The AGN/Starburst connection in luminous infrared galaxies

We are addressing the issue of whether there exists an evolutionaryconnection between starburst and AGN in luminous infrared galaxies. We are combining theoretical modelling with optical, radio and infrared data from IRAS for a large sample of 285 infrared galaxies with a range ofluminosities. In this paper, we present a comparison between the optical spectroscopic data with the incidence of compact radio cores for a subsample of these galaxies. We find 90% of AGN type galaxies contain compact radio cores, while 37% of starburst galaxies contain compact radio cores. The compact radio cores in the starburst galaxies have a minimum brightness temperature of 3 × 10⁵ K, higher than those of standard extended HII regions and may be obscured AGN or complexes of extremely luminous supernovae such as that seen in Arp 220.     

Radio Intra-Day Variability: Answers and Questions

Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as10¹⁸ to 10²¹ K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high. This revised version was published online in July 2006 with corrections to the Cover Date.     

EVN observations of low-luminosity flat-spectrum active galactic nuclei

We present and discuss the results of very-long baseline interferometry (VLBI, EVN) observations of three low-luminosity     broad emission line active galactic nuclei (AGNs) carefully selected from a sample of flat-spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about     . This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years' time.     

The most compact bright radio-loud AGN—I. A new target sample selected for the space VLBI

We investigated the archival ground-based VLBI images of the extragalactic radio sources included in both the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck catalogues, and selected 49 bright and compact sources as potential targets for space Very Long Baseline Interferometry (VLBI) observations at mm wavelengths. These sources have a flat radio continuum spectrum between 33 and 94 GHz. They are identified as core-dominated active galactic nuclei (AGN), located at declinations above ?40^°, and have never been observed with ground-based VLBI at 86 GHz. The radio properties of the 49 new sources are presented. We compare this new sample with similar samples of compact AGN available from earlier studies. The new candidates, together with the existing bright compact AGN sample identified from 86-GHz ground-based VLBI imaging surveys, form a catalogue of more than 160 AGN. These could be primary targets for mm-VLBI observations on the ground, as well as for future mm-wavelength space VLBI missions such as the project with two satellites currently under study in China.     

致密射电核

活动星系统的多波段观测和研究，早已成为天体物理学最热门的前沿之一。在射电天文学中，按其观测形态又常将ＡＧＮ和致密射电核等同看待。对ＡＮＧ及ＣＲＣ的含义、分类、总的频谱特征、射电结构和射电性质，模型研究及ＶＬＢＩ的重要作用、最新的观测结果、问题和前景作一评述。评述中将侧重于用射电天文手段观测研究ＣＲＣ的结果。     

VLBI Imaging of Luminous Infrared Galaxies: Starbursts & AGN

As described by Colin Lonsdale in this Workshop, VLBI observations reveal thepresence of both AGN and Starburst activity in Luminous Infrared Galaxies.We highlight new results on two classical LIGs which span the range of LIGradio activity: Arp 220, which reveals luminous radio supernovae (RSN)produced in an intense starburst, and Mrk231, which has a powerful compact,AGN radio core. Second epoch observations of the compact radio sources inArp 220 confirm their nature as luminous Radio Supernovae, but indicate alower luminous RSN frequency (_LRSN 0.3 yr^-1) andconsequent slower decay rate than previously suggested. We interpret thisas due to the dense starburst medium into which the supernovaedetonate. The compact radio morphology of Mrk231 places it among theCompact Symmetric Objects (CSOs) which are suggested to be young radio sources in which asymmetric lobes or hotspots reveal theworking surface of a relativistic jet upon the ambient medium. Assumingthat the lobes in Mrk 231 are confined by ram pressure, we estimate an agefor the jet/compact source, 10⁶yr. We interpret Mrk 231as a newly formed QSO emerging from a starburst.     

Properties of OH Megamaser Galaxies in the Radio Continuum. II. Data Analysis

The properties of OH megamaser galaxies in the radio continuum are discussed. Many radio sources in OH megamaser galaxies exhibit relatively flat (α ≥ −0.5) radio spectra between frequencies of 1.49 and 8.44 GHz along with high brightness temperatures (T_b ≥ 10⁴ K). In these galaxies the line and radio continuum fluxes are not correlated. The continuum radio emission of OH megamasers is predominantly nonthermal and is associated either with an active nucleus or with compact star formation. The thermal component of the radio emission from these galaxies can be neglected. The observed flat radio spectra and high brightness temperatures imply the existence of an active galactic nucleus, although some megamasers may be associated with compact star formation.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 281–290 (May 2005).     

