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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.
| Constraints on evolutionary properties of GHz Peaked Spectrum galaxies
We have used the available samples of Gigahertz Peaked Spectrum (GPS)galaxies to investigate their evolution properties in the framework ofthe "youth" scenario. Care was taken to properly allow for the differentselection criteria used to define the samples. We find that the observedredshift and peak frequency distributions can be satisfactorilyaccounted for in terms of simple luminosity evolution of individualsources, along the lines discussed by Fanti et al. (1995, A&A, 302,317) and Begelman (1996, in Proc. Cygnus A, Study of Radio Galaxy; 1999,in Proc. of KNAW Colloq.), although the derived parameter values havelarge uncertainties due to ambiguities in the selection of GPS sourcesand to the incompleteness of redshift measurements. However the simplestself-similar model, whereby the evolution is controlled only by theradial profile of the density of the ambient medium is not good enoughand one additional parameter needs to be introduced. The fit requires adecrease of the emitted power and of the peak luminosity with source ageor with decreasing peak frequency, at variance with the Snellen et al.(2000, MNRAS, 319, 445) model. On the other hand, our analysis confirmsthe rather flat slope of the luminosity function, found by Snellen etal. (2000) who also report indications of a high luminosity break, notrequired by the data sets we have used. Our results suggest that the GPSgalaxies are the precursors of extended radio sources with luminositiesbelow the break of the luminosity function. No cosmological evolution ofthe GPS galaxy population is required by presently available data.
| High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars
We discuss our ATCA 18.5 and 22 GHz flux density measurements ofSouthern extragalactic sources in the complete 5 GHz sample of Kühret al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz)spectral indices of steep-spectrum sources for which we have 18.5 GHzdata (66% of the complete sample) are systematically steeper than thelow frequency (2.7-5 GHz) ones, with median α^52.7 =0.76, median α18.55 = 1.18(Sν∝ ν-α), and median steepeningΔα = 0.32, and there is evidence of an anti-correlation ofΔα18.55 with luminosity. Thecompleteness of 18.5 GHz data is much higher (89%) for flat-spectrumsources (mostly quasars), which also exhibit a spectral steepening:median α^52.7=-0.14, medianα18.55=0.16 (Sν∝ν-α), and median Δα = 0.19. Takingadvantage of the almost complete redshift information on flat-spectrumquasars, we have estimated their 5 GHz luminosity function in severalredshift bins. The results confirm that their radio luminosity densitypeaks at z_peak ≃ 2.5 but do not provide evidence for deviationsfrom pure luminosity evolution as hinted at by other data sets. Acomparison of our 22 GHz flux densities with WMAP K-band data forflat-spectrum sources suggests that WMAP flux densities may be low by amedian factor of ≃1.2. The extrapolations of 5 GHz counts andluminosity functions of flat-spectrum radio quasars using the observeddistribution of the 5-18.5 GHz spectral indices match those deriveddirectly from WMAP data, indicating that the high frequency WMAP surveydoes not detect any large population of FSRQs with anomalous spectra.
| Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations
CG J1720-67.8 is an ultra compact group of several galaxies with alow-velocity dispersion, and displaying the hallmarks of mutualinteraction and possible tidal dwarf galaxy formation. In hierarchicalmodels, the system is a possible precursor to a massive ellipticalgalaxy. In this paper, we use new optical integral field spectroscopicand radio observations to investigate the evolutionary status of thegroup in more detail: global star formation rates are estimated usingHα and 1.4-GHz radio continuum measurements; HI observationsprovide an upper limit to the global neutral gas content; opticalbroadband colours and spectra provide ages and stellar mass estimatesfor the tidal dwarf candidates; the bi-dimensional Hα velocityfield is used to trace the kinematics of the group and its members,which are compared with numerical simulations of galaxy encounters. Theobservations suggest a model in which multiple interactions haveoccurred, with the latest strong encounter involving at least two majorcomponents within the last 200Myr. Debris from the encounter fuelsongoing star formation at the global level of~20Msolaryr-1, with self-gravity within the tidaltail possibly providing a mechanism to enhance the star formation rateof the tidal dwarf candidates, with bursts of star formation in clumpsof mass ~2 × 107 Msolar appearing within thelast 10Myr. The amount of time required for final merging of all groupcomponents remains uncertain.
