We introduce the Mass-Excitation (MEx) diagnostic to identify active galactic nuclei (AGN) in galaxies at intermediate redshift. In the absence of near-infrared spectroscopy, necessary to use traditional nebular line diagrams at z>0.4, we demonstrate
that combining [OIII]5007/Hbeta and stellar mass successfully distinguishes between star formation and AGN emission. The MEx classification scheme relies on a novel probabilistic approach splitting galaxies into sub-categories with more confidence than alternative high-z diagnostic diagrams. It recognizes that galaxies near empirical boundaries on traditional diagrams have an uncertain classification and thus a non-zero probability of belonging to more than one category. An outcome of this work is a system of statistical weights that can be used to compute global properties of galaxy samples. We apply the MEx diagram to 2,812 galaxies at 0.3<z<1 in the Great Observatories Origins Deep Survey North and Extended Groth Strip fields, and compare it to an independent X-ray classification scheme. We identify Compton-thick AGN candidates with large X-ray absorption, which we infer from the luminosity ratio between hard X-ray emission and [OIII]5007, a nearly isotropic tracer of AGN. X-ray stacking of sources that were not detected individually supports the validity of the MEx diagram and yields a very flat spectral slope for the Compton-thick candidates (Gamma~0.4, unambiguously indicating absorbed AGN). We present evidence that composite galaxies, which are difficult to identify with alternative high-redshift diagrams, host the majority of the highly-absorbed AGN. Our findings suggest that the interstellar medium of the host galaxy provides significant absorption in addition to the torus invoked in AGN unified models.
We report results from a pilot program to obtain spectroscopy for objects detected in the Galaxy Evolution Explorer (GALEX) Medium Imaging Survey (MIS). Our study examines the properties of galaxies detected by GALEX fainter than the Sloan Digital Sk
y Survey (SDSS) spectroscopic survey. This is the first study to extend the techinques of Salim et al. 2005 to estimate stellar masses, star formation rates (SFR) and the b (star formation history) parameter for star-forming galaxies out to z~0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV=23.9 (AB mag), having counterparts in the SDSS Data Release 4 (DR4). Of our sample, 43 are starforming galaxies with z<0.7, 3 have emission line ratios indicative of AGN with z<0.7, and 4 objects with z~1 are QSOs, 3 of which are not previously cataloged. We compare our sample to a much larger sample of ~50,000 matched GALEX/SDSS galaxies with SDSS spectroscopy; while our survey is shallow, the optical counterparts to our sources reach ~3 magnitudes fainter in SDSS r magnitude than the SDSS spectroscopic sample. We use emission line diagnostics for the galaxies to determine that the sample contains mostly star-forming galaxies. The galaxies in the sample populate the blue sequence in the NUV-r vs M_r color-magnitude diagram. Our sample has SFRs, luminosities, and velocity dispersions that are similar to the samples of faint compact blue galaxies studied previously in the same redshift range by Koo et. al 1995, Guzman et. al 1996 & Phillips et. al 1997. However, our sample is ~2 mag fainter in surface brightness than the compact blue galaxies. We find that the star-formation histories for a majority of the galaxies are consistent with a recent starburst within the last 100 Myr.
We measure star formation rates of ~50,000 optically-selected galaxies in the local universe (z~0.1), spanning a range from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS (optical) photometry
to a library of population synthesis models that include dust attenuation. For star-forming galaxies, our UV-based SFRs compare remarkably well with those derived from SDSS H alpha. Deviations from perfect agreement between these two methods are due to differences in the dust attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for galaxies with weak or no H alpha emission, and where H alpha is contaminated with an emission from an AGN. We use full-SED SFRs to calibrate a simple prescription that uses GALEX UV magnitudes to produce good SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and for (3) galaxies without H alpha emission. We find that the three have distinct star formation histories, with AGN lying intermediate between the star-forming and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. Weak AGN, while also massive, have lower SFR, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent (red sequence) galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at z=0.1 with smaller total error than previous measurements (abridged).
