Astronomy and Astrophysics, volume 516, A9-9 (2010/6-2)
X-ray imaging of the ionisation cones in NGC 5252.
DADINA M., GUAINAZZI M., CAPPI M., BIANCHI S., VIGNALI C., MALAGUTI G. and COMASTRI A.
Abstract (from CDS):
The physical conditions of the gas forming the narrow-line regions (NLR) in active galactic nuclei (AGN) have been extensively studied in the optical band. Recently detailed X-ray studies have shown how the emission in the 0.1-2keV band detected in Seyfert 2 galaxies is associated to gas lying close to or associated with the NLR. We take advantage of the spectacular extension (∼15'') of the NLR in the type II Seyfert galaxy NGC 5252 and of the complementary characteristics of XMM-Newton and Chandra to investigate the physical conditions of the gas in this galaxy. The X-ray data from XMM-Newton are used to define the spectral properties of the ionising nuclear source. The Chandra data are used to trace the spatial characteristics of the soft X-ray emission. This information is then compared to the optical HST characteristics of the NLR in NGC 5252. The X-ray spectrum of the nucleus of NGC 5252 is intrinsically flat (Γ∼1.4-1.5) and absorbed by neutral gas with a column density NH∼1022cm–2. Below ∼1keV a soft excess is detected. The high-resolution spectrum obtained with the XMM-Newton RGS shows emission lines in the 0.2-1.5keV range which strongly indicate that the soft X-ray component is essentially due to ionised gas. Moreover, the soft X-ray emission is spatially resolved around the nucleus and well overlaps the images obtained in narrow optical bands centred around the [Oiii] emission line at 5007 Å. The [Oiii]/soft-X flux ratios along the ionisation cones are basically constant. This indicates that the electron density does not significantly deviate from the r–2 law (constant ionisation parameter) moving outward from the nucleus. This result combined with previous optical studies suggests two plausible but different scenarios in the reconstruction of the last ∼30000 years of history of the central AGN. The most promising one is that the source is indeed a ``quasar relic'' with a steady and inefficient energy release from the accretion of matter onto the central super-massive black-hole. This scenario is suggested also by the flat nuclear X-ray spectrum that evokes an advection dominated accretion flow (ADAF) like emission mechanism.
galaxies: active - galaxies: individual: NGC 5252 - galaxies: Seyfert - X-ray: galaxies