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.