Hidden Active Galactic Nuclei Research Articles

Pure Spectroscopic Constraints on UV Luminosity Functions and Cosmic Star Formation History from 25 Galaxies at z spec = 8.61–13.20 Confirmed with JWST/NIRSpec

We present pure spectroscopic constraints on the UV luminosity functions and cosmic star formation rate (SFR) densities from 25 galaxies at z spec = 8.61–13.20. By reducing the JWST/NIRSpec spectra taken in multiple programs of Early Release Observation, Early Release Science, General Observer, and Director’s Discretionary Time observations with our analysis technique, we independently confirm 16 galaxies at z spec = 8.61–11.40, including new redshift determinations, and a bright interloper at z spec = 4.91 that was claimed as a photometric candidate at z ∼ 16. In conjunction with nine galaxies at redshifts up to z spec = 13.20 in the literature, we make a sample of 25 spectroscopically confirmed galaxies in total and carefully derive the best estimates and lower limits of the UV luminosity functions. These UV luminosity function constraints are consistent with the previous photometric estimates within the uncertainties and indicate mild redshift evolution toward z ∼ 12, showing tensions with some theoretical models of rapid evolution. With these spectroscopic constraints, we obtain firm lower limits of the cosmic SFR densities and spectroscopically confirm a high SFR density at z ∼ 12 beyond the constant star formation efficiency models, which supports earlier claims from the photometric studies. While there are no spectroscopically confirmed galaxies with very large stellar masses violating the ΛCDM model due to the removal of the bright interloper, we confirm star-forming galaxies at z spec = 11–13 with stellar masses much higher than model predictions. Our results indicate possibilities of high star formation efficiency (>5%), a hidden active galactic nucleus, a top-heavy initial mass function (possibly with Population III), and large scatter/variance. Having these successful and unsuccessful spectroscopy results, we suggest observational strategies for efficiently removing low-redshift interlopers for future JWST programs.

The Unusual AGN Host NGC 1266: Evidence for Shocks in a Molecular Gas Rich S0 Galaxy with a Low Luminosity Nucleus

NGC 1266 is a lenticular galaxy (S0) hosting an active galactic nucleus (AGN), and known to contain a large amount of shocked gas. We compare the luminosity ratio of mid-J CO lines to IR continuum with star-forming galaxies (SFGs), and then model the CO spectral line energy distribution (SLED). We confirm that in the mid- and high-J regions (J up = 4–13), the C-type shock (v s = 25 km s−1, n H = 5 × 104 cm−3) can reproduce the CO observations well. The galaxy spectral energy distribution (SED) is constructed and modeled by the code X-CIGALE and obtains a set of physical parameters including the star formation rate (SFR, 1.17 ± 0.47 M ⊙ yr−1). Also, our work provides SFR derivation of [C ii] from the neutral hydrogen regions only (1.38 ± 0.14 M ⊙yr−1). Previous studies have illusive conclusions on the AGN or starburst nature of the NGC 1266 nucleus. Our SED model shows that the hidden AGN in the system is intrinsically low-luminosity, consequently the infrared luminosity of the AGN does not reach the expected level. Archival data from NuSTAR hard X-ray observations in the 3–79 keV band shows a marginal detection, disfavoring presence of an obscured luminous AGN and implying that a compact starburst is more likely dominant for the NGC 1266 nucleus.

Chandra reveals a luminous Compton-thick QSO powering a Lyα blob in a z = 4 starbursting protocluster