| Stability study of realization of the celestial reference frame
We present a short overview of the activity of the IERS as well as theMain Astronomical Observatory (MAO) of the National Academy of Sciencesof Ukraine for maintenance and extention of the International CelestialReference Frame (ICRF). Special attention is given to the timestabilities of positions of radio sources (RS) and to the selection of asubset of RS to be used for maintenance of the ICRF. It is shown thatseven RS qualified by the IERS as defining sources are unstable.
| 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 VSOP 5 GHz Active Galactic Nucleus Survey. III. Imaging Results for the First 102 Sources
The VLBI Space Observatory Programme (VSOP) mission is a Japanese-ledproject to study radio sources with sub-milliarcsec resolution using anorbiting 8 m telescope, HALCA, along with global arrays of Earth-basedtelescopes. Approximately 25% of the observing time is devoted to asurvey of compact active galactic nuclei (AGNs) that are stronger than 1Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series,presents the results from the analysis of the first 102 Survey sources.We present high-resolution images and plots of visibility amplitudeversus projected baseline length. In addition, model-fit parameters tothe primary radio components are listed, and from these the angular sizeand brightness temperature for the radio cores are calculated. For thosesources for which we were able to determine the source frame corebrightness temperature, a significant fraction (53 out of 98) have asource frame core brightness temperature in excess of 1012 K.The maximum source frame core brightness temperature we observed was1.2×1013 K. Explaining a brightness temperature thishigh requires an extreme amount of relativistic Doppler beaming. Sincethe maximum brightness temperature one is able to determine using onlyground-based arrays is of the order of 1012 K, our resultsconfirm the necessity of using space VLBI to explore the extremely highbrightness temperature regime.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| The Radio Variability of the Gravitational Lens PMN J1838-3427
We present the results of a radio variability study of the gravitationallens PMN J1838-3427. Our motivation was to determine the Hubble constantby measuring the time delay between variations of the two quasar images.We monitored the system for 4 months (approximately 5 times longer thanthe expected delay) using the Australia Telescope Compact Array at 9GHz. Although both images were variable on a timescale of a few days, nocorrelated intrinsic variability could be identified, and therefore notime delay could be measured. Notably, the fractional variation of thefainter image (8%) was greater than that of the brighter image (4%),whereas lensed images of a point source would have the same fractionalvariation. This effect can be explained, at least in part, as therefractive scintillation of both images due to the turbulentinterstellar medium of the Galaxy.
| VLBI Observations of Southern Hemisphere ICRF Sources. I.
We present 8.4 GHz very long baseline interferometry (VLBI) observationsof 69 southern hemisphere extragalactic sources in the InternationalCelestial Reference Frame. These are the first in a series ofobservations intended to image all such sources at milliarcsecondresolution in order to determine their continued suitability forreference-frame use based on intrinsic structure. We use the resultantimages to calculate a core fraction, that is, the ratio of core fluxdensity to total flux density, for all observed sources. The resultingdistribution, with a mean value of 0.83, suggests that most sources arerelatively compact. However, just over half the observed sources showsignificant extended emission in the form of multiple compactcomponents. These sources are probably poorly suited for high-accuracyreference-frame use unless intrinsic structure and potential variabilitycan be taken into account. Our observations represent the first large,comprehensive VLBI imaging survey in the southern hemisphere,significantly extending the existing limited VLBI surveys and, alongwith some well-known objects, containing many sources that have neverbeen imaged at milliarcsecond resolution. The overlap with Very LongBaseline Array images of sources between 0° and -35° declinationhelps determine the limits to imaging with the southernhemisphere-accessible telescopes.
| New candidate GHz peaked spectrum and compact steep spectrum sources
Data from a recent Australia Telescope Compact Array (ATCA) program ofmulti-frequency, multi-epoch monitoring of 202 active galactic nucleiwith declinations <+10o have been searched for GHz PeakedSpectrum (GPS) and Compact Steep Spectrum (CSS) radio sources.Supplementary data at higher and lower frequencies, where available,have been used to further examine the spectral properties of previouslyreported and new candidate GPS and CSS sources. The ATCA monitoringprogram has allowed the variability and polarization properties ofsources previously reported as GPS and CSS sources, and the majority ofnew GPS and CSS candidates, to be investigated, confirming that theseare useful diagnostics in discriminating genuine GPS and CSS sourcesfrom variable sources that display similar spectra only temporarily. GPSsources are confirmed to be generally more compact, and less polarized,than CSS sources, although CSS sources show evidence for being somewhatless variable than GPS sources at 1.4 and 2.5 GHz. In addition, thewidths of GPS spectra are examined, and a significant difference isfound in the GPS sample of Snellen et al. (2000) between sources withcompact double (CD) or compact symmetric object (CSO) morphologies andsources with other morphologies, in that CD and CSO sources havegenerally narrower spectra. Possible reasons for this difference areconsidered.
