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Register a new userThe OPTX Project V: Identifying Distant AGNs
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The Baldwin, Phillips, and Terlevich emission-line ratio diagnostic ([OIII]/H{beta} versus [NII]/H{alpha}, hereafter BPT diagram) efficiently separates galaxies whose signal is dominated by star formation <BPT-SF> from those dominated by AGN activity (BPT-AGN). Yet this BPT diagram is limited to z < 0.5, the redshift at which [NII]{lambda}6584 leaves the optical spectral window. Using the Sloan Digital Sky Survey (SDSS), we construct a new diagnostic, or TBT diagram, that is based on rest-frame g-z color, [NeIII]{lambda}3869, and [OII]{lambda}{lambda}3726 + 3729 and can be used for galaxies out to z < 1.4. The TBT diagram identifies 98.7% of the SDSS BPT-AGN as TBT-AGN and 97% of the SDSS BPT-SF as TBT-SF. Furthermore, it identifies 97% of the OPTX Chandra X-ray selected AGNs as TBT-AGN. This is in contrast to the BPT diagram, which misidentifies 20% of X-ray selected AGNs as BPT-SF. We use the GOODS-N and Lockman Hole galaxy samples, with their accompanying deep Chandra imaging, to perform X-ray and infrared stacking analyses to further validate our TBT-AGN and TBT-SF selections; that is, we verify the dominance of AGN activity in the former and star formation activity in the latter. Finally, we address the inclusion of the majority of the BPT-comp (sources lying between the BPT-SF and BPT-AGN regimes) in our TBT-AGN regime. We find that the stacked BPT-comp source is X-ray hard (<{Gamma}eff> = 1.0 +/-0.4) and has a high X-ray luminosity to total infrared luminosity ratio. This suggests that, on average, the X-ray signal in BPT-comp is dominated by obscured or low accretion rate AGN activity rather than by star formation, supporting their inclusion in the TBT-AGN regime.
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We compare the optical spectral types with the X-ray spectral properties for a uniformly selected (sources with fluxes greater than the 3 sigma level and above a flux limit of f_2-8 keV > 3.5x10^-15 erg/cm2/s), highly spectroscopically complete (>80% for f_2-8 keV > 10^-14 erg/cm2/s and >60% below) 2-8 keV X-ray sample observed in three Chandra fields (CLANS, CLASXS, and the CDF-N) that cover ~1.2 deg^2. For our sample of 645 spectroscopically observed sources, we confirm that there is significant overlap of the X-ray spectral properties, as determined by the effective photon indices, Geff, obtained from the ratios of the 0.5-2 keV to 2-8 keV counts, for the different optical spectral types. For example, of the broad-line AGNs (non-broad-line AGNs), 20% +/- 3% (33% +/- 4%) have Geff<1.2 (Geff > 1.2). Thus, one cannot use the X-ray spectral classifications and the optical spectral classifications equivalently. Since it is not understood how X-ray and optical classifications relate to the obscuration of the central engine, we strongly advise against a mixed classification scheme, as it can only complicate the interpretation of X-ray AGN samples. We confirm the dependence of optical spectral type on X-ray luminosity, and for z<1, we find a similar luminosity dependence of Geff. However, this dependence breaks down at higher redshifts due to the highly redshift-dependent nature of Geff. We therefore also caution that any classification scheme which depends on Geff is likely to suffer from serious redshift bias.
It is well established that a dominant phase in the growth of massive galaxies occurred at high redshift and was heavily obscured by gas and dust. Many studies have explored the stellar growth of massive galaxies but few have combined these constraints with the growth of the supermassive black hole (SMBH; i.e., identified as AGN activity). In this brief contribution we highlight our work aimed at identifying AGNs in z~2 luminous dust-obscured galaxies. Using both sensitive X-ray and infrared (IR)-submillimeter (submm) observations, we show that AGN activity is common in z~2 dust-obscured systems. With a variety of techniques we have found that the majority of the AGN activity is heavily obscured, and construct diagnostics based on X-ray-IR data to identify some of the most heavily obscured AGNs in the Universe (i.e., AGNs obscured by Compton-thick material; N_H>1.5x10^24 cm^-2). On the basis of these techniques we show that SMBH growth was typically heavily obscured (N_H>10^23 cm^-2) at z~2, and find that the growth of the SMBH and spheroid was closely connected, even in the most rapidly evolving systems.
