More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys .
We present a study based on our co-added image stacking technique , in which our aim is to detect the optical emission from unresolved , isolated radio sources of the Very Large Array ( VLA ) Faint Images of the Radio Sky at Twenty-cm ( FIRST ) survey that have no identified optical counterparts in the Sloan Digital Sky Survey ( SDSS ) Stripe 82 co-added data set .
From the FIRST catalogue , 2116 such radio point sources were selected , and cut-out images , centred on the FIRST coordinates , were generated from the Stripe 82 images .
The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio , in the hope that optical emission from the radio sources would become detectable .
Multiple stacks were generated , based on the radio luminosity of the point sources .
The resulting stacked images show central peaks similar to point sources .
The peaks have very red colours with steep optical spectral energy distributions .
We have found that the optical spectral index \alpha _ { \nu } falls in the range -2.9 \leq \alpha _ { \nu } \leq - 2.2 ( S _ { \nu } \propto \nu ^ { \alpha _ { \nu } } ) , depending only weakly on the radio flux .
The total integration times of the stacks are between 270 and 300 h , and the corresponding 5 \sigma detection limit is estimated to be about m _ { r } \simeq 26.6 mag .
We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei .
Dust-reddened quasars might represent an early phase of quasar evolution , and thus they can also give us an insight into the formation of massive galaxies .
The data used in the paper are available on-line at http : //www.vo.elte.hu/doublestacking .