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The complete census of optically selected AGNs in the Coma supercluster: the dependence of AGN activity on the local environment
Aims: To investigate the dependence of the occurrence of activegalactic nuclei (AGNs) on local galaxy density, we study the nuclearproperties of ~5000 galaxies in the Coma supercluster whose densityspans two orders of magnitude from the sparse filaments to the cores ofrich clusters. Methods: We obtained optical spectra of the nucleiof 283 galaxies using the 1.5 m Cassini telescope of Bolognaobservatory. Among these galaxies, 177 belong to the Coma superclusterand are added to the 4785 spectra available from SDSS (DR7) to fill-inthe incomplete coverage by SDSS of luminous galaxies. We perform aspectral classification of the nuclei of galaxies in this region (with acompleteness of 98% at r ≤ 17.77), classifying the nuclear spectra insix classes: three of them (SEY, sAGN, LIN) refer to AGNs and theremaining three (HII, RET, PAS) refer to different stages of starburstactivity. We perform this classification as recommended by Cid Fernandesand collaborators using the ratio of λ 6584 [NII] to Hαlines and the equivalent width of Hα (WHAN diagnostic), aftercorrecting the last quantity by 1.3 Å for underlying absorption. Results: We find that 482 (10%) of 5027 galaxies host an AGN:their frequency strongly increases with increasing luminosity of theparent galaxies, such that 32% of galaxies withLog(Li/L⊙) ≥ 10.2 harbor an AGN at theirinterior. In addition to their presence in luminous galaxies, AGNs arealso found in red galaxies with ⟨g - i⟩ ≃ 1.15 ±0.15 mag. The majority of SEY and sAGN (strong AGNs) are associated withluminous late-type (or S0a) galaxies, while LIN (weak AGNs) and RET("retired": nuclei that have experienced a starburst phase in the pastand are now ionized by their hot evolved low-mass stars), are mostlyfound among E/S0as. The number density of AGNs, HII region-like, andretired galaxies is found to anti-correlate with the local density ofgalaxies, such that their frequency drops by a factor of two near thecluster cores, while the frequency of galaxies containing passive nucleiincreases by the same amount towards the center of rich clusters. Thedependence of AGN number density on the local galaxy density is greaterthan the one implied by morphology segregation alone.Table 4 is available in electronic form at http://www.aanda.org
| The galaxy population of Abell 1367: photometric and spectroscopic data
Aims: Photometric and spectroscopic observations of the galaxypopulation of the galaxy cluster Abell 1367 have been obtained, over afield of 34' × 90', covering the cluster centre out to a radius of~2.2 Mpc. Optical broad- and narrow-band imaging was used to determinegalaxy luminosities, diameters and morphologies, and to study currentstar formation activity of a sample of cluster galaxies. Near-infraredimaging was obtained to estimate integrated stellar masses, and to aidthe determination of mean stellar ages and metallicities for the futureinvestigation of the star formation history of those galaxies. Opticalspectroscopic observations were also taken, to confirm clustermembership of galaxies in the sample through their recession velocities.Methods.U, B and R broad-band and Hα narrow-band imagingobservations were carried out using the Wide Field Camera (WFC) on the2.5 m Isaac Newton Telescope on La Palma, covering the field describedabove. J and K near-infrared imaging was obtained using the Wide FieldCamera (WFCAM) on the 3.8 m UK Infrared Telescope on Mauna Kea, coveringa somewhat smaller field of 0.75 square degrees on the cluster centre.The spectroscopic observations were carried out using a multifibrespectrograph (WYFFOS) on the 4.2 m William Herschel Telecope on LaPalma, over the same field as the optical imaging observations. Results: Our photometric data give optical and near-infrared isophotalmagnitudes for 303 galaxies in our survey regions, down to stateddiameter and B-band magnitude limits, determined within R24isophotal diameters. Our spectroscopic data of 328 objects provide 84galaxies with detections of emission and/or absorption lines. Combiningthese with published spectroscopic data gives 126 galaxies within oursample for which recession velocities are known. Of these, 72 galaxiesare confirmed as cluster members of Abell 1367, 11 of which areidentified in this study and 61 are reported in