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Astron. Astrophys. 360, 457-462 (2000)


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Sub-mm and X-ray background: Two unrelated phenomena?

P. Severgnini 1, R. Maiolino 2, M. Salvati 2, D. Axon 3, A. Cimatti 2, F. Fiore 4, R. Gilli 1,5, F. La Franca 6, A. Marconi 2, G. Matt 6, G. Risaliti 1 and C. Vignali 7,8

1 Università di Firenze, Dipartimento di Astronomia, Largo E. Fermi 5, 50125 Firenze, Italy
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3 University of Hertfordshire, Department of Physical Sciences, College Lane, Hatfield, Herts AL10 8 AB, UK
4 Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Monteporzio Catone, Italy
5 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Postsdam, Germany
6 Università degli Studi "Roma Tre", Dipartimento di Fisica, Via della Vasca Navale 84, 00146 Roma, Italy
7 Università di Bologna, Dipartimento di Astronomia, Via Ranzani 1, 40127 Bologna, Italy
8 Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy

Received 30 March 2000 / Accepted 13 June 2000

Abstract

Obscured AGNs are thought to contribute a large fraction of the hard X-ray background (2-10 keV), and have also been proposed as the powerhouse of a fraction of the SCUBA sources which make most of the background at 850µm, thus providing a link between the two spectral windows. We have tackled this issue by comparing data at 2-10 keV and at 850µm for a sample of 34 sources at fluxes (or limiting fluxes) which resolve most of the background in the two bands. We present here new SCUBA observations, and new correlations between separate data sets retrieved from the literature. Similar correlations presented by others are added for completeness. None of the 11 hard X-ray (2-10 keV) sources has a counterpart at 850 µm, with the exception of a Chandra source in the SSA13 field, which is a candidate type 2, heavily absorbed QSO at high redshift. The ratios [FORMULA] (mostly upper limits) of the X-ray sources are significantly lower than the value observed for the cosmic background. In particular, we obtain that 2-10 keV sources brighter than [FORMULA], which make at least 75% of the background in this band, contribute for less than 7% to the submillimeter background. Out of the 24 SCUBA sources, 23 are undetected by Chandra. The ratios [FORMULA] (mostly lower limits) of these SCUBA sources indicate that most of them must be powered either by starburst activity, or by an AGN which is obscured by a column [FORMULA], with a reflection efficiency in the hard X rays significantly lower than 1% in most cases. However, AGNs of this type could not contribute significantly to the 2-10 keV background.

Key words: galaxies: active – submillimeter – infrared: galaxies – X-rays: galaxies – cosmology: diffuse radiation

Send offprint requests to: P. Severgnini (paolas@arcetri.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000

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