Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
An Hα survey of cluster galaxies - V. Cluster-field comparison for early-type galaxies
We have extended our Hα objective prism survey of eightlow-redshift clusters (viz. Abell 262, 347, 400, 426, 569, 779, 1367 and1656) to include a complete sample of early-type galaxies within 1.5Abell radii of the cluster centres. Of the 379 galaxies surveyed, 3 percent of E, E-S0 galaxies, 6 per cent of S0 galaxies, and 9 per cent ofS0/a galaxies were detected in emission. From a comparison of clusterand supercluster field galaxies, we conclude that the frequency ofemission-line galaxies (ELGs; Wλ>= 20 Å) issimilar for field and cluster early-type galaxies. A similar result haspreviously been obtained for galaxies of types Sa and later. Together,these results confirm the inference of Biviano et al. that the relativefrequency of ELGs in clusters and the field can be entirely accountedfor by the different mix of morphological types between the differingenvironments, and that, for galaxies of a given morphological type, thefraction of ELGs is independent of environment. Detected emission isclassified as `compact' or `diffuse', identified as circumnuclearstarburst or active galactic nucleus (AGN) emission and disc emission,respectively. By comparing spectroscopic data for cluster early-typeELGs with data for field galaxies from the Palomar spectroscopic surveyof nearby galactic nuclei, we demonstrate that there is modest evidencefor an enhancement of compact HII emission relative to AGN emission inthe early-type cluster ELGs as compared to the field. For the clusterearly-type galaxies, compact HII emission correlates strongly with adisturbed morphology. This suggests that, as for later-type clustergalaxies, this enhanced compact HII emission can readily be explained asan enhancement of circumnuclear starburst emission due to gravitationaltidal interactions, most likely caused by subcluster merging and otheron-going processes of cluster virialization.
| M/L, Hα Rotation Curves, and H I Measurements for 329 Nearby Cluster and Field Spirals. I. Data
A survey of 329 nearby galaxies (redshift z<0.045) has been conductedto study the distribution of mass and light within spiral galaxies overa range of environments. The 18 observed clusters and groups span arange of richness, density, and X-ray temperature and are supplementedby a set of 30 isolated field galaxies. Optical spectroscopy taken withthe 200 inch (5 m) Hale Telescope provides separately resolved Hαand [N II] major-axis rotation curves for the complete set of galaxies,which are analyzed to yield velocity widths and profile shapes, extents,and gradients. H I line profiles provide an independent velocity widthmeasurement and a measure of H I gas mass and distribution. I-bandimages are used to deconvolve profiles into disk and bulge components,to determine global luminosities and ellipticities, and to checkmorphological classification. These data are combined to form a unifieddata set ideal for the study of the effects of environment upon galaxyevolution.
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| Tidally Triggered Star Formation in Close Pairs of Galaxies. II. Constraints on Burst Strengths and Ages
Galaxy-galaxy interactions rearrange the baryons in galaxies and triggersubstantial star formation; the aggregate effects of these interactionson the evolutionary histories of galaxies in the universe are poorlyunderstood. We combine B- and R-band photometry and optical spectroscopyto estimate the strengths and timescales of bursts of triggered starformation in the centers of 190 galaxies in pairs and compact groups.Based on an analysis of the measured colors and EW(Hα), wecharacterize the preexisting and triggered populations separately. Thebest-fitting burst scenarios assume stronger reddening corrections forline emission than for the continuum and continuous star formationlasting for >~100 Myr. The most realistic scenarios require aninitial mass function that is deficient in the highest mass stars. Thecolor of the preexisting stellar population is the most significantsource of uncertainty. Triggered star formation contributessubstantially (probably >~50%) to the R-band flux in the centralregions of several galaxies; tidal tails do not necessarily accompanythis star formation. Many of the galaxies in our sample have bluercenters than outskirts, suggesting that pre- or nonmerger interactionsmay lead to evolution along the Hubble sequence. These objects wouldappear blue and compact at higher redshifts; the older, redder outskirtsof the disks would be difficult to detect. Our data indicate thatgalaxies with larger separations on the sky contain weaker, and probablyolder, bursts of star formation on average. However, confirmation ofthese trends requires further constraints on the colors of the olderstellar populations and on the reddening for individual galaxies.
