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Flat-spectrum symmetric objects with ~1kpc sizes - I. The candidates
In order to understand the origin and evolution of radio galaxies,searches for the youngest such sources have been conducted. Compactsymmetric objects (CSOs)/medium symmetric objects (MSOs) are thought tobe the earliest stages of radio sources, with possible ages of<~103yr for CSOs (<1kpc in size) and104-105yr for MSOs (1-15kpc). From a literatureselection in heterogeneous surveys, we have established a sample of 37confirmed CSOs. In addition, we only found three confirmed flat-spectrumMSOs in the literature. The typical CSO resides on a z <~ 0.5 galaxy,has a flat radio spectrum (αthin < 0.5;Sν ~ ν-α), is <0.3kpc in size, hasan arm length ratio <= 2, and well-aligned (θ <= 20°)opposite lobes with a flux density ratio <=10. In order to populatethe 0.3-1kpc size range (large CSOs) and also in order to find moreflat-spectrum MSOs, we have built a sample of 157 radio sources withα4.851.40 < 0.5 that were resolved withthe Very Large Array A configuration (VLA-A) 8.4GHz. As first results,we have `rediscovered' nine of the known CSO/MSOs while identifying twonew ~14kpc MSOs and two candidate CSO/MSOs (which only lack redshiftsfor final classification). We were able to reject 61 of the remaining144 objects from literature information alone. In the series of papersthat starts with this one we plan to classify the remaining 83 CSO/MSOcandidates (thanks to radio and optical observations) as well ascharacterize the physical properties of the (likely) many 0.3-15kpcflat-spectrum CSO/MSOs to be found.
| Merger origin of radio galaxies investigated with H I observations
We present results of an H I study of a complete sample of nearby radiogalaxies. Our goal is to investigate whether merger or interactionevents could be at the origin of the radio-AGN activity. Around five ofour radio galaxies, hosted mainly by early-type galaxies, we detectextended H I in emission. In most cases this H I is distributed in large(up to 190 kpc) and massive (up to M_HI 1010Mȯ) disk- or ring-like structures, that show fairlyregular rotation around the host galaxy. This suggests that in thesesystems a major merger likely occurred, but at least several Gyr ago.For the H I-rich radio galaxy B2 0648+27 we confirm such a merger originthrough the detection of a post-starburst stellar population thatdominates the visible light throughout this system. The timescale of thecurrent episode of radio-AGN activity in our H I- rich radio galaxies isseveral orders of magnitude smaller than the merger timescales.Therefore the radio-AGN activity either started late in the lifetime ofthe merger event, or is not directly related to the merger event at all.Another intriguing result is that the H I- rich (> 109Mȯ) radio galaxies in our sample all have compact radiosources, while none of the extended radio sources contain these amountsof extended H I. This strongly suggests that there is a relation betweenthe size of the radio jet and the presence of large amounts of neutralgas associated with the host galaxy.
