Approximately 20–30 % of 1.4 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } z \lower 2.15 pt \hbox { $ % \buildrel < \over { \sim } $ } 2.5 galaxies with K _ { Vega } < 22 detected with Spitzer MIPS at 24 \mu m show excess mid-IR emission relative to that expected based on the rates of star formation measured from other multiwavelength data .
These galaxies also display some near-IR excess in Spitzer IRAC data , with a spectral energy distribution peaking longward of 1.6 \mu m in the rest frame , indicating the presence of warm-dust emission usually absent in star forming galaxies .
Stacking Chandra data for the mid-IR excess galaxies yields a significant hard X-ray detection at rest-frame energies > 6.2 keV .
The stacked X-ray spectrum rises steeply at > 10 keV , suggesting that these sources host Compton-thick Active Galactic Nuclei ( AGNs ) with column densities N _ { H } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { 24 } cm ^ { -2 } and an average , unobscured X-ray luminosity L _ { 2 - 8 keV } \approx ( 1 – 4 ) \times 10 ^ { 43 } erg s ^ { -1 } .
Their sky density ( \sim 3200 deg ^ { -2 } ) and space density ( \sim 2.6 \times 10 ^ { -4 } Mpc ^ { -3 } ) are twice those of X-ray detected AGNs at z \approx 2 , and much larger than those of previously-known Compton thick sources at similar redshifts .
The mid-IR excess galaxies are part of the long sought-after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background .
The fraction of mid-IR excess objects increases with galaxy mass , reaching \sim 50 –60 % for M \sim 10 ^ { 11 } M _ { \odot } , an effect likely connected with downsizing in galaxy formation .
The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z = 2 is similar to the mass ratio of black holes to stars in local spheroids , implying concurrent growth of both within the precursors of today ’ s massive galaxies .