Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Unifying B2 radio galaxies with BL Lacertae objects
In an earlier paper we presented nuclear X-ray flux densities, measuredwith ROSAT, for the B2 bright sample of nearby low-luminosity radiogalaxies. In this paper we construct a nuclear X-ray luminosity functionfor the B2 radio galaxies, and discuss the consequences of our resultsfor models in which such radio galaxies are the parent population of BLLacertae (BL Lac) objects. Based on our observations of the B2 sample,we use Monte Carlo techniques to simulate samples of beamed radiogalaxies, and use the selection criteria of existing samples of BL Lacobjects to compare our simulated results to what is observed. We findthat previous analytical results are not applicable since the BL Lacsamples are selected on beamed flux density. A simple model in which BLLacs are the moderately beamed (γ~ 3) counterparts of radiogalaxies, with some random dispersion (~0.4 decades) in the intrinsicradio-X-ray relationship, can reproduce many of the features of theradio-selected and X-ray-selected BL Lac samples, including their radioand X-ray luminosity functions and the distributions of theirradio-to-X-ray spectral indices. In contrast, models in which the X-rayand radio emission have systematically different beaming parameterscannot reproduce important features of the radio-galaxy and BL Lacpopulations, and recently proposed models in which the radio-to-X-rayspectral index is a function of source luminosity cannot in themselvesaccount for the differences in the slopes of the radio- andX-ray-selected BL Lac luminosity functions. The redshift distributionand number counts of the X-ray-selected Einstein Medium SensitivitySurvey (EMSS) sample are well reproduced by our best models, supportinga picture in which these objects are beamed Fanaroff-Riley type I radiogalaxies with intrinsic luminosities similar to those of the B2 sample.However, we cannot match the redshift distribution of the radio-selected1-Jy sample, and it is likely that a population of Fanaroff-Riley typeII radio galaxies is responsible for the high-redshift objects in thissample, in agreement with previously reported results on the sample'sradio and optical emission-line properties.
| Redshifts for a Sample of Radio-selected Poor Clusters
Multifiber optical spectroscopy has been performed on galaxies in thevicinity of strong, nearby radio galaxies. These radio galaxies wereselected from the 3CR and B2 catalogs based on their exclusion from theAbell catalog, which is puzzling given the hypothesis that an externalmedium is required to confine the radio plasma of such galaxies.Velocities derived from the spectra were used to confirm the existenceof groups and poor clusters in the fields of most of the radio galaxies.We find that all radio galaxies with classical Fanaroff-Riley type Imorphologies prove to reside in clusters, whereas the other radiogalaxies often appear to be recent galaxy-galaxy mergers in regions oflow galaxy density. These findings confirm the earlier result that theexistence of extended X-ray emission combined with a statistical excessof neighboring galaxies can be used to identify poor clusters associatedwith radio galaxies.
| 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
| Circumnuclear Stellar Population, Morphology, and Environment of Seyfert 2 Galaxies: An Evolutionary Scenario
We investigate the relation between the characteristics of thecircumnuclear stellar population and both the galaxy morphology and thepresence of close companions for a sample of 35 Seyfert 2 nuclei.Fifteen galaxies present unambiguous signatures of recent episodes ofstar formation within ~300 pc of the nucleus. When we relate thisproperty to the Hubble type of the host galaxy, we find that theincidence of recent circumnuclear star formation increases along theHubble sequence; it seems to be greater than that in non-Seyfertgalaxies for the early Hubble types S0 and Sa but similar to that innon-Seyfert galaxies for later Hubble types. In both early-type andlate-type Seyfert galaxies, the presence of recent circumnuclear starformation is related to the galaxy morphology in the inner fewkiloparsecs, as observed in Hubble Space Telescope images through thefilter F606W by Malkan et al., who have assigned a late ``inner Hubbletype'' to most Seyfert 2 galaxies with recent circumnuclear starformation. This new classification is due to the presence of dust lanesand spiral structures in the inner region. The presence of recent starformation around Seyfert 2 nuclei is also related to interactions: amongthe 13 galaxies of the sample with close companions or in mergers, ninehave recent star formation in the nuclear region. These correlationsbetween the presence of companions, the inner morphology, and theincidence of recent star formation suggest an evolutionary scenario inwhich the interaction is responsible for sending gas inward, which bothfeeds the active galactic nucleus and triggers star formation. Thestarburst then fades with time and the composite Seyfert 2+starburstnucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellarpopulation. This scenario can reconcile the hypothesis that the activenucleus in Seyfert galaxies is triggered by interactions with theresults of previous studies, which find only a small excess ofinteracting galaxies in Seyfert samples when compared with non-Seyfertsamples. The large excess can only be found early after the interaction,in the phase in which a composite (Seyfert+starburst) nucleus isobserved.