| Evolution of Star-forming and Active Galaxies in Nearby Clusters
We have used optical spectroscopy to investigate the active galaxypopulations in a sample of 20 nearby Abell clusters. The targets wereidentified on the basis of 1.4 GHz radio emission, which identifies themas either active galactic nuclei (AGNs) or galaxies forming stars atrates comparable to or greater than that of the Milky Way. The spectrawere used to characterize the galaxies via their emission and absorptionfeatures. The spectroscopy results reveal a significant population ofstar-forming galaxies with large amounts of nuclear dust extinction.This extinction eliminates bluer emission lines such as [O II] from thespectra of these galaxies, meaning their star formation could easily beoverlooked in studies that focus on such features. Around 20% of thecluster star-forming galaxies have spectra of this type. The radialdistributions of active galaxies in clusters show a strong segregationbetween star-forming galaxies and AGNs, with star-forming galaxiesbroadly distributed and AGNs preferentially in the cluster cores. Theradial distribution of the dusty star-forming galaxies is more centrallyconcentrated than the star-forming galaxies in general, which arguesthat they are a consequence of some cluster environmental effect.Furthermore, we note that such galaxies may be identified using their4000 Å break strengths. We find that discrepancies in reportedradio luminosity functions for AGNs are likely the result ofclassification differences. There exists a large population of clustergalaxies whose radio fluxes, far-infrared fluxes, and optical magnitudessuggest their radio emission may be powered by stars yet whose spectralack emission lines. Understanding the nature of these galaxies iscritical to assessing the importance of AGNs in the radio luminosityfunction at low luminosities. We also find that regardless of thispopulation, the crossover point where the radio luminosity function iscomposed equally of star-forming galaxies and AGNs occurs at lowerluminosities in clusters than in the field. This is likely a simpleconsequence of the reduction in star formation in cluster galaxies andthe morphological mix in clusters compared with the field.Based in part on observations obtained with the Apache Point Observatory(APO) 3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| The Radio Galaxy Populations of Nearby Northern Abell Clusters
We report on the use of the NRAO VLA Sky Survey (NVSS) to identify radiogalaxies in 18 nearby Abell clusters. The listings extend from the coresof the clusters out to radii of 3 h-175 Mpc, whichcorresponds to 1.5 Abell radii and approximately 4 orders of magnitudein galaxy density. To create a truly useful catalog, we have collectedoptical spectra for nearly all of the galaxies lacking public velocitymeasurements. Consequently, we are able to discriminate between thoseradio galaxies seen in projection on the cluster and those that are inactuality cluster members. The resulting catalog consists of 329 clusterradio galaxies plus 138 galaxies deemed foreground or backgroundobjects, and new velocity measurements are reported for 273 of theseradio galaxies. The motivation for the catalog is the study of galaxyevolution in the cluster environment. The radio luminosity function is apowerful tool in the identification of active galaxies, as it isdominated by star-forming galaxies at intermediate luminosities andactive galactic nuclei (AGNs) at higher luminosities. The flux limit ofthe NVSS allows us to identify AGNs and star-forming galaxies down tostar formation rates less than 1 Msolar yr-1. Thissensitivity, coupled with the all-sky nature of the NVSS, allows us toproduce a catalog of considerable depth and breadth. In addition tothese data, we report detected infrared fluxes and upper limits obtainedfrom IRAS data. It is hoped that this database will prove useful in anumber of potential studies of the effect of environment on galaxyevolution. Based in part on observations obtained with the Apache PointObservatory 3.5 m telescope, which is owned and operated by theAstrophysical Research Consortium (ARC).
| The calibration of the extragalactic distance scale: methods and problems.
Not Available
| An Hα survey of eight Abell clusters: the dependence of tidally induced star formation on cluster density
We have undertaken a survey of Hα emission in a substantiallycomplete sample of CGCG galaxies of types Sa and later within 1.5 Abellradii of the centres of eight low-redshift Abell clusters (Abell 262,347, 400, 426, 569, 779, 1367 and 1656). Some 320 galaxies weresurveyed, of which 116 were detected in emission (39 per cent ofspirals, 75 per cent of peculiars). Here we present previouslyunpublished data for 243 galaxies in seven clusters. Detected emissionis classified as `compact' or `diffuse'. From an analysis of the fullsurvey sample, we confirm our previous identification of compact anddiffuse emission with circumnuclear starburst and disc emissionrespectively. The circumnuclear emission is associated either with thepresence of a bar, or with a disturbed galaxy morphology indicative ofongoing tidal interactions (whether galaxy-galaxy, galaxy-group, orgalaxy-cluster). The frequency of such tidally induced (circumnuclear)starburst emission in spirals increases from regions of lower to higherlocal galaxy surface density, and from clusters with lower to highercentral galaxy space density. The percentages of spirals classed asdisturbed and of galaxies classified as peculiar show a similar trend.These results suggest that tidal interactions for spirals are morefrequent in regions of higher local density and for clusters with highercentral galaxy density. The prevalence of such tidal interactions inclusters is expected from recent theoretical modelling of clusters witha non-static potential undergoing collapse and infall. Furthermore, inaccord with this picture, we suggest that peculiar galaxies arepredominantly ongoing mergers. We conclude that tidal interactions arelikely to be the main mechanism for the transformation of spirals to S0sin clusters. This mechanism operates more efficiently in higher densityenvironments, as is required by the morphological type-local surfacedensity (T-Σ) relation for galaxies in clusters. For regions ofcomparable local density, the frequency of tidally induced starburstemission is greater in clusters with higher central galaxy density. Thisimplies that, for a given local density, morphological transformation ofdisc galaxies proceeds more rapidly in clusters of higher central galaxydensity. This effect is considered to be the result of subclustermerging, and could account for the previously considered anomalousabsence of a significant T-Σ relation for irregular clusters atintermediate redshift.
