Radio-loud AGN Research Articles

A TALE OF TWO FEEDBACKS: STAR FORMATION IN THE HOST GALAXIES OF RADIO AGNs

Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star-formation activity in these galaxies. The exact nature of this link is still under debate. Radio jets have long been argued to be an ideal mechanism that allows AGN to interact with their host galaxy and regulate star-formation. In this context, we are using a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of the putative link between AGN activity and star-formation. This is done by means of spectral energy distribution (SED) fitting. We use the excellent spectral coverage of the AKARI infrared space telescope together with the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. Through SED fitting we constrain both the AGN and host galaxy components. We find a significant AGN component in our sample of relatively faint radio-sources ($<$mJy), that increases in power with increasing radio-luminosity. At the highest radio-luminosities, the presence of powerful jets dominates the radio emission of these sources. A positive correlation is found between the luminosity of the AGN component and that of star-formation in the host galaxy, independent of the radio luminosity. By contrast, for a given redshift and AGN luminosity, we find that increasing radio-luminosity leads to a decrease in the specific star-formation rate. The most radio-loud AGN are found to lie on the main sequence of star-formation for their respective redshifts. For the first time, such a two-sided feedback process is seen in the same sample. We conclude that radio jets do suppress star-formation in their host galaxies but appear not to totally quench it. Our results therefore support the maintenance nature of "radio-mode" feedback from radio-AGN jets.

An X-ray survey of the 2 Jy sample – I. Is there an accretion mode dichotomy in radio-loud AGN?

We carry out a systematic study of the X-ray emission from the active nuclei of the 0.02<z<0.7 2Jy sample, using Chandra and XMM-Newton observations. We combine our results with those from mid-IR, optical emission line and radio observations, and add them to those of the 3CRR sources. We show that the low-excitation objects in our samples \redit{show signs} of radiatively inefficient accretion. We study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. We also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. We derive the bolometric and jet kinetic luminosities for the samples and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at ~1 per cent and the latter at ~20 per cent Eddington. Our results are consistent with a simple Eddington switch when the effects of environment on radio luminosity and black hole mass calculations are considered. The apparent independence of jet kinetic power and radiative luminosity in the high-excitation population in our plots supports a model in which jet production and radiatively efficient accretion are not strongly correlated in high-excitation objects, though they have a common underlying mechanism.

Long-term variability of AGN at hard X-rays

Variability at all observed wavelengths is a distinctive property of AGN. Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Swift/BAT offers the unique opportunity to follow, on time scales of days to years and with a regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. We study the amplitude of the variations, and their dependence on sub-class and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of the AGN in the sample are found to exhibit significant variability on months to years time scales, radio loud sources being the most variable. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes of the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are well correlated, suggesting a common origin of the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on month time scale. In addition, sources with harder spectra are found to be more variable than softer ones. These properties are generally consistent with a variable power law continuum, in flux and shape, pivoting at energies >~ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).

Radio-loud active galactic nuclei at high redshifts and the cosmic microwave background

We discuss how the interaction between the electrons in a relativistic jet and the Cosmic Microwave Background (CMB) affects the observable properties of radio-loud AGN at early epochs. At high z the magnetic energy density in the radio lobes of powerful radio-loud quasars can be exceeded by the energy density of the CMB (because of its (1+z)^4 dependance). In this case, relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron. Thus, sources sharing the same intrinsic properties have different extended radio and X-ray luminosities when located at different z: more distant sources are less luminous in radio and more luminous in X-rays than their closer counterparts. Instead, in compact regions where the local magnetic field still exceeds the CMB in terms of energy density, synchrotron radiation would be unaffected by the presence of the CMB. Such regions include the compact inner jet and the so-called hot spots in the radio lobes. The decrease in radio luminosity is larger in misaligned sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux limited radio surveys, and therefore they are possibly under-represented in the associated samples. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB photons is less important at low radio frequencies. Therefore objects not detected so far in current surveys at a few GHz could be picked up by low frequency deep surveys, such as LOFAR and SKA. Until then, we can estimate the number of high redshift radio-loud AGNs through the census of their aligned proxies, i.e., blazars. Indeed, their observed radio emission arises in the inner and strongly magnetized compact core of the relativistic jet, and not affected by inverse Compton scattering off CMB photons.