Context. Galaxy clusters in the local universe descend from high-redshift overdense regions known as protoclusters. The large gas reservoirs and high rate of galaxy interaction in protoclusters are expected to enhance star-formation activity and trigger luminous supermassive black-hole accretion in the nuclear regions of the host galaxies. Aims. We investigated the active galactic nucleus (AGN) content of a gas-rich and starbursting protocluster at z = 4.002, known as the Distant Red Core (DRC). In particular, we search for luminous and possibly obscured AGN in 13 identified members of the structure, and compare the results with protoclusters at lower redshifts. We also test whether a hidden AGN can power the Lyα blob (LAB) detected with VLT/MUSE in the DRC. Methods. We observed all of the identified members of the structure with 139 ks of Chandra ACIS-S imaging. Being less affected by absorption than optical and IR bands, even in the presence of large column densities of obscuring material, X-ray observations are the best tools to detect ongoing nuclear activity in the DRC galaxies. Results. We detect obscured X-ray emission from the two most gas-rich members of the DRC, named DRC-1 and DRC-2. Both of them are resolved into multiple interacting clumps in high-resolution Atacama Large Millimeter Array and Hubble Space Telescope observations. In particular, DRC-2 is found to host a luminous (L2−10 keV ≈ 3 × 1045 erg s−1 ) Compton-thick (NH ≳ 1024 cm−2) quasar (QSO) candidate, comparable to the most luminous QSOs known at all cosmic times. The AGN fraction among DRC members is consistent with results found for lower redshift protoclusters. However, X-ray stacking analysis reveals that supermassive black hole (SMBH) accretion is likely also taking place in other DRC galaxies that are not detected individually by Chandra. Conclusions. The luminous AGN detected in the most gas-rich galaxies in the DRC and the widespread SMBH accretion in the other members, which is suggested by stacking analysis, point toward the presence of a strong link between large gas reservoirs, galaxy interactions, and luminous and obscured nuclear activity in protocluster members. The powerful and obscured QSO detected in DRC-2 is likely powering the nearby LAB detected with VLT/MUSE, possibly through photoionization; however, we propose that the diffuse Lyα emission may be due to gas shocked by a massive outflow launched by DRC-2 over a ≈10 kpc scale.

The next-generation X-ray galaxy survey with eROSITA

ABSTRACT eROSITA, launched on 2019 July 13, will be completing the first all-sky survey in the soft and medium X-ray band in nearly three decades. This 4-yr survey, finishing in late 2023, will present a rich legacy for the entire astrophysics community and complement upcoming multiwavelength surveys (with, e.g. the Large Synoptic Survey Telescope and the Dark Energy Survey). Besides the major scientific aim to study active galactic nuclei (AGNs) and galaxy clusters, eROSITAwill contribute significantly to X-ray studies of normal (i.e. not AGN) galaxies. Starting from multiwavelength catalogues, we measure star formation rates and stellar masses for 60 212 galaxies constrained to distances of 50–200 Mpc. We chose this distance range to focus on the relatively unexplored volume outside the local Universe, where galaxies will be largely spatially unresolved and probe a range of X-ray luminosities that overlap with the low luminosity and/or highly obscured AGN population. We use the most recent X-ray scaling relations as well as the on-orbit eROSITA instrument performance to predict the X-ray emission from XRBs and diffuse hot gas and to perform both an analytic prediction and an end-to-end simulation using the mission simulation software, sixte. We consider potential contributions from hidden AGN and comment on the impact of normal galaxies on the measurement of the faint end of the AGN luminosity function. We predict that the eROSITA 4-yr survey, will detect ≳15 000 galaxies (3σ significance) at 50–200 Mpc, which is ∼100 × more normal galaxies than detected in any X-ray survey to date.

Planck Far-infrared Detection of Hyper Suprime-Cam Protoclusters at z ∼ 4: Hidden AGN and Star Formation Activity

Abstract We perform a stacking analysis of Planck, AKARI, Infrared Astronomical Satellite, Wide-field Infrared Survey Explorer, and Herschel images of the largest number of (candidate) protoclusters at z ∼ 3.8 selected from the Hyper Suprime-Cam Subaru Strategic Program. Stacking the images of the 179 candidate protoclusters, the combined infrared (IR) emission of the protocluster galaxies in the observed 12–850 μm wavelength range is successfully detected with >5σ significance (at Planck). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at z ∼ 4. The observed IR SEDs of the protoclusters exhibit significant excess emission in the mid-IR compared to that expected from typical star-forming galaxies (SFGs). They are reproduced well using SED models of intense starburst galaxies with warm/hot dust heated by young stars, or by a population of active galactic nucleus (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8–1000 μm) luminosity of 19.3 − 4.2 + 0.6 × 10 13 L ⊙ and a star formation rate of 16.3 − 7.8 + 1.0 × 10 3 M ⊙ yr−1 are found, whereas for the AGN/SFG composite model, 5.1 − 2.5 + 2.5 × 10 13 L ⊙ and 2.1 − 1.7 + 6.3 × 10 3 M ⊙ yr−1 are found. Uncertainty remains in the total SFRs; however, the IR luminosities of the most massive protoclusters are likely to continue increasing up to z ∼ 4. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the z ∼ 4 protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical.

