David Harold. Whysong

THE NUCLEAR REDDENING CURVE FOR ACTIVE GALACTIC NUCLEI AND THE SHAPE OF THE INFRARED TO X-RAY SPECTRAL ENERGY DISTRIBUTION

We present extinction curves derived from the broad emission lines and continua of samples of 72 radio-loud and 1018 radio-quiet active galactic nuclei (AGNs). The curves are significantly flatter in the UV than curves for the local interstellar medium. The reddening curves for the radio-quiet Large Bright Quasar Survey quasars are slightly steeper than those of the radio-loud quasars in the UV, probably because of additional reddening by dust farther out in the host galaxies of the former. The UV extinction curves for the radio-loud AGNs are very flat. This is explicable with slight modifications to standard Mathis-Rumpl-Nordsieck dust models: there is a relative lack of small grains in the nuclear dust. Our continuum and broad emission line reddening curves agree in both shape and amplitude, confirming that the continuum shape is indeed profoundly affected by reddening for all but the bluest AGNs. With correction by our generic extinction curve, all of the radio-loud AGNs have optical-UV continuous spectra consistent with a single shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical shape over orders of magnitude in luminosity. We also argue that radio-loud and radio-quiet AGNs probably share the same underlying continuum shape and that most of the systematic differences between their observed continuum shapes are due to higher nuclear reddening in radio-selected AGNs and additional reddening from dust farther out in the host galaxies in radio-quiet AGNs. Our conclusions have important implications for the modeling of quasar continua and the analysis of quasar demographics.

THERMAL EMISSION AS A TEST FOR HIDDEN NUCLEI IN NEARBY RADIO GALAXIES

The clear sign of a hidden quasar inside a radio galaxy is the appearance of quasar spectral features in its polarized (scattered) light. However, that observational test requires suitably placed scattering material to act as a mirror, allowing us to see the nuclear light. A rather robust and more general test for a hidden quasar is to look for the predicted high mid-IR luminosity from the nuclear obscuring matter. The nuclear waste heat is detected and well isolated in the nearest narrow-line radio galaxy, Cen A. This confirms other indications that Cen A does contain a modest quasar-like nucleus. However, we show here that M87 does not: at high spatial resolution, the mid-IR nucleus is seen to be very weak and consistent with simple synchrotron emission from the base of the radio jet. This fairly robustly establishes that there are real narrow-line radio galaxies, without the putative accretion power and with essentially all the luminosity in kinetic form. Next, we show the intriguing morphology of Cygnus A, where all of the mid-IR emission is consistent with reprocessing by the hidden quasar known to exist from spectropolarimetry and other evidence.

Shocked Molecular Hydrogen in the 3C 326 Radio Galaxy System

The Spitzer spectrum of the giant FR II radio galaxy 3C 326 is dominated by very strong molecular hydrogen emission lines on a faint IR continuum. The H2 emission originates in the northern component of a double-galaxy system associated with 3C 326. The integrated luminosity in H2 pure rotational lines is 8.0 × 1041 erg s-1, which corresponds to 17% of the 8-70 μm luminosity of the galaxy. A wide range of temperatures (125-1000 K) is measured from the H2 0-0 S(0)-S(7) transitions, leading to a warm H2 mass of 1.1 × 109 M☉. Low-excitation ionic forbidden emission lines are consistent with an optical LINER classification for the active nucleus, which is not luminous enough to power the observed H2 emission. The H2 could be shock heated by the radio jets, but there is no direct indication of this. More likely, the H2 is shock heated in a tidal accretion flow induced by interaction with the southern companion galaxy. The latter scenario is supported by an irregular morphology, a tidal bridge, and a possible tidal tail imaged with IRAC at 3-9 μm. Unlike ultraluminous infrared galaxies, which in some cases exhibit H2 line luminosities of comparable strength, 3C 326 shows little star formation activity (~0.1 M☉ yr-1). This may represent an important stage in galaxy evolution. Starburst activity and efficient accretion onto the central supermassive black hole may be delayed until the shock-heated H2 can kinematically settle and cool.

New insights on selected radio galaxy nuclei

Abstract We are performing a survey of powerful 3CR radio galaxies and quasars in the mid-infrared. The purpose is to test for the presence of a powerful hidden quasar-like thermal nucleus by measuring the ‘waste heat’ that must be emitted by any obscuring dust. The dust is treated as a calorimeter for the central engine. Three early mid-IR detections are particularly interesting: Cygnus A, M87, and Centaurus A. These are notable both as a demonstration of our technique and as an example of the great variety of objects that are classified as radio galaxies. We confirm the presence of hidden quasar-like nuclei in Cyg A and Cen A, but M87 shows only weak mid-IR emission, indicating its AGN is non-thermal, and consistent with only synchrotron emission. We also present a new near-infrared adaptive optics image of Cygnus A which shows a secondary point source.

Cosmological Halos: A Search for the Ionized Intergalactic Medium

Standard big bang nucleosynthesis predicts the average baryon density of the universe to be a few percent of the critical density. Only about one-tenth of the predicted baryons have been seen. A plausible repository for the missing baryons is in a diffuse ionized intergalactic medium (IGM). In an attempt to measure the IGM, we searched for Thomson-scattered halos around strong high-redshift radio sources. Observations of the radio source 1935-692 were made with the Australia Telescope Compact Array. We assumed a uniform IGM, and isotropic steady emission of 1935-692 for a duration between 107 and 108 yr. A model of the expected halo visibility function was used in χ2 fits to place upper limits on ΩIGM. The upper limits varied depending on the methods used to characterize systematic errors in the data. The results are 2 σ limits of ΩIGM < 0.65. While not yet at the sensitivity level to test primordial nucleosynthesis, improvements in the technique will probably allow this in future studies.