VLA Detection of RRLs from the radio nucleus of NGC 253 : Ionization by a weak AGN, an obscured SSC or a compact SNR ?

TL;DR

This study uses radio recombination line imaging to investigate the ionizing source in NGC 253's nucleus, favoring an AGN with a hybrid accretion disk over supernova remnants or young star clusters.

Contribution

It provides the first detailed RRL imaging of NGC 253's nucleus and proposes a hybrid ADAF and thin disk model as the ionizing source, challenging previous star cluster or SNR hypotheses.

Findings

RRL emission peaks at the unresolved radio nucleus.

Ionization likely caused by an AGN with a hybrid accretion disk.

Star cluster and SNR models are inconsistent with observed data.

Abstract

We have imaged the H92alpha and H75alpha radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of ~4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed towards the nucleus are extremely wide, with FWHM of ~200 km /s. Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ~10^4 \cc and mass is few thousand Msun, which requires an ionizing flux of (6-20)x10^{51} photons /s. We consider a SNR expanding in a dense medium, a star cluster and also an AGN as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model was considered and the dynamics of the ionized gas in a stellar-wind driven structure was investigated. Such a model is consistent with the properties of the ionized gas only for a cluster younger than ~10^5 years. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it was shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of a inner ADAF disk and an outer thin disk is suggested, which could explain the radio, UV and the X-ray luminosities of the nucleus.