| Radio Spectral Variability of the Nearby GPS Galaxy PKS 1718-649
Data from new Australia Telescope Compact Array observations of a nearbygigahertz peaked spectrum radio source, PKS 1718-649, show significantvariability both below and above the spectral turnover frequency duringa nine month period. Simple models of synchrotron self-absorption (SSA)and free-free absorption (FFA) cannot explain the individual spectra orthe spectral variability. Novel SSA or FFA models will need to beinvestigated in order to explain the complex variability in this source.Some potential explanations for the spectral variability are exploredhere but we can make no strong conclusion about which of SSA or FFA (oralternative mechanisms) causes the peaked spectrum in PKS 1718-649.
| Proper Motions in Compact Symmetric Objects
We discuss recent measurements of proper motions of the hotspots ofcompact symmetric objects (CSOs). Source expansion has been detected in10 CSOs so far and all these objects are very young (<=3 ×103yr). In a few sources ages have also been estimated fromenergy supply and spectral ageing arguments and these estimates arecomparable. This argues that these sources are close to equipartitionand that standard spectral ageing models apply. Proper motion studiesare now constraining hotspot accelerations, side-to-side motions, anddifferences in hotspot advance speeds between the two hotspots of asource. Although most CSOs are young sources their evolution is unclear.There is increasing evidence that in some objects the CSO structurerepresents a new phase of activity within a recurrent source.
| Large-scale HI structures and the origin of radio galaxies
We present the first results of a study aimed to detect large HIstructures in radio galaxies. In two of the three cases presented (ComaA and B2 0648+27), the detection of a large amount of HI distributedover several tens of kpc suggests a major merger as the origin of theobserved system and allows us to infer when this merger must haveoccurred. The situation is less clear for the third object (3C 433). Wepropose an evolutionary sequence for one of the radio galaxies studied.The implications of this study of neutral hydrogen in nearby radiogalaxies for high-/z objects and `normal' (i.e. radio-quiet) ellipticalgalaxies are also discussed.
| Identification of a new low-redshift GHz-peaked spectrum radio source and implications for the GHz-peaked spectrum class
The extragalactic radio source PKS B2254-367, associated with the galaxyIC 1459, is identified as a GHz-peaked spectrum (GPS) radio source. At adistance of 19 Mpc, PKS B2254-367 is one of the closest known GPS radiosources. IC 1459, NGC 1052 (18 Mpc) and NGC 6328 (56 Mpc) are the onlyGPS radio source host galaxies for which the detailed kinematics of thehost galaxies can be studied. All three galaxies present strong evidencefor merger activity, an actively fuelled black hole and high-densityenvironments with which the radio sources interact. Although radioluminosity evolution is generally invoked to explain the apparentoverabundance of GPS sources relative to radio galaxies, such strongevidence for merger-induced activity in the nuclei of nearby GPS hostgalaxies argues that at least some GPS radio sources are limited intheir development by the effects of merger activity and the resultinglikely sporadic fuelling of the central black holes and accretion discsthat power the radio sources. The radio structure associated with NGC1052 may directly suggest such a scenario.
| HST/WFPC2 snapshot imaging of symbiotic stars
The results of a HST/WFPC2 snapshot imaging survey of selected symbioticstars in 1999/2000 are presented. Seven sources - HD 149427 (PC 11), PUVul, RT Ser, He2-104 (Southern Crab), V1329 Cyg (HBV 475), V417 Cen andAS 201 - were observed in filters F218W (ultraviolet continuum), F502N([O III]λλ 4959, 5007) and F656N (Hαλ 6563);an eighth source, RS Oph, was observed in F437N ([O III]λ 4363),F502N and F656N. The presence of extended emission was detected inHe2-104, V1329 Cyg and possibly HD 149427. In He2-104, we detected the[O III] and Hα counterparts to the inner lobes found in [N II] byCorradi et al. For V1329 Cyg, comparison with previously publishedHST/FOC results indicates expanding ejecta which may be associated withan ejection event in 1982 (+/-2 yr) at a velocity of 260 +/- 50 kms-1 in the plane of the sky and at an assumed distance of 3.4kpc. We also present previously unpublished radio images of HD 149427,which we have obtained from the archives of the Australia TelescopeCompact Array and which reveal the presence of extended emission at asimilar orientation to that of the possible optical extension. Finally,we also include HST/WFPC2 GO observations of AG Peg and detect possibleextended emission in the F218W filter.
