Andrew S. Wilson

The difference between radio-loud and radio-quiet active galaxies

The recent development of unified theories of active galactic nuclei (AGNs) has indicated that there are two physically distinct classes of these objects--radio-loud and radio-quiet. Despite differences, the (probable) thermal emissions from the AGNs (continua and lines from X-ray to infrared wavelengths) are quite similar to the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes and propose that galaxies (e.g., spirals) which have not suffered a recent major merger event contain nonrotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. The ratio of the number of radio-loud to radio-quiet AGNs at a given thermal (e.g., optical) luminosity is determined by the galaxy merger rate. Comparisons between the predicted and observed radio luminosity functions constrain the efficiencies with which jet power is extracted from the spinning hole and radio emission is produced by the jet.

Chandra Snapshot Observations of Low-Luminosity Active Galactic Nuclei with a Compact Radio Source

The results of Chandra snapshot observations of 11 low-ionization nuclear emission-line regions (LINERs), three low-luminosity Seyfert galaxies, and one H II-LINER transition object are presented. Our sample consists of all the objects with a flat- or inverted-spectrum, compact radio core in the Very Large Array survey of 48 low-luminosity AGNs (LLAGNs) by Nagar and coworkers in 2000. An X-ray nucleus is detected in all galaxies except one, and their X-ray luminosities are in the range 5 ? 1038-8 ? 1041 ergs s-1. The X-ray spectra are generally steeper than expected from thermal bremsstrahlung emission from an advection-dominated accretion flow. The X-ray-to-H? luminosity ratios for 11 of 14 objects are in good agreement, with the value characteristic of LLAGNs and more luminous AGNs, and indicate that their optical emission lines are predominantly powered by an LLAGN. For three objects, this ratio is less than expected. Comparing with properties in other wavelengths, we find that these three galaxies are most likely to be heavily obscured AGNs. We use the ratio RX = ?L?(5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure of radio loudness. In contrast to the usual definition of radio loudness [Ro = L?(5 GHz)/L?(B)], RX can be used for heavily obscured (NH 1023 cm-2, AV > 50 mag) nuclei. Further, with the high spatial resolution of Chandra, the nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert galaxies, quasars, and radio galaxies and confirm the suggestion that a large fraction of LLAGNs are radio-loud.

ACCRETION ONTO THE SUPERMASSIVE BLACK HOLE IN M87

Chandra X-ray observations of the giant elliptical galaxy M87 resolve the thermal state of the hot interstellar medium into the accretion (Bondi) radius of its central 3 × 109 M☉ black hole. We measure the X-ray gas temperature and density profiles and calculate the Bondi accretion rate, Bondi ~ 0.1 M☉ yr-1. The X-ray luminosity of the active nucleus of M87 observed with Chandra is LX,0.5-7 keV ~ 7 × 1040 ergs s-1. This value is much less than the predicted nuclear luminosity, LBondi ~ 5 × 1044 ergs s-1, for accretion at the Bondi rate with a canonical accretion radiative efficiency of 10%. If the black hole in M87 accretes at this rate it must do so at a much lower radiative efficiency than the canonical value. The multiwavelength spectrum of the nucleus is consistent with that predicted by an advection-dominated flow. However, as is likely, the X-ray nucleus is dominated by jet emission then the properties of flow must be modified, possibly by outflows. We show that the overall energetics of the system are just consistent with the predicted Bondi nuclear power. This suggests that either most of the accretion energy is released in the relativistic jet or the central engine of M87 undergoes on-off activity cycles. We show that, at present, the energy dumped into the ISM by the jet may reduce the accretion rate onto the black hole by a factor ∝ (vj/cs)-2, where vj is the jet velocity and cs the ISM sound speed, and that this is sufficient to account for the low nuclear luminosity.