| X-ray observations of low-power radio galaxies from the B2 catalogue
We present an analysis of X-ray data, taken with ROSAT, for awell-defined sample of low-power radio galaxies from the Bologna B2catalogue. Where possible, the HRI has been used in order to takeadvantage of the high spatial resolution provided by this instrument. Avariety of models are fitted to radial profiles in order to separate theresolved and unresolved X-ray emission from the galaxies. We demonstratea strong, approximately linear, correlation between the luminosities ofthe unresolved X-ray components and the 5-GHz luminosities of the radiocores in this sample. This suggests a physical relationship between thesoft X-ray emission of radio galaxies and the jet-generated radio coreemission. We infer a nuclear jet-related origin for at least some of theX-ray emission.
| Asymmetries in the jets of weak radio galaxies
We describe a study of the side-to-side asymmetries on kpc scales inthe jets of FR I radio galaxies selected from the B2 sample. The basicdata are jet surface brightnesses and widths determined by fittingtransverse profiles to Very Large Array (VLA) images at a range ofdistances from the core. Differences between the jets at a givendistance from the nucleus are interpreted as effects of Doppler beamingon intrinsically symmetrical flows and are compared with the modelderived for 3C31 by Laing & Bridle and with simpler variants. Thejet/counterjet brightness ratios where the main jet first brightens arecorrelated with core prominence, as expected for a relativistic flow.From the distribution of brightness ratios, we infer that jets have amaximum velocity ~0.9c where they first flare and brighten, but there isalso evidence for additional slower material. Deceleration tosubrelativistic speeds occurs on scales which increase with radio power.Jets in the majority of sources with luminosities <1024WHz-1 at 1.4 GHz become essentially symmetrical (andtherefore subrelativistic) within 2 kpc of the core. In more powerfulsources, jets that flare within the first 2 kpc become symmetrical by 10kpc, but a subset of the most luminous objects has jets which remainasymmetrical to larger distances. The point at which the brighter jetflares appears to correspond to a sudden increase in rest-frameemissivity, but the ratio of distances to the flaring point in main andcounterjets is anticorrelated with brightness ratio, as expected for adecelerating relativistic flow. Brightness and full width athalf-maximum (FWHM) ratios are also anticorrelated, an effect which weinterpret as a result of Doppler beaming for a flow in which thevelocity decreases radially outwards from the jet axis. Jet decelerationby entrainment of external material provides a natural explanation forthese velocity gradients. The jet energy flux is roughly consistent withenergy supply to the lobes over a source lifetime estimated fromspectral index measurements. Our results are qualitatively consistentwith unified models of FR I radio galaxies and BL Lac objects, butrequire some modifications to the standard picture.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| H_2O megamaser emission from FR I radio galaxies
A systematic search for 22 GHz H_2O megamaser emission is reported for50 nearby (z la 0.15) FR I galaxies. No detection was obtained, implyingthat ultraluminous H_2O masers (L_H_2O > 10(3) Lsun) mustbe rare in early-type galaxies with FR I radio morphology. Despitehigher radio core luminosities the detection rate for our sample islower than in similar surveys of late-type Seyfert galaxies. Thispuzzling difference between Seyferts and low-power radio galaxies couldbe explained in several ways: a) the maser emission is saturated andtherefore independent of the radio core luminosity, b) the masers areunsaturated and originate in a thin circumnuclear gas disk, so the`seed' radio continuum would come from the far jet which isrelativistically dimmed or c) the amount, kinematics, or thedistribution of the molecular gas in the nuclei of Seyferts and radiogalaxies is different. Further studies of maser properties may provideclues to the differences between radio-loud and radio-quiet AGN.
