Astron. Astrophys. 355, L47-L51 (2000)

Letter to the Editor

Optically dim counterparts of hard X-ray selected AGNs ^*

R. Maiolino ¹, M. Salvati ¹, L.A. Antonelli ², A. Comastri ³, F. Fiore ², F. Ghinassi ⁴, R. Gilli ⁵, F. La Franca ⁶, F. Mannucci ⁷, G. Risaliti ⁵, D. Thompson ⁸ and C. Vignali <sup>3,9

1</sup> Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
³ Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Telescopio Nazionale Galileo, Apartado de Correos 565, 38700 Santa Cruz de La Palma, Canary Islands, Spain
⁵ Università di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Università "Roma Tre", Dipartimento di Fisica, via della Vasca Navale 84, 00146 Roma, Italy
⁷ CAISMI-CNR, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ California Institute of Technology, MS 320-47, Pasadena, CA, 91125, USA
⁹ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy

Received 1 February 2000 / Accepted 25 February 2000

Abstract

We present near-IR photometry and imaging observations of a small sample of sources identified in the BeppoSAX 5-10 keV survey (HELLAS) which resolves % of the X-ray background at these energies. The near-IR data are combined with optical spectra and photometry. Only 40% of the sources in our sample have the blue, power law continuum typical of color-selected QSOs. The remaining 60% are dominated by a galactic component which, on the basis of the continuum colors and shape, have ages ranging from 10⁹ to 10¹⁰ years. The images show that the blue QSOs are pointlike at our angular resolution, while all the other sources are extended, consistent with their spectral appearance and low redshift. Since down to R = 20 only about two thirds of the HELLAS sources have a counterpart, the preliminary HELLAS census comprises in roughly equal parts: i) blue QSOs (mostly at high redshifts); ii) optically dim, galaxy-dominated active nuclei (mostly at modest redshifts); and iii) empty fields (possibly highly absorbed QSOs at high redshifts).

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

* Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the CNAA (Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Send offprint requests to: R. Maiolino

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000

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