| The Effects of Interactions on the Structure and Morphology of Elliptical/Lenticular Galaxies in Pairs
We present a structural and photometric analysis of 42elliptical/lenticular galaxies in E/S0 + S pairs observed in the BVRIcolor bands. The aim of the analysis is to empirically determine theeffects of interactions on the galaxies' morphology, structure, andstellar populations as seen from the CAS parameters (light concentrationC, asymmetry A, and clumpiness S). We further compare these values to acontrol sample of 67 mostly isolated noninteracting E/S0 galaxies. Wefind that the paired E/S0 galaxies occupy more scattered loci in CASspace than noninteracting E/S0s and that the structural effects ofinteractions on E/S0s are minor, in contrast to disk galaxies involvedin interactions. This suggests that observational methods forrecognizing interactions, such the CAS methodology of Conselice, wouldnot detect E/S0s involved in interactions (related to early phases ofthe so-called dry mergers), and that the majority of interactinggalaxies identified at high redshift must be gas-dominated systems.However, we find statistical differences in the asymmetry index whencomparing isolated and interacting E/S0s. On average, paired E/S0galaxies have A-values 2.96+/-0.72 times larger than those ofnoninteracting E/S0s. For the subset of presumably strongly interactingE/S0s, A and S can be several times larger than the typical values ofthe isolated E/S0s. We show that the asymmetries are consistent withseveral internal and external morphological distortions. We concludethat the subsample of interacting E/S0s should be dense, gas-poorgalaxies in systems spanning a wide range of interaction stages, withtypical merging timescales >~0.1-0.5 Gyr. We use the observedphenomenology of this subsample to predict the approximate loci of drypremergers in the CAS parameter space.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| The Size of the Radio-Emitting Region in Low-Luminosity Active Galactic Nuclei
We have used the VLA to study radio variability among a sample of 18low-luminosity active galactic nuclei (LLAGNs) on timescales of a fewhours to 10 days. The goal was to measure or limit the sizes of theLLAGN radio-emitting regions in order to use the size measurements asinput to models of the radio emission mechanisms in LLAGNs. We detectvariability on typical timescales of a few days at a confidence level of99% in half the target galaxies. Either variability that is intrinsic tothe radio-emitting regions or that is caused by scintillation in theGalactic interstellar medium is consistent with the data. For eitherinterpretation, the brightness temperature of the emission is below theinverse Compton limit for all our LLAGNs and has a mean value of about1010 K. The variability measurements plus VLBI upper limitsimply that the typical angular size of the LLAGN radio cores at 8.5 GHzis 0.2 mas, plus or minus a factor of 2. The ~1010 Kbrightness temperature strongly suggests that a population ofhigh-energy nonthermal electrons must be present, in addition to ahypothesized thermal population in an accretion flow, in order toproduce the observed radio emission.
| The Two-sided Parsec-Scale Structure of the Low-Luminosity Active Galactic Nucleus in NGC 4278
We present new very long baseline interferometry observations of theLINER galaxy NGC 4278. The observations were taken with the Very LongBaseline Array (VLBA) and a single antenna of the Very Large Array (VLA)at 5 and 8.4 GHz and have a linear resolution of <~0.1 pc. Our radiodata reveal a two-sided structure, with symmetric S-shaped jets emergingfrom a flat-spectrum core. We fit the jet brightness with Gaussiancomponents, which we identify from a previous observation taken 5 yearsbefore. By comparing the positions of the components in the two epochs,we measure motions between 0.45+/-0.14 and 3.76+/-0.65 mas,corresponding to apparent velocities <~0.2c and to ages in the range8.3-65.8 yr. Assuming that the radio morphology is intrinsicallysymmetric and its appearance is governed by Doppler beaming effects, wefind that NGC 4278 has mildly relativistic jets (β~0.75), closelyaligned to the line of sight(2deg<=θ<=4deg). Alternatively, thesource could be oriented at a larger angle and asymmetries could berelated to the jet interaction with the surrounding medium. We alsopresent new simultaneous VLA observations between 1.4 and 43 GHz, and a5 GHz light curve between 1972 and 2003. The radio spectrum can befitted by a relatively steep power law (α=0.54). We findsignificant variability at 5 GHz. All these arguments indicate that theradiation from NGC 4278 is emitted via the synchrotron process byrelativistic particles accelerated by a supermassive black hole. Despitea much lower power, this is the same process that takes place inordinary radio-loud active galactic nuclei (AGNs).