| 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.
| The Dynamics of Poor Systems of Galaxies
We assemble and observe a sample of poor galaxy systems that is suitablefor testing N-body simulations of hierarchical clustering and otherdynamical halo models. We (1) determine the parameters of the densityprofile rho(r) and the velocity dispersion profile sigma_p(R), (2)separate emission-line galaxies from absorption-line galaxies, examiningthe model parameters and as a function of spectroscopic type, and (3)for the best-behaved subsample, constrain the velocity anisotropyparameter, beta, which determines the shapes of the galaxy orbits. Oursample consists of 20 systems, 12 of which have extended X-ray emissionin the ROSAT All-Sky Survey. We measure the 877 optical spectra ofgalaxies brighter than m_R~15.4 within 1.5 h^-1 Mpc of the systemcenters (we take H_0=100 h km s^-1 Mpc^-1). Thus, we sample the systemmembership to a radius typically three times larger than other recentoptical group surveys. The average system population is 30 galaxies, andthe average line-of-sight velocity dispersion is ~300 km s^-1. TheNavarro, Frenk, & White universal profile and the Hernquist modelboth provide good descriptions of the spatial data. In most cases anisothermal sphere is ruled out. Systems with declining sigma_p(R) arewell-matched by theoretical profiles in which the star-forming galaxieshave predominantly radial orbits (beta>0) many of these galaxies areprobably falling in for the first time. There is significant evidencefor spatial segregation of the spectroscopic classes regardless ofsigma_p(R).
| Seeking the Local Convergence Depth. IV. Tully-Fisher Observations of 35 Abell Clusters
We present Tully-Fisher observations for 35 rich Abell clusters ofgalaxies. Results from I-band photometry and optical rotation curve workconstitute the bulk of this paper. This is the third such datainstallment of an all-sky survey of 52 clusters in the distance range~50 to 200 h^-1 Mpc. The complete data set provides the basis fordetermining an accurate Tully-Fisher template relation and forestimating the amplitude and direction of the local bulk flow on a 100h^-1 Mpc scale.
| The I-Band Tully-Fisher Relation for SC Galaxies: Optical Imaging Data
Properties derived from the analysis of photometric I-band imagingobservations are presented for 1727 inclined spiral galaxies, mostly oftypes Sbc and Sc. The reduction, parameter extraction, and errorestimation procedures are discussed in detail. The asymptotic behaviorof the magnitude curve of growth and the radial variation in ellipticityand position angle are used in combination with the linearity of thesurface brightness falloff to fit the disk portion of the profile. TotalI-band magnitudes are calculated by extrapolating the detected surfacebrightness profile to a radius of eight disk scale lengths. Errors inthe magnitudes, typically ~0.04 mag, are dominated by uncertainties inthe sky subtraction and disk-fitting procedures. Comparison is made withthe similar imaging database of Mathewson, Ford, & Buchhorn, both aspresented originally by those authors and after reanalyzing theirdigital reduction files using identical disk-fitting procedures. Directcomparison is made of profile details for 292 galaxies observed incommon. Although some differences occur, good agreement is found,proving that the two data sets can be used in combination with onlyminor accommodation of those differences. The compilation of opticalproperties presented here is optimized for use in applications of theTully-Fisher relation as a secondary distance indicator in studies ofthe local peculiar velocity field.
| Dynamics of cD Clusters of Galaxies. III. Redshift Data for 11 Abell Clusters
We present the final observational data for a spectroscopic study of asample of cD galaxy clusters. The goal of this program has been to studythe dynamics of the clusters, with emphasis on determining the natureand frequency of peculiar-velocity cD galaxies. In this paper we presentredshifts for 762 galaxies in the fields of the rich Abell clustersA779, A1691, A1749, A1767, A1837, A1927, A2067, A2079, A2089, A2199, andA2666. We also present preliminary dynamical properties for theseclusters using our measured redshifts.