A Powerful Nuclear Starburst in the Seyfert Galaxy Markarian 477: Implications for the Starburst-Active Galactic Nucleus Connection

There have been many suggestions that starbursts may play an important role in Seyfert galaxies. Recent analyses of the UV, optical, and near-IR featureless continuum (FC) in type 2 Seyfert nuclei have provided indirect evidence that the FC may be produced by a compact starburst and that this starburst may provide a power source for the pronounced far-IR excess in type 2 Seyfert galaxies. In the present paper, we present and discuss Hubble Space Telescope (HST) images and a UV spectrum plus new ground-based near-UV through near-IR spectra of Mrk 477 (=I Zw 92). This object is arguably the most powerful type 2 Seyfert nucleus in the local universe and thus offers a unique opportunity for detailed investigation of the nature of the FC. The data provide direct evidence that the observed UV through near-IR continuum in the nucleus of Mrk 477 is dominated by light from a dusty starburst. An HST image of the space-UV continuum shows that it arises in a compact (few hundred parsec) region, much smaller than the sizes of previously studied circumnuclear starbursts in Seyfert galaxies like NGC 1068. In the HST space-UV spectrum of this region, we detect strong N V λ1240 and Si IV λ1400 stellar wind lines and other weak photospheric lines from hot stars. In the optical spectrum we detect a broad emission complex around the He II λ4686 line that may be produced by an ensemble of about 30,000 Wolf-Rayet stars (WN subtype). In the near-IR spectrum we detect a strong Ca II triplet feature produced by red supergiants. Detailed comparison of the data with models implies that the nuclear starburst in Mrk 477 has an age of about 6 Myr, was of short duration (an instantaneous burst), and has solar or higher metallicity and a bolometric luminosity of ~(3 × 1010)-1011 L☉. This nuclear starburst must therefore make a significant contribution to the overall energetics of Mrk 477, even though optical spectropolarimetry demonstrates that a powerful hidden type 1 Seyfert nucleus is also present. We speculate that the compact starburst is occurring in the outer reaches of the obscuring torus that hides the type 1 Seyfert nucleus. An observational program that is underway will determine whether Mrk 477 is typical of Seyfert 2 nuclei and, thus, whether nuclear starbursts are an important part of the Seyfert phenomenon.

Multiwavelength tests of the dusty torus model for Seyfert galaxies

We present a compilation of emission properties for a sample of 116 Seyfert galaxies based on both previously unpublished data and measurements available in the literature. These measurements include fluxes in the emission lines (O III) lambda(5007) and H-beta, as well as the infrared (25-60 microns), ultraviolet (1450 A), soft (0.2-4 keV), and hard (2-10 keV) X-ray continua. These are used to try to distinguish between isotropic and anisotropic emission properties of Seyfert galaxies. The distribution functions of (O III) lambda 5007 infrared, and hard X-ray continuum are similar for Seyfert 1s and Seyfert 2s, consistent with these properties being isotropic. The ultraviolet and soft X-ray continua of Seyfert 2s are underluminous relative to the type 1s suggesting photons at these energies escape from the central source anisotropically. There is a correlation between the ultraviolet continuum and emission-line fluxes in Seyfert 1s consistent with the idea that the central engine is responsible for powering the line emission. No such correlation is found for the Seyfert 2s. Instead, the scatter in the plot of ultraviolet continuum versus line emission suggests the true nuclear continuum luminosity is not seen at Earth in these objects. These properties are consistent with those expected in the dusty torus model.

Hubble Space Telescope and VLA Observations of Seyfert 2 Galaxies: The Relationship between Radio Ejecta and the Narrow-Line Region*

We present HST/WFPC2 images, in narrowband filters containing the [O III] λ5007 and Hα + [N II] emission lines and their adjacent continua, of a sample of seven Seyfert 2 galaxies selected because they possess either extended emission-line regions in ground-based observations or a hidden broad-line region in polarized light. Six of the galaxies have also been observed with the VLA in order to obtain radio maps of better quality and angular resolution than those in the literature. We find detailed correspondences between features in the radio and emission-line images, clearly indicating strong interactions between the radio jets and the interstellar medium. Such interactions play a major role in determining the morphology of the NLR, as the radio jets sweep up and compress ambient gas, producing ordered structures with enhanced surface brightness in line emission. In at least three galaxies, namely, Mrk 573, ESO 428-G14, and Mrk 34 (and perhaps also NGC 7212), off-nuclear radio lobes coincide with regions of low gaseous excitation (as measured by the [O III]/(Hα + [N II]) ratio). In Mrk 573 and NGC 4388, there is a clear trend for low-brightness ionized gas to be of higher excitation. These results may be understood if radio lobes and regions of high emission-line surface brightness are associated with high gas densities, reducing the ionization parameter. [O III]/(Hα + [N II]) excitation maps reveal bipolar structures that can be interpreted as either the ionization cones expected in the unified scheme or widening, self-excited gaseous outflows. Only NGC 4388 and Mrk 573 show a clearly defined, straight-edged ionization cone.