| An Investigation of Synchrotron Self-absorption and Free-Free Absorption Models in Explanation of the Gigahertz-peaked Spectrum of PKS 1718-649
We have undertaken the most detailed investigation yet of the shape andvariability of the radio spectrum near the spectral peak of agigahertz-peaked spectrum radio galaxy, using the Australia TelescopeCompact Array. The radio spectrum of PKS 1718-649 was measured nearlysimultaneously at 30-40 different frequencies over the range of 1-9 GHzat four epochs over a 14 month period, with the aim of constrainingdifferent physical models for the absorption process that causes thespectral peak at gigahertz frequencies. We find that the two mostplausible absorption processes, synchrotron self-absorption andfree-free absorption, can each explain most aspects of the data.However, each process also has difficulty explaining some aspects of thedata, in particular the variability of the source at frequencies belowthe peak. In either case, models based on homogeneous absorbers areclearly inadequate and inhomogeneities must be introduced into eitherthe synchrotron self-absorbed source or the free-free absorbing screen.Based on our analysis we cannot strongly prefer either synchrotronself-absorption or free-free absorption as the sole process responsiblefor the gigahertz-peaked spectrum of PKS 1718-649. However, theconsistency of the measured source size, from VLBI observations, withthe observed turnover frequency favors synchrotron self-absorption as asignificant effect in PKS 1718-649.
| B2 0648+27: A radio galaxy in a major merger
We present WSRT observations of the neutral hydrogen in the nearby radiogalaxy B2 0648+27. In emission, we detect a very large amount of H I(MH I = 1.1 x 1010 Msun) that isdistributed in a very extended disk, or ring-like structure, of about160 kpc in size. We also detect H I absorption against the central radiocontinuum component. The detection of the H I, its structure andkinematics, give us key information for building a possible evolutionaryscenario. The characteristics of the detected H I are explained as theresult of a major merger event that is likely to have occurred <≈109 yr ago. Interestingly, we find that, when observed inradio continuum at higher resolution, this galaxy has a double lobed,steep spectrum structure of about 1 kpc in size. Thus, despite its lowradio power, B2 0648+27 bears striking similarities with CompactSymmetric Objects, i.e. objects believed to represent the early phase ofradio galaxies (la few thousand yrs old). B2 0648+27 is one of the fewnearby radio galaxies where extended neutral hydrogen has been detectedso far. This, and other recent results, appear however to indicate thatnearby radio galaxies are more often gas rich than commonly assumed. Thephenomena described are likely to be much more common at high redshiftand galaxies like B2 0648+27 may provide valuable information on theevolution of high redshift radio sources.Based on observations with the Westerbork Synthesis Radio Telescope(WSRT) and the Very Large Array (VLA).
| VSOP Space VLBI and Geodetic VLBI Investigations of Southern Hemisphere Radio Sources
We present images from VLBI Space Observatory Programme (VSOP)observations of 14 compact extragalactic southern hemisphere radiosources, including a description of the observations, the data reductiontechniques, and the parameters of the resulting images and model fits.These images provide the highest resolution information to date for manyof these objects. Comparisons are made between VSOP and previousground-based VLBI results, including images from data extracted from thegeodetic VLBI archive at the United States Naval Observatory. From theVSOP data, we find that the two radio galaxies observed have lower peakbrightness temperatures than the 12 quasars. Also, these data show (1)no evidence for obvious differences between the brightness temperaturedistributions of gamma-ray-loud and gamma-ray-quiet radio-loud activegalactic nuclei and (2) no evidence for obvious correlations betweenbrightness temperature and spectral index, radio polarization, fluxdensity, or month timescale modulation index. These results areconsistent with previous work by Lister, Tingay, & Preston, whofound that the only observable significantly correlated withVSOP-derived brightness temperature is intraday variability, which isstrongly correlated with many relativistic beaming indicators. For onesource, PKS 1127-145, we undertake a detailed investigation of themilliarcsecond-scale component positions as a function of time, takingdata from the literature and the current work, to estimate propermotions. As a result, we suggest that two components previously reportedas stationary, C1 and C2, have apparent transverse speeds of (9.1+/-3.8)and (5.3+/-2.3) h-1c, respectively. We also make the firstinvestigation of the apparent motion in the nearest GHz-peaked spectrumradio galaxy, PKS 1718-649, finding an upper limit on the apparentseparation speed of 0.08c. Comparison of geodetic VLBI and VSOP datashow no significant detection of component motion in PKS 0208-512,(2.4+/-3.1) h-1c, and only a tentative detection in PKS0537-441, (2.8+/-2.2) h-1c. A significant detection ofcomponent motion is found in PKS 1610-771, solely from the geodetic VLBIdata, (9.4+/-3.5) h-1c.
