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First Results from SAPAC: Toward a Three-dimensional Picture of the Fornax Cluster Core
A sophisticated surface brightness fluctuation (SBF) analysis packagehas been developed, designed to measure distances of early-type galaxiesby means of SBFs of unresolved stars. This suite of programs, calledSAPAC, is made readily available to the astronomical community forextensive testing, with the long-term goal of providing the necessarytools for systematic distance surveys of early-type galaxies usingmodern optical/near-IR telescopes equipped with wide-field cameras. Wediscuss the technical and scientific concepts of SAPAC and demonstrateits capabilities by analyzing deep B- and R-band CCD images of 10 dwarfelliptical galaxy candidates in the Fornax Cluster obtained with FORS1at the Very Large Telescope. All candidates are confirmed as clustermembers. We then turn our attention to the innermost region of theFornax Cluster. A total of 29 early-type galaxies closer than threecluster core radii (2deg) to the central galaxy NGC 1399 haveradial velocities and SBF distances. Their Hubble diagram exhibits apronounced S-shaped infall pattern, suggesting that Fornax is still inthe process of formation during the present epoch through a generalcollapse and possible accretion of distinct groups of galaxies. Fromfitting a model we estimate the cluster mass within 720 kpc projecteddistance of NGC 1399 to be 2.3+/-0.3×1014Msolar. The associated collapse time istcoll=2.9+1.6-0.9 Gyr. After cleansing our galaxy sample of afew kinematical outliers, the true distance of the Fornax Cluster coreis determined at 20.13+/-0.40 Mpc [(m-M)0=31.51+/-0.04 mag].Applying a bootstrap resampling technique on the distance distributionwith individual distance errors taken into account further reveals asmall intrinsic cluster depth of σint=0.74+0.52-0.74Mpc, in best agreement with the cluster's linear extension in the sky:σR.A.=σdecl.~0.5 Mpc. We conclude thatthe early-type galaxy population in the Fornax Cluster must be spatiallywell constrained, with no evidence of elongation along the line ofsight, in contrast to the Virgo Cluster. Moreover, we find marginalevidence for substructure, a result that is consistent with the youngevolutionary state of the cluster and the overall galaxy infall.Combining the kinematically defined cluster distance with the meancosmological velocity for the central cluster galaxy sample yields aHubble constant of H0=63+/-5 km s-1Mpc-1.Based on observations collected at the ESO Very Large Telescope, underprogram ESO 68.A-0176.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| The old globular cluster system of the dIrr galaxy NGC 1427A in the Fornax cluster
We present a study of the old globular cluster (GC) population of thedwarf irregular galaxy NGC 1427A using multi-wavelength VLT observationsin U,B,V,I, Hα and J bands under excellent observing conditions.We applied color and size selection criteria to select old GC candidatesand made use of archival ACS images taken with the Hubble SpaceTelescope to reject contaminating background sources and blended objectsfrom the GC candidates' list. The Hα observations were used tocheck for contamination due to compact, highly reddened young starclusters whose colors and sizes could mimic those of old GCs. Afteraccounting for contamination we obtain a total number of 38±8 GCcandidates with colors consistent with an old (~10 Gyr) and metal-poor(Z<0.4× Zȯ) population as judged by simplestellar population models. Our contamination analysis indicates that thedensity distribution of GCs in the outskirts of the Fornax central cDgalaxy NGC 1399 may not be spherically symmetric. Wederive a present-day specific frequency SN of 1.6 for NGC1427A, a value significantly larger than what is observed in the LocalGroup dwarf irregular galaxies and comparable with the values found forthe same galaxy types in the Virgo and Fornax clusters. Assuming auniversal globular cluster luminosity function turnover magnitude, wederive a distance modulus to NGC 1427A of 31.01±0.21 mag whichplaces it 3.2±2.5 (statistic)±1.6 (systematic) Mpc infront of the Fornax central cD galaxy NGC 1399. The implications of thisresult for the relationship between NGC 1427A and the clusterenvironment are briefly discussed.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| NGC 1427A - the beginning of the end: a lonely dwarf irregular entering the dense core of the Fornax cluster.