The Wide-field Infrared Survey Explorer properties of complete samples of radio-loud active galactic nucleus

We present an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of 79 quasars and 273 radio galaxies, and covers a redshift range 0.003<z<3.395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed for the first time using large complete samples. Our results demonstrate that a division in the accretion modes of LERGs and HERGs clearly stands out in the mid-IR-radio plane (L_(22 \mu m) = 5x10^(43) erg s^(-1)). This means that WISE data can be effectively used to diagnose accretion modes in radio-loud AGN. The mid-IR properties of all objects were analysed to test the unification between quasars and radio galaxies, consistent with earlier work and we argue that smooth torus models best reproduce the observation. Quasars are found to have higher mid-IR luminosities than radio galaxies. We also studied all the sources in the near-IR to gain insights into evolution of AGN host galaxies. A relation found between the near-IR luminosity and redshift, well-known in the near-IR, is apparent in the two near-IR WISE bands, supporting the idea that radio sources are hosted by massive elliptical galaxies that formed their stars at high redshifts and evolved passively thereafter. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN.

Do the spectral energy distributions of type 1 active galactic nuclei show diversity?

We create broadband SEDs of 761 type 1 AGN. The Scott et al. sample, created by a cross-correlation of the optical SDSS DR5 quasar catalogue and the 2XMMi catalogue of serendipitous X-ray sources, is further matched with the FIRST catalogue of radio sources, the WISE MIR all-sky data release, the 2MASS NIR point source catalogue, the UKIDSS DR9 Large Area Survey and the GALEX all-sky and medium UV imaging surveys. This allows broadband SEDs covering log(nu)~9.2-18.1 to be created. We investigate variations in the SED shape by binning a subsample of 237 AGN with the best quality SEDs according to their X-ray spectral parameters, their AGN sub-type and their luminosity, black hole mass and Eddington ratio. The AGN sub-populations show some significant differences in their SEDs; X-ray absorbed AGN show a deficit of emission at X-ray/UV frequencies and an excess in the MIR consistent with absorption and re-emission, radio-loud AGN show increased radio and X-ray emission, consistent with the presence of a jet component in addition to the emission seen from radio-quiet AGN and the SEDs of NLS1s only differ from other type 1s in the X-ray regime, suggesting any physical differences are limited to their X-ray emitting region. Binning the AGN according to underlying physical parameters reveals more subtle differences in the SEDs. The X-ray spectral slope does not appear to have any influence or dependence on the multiwavelength emission in the rest of the SED. The contribution of X-rays to Lbol is lower in higher luminosity sources, and relatively more emission in the optical/UV is seen in AGN with higher Lx. Variations in the relative flux and peak frequency of the BBB are observed and may suggest higher inner disc temperatures with increasing accretion rates. Overall, we find that the diversity in the SED shapes is relatively small, and we find no apparent single driver for the variations.

MULTIWAVELENGTH PROBES OF THE ENVIRONS OF RELATIVISTIC SHOCKS IN BLAZAR JETS

Diffusive shock acceleration (DSA) at relativistic shocks is likely to be an important acceleration mechanism in various astrophysical jet sources, including radio-loud AGN. An important recent development for blazar science is the ability of Fermi-LAT data to pin down the power-law index of the high energy portion of emission in these sources, and therefore also the index of the underlying non-thermal particle population. This diagnostic potential was not possible prior to Fermi launch, when gamma-ray information was dominated by the highly-absorbed TeV band. This paper highlights how multiwavelength spectra including X-ray band and Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazar jets. The spectral index of the non-thermal particle distributions resulting from Monte Carlo simulations of DSA, and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles' mean free path scale, and also on the magnetic field obliquity to the shock normal. We investigate self-consistently the radiative synchrotron/Compton signatures of the resulting thermal and non-thermal particle distributions. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for the blazar AO 0235+164. The possible interpretation that turbulence levels decline with remoteness from jet shocks, and a significant role for non-gyroresonant diffusion, are discussed.

First X-ray observations of low-power compact steep spectrum sources

We report first X-ray Chandra observations of a sample of seven low luminosity compact (LLC) sources. They belong to a class of young compact steep spectrum (CSS) radio sources. Four of them have been detected, the other three have upper limit estimations for X-ray flux, one CSS galaxy is associated with an X-ray cluster. We have used the new observations together with the observational data for known strong CSS and gigahertz-peaked spectrum (GPS) objects and large scale FRIs and FRIIs to study the relation between morphology, X-ray properties and excitation modes in radio-loud AGNs. We found that: (1) The low power objects fit well to the already established X-ray - radio luminosity correlation for AGNs and occupy the space among, weaker in the X-rays, FRI objects. (2) The high excitation galaxies (HEG) and low excitation galaxies (LEG) occupy distinct locus in the radio/X-ray luminosity plane, notwithstanding their evolutionary stage. This is in agreement with the postulated different origin of the X-ray emission in these two group of objects. (3) We have tested the AGN evolution models by comparing the radio/X-ray luminosity ratio with the size of the sources, and indirectly, with their age. We conclude that the division for two different X-ray emission modes, namely originate in the base of the relativistic jet (FRIs) or in the accretion disk (FRIIs) is already present among the younger compact AGNs. (4) Finally, we found that the CSS sources are less obscured than the more compact GPSs in X-rays. However, the anti-correlation between X-ray column density and radio size does not hold for the whole sample of GPS and CSS objects.