Ultraviolet-Optical Observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the Starburst-Active Galactic Nucleus Connection

We present and discuss Hubble Space Telescope (HST) (WFPC2 and FOC) images and ultraviolet (GHRS) spectra plus ground-based optical spectra of three Seyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies, together with Mrk 477 (Heckman et al.), were selected on the basis of ultraviolet brightness from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the goal of studying the starburst-active galactic nucleus (AGN) connection and the origin of the so-called featureless continuum in Seyfert 2 nuclei. The data provide direct evidence of the existence of nuclear starbursts that dominate the ultraviolet light and that are responsible for the featureless continuum in these type 2 Seyfert nuclei. The GHRS spectra show absorption features formed in the photospheres (S V λ1501, C III λλ1426, 1428, Si III λ1417, and Si III + P III λ1341-1344) and in the stellar winds (C IV λ1550, Si IV λ1400, and N V λ1240) of massive stars. Signatures of massive stars are also clearly detected in their optical and near-UV spectra where the high-order Balmer series and He I lines are observed in absorption. These lines are formed in the photospheres of O and B stars, and thus they also provide strong independent evidence of the presence of massive stars in the nuclei of these Seyfert 2 nuclei. Interstellar absorption lines similar to those formed in the interstellar medium of starbursts are also observed. They are blueshifted by a few hundred km s-1 with respect to the systemic velocity, indicating that the interstellar gas is outflowing. These outflows are most likely driven by the nuclear starburst. These starbursts are dusty, compact, and powerful. They have sizes ranging from less than 100 pc to a few hundred parsecs (much smaller than that seen in the prototype Seyfert 2 galaxy NGC 1068). Their UV colors imply that they are heavily reddened (by 2 to 3 mag in the UV), and the implied bolometric luminosities are of order 1010 L☉. The bolometric luminosities of these starbursts are similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei. The data on this small sample suggest that more powerful AGNs may be related to more powerful central starbursts. Comparing the HST spectra to IUE spectra obtained through apertures with projected sizes of 3-11 kpc (and to IRAS far-IR data) we estimate that the nuclear starbursts account for 6%-25% of the total intrinsic UV luminosity of the entire galaxy.

CHANDRA X-RAY OBSERVATIONS OF PICTOR A: HIGH-ENERGY COSMIC RAYS IN A RADIO GALAXY?