Not Available
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample
We investigate the number of physical companion galaxies for a sample ofrelatively isolated elliptical galaxies. The NASA/IPAC ExtragalacticDatabase (NED) has been used to reinvestigate the incidence of satellitegalaxies for a sample of 34 elliptical galaxies, first investigated byBothun & Sullivan using a visual inspection of Palomar Sky Surveyprints out to a projected search radius of 75 kpc. We have repeatedtheir original investigation using data cataloged in NED. Nine of theseelliptical galaxies appear to be members of galaxy clusters; theremaining sample of 25 galaxies reveals an average of +1.0+/-0.5apparent companions per galaxy within a projected search radius of 75kpc, in excess of two equal-area comparison regions displaced by 150-300kpc. This is significantly larger than the +0.12+/-0.42companions/galaxy found by Bothun & Sullivan for the identicalsample. Making use of published radial velocities, mostly availablesince the completion of the Bothun-Sullivan study, identifies thephysical companions and gives a somewhat lower estimate of +0.4companions per elliptical galaxy. This is still 3 times larger than theoriginal statistical study, but given the incomplete and heterogeneousnature of the survey redshifts in NED, it still yields a firm lowerlimit on the number (and identity) of physical companions. An expansionof the search radius out to 300 kpc, again restricted to sampling onlythose objects with known redshifts in NED, gives another lower limit of4.5 physical companions per galaxy. (Excluding five elliptical galaxiesin the Fornax Cluster, this average drops to 3.5 companions perelliptical.) These physical companions are individually identified andlisted, and the ensemble-averaged radial density distribution of theseassociated galaxies is presented. For the ensemble, the radial densitydistribution is found to have a falloff consistent withρ~R-0.5 out to approximately 150 kpc. For non-FornaxCluster companions the falloff continues out to the 300 kpc limit of thesurvey. The velocity dispersion of these companions is found to reach amaximum of 350 km s-1 at around 120 kpc, after which theyfall at a rate consistent with Keplerian falloff. This falloff may thenindicate the detection of a cut-off in the mass-density distribution inthe elliptical galaxies' dark matter halo at ~100 kpc.
| The large-scale distribution of neutral hydrogen in the Fornax region
Using data from the HI Parkes All Sky Survey (HIPASS), we have searchedfor neutral hydrogen in galaxies in a region ~25 × 25deg2 centred on NGC 1399, the nominal centre of the Fornaxcluster. Within a velocity search range of 300-3700 km s-1and to a 3σ lower flux limit of ~40 mJy, 110 galaxies with HIemission were detected, one of which is previously uncatalogued. None ofthe detections has early-type morphology. Previously unknown velocitiesfor 14 galaxies have been determined, with a further four velocitymeasurements being significantly dissimilar to published values.Identification of an optical counterpart is relatively unambiguous formore than ~90 per cent of our HI galaxies. The galaxies appear to beembedded in a sheet at the cluster velocity which extends for more than30° across the search area. At the nominal cluster distance of ~20Mpc, this corresponds to an elongated structure more than 10 Mpc inextent. A velocity gradient across the structure is detected, withradial velocities increasing by ~500 km s-1 from south-eastto north-west. The clustering of galaxies evident in optical surveys isonly weakly suggested in the spatial distribution of our HI detections.Of 62 HI detections within a 10° projected radius of the clustercentre, only two are within the core region (projected radius<1°) and less than 30 per cent are within 3.5°, suggesting aconsiderable deficit of HI-rich galaxies in the centre of the cluster.However, relative to the field, there is a 3(+/-1)-fold excess ofHI-rich galaxies in the outer parts of the cluster where galaxies may beinfalling towards the cluster for the first time.
