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.