| The Parkes quarter-Jansky flat-spectrum sample. I. Sample selection and source identifications
We present a new sample of quarter-Jansky flat-spectrum radio sourcesselected to search for high-redshift quasars and to study the evolutionof the flat-spectrum quasar population. The sample comprises 878 radiosources selected from the Parkes catalogues with spectral indices alpha5 GHz_2.7 GHz >=-0.4 where Snu ~ nu alpha. The sample covers all right ascensions and the declination rangefrom -80fdg0 to +2fdg5 , excluding low galactic latitudes (mid b mid< 10deg) and the Magellanic Cloud regions. We haveobtained improved radio source positions, firstly to reconfirm themajority of the existing identifications, and secondly, using digitizedsky-survey data and deep B, Gunn-i and Gunn-z CCD-imaging, to findoptical identifications for 223 previously-unidentified sources. Wepresent the final catalogue of 878 flat-spectrum sources: 827 arecompact radio sources identified with galaxies, quasars and BL Lacobjects, 38 have either extended radio structure or are identified withGalactic objects (PN, HII or non-compact radio source), 4 are obscuredby Galactic stars, and 9 (1 per cent of the total sample) remainunidentified. Full Appendices A-D are only available in electronic format http://www.edpsciences.org
| The ATCA intraday variability survey of extragalactic radio sources
We present the results of an Australia Telescope Compact Array (ATCA)survey for intraday variability (IDV) of the total and polarized fluxdensities of 118 compact, flat-spectrum, extragalactic radio sourcesfrom the Parkes 2.7-GHz Survey. A total of 22 total flux density IDVsources were discovered and 15 sources were found to show IDV of theirpolarized flux density. We discuss the statistical properties of the IDVsources, including the distribution of source modulation indices, andthe dependence of the variability amplitude on source spectral index andon Galactic position. We suggest interstellar scintillation (ISS) in theGalactic interstellar medium as the most likely mechanism for IDV. Evenso, the inferred high brightness temperatures cannot be easilyexplained.
| Where is the[O III] λ4363 Emitting Region in Active Galactic Nuclei?
The emission-line flux ratio of [O III] λ4363/[O III]λ5007 (RO III) is a useful diagnostic for theionization mechanism and physical properties of emission-line regions inactive galactic nuclei (AGNs). However, it is known that simplephotoionization models underpredict the [O III] λ4363 intensity,being inconsistent with observations. In this paper we report on severalpieces of evidence that a large fraction of the [O III] λ4363emission arises from the dense gas obscured by putative tori: (1) thevisibility of high-RO III regions is correlated to that ofbroad-line regions, (2) higher RO III objects show hottermid-infrared colors, (3) higher RO III objects show strongerhighly ionized emission lines such as [Fe VII] λ6087 and [Fe X]λ6374, and (4) higher RO III objects have broader linewidth of [O III] λ4363 normalized by that of [O III]λ5007. To estimate how such a dense component contributes to thetotal emission-line flux, dual-component photoionization modelcalculations are performed. It is shown that the observed values ofRO III of type 1 AGNs may be explained by introducing a5%-20% contribution from the dense component, while those of type 2 AGNsmay be explained by introducing a 0%-2% contribution. We also discussthe [O III] λ4363 emitting regions in LINERs in the framework ofour dual-component model.
