Very Large Array Detection of Radio Recombination Lines from the Radio Nucleus of NGC 253: Ionization by a Weak Active Galactic Nucleus, an Obscured Super Star Cluster, or a Compact Supernova Remnant?

Abstract

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 −1 . Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ∼10 4 cm −3 and mass is few thousand M ⊙ , which requires an ionizing flux of 6-20×10 51 photons s −1 . 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.

ALMA observations of 99 GHz free–free and H40α line emission from star formation in the centre of NGC 253

“…It is unlikely that an active galactic nucleus is present, as the H40α line would appear > ∼ 1000 km s −1 in width. We do not detect any asymmetry in the line emission from the central source as had been reported in some prior recombination line observations (Mohan et al 2002;Kepley et al 2011) but not others. The reason for this discrepancy is unclear, although one possibility is that the line shapes differ because the lower-frequency lines are probing less dense gas than the H40α line.…”

“…Seaquist & Bell 1977;Mebold et al 1980;Anantharamaiah & Goss 1996;Puxley et al 1997;Mohan et al 2002Mohan et al , 2005Rodríguez-Rico et al 2006;Kepley et al 2011). This includes the detection of H40α emission by Puxley et al (1997).…”

PAH emission variations within the resolved starbursts of NGC 253 and NGC 1808

“…However, the lack of detected source variability at radio wavelengths over 10 year timescales implies that if it is a SNR it is likely not expanding (Ulvestad & Antonucci 1997). Further, radio observations of the nucleus have detected both H 2 O maser emission (Nakai et al 1995) and broad (∆V ∼ 200 km s −1 ) H92α and H75α recombination lines (Mohan et al 2002). Although the latter can be interpreted in the context of a young compact stellar cluster Mohan et al favour a low luminosity AGN model in light of the radio continuum observations of Turner & Ho (1985) and Ulvestad & Antonucci (1997).…”

H2O Maser Emission in the Starburst Galaxy NGC 253

“…Turner (1985) reported OH maser emission tracing a collimated outflow from the nucleus, and Turner & Ho (1985) measured a 2 cm brightness temperature of !9 ; 10 4 K for the central radio continuum source. Mohan et al (2002) reported the detection of hydrogen radio recombination lines from the nucleus that are best explained by an AGN as the ionizing source. The hard X-ray spectrum measured by Weaver et al (2002) at the position of the nucleus is also best explained by a black hole surrounded by a dusty molecular torus 150 pc in size.…”