An optical spectroscopic survey of the 3CR sample of radio galaxies withz&lt; 0.3

We explore the implications of our optical spectroscopic survey of 3CR radio sources with z<0.3 for the unified model (UM) for radio-loud AGN, focusing on objects with a FRII radio morphology. The sample contains 33 high ionization galaxies (HIGs) and 18 broad line objects (BLOs). According to the UM, HIGs, the narrow line sources, are the nuclearly obscured counterparts of BLOs. The fraction of HIGs indicates a covering factor of the circumnuclear matter of 65% that corresponds, adopting a torus geometry, to an opening angle of 50+/-5 degree. No dependence on redshift and luminosity on the torus opening angle emerges. We also consider the implications for a 'clumpy' torus. The distributions of radio luminosity of HIGs and BLOs are not statistically distinguishable, as expected from the UM. Conversely, BLOs have a radio core dominance, R, more than ten times larger than HIGs, as expected in case of jet Doppler boosting. Modeling the R distributions leads to an estimate of the jet bulk Lorentz factor of Gamma ~3-5. The test of the UM based on the radio source size is not conclusive due to the limited number of objects. Studying the emission line ([OIII], [OII]and [OI]) properties of HEGs and BLOs, we find evidences of a narrow line region (NLR) density stratification and its partial obscuration from the torus. In conclusion, the radio and NLR properties of HIGs and BLOs are consistent with the UM predictions. We also explored the radio properties of 21 3CR FRII low-ionization galaxies (LIGs) at z<0.3. We find that they cannot be part of the model that unifies HIGs and BLOs, but they are instead intrinsically different source, still reproduced by a randomly oriented population.

The host galaxy of the z = 2.4 radio-loud AGN MRC 0406−244 as seen by HST

We present multicolour Hubble Space Telescope images of the powerful z=2.4 radio galaxy MRC 0406-244 and model its complex morphology with several components including a host galaxy, a point source, and extended nebular and continuum emission. We suggest that the main progenitor of this radio galaxy was a normal, albeit massive (M ~10^{11} solar masses), star-forming galaxy. The optical stellar disc of the host galaxy is smooth and well described by a S\'ersic profile, which argues against a recent major merger, however there is also a point-source component which may be the remnant of a minor merger. The half-light radius of the optical disc is constrained to lie in the range 3.5 to 8.2kpc, which is of similar size to coeval star forming galaxies. Biconical shells of nebular emission and UV-bright continuum extend out from the host galaxy along the radio jet axis, which is also the minor axis of the host galaxy. The origin of the continuum emission is uncertain, but it is most likely to be young stars or dust-scattered light from the AGN, and it is possible that stars are forming from this material at a rate of 200^{+1420}_{-110} solar masses per year.

LOFAR and Radio Loud AGN

AbstractAt very low frequencies, the new pan-European radio telescope, LOFAR, is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. LOFAR will deeply survey the northern sky from ~ 10 up to 240 MHz. In this contribution we briefly describe some of the capabilities of LOFAR and the surveys planned to study fundamental issues related to the formation and evolution of galaxies and clusters of galaxies. We describe some of the challenges of low frequency observations with LOFAR and our progress in overcoming them. Further, we present some recent results from the ongoing imaging commissioning efforts. In the second part we discuss our studies of Low Excitation and High Excitation Radio Galaxies in the Boötes field and how LOFAR Surveys will help in studying their evolution.

Radio Properties of AGN

AbstractTo study the radio properties of AGN, we cross-correlate and investigate Veron-Cetty &amp; Veron catalog of QSOs and Active Galaxies (v.13, 2010) with a number of radio catalogs: NVSS, FIRST, GB6, 87GB, SUMSS, WISH, WENSS, and 7C. This catalog contains 168,940 objects with positional accuracy of mostly 1 arcsec, though many positions have larger errors. We use new cross-correlation software based on accuracy of each object independently. In this software we take into account errors for each source and take identifications with errors within 3 sigma. Altogether, we find ~16,000 AGN having radio detection in any of the listed catalogs. Using all data from radio catalogs, we derive a homogeneous sample of radio AGN. The sample allows accomplish several tasks, including study of the distribution of radio sources by activity types, differences in physical properties of radio-loud and radio-quiet AGN, luminosity functions for various types of radio AGN, study of the q parameter by AGN types and its evolution, etc.

