AChandraObservation of M51: Active Nucleus and Nuclear Outflows

Abstract

We present a Chandra Advanced CCD Imaging Spectrometer (ACIS-S) observation of the nuclear region of the nearby spiral galaxy M51 (NGC 5194), which has a low-luminosity Seyfert 2 nucleus. The X-ray image shows the nucleus, southern extranuclear cloud, and northern loop, the morphology of the extended emission being very similar to those seen in radio continuum and optical emission-line images. The X-ray spectrum of the nucleus is well represented by a model consisting of soft thermal plasma with kT ? 0.5 keV, a very hard continuum, and an Fe K? emission line at 6.45 keV with an equivalent width of greater than 2 keV. The very strong Fe line and the flat continuum indicate that the nucleus is obscured by a column density in excess of 1024 cm-2 and that the spectrum is dominated by reflected emission from cold matter near the nucleus. The X-ray spectra of the extranuclear clouds are well fitted by a thermal plasma model with kT ? 0.5 keV. This spectral shape and morphology strongly suggest that the clouds are shock-heated by the bipolar outflow from the nucleus. The shock velocities of the extranuclear cloud and northern loop inferred from the temperatures of the X-ray gas are 690 and 660 km s-1, respectively. By assuming a steady state situation in which the emission of the extranuclear clouds is powered by the jets, the mechanical energy in the jets is found to be comparable to the bolometric luminosity of the nucleus.