X-Ray Emission from the Nuclear Region of Arp 220

Abstract We present an imaging and spectral analysis of the nuclear region of the ultraluminous infrared galaxy merger of Arp 220, using deep Chandra-ACIS observations summing up to ∼ 300 ks . Narrowband imaging with subpixel resolution of the innermost nuclear region reveals two distinct Fe–K emitting sources, coincident with the infrared and radio nuclear clusters. These sources are separated by 1′ (∼380 pc). The X-ray emission is extended and elongated in the eastern (E) nucleus, like the disk emission observed in millimeter radio images, suggesting a starburst dominance in this region. We estimate an Fe–K equivalent width of ≳ 1 keV for both sources and observe 2–10 keV luminosities of ∼ 2 × 10 40 erg s − 1 (western, W) and ∼ 3 × 10 40 erg s − 1 (E). In the 6–7 keV band the emission from these regions is dominated by the 6.7 keV Fe xxv line, suggesting a contribution from collisionally ionized gas. The thermal energy content of this gas is consistent with the kinetic energy injection in the interstellar medium by SNe II. However, nuclear winds from a hidden active galactic nucleus (AGN) ( υ ∼ 2000 km s − 1 ) cannot be excluded. The 3 σ upper limits on the neutral Fe–Kα flux of the nuclear regions correspond to the intrinsic AGN 2–10 keV luminosities of < 1 × 10 42 erg s − 1 (W) and < 0.4 × 10 42 erg s − 1 (E). For typical AGN spectral energy distributions the bolometric luminosities are < 3 × 10 43 erg s − 1 (W) and < 8 × 10 43 erg s − 1 (E), and black hole masses of < 1 × 10 5 M ☉ (W) and < 5 × 10 5 M ☉ (E) are evaluated for Eddington limited AGNs with a standard 10% efficiency.

A large sample of Kohonen selected E+A (post-starburst) galaxies from the Sloan Digital Sky Survey

We aim to create a large sample of local post-starburst (PSB) galaxies to study their characteristic properties, particularly morphological features indicative of gravitational distortions and indications for active galactic nuclei (AGNs). The selection is based on a huge Kohonen self-organising map (SOM) of about one million SDSS spectra. The SOM is made fully available for the astronomical community, in combination with an interactive user interface. We compiled a catalogue of 2665 PSB galaxies with redshifts z < 0.4. In the colour-mass diagram, the PSB sample is found to be clearly concentrated towards the region between the red and the blue cloud, in agreement with the idea that PSB galaxies represent the transitioning phase between actively and passively evolving galaxies. The relative frequency of morphologically distorted PSB galaxies is at least 57%, significantly higher than in a comparison sample. The search for AGNs based on conventional selection criteria in the radio and MIR results in a low AGN fraction of 2 - 3%. We confirm an MIR excess in the mean SED of the PSB galaxy sample that may indicate hidden AGNs, though other sources are also possible. (Abstract modified to match the arXiv format.)

HST/WFC3 OBSERVATIONS OF AN OFF-NUCLEAR SUPERBUBBLE IN ARP 220

We present a high spatial resolution optical and infrared study of the circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging using HST/WFC3 has resolved the previously observed peak in H$\alpha$+[NII] emission into a bubble-shaped feature. This feature measures 1.6" in diameter, or 600 pc, and is only 1" northwest of the western nucleus. The bubble is aligned with the western nucleus and the large-scale outflow axis seen in X-rays. We explore several possibilities for the bubble origin, including a jet or outflow from a hidden active galactic nucleus (AGN), outflows from high levels of star formation within the few hundred pc nuclear gas disk, or an ultraluminous X-ray source. An obscured AGN or high levels of star formation within the inner $\sim$100 pc of the nuclei are favored based on the alignment of the bubble and energetics arguments.