On the nature of some active regions in the microwave range

The radio emission of a selected number of solar active regions has been investigated with high angular resolution at two frequencies: 10 and 17 GHz. By comparing the results of the two observations the following conclusions can be drawn:

1. The brightness temperature distribution of an active region is often composed of very bright cores of small dimension (angular extent θ?20″) imbedded in extended halos of lower brightness.
2. The radio emission of such structures as well as the degree of polarization can be explained with a thermal process. The halos can originate by pure thermal bremsstrahlung while in the case of the very bright cores found at 10 GHz (brightness temperature T _b?1–9 × 10⁶K) the emission at the harmonics of the gyrofrequency is needed.

     

Searching for dual active galactic nuclei

Binary or dual active galactic nuclei (DAGN) are expected from galaxy formation theories. However, confirmed DAGN are rare and finding these systems has proved to be challenging. Recent systematic searches for DAGN using double-peaked emission lines have yielded several new detections, as have the studies of samples of merging galaxies. In this paper, we present an updated list of DAGN compiled from published data. We also present preliminary results from our ongoing Expanded Very Large Array (EVLA) radio study of eight double-peaked emission-line AGN (DPAGN). One of the sample galaxy shows an S-shaped radio jet. Using new and archival data, we have successfully fitted a precessing jet model to this radio source. We find that the jet precession could be due to a binary AGN with a super-massive black-hole (SMBH) separation of \(\sim \) 0.02 pc or a single AGN with a tilted accretion disk. We have found that another sample galaxy, which is undergoing a merger, has two radio cores with a projected separation of 5.6 kpc. We discuss the preliminary results from our radio study.     

Doppler boosting, superluminal motion, and the kinematics of AGN jets

We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to γ∼30 and intrinsic luminosity up to ∼10²⁶ W Hz⁻¹. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ∼5×10¹³ K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ∼2×10¹¹ K, i.e., closer to equipartition.     

Radio AGN in the local universe: unification,triggering and evolution

Associated with one of the most important forms of active galactic nucleus (AGN) feedback, and showing a strong preference for giant elliptical host galaxies, radio AGN (\(L_{1.4\,\mathrm{GHz}} > 10^{24}\) W \(\hbox {Hz}^{-1}\)) are a key sub-class of the overall AGN population. Recently their study has benefitted dramatically from the availability of high-quality data covering the X-ray to far-IR wavelength range obtained with the current generation of ground- and space-based telescope facilities. Reflecting this progress, here I review our current state of understanding of the population of radio AGN at low and intermediate redshifts (\(z < 0.7\)), concentrating on their nuclear AGN and host galaxy properties, and covering three interlocking themes: the classification of radio AGN and its interpretation; the triggering and fuelling of the jet and AGN activity; and the evolution of the host galaxies. I show that much of the observed diversity in the AGN properties of radio AGN can be explained in terms of a combination of orientation/anisotropy, mass accretion rate, and variability effects. The detailed morphologies of the host galaxies are consistent with the triggering of strong-line radio galaxies (SLRG) in galaxy mergers. However, the star formation properties and cool ISM contents suggest that the triggering mergers are relatively minor in terms of their gas masses in most cases, and would not lead to major growth of the supermassive black holes and stellar bulges; therefore, apart from a minority (<20 %) that show evidence for higher star formation rates and more massive cool ISM reservoirs, the SLRG represent late-time re-triggering of activity in mature giant elliptical galaxies. In contrast, the host and environmental properties of weak-line radio galaxies (WLRG) with Fanaroff–Riley class I radio morphologies are consistent with more gradual fuelling of the activity via gas accretion at low rates onto the supermassive black holes.     

Star formation in southern Seyfert galaxies

We have produced radio maps, using the Australia Telescope Compact Array, of the central regions of six southern type 2 Seyfert galaxies (NGC 1365, 4945, 6221, 6810, 7582 and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self-absorption. In each case the radio to far-infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [Fe  II ] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR and [Fe  II ] line emissions are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and H  II regions) rather than with the AGN.     

Evidence for cospatial optical and radio polarized emission in active galactic nuclei

We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles χ_opt. A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0° is visible in the distribution of |χ_opt−χ_43 GHz|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components.     

The evolution of active galaxies in clusters

The results of deep radio, sub-mm and X-ray observations of samples of high redshift (z∼1) clusters are presented. These reveal significant excesses of active galaxies associated with the clusters at all three wavelengths. The cluster radio source population shows evolution consistent with the (1+z)³ evolution typical of many AGN classes. A large fraction of the AGN are hosted by apparently passive early-type galaxies, often with a close companion. These results essentially constitute the detection of a counterpart of the Butcher-Oemler effect for both strongly star bursting galaxies and AGN. This revised version was published online in July 2006 with corrections to the Cover Date.     

