Radio continuum and X-ray emission from the most extreme far-IR-excess galaxy NGC 1377

An extremely obscured AGN revealed

¹ Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: costagli@chalmers.se
² INAF−Istituto di Radioastronomia – Italian ARC, via Gobetti 101, 40129 Bologna, Italy
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, s/n, 18008 Granada, Spain
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30 HA, UK
⁵ Visiting Scientist: Departamento de Física Teorica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain

Received: 25 May 2016
Accepted: 8 July 2016

Abstract

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.

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.

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.

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 45 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.

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.