| Weak Radio Galaxies. I. Broad-Band Optical Imaging
We report on a study of the optical properties of weak radio galaxies(WRGs) from the B2 survey, to obtain the broad-band photometricproperties and morphology information crucial for our narrow-bandimaging at Hα and [O III] study (Carrillo et al. 1997). This papercontains optical CCD images of 30 radio galaxies obtained at V, R and I.We present the morphological and photometric results and discuss theirrelationship to the radio structure and environmental properties. Wefind that most WRGs are E galaxies, have peculiar morphologies and arelocated in high galaxy density environments. Optical colors of WRGs areunusual if compared to colors of normal ellipticals, but similar to AGNcolors. The surface brightness profiles of most WRGs follow the deVaucouleurs law expected for ellipticals at most radii, but in all casesa turnover or flatness is observed in the innermost portion of theprofiles, possibly produced by an additional nuclear emission source.The tidal effects produced by galaxy companions described by Kormendy(1977) for ellipticals, are clearly evident in the outer parts of theWRGs profiles and can be associated to interaction effects.
| Optical and Far-Infrared Emission of IRAS Seyfert Galaxies
This paper presents an analysis of moderately large samples of type 1and 2 Seyfert galaxies through optical observations and far-infraredIRAS data, also taking into account theoretical color indices derivedfrom dust emission models. The galaxies in the samples cover a ratherlarge interval in far-infrared luminosity, i.e., 7.6 <= log(LIR/Lȯ) <= 12.6. We show that both types of Seyferts haveapproximately the same distribution of number of objects with a givenLIR. Galaxies with similar far-infrared color indices alpha (100, 60)are grouped together, and the corresponding average color indices areinterpreted in terms of a simple model in which the observed colorsresult from the combination of dust directly heated by the activegalactic nucleus with a component from the host galaxy represented bythe emission of cool dust. On the basis of the average IRAS colors ofthe derived groups, we show that type 1 and 2 Seyfert galaxies areundistinguishable from each other. From the luminosity ratios LIR/LHalpha and LIR/L[O III], we show that basically the same model can beapplied to both types of Seyfert, only allowing for the variation ofmodel conditions: type 2 Seyferts would be like type 1 Seyferts but withthe Seyfert nucleus and broad line region more effectively "hidden" bydust.
| Polarization of low-luminosity radio galaxies: The 6 CM data
Polarization data are presented for 62 B2 radio galaxies. The sourceswere observed at 6 cm in different configurations of the VLA. Theresults are given as integrated and mean component parameters. We alsopresent maps of total intensity with superimposed vectors representingthe fractional polarization and position angle. We compare these newdata with older data at 20 cm and discuss briefly depolarization,rotation of polarization angle and geometry of the magnetic field.
| Corrections and additions to the third reference catalogue of bright galaxies
List of corrections and additions to the Third Reference Catalogue ofBright Galaxies (RC3) are given. The corrected version of the catalogue(RC3.9b), dated April 1994, is currently available through the nationaldata centers.
| Optical positions and 327 MHz flux-densities of UGC galaxies in selected Westerbork fields
The study presents accurate optical positions of 421 UGC galaxies whichare used to search for 30 92-cm WSRT fields observed for emission fromthese galaxies. Good 92-cm flux densities were obtained for 140galaxies, marginal flux densities for 71 galaxies, and upper limits for210 galaxies. For 35 galaxies, spectral indices in the decimeterwavelength range are determined. The mean spectral index for spiralgalaxies (0.72 +/- 0.03) is very similar to that of elliptical galaxies(0.64 +/- 0.10). The four multiple systems in the sample have a muchflatter spectral index (-0.21 +/- 0.07), from which the presence of asignificant thermal component in their total radio emission issuggested. Comparison with IRAS results show that about half of thegalaxies detected at radio wavelengths are detected in the FIR. It isproposed that some spiral galaxies are anomalously weak in the IR ascompared with their radio brightness.