| The Fornax Cluster Spectroscopic Survey: a sample of confirmed cluster dwarfs
The Fornax Cluster Spectroscopic Survey (FCSS) project utilizes theTwo-degree Field (2dF) multi-object spectrograph on the Anglo-AustralianTelescope (AAT). Its aim is to obtain spectra for a complete sample ofall 14000 objects with 16.5 <=bj<= 19.7 irrespective oftheir morphology in a 12 deg2 area centred on the Fornaxcluster. A sample of 24 Fornax cluster members has been identified fromthe first 2dF field (3.1 deg2 in area) to be completed. Thisis the first complete sample of cluster objects of known distance withwell-defined selection limits. Nineteen of the galaxies (with -15.8 ~ 3 arcsec (~=300 pc). This apparent minimum scale sizeimplies an equivalent minimum luminosity for a dwarf of a given surfacebrightness. This produces a limit on their distribution in themagnitude-surface brightness plane, such that we do not observe dEs withhigh surface brightnesses but faint absolute magnitudes. Above thisobserved minimum scale size of 3 arcsec, the dEs and dS0s fill the wholearea of the magnitude-surface brightness plane sampled by our selectionlimits. The observed correlation between magnitude and surfacebrightness noted by several recent studies of brighter galaxies is notseen with our fainter cluster sample. A comparison of our results withthe Fornax Cluster Catalog (FCC) of Ferguson illustrates that attemptsto determine cluster membership solely on the basis of observedmorphology can produce significant errors. The FCC identified 17 of the24 FCSS sample (i.e. 71 per cent) as being `cluster' members, inparticular missing all five of the UCDs. The FCC also suffers fromsignificant contamination: within the FCSS's field and selection limits,23 per cent of those objects described as cluster members by the FCC areshown by the FCSS to be background objects.
| A Catalog of Candidate Intermediate-Luminosity X-Ray Objects
ROSAT, and now Chandra, X-ray images allow studies of extranuclear X-raypoint sources in galaxies other than our own. X-ray observations ofnormal galaxies with ROSAT and Chandra have revealed that off-nuclear,compact, intermediate-luminosity(LX[2-10keV]>=1039.0 ergs s-1) X-rayobjects (IXOs, a.k.a. ULXs [ultraluminous X-ray sources]) are quitecommon. Here we present a catalog and finding charts for 87 IXOs in 54galaxies, derived from all of the ROSAT HRI imaging data for galaxieswith cz<=5000 km s-1 from the Third Reference Catalog ofBright Galaxies. We have defined the cutoff LX for IXOs sothat it is well above the Eddington luminosity of a 1.4Msolar black hole (1038.3 ergs s-1), soas not to confuse IXOs with ``normal'' black hole X-ray binaries. Thiscatalog is intended to provide a baseline for follow-up work withChandra and XMM-Newton, and with space- and ground-based survey work atwavelengths other than X-ray. We demonstrate that elliptical galaxieswith IXOs have a larger number of IXOs per galaxy than nonellipticalgalaxies with IXOs and note that they are not likely to be merelyhigh-mass X-ray binaries with beamed X-ray emission, as may be the casefor IXOs in starburst galaxies. Approximately half of the IXOs withmultiple observations show X-ray variability, and many (19) of the IXOshave faint optical counterparts in DSS optical B-band images. Follow-upobservations of these objects should be helpful in identifying theirnature.