ACTIVE GALACTIC NUCLEUS FEEDBACK AT z ∼ 2 AND THE MUTUAL EVOLUTION OF ACTIVE AND INACTIVE GALAXIES

The relationships between galaxies of intermediate stellar mass and moderate luminosity active galactic nuclei (AGNs) at 1<z<3 are investigated with the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample complemented with public data in the GOODS-South field. Using X-ray data, hidden AGNs are identified in unsuspected star-forming galaxies with no apparent signs of non-stellar activity. In the color-mass plane, two parallel trends emerge during the ~2 Gyr between the average redshifts z~2.2 and z~1.3: while the red sequence becomes significantly more populated by ellipticals, the majority of AGNs with L(2-10 keV)>10^42.3 erg s^-1 disappear from the blue cloud/green valley where they were hosted predominantly by star-forming systems with disk and irregular morphologies. These results are even clearer when the rest-frame colors are corrected for dust reddening. At z~2.2, the ultraviolet spectra of active galaxies (including two Type 1 AGNs) show possible gas outflows with velocities up to about -500 km s^-1 that are not observed neither in inactive systems at the same redshift, nor at lower redshifts. Such outflows indicate the presence of gas that can move faster than the escape velocities of active galaxies. These results suggest that feedback from moderately luminous AGNs (logL_X<44.5 erg s^-1) played a key role at z>~2 by contributing to outflows capable of ejecting part of the interstellar medium and leading to a rapid decrease in the star formation in host galaxies with stellar masses 10<logM<11 M_Sun.

Herschelunveils a puzzling uniformity of distant dusty galaxies

The Herschel Space Observatory enables us to accurately measure the bolometric output of starburst galaxies and active galactic nuclei (AGN) by directly sampling the peak of their far-infrared (IR) emission. Here we examine whether the spectral energy distribution (SED) and dust temperature of galaxies have strongly evolved since z~2.5. We use Herschel deep extragalactic surveys from 100 to 500um to compute total IR luminosities in galaxies down to the faintest levels, using PACS and SPIRE in the GOODS-North field (PEP and HerMES key programs). We show that measurements in the SPIRE bands can be used below the statistical confusion limit if information at higher spatial resolution is used to identify isolated galaxies whose flux is not boosted by bright neighbors. Below z~1.5, mid-IR extrapolations are correct for star-forming galaxies with a dispersion of only 40% (0.15dex), therefore similar to z~0 galaxies. This narrow distribution is puzzling when considering the range of physical processes that could have affected the SED of these galaxies. Extrapolations from only one of the 160um, 250um or 350um bands alone tend to overestimate the total IR luminosity. This may be explained by the lack of far-IR constraints around and above ~150um (rest-frame) on local templates. We also note that the dust temperature of luminous IR galaxies around z~1 is mildly colder by 10-15% than their local analogs and up to 20% for ULIRGs at z~1.6. Above z=1.5, distant galaxies are found to exhibit a substantially larger mid- over far-IR ratio, which could either result from stronger broad emission lines or warm dust continuum heated by a hidden AGN. Two thirds of the AGNs identified in the field with a measured redshift exhibit the same behavior as purely star-forming galaxies. Hence a large fraction of AGNs harbor star formation at very high SFR and in conditions similar to purely star-forming galaxies.