| Polarization in low luminosity radio galaxies
Polarization data are presented for 69 B2 radiogalaxies. The sourceswere observed at 20 cm in different configurations of the VLA. Theresults are given as integrated and mean component parameter and arecompared with those from similar studies. We also present maps of totalintensity with superimposed vectors representing the fractionalpolarization and position angle for the most interesting sources.Information on individual sources are given when interesting structuresare present.
| Optical line-emitting gas and radio emission - Evidence for correlation in low-luminosity radio galaxies
Narrow-band H-alpha + (N II) images of a sample of low-luminosity B2radio sources are used to investigate the correlations between radio andoptical emission-line properties. It is shown that the correlationbetween radio and emission-line luminosity - previously established forhigh-power radio galaxies - extends over five decades in radioluminosity and includes the low-radio power sources. The compact radiosources have emission-line excesses relative to the other sources in thesample of the same radio power, as well as far-IR emissioncharacteristics of emission from cool dust. Although continuityarguments can be used to show that the ionized gas in most of thelow-luminosity sources is likely to be photoionized by the ActiveGalactic Nucleus, it is possible that additional ionizing sourcescontribute in the compact, emission-line excess objects. A program ofdetailed spectroscopic observations is required to investigate theunderlying physical mechanisms.
| A finding list of extragalactic radio jets and statistical results
Extragalactic radio jets are a common phenomenon. Many more jets havebeen found since Bridle and Perley (1984) reviewed the subject. In thispaper, we list 276 radio jets known in December 1989. We investigate theratio of jet emission to core flux as a function of core luminosity andcompare it with the fraction of detections of jets given by Bridle andPerley. We find them to be consistent.
| Very large array observations of radio-selected dumbbell galaxies
An unbiased sample of radio sources associated with optical dumbbellgalaxies is presented. This sample has been assembled to study therelationship between the radio and optical properties of radio-louddumbbell galaxies. High-quality radio data already exist for a number ofthe sources in the sample, but those sources without good data have beenobserved with the very large array at 20 or 6 cm. These new observationsare described and radio images are presented. Analysis of both the radiostructure of the sources and their radio luminosity has been carriedout, and a comparison is made with the properties of a complete sampleof radio sources associated with single galaxies. Radio sourcesassociated with dumbbell galaxies are found on average to have moredistorted structures than sources associated with single galaxies,demonstrating the influence of the dumbbell dynamics in shaping thelarge-scale structure of the radio sources. It is shown that in therange 10 exp 24 - 10 exp 26 W/Hz at 408 MHz the radio luminosityfunction of dumbbell systems is flatter than that of single-galaxy radiosources, indicating that a close companion may trigger a radio source inthe main galaxy, or alternatively increase the luminosity of an existingradio source.
| A dynamical analysis of twelve clusters of galaxies
Four-hundred-twenty-eight new redshift measurements for galaxies in thevicinity of 12 Abell clusters are presented. The data are supplementedby previously published data with 3 deg of each cluster center. Thecluster selection, the variety of telescopes and instrumentation used toobtain the galaxy redshifts, and the available X-ray observations arediscussed. Each cluster is exmained in some detail, with the emphasisplaced on the nature of the observed velocity distributions. Robust andresistant estimators of the velocity location and scale are applied inorder to quantify these distributions. The offset in velocity space ofthe dominant galaxy in each cluster or subcluster is considered withrespect to the central location in the velocity space of the cluster asa whole, and the physical implications of significant offsets found inseveral clusters are discussed. Dynamical estimates of the masses ofclusters and/or subclusters are obtained; for clearly bimodal systems,two-body models are employed to specify their likely dynamical state.