| Multi stage three-dimensional sweeping and annealing of disc galaxies in clusters
We present new three-dimensional, hydrodynamic simulations of the rampressure stripping of disc galaxies via interaction with a hotintracluster medium (ICM). The simulations were carried with thesmoothed-particle hydrodynamics, adaptive mesh `hydra' code(SPH-AP3M), with model galaxies consisting of gas and stellardisc components and dark haloes. The simulations also include radiativecooling, which is important for keeping the warm, diffuse gas ofmoderate density from being unrealistically heated by the ICM. Weexamine the role that wind velocity, density and galaxy tilt play in gasstripping. We include cases with lower ram pressures than other recentstudies. In accord with previous studies, we find that low columndensity gas is promptly removed from the outer disc. However, we alsofind that not all of the gas stripped from the disc escapes immediatelyfrom the halo, some of material can linger for times of order108yr. We use a simple analytic model to demonstrate that gaselements in the ICM wind feel an effective potential with a minimumdisplaced downstream from the halo centre. The onset of the ICM wind hasa profound effect on the disc gas that is not immediately stripped. Thisremnant disc is displaced relative to the halo centre and compressed.This can trigger gravitational instability and the formation of numerousflocculent spirals. These waves transport angular momentum outward,resulting in further compression of the inner disc and the formation ofa prominent gas ring. This `annealing' process makes the inner disc,which contains much of the total gas mass, resistant to furtherstripping, but presumably susceptible to global starbursts. Spirals inthe outer disc stretch, shear and are eventually stripped on time-scalesof a few times 108yr, after which time, mass and angularmomentum loss effectly cease. For inclined galaxies, these effects areconsiderably modified over the same time-scale. The amount of mass lossis reduced. In addition, we find that a higher galaxy tilt couples thewind and the rotating disc, and produces a higher degree of angularmomentum removal. Temperature and line-of-sight velocity maps fromseveral of the simulations are presented for comparison withobservation. When the mass loss and annealing processes go tocompletion, we find that the total amount of mass lost from a fixedtarget galaxy is well-fitted by a simple power-law function of adimensionless parameter that combines the ram pressure and internalproperties of the galaxy. Ramifications for the cluster galaxy evolutionare discussed.
| The neutral hydrogen content of Fornax cluster galaxies
We present a new set of deep H I observations of member galaxies of theFornax cluster. We detected 35 cluster galaxies in HI. The resulting sample, the most comprehensive to date, is used toinvestigate the distribution of neutral hydrogen in the clustergalaxies. We compare the H I content of the detected cluster galaxieswith that of field galaxies by measuring H I mass-to-light ratios andthe H I deficiency parameter of Solanes et al. (\cite{Sol96}). The meanH I mass-to-light ratio of the cluster galaxies is 0.68+/- 0.15,significantly lower than for a sample of H I-selected field galaxies(1.15+/- 0.10), although not as low as in the Virgocluster (0.45+/- 0.03). In addition, the H I content of twocluster galaxies (NGC 1316C and NGC1326B) appears to have been affected by interactions. The meanH I deficiency for the cluster is 0.38+/-0.09 (for galaxy types T=1-6),significantly greater than for the field sample (0.05+/-0.03). Boththese tests show that Fornax cluster galaxies are HI-deficient compared to field galaxies. The kinematics of the clustergalaxies suggests that the H I deficiency may be caused by ram-pressurestripping of galaxies on orbits that pass close to the cluster core. Wealso derive the most complete B-band Tully - Fisher relation of inclinedspiral galaxies in Fornax. A subcluster in theSouth-West of the main cluster contributes considerably to the scatter.The scatter for galaxies in the main cluster alone is 0.50 mag, which isslightly larger than the intrinsic scatter of 0.4 mag. We use the Tully- Fisher relation to derive a distance modulus ofFornax relative to the Virgocluster of -0.38+/- 0.14 mag. The galaxies in the subclusterare (1.0+/-0.5) mag brighter than the galaxies of the main cluster,indicating that they are situated in the foreground. With their meanvelocity 95 km s-1 higher than that of the main cluster weconclude that the subcluster is falling into the main Fornaxcluster.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| Distances to Galaxies from the Correlation between Luminosities and Line Widths. III. Cluster Template and Global Measurement of H0
