Research Note

The ALHAMBRA survey: Discovery of a faint QSO at z = 5.41

¹ Instituto de Astrofísica de Andalucía (CSIC) Glorieta de la Astronomía s/n 18008 Granada Spain
e-mail: [matute;isabel;pepa;cesar;chony;jaime;mpovic;emilio;benitez;mcs;rosa;amb;fprada;quintana]@iaa.es
² Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
e-mail: fsoto@ifca.unican.es
³ Unidad Asociada Observatori Astronòmic (IFCA – Universitat de València), Valencia, Spain
⁴ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), 44001 Teruel, Spain
e-mail: [moles;dch]@cefca.es
⁵ Instituto de Astrofísica de Canarias, La Laguna, 38200 Tenerife, Spain
e-mail: jalfonso@iac.es
⁶ Observatrio Nacional-MCT, CEP 20921-400, Rio de Janeiro - RJ, Brazil
e-mail: villegas@on.br
⁷ School of Physics and Astronomy, Tel Aviv University, Israel
e-mail: tjb@wise.tau.ac.il
⁸ Facultad de Física. Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, 41012 Sevilla, Spain
e-mail: jcc-famn@us.es
⁹ Institut de Ciències de l’Espai, IEEC-CSIC, 08193 Barcelona, Spain
e-mail: fjc@ieec.fcr.es
¹⁰ Departamento de Astrofísica, Facultad de Física, Universidad de la Laguna, Spain
e-mail: jcn@iac.es
¹¹ Departamento de Astronomía, Pontificia Universidad Católica, 782-0436 Macul, Santiago, Chile
e-mail: linfante@astro.puc.cl
¹² Departament d’Astronomía i Astrofísica, Universitat de València, 46100 Valencia, Spain
e-mail: vicent.martinez@uv.es
¹³ Observatori Astronòmic de la Universitat de València, 46071 Valencia, Spain

Received: 19 May 2013
Accepted: 12 July 2013

Abstract

Aims. We aim to illustrate the potentiality of the Advanced Large, Homogeneous Area, Medium-Band Redshift Astronomical (ALHAMBRA) survey to investigate the high-redshift universe through the detection of quasi stellar objects (QSOs) at redshifts higher than 5.

Methods. We searched for QSOs candidates at high redshift by fitting an extensive library of spectral energy distributions – including active and non-active galaxy templates, as well as stars – to the photometric database of the ALHAMBRA survey (composed of 20 optical medium-band plus the 3 broad-band JHK_s near-infrared filters).

Results. Our selection over ≈1 square degree of ALHAMBRA data (~1/4 of the total area covered by the survey), combined with GTC/OSIRIS spectroscopy, has yielded identification of an optically faint QSO at very high redshift (z = 5.41). The QSO has an absolute magnitude of ~−24 at the 1450 Å continuum, a bolometric luminosity of ≈2 × 10⁴⁶ erg s^-1, and an estimated black hole mass of ≈10⁸ M_⊙. This QSO adds itself to a reduced number of known UV faint sources at these redshifts. The preliminary derived space density is compatible with the most recent determinations of the high-z QSO luminosity functions. This new detection shows how ALHAMBRA, as well as forthcoming well-designed photometric surveys, can provide a wealth of information on the origin and early evolution of this kind of object.