ABSTRACT We present an all-sky sample of 1.4 million active galactic nuclei (AGNs) meeting a two-color infrared photometric selection criteria for AGNs as applied to sources from the Wide-field ...Infrared Survey Explorer final catalog release (AllWISE). We assess the spatial distribution and optical properties of our sample and find that the results are consistent with expectations for AGNs. These sources have a mean density of 38 AGNs per square degree on the sky, and their apparent magnitude distribution peaks at g 20, extending to objects as faint as g 26. We test the AGN selection criteria against a large sample of optically identified stars and determine the "leakage" (that is, the probability that a star detected in an optical survey will be misidentified as a quasi-stellar object (QSO) in our sample) rate to be ≤4.0 × 10−5. We conclude that our sample contains almost no optically identified stars (≤0.041%), making this sample highly promising for future celestial reference frame work as it significantly increases the number of all-sky, compact extragalactic objects. We further compare our sample to catalogs of known AGNs/QSOs and find a completeness value of 84% (that is, the probability of correctly identifying a known AGN/QSO is at least 84%) for AGNs brighter than a limiting magnitude of R 19. Our sample includes approximately 1.1 million previously uncataloged AGNs.
Archival observations of 18 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the ratio of emission-line fluxes of Fe II at 26 km and Ne II at ...12.8 km and the 7.7 km PAH strength, with the Fe II/Ne II flux ratio decreasing with increasing PAH strength. We also find a strong correlation between the Fe II/Ne II flux ratio and the host galaxy metallicity, with the flux ratio decreasing with increasing metallicity. Since Fe II emission has been linked primarily to supernova shocks, we attribute the high Fe II/Ne II ratios in low-metallicity galaxies to enhanced supernova activity. We consider this to be a dominant mechanism for PAH destruction, rather than grain destruction in photoionized regions surrounding young massive stars. We also consider whether the extreme youth of the low-metallicity galaxies is responsible for the lack of PAH emission.
We report the serendipitous discovery of the first gravitationally lensed quasar candidate from Pan-STARRS. The grizy images reveal four point-like images with magnitudes between 14.9 and 18.1 mag. ...The colors of the point sources are similar, and they are more consistent with quasars than with stars or galaxies. The lensing galaxy is detected in the izy bands, with an inferred photometric redshift of ∼0.6, lower than that of the point sources. We successfully model the system with a singular isothermal ellipsoid with shear, using the relative positions of the five objects as constraints. While the brightness ranking of the point sources is consistent with that of the model, we find discrepancies between the model-predicted and observed fluxes, likely due to microlensing by stars and millilensing due to the dark matter substructure. In order to fully confirm the gravitational lens nature of this system and add it to the small but growing number of the powerful probes of cosmology and astrophysics represented by quadruply lensed quasars, we require further spectroscopy and high-resolution imaging.
We have conducted a comprehensive mid-infrared (IR) spectroscopic investigation of 67 low-ionization nuclear emission line regions (LINERs) using archival observations from the high-resolution ...modules of the Infrared Spectrograph on board the Spitzer Space Telescope. Using the Ne V 14 and 24 is a subset of m lines as active galactic nuclei (AGNs) diagnostics, we detect active black holes in 39% of the galaxies in our sample, many of which show no signs of activity in either the optical or X-ray bands. In particular, a detailed comparison of multiwavelength diagnostics shows that optical studies fail to detect AGNs in galaxies with large far-IR luminosities. These observations emphasize that the nuclear power source in a large percentage of LINERs is obscured in the optical. Indeed, the majority of LINERs show mid-IR Ne V 14/Ne V 24 is a subset of m flux ratios well below the theoretical low-density limit, suggesting that there is substantial extinction toward even the Ne V-emitting region. Combining optical, X-ray, and mid-IR diagnostics, we find an AGN detection rate in LINERs of 74%, higher than previously reported statistics of the fraction of LINERs hosting AGNs. The Ne V 24 is a subset of m/O IV 26 is a subset of m mid-IR line flux ratio in AGN-LINERs is similar to that of standard AGNs, suggesting that the spectral energy distribution of the intrinsic optical/UV continuum is similar in the two. This result is in contrast to previous suggestions of a UV deficit in the intrinsic broadband continuum emission in AGN-LINERs. Consistent with our finding of extinction to the Ne V-emitting region, we propose that extinction may also be responsible for the observed optical/UV deficit seen in at least some AGN-LINERs.
