Probing nuclear activity versus star formation at z~0.8 using
  near-infrared multi-object spectroscopy
  We present near-infrared (NIR) spectroscopic observations of 28 X-ray and
mid-infrared selected sources at a median redshift of z~0.8 in the Extended
Groth Strip (EGS). To date this is the largest compilation of NIR spectra of
active galactic nuclei (AGN) at this redshift. The data were obtained using the
multi-object spectroscopic mode of the Long-slit Intermediate Resolution
Infrared Spectrograph (LIRIS) at the 4.2m William Herschel Telescope (WHT).
These galaxies are representative of a larger sample studied in a previous
work, consisting of over a hundred X-ray selected sources with mid-infrared
counterparts, which were classified either as AGN-dominated or host
galaxy-dominated, depending on the shape of their spectral energy distributions
(SEDs). Here we present new NIR spectra of 13 and 15 sources of each class
respectively. We detect the H alpha line at > 1.5 sigma above the continuum for
the majority of the galaxies. Using attenuation-corrected H alpha luminosities
and observed Spitzer/MIPS 24 micron fluxes, and after subtracting an AGN
component estimated using an AGN empirical correlation and multi-frequency SED
fits, we obtain average star formation rates (SFRs) of 7+/-7 and 20+/-50
Msun/yr respectively (median SFRs = 7 and 5 Msun/yr). These values are lower
than the SFRs reported in the literature for different samples of non-active
star-forming galaxies of similar stellar masses and redshifts (M* ~ 10^11 Msun
and z~1). In spite of the small size of the sample studied here, as well as the
uncertainty affecting the AGN-corrected SFRs, we speculate with the possibility
of AGN quenching the star formation in galaxies at z~0.8. Alternatively, we
might be seeing a delay between the offset of the star formation and AGN
activity, as observed in the local universe.
