Active galactic nuclei in the mid-IR

Abstract

Aims. We study the evolution of the luminosity function (LF) of type-1 and type-2 Active Galactic Nuclei (AGN) in the mid-infrared, derive the contribution of the AGN to the Cosmic InfraRed Background (CIRB) and the expected source counts to be observed by Spitzer at 24 µm. Methods. We used a sample of type-1 and type-2 AGN selected at 15 µm (ISO) and 12 µm (IRAS), and classified on the basis of their optical spectra. Local spectral templates of type-1 and type-2 AGN have been used to derive the intrinsic 15 µm luminosities. We adopted an evolving smooth two-power law shape of the LF, whose parameters have been derived using an un-binned maximum likelihood method. Results. We find that the LF of type-1 AGN is compatible with a pure luminosity evolution. A small flattening of the faint (L 15 < L * 15 ) slope of the LF with increasing redshift is favoured by the data. A similar evolutionary scenario is found for the type-2 population with a rate k L ranging from ∼1.8 to 2.6, depending significantly on the adopted mid-infrared spectral energy distribution. For type-2 AGN a flattening of the LF with increasing redshift is suggested by the data, possibly caused by the loss of a fraction of type-2 AGN hidden within the optically classified starburst and normal galaxies. The type-1 AGN contribution to the CIRB at 15 µm is (4.2-12.1) × 10 −11 W m −2 sr −1 , while the type-2 AGN contribution is (5.5-11.0) × 10 −11 W m −2 sr −1 . We expect that Spitzer will observe, down to a flux limit of S 24 µm = 0.01 mJy, a density of ∼1200 deg −2 type-1 and ∼1000 deg −2 type-2 optically classified AGN. Conclusions. AGN evolve in the mid-infrared with a rate similar to the ones found in the optical and X-rays bands. The derived total contribution of the AGN to the CIRB (4-10%) and Spitzer counts should be considered as lower limits, because of a possible loss of type-2 sources caused by the optical classification.

AGN counts at 15$\mu{\rm m}$

“…Gruppioni et al 2005;Le Floc'h et al 2005;Pérez-González et al 2005;Babbedge et al 2006;Caputi et al 2007). On the contrary, the evolution of AGN at MIR wavelengths are similar to what has been measured in the optical and X-ray bands for type 1 AGN (AGN1) and even slower for type 2 AGN (Matute et al 2002(Matute et al , 2006.…”

“…3). In contrast, all the newly classified AGN2 have IR luminosities in the range 10 9.5 L < L 15 < 10 11 L , exactly in the region of the AGN2 LF where a flattening has been observed (Matute et al 2006). This result confirms the hypothesis of Matute et al (2006) that the observed flattening of the AGN2 LF at redshift ∼0.35 was due to an incompleteness in the AGN identification.…”

“…In contrast, all the newly classified AGN2 have IR luminosities in the range 10 9.5 L < L 15 < 10 11 L , exactly in the region of the AGN2 LF where a flattening has been observed (Matute et al 2006). This result confirms the hypothesis of Matute et al (2006) that the observed flattening of the AGN2 LF at redshift ∼0.35 was due to an incompleteness in the AGN identification. As previously pointed out by La Franca et al (2004), the origin of this incompleteness has to be ascribed to the fact that a relevant fraction of AGN2 do not show clear AGN signatures in their optical spectra.…”

“…As discussed by Matute et al (2006), the absence of AGN2 in this luminosity range is mainly due to selection effects. The AGN1 SEDs have a fairly constant infrared to optical ratio, while for AGN2 the infrared-to-optical ratio increases with the increase in the luminosity (Log (L 15 /L R ) ∝ 0.5 Log L 15 ).…”

“…As a consequence, AGN2 have on average fainter optical counterparts than AGN1 at high luminosities and then miss the spectroscopic identification (see Fig. 1 in Matute et al 2006).…”