We report X-ray observations of the nearby, powerful radio galaxy Pictor A with the Chandra Observatory and optical and near-UV observations of its western radio hot spot with the Hubble Space Telescope. X-ray emission is detected from the nucleus, a 19 (110 kpc) long jet to the west of the nucleus, the western radio hot spot some 42 (240 kpc) from the nucleus, and the eastern radio lobe. The morphology of the western hot spot is remarkably similar to that seen at radio and optical wavelengths, where the emission is known to be synchrotron radiation. The X-ray spectrum of the hot spot is well described by an absorbed power law with photon index Γ = 2.07 ± 0.11. The X-ray jet coincides with a weak radio jet and is laterally extended by 20 (1.9 kpc). The observed jet is up to 15 times brighter in X-rays than any counterjet, a difference ascribed to relativistic boosting since the western radio lobe is probably the closer. The jets spectrum is well modeled by an absorbed power law with Γ = 1.94 and poorly fitted by a Raymond-Smith thermal plasma model. The emission processes responsible for the X-rays are discussed in detail. The radio-to-optical spectrum of the hot spot breaks or turns down at 1013-1014 Hz, and its X-ray spectrum is not a simple extension of the radio-to-optical spectrum to higher frequencies. Thermal models for the hot spots X-ray emission are ruled out. Synchrotron self-Compton models involving scattering from the known population of electrons give the wrong spectral index for the hot spots X-ray emission and are also excluded. A composite synchrotron plus synchrotron self-Compton model can match the X-ray observations but requires similar contributions from the two components in the Chandra band. We show that the hot spots X-ray emission could be synchrotron self-Compton emission from a hitherto unobserved population of electrons emitting at low radio frequencies but do not favor this model in view of the very weak magnetic field required. An inverse Compton model of the jet, in which it scatters microwave background photons but moves nonrelativistically, requires a magnetic field a factor of 30 below equipartition and ad hoc conditions to explain why the radio lobes are fainter than the jet in X-rays but brighter in the radio. These problems are alleviated if the jet moves relativistically, but models with an equipartition field require an implausibly small angle (θ) between the jet and the line of sight. A model with θ 23° and a field a factor of 6 below equipartition seems viable. Synchrotron radiation is an alternative process for the X-ray emission. The expected synchrotron spectrum from relativistic electrons accelerated by strong shocks and subject to synchrotron radiation losses is in very good agreement with that observed for both the hot spot and jet. The possibility that the relativistic electrons result via photopion production by high-energy protons accelerated in shocks (a proton-induced cascade) is briefly discussed.

Double-peaked broad line emission from the LINER nucleus of NGC 1097

We report the recent appearance of a very broad component in the H-alpha and H-beta emission lines of the weakly active nucleus of the Sersic-Pastoriza galaxy NGC 1097. The FWZI of the broad component is about 21,000 km/s, and its profile is double-peaked; the presence of a blue, featureless continuum in the nucleus is also suggested. The broad component was first observed in H-alpha in November 2, 1991, and confirmed 11 months later. The H-alpha profile and flux did not change in this time interval. Comparison with previously published spectral data indicates that the broad lines have only recently appeared. Together with the relatively high X-ray luminosity and the compact nuclear radio source, our results characterize the presence of a Seyfert 1 nucleus in a galaxy which had previously shown only LINER characteristics. Obscuring material along our line of sight to the nucleus appears to have recently cleared, permitting a direct view of the active nucleus. We discuss two possible structures for the broad line region, biconical outflow and an accretion disk, that could give rise to the observed profile.

A Survey for H2O Megamasers in Active Galactic Nuclei. II. A Comparison of Detected and Undetected Galaxies

A survey for H2O megamaser emission from 354 active galaxies has resulted in the detection of 10 new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities less than 7000 km s-1) and a magnitude-limited sample (covering Seyferts and LINERs with mB ≤ 14.5). In order to determine whether the H2O-detected galaxies are typical active galactic nuclei (AGNs) or have special properties that facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic properties of the observed galaxies. The most significant finding is that H2O megamasers are detected only in Seyfert 2 and LINER galaxies, not Seyfert 1s. This lack of detection in Seyfert 1s indicates either that they do not have molecular gas in their nuclei with physical conditions appropriate to produce 1.3 cm H2O masers or that the masers are beamed away from Earth, presumably in the plane of the putative molecular torus that hides the Seyfert 1 nucleus in Seyfert 2s. LINERs are detected at a similar rate to Seyfert 2s, which constitutes a strong argument that at least some nuclear LINERs are AGNs rather than starbursts, since starbursts have not been detected as H2O megamasers. We preferentially detect H2O emission from the nearer galaxies and from those that are apparently brighter at mid- and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H2O-detected galaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected ones, though these data are incomplete. We find evidence that Seyfert 2s with very high (NH > 1024 cm-2) X-ray-absorbing columns of gas are more often detected as H2O maser emitters than Seyfert 2s with lower columns. It may be that the probability of detecting H2O maser emission in Seyfert galaxies increases with increasing column of cool gas to the nucleus, from Seyfert 1s through narrow-line X-ray galaxies to Seyfert 2s.