10 A two-phase molecular interstellar medium with a warm and cold (>70 and ∼19 K) component is also an applicable model for the starburst ring. A possible source for heating the warm gas component is mechanical heating due to stellar feedback rather than PDR. Furthermore, we find evidence for non-circular motions along the north-south optical bar in the lower-J CO images, suggesting a cold gas inflow. We suggest that star formation in the starburst ring is sustained by the bardriven cold gas inflowand that starburst activities radiatively and mechanically power the CO excitation. The absence of a bright active galactic nucleus can be explained by a scenario wherecold gas accumulating on the starburst ring is exhausted as the fuel for star formationor is launched as an outflow before being able to feed to the nucleus.

No AGN evidence in NGC 1614 from deep radio VLBI observations

“…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). …”

“…The scenario of a powerful star formation ring surround-ing a non-active nucleus can be explained by a balance between the in-falling and out-falling gas or by the molecular gas exhaustion in the ring due to the intense star formation (Pérez-Torres et al 2010;Saito et al 2017), preventing the supermassive black hole from being fed. This is in agreement with the SF-powered molecular outflow found in NGC 1614 by García-Burillo et al (2015).…”

“…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).…”

Imaging the molecular outflows of the prototypical ULIRG NGC 6240 with ALMA

“…This is also supported by the maximum velocity of the CO (2-1) emission (FWZI/2 ∼ 1000 km s −1 ; Figure 1b), which is consistent with the maximum velocity of a warm molecular outflow traced by blueshifted OH absorption (∼1200 km s −1 ; Veilleux et al 2013) and Hα emission (≥1000 km s −1 ; Heckman et al 1990). As seen in other LIRGs (e.g., NGC 1068; García-Burillo et al 2014, NGC 3256;Sakamoto et al 2014, andNGC 1614;Saito et al 2017), molecular outflows are often spatially extended (a few kpc) and concomitant with (or entrained by) ionized gas outflow. Considering all evidences described above and some similarities with other LIRGs, we suggest that the four peaks of the broad CO (2-1) components trace molecular outflows in NGC 6240.…”

Circumnuclear Multi-phase Gas in the Circinus Galaxy. I. Non-LTE Calculations of CO Lines

“…In starburst regions, the local value of the FUV should vary over several orders of magnitude (Rosenberg et al 2014), e.g., G 0 = 100 − 10 5 , depending on the local structures of the ISM and the distance to the radiation sources. Assuming the observed star formation rate (SFR) in the Circinus galaxy (Hicks et al 2009), i.e., ∼ 100M yr −1 kpc −2 , and the relation between G 0 and SFR density in the IR-luminous merger NGC 1614 (Saito et al 2017), the FUV in Circinus can be estimated as G 0 = 10 2 − 10 3 on average (note that the scatter could be extremely large, ranging from G 0 = 10 to 10 6 ).…”