We report the discovery using Spitzer's high-resolution spectrograph of seven active galactic nuclei (AGNs) in a sample of 32 late-type galaxies that show no definitive signatures of AGNs in their ...optical spectra. These observations suggest that the AGN detection rate in late-type galaxies is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with an input AGN and an extreme UV-bright starburst ionizing radiation field that the observed mid- infrared line spectrum, which includes the high-ionization Ne v 14 mum and/or 24 mum lines, cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGNs missed by optical spectroscopy. The AGN bolometric luminosities in our sample range from image to image ergs s super(-1), which, based on the Eddington limit, corresponds to a lower mass limit for the black hole that ranges from image to as high as image M sub(image). These lower mass limits, however, do not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.
We present Spitzer Infrared Spectrograph observations of the ultraluminous X-ray source (ULX) NGC 6946 X-1 and its associated nebula MF 16. This ULX has very similar properties to the famous Holmberg ...II ULX, the first ULX to show a prominent infrared OIV emission line comparable to those found in active galactic nuclei. This paper attempts to constrain the ULX spectral energy distribution (SED) given the optical/UV photometric fluxes and high-resolution X-ray observations. Specifically, Chandra X-ray data and published Hubble optical/UV data are extrapolated to produce a model for the full optical to X-ray SED. The photoionization modeling of the IR lines and ratios is then used to test different accretion spectral models. While either an irradiated disk model or an O-supergiant plus accretion disk model fits the data very well, we prefer the latter because it fits the nebular parameters slightly better. In this second case the accretion disk alone dominates the extreme-UV and X-ray emission, while an O-supergiant is responsible for most of the far-UV emission.
We present a comprehensive comparative high resolution mid-IR spectroscopic and X-ray imaging investigation of LINERs using archival observations from the ISO-SWS and the Chandra Advanced CCD Imaging ...Spectrometer. Although the sample is heterogenous and incomplete, this is the first comprehensive study of the mid-infrared fine structure line emission of LINERs. These results have been compared with similar observations of starburst galaxies and AGN. We find that LINERs very clearly fall between starbursts and AGN in their mid-IR fine structure line spectra, showing $L_{\rm OIV26~{\mu}m}$/LFIR and $L_{\rm OIV26~{\mu}m}$/$L_{\rm NeII12.8~{\mu}m}$ ratios, both measures of the dominant nuclear energy source in dust-enshrouded galaxies, intermediate between those of AGN and starbursts. Chandra imaging observations of the LINERs reveal hard nuclear point sources morphologically consistent with AGN in most (67%) of the sample, with a clear trend with IR-brightness. Most LINERs that show a single dominant hard compact X-ray core are IR-faint (LFIR/$L_{B} < 1$), whereas most LINERs that show scattered X-ray sources are IR-bright. A comparative X-ray/mid-IR spectroscopic investigation of LINERs reveals some puzzling results. Objects that display strong hard nuclear X-ray cores should also display high excitation lines in the IR. However, we find two LINERs disagree with this expectation. The galaxy NGC 404 shows weak soft X-ray emission consistent with a starburst but has the most prominent highest excitation mid-IR spectrum of our entire sample. Using IR emission line diagnostics alone, this galaxy would be classified as hosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 has an extremely luminous binary AGN as revealed by the X-rays but shows weak IR emission lines. With the advent of SIRTF, and future IR missions such as Herschel and JWST, it is increasingly critical to determine the origin of these multiwavelength anomalies.
We compare mid-infrared emission-line properties from high-resolution Spitzer spectra of a hard X-ray (14-195 keV) selected sample of nearby (z < 0.05) active galactic nuclei (AGNs) detected by the ...Burst Alert Telescope (BAT) aboard Swift. The luminosity distribution for the mid-infrared emission lines, O IV 25.89 {mu}m, Ne II 12.81 {mu}m, Ne III 15.56 {mu}m, and Ne V 14.32/24.32 {mu}m, and hard X-ray continuum show no differences between Seyfert 1 and Seyfert 2 populations; however, six newly discovered BAT AGNs are under-luminous in O IV, most likely the result of dust extinction in the host galaxy. The overall tightness of the mid-infrared correlations and BAT fluxes and luminosities suggests that the emission lines primarily arise in gas ionized by the AGNs. We also compare the mid-infrared emission lines in the BAT AGNs with those from published studies of ULIRGs, Palomar-Green quasars, star-forming galaxies, and LINERs. We find that the BAT AGN sample falls into a distinctive region when comparing the Ne III/Ne II and the O IV/Ne III ratios. These line ratios are lower in sources that have been previously classified in the mid-infrared/optical as AGNs than those found for the BAT AGNs, suggesting that, in our X-ray selected sample, the AGNs represent the main contribution to the observed line emission. These ratios represent a new emission line diagnostic for distinguishing between AGNs and star-forming galaxies.
