I present a generalized power-law diagnostic that allows to identify the presence of active galactic nuclei ( AGN ) in infrared ( IR ) galaxies at z > 1 , down to flux densities at which the extragalactic IR background is mostly resolved .
I derive this diagnostic from the analysis of 174 galaxies with S _ { \nu } ( 24 \mu m ) > 80 \mu Jy and spectroscopic redshifts z _ { spec } > 1 in the Chandra Deep Field South , for which I study the rest-frame UV/optical/near-IR spectral energy distributions ( SEDs ) , after subtracting a hot-dust , power-law component with three possible spectral indices \alpha = 1.3 , 2.0 and 3.0 .
I obtain that 35 % of these 24 \mu m sources are power-law composite galaxies ( PLCGs ) , which I define as those galaxies for which the SED fitting with stellar templates , without any previous power-law subtraction , can be rejected with > 2 \sigma confidence .
Subtracting the power-law component from the PLCG SEDs produces stellar-mass correction factors < 1.5 in > 80 \% of cases .
The PLCG incidence is especially high ( 47 % ) at 1.0 < z < 1.5 .
To unveil which PLCGs host AGN , I conduct a combined analysis of 4Ms X-ray data , galaxy morphologies , and a greybody modelling of the hot dust .
I find that : 1 ) 77 % of all the X-ray AGN in my 24 \mu m sample at 1.0 < z < 1.5 are recognised by the PLCG criterion ; 2 ) PLCGs with \alpha = 1.3 or 2.0 have regular morphologies and T _ { dust } \raisebox { -3.698858 pt } { ~ { } \shortstack { $ > $ \ [ -0.07 cm ] $ \sim$ } } ~ { } 1000 K , indicating nuclear activity .
Instead , PLCGs with \alpha = 3.0 are characterised by disturbed galaxy dynamics , and a hot interstellar medium can explain their dust temperatures T _ { dust } \sim 700 - 800 K .
Overall , my results indicate that the fraction of AGN among 24 \mu m sources is between \sim 30 % and 52 % at 1.0 < z < 1.5 .