Alma Observations of Warm Dense Gas in NGC 1614—breaking of the Star Formation Law in the Central Kiloparsec

Abstract

We present ALMA Cycle-0 observations of the CO (6-5) line emission and of the 435 μm dust continuum emission in the central kiloparsec of NGC 1614, a local luminous infrared galaxy at a distance of 67.8 Mpc (1 = 329 pc). The CO emission is well resolved by the ALMA beam (0. 26 × 0. 20) into a circumnuclear ring, with an integrated flux of f = 898 (±153) Jy km s −1 , which is 63(±12)% of the total CO (6-5) flux measured by Herschel. The molecular ring, located between 100 pc < r < 350 pc from the nucleus, looks clumpy and includes seven unresolved (or marginally resolved) knots with median velocity dispersion of ∼40 km s −1 . These knots are associated with strong star formation regions with Σ SFR ∼ 100 M yr −1 kpc −2 and Σ Gas ∼ 10 4 M pc −2 . The nondetections of the nucleus in both the CO (6-5) line emission and the 435 μm continuum rule out, with relatively high confidence, a Compton-thick active galactic nucleus in NGC 1614. Comparisons with radio continuum emission show a strong deviation from an expected local correlation between Σ Gas and Σ SFR , indicating a breakdown of the Kennicutt-Schmidt law on the linear scale of ∼100 pc.

High-resolution imaging of the molecular outflows in two mergers: IRAS 17208-0014 and NGC 1614

“…The recent 0. 5-CO(2-1) map of the SMA, published by König et al (2013), spatially resolved the nuclear disk into an asymmetric ring of ∼230 pc radius, similar to the CO(6-5) image of the central region obtained with ALMA by Xu et al (2015). The disk shows an extension to the northwest in the SMA image.…”

“…In their multiwavelength study, Herrero-Illana et al (2014) concluded that there is no evidence of an AGN in the nucleus of NGC 1614 down to an AGN-to-bolometric luminosity ratio of <10%. In agreement with this picture, not detecting the nuclear source in the 435 µm continuum image of the galaxy obtained with ALMA led Xu et al (2015) to rule out a Compton-thick AGN in NGC 1614.…”

“…Among the objects studied by García-Burillo et al (2012), NGC 1614 shows the most extreme (high) star formation efficiency of the dense molecular gas traced by the 1-0 line of HCN (S FE dense ≡ L FIR /L HCN = 3800 L /(K km s −1 pc 2 )). Furthermore, Xu et al (2015) find a breakdown of the Kennicutt-Schmidt (KS) law on the linear scale of 100 pc in the nuclear ring, based on the gas column densities derived from the CO(6-5) map.…”

“…The molecular gas in this source, imaged by the OVRO, SMA, CARMA, and ALMA interferometers, is distributed in a nuclear disk and a fainter extension (Wilson et al 2008;Olsson et al 2010;König et al 2013;Sliwa et al 2014;Xu et al 2015;Usero et al, in prep.). The recent 0.…”

No AGN evidence in NGC 1614 from deep radio VLBI observations

“…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).…”