Discovery of an active supermassive black hole in the bulgeless galaxy NGC4561

Abstract

We present XMM-Newton observations of the Chandra-detected nuclear X-ray source in NGC4561. The hard X-ray spectrum can be described by a model composed of an absorbed power law with Γ= 2.5+0.4- 0.3 and column density NH = 1.9+0.1- 0.2 × 1022 atomscm-2. The absorption-corrected luminosity of the source is L(0.2-10.0keV) =2.5 × 1041ergs-1, with bolometric luminosity over 3 × 1042ergs-1. Based on the spectrum and the luminosity, we identify the nuclear X-ray source in NGC4561 to be an active galactic nucleus (AGN), with a black hole (BH) of mass M BH >2 × 104 M ⊙. The presence of a supermassive black hole at the center of this bulgeless galaxy shows that BH masses are not necessarily related to bulge properties, contrary to general belief. Observations such as these call into question several theoretical models of BH-galaxy coevolution 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 parameterized 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. © 2012. The American Astronomical Society. All rights reserved.