| The neutral hydrogen content of early type disk galaxies
This paper presents the results of a sensitive 21-cm survey of massiveearly type galaxies made with the Arecibo radio telescope. Of the 81galaxies observed, the detections comprise 48 percent of the S0s, 73percent of the S0a's, and 96 percent of the Sa's. The values of thehybrid, distance-independent H I surface densities of the S0 galaxies inthe sample ranged continuously from amounts comparable to the mostgas-rich Sa galaxies to low estimated upper limts of the H I content.CCD images of most of the gas-rich S0s revealed either faint spiralfeatures or patchy structure in the disks. While no firm correlationbetween H I content and environmental density is apparent for thegalaxies in the sample, two-sample statistics suggest a differencebetween the highest and the lowest density bins. Early-type diskgalaxies within low density environments tend to have higher gas surfacedensities than those within high-density environments.
| UGC galaxies stronger than 25 mJy at 4.85 GHz
UGC galaxies in the declination band +5 to +75 deg were identified byposition coincidence with radio sources stronger than 25 mJy on theGreen Bank 4.85 GHz sky maps. Candidate identifications were confirmedor rejected with the aid of published aperture-synthesis maps and new4.86 GHz VLA maps having 15 or 18 arcsec resolution, resulting in asample of 347 nearby radio galaxies plus five new quasar-galaxy pairs.The radio energy sources in UGC galaxies were classified as 'starbursts'or 'monsters' on the basis of their infrared-radio flux ratios, infraredspectral indices, and radio morphologies. The rms scatter in thelogarithmic infrared-radio ratio q is not more than 0.16 for starburstgalaxies selected at 4.85 GHz. Radio spectral indices were obtained fornearly all of the UGC galaxies, and S0 galaxies account for adisproportionate share of the compact flat-spectrum (alpha less than0.5) radio sources. The extended radio jets and lobes produced bymonsters are preferentially, but not exclusively, aligned within about30 deg of the optical minor axes of their host galaxies. The tendencytoward minor-axis ejection appears to be independent of radio-sourcesize and is strongest for elliptical galaxies.
| Observations of faint radio galaxies with the Ratan-600 radio telescope
The results of observations of 79 faint radio galaxies from the BolognaSurvey with the RATAN-600 radio telescope at the frequency 3.95 GHz arepresented. The dependences of spectral indices of radio emission on theratio of radio and optical luminosities are analyzed. The dependence oflinear sizes of radio galaxies on these ratios and on spectral indiceswas studied. It is shown that median angular sizes and spectral indicesincrease with flux density. It is also shown that among radio galaxieswith flat spectra objects of small linear sizes (less than 10 kpc) occurmuch more frequently than among radio galaxies with steep spectra.
| Infrared studies of elliptical galaxies. II - A radio-selected sample
An IR survey of radio galaxies in the Bologna B2 catalog is reported. Itis found that 40 percent of the sample has IR luminosities of at leastone billion solar luminosities, as opposed to about 8 pecent of normalellipticals. The galaxies are inhomogeneous in their IR properties. Themost IR-luminous galaxies are those listed as peculiar by Zwicky.Statistically, these galaxies are strikingly different from Seyfertgalaxies in their IR properties in that they show much more radioemission in comparison with their IR emission than do Seyferts, evenwhen the emission from the extended radio lobes has been discounted.
| Groups of galaxies in the Center for Astrophysics redshift survey
By applying the Huchra and Geller (1982) objective group identificationalgorithm to the Center for Astrophysics' redshift survey, a catalog of128 groups with three or more members is extracted, and 92 of these areused as a statistical sample. A comparison of the distribution of groupcenters with the distribution of all galaxies in the survey indicatesqualitatively that groups trace the large-scale structure of the region.The physical properties of groups may be related to the details oflarge-scale structure, and it is concluded that differences among groupcatalogs may be due to the properties of large-scale structures andtheir location relative to the survey limits.
| Radio galaxy jets as probes of galactic structure
It has been noted that the central source of an asymmetric nucleargalactic radio jet may experience considerable net thrust andconsequently behave like a massive rocket. In this paper, simple modelsfor the motion of a rocket through a galaxy are examined. It is foundthat the density distribution of the galaxy is important, and determineswhether a given source can escape. Thus, observations of the locationand velocity of a source relative to its galactic center may provide newconstraints on models of the density distribution in galaxies.