| Mixed-Morphology Pairs as a Breeding Ground for Active Nuclei
Mixed-morphology pairs offer a simplification of the interactionequation that involves a gas-rich fast rotator paired with a gas-poorslow rotator. In past low-resolution IRAS studies it was assumed thatthe bulk of the far-infrared (FIR) emission originated in the spiralcomponent. However, our Infrared Space Observatory studies revealed asurprising number of early-type components with significant IR emission,some of which turned out to show active nuclei. This motivated us tolook at the current statistics of active nuclei in mixed pairs using theradio-FIR continuum correlation as a diagnostic. We find a clear excessof early-type components with radio continuum emission and activenuclei. We suggest that they arise more often in mixed pairs viacross-fueling of gas from the spiral companion. This fuel is moreefficiently channeled into the nucleus of the slow-rotating receptor. Ina sample of 112 mixed-morphology pairs from the Karachentsev catalog, wefind that about 25%-30% of detected mixed pairs show a displacement fromthe radio-FIR relation defined by normal star-forming galaxies. Thelatter objects show excess radio continuum emission, while others extendthe relation to unusually high radio and FIR flux levels. Many of theoutliers or extreme emitters involve an early-type component with anactive nucleus. The paired E/S0 galaxies in the sample exhibit asignificant excess detection fraction and a marginal excess luminositydistribution compared to those of isolated unpaired E/S0 galaxies.
| A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies
We examine the properties of central dust in nearby quiescent and activeearly-type galaxies. The active galaxies are low-power radio galaxieswith Fanaroff & Riley type I or I/II radio jets. We focus on (a) thecomparison of the dust distributions in the active and quiescent galaxysamples; and (b) the relation between the radio jet and dustorientations. Our main observational conclusions are: (i) in line withprevious studies, the dust detection rate is higher in radio-jetgalaxies than in non radio-jet galaxies; (ii) radio galaxies contain ahigher fraction of regular dust “ellipses” compared toquiescent galaxies which contain more often irregular dustdistributions; (iii) the morphology, size and orientation of dustellipses and lanes in quiescent early-types and active early-types withkpc-scale radio jets is very similar; (iv) dust ellipses are alignedwith the major axis of the galaxy, dust lanes do not show a preferredalignment except for large (>kpc) dust lanes which are aligned withthe minor axis of the galaxy; and (v) as projected on the sky, jets donot show a preferred orientation relative to the galaxy major axis (andhence dust ellipses), but jets are preferentially perpendicular to dustlanes. We show that the dust ellipses are consistent with being nearlycircular thin disks viewed at random viewing angles. The lanes arelikely warped dust structures, which may be in the process of settlingdown to become regular disks or are being perturbed by anon-gravitational force. We use the observed dust-jet orientations toconstrain the three-dimensional angle θDJ between jetand dust. For dust-lane galaxies, the jet is approximately perpendicularto the dust structure, while for dust-ellipse galaxies there is a muchwider distribution of θDJ. We discuss two scenariosthat could explain the dust/jet/galaxy orientation dichotomy. If lanesare indeed settling, then the jet orientation apparently is roughlyaligned with the angular momentum of the dust before it settles. Iflanes are perturbed by a jet-related force, it appears that it causesthe dust to move out of its equilibrium plane in the galaxy into a planewhich is perpendicular to the jet.
| Disks, tori, and cocoons: emission and absorption diagnostics of AGN environments
One of the most important problems in the study of active galaxies isunderstanding the detailed geometry, physics, and evolution of thecentral engines and their environments. The leading models involve anaccretion disk and torus structure around a central dense object,thought to be a supermassive black hole. Gas found in the environment ofactive galactic nuclei (AGN) is associated with different structures:molecular accretion disks, larger scale atomic tori, ionized and neutral“cocoons” in which the nuclear regions can be embedded. Allof them can be studied at radio wavelengths by various means. Here, wesummarize the work that has been done to date in the radio band tocharacterize these structures. Much has been learned about the centralfew parsecs of AGN in the last few decades with contemporary instrumentsbut the picture remains incomplete. In order to be able to define a moreaccurate model of this region, significant advances in sensitivity,spectral and angular resolution, and bandpass stability are required.The necessary advances will only be provided by the Square KilometerArray and we discuss the possibilities that these dramatic improvementswill open for the study of the gas in the central region of AGN.
