Vla Detection of Rrls from the Radio Nucleus of Ngc 253 : Ionization by a Weak Agn, an Obscured Ssc or a Compact Snr ?

We have imaged the H92α and H75α 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 cm and mass is few thousand M⊙, which requires an ionizing flux of 6–20×10 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 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. Subject headings: galaxies: individual (NGC 253) — galaxies: ISM — galaxies: nuclei — galaxies: starburst — radio lines: galaxies