| Relative velocities of dumbbell galaxies
A systematic study of a morphologically-selected sample of dumbbellgalaxies is presented. A dumbbell/multiple nuclei subclassificationscheme is introduced. The single-object velocity dispersion of dumbbellcomponents in Abell-type clusters is 436 + or - 88 km/sec, which issignificantly smaller than that of normal cluster galaxies and ofmultiple nuclei systems. A detailed analysis of this sample, taking intoaccount various projection effects, indicates that the two components ofdumbbell pairs are physically associated, and probably in circularorbits around a common center. A relative rotation velocity of 1200km/sec at a separation of 40 kpc is indicated. This translates into asingle-object velocity of 600 km, intermediate between the typicalrotation velocity in the central part of a cD galaxy and that in thecluster-at-large. Furthermore, the rotation curve appears to be risingin this radial range. The data provide direct evidence of the existenceof a dark intracluster medium on a scale of 20-50 kpc.
| A morphological effect in pairs of elliptical galaxies
A morphological signature of gravitational interaction has been found ina sample of 50 close pairs of elliptical galaxies. The halos of bothgalaxies in five, and possibly more, pairs are off-center and thisoffset is symmetric with respect to the center of the pair. Among thephysical mechanisms that could be responsible for this morphologicaleffect, the displacement of the nucleus with respect to the halo in eachgalaxy or the formation of an asymmetric tidal bulge are the mostlikely. The observation of this effect should provide constraints on theinternal dynamics of elliptical galaxies, as well as on the dynamics ofpairs.
| A catalogue of early-type galaxies with emission lines
Spectroscopic and photometric data on 289 early-type galaxies (E and S0)with optical emission lines are presented and possible correlationsamong properties of the galaxies in the sample are investigated. Theoccurrence of phenomena as radio emission, presence of neutral hydrogenand dust shows an increase in comparison with the occurrence of the samephenomena in these morphological classes as a whole. There is noevidence of a relationship between apparent shape and presence ofionized gas in the central regions.
| VLA observations of low-luminosity radio galaxies. VI - Discussion of radio jets
The B2 source sample of low luminosity radio galaxies, which is used forthe present statistical study of the properties of jets, contains about100 objects, and has been reobserved at 1.4 GHz using three differentconfigurations of the VLA. Unambiguous evidence is found for thepresence of jets in 45 percent of the sources. The fraction of sourceswith jets decreases with increasing source power; the less powerfulradio galaxies are noted to have the more symmetric jets. Jet propertiesare discussed in terms of the Bicknell (1984, 1985) model.
| The statistical distribution of the neutral-hydrogen content of S0 galaxies
The distribution of relative global H I content M(H I)/L(B) has beenderived for galaxies of types S0 and S0/a using a data set derived fromrecent H I observations in the literature. The relative H I content ofthese galaxies is found to show transitional properties betweenelliptical and spiral galaxies. The distribution of M(H I)/L(B) forS0/a's resembles that for spirals, and these galaxies may represent'fossil' spirals, i.e., galaxies whose gas has been severely depleted bystar formation. The distribution for S0's, however, resembles that forellipticals. The form of this distribution strongly suggests an externalorigin for most of the H I in S0 galaxies.
| VLA observations of low luminosity radio galaxies. I - Sources with angular size smaller than two arcminutes
Fifty-seven radio sources with angular size smaller than about twoarcmin, selected from two complete samples of low luminosity B2 radiogalaxies, have been observed at 20 cm with the VLA in the Bconfiguration, with an angular resolution of about 3.5 arcsec. Thereduction procedure is described and the quality of the radio maps isdiscussed. The dynamic range in the final CLEANed maps is usually around100:1 or better. Observational parameters and intrinsic propertiesderived for each source are given in separate tables. Contour maps arepresented for resolved sources. Information on individual sources isprovided, particularly in case of interesting structures.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
Katalógusok és elnevezések:
|