| The spectral energy distributions of the revised 200-mJy sample
We address the question of why low-luminosity radio sources withsimilar flat radio spectra show a range of optical activity. Theinvestigation is based on the spectral energy distributions (SEDs) ofobjects from the 200-mJy sample. We gathered new data from the VLA at 43GHz, from SCUBA in the JCMT at 2000, 1350 and 850 μm, and from theISOPHOT instrument on ISO at 170, 90, 60 and 25 μm. There isconsiderable diversity amongst the SEDs of the objects: there areobjects with steep broad-band spectra between centimetre and millimetrebands (14 per cent of the sample); there are those with flat broad-bandspectra over most of the spectral range (48 per cent of the sample); andthere are those which show pronounced submillimetre/infrared excesses(27 per cent of the sample). Some objects of the first group havetwo-sided radio morphology, indicating that their parsec-scale emissionis not dominated by beamed jet emission. Amongst the objects that havesmooth broad-band spectra from the radio to the infrared, there arepassive elliptical galaxies as well as the expected BL Lacertae objects.The most pronounced submillimetre/infrared excesses are shown by thebroad-emission-line objects.
| Construction of a Celestial Coordinate Reference Frame from VLBI Data
A large number (2 million) of VLBI observations have been reducedin order to refine the measured coordinates of the observed radiosources. The data reduction was carried out in the OCCAM package usingthe least squares colocation method. Corrections to the coordinates of642 objects were derived. The accuracy of the catalog is no worse than0.2 milliseconds of arc for stable sources.
| The nuclear region of low luminosity flat radio spectrum sources. II. Emission-line spectra
We report on the spectroscopic study of 19 low luminosity Flat RadioSpectrum (LL FRS) sources selected from Marchã's et al.(\cite{March96}) 200 mJy sample. In the optical, these objects aremainly dominated by the host galaxy starlight. After correcting the datafor this effect, we obtain a new set of spectra clearly displaying weakemission lines; such features carry valuable information concerning theexcitation mechanisms at work in the nuclear regions of LL FRS sources.We have used a special routine to model the spectra and assess theintensities and velocities of the emission lines; we have analyzed theresults in terms of diagnostic diagrams. Our analysis shows that 79% ofthe studied objects harbour a Low Ionization Nuclear Emission-lineRegion (or LINER) whose contribution was swamped by the host galaxystarlight. The remaining objects display a higher ionization spectrum,more typical of Seyferts; due to the poor quality of the spectra, it wasnot possible to identify any possible large Balmer components. The factthat we observe a LINER-type spectrum in LL FRS sources supports theidea that some of these objects could be undergoing an ADAF phase; inaddition, such a low ionization emission-line spectrum is in agreementwith the black hole mass values and sub-Eddington accretion ratespublished for some FRS sources.Based on observations collected at the Multiple Mirror Telescope on Mt.Hopkins.Full Fig. 1 is only available in electronic form athttp://www.edpsciences.org
| The nuclear region of low luminosity flat radio spectrum sources. I. Stellar content
In this work we have examined the spectroscopic properties of a sampleof 19 optically bright, low luminosity Flat Radio Spectrum (LL FRS)sources. Our study focuses on the properties of their host galaxies,namely the nuclear stellar populations and dust content. In the optical- spectral region covered by our data - the objects in the sample aremainly dominated by the host galaxy starlight, which strongly dilutesthe non-thermal continuum as well as possible emission-line featuresrelated to the active nucleus. We have computed the nuclear stellarpopulations contributing to the spectra of the objects in our sample.The stellar population synthesis has been performed by using a veryreliable mathematical method, which yields a Global PrincipalGeometrical solution. Our results show that, for most of the objects inthe sample, the populations are composed of old stars of solarmetallicity, or lower; the populations are mainly composed of late-typestars, i.e. G, K and M spectral types, the young component coming thusfrom supergiant stars; the dust content