THE INCIDENCE OF ACTIVE GALACTIC NUCLEI IN PURE DISK GALAXIES: THESPITZERVIEW

Using the Spitzer telescope, we have conducted a high-resolution spectroscopic study of 18 bulgeless (Hubble type of Sd or Sdm) galaxies that show no definitive signatures of nuclear activity in their optical spectra. This is the first systematic mid-infrared (MIR) search for weak or hidden active galactic nuclei (AGNs) in a statistically significant sample of bulgeless (Sd/Sdm) disk galaxies. Based on the detection of the high-ionization [Ne v] 14.3 μm line, we report the discovery of an AGN in 1 out of the 18 galaxies in the sample. This galaxy, NGC 4178, is a nearby edge-on Sd galaxy, which likely hosts a prominent nuclear star cluster (NSC). The bolometric luminosity of the AGN inferred from the [Ne v] line luminosity is ∼8 × 1041 ergs s−1. This is almost 2 orders of magnitude greater than the luminosity of the AGN in NGC 4395, the best studied AGN in a bulgeless disk galaxy. Assuming that the AGN in NGC 4178 is radiating below the Eddington limit, the lower mass limit for the black hole is ∼6 × 103 M☉. The fact that none of the other galaxies in the sample shows any evidence for an AGN demonstrates that while the AGN detection rate based on MIR diagnostics is high (30%–40%) in optically quiescent galaxies with pseudobulges or weak classical bulges (Hubble type Sbc and Sc), it drops drastically in Sd/Sdm galaxies. Our observations, therefore, confirm that AGNs in completely bulgeless disk galaxies are not hidden in the optical but truly are rare. Of the three Sd galaxies with AGNs known so far, all have prominent NSCs, suggesting that in the absence of a well-defined bulge, the galaxy must possess an NSC in order to host an AGN. On the other hand, while the presence of an NSC appears to be a requirement for hosting an AGN in bulgeless galaxies, neither the properties of the NSC nor those of the host galaxy appear exceptional in late-type AGN host galaxies. The recipe for forming and growing a central black hole in a bulgeless galaxy therefore remains unknown.

A Compton-thick active galactic nucleus powering the hyperluminous infrared galaxy IRAS 00182−7112

Abstract We present X-ray observations of the hyperluminous infrared galaxy (HLIRG) IRAS 00182− 7112 obtained using the XMM–Newton EPIC camera. A luminous hard X-ray source coincident with the nucleus is revealed, along with weaker soft X-ray emission which may be extended or offset from the hard emission. The EPIC spectrum is extremely flat and shows Fe Kα emission with very high equivalent width: both are typical characteristics of a buried, Compton-thick active galactic nucleus (AGN) which is seen only in scattered light. Perhaps the most remarkable characteristic of the spectrum is that the Fe Kα line energy is that of He-like iron, making IRAS 00182−7112 the first hidden AGN known to be dominated by ionized, Compton-thick reflection. Taking an appropriate bolometric correction, we find that this AGN could easily dominate the far-infrared (FIR) energetics. The nuclear reflection spectrum is seen through a relatively cold absorber with column density consistent with recent Spitzer observations. The soft X-ray emission, which may be thermal in nature and associated with star-forming activity, is seen unabsorbed. The soft X-rays and weak polycyclic aromatic hydrocarbon features both give estimates of the star formation rate ∼300 M⊙ yr−1, insufficient to power the FIR emission and supportive of the idea that this HLIRG is AGN-dominated.

The AGN and gas disc in the low surface brightness galaxy PGC 045080

We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disc and massive H I gas content. Radio continuum observations of the galaxy were carried out at 320, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disc at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with active galactic nucleus (AGN) activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the Hα flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centres can host significant AGN activity. We also mapped the H I gas disc and velocity field in PGC 045080. The H I disc extends well beyond the optical disc and appears warped. In the H I intensity maps, the disc appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disc but is fairly uniform in the other half. We derived the H I rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of ∼190 km s −1 .