Study of Spectral Energy Distributions of low-luminosity radio galaxies at z ~ 1–3: a high-z view of the host/AGN connection

AbstractLow-luminosity radio galaxies, common in the local Universe, are associated with giant elliptical galaxies and typically with a FR I radio morphology. However, they are rare in flux-limited samples of distant radio-loud (RL) AGN due to a selection bias. Chiaberge et al. (2009) selected the first sizeable sample of FRI candidates at 1&lt;z&lt;3, in the COSMOS field. We study the Spectral Energy Distributions (SEDs) of this low radio power sample from the far-UV to the Mid-IR wavelengths. Our results show that the hosts of these high-z low-luminosity radio sources are old massive galaxies, similar to the local FR Is. However, for half of the sample the UV and MIR excesses indicate the possible significant contribution from star formation and/or nuclear activity, not seen in low-z FR Is. Our sources display a wide variety of properties: from possible quasars at the highest luminosities, to low-luminosity old galaxies.

Very Long Baseline Interferometry observations: the closest look at the cores of AGN

AbstractVery Long Baseline Interferometry (VLBI) provides the highest angular resolution achievable in astronomy, reaching sub-milliarcsecond scales. For radio loud AGNs, this offers the unique opportunity to directly image and monitor the fine details of the jet structure, approaching the event horizon in the nearest and most supermassive black holes, like in M87. After a quick review of various VLBI facilities, we present results from recent VLBI monitoring projects on two remarkable radio and high energy sources: the radio galaxy M87 and the BL Lac object Mrk421. For the latter, we present a detailed analysis of the jet structure in total intensity and polarization through the whole 2011, during which a multi-wavelength campaign took place. We reveal flux density variability but no significant changes in the jet structure. In M87, we detect and follow the evolution of the core and of the jet feature HST-1; we reveal superluminal motion of components within HST-1, with a possible connection between ejection of new such components and the occurrence of very high energy flares. Estimates on the physical parameters for both sources are given (Doppler factor, viewing angle, magnetic field, etc.).

The AGN content of deep radio surveys and radio emission in radio-quiet AGN. Why every astronomer should care about deep radio fields

AbstractWe present our very recent results on the sub-mJy radio source populations at 1.4 GHz based on the Extended Chandra Deep Field South VLA survey, which reaches ~ 30 μJy, with details on their number counts, evolution, and luminosity functions. The sub-mJy radio sky turns out to be a complex mix of star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate and declining radio-loud AGN. While the well-known flattening of the radio number counts below 1 mJy is mostly due to star-forming galaxies, these sources and AGN make up an approximately equal fraction of the sub-mJy sky. Our results shed also light on a fifty-year-old issue, namely radio emission from radio-quiet AGN, and suggest that it is closely related to star formation, at least at z ~ 1.5 − 2. The implications of our findings for future, deeper radio surveys, including those with the Square Kilometre Array, are also discussed. One of the main messages, especially to non-radio astronomers, is that radio surveys are reaching such faint limits that, while previously they were mainly useful for radio quasars and radio galaxies, they are now detecting mostly star-forming galaxies and radio-quiet AGN, i.e., the bulk of the extragalactic sources studied in the infrared, optical, and X-ray bands.

The MASIV Legacy: Surveying AGN Intra-day Variability at Radio Wavelengths

AbstractThe Micro-arcsecond Scintillation-Induced Variability (MASIV) Survey and its follow-up observations have provided large datasets of AGN intra-day variability (IDV) at radio wavelengths. These data have shown that IDV arises mainly from scintillation caused by scattering in the ionized interstellar medium (ISM) of our Galaxy, based on correlation with Galactic latitudes and line-of-sight Galactic electron column densities. The sensitivity of interstellar scintillation (ISS) towards source angular sizes has provided a new tool for studying the most compact components of radio-loud AGNs at microarcsecond (μas) scale resolution - much higher than any ground-based radio interferometer. We present here key results from the MASIV Survey and its follow-up observations, and point to relevant papers where these results have been published.