We compare optical and hard X-ray identifications of AGNs using a uniformly selected (above a flux limit of f_2-8 keV = 3.5e-15 erg/cm2/s) and highly optically spectroscopically complete ( > 80% for f_2-8 keV > 1e-14 erg/cm2/s and > 60% below) 2-8 keV sample observed in three Chandra fields (CLANS, CLASXS, and the CDF-N). We find that empirical emission-line ratio diagnostic diagrams misidentify 20-50% of the X-ray selected AGNs that can be put on these diagrams as star formers, depending on which division is used. We confirm that there is a large (2 orders in magnitude) dispersion in the log ratio of the [OIII]5007A to hard X-ray luminosities for the non-broad line AGNs, even after applying reddening corrections to the [OIII] luminosities. We find that the dispersion is similar for the broad-line AGNs, where there is not expected to be much X-ray absorption from an obscuring torus around the AGN nor much obscuration from the galaxy along the line-of-sight if the AGN is aligned with the galaxy. We postulate that the X-ray selected AGNs that are misidentified by the diagnostic diagrams have low [OIII] luminosities due to the complexity of the structure of the narrow-line region, which causes many ionizing photons from the AGN not to be absorbed. This would mean that the [OIII] luminosity can only be used to predict the X-ray luminosity to within a factor of ~3 (one sigma). Despite selection effects, we show that the shapes and normalizations of the [OIII] and transformed hard X-ray luminosity functions show reasonable agreement, suggesting that the [OIII] samples are not finding substantially more AGNs at low redshifts than hard X-ray samples.
We have performed accurate iron abundance measurements for 44 red giants (RGs) in the Carina dwarf spheroidal (dSph) galaxy. We used archival, high-resolution spectra (R~38,000) collected with UVES at ESO/VLT either in slit mode (5) or in fiber mode (39, FLAMES/GIRAFFE-UVES). The sample is more than a factor of four larger than any previous spectroscopic investigation of stars in dSphs based on high-resolution (R>38,000) spectra. We did not impose the ionization equilibrium between neutral and singly-ionized iron lines. The effective temperatures and the surface gravities were estimated by fitting stellar isochrones in the V, B-V color-magnitude diagram. To measure the iron abundance of individual lines we applied the LTE spectrum synthesis fitting method using MARCS model atmospheres of appropriate metallicity. We found evidence of NLTE effects between neutral and singly-ionized iron abundances. Assuming that the FeII abundances are minimally affected by NLTE effects, we corrected the FeI stellar abundances using a linear fit between FeI and FeII stellar abundance determinations. We found that the Carina metallicity distribution based on the corrected FeI abundances (44 RGs) has a weighted mean metallicity of [Fe/H]=-1.80 and a weighted standard deviation of sigma=0.24 dex. The Carina metallicity distribution based on the FeII abundances (27 RGs) gives similar estimates ([Fe/H]=-1.72, sigma=0.24 dex). The current weighted mean metallicities are slightly more metal poor when compared with similar estimates available in the literature. Furthermore, if we restrict our analysis to stars with the most accurate iron abundances, ~20 FeI and at least three FeII measurements (15 stars), we found that the range in iron abundances covered by Carina RGs (~1 dex) agrees quite well with similar estimates based on high-resolution spectra.
Observing the neutral Hydrogen (HI) distribution across the Universe via redshifted 21-cm line Intensity Mapping (IM) constitutes a powerful probe for cosmology. However, this redshifted 21cm signal is obscured by the foreground emission. This paper addresses the capabilities of the BINGO survey to separate such signals. Specifically, this paper (a) looks in detail at the different residuals left over by foreground components, (b) shows that a noise-corrected spectrum is unbiased and (c) shows that we understand the remaining systematic residuals by analyzing non-zero contributions to the three point function. We use the Generalized Needlet Internal Linear Combination (GNILC), which we apply to sky simulations of the BINGO experiment for each redshift bin of the survey. We present our recovery of the redshifted 21-cm signal from sky simulations of the BINGO experiment including foreground components. We test the recovery of the 21-cm signal through the angular power spectrum at different redshifts, as well as the recovery of its non-Gaussian distribution through a bispectrum analysis. We find that non-Gaussianities from the original foreground maps can be removed down to, at least, the noise limit of the BINGO survey with such techniques. Our bispectrum analysis yields strong tests of the level of the residual foreground contamination in the recovered 21-cm signal, thereby allowing us to both optimize and validate our component separation analysis. (Abridged)
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