The correlation between the luminosities and rotation velocities ofgalaxies can be used to estimate distances to late-type galaxies. It isan appropriate moment to reevaluate this method given the great deal ofnew information available. The major improvements described hereinclude: (1) the template relations can now be defined by large,complete samples, (2) the samples are drawn from a wide range ofenvironments, (3) the relations are defined by photometric informationat the B, R, I, and K' bands, (4) the multiband information clarifiesproblems associated with internal reddening, (5) the template zeropoints are defined by 24 galaxies with accurately known distances, and(6) the relations are applied to 12 clusters scattered across the skyand out to velocities of 8000 km s-1. The biggest change fromearlier calibrations are associated with point 5. Roughly a 15% increasein the distance scale has come about with the fivefold increase in thenumber of zero-point calibrators. The overall increase in the distancescale from the luminosity-line width methodology is about 10% afterconsideration of all factors. Modulo an assumed distance to the LargeMagellanic Cloud of 50 kpc and no metallicity corrections to the Cepheidcalibration, the resulting value of the Hubble constant isH0=77+/-8 km s-1 Mpc-1, where the erroris the 95% probable statistical error. Cumulative systematic errorsinternal to this analysis should not exceed 10%. Uncertainties in thedistance scale ladder external to this analysis are estimated at ~10%.If the Cepheid calibration is shifted from the LMC to NGC 4258 with adistance established by observations of circumnuclear masers, thenH0 is larger by 12%.
| The Ionized Gas Kinematics of the LMC-Type Galaxy NGC 1427A in the Fornax Cluster
NGC 1427A is a LMC-like irregular galaxy in the Fornax Cluster with anextended pattern of strong star formation around one of its edges, whichis probably due to some kind of interaction with the clusterenvironment. We present Hα velocities within NGC 1427A, obtainedthrough long-slit spectroscopy at seven different positions, chosen tofall on the brightest H II regions of the galaxy. Because of itslocation very near the center of the cluster this object is an excellentcandidate to study the effects that the cluster environment has ongas-rich galaxies embedded in it. The rotation of NGC 1427A is modeledin two different ways. The global ionized gas kinematics is reasonablywell described by solid-body rotation, although on small scales it showsa chaotic behavior. In this simple model, the collision with a smallermember of the cluster as being responsible for the peculiar morphologyof NGC 1427A is very unlikely, since the only candidate intruder fallssmoothly into the general velocity pattern of the main galaxy. In a moreelaborate model, for which we obtain a better solution, this object doesnot lie in the same plane of NGC 1427A, in which case we identify it asa satellite bound to the galaxy. These results are discussed in thecontext of a normal irregular versus one interacting with some externalagent. Based on several arguments and quantitative estimates, we arguethat the passage through the hot intracluster gas of the Fornax Clusteris a very likely scenario to explain the morphological properties of NGC1427A, although our kinematical data are not enough to support it morefirmly nor rule out the possibility of a normal irregular. Based on datacollected at Las Campanas Observatory, Chile, run by the CarnegieInstitution of Washington.
| Morphology of star formation regions in irregular galaxies
The location of HII regions, which indicates the locus of present starformation in galaxies, is analysed for a large collection of 110irregular galaxies (Irr) imaged in Hα and nearby continuum. Theanalysis is primarily by visual inspection, although a two-dimensionalquantitative measure is also employed. The two different analyses yieldessentially identical results. HII regions appear preferentially at theedges of the light distribution, predominantly on one side of thegalaxy, contrary to what is expected from stochastic self-propagatingstar formation scenarios. This peculiar distribution of star-formingregions cannot be explained by a scenario of star formation triggered byan interaction with extragalactic gas, or by a strong one-armed spiralpattern.
