Open AccessDo nuclear starbursts obscure the X‐ray background?
Author(s) -
A. C. Fabian,
X. Barcons,
O. Almaini,
K. Iwasawa
Publication year - 1998
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1998.01645.x
Subject(s) - physics , astrophysics , eddington luminosity , galaxy , luminosity , accretion (finance) , astronomy , supernova , active galactic nucleus , star formation , sky , velocity dispersion , supermassive black hole
We propose a model for the source of the X‐ray background (XRB) in which low‐luminosity active nuclei ( L ∼ 10 43 erg s −1 ) are obscured ( N ∼ 10 23 cm −2 ) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X‐ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s −1 and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star‐forming cluster itself and the central black hole. A hot ( T ∼ 10 6 − 10 7 K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 10 7 M⊙ through Bondi accretion, at a sub‐Eddington rate appropriate for the luminosity of these objects. If starburst‐obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.