is weak. Both the stellarpopulations and the dust content are in agreement with what is usuallyobserved in ``normal'' elliptical galaxies. Similar stellar content hasequally been found in the nuclear regions of galaxies hosting a LowIonization Nuclear Emission Line Region, or LINER.The present work is important in illustrating the different applicationsof stellar population synthesis in the study of low luminosity radiosources. In fact, the synthesis allows us not only to obtain valuableinformation about the stellar populations and dust content of the hostgalaxies, therefore providing material for further studies on theconnection between host galaxy and active nucleus, but also to revealthe so-far unstudied optical emission-line features present in thespectrum of our objects.Based on observations collected at the Multiple Mirror Telescope on Mt.Hopkins.Tables 2, 3 and full Fig. 1 are only available in electronic form athttp://www.edpsciences.org
| Infrared Observations of Galaxies in the Local Universe. II. 391 Calibrated Images with Photometric and Structural Measurements
This paper presents empirical results from a deep imaging survey ofgalaxies in the local universe at the J and Ks wavelengths.Three hundred ninety-one images have been obtained and calibrated usingthe same camera and filter set with the Steward Observatory 1.6 m KuiperTelescope on Mount Bigelow and the 2.3 m Bok Telescope on Kitt Peak. Thelimiting magnitude is typically 22 mag arcsec-1 at J and 21mag arcsec-1 at Ks. The central surfacebrightness, apparent magnitudes, sizes, scale lengths, and inclinationsare tabulated from measurements made using these data. The purpose ofthis paper is to provide basic near-infrared data on a variety of galaxytypes.
| Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| HST imaging of nearby CSOs: obscuration and nuclear structures
We present 3-band HST imaging of three nearby (/z<0.1) compactsymmetric objects: 4C31.04, 1946+708 and 1146+596 (NGC 3894). Theseobjects were chosen for HST observation on the basis of detected HI andmolecular line absorption. The images show large amounts of obscurationin each source, well distributed throughout the host galaxies, butsomewhat concentrated in the nuclear regions. All three also showevidence of nuclear structures which resemble disks or tori. We discussthe possible association of the nuclear structures and obscuration withtheir radio structures, and compare with other HST observations of GPS,CSS and large-scale radio galaxies.
| Imaging HI absorption toward symmetric radio galaxies-evidence for a circumnuclear torus
Recent VLBI observations have identified several compact radio sourceswhich have symmetric structures on parsec scales, and exhibit HIabsorption which appears to be associated with the active nucleus. Thesesources are uniquely well suited to investigations into the physics ofthe central engines, in particular to studies of the kinematics of thegas within 100 pc of the core. In these compact sources, it isreasonable to assume that this circumnuclear material is accreting onto,and ``feeding,'' the central engine. We present results of HI imagingstudies of 3 symmetric radio galaxies which show evidence of acircumnuclear torus.
| Compact symmetric objects-newborn radio galaxies?
We review the question of the age of Compact Symmetric Objects (CSOs);defined as lobe-dominated sources smaller than 1 kpc in overall size. Weshow that the evidence increasingly points to these objects being veryyoung (<104 yr old). Evidence from spectral aging, energysupply arguments and, most convincingly, from long term VLBI kinematicstudies, is all consistent with the `youth' scenario for CSOs. From VLBIkinematic studies hotspot advance speeds in CSOs are found to be 0.1 to0.3c and external densities estimated from ram pressure balance are /~1cm-3. The separate question of the subsequent evolution ofCSOs and whether they are the progenitors of classical double sourcesis, in contrast, not yet definitively answered. However it is found thatthe numbers of CSOs in flux limited samples is to first order what wouldbe expected under such a scenario. The detailed differences in CSOpopulation density between the data and model predictions might beresolved in various ways. Possibly not all CSOs evolve into largesources, or some sources show recurrent activity or, most likely, thesimplest source evolution models need modification.
