The Radio Variability of the Gravitational Lens PMN J1838-3427

We present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a time scale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.Comment: To appear in AJ (8 pages, including 4 figures

Spitzer mid-infrared spectroscopy of compact symmetric objects: What powers radio-loud active galactic nuclei?

We present low- and high-resolution mid-infrared (mid-IR) spectra and photometry for eight compact symmetric objects (CSOs) taken with the Infrared Spectrograph on the Spitzer Space Telescope. The hosts of these young, powerful radio galaxies show significant diversity in their mid-IR spectra. This includes multiple atomic fine-structure lines, H2 gas, polycyclic aromatic hydrocarbon (PAH) emission, warm dust from T = 50 to 150 K, and silicate features in both emission and absorption. There is no evidence in the mid-IR of a single template for CSO hosts, but 5/8 galaxies show similar moderate levels of star formation (<10 M_sun/yr from PAH emission) and silicate dust in a clumpy torus. The total amount of extinction ranges from A_V ~ 10 to 30, and the high-ionization [Ne V] 14.3 and 24.3 um transitions are not detected for any galaxy in the sample. Almost all CSOs show contributions both from star formation and active galactic nuclei (AGNs), suggesting that they occupy a continuum between pure starbursts and AGNs. This is consistent with the hypothesis that radio galaxies are created following a galactic merger; the timing of the radio activity onset means that contributions to the IR luminosity from both merger-induced star formation and the central AGN are likely. Bondi accretion is capable of powering the radio jets for almost all CSOs in the sample; the lack of [Ne V] emission suggests an advection-dominated accretion flow mode as a possible candidate. Merging black holes (BHs) with M_BH > 10^8 M_sun likely exist in all of the CSOs in the sample; however, there is no direct evidence from these data that BH spin energy is being tapped as an alternative mode for powering the radio jets.Comment: 22 pages, 14 figures; published in Ap