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| ROSAT HRI observations of six early-type galaxies
High-resolution ROSAT HRI soft X-ray observations of four E/S0 galaxieswere conducted by us. The data show no signs of Seyfert activity in theX-ray regime. The central emission peaks of the four galaxies, NGC533,NGC2832, NGC4104 and NGC6329, are associated with their cooling flows.The half intensity radii of the cooling flows range from 0.8 to 3.5 kpc.We find a trend (based up to now on only five objects) of the radiopower of the cores in E/S0 galaxies to increase with the size and theaccretion rates of their cooling flows. In one galaxy, NGC4921, nocentrally peaked extended gaseous envelope was found, which is mostlikely due to the fact that it is not an E/S0 galaxy, but an early-typespiral. NGC2885, the sixth galaxy in our initial sample, shows signs ofX-ray emission from an AGN. It has also been classified as a Sy-1 AGN byBade et al. (1995). However, optical imaging suggests that this galaxyis probably not an E or S0 type system either, but rather an early-typespiral galaxy. Thus, in the context of accretion rate vs. galaxy typemodels of low-luminosity AGNs, the presence of an X-ray luminous Sy-1nucleus in NGC2885 is no surprise.
| 21 CM H1 Line Spectra of Galaxies in Nearby Clusters
A compilation of HI line fluxes, systemic velocities and line widths ispresented for \Ndet detected galaxies, mostly in the vicinities of 30nearby rich clusters out to a redshift of z ~ .04, specifically for usein applications of the Tully-Fisher distance method. New 21 cm HI lineprofiles have been obtained for ~ 500 galaxies in 27 Abell clustersvisible from Arecibo. Upper limits are also presented for \Nnod galaxiesfor which HI emission was not detected. In order to provide ahomogeneous line width determination optimized for Tully-Fisher studies,these new data are supplemented by the reanalysis of previouslypublished spectra obtained both at Arecibo and Green Bank that areavailable in a digital archive. Corrections for instrumental broadening,smoothing, signal-to-noise and profile shape are applied, and anestimate of the error on the width is given. When corrected forturbulent broadening and viewing angle, the corrected velocity widthspresented here will provide the appropriate line width parameter neededto derive distances via the Tully-Fisher relation.
| Properties of nearby clusters of galaxies. III. A 76, A 157, A 407, A 505, A 671, A 779, A 1700, A 2028, A 2040, A 2052 A 2063, A 2065, A 2593, A 2657, A 2670
We present F band photometry, from digitized 48-inch Palomar plates, of2818 galaxies brighter than m_3+3 in 15 Abell clusters. For each galaxy,absolute coordinates, magnitude, size, ellipticity and orientation aregiven. For each cluster we provide finding charts and contour maps ofthe galaxy surface density. The absolute coordinates of the galaxies ofother 8 clusters presented in the first paper of this series are alsoincluded.
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| The CfA Redshift Survey: Data for the NGP +36 Zone
We have assembled redshifts for a complete sample of 719 galaxies withm_zw_ <= 15.5 in the declination range 32.5^deg^ <= δ <=38.5^deg^ and right ascension range 8^h^ <= α <= 17^h^. Wehave determined morphological types for all galaxies in the magnitudelimited sample by direct inspection of the POSS-O plates. 576 of theredshifts are measurements from Mount Hopkins, and 405 are newredshifts. We also include new redshifts for 77 fainter galaxies in thesame strip.
| The velocity field of clusters of galaxies within 100 megaparsecs. II - Northern clusters
Distances and peculiar velocities for galaxies in eight clusters andgroups have been determined by means of the near-infrared Tully-Fisherrelation. With the possible exception of a group halfway between us andthe Hercules Cluster, we observe peculiar velocities of the same orderas the measuring errors of about 400 km/s. The present sample is drawnfrom the northern Galactic hemisphere and delineates a quiet region inthe Hubble flow. This contrasts with the large-scale flows seen in theHydra-Centaurus and Perseus-Pisces regions. We compare the observedpeculiar velocities with predictions based upon the gravity fieldinferred from the IRAS redshift survey. The differences between theobserved and predicted peculiar motions are generally small, except neardense structures, where the observed motions exceed the predictions bysignificant amounts. Kinematic models of the velocity field are alsocompared with the data. We cannot distinguish between parameterizedmodels with a great attractor or models with a bulk flow.