| Automated optical identification of a large complete northern hemisphere sample of flat-spectrum radio sources with [formmu1]S6 cm>200 mJy
This paper describes the automated optical APM identification of radiosources from the Jodrell Bank-VLA Astrometric Survey (JVAS), as used forthe search for distant radio-loud quasars. Since JVAS was not intendedto be complete, a new complete sample, JVAS++, has been constructed withselection criteria similar to those of JVAS (S5GHz>200mJy,α1.4-5GHz>-0.5), and with the use of the moreaccurate GB6 and NVSS surveys. Comparison between this sample and JVASindicates that the completeness and reliability of the JVAS survey are~90 and ~70 per cent respectively. The complete sample has been used toinvestigate possible relations between optical and radio properties offlat-spectrum radio sources. From the 915 sources in the sample, 756have an optical APM identification on a red (e) and/or blue (o) plate,resulting in an identification fraction of 83 per cent with acompleteness and reliability of 98 and 99 per cent respectively. About20 per cent are optically identified with extended APM objects on thered plates, e.g., galaxies. However, the distinction between galaxiesand quasars can not be made properly near the magnitude limit of thePOSS-I plates. The identification fraction appears to decrease from>90 per cent for sources with a 5-GHz flux density of >1Jy, to<80 per cent for sources at 0.2Jy. The identification fraction, inparticular that for unresolved quasars, is found to be lower for sourceswith steeper radio spectra. In agreement with previous studies, we findthat the quasars at low radio flux density levels also tend to havefainter optical magnitudes, although there is a large spread. Inaddition, objects with a steep radio-to-optical spectral index are foundto be mainly highly polarized quasars, supporting the idea that in theseobjects the polarized synchrotron component is more prominent. It isshown that the large spread in radio-to-optical spectral index ispossibly caused by source-to-source variations in the Doppler boostingof the synchrotron component.
| The VLBA Calibrator Survey-VCS1
A catalog containing milliarcsecond-accurate positions of 1332extragalactic radio sources distributed over the northern sky ispresented-the Very Long Baseline Array Calibrator Survey (VCS1). Thepositions have been derived from astrometric analysis of dual-frequency2.3 and 8.4 GHz VLBA snapshot observations; in a majority of cases,images of the sources are also available. These radio sources aresuitable for use in geodetic and astrometric experiments, and asphase-reference calibrators in high-sensitivity astronomical imaging.The VCS1 is the largest high-resolution radio survey ever undertaken andtriples the number of sources available to the radio astronomy communityfor VLBI applications. In addition to the astrometric role, this surveycan be used in active galactic nuclei, Galactic, gravitational lens, andcosmological studies.
| A Determination of H0 with the CLASS Gravitational Lens B1608+656. III. A Significant Improvement in the Precision of the Time Delay Measurements
The gravitational lens CLASS B1608+656 is the only four-image lenssystem for which all three independent time delays have been measured.This makes the system an excellent candidate for a high-qualitydetermination of H0 at cosmological distances. However, theoriginal measurements of the time delays had large (12%-20%)uncertainties, due to the low level of variability of the backgroundsource during the monitoring campaign. In this paper, we present resultsfrom two additional VLA monitoring campaigns. In contrast to the ~5%variations seen during the first season of monitoring, the source fluxdensity changed by 25%-30% in each of the subsequent two seasons. Weanalyzed the combined data set from all three seasons of monitoring toimprove significantly the precision of the time delay measurements; thedelays are consistent with those found in the original measurements, butthe uncertainties have decreased by factors of 2-3. We combined thedelays with revised isothermal mass models to derive a measurement ofH0. Depending on the positions of the galaxy centroids, whichvary by up to 0.1" in Hubble Space Telescope (HST) images obtained withdifferent filters, we obtain H0=61-65 km s-1Mpc-1, for(ΩM,ΩΛ)=(0.3,0.7). The value ofH0 decreases by 6% if(ΩM,ΩΛ)=(1.0,0.0). The formaluncertainties on H0 due to the time-delay measurements are+/-1 (+/-2) km s-1 Mpc-1 for the 1 σ (2σ) confidence limits. Thus, the systematic uncertainties due tothe lens model, which are on the order of +/-15 km s-1Mpc-1, now dominate the error budget for this system. Inorder to improve the measurement of H0 with this lens, newmodels that incorporate the constraints provided by stellar dynamics andthe optical/infrared Einstein ring seen in HST images must be developed.
