A Multi-Wavelength View of the Central Kiloparsec Region in the Luminous Infrared Galaxy NGC 1614

Abstract

The Luminous Infrared Galaxy NGC 1614 hosts a prominent circumnuclear ring of star formation. However, the nature of the dominant emitting mechanism in its central ∼ 100 pc is still under debate. We present sub-arcsecond angular resolution radio, mid-infrared, Paα, optical, and X-ray observations of NGC 1614, aimed at studying in detail both the circumnuclear ring and the nuclear region. The 8.4 GHz continuum emission traced by the Very Large Array (VLA) and the Gemini/T-ReCS 8.7 micron emission, as well as the Paα line emission, show remarkable morphological similarities within the star-forming ring, suggesting that the underlying emission mechanisms are tightly related. We used an HST /NICMOS Paα map of similar resolution to our radio maps to disentangle the thermal freefree and non-thermal synchrotron radio emission, from which we obtained the intrinsic synchrotron power-law for each individual region within the central kpc of NGC 1614. The radio ring surrounds a relatively faint, steep-spectrum source at the very center of the galaxy, suggesting that the central source is not powered by an AGN, but rather by a compact (r < ∼ 90 pc) starburst. Chandra X-ray data also show that the central kpc region is dominated by starburst activity, without requiring the existence of an AGN. We also used publicly available infrared data to model-fit the spectral energy distribution of both the starburst ring and a putative AGN in NGC 1614. In summary, we conclude that there is no need to invoke an AGN to explain the observed bolometric properties of the galaxy.

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

“…Numerical simulations suggest that the burst was triggered after a minor merger (Väisäinen et al 2012). 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.…”

No AGN evidence in NGC 1614 from deep radio VLBI observations

“…(iii) As found in Herrero-Illana et al (2014) based on the thermal vs non-thermal emission relation, regions A and B (see Fig. 1, top) are the most prone to harbor supernovae.…”

“…From the dozens of compact sources detected in Arp 299 (Pérez-Torres et al 2009;Bondi et al 2012), only eight are above our 5σ detection threshold (7.4 × 10 26 erg s −1 Hz −1 ) and have a SN origin. Furthermore, the supernova rate obtained through consistent SED multiwavelength modeling is higher for Arp 299 ( 0.8 SN yr −1 ; Mattila et al 2012) than for NGC 1614 ( 0.4 SN yr −1 ; Herrero-Illana et al 2014). Therefore, we would expect four SN detections in our observations.…”

“…In Figure 1 we show our VLA image of the circumnuclear ring of NGC 1614 with overlaid regions (for the formal definition of the regions, see Table 2 from Herrero-Illana et al 2014), together with the EVN observations presented in this study of the innermost nuclear region (N). We do not find any emission in that region above 5 × rms at any of the observed frequencies.…”

“…In Herrero-Illana et al (2014) we used multiwavelength observations aimed at characterizing the circumnuclear region, finding an average star formation rate (SFR) of 58 M yr −1 and a core-collapse SN rate of 0.4 SN yr −1 , obtained through spectral energy distribution (SED) modeling. We also disentangled the radio thermal free-free and nonthermal synchrotron emission to determine the age of the starburst, finding an age differentiation in the ring, with the starburst in the southeast regions being younger ( 5.5 Myr) than that in the northwest ( 8 Myr).…”