| Simultaneous Observations of Variability at All Atmospheric Levels of V824 Arae (HD 155555)
We conducted a multiwavelength campaign observing V824 Ara (HD 155555,G5 IV+K0 IV-V) continuously throughout one complete orbital cycle (~1.7days) in early May of 1996. At the core of this campaign wereobservations using the GHRS on the Hubble Space Telescope (HST). In all,about 48,000 spectra, many in rapid readout mode, were obtained with theGHRS covering the C IV, Mg II, and Fe XXI wavelength regions at 11-15separate phases. Simultaneous observations were made with the ExtremeUltraviolet Explorer (EUVE). Radio observations (3.5 and 6 cm) wereconducted at the Australian Telescope, while ground-based visualspectroscopic and photometric observations were made at EuropeanSouthern Observatory, Cerro Tololo Inter-American Observatory, theAnglo-Australian Telescope, and South African Astronomical Observatory.Additional ground-based observations were obtained before, during, andafter the campaign. Our primary intent was to obtain a three-dimensionalmodel of the atmosphere extending from the photosphere to the corona.Variability was clearly detected, including several flares observed inthe HST, EUVE, and radio data. We present results from modeling theultraviolet transition region lines using an anisotropic macroturbulencemodel. Previous studies of transition region lines in late-type activestars have used multiple Gaussians to fit the observed line profiles,adding broad components to account for the extended wings observed inseveral active systems, including V711 Tau (HR 1099). This broadcomponent has been interpreted as arising from the continuous presenceof microflaring. We demonstrate that anisotropic macroturbulence modelscan also explain the observed Mg II profiles. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555.
| Extragalactic Radio Source Selection for Use in Directly Linking Optical Astrometric Observations to the Radio Reference Frame
Future space-based optical astrometric satellite missions present thepossibility of directly linking the radio and optical reference framesat the microarcsecond level. We have evaluated the current database ofradio observations of the extragalactic objects that make up theInternational Celestial Reference Frame (ICRF) to determine the optimumcandidates in terms of their radio properties for use as radio-opticalframe tie sources. We use up-to-date radio astrometric and ancillarydata to evaluate the sources in terms of their suitability for use bythe Space Interferometry Mission (SIM), but the results apply equallywell to any optical astrometric satellite or optical ground-basedobservations with sufficient brightness sensitivity to measure preciseastrometric positions of extragalactic objects directly. Currently, theICRF limits the accuracy of any radio-optical frame tie based on futureoptical astrometric satellite observations if the projected accuraciesfor these missions are realized. We have attempted to construct asuitable set of frame tie sources but found a significant deficit ofcandidate sources in the southern hemisphere. If the SIM optical frameis to be successfully tied to the ICRF, additional radio observationsare necessary to overcome the limitations of the radio frame.
| Radio Spectra and Variability of Gigahertz-Peaked Spectrum Radio Sources and Candidates
We have made long-term, high radio frequency observations of southernand equatorial active galactic nuclei (AGNs). After complementing thesedata with data from the literature, we have constructed the radiospectra of these sources and searched for sources with inverted spectrain the gigahertz range. We have identified 12 new sources with spectralshapes resembling those of the gigahertz-peaked spectrum (GPS) sourcesand eight other sources with inverted spectral parts in the gigahertzregion. Several of these new GPS-source candidates have high (>=10GHz) peak frequencies in the observer's frame, and they all exhibitstrong long-term variability. We have also studied the variabilitybehavior of known GPS sources, mainly quasar-type sources. All of the 14sources included in our sample show moderate to extremely high radiovariability, and at least 12 of them are also variable in the millimeterdomain. Long-term monitoring of these sources shows that some of themhave spectral shapes resembling those of classical variableflat-spectrum sources, indicating that some of these sources have beenmisidentified as GPS sources when only sparsely sampled data have beenavailable. On the other hand, four of the variable sources showpersistent GPS-type spectra at all stages of activity, suggesting thatat least some of the GPS sources can be variable.