| H I Absorption in the Gigamaser Galaxy TXS 2226-184 and the Relation between H I Absorption and Water Emission
We report on the discovery of H I in absorption toward the gigamasergalaxy TXS 2226-184 using the Very Large Array. The absorption appearsto consist of two components-one with a width of 125 km s-1and one broader (420 km s-1)-both toward the compact radiosource in the nucleus of the galaxy. Based on these large velocitywidths, we suggest that the H I absorption is produced in the centralparsecs of the galaxy, on a scale similar to that which gives rise tothe water maser emission. This brings to eight the number of galaxiesknown to exhibit both water masers and H I absorption. We explore therelationship between these two phenomena and present a physicallymotivated (but unfruitful) search for water maser emission in five radiogalaxies known to exhibit strong H I absorption.
| Search for point sources of gamma radiation above 15 TeV with the HEGRA AIROBICC array
A search for potential point sources of very high energy gamma rays hasbeen carried out on the data taken simultaneously by the HEGRA AIROBICCand Scintillator arrays from August 1994 to March 2000. The list ofsought sources includes supernova remnants, pulsars, AGNs and binarysystems. The energy threshold is around 15 TeV. For the Crab Nebula, amodest excess of 2.5 standard deviations above the cosmic ray backgroundhas been observed. Flux upper limits (at 90% c.l.) of around 1.3 timesthe flux of the Crab Nebula are obtained, in average, for the candidatesources. A different search procedure has been used for an all-skysearch which yields absolute flux upper limits between 4 and 9 crabsdepending on declination, in the band from delta = 0 to delta =60o. The full versions of Tables 1 and 2, including thecoordinates of the sources, are available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/39
| A new list of extra-galactic radio jets
A catalogue of extra-galactic jets is very useful both in observationaland theoretical studies of active galaxies. With the use of new powerfulradio instruments, the detailed structures of very compact or weak radiosources are investigated observationally and many new radio jets aredetected. In this paper, we give a list of 661 radio sources withdetected radio jets known to us prior to the end of December 2000. Allreferences are collected for the observations of jets in radio, IR,optical, UV and X-ray wave-bands. Table 1 and references to Table 1 areonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/757
| Twin Jets as a Potential Distance Detector
A preview study of using observational data of the proper motions andDoppler-shifted velocities of twin jets to determine the distance ofsources inside and outside our Galaxy is made. We investigate thefeasibility of this method by studying the uncertainty of the distancecaused by the uncertainties of the measured quantities. It shows that,when the motion of components of the jet is relativistic, theuncertainty of the distance is within the same order of theuncertainties of the measured values of the proper motions andDoppler-shifted velocities. In particular, when assuming that thepattern speed equals the flow speed in the jet, for 10% uncertainties ofthe measured quantities, the uncertainty of the distance caused by themwould be well within 13%. With the current technique, this method isrealizable. For the convenience of choosing the sources to observe, somesources as potential targets are also listed in this paper.
| A catalogue and analysis of X-ray luminosities of early-type galaxies
We present a catalogue of X-ray luminosities for 401 early-typegalaxies, of which 136 are based on newly analysed ROSAT PSPC pointedobservations. The remaining luminosities are taken from the literatureand converted to a common energy band, spectral model and distancescale. Using this sample we fit the LX:LB relationfor early-type galaxies and find a best-fit slope for the catalogue of~2.2. We demonstrate the influence of group-dominant galaxies on the fitand present evidence that the relation is not well modelled by a singlepower-law fit. We also derive estimates of the contribution to galaxyX-ray luminosities from discrete-sources and conclude that they provideLdscr/LB~=29.5ergs-1LBsolar-1. Wecompare this result with luminosities from our catalogue. Lastly, weexamine the influence of environment on galaxy X-ray luminosity and onthe form of the LX:LB relation. We conclude thatalthough environment undoubtedly affects the X-ray properties ofindividual galaxies, particularly those in the centres of groups andclusters, it does not change the nature of whole populations.