| The Southern Sky Redshift Survey
We report redshifts, magnitudes, and morphological classifications for5369 galaxies with m_B <= 15.5 and for 57 galaxies fainter than thislimit, in two regions covering a total of 1.70 sr in the southerncelestial hemisphere. The galaxy catalog is drawn primarily from thelist of nonstellar objects identified in the Hubble Space TelescopeGuide Star Catalog (GSC). The galaxies have positions accurate to ~1"and magnitudes with an rms scatter of ~0.3 mag. We compute magnitudes(m_SSRS2) from the relation between instrumental GSC magnitudes and thephotometry by Lauberts & Valentijn. From a comparison with CCDphotometry, we find that our system is homogeneous across the sky andcorresponds to magnitudes measured at the isophotal level ~26 magarcsec^-2. The precision of the radial velocities is ~40 km s^-1, andthe redshift survey is more than 99% complete to the m_SSRS2 = 15.5 maglimit. This sample is in the direction opposite that of the CfA2; incombination the two surveys provide an important database for studies ofthe properties of galaxies and their large-scale distribution in thenearby universe. Based on observations obtained at Cerro TololoInter-American Observatory, National Optical Astronomy Observatories,operated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation;Complejo Astronomico El Leoncito, operated under agreement between theConsejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; the European Southern Observatory, LaSilla, Chile, partially under the bilateral ESO-ObservatórioNacional agreement; Fred Lawrence Whipple Observatory;Laboratório Nacional de Astrofísica, Brazil; and the SouthAfrican Astronomical Observatory.
| X-Ray Emission from the Fornax Cluster
We have analyzed the ROSAT PSPC observations of the central region ofthe Fornax cluster, a relatively poor group of galaxies at a distance ofabout 24 Mpc. The brightest X-ray and optical galaxy in the group is NGC1399, an E1 galaxy located near the center of the Fornax cluster. Wecharacterize the hot gas around the galaxy, derived from a 2' to 18'annulus around NGC 1399, as having a mean temperature of 1.30 +/- 0.05keV and a heavy element abundance of 0.6 +/- 0.1 with respect to solarabundance (Fe/H = 4.68 x 10-5 by number). Spatially resolved spectraldata provide both gas temperature and gas abundance profiles extendingto 125 kpc (18') from the galaxy. The temperature distribution, combinedwith the X-ray surface brightness profile, yields an accuratedetermination of the gravitating mass within 125 kpc, which falls in therange (4.3--8.1) x 1012 Mȯ (95% confidence range, includingsystematic uncertainties). If we include the extended optical haloaround NGC 1399, the mass-to-light ratio increases with radius from 33+/- 8 Mȯ/Lȯ at 18 kpc to 70 +/- 22 Mȯ/Lȯ at 110 kpc.We compare the heavy element abundance distribution measured around NGC1399 with that measured around the Virgo galaxy NGC 4472, as well as tomodels for hot coronae. We find that the abundance distribution is ingood agreement with that previously measured for NGC 4472 by Forman etal. in 1993. For both galaxies, the observed abundance profiles requireboth a weak evolution of the type Ia supernova rate with time and apresent epoch rate which agrees with that of Cappellaro et al. Wecompare mass measurements in NGC 1399 to those for M87. The similarityof the optical masses in these systems and their differences in gasmasses and gravitating masses lead us to suggest that the opticalgalaxies formed at an early stage when the central potentials of thesetwo systems were similar. Subsequent infall of gas and dark matter intothe larger, deeper Virgo potential resulted in the greater mass of theVirgo cluster compared to Fornax. We also report on X-ray properties ofthirteen other Fornax galaxies. Eight of these were detected in ROSATimages with luminosities in the 0.2 to 2 keV energy band from 1 x 1039to 1.6 x 1041 ergs s-1. Five galaxies were sufficiently bright to permitspectral analyses and all but one (NGC 1380) had spectra consistent withthermal emission. Two (NGC 1404 and NGC 1387) of the four galaxies withwell-constrained spectral parameters have hot coronae withcharacteristic gas temperatures of about 0.5 keV and iron abundancesless than that found around NGC 1399 and other bright ellipticals. Tomaintain these hot coronae, the absolute magnitudes of these galaxiesmust be brighter than -19. Thus the distance to Fornax must be at least18 Mpc, and, if there are no large peculiar velocities, the Hubbleconstant should be less than 75 km s-1 Mpc-1. Since these galaxies areall members of Fornax, distance uncertainties do not affect therelationship between their optical magnitude and X-ray luminosity.Analysis of the Fornax galaxies supports the contention that the scatterin the X-ray and optical relationship is intrinsic and does not arisesolely from distance uncertainties. For the elliptical galaxy NGC 1404,the X-ray images show that the hot corona is distorted and likely isbeing stripped, indicating infall of the galaxy toward NGC 1399 and thecluster center.
