Sub-arcsec mid-IR observations of NGC 1614: Nuclear star-formation or an intrinsically X-ray weak AGN?

TL;DR

This study uses high-resolution mid-infrared observations to investigate the nuclear activity and star formation in NGC 1614, finding evidence for a weak AGN and analyzing star formation rate tracers at small scales.

Contribution

It provides the first detailed mid-IR spectral analysis of NGC 1614's nucleus and circumnuclear ring, exploring the nature of its nuclear activity and comparing multiple SFR tracers.

Findings

The nucleus shows characteristics consistent with a weak or obscured AGN.

Star formation rate estimates vary significantly depending on the tracer used.

The nuclear luminosity is less than 5% of the galaxy's total bolometric luminosity.

Abstract

We present new mid-infrared N-band spectroscopy and Q-band photometry of the local luminous infrared galaxy NGC1614, one of the most extreme nearby starbursts. We analyze the mid-IR properties of the nucleus (central 150 pc) and four regions of the bright circumnuclear (diameter~600 pc) star-forming (SF) ring of this object. The nucleus differs from the circumnuclear SF ring by having a strong 8-12 micron continuum (low 11.3 micron PAH equivalent width). These characteristics, together with the nuclear X-ray and sub-mm properties, can be explained by an X-ray weak active galactic nucleus (AGN), or by peculiar SF with a short molecular gas depletion time and producing an enhanced radiation field density. In either case, the nuclear luminosity (L(IR) < 6e43 erg/s) is only <5% of the total bolometric luminosity of NGC1614. So this possible AGN does not dominate the energy output in this object. We also compare three star-formation rate (SFR) tracers (Pa$\alpha$, 11.3 micron PAH, and 24 micron emissions) at 150 pc scales in the circumnuclear ring. In general, we find that the SFR is underestimated (overestimated) by a factor of 2-4 (2-3) using the 11.3 micron PAH (24 micron) emission with respect to the extinction corrected Pa$\alpha$ SFR. The former can be explained because we do not include diffuse PAH emission in our measurements, while the latter might indicate that the dust temperature is particularly warmer in the central regions of NGC1614.