The Unusual Galaxy J2310-43: An Active Nucleus Without Optical Emission Lines and Without a Substantial Optical Continuum

X-ray, optical, and radio observations show the galaxy J2310-43 (Einstein Observatory Source Catalog number 4689, z = 0.0886) to be a very unusual object. Our analysis of ROSAT PSPC data for 2310-43 (23^h10^m41.7s, -43°47'38", J2000) indicates that the X-ray emission is extended spatially out to ~3.5 arcmin. Spectral analysis of the PSPC data finds that a simple power-law model with line-of-sight Galactic absorption can fit the observed PSPC energy distribution, as can two-component Raymond-Smith models, and composite power-law and Raymond-Smith models. Analysis of CCD optical images indicates that 2310-43 is located within a cluster that is probably Abell richness class 0, and a search of the NASA Extragalactic Database finds a 5 GHz radio source of 62 mJy associated with 2310-43.

While the data support a variety of interpretations, we note that 2310-43 differs significantly from clusters and groups of galaxies as well as from normal elliptical galaxies. We can interpret ~20% of the observed X-ray emission as an extended component emitting ~3.4 × 10⁴³ ergs s^-1 (0.1-2.4 keV), associated with the cluster around 2310-43. The bulk of the X-ray emission, corresponding to ~1.45 × 10⁴⁴ ergs s^-1, can then be associated with a nuclear point source consistent with the PSPC spatial and spectral data. However, the (B - V) color of 2310-43 is typical of a normal elliptical galaxy, making a BL Lac interpretation questionable, and the absence of emission lines in the optical spectrum rules out a standard quasar/Seyfert identification for the source.

Our data show that 2310-43 is most similar to the optically dull, X-ray, and radio-loud galaxy 3C 264 (first characterized as ``optically dull'' by Elvis et al.). The location of these two objects in the α_ro - α_ox, color-color diagram is at the extreme for BL Lac objects, implying a substantial deficiency in the optical continuum emission. We assume that the nuclear emissions from 2310-43 and 3C 264 are primarily nonthermal radiation, possibly associated with a nuclear jet. Then, we examine our observations in the context of several current models with the goal of understanding how a process (such as jet-disk symbiosis) can result in depressed optical continuum compared to most active galactic nuclei (AGNs) and BL Lac objects. We also discuss future observations (some already underway) that can shed additional light on these unusual sources.