Discovery of an active supermassive black hole in the bulge-less galaxy NGC 4561

Abstract

We present XMM-Newton observations of the Chandra-detected nuclear X-ray source in NGC 4561. The hard X-ray spectrum can be described by a model composed of an absorbed power-law with Gamma= 2.5^{+0.4}_{-0.3}, and column density N_H=1.9^{+0.1}_{-0.2} times 10^{22} atoms cm^{-2}. The absorption corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5 times 10^{41} ergs s^{-1}, with bolometric luminosity over 3 \times 10^{42} ergs s^{-1}. Based on the spectrum and the luminosity, we identify the nuclear X-ray source in NGC 4561 to be an AGN, with a black hole of mass M_BH > 20,000 solar masses. The presence of a supermassive black hole at the center of this bulge-less galaxy shows that black hole masses are not necessarily related to bulge properties, contrary to the general belief. Observations such as these call into question several theoretical models of BH--galaxy co-evolution that are based on merger-driven BH growth; secular processes clearly play an important role. Several emission lines are detected in the soft X-ray spectrum of the source which can be well parametrized by an absorbed diffuse thermal plasma with non-solar abundances of some heavy elements. Similar soft X-ray emission is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs, suggesting an origin in the circumnuclear plasma.