The most obscured AGN in the COSMOS field

¹ Dipartimento di Fisica e AstronomiaUniversità di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
e-mail: giolanzu@gmail.com
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Physics Department, University of Zagreb, Bijenicka cesta 32, 10002 Zagreb, Croatia
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁵ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (RM), Italy

Received: 6 March 2015
Accepted: 15 April 2015

Abstract

Highly obscured active galactic nuclei (AGN) are common in nearby galaxies, but are difficult to observe beyond the local Universe, where they are expected to significantly contribute to the black hole accretion rate density. Furthermore, Compton-thick (CT) absorbers (N_H ≳ 10²⁴ cm^-2) suppress even the hard X-ray (2−10 keV) AGN nuclear emission, and therefore the column density distribution above 10²⁴ cm^-2 is largely unknown. We present the identification and multi-wavelength properties of a heavily obscured (N_H ≳ 10²⁵ cm^-2), intrinsically luminous (L₂₋₁₀ > 10⁴⁴ erg s^-1) AGN at z = 0.353 in the COSMOS field. Several independent indicators, such as the shape of the X-ray spectrum, the decomposition of the spectral energy distribution and X-ray/[NeV] and X-ray/6 μm luminosity ratios, agree on the fact that the nuclear emission must be suppressed by a ≳10²⁵ cm^-2 column density. The host galaxy properties show that this highly obscured AGN is hosted in a massive star-forming galaxy, showing a barred morphology, which is known to correlate with the presence of CT absorbers. Finally, asymmetric and blueshifted components in several optical high-ionization emission lines indicate the presence of a galactic outflow, possibly driven by the intense AGN activity (L_Bol/L_Edd = 0.3−0.5). Such highly obscured, highly accreting AGN are intrinsically very rare at low redshift, whereas they are expected to be much more common at the peak of the star formation and BH accretion history, at z ~ 2−3. We demonstrate that a fully multi-wavelength approach can recover a sizable sample of such peculiar sources in large and deep surveys such as COSMOS.