| Extension of the ICRF for selected areas down to the 15th magnitude
In this paper, we present our results towards the extension of the ICRFto the optical domain down to the 15{th} V magnitude, forsome regions of special astronomical interest. This extension is givenby accurate positions and proper motions in the ICRS. Present epochpositions were obtained with the CCD transit circle of theAbrahão de Moraes Observatory, in the city of Valinhos -São Paulo. Proper motion derivation is achieved by combining ourmeridian circle positions with those from available astrometriccatalogues and SERC-J plate measurements performed with the MAMAmeasuring machine (Paris). The final result is presented as a cataloguecontaining positions, proper motions, magnitudes andcross-identifications with major catalogues for 41 721 objects. A fulldescription of the employed data is given along with an analysis of theuncertainties on positions and proper motions in our final catalogue. Onaverage, stars with V<= 14.0 have positional precisions better than50 mas in both coordinates, and 100 mas at the detection limit of ourinstrument (V ~ 16.0). For proper motions, average precisions are betterthan 4 mas/year, whatever the magnitude, when delta >=-17{o}. To the south of this declination, precisions becomemagnitude-dependent, providing figures of 3 mas/year when V<=12.0 andreaching 17 mas/year at the detection limit. The declination dependenceaffects the fainter stars and reflects a feature of our main first epochmaterial for this magnitude range, the USNO-A2.0 catalogue. The ValinhosCCD transit circle observations cover a variety of regions of greatinterest. Here, we consider those containing extragalactic radiosources, mostly from ICRF, and pre-main sequence stars in southernstar-forming regions (Chamaeleon, Lupus and Upper Scorpius - Ophiuchus),where positions from the SERC-J plates were employed. Based onobservations made with the CCD transit circle at the Abrahão deMoraes Observatory, Valinhos - São Paulo. Based on measurementsmade with the MAMA automatic measuring machine. Table 6 is onlyavailable 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/375/308
| HI deficiency in the galaxy cluster ACO 3627.
ATCA 21 cm Hi observations of the rich galaxy cluster ACO 3627 in theGreat Attractor region are presented. Three fields of 30' diameterlocated within one Abell radius of ACO 3627 were observed with aresolution of 15'' and an rms noise of ~ 1 mJy/beam. Only two galaxieswere detected in these fields. We compare their Hi distribution to newoptical R-band images and discuss their velocity fields. The firstgalaxy is a gas-rich unperturbed spiral whereas the second shows apeculiar Hi distribution. The estimated 3sigma Hi mass limit of ourobservations is ~ 7 108 Msun for a line width of150 km s-1. The non-detection of a considerable number ofluminous spiral galaxies indicates that the spiral galaxies are Hideficient. The low detection rate is comparable to the Hi deficient Comacluster (Bravo-Alfaro et al. \cite{q3}). ACO 3627 is a bright X-raycluster. We therefore suspect that ram pressure stripping is responsiblefor the Hi deficiency of the bright cluster spirals. The AustraliaTelescope Compact Array (ATCA) is operated by the Australia TelescopeNational Facility, CSIRO, as a National Research Facility.
| The VSOP 5 GHz AGN Survey I. Compilation and Observations
The VSOP mission is a Japanese-led project to image radio sources withsub-milliarcsec resolution by correlating the signal from the orbiting8-m telescope, HALCA, with a global array of telescopes. Twenty-fivepercent of the scientific time of this mission is devoted to a survey of402 bright, small-diameter extra-galactic radio sources at 5 GHz. Themajor goals of the VSOP Survey are statistical in nature: to determinethe brightness temperature and approximate structure; to provide asource list for use with future space VLBI missions; and to compareradio properties with other data throughout the EM spectrum. This paperdescribes: the compilation of a complete list of radio sourcesassociated with active galactic nuclei (AGN); the selection of thesubsample of sources to be observed with VSOP; the extensive groundresources used for the Survey; the status of the observations as of 2000July; the data-analysis methods; and several examples of results fromthe VSOP Survey. More detailed results from the full sample will begiven in future papers.
| Asymmetric Free-Free Absorption towards a Double Lobe of OQ 208
We present 1.66 GHz VSOP (VLBI Space Observatory Programme) observationsof a GHz-peaked spectrum (GPS) source, OQ 208. Two compact lobes with a10 pc separation in the NE-SW direction are identified with thoseobserved with ground VLBI at higher frequencies. We find that the twolobes are highly asymmetric in terms of a flux density ratio NE:SW of ~60:1 at 1.66 GHz, larger than that at higher frequencies. Both lobesshow a steeply rising spectrum between 1.66 and 2.32 GHz, which impliesfree-free absorption by an ambient plasma, rather than synchrotronself-absorption. The absorption features show different peakfrequencies, and indicate a larger optical depth towards the fainter SWlobe. This suggests the existence of an external absorber embedding thelobes. Hence, we can estimate the electron temperature, T_e, and theelectron density, n_e, of the absorber. Considering the optical depthand X-ray luminosity, we derive the electron temperature and density inthe ranges of 104 < T_e < 6 x 107 K and 600< n_e < 7 x 105 cm-3, respectively. Theseproperties coincide with those of NLR (Narrow Line Region). We point outthat the NLR plasma affects evolution of radio galaxies, paticularly GPSsources.
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