Radio continuum and X-ray emission from the most extreme far-IR-excess galaxy NGC 1377: An extremely obscured AGN revealed

<p>Context. Galaxies which strongly deviate from the radio-far infrared (FIR) correlation are of great importance for studies of galaxy evolution as they may be tracing early, short-lived stages of starbursts and active galactic nuclei (AGNs). The most extreme FIR-excess galaxy NGC 1377 has long been interpreted as a young dusty starburst, but millimeter observations of CO lines revealed a powerful collimated molecular outflow which cannot be explained by star formation alone.<br /> Aims: This paper aims to determine the nature of the energy source in the nucleus of NGC 1377 and to study the driving mechanism of the collimated CO outflow.<br /> Methods: We present new radio observations of NGC 1377 at 1.5 and 10 GHz obtained with the Jansky Very Large Array (JVLA) and Chandra X-ray observations. The observations are compared to synthetic starburst models to constrain the properties of the central energy source.<br /> Results: We obtained the first detection of the cm radio continuum and X-ray emission in NGC 1377. We found that the radio emission is distributed in two components, one on the nucleus and another offset by 4.5 arcsec to the south-west. We confirm the extreme FIR-excess of the galaxy, with a q_FIR ≃ 4.2, which deviates by more than 7σ from the radio-FIR correlation. Soft X-ray emission is detected on the off-nucleus component. From the radio emission we estimated for a young (〈10 Myr) starburst a star formation rate (SFR) of 〈0.1 M_☉ yr^-1. Such a SFR is not sufficient to power the observed IR luminosity and to drive the CO outflow.<br /> Conclusions: We found that a young starburst cannot reproduce all the observed properties of the nucleus of NGC 1377. We suggest that the galaxy may be harboring a radio-quiet, obscured AGN of 10⁶M_☉, accreting at near-Eddington rates. We speculate that the off-nucleus component may be tracing an hot-spot in the AGN jet.</p>