| The Peculiar Morphology of the Irregular Galaxy NGC 1427A
Multicolor surface photometry and fragmentary low-resolutionspectroscopy of the irregular galaxy NGC 1427A are presented. Thisgalaxy, a member of the Fornax Cluster, has not been studied in detailpreviously. It shows a very distinctive morphology: a low surfacebrightness stellar background with several bright knots forming adistorted ring-like structure. A faint plume and several diffusedfilaments connect the northern part of the main body of the galaxy withan object having elliptical isophotes. The overall aspect of NGC 1427A,then, resembles that of known interacting galaxies. The very blue colorsof the bright knots show that they are composed by young stars, andseveral of them also show emission lines. The northern object, which hasitself a couple of these blue knots, is bluer than the background of themain body of the galaxy. This fact indicates that the star formationhistories in both objects have been different. Morphologically, thenorthern object is similar to the numerous dwarf elliptical or irregulargalaxies that populate the Fornax Cluster. The possibility of aninteraction being the cause of the particular structure of this galaxyis discussed.
| The I band Tully-Fisher relation for cluster galaxies: data presentation.
Observational parameters which can be used for redshift-independentdistance determination using the Tully-Fisher (TF) technique are givenfor \ntot spiral galaxies in the fields of 24 clusters or groups. I bandphotometry for the full sample was either obtained by us or compiledfrom published literature. Rotational velocities are derived either from21 cm spectra or optical emission line long-slit spectra, and convertedto a homogeneous scale. In addition to presenting the data, a discussionof the various sources of error on TF parameters is introduced, and thecriteria for the assignment of membership to each cluster are given.
| NGC 1427A - an LMC type galaxy in the Fornax cluster.
We have discovered that the Fornax irregular galaxy NGC 1427A is in verymany respects a twin of the Large Magellanic Cloud. Based on B, V, I,and Hα images, we find the following. The light of the galaxy isdominated by high surface brightness regions in the south-west that aresuperimposed by a half-ring of OB associations and Hii regionsindicating recent star formation. The colors of the main stellar bodyare (V-I)=0.8mag and (B-V)=0.4mag, comparable to the LMC colors. A lowsurface brightness cloud north of the main body as well as a LSB tail inthe west have colors (V-I =~0.2 mag) that are more typical for bluecompact dwarf galaxies. We identified a system of cluster candidateswith mean ages <=2Gyr (assuming a LMC metallicity) comprising therichest cluster system in an irregular galaxy observed up to now outsidethe Local Group. In X-ray wavelengths NGC 1427A appears with arelatively soft and complex spectrum.
| A New I-Band Tully-Fisher Relation for the Fornax Cluster: Implication for the Fornax Distance and Local Supercluster Velocity Field
The Fornax Cluster represents an important step in the extragalacticdistance scale. Here we present a new I-band luminosity-H I velocitywidth (I-band Tully-Fisher) study of the cluster using an enlargedsampled of spiral galaxies in Fornax. I-band CCD photometry and 21 cmparameters are measured for 23 members of Fornax and compared with datafor the Virgo Cluster. We obtain an accurate distance modulus of Fornaxrelative to Virgo of -0.06 +/- 0.15 mag. The low scatter of Fornaxgalaxies around the Tully-Fisher relation will make the cluster an idealcalibrator once a direct measurement of its distance is obtained.Furthermore, the H I content of the galaxies does not seem affected bythe cluster environment. Here we use two different absolute calibrationmethods which yield an absolute distance to Fornax of 15.4 +/- 2.3 Mpc(absolute distance modulus of 30.94 +/- 0.33 mag). A simple model yieldsa Local Group Virgocentric flow velocity of 224 +/- 90 km s^-1^ whichcorresponds to a Hubble constant of H_0_ = 74 +/- 11 km s^-1^ Mpc^-1^from the Fornax data.