We present results from a high-resolution X-ray imaging study of nearby LINERs observed by ACIS on board Chandra. This study complements and extends previous X-ray studies of LINERs, focusing on the ...underexplored population of nearby dust-enshrouded infrared-bright LINERs. The sample consists of 15 IR-bright LINERs (L sub(FIR)/L sub(B) > 3), with distances that range from 11 to 26 Mpc. Combining our sample with previous Chandra studies, we find that similar to 51% (28/55) of the LINERs display compact hard X-ray cores. The nuclear 2-10 keV luminosities of the galaxies in this expanded sample range from similar to 2 x 10 super(38) to similar to 2 x 10 super(44) ergs s super(-1). We find that the most extreme IR-faint LINERs are exclusively active galactic nuclei (AGNs). The fraction of LINERs containing AGNs appears to decrease with IR brightness and increase again at the highest values of L sub(FIR)/L sub(B). We find that of the 24 LINERs showing compact nuclear hard X-ray cores in the expanded sample that were observed at H alpha wavelengths, only eight actually show evidence of a broad line. Similarly, of the 14 LINERs showing compact nuclear hard X-ray cores with corresponding radio observations, only eight display a compact flat spectrum radio core. These findings emphasize the need for high-resolution X-ray imaging observations in the study of IR-bright LINERs. Finally, we find an intriguing trend in the Eddington ratio versus L sub(FIR) and L sub(FIR)/L sub(B) for the AGN-LINERs in the expanded sample that extends over 7 orders of magnitude in L/L sub(Edd). This correlation may imply a link between black hole growth, as measured by the Eddington ratio, and the star formation rate, as measured by the far-IR luminosity and IR-brightness ratio. If the far-IR luminosity is an indicator of the molecular gas content in our sample of LINERs, our results may further indicate that the mass accretion rate scales with the host galaxy's fuel supply. We discuss the potential implications of our results in the framework of black hole growth and AGN fueling in low-luminosity AGNs.
We present archival high-resolution X-ray imaging observations of 25 nearby LINERs observed by ACIS on board Chandra. This sample builds on our previously published proprietary and archival X-ray ...observations and includes the complete set of LINERs with published black hole masses and FIR luminosities that have been observed by Chandra. Of the 82 LINERs observed by Chandra, 41 (50%) display hard nuclear cores consistent with an AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from 62 x 10 super(38) to 61 x 10 super(44) ergs s super(-1). Reinforcing our previous work, we find a significant correlation between the Eddington ratio, L sub(bol)/L sub(Edd), and the FIR luminosity, L sub(FIR), as well as the IR brightness ratio, L sub(FIR)/L sub(B), in the host galaxy of AGN-LINERs that extends over 7 orders of magnitude in L sub(bol)/L sub(Edd). Combining our AGN-LINER sample with galaxies from other AGN subclasses, we find that this correlation is reinforced in a sample of 129 AGNs, extending over almost 9 orders of magnitude in L sub(bol)/L sub(Edd). Using archival and previously published observations of the 6.2 km PAH feature from ISO, we find that it is unlikely that dust heating by the AGN dominates the FIR luminosity in our sample of AGNs. Our results may therefore imply a fundamental link between the mass accretion rate (M), as measured by the Eddington ratio, and the star formation rate (SFR), as measured by the FIR luminosity. Apart from the overall correlation, we find that the different AGN subclasses occupy distinct regions in the L sub(FIR) and L sub(bol)/L sub(Edd) plane. Assuming a constant radiative efficiency for accretion, our results may imply a variation in the SFR/M ratio as a function of AGN activity level, a result that may have significant consequences for our understanding of galaxy formation and black hole growth.
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