Spatially resolved X-ray spectroscopy of the archetype type 2 active galactic nucleus NGC 1068 with Chandra

Abstract

Abstract We investigate spatial distributions of iron Kα (Fe-Kα) lines in the cental 100 pc of the Seyfert 2 galaxy NGC 1068 observed with Chandra. The spatial distributions of Fe-Kα lines, neutral and highly ionized, around the center of the galactic nucleus are not isotropic, as consistently confirmed in both image and spectral analyses. The hydrogen number density of the gas clouds responsible for the neutral Fe-Kα line emission is estimated to be 102–103 cm−3 for the sampled regions near the galactic core. The photo-ionization model, where iron is assumed to be ionized by X-rays from the galactic nucleus, yields ionization parameters lower than 19 for these clouds. The range of this ionization parameter is two or three orders of magnitude lower than the theoretically expected value to produce the observed helium-like Fe-Kα line intensities. Therefore, the photo-ionization model is excluded from the explanation of the amount of highly ionized iron that is responsible for the observed Fe-Kα lines. Also, we find anti-correlation in the spatial distributions between the molecular cloud in the area observed with ALMA and that of the Fe-Kα lines, including that from neutral iron. We suggest that X-ray iron-line and radio molecular cloud observations are complementary to probe the distribution of matters in the central regions around the cores of active galactic nuclei.