Infrared Properties of Close Pairs of Galaxies

We discuss spectroscopy and IR photometry for a complete sample of ~800 galaxies in close pairs objectively selected from the second Center for Astrophysics redshift survey. We use the Two Micron All Sky Survey to compare near-IR color-color diagrams for our sample with the Nearby Field Galaxy Sample and with a set of IRAS flux-limited pairs from Surace and coworkers. We construct a basic statistical model to explore the physical sources of the substantial differences among these samples. The model explains the spread of near-IR colors and is consistent with a picture in which central star formation is triggered by the galaxy-galaxy interaction before a merger occurs. For 160 galaxies we report new, deep JHK photometry within our spectroscopic aperture, and we use the combined spectroscopic and photometric data to explore the physical conditions in the central bursts. We find a set of objects with H - K ≥ 0.45 and with a large F_(FIR)/F_H. We interpret the very red H - K colors as evidence for 600-1000 K dust within compact star-forming regions, perhaps similar to super star clusters identified in individual well-studied interacting galaxies. The galaxies in our sample are candidate hidden bursts or, possibly, hidden active galactic nuclei. Over the entire pair sample, both spectroscopic and photometric data show that the specific star formation rate decreases with the projected separation of the pair. The data suggest that the near-IR color-color diagram is also a function of the projected separation; all of the objects with central near-IR colors indicative of bursts of star formation lie at small projected separation.

Emission-line properties of Seyfert 2 nuclei

This is the third paper of a series devoted to the study of the global properties of Joguet's sample of 79 nearby galaxies observable from the southern hemisphere, of which 65 are Seyfert 2 galaxies. We use the population synthesis models of Paper II to derive ‘pure’ emission-line spectra for the Seyfert 2 galaxies in the sample, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission-line clouds suffer substantially more extinction than the starlight, and we confirm the correlations between stellar and nebular velocity dispersions and between emission-line luminosity and velocity dispersions, although with substantial scatter. Nuclear luminosities correlate with stellar velocity dispersions, but Seyferts with conspicuous star-forming activity deviate systematically towards higher luminosities. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, L∝σ4★, consistent with the Faber-Jackson law. Emission-line ratios indicative of the gas excitation such as [O iii]/Hβ and [O iii]/[O ii] are statistically smaller for Seyferts with significant star formation, implying that ionization by massive stars is responsible for a substantial and sometimes even a dominant fraction of the Hβ and [O ii] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden active galactic nucleus (AGN). We correlate this fraction with classical indicators of AGN photoionization (i.e. X-ray luminosity and nebular excitation), but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays.

ChandraObservations of NGC 4261 (3C 270): Revealing the Jet and Hidden Active Galactic Nucleus in a Type 2 LINER

Chandra Observations of NGC 4261 (3C 270): Revealing the Jet and Hidden Active Galactic Nucleus in a Type 2 LINER A. Zezas1, M. Birkinshaw2, D. M. Worrall2, A. Peters3, and G. Fabbiano1 © 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. The Astrophysical Journal, Volume 627, Number 2 Article PDF View article References Citations 318 Total downloads Cited by 10 articles Turn on MathJax Share this article Get permission to re-use this article Article information Abstract We present results from a Chandra ACIS-S observation of the type 2 low-ionization nuclear emission-line region (LINER) radio galaxy NGC 4261 (3C 270). The X-ray data show that this galaxy hosts a heavily obscured (NH ~ 8 × 1022 cm-2) active galactic nucleus (AGN) with an intrinsic luminosity of 2.4 × 1041 ergs s-1, which dominates the energy output of the nucleus but is much less than the Eddington luminosity of its central massive black hole. The multiwavelength properties of this component are consistent with either radiatively inefficient accretion flows or emission from the inner part of a jet within 20 mas (3.4 pc) from the nucleus. A softer, unobscured power-law component, which we identify as synchrotron X-ray emission related to the unobscured part of the inner jet, produces ~10% of the 0.1-10.0 keV emission from the nucleus and is likely to be associated with the arcsecond-scale optical/UV emission measured with the Hubble Space Telescope (HST). These results indicate that the energy source of some type 2 LINERs could be a heavily obscured AGN, when obscuration of the ionizing continuum might be responsible for the low ionization state of the line-emitting gas.