The history and evolution of young and distant radio sources

We study two classes of object to gain a better understanding of the evolution of Active Galactic Nuclei (AGN): Infrared-Faint Radio Sources (IFRSs) and Gigahertz Peaked Spectrum (GPS) / Compact Steep Spectrum (CSS) sources. IFRSs are a recently discovered rare class of object, which were found to be strong in the radio but undetectable in extremely sensitive infrared observations from the Spitzer Space Telescope, even in stacked images with σ 3. Therefore, IFRSs may significantly increase the number of known high-redshift galaxies. However, their non-detections in the optical and infrared prevented confirmation of their nature. Previous studies of IFRSs focused on very sensitive observations of a few small regions of the sky, and the largest sample consisted of 55 IFRSs. However, we follow the strategy of combining radio data with IR and optical data for a large region of the sky. Using these data, we discover a population of >1300 brighter IFRSs which are, for the first time, reliably detected in the infrared and optical. We present the first spectroscopic redshifts of IFRSs and show that the brightest IFRSs are at z > 2. Furthermore, we rule out that IFRSs are Star Forming Galaxies, hotspots, lobes or misidentifications. We find the first X-ray counterparts of IFRSs, and increase the number of known polarised IFRSs five-fold. We present an analysis of their radio spectra and show that IFRSs consist of GPS, CSS and ultra-steep-spectrum sources. We follow up >50 of these using VLBI observations, and confirm the AGN status of IFRSs. We conclude that IFRSs represent a new population of high-redshift radio galaxies, which, for the faintest IFRSs, may have redshifts as high as z = 7 and consist of a few hundred thousand objects across the μJy sky. GPS and CSS sources are compact radio sources with a convex radio spectrum. They are widely thought to represent young and evolving radio galaxies that have recently launched their jets. However, good evidence exists in individual cases that GPS and CSS sources are one of the following: 1) frustrated by interactions with dense gas and dust in their environment; 2) prematurely dying radio sources; 3) recurrent radio galaxies. Their convex spectrum is generally thought to be caused by Synchrotron Self Absorption (SSA), an internal process in which the same population of electrons is responsible for the synchrotron emission and self-absorption. However, recent studies have shown that the convex spectrum may be caused by Free-Free Absorption (FFA), an external process in which an inhomogeneous screen absorbs the synchrotron emission. The majority of GPS and CSS samples consist of Jy-level and therefore, high-luminosity sources. VLBI images show that GPS and CSS sources typically have double-lobed, edge-brightened morphologies on mas scales, appearing as scaled down versions of Fanaroff-Riley Class II (FR II) galaxies. Recently, two low-luminosity GPS sources were found to have jet-brightened morphologies, which appeared as scaled down versions of Fanaroff-Riley Class I (FR I) galaxies. From this, it was proposed that there exists a morphology-luminosity break analogous to the FR I/II break and that low-luminosity GPS and CSS sources are the compact counterparts of FR I galaxies. However, this hypothesis remains unconfirmed, since very few samples of low-luminosity GPS and CSS sources exist. We have observed the faintest population of GPS and CSS sources to date, consisting ⇠150 sources, many of which are low-luminosity. We use high-resolution radio observations to determine their linear size, resolve their jets and observe their small-scale morphology. We combine these data with a large number of radio observations at other frequencies to model their radio spectra using SSA and FFA models. In particular, we use very low frequency observations that have only recently become available to constrain their spectral peaks. We follow up eight of the most compact sources with VLBI and detect six of them. We find that our GPS and CSS sources are well modelled by an inhomogeneous FFA model (hereafter ‘FFA’). Furthermore, we find a number of very compact GPS and CSS sources that are inconsistent with SSA theory. We show that a single inhomogeneous SSA model fits poorly to the majority of radio spectra, predicting far too steep a slope below the peak. We resolve all of the sources with VLBI and derive their kinematic ages based on the jet sizes. Even when assuming more complex SSA models, we derive magnetic field strengths several orders of magnitude too high for one source. A few sources are well modelled by an FFA model in which the inhomogeneous absorption is dominated by clouds of high density, consistent with the frustration hypothesis. However, the majority of sources are well modelled by FFA models with low-density clouds. These models suggest that an inhomogeneous and clumpy medium surrounds the sources, implying they may undergo recurrent activity. Furthermore, the spectral model of one CSS source suggests it is a prematurely dying radio galaxy whose jets have been switched off for ∽600 years. However, we find no evidence of restarted radio galaxies within our high resolution observations. The kinematic and spectral ages we derive are consistent with the hypothesis that GPS and CSS sources are young and evolving. We find tentative evidence that at mJy-levels, the fraction of CSS sources is smaller than the fraction estimated for Jy-level sources. This may be accounted for as a selection effect or if a smaller fraction of mJy-level GPS sources evolve into CSS sources as compared to the Jy-level GPS sources. We find a few GPS sources with low luminosities, which we will follow up with VLBI to test whether they are the compact counterparts of FR I galaxies. We conclude that, despite being historically favoured, single inhomogeneous SSA is not the dominant form of absorption amongst a large fraction of GPS and CSS sources. We find that FFA provides a good model for the majority of the spectra with observable turnovers, suggesting an inhomogeneous and clumpy ambient medium. Furthermore, we conclude that the majority of our GPS and CSS sources are young and evolving and may undergo recurrent activity over small time scales. We conclude that a very small fraction of GPS and CSS sources consists of frustrated, dying or restarted radio galaxies

Enormous disc of cool gas surrounding the nearby powerful radio galaxy NGC 612 (PKS 0131-36)

We present the detection of an enormous disc of cool neutral hydrogen (HI) gas surrounding the S0 galaxy NGC 612, which hosts one of the nearest powerful radio sources (PKS 0131-36). Using the Australia Telescope Compact Array, we detect M_HI = 1.8 x 10^9 M_sun of HI emission-line gas that is distributed in a 140 kpc wide disc-like structure along the optical disc and dust-lane of NGC 612. The bulk of the gas in the disc appears to be settled in regular rotation with a total velocity range of 850 km/s, although asymmetries in this disc indicate that perturbations are being exerted on part of the gas, possibly by a number of nearby companions. The HI disc in NGC 612 suggests that the total mass enclosed by the system is M_enc ~ 2.9 x 10^12 sin^-2(i) M_sun, implying that this early-type galaxy contains a massive dark matter halo. We also discuss an earlier study by Holt et al. that revealed the presence of a prominent young stellar population at various locations throughout the disc of NGC 612, indicating that this is a rare example of an extended radio source that is hosted by a galaxy with a large-scale star-forming disc. In addition, we map a faint HI bridge along a distance of 400 kpc in between NGC 612 and the gas-rich (M_HI = 8.9 x 10^9 M_sun) barred galaxy NGC 619, indicating that likely an interaction between both systems occurred. From the unusual amounts of HI gas and young stars in this early-type galaxy, in combination with the detection of a faint optical shell and the system's high infra-red luminosity, we argue that either ongoing or past galaxy interactions or a major merger event are a likely mechanism for the triggering of the radio source in NGC 612. This paper is part of an ongoing study to map the large-scale neutral hydrogen properties of nearby radio galaxies. --abridged--Comment: 13 pages, 9 figures. MNRAS in press. See http://www.blackwell-synergy.com/doi/full/10.1111/j.1365-2966.2008.13142.x for a full resolution versio