EGRET检测到的γ射线活动星系核的射电图像

研究表明活动星系核的高能γ射线辐射和低频射电辐射有着某些内在的联系。ＥＧＲＥＴ已检测到６６ 颗可信度很高的活动星系核。而这些高能的活动星系核有部分缺少足够的射电图象观测。为研究这些河外剧变源的射电辐射性质及致密结构,并研究活动量系核射电喷流弯曲与高能辐射的内在联系。我们利用国际欧洲ＶＬＢＩ网,英国的ＭＥＲＬＩＮ 和美国的ＶＬＡ对近２０ 颗γ射线活动星系核进行了观测。本文给出部分高能γ射线活动星系核在８ ．５ＧＨｚ的ＶＬＡ 观测图象。     

Radio AGNs among Luminous Infrared Galaxies

An analysis of radio and FIR emission in over 1500IRAS selected galaxies produces a good linear correlationbetween radio and FIR luminosity, indicating that star formationin normal field galaxies dominates the infrared luminosityin the local volume. Galaxies with clear radio-excess (definedas having at least5 times larger radio flux over expected from FIR) are identified as hosting a radio AGN, and they account for onlyabout 1% of the whole sample. This fraction increases to 10% among themore luminous galaxies with L _1.4GHz 10²³ W Hz^-1 (equivalently L _60m 10¹¹ L), however. The characteristic mid-IR excess of a Seyfert nucleus is ubiquitously present amongthe radio-excess objects, suggesting that mid-IR excess isa robust tracer of an AGN despite the high mid-IR opacity.We conclude that about 30% of the luminous infrared galaxies(L _60m 10¹¹ L) host an AGN based on themid-IR excess, and about 40% of the mid-IR excess AGNs alsohost a radio AGN. A VLA imaging survey of a distance limited sample of IR luminousgalaxies has revealed the presence of 100 kpc scale giant radioplumes in 3 out of 9 cases (Mrk 231, Mrk 273, NGC 6240). Theirlarge spatial extent, energetics, and presence of a powerful AGN in each case suggests that an AGN is the power source. Such plumesare not detected in other ultraluminous infrared galaxies which lack clear evidence for an AGN, such as Arp 220.     

Seyfert galaxies and the radio–far-infrared correlation

We have observed a sample of 149 Seyfert galaxies and radio-quiet quasars at 13 cm with both a 275-km radio interferometer and the 6-km compact array of the Australia Telescope. The high-resolution observations searched for the presence of compact, high-brightness-temperature radio emission from the active nucleus. The low-resolution observations measured the total radio emission from the galaxy disc and Seyfert core and lobes. From these we draw the following conclusions. (i) Seyfert galaxies that lack compact radio cores display a correlation between radio and far-infrared (FIR) emission similar to the correlation displayed by normal spirals, albeit with greater scatter. The correlation is found to be intrinsic and is not an artefact of the richness effect. (ii) A very different radio–FIR correlation is displayed by those Seyferts that harbour compact radio cores. These tend to be more radio-loud than either normal spirals or the Seyferts that lack compact cores. The compact core emission thus seems to be responsible for the generally poor radio–FIR correlation displayed by Seyfert galaxies. (iii) The radio–FIR correlation is not significantly improved by subtracting off the 0.1-arcsec (20- to 200-pc) compact radio emission from the total radio emission. This suggests that the emission from the active galactic nucleus has significant structure on scales larger than 0.1 arcsec. Perhaps these structures are the 'linear' radio features that have been seen previously in Seyfert nuclei.     

High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample – II. The properties of compact radio components

We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-μm AGN sample of Rush et al. Our main results can be summarized as follows.
Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike, as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extranuclear emission regions; extranuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of 'ionization cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm flux densities have well-matched properties.     

High Resolution Radio Observations of Luminous IR Galaxies

High resolution radio observations provide a unique tool for investigation ofultraluminous infrared galaxies, due to the combination of high angularresolution and source transparency available at centimeter wavelengths. Thetypical angular size of the IR emitting region in the most luminous systems isof order 0.1 arcsec, while dust absorption appears to obscure this region fromdirect view at wavelengths shorter than 100 microns. Radio observationswith VLBI and MERLIN are revealing compact radio structures indicative of bothstarburst and AGN activity in these systems. Additional observations arerequired to clarify relationships between radio structures and the energygeneration mechanisms, and to realize the full potential of radio interferometryfor ULIRG studies.     

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.