| General study of group membership. II - Determination of nearby groups
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
| Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members
This paper gives a catalog of the groups and associations obtained bymeans of a revised hierarchical algorithm applied to a sample of 4143galaxies with diameters larger than 100 arcsec and redshifts smallerthan 6000 km/s. The 264 groups of galaxies obtained in this way (andwhich contain at least three sample galaxies) are listed, with the looseassociations surrounding them and the individual members of eachaggregate as well; moreover, the location of every entity among 13regions corresponding roughly to superclusters is specified. Finally,1729 galaxies belong to the groups, and 466 to the associations, i.e.,the total fraction of galaxies within the various aggregates amounts to53 percent.
| Southern Sky Redshift Survey - The catalog
The catalog of radial velocities for galaxies which comprise thediameter-limited sample of the Southern Sky Redshift Survey ispresented. It consolidates the data of observations carried out at theLas Campanas Observatory, Observatorio Nacional, and South AfricanAstronomical Observatory. The criteria used for the sample selection aredescribed, as well as the observational procedures and the techniqueutilized to obtain the final radial velocities. The intercomparisonbetween radial velocity measurements from different telescopes indicatesthat the final data base is fairly homogeneous with a typical error ofabout 40 km/s. The sample is at present 90 percent complete, and themissing galaxies are predominantly objects with very low surfacebrightness for which it is very difficult to obtain optical redshifts.
| A catalog of southern groups of galaxies
A catalog of groups of galaxies identified in the southern Galactic capis presented. This catalog was constructed utilizing the group-findingalgorithm developed by Huchra and Geller (1982) to analyze galaxysamples with well-defined selection criteria and complete velocityinformation.
| Population studies in groups and clusters of galaxies. II - A catalog of galaxies in the central 3.5 deg of the Fornax Cluster
A catalog of 2678 galaxies within an area of about 40 sq deg centered onthe Fornax Cluster has been compiled based on 26 deep large-scale platesobtained with the 2.5-m Las Campanas Observatory reflector. The catalogincludes 340 likely cluster members and 2338 likely background galaxies.Radial velocities are given for 89 of the galaxies. The spatialdistributions of various types of galaxies have been modeled as the sumof a King (1962) model cluster component superimposed on a uniformbackground. Using maximum-likelihood fits to these distributions, a coreradius of 0.7 deg is found for a King model fit to the cluster,suggesting that there are few cluster members contained in the sample ofbackground galaxies.
| Latel-type galaxies - Star formation rates
A simple method for determining star formation rates (SFRs) in externalgalaxies is presented which is based on surface photometry in one coloronly. The intensity distributions of 59 late-type spiral and irregulargalaxies were divided into two components each, one containing the lightof the smoothly distributed old disk population, the other containingthe light of the clumped extreme Population I. This makes it possible todetermine the ratios of the 'present' SFRs (for stars with ages lessthan 50 million yr) to the average SFRs. Among other results, this ratiois found to be higher in asymmetric galaxies than in symmetric ones.
| Southern Galaxy Catalogue.
Not Available
| A catalogue of dwarf galaxies south of delta = -17.5 deg
A catalog of 584 suspected dwarf galaxies south of declination -17.5 degis presented. The survey was done on ESO(B) and SRC(J) plates and films.Position, major and minor axes, luminosity classes, and types(irregular, spiral, elliptical) are given.
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