Letter to the Editor

Herschel FIR counterparts of selected Lyα emitters at z ~ 2.2^*,**

Fast evolution since z ~ 3 or missed obscured AGNs?

¹ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain e-mail: bongio@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
³ Herschel Science Centre (ESAC). Villafranca del Castillo, Spain
⁴ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
⁸ School of Physics and Astronomy, Tel Aviv University, Israel
⁹ Departamento de Física Atómica, Molecular y Nuclear, Facultad de Física, Universidad de Sevilla, Spain
¹⁰ Institut de Ciències de l'Espai (CSIC), Barcelona, Spain
¹¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹² Centro de Estudios de Física del Cosmos de Aragón, CEFCA, 44001 Teruel, Spain
¹³ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹⁴ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
¹⁵ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁶ Departamento de Astronomía, Pontificia Universidad Católica, Santiago, Chile
¹⁷ Departament d'Astronomía i Astrofísica, Universitat de València, València, Spain
¹⁸ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy
¹⁹ Centro Astronómico Hispano-Alemán, Almería, Spain

Received: 2 April 2010
Accepted: 3 August 2010

Abstract

Lyα emitters (LAEs) are seen everywhere in the redshift domain from local to z ~ 7. Far-infrared (FIR) counterparts of LAEs at different epochs could provide direct clues on dust content, extinction, and spectral energy distribution (SED) for these galaxies. We search for FIR counterparts of LAEs that are optically detected in the GOODS-North field at redshift z ~ 2.2 using data from the Herschel Space Telescope with the Photodetector Array Camera and Spectrometer (PACS). The LAE candidates were isolated via color-magnitude diagram using the medium-band photometry from the ALHAMBRA Survey, ancillary data on GOODS-North, and stellar population models. According to the fitting of these spectral synthesis models and FIR/optical diagnostics, most of them seem to be obscured galaxies whose spectra are AGN-dominated. From the analysis of the optical data, we have observed a fraction of AGN or composite over source total number of ~0.75 in the LAE population at z ~ 2.2, which is marginally consistent with the fraction previously observed at z = 2.25 and even at low redshift (0.2 < z < 0.45), but significantly different from the one observed at redshift ~3, which could be compatible either with a scenario of rapid change in the AGN fraction between the epochs involved or with a non detection of obscured AGN in other z = 2–3 LAE samples due to lack of deep FIR observations. We found three robust FIR (PACS) counterparts at z ~ 2.2 in GOODS-North. This demonstrates the possibility of finding dust emission in LAEs even at higher redshifts.