| Magnetism in the local Virgo-centered supercluster of galaxies (radius about 24 Mpc)
The warp excess in nearby spiral galaxies is investigated as well as itsearlier interpretation in terms of a magnetic field on a distance scaleout to a redshift of 0.3 (out to 1200 Mpc, for H(0) = 75 km/s Mpc). Itis shown from a study of the rotation measure data as a function ofredshift that such a large scale is unwarranted, and that any warpexcess may be rather local and spurious with a small S/N of 1.7. It isinferred that a local intergalactic magnetic field must be less than 50microG. To facilitate studies on the local supercluster of galaxies, aVirgo-center supercluster coordinate system is proposed, where longitude0 deg goes through the center of the Virgo cluster of galaxies at thecenter of the local supercluster.
| Magnetic fields in the Milky Way neighbourhood as deduced from WARPS in spiral galaxies
It is shown that warps of spiral galaxies are not randomly oriented inthe Milky Way neighborhood. By adopting a previous model, in which warpsare produced by intergalactic magnetic fields, and considering allNorthern Hemisphere warped edge-on NGC spiral galaxies, an analysis ofthe intergalactic magnetic field in the 100 Mpc neighborhood of theGalaxy is carried out. At the 100 Mpc scale the magnetic field is stillrather homogeneous, having a direction given by (alpha = 289 deg, delta= 8 deg), but a characteristic scale of about 25 Mpc is found, insidewhich the dispersion is very low. The region containing the VirgoCluster has a direction of the magnetic field different from thedirection found in adjacent regions.
| Frequency of Warped Spiral Galaxies at Visible Wavelengths
A list of optical warped spiral galaxies with basic position andgeometrical parameters is presented. From this list an attempt todetermine the frequency of warped discs has been made. All northern NGChigh inclined spirals were examined in the Palomar Observatory SkySurvey. Warps were observed in about half of these galaxies, whichindicate that nearly all spiral galaxies are warped.
| The kinematics of Abell clusters
Velocity histograms, galaxy positions, and velocity dispersions arepresented for 69 nearby Abell clusters. The shape of the cumulativedistribution for R = 1 or more clusters does not match the predictionsof standard CDM models for any biasing parameter b. The only modelsconsistent with the median and maximum dispersions of the sample arethose of b about 1.6-2.0 and in which clusters are identified in threedimensions. Velocity dispersions of high-redshift systems appearfundamentally different from those of nearby clusters. The mediandispersion of the subset of 25 cD clusters is similar to that of non-cDsystems. A substantial fraction of the cD galaxies have velocitiessignificantly different from the mean of their parent clusters. Three ofthese have 4 s or more measured redshifts. These last two resultssuggest that the formation of cD galaxies is dominated by the local,rather than global, cluster environment.
| Statistical study of optical galactic WARPS
A list of northern spiral NGC galaxies, with inclinations higher than 74deg, and exhibiting warps have been obtained from the Palomar Sky Survey(POSS). Approximate position angles and 'warping angles' are given foreach galaxy, in order to have a first reference for statistical studiesof the warping phenomenon. When it is taken into account that warps arenot always observed (depending on their relative position with respectto the observer) the resulting frequency of warps was so high that atentative conclusion is that all disks are warped.
| The morphology of multiple-nucleus brightest cluster galaxies
The morphology of high SNR CCD images of 16 multiple-nucleus brightestcluster galaxies is studied using an algorithm that models images of thesystems as the line-of-sight superposition of normal ellipticalgalaxies. The algorithm is applied initially to the classicmultiple-nucleus cD galaxy in A2199. Evidence is found suggestive ofdeep interpenetrating high-speed encounters by its secondaries. Theinteractions effects studied include noncentric isophotes, brightnessprofile effects, excess light around primary galaxies, and dynamicalfriction wakes. The results show that in many cases multiple systems areinteracting systems.
| Photométrie des Galaxies
Not Available
| Correlation between the radio power and the X-ray luminosity for rich clusters of galaxies
On the basis of data from a nearly complete sample of 140 Abell clustersof galaxies with z in the range of 0.02-0.075, the correlation betweenthe clusters' radio and X-ray luminosities is studied. Only a weakindirect correlation is found between these parameters. Consideration isgiven to hypotheses pertaining to the interaction between radio galaxiesand the intergalactic gas of clusters.
| Relative velocities of multiple-nucleus galaxies
Relative velocities of the components in 17 multiple-nucleus galaxieshave been measured. Combining these results with previously publisheddata, it was found that the distribution of velocity differences isconsistent with random sampling from a gaussian with a disperison ofabout 800 km/s, typical of that found in rich clusters. These resultsare compatible with Merritt's recent (1984) suggestion that the majorityof the secondary components in multiple-nucleus systems are not bound tothe central galaxy and will not merge on a dynamical time-scale.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
