Herschel FIR counterparts of selected Ly-alpha emitters at z~2.2. Fast
  evolution since z~3 or missed obscured AGNs?
  Ly-alpha emitters (LAEs) are seen everywhere in the redshift domain from
local to z~7. Far-infrared (FIR) counterparts of LAEs at different epochs could
provide direct clues on dust content, extinction, and spectral energy
distribution (SED) for these galaxies. We search for FIR counterparts of LAEs
that are optically detected in the GOODS-North field at redshift z~2.2 using
data from the Herschel Space Telescope with the Photodetector Array Camera and
Spectrometer (PACS). The LAE candidates were isolated via color-magnitude
diagram using the medium-band photometry from the ALHAMBRA Survey, ancillary
data on GOODS-North, and stellar population models. According to the fitting of
these spectral synthesis models and FIR/optical diagnostics, most of them seem
to be obscured galaxies whose spectra are AGN-dominated. From the analysis of
the optical data, we have observed a fraction of AGN or composite over source
total number of ~0.75 in the LAE population at z~2.2, which is marginally
consistent with the fraction previously observed at z=2.25 and even at low
redshift (0.2<z<0.45), but significantly different from the one observed at
redshift ~3, which could be compatible either with a scenario of rapid change
in the AGN fraction between the epochs involved or with a non detection of
obscured AGN in other z=2-3 LAE samples due to lack of deep FIR observations.
We found three robust FIR (PACS) counterparts at z~2.2 in GOODS-North. This
demonstrates the possibility of finding dust emission in LAEs even at higher
redshifts.