the literature. Hαequivalent widths and fluxes are presented for all cluster galaxies withdetected line emission. Conclusions: Spectroscopic andphotometric data are presented for galaxies in the nearby cluster Abell1367, as the first stage of a study of their stellar population and starformation properties.Based on observations made with the Isaac Newton Telescope and theWilliam Herschel Telescope, operated on the island of La Palma by theIsaac Newton Group in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofísica de Canarias and theUnited Kingdom Infra-Red Telescope operated on Mauna Kea, Hawaii by theJoint Astronomy Centre.Tables 8-11 are also available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/527/A101
| Probing evolutionary mechanisms in galaxy clusters: neutral atomic hydrogen in Abell1367
We present Very Large Array HI imaging data for a field in the NW of thegalaxy cluster Abell 1367 (z =0.02) in an attempt to probe the effectenvironment has on the interstellar medium of late-type spiral galaxies.Several galaxies, like CGCG 097-087, show pronounced tails andasymmetries, and seven out of 10 show significant, several kpc offsetsbetween the HI centroid and the optical. We compare our results againsta sample of optically bright, late-type galaxies (spirals) across thecentral 1.5 Mpc of the cluster taken from the Arecibo Galaxy EnvironmentSurvey. We find that these late-type spirals are predominantly found inthe northern half of the cluster, especially those that are relativelygas rich. We calculate the HI deficiency and find that the expectedglobal trend for the HI deficiency of these spirals to increase withprojected proximity to the cluster core, seen in clusters like Coma andVirgo, is not observed. We classified the spirals into four evolutionarystates, with the galaxies in each state sharing a similar degree of HIdeficiency and optical colour. The common characteristics of the spiralsin each evolutionary state suggest they have been subject to similarenvironmental processes. Many of the spirals in the most commonevolutionary state (moderate HI deficiency and blue colour) have an HIintensity maximum which is displaced relative to its opticalcounterpart. The orientation of these offsets and magnitude of their HIdeficiencies together with data from other wavelengths provideobservational evidence in support of varying degrees of ram pressurestripping and tidal interaction. In general, our results indicate thatthe HI discs of bright late-type galaxies in the central part of thecluster are subject to both gas loss and morphological disturbance as aresult of their interaction with the cluster environment. This providesfurther observational evidence of a more complex environment in Abell1367 as compared to Virgo and Coma.
| Clustering of galaxies on scales of ?10 h -1 based on the coma, bridge, and A1367 clusters
Not Available
| Star-forming galaxies in low-redshift clusters: data and integrated galaxy properties
Aims. This paper is a continuation of an ongoing study of theevolutionary processes affecting cluster galaxies. Methods: Both CCD Rband and Hα narrow-band imaging was used to determine photometricparameters (mr, r24, Hα flux, and equivalentwidth) and derive star formation rates for 227 CGCG galaxies in 8low-redshift clusters. The galaxy sample is a subset of CGCG galaxies inan objective prism survey (OPS) of cluster galaxies for Hαemission. Results: It is found that detection of emission-line galaxiesin the OPS is 85%, 70%, and 50% complete at the mean surface brightnessvalues of 1.25 × 10-19, 5.19 × 10-20,and 1.76 × 10-20 W m-2 arcsec-2,respectively, measured within the R band isophote of 24 magarcsec-2 for the galaxy. Conclusions: The CCD data, togetherwith matched data from a recent Hα galaxy survey of UGC galaxieswithin v ≤ 3000 km s-1, will be used for a comparativestudy of R band and Hα surface photometry between cluster andfield spirals.Table 2 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/486/755
| Dependence of Galaxy Structure on Rest-Frame Wavelength and Galaxy Type
We present a quantitative analysis of the morphologies for 199 nearbygalaxies as parameterized with measurements of the concentration,asymmetry, and clumpiness (CAS) parameters at wavelengths from 0.15 to0.85 μm. We find that these CAS parameters depend on both galaxy typeand the wavelength of observation. As such, we use them to obtain aquantitative measure of the ``morphological k-