Gamma-ray emitting narrow-line Seyfert 1 galaxies. New discoveries and open questions

AbstractThe discovery of γ-ray emission from 5 radio-loud narrow-line Seyfert 1 galaxies revealed the presence of a possible emerging third class of AGNs with relativistic jets, in addition to blazars and radio galaxies. The existence of relativistic jets also in this subclass of Seyfert galaxies opened an unexplored research space for our knowledge of the radio-loud AGNs. Here, we discuss the radio-to-γ-rays properties of the γ-ray emitting narrow-line Seyfert 1 galaxies, also in comparison with the blazar scenario.

Evolution of faint radio sources in the VIDEO-XMM3 field

It has been speculated that low luminosity radio-loud AGN have the potential to serve as an important source of AGN feedback, and may be responsible for suppressing star-formation activity in massive elliptical galaxies at late times. As such the cosmic evolution of these sources is vitally important to understanding the significance of such AGN feedback processes and their influence on the global star-formation history of the universe. In this paper we present a new investigation of the evolution of faint radio sources out to $z{\sim}2.5$. We combine a 1 square degree VLA radio survey, complete to a depth of 100 $\mu$Jy, with accurate 10 band photometric redshifts from the VIDEO and CFHTLS surveys. The results indicate that the radio population experiences mild positive evolution out to $z{\sim}1.2$ increasing their space density by a factor of $\sim$3, consistent with results of several previous studies. Beyond $z$=1.2 there is evidence of a slowing down of this evolution. Star-forming galaxies drive the more rapid evolution at low redshifts, $z{<}$1.2, while more slowly evolving AGN populations dominate at higher redshifts resulting in a decline in the evolution of the radio luminosity function at $z{>}$1.2. The evolution is best fit by pure luminosity evolution with star-forming galaxies evolving as $(1+z)^{2.47\pm0.12}$ and AGN as $(1+z)^{1.18\pm0.21}$.

X-ray emission around the z = 4.1 radio galaxy TN J1338−1942 and the potential role of far-infrared photons in AGN feedback

We report the discovery in an 80-ks observation of spatially-extended X-ray emission around the high-redshift radio galaxy TNJ1388-1942 (z=4.11) with the Chandra X-ray Observatory. The X-ray emission extends over a ~30-kpc diameter region and although it is less extended than the GHz-radio lobes, it is roughly aligned with them. We suggest that the X-ray emission arises from Inverse Compton (IC) scattering of photons by relativistic electrons around the radio galaxy. At z=4.11 this is the highest redshift detection of IC emission around a radio galaxy. We investigate the hypothesis that in this compact source, the Cosmic Microwave Background (CMB), which is ~700x more intense than at z~0 is nonetheless not the relevant seed photon field for the bulk of the IC emission. Instead, we find a tentative correlation between the IC emission and far-infrared luminosities of compact, far-infrared luminous high-redshift radio galaxies (those with lobe lengths of <100kpc). Based on these results we suggest that in the earliest phases of the evolution of radio-loud AGN at very high redshift, the far-infrared photons from the co-eval dusty starbursts occuring within these systems may make a significant contribution to their IC X-ray emission and so contribute to the feedback in these massive high-redshift galaxies.

KEPLERPHOTOMETRY OF FOUR RADIO-LOUD ACTIVE GALACTIC NUCLEI IN 2010-2012

We have used Kepler photometry to characterize variability in four radio-loud active galactic nuclei (three quasars and one object tentatively identified as a Seyfert 1.5 galaxy) on timescales from minutes to months, comparable to the light crossing time of the accretion disk around the central supermassive black hole or the base of the relativistic jet. Kepler's almost continuous observations provide much better temporal coverage than is possible from ground-based observations. We report the first such data analyzed for quasars. We have constructed power spectral densities using 8 Kepler quarters of long-cadence (30-minute) data for three AGN, 6 quarters for one AGN and 2 quarters of short-cadence (1-minute) data for all four AGN. On timescales longer than about 0.2-0.6 day, we find red noise with mean power-law slopes ranging from -1.8 to -1.2, consistent with the variability originating in turbulence either behind a shock or within an accretion disk. Each AGN has a range of red noise slopes which vary slightly by month and quarter of observation. No quasi-periodic oscillations of astrophysical origin were detected. We detected several days-long flares when brightness increased by 3% - 7% in two objects. No flares on timescales of minutes to hours were detected. Our observations imply that the duty cycle for enhanced activity in these radio-loud AGN is small. These well-sampled AGN light curves provide an impetus to develop more detailed models of turbulence in jets and instabilities in accretion disks.