| Intermediate BL Lac objects
The 200-mJy sample, defined by Marchã et al., contains about 60nearby, northern, flat-spectrum radio sources. In particular, the samplehas proved effective at finding nearby radio-selected BL Lac objectswith radio luminosities comparable to those of X-ray-selected objects,and low-luminosity flat-spectrum weak emission-line radio galaxies(WLRGs). The 200-mJy sample contains 23 BL Lac objects (including 6 BLLac candidates) and 19 WLRGs. We will refer to these subsamples as the200-mJy BL Lac sample and the 200-mJy WLRG sample, respectively. We havestarted a systematic analysis of the morphological pc-scale propertiesof the 200-mJy radio sources using VLBI observations. This paperpresents VLBI observations at 5 and 1.6GHz of 14 BL Lac objects andWLRGs selected from the 200-mJy sample. The pc-scale morphology of theseobjects is briefly discussed. We derive the radio beaming parameters ofthe 200-mJy BL Lac objects and WLRGs and compare them with those ofother BL Lac samples and with a sample of FR I radio galaxies. Theoverall broad-band radio, optical and X-ray properties of the 200-mJy BLLac sample are discussed and compared with those of other BL Lacsamples, radio- and X-ray-selected. We find that the 200-mJy BL Lacobjects fill the gap between HBL and LBL objects in the colour-colourplot, and have intermediate αXOX as expected in thespectral energy distribution unification scenario. Finally, we brieflydiscuss the role of the WLRGs.
| Relativistic Beaming and Flux Variability in Active Galactic Nuclei
We discuss the impact of special relativistic effects on the observedlight curves and variability duty cycles of radio-loud active galacticnuclei (AGNs). We model the properties of AGN light curves at radiowavelengths using a shot-noise process in which the occurrence of majorflaring events in a relativistic jet is governed by Poisson statistics.We show that flaring sources whose radiation is highly beamed toward usare able to reach very high flux levels, but will in fact spend most oftheir time in relatively low flaring states. This is primarily due torelativistic Doppler contraction of flaring timescales in the observerframe. The fact that highly beamed AGNs are not observed to return to asteady state quiescent level between flares implies that their weaklybeamed counterparts should have highly stable flux densities that resultfrom a superposition of many long-term, low-amplitude flares. The``apparent'' quiescent flux levels of these weakly beamed AGNs(identified in most unified models as radio galaxies) will besignificantly higher than their ``true'' quiescent (i.e., nonflaring)flux levels. We have also performed Monte Carlo simulations to examinehow relativistic beaming and source variability bias the selectionstatistics of flat-spectrum AGN samples. We find that in the case of theCaltech-Jodrell Flat-Spectrum Survey, the predicted orientation biastoward jets seen end-on is weakened if the parent population isvariable, because highly beamed sources have a stronger tendency to befound in low flaring states. This effect is small, however, becausehighly beamed sources are relatively rare, and in most cases their fluxdensities will be boosted sufficiently above the survey limit that theywill be selected regardless of their flaring level. We find that, forlarger flat-spectrum AGN surveys with fainter flux density cutoffs,variability should not be an appreciable source of selection bias.
| Compact Symmetric Objects as Radio Flux Density Calibrators
We present results from the first intensive monitoring campaign of asample of compact symmetric objects (CSOs). We observed seven CSOs at8.5 GHz over a period of eight months, with an average spacing betweenobservations of 2.7 days. Our results show that, as predicted, the fluxdensities of the CSOs are extremely stable; the mean rms variability ofthe sample was 0.7% in flux density. The low variability of the CSOsmakes them excellent flux density calibrators at this frequency. Werecommend that at least four CSOs be included in any VLA monitoringcampaign that requires precise epoch-to-epoch calibration, such as thoseto measure gravitational lens time delays. The CSO data enable thecorrection of small systematic errors in the primary flux calibration.
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