A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei

Using the Green Bank Telescope, we have conducted a survey for 1.3 cm water emission toward the nuclei of nearby active galaxies, the most sensitive large survey for H2O masers to date. Among 145 galaxies observed, emission was newly detected in 11 sources and confirmed in one other. Our survey targeted nearby (v < 12,000 km s-1), mainly type 2 active galactic nuclei (AGNs) north of δ = -20° and includes a few additional sources as well. We find that more than one-third of Seyfert 2 galaxies have strong emission, although the detection rate declines beyond v ~ 5000 km s-1 because of sensitivity limits. Two of the masers discovered during this survey are found in unexpected hosts: NGC 4151 (Seyfert 1.5) and NGC 2782 (starburst). We discuss the possible relations between the large X-ray column to NGC 4151 and a possible hidden AGN in NGC 2782 to the detected masers. Four of the masers discovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, have high-velocity lines symmetrically spaced about the systemic velocity, a likely signature of molecular gas in a nuclear accretion disk. The source in NGC 6323, in particular, reveals the classic spectrum of a disk maser represented by three distinct groups of Doppler components. Future single-dish and VLBI observations of these four galaxies could provide a measurement of the distance to each galaxy and of the Hubble constant, independent of standard candle calibrations.

The star formation history of Seyfert 2 nuclei

We present a study of the stellar populations in the central ∼200 pc of a large and homogeneous sample comprising 79 nearby galaxies, most of which are Seyfert 2s. The star formation history of these nuclei is reconstructed by means of state‐of‐the‐art population synthesis modelling of their spectra in the 3500–5200 Å interval. A quasar‐like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden active galactic nucleus (AGN). We find the following. (1) The star formation history of Seyfert 2 nuclei is remarkably heterogeneous: young starbursts, intermediate‐age and old stellar populations all appear in significant and widely varying proportions. (2) A significant fraction of the nuclei show a strong FC component, but this FC is not always an indication of a hidden AGN: it can also betray the presence of a young, dusty starburst. (3) We detect weak broad Hβ emission in several Seyfert 2s after cleaning the observed spectrum by subtracting the synthesis model. These are most likely the weak scattered lines from the hidden broad‐line region envisaged in the unified model, given that in most of these cases independent spectropolarimetry data find a hidden Seyfert 1. (4) The FC strengths obtained by the spectral decomposition are substantially larger for the Seyfert 2s which present evidence of broad lines, implying that the scattered non‐stellar continuum is also detected. (5) There is no correlation between the star formation in the nucleus and either the central or overall morphology of the parent galaxies.

ChandraObservations of Arp 220: The Nuclear Source

We present the first results from 60 ks of observations of Arp 220 using the ACIS-S instrument on Chandra. We report the detection of several sources near the galaxy's nucleus, including a point source with a hard spectrum that is coincident with the western radio nucleus B. This point source is mildly absorbed (NH ~ 3 × 1022 cm-2) and has an estimated luminosity of 4 × 1040 ergs s-1. In addition, a fainter source may coincide with the eastern nucleus A. Extended hard X-ray emission in the vicinity raises the total estimated nuclear 2-10 keV X-ray luminosity to 1.2 × 1041 ergs s-1, but we cannot rule out a hidden active galactic nucleus behind columns exceeding 5 × 1024 cm-2. We also detect a peak of soft X-ray emission to the west of the nucleus and a hard point source 2.5 kpc from the nucleus with a luminosity of 6 × 1039 ergs s-1.

Discovery of a Binary Active Galactic Nucleus in the Ultraluminous Infrared Galaxy NGC 6240 Using Chandra

Ultraluminous infrared galaxies (ULIRGs) are outstanding due to their huge luminosity output in the infrared, which is predominantly powered by superstarbursts and/or hidden active galactic nuclei (AGNs). NGC 6240 is one of the nearest ULIRGs and is considered a key representative of its class. Here we report the first high-resolution imaging spectroscopy of NGC 6240 in X-rays. The observation, performed with the ACIS-S detector aboard the Chandra X-Ray Observatory, led to the discovery of two hard nuclei, coincident with the optical-IR nuclei of NGC 6240. The AGN character of both nuclei is revealed by the detection of absorbed, hard, luminous X-ray emission and two strong neutral Fe Kα lines. In addition, extended X-ray emission components are present, changing their rich structure in dependence of energy. The close correlation of the extended emission with the optical Hα emission of NGC 6240, in combination with the softness of its spectrum, clearly indicates its relation to starburst-driven superwind activity.