XMM-newton observations of the dwarf elliptical galaxy NGC 3226

Abstract

We report on an XMM-Newton observation of the dwarf elliptical galaxy NGC 3226 performed in November 2000. The analysis of the 0.4-10 keV spectrum of its nucleus is consistent with a power law continuum (Γ ≈ 1.96) absorbed at low energies by neutral gas (with a hydrogen column density NH ≈ 3-8×1021 cm-2 and a covering fraction greater than 85%) and by weakly ionized gas (NW ≈ 4-6×1021 cm-2 with ξ ≈ 1-7 erg s-1 cm). However, the study indicates that a bremstrahlung model absorbed by neutral material is a better description reminiscent of the X-ray emission from advection-dominated accretion flows (ADAFs). The temperature of the best fit bremstrahlung model (T ≈ 108 K) and the absence of short and mid time-scale variability suggests that the X-ray emission originates from regions relatively far from the nucleus as in convection-dominated accretion flows (CDAFs) or in the so-called "wind" models. By comparing the 2-10 keV luminosity of the central object (LX = 2.58×1040 erg s-1 after correction from Galactic and intrinsic absorption) with radio flux measurements, we find a mass in the range (1.7-50) ×107 M⊙ for the accreting black hole, a value comparable with an independent estimate from the dispersion of radial velocities. We argue that the dwarf elliptical galaxy NGC 3226 may harbor a sub-Eddington, super-massive black hole in a radiative inefficient state which is distinct from the near-Eddington, thermally dominated black holes presumably found in quasars and luminous Seyfert galaxies.