Compact Symmetric Objects -- I Towards a Comprehensive Bona Fide Catalog

Compact Symmetric Objects (CSOs) are jetted Active Galactic Nuclei (AGN) with overall projected size <1 kpc. The classification was introduced to distinguish these objects from the majority of compact jetted-AGN in centimeter wavelength very long baseline interferometry observations, where the observed emission is relativistically boosted towards the observer. The original classification criteria for CSOs were: (i) evidence of emission on both sides of the center of activity, and (ii) overall size <1 kpc. However some relativistically boosted objects with jet axes close to the line of sight appear symmetric and have been mis-classified as CSOs, thereby undermining the CSO classification. This is because two essential CSO properties, pointed out in the original papers, have been neglected: (iii) low variability, and (iv) low apparent speeds along the jets. As a first step towards creating a comprehensive catalog of ``bona fide'' CSOs, we identify 79 bona fide CSOs, including 15 objects claimed as confirmed CSOs here for the first time, that match the CSO selection criteria. This sample of bona fide CSOs can be used for astrophysical studies of CSOs without contamination by mis-classified CSOs. We show that the fraction of CSOs in complete flux density limited AGN samples with S$_{\rm 5\,GHz}$ >700 mJy is between $(6.8\pm1.6)$% and $(8.5\pm1.8)$%.Comment: 28 pages, 9 figures, 3 tables, accepted for publicatio

Investigating high-energy emission in young radio galaxies

Some of the most intriguing questions about the physics and the evolution of radio galaxies are related to the early stages of their life. The so-called Compact Symmetric Objects (CSOs), defined to be those with radio lobe emission on both sides of an active nucleus and an overall size less than about one kpc, are thought to represent the first stage in the evolutionary path of radio galaxies. CSOs are considered a perfect test bench to study the accretion and ejection processes in the early-born radio galaxies and, in view of their compact dimensions, to investigate the interaction between the expanding radio source and the host galaxy. In fact, an alternative scenario ascribes the compactness of CSOs to an extremely dense interstellar medium of the host galaxy, which can slow down or even completely halt the radio source expansion. For this Thesis work, a mini-sample of three CSOs has been selected based on their detection in the X- and gamma-ray band: 1718-649, 1146+596, and 1843+356. The aim of this work was to investigate the origin of the high-energy emission in these objects, infer their physical properties using the broadband spectral energy distribution, and understand the role of the ambient medium in their evolution. Thanks to the high quality of the X-ray data, I was able to: (i) characterize the spectral properties of the unresolved X-ray emission associated with the CSOs; (ii) investigate the hot gaseous component co-spatial with the CSOs; (iii) measure the intrinsic obscuration of the X-ray emission, hence probe the presence of dense gas. In the second part of the study, I considered the broadband emission of two of the targets against theoretical models for the non-thermal emission of lobes and jets. Modeling of the radio-to-gamma-ray emission has provided estimates of the physical parameters of the young sources (e.g. magnetic field, particle contents, etc.) and of the total jet power, allowing to make predictions on the fate of the sources

Looking Through Rainbow Coloured Glasses: Radio Spectral Variability and its Physical Origins

This thesis investigates supermassive black holes in the centres of galaxies producing large, mushroom shaped clouds up to millions of light years across. We focus on baby black holes, which are smaller than typical radio galaxies, often just tens of thousands of light years across. Using telescopes around Australia, this work discovered these black holes are not as young as previously thought, but frustrated teens being restricted by a surrounding cloud of gas

PKS 1718–649, the Nearest GPS Radio Source

AbstractPKS 1718–649 has a classic GPS spectrum and is identified with the galaxy NGC 6328. At a distance of 56 Mpc, PKS 1718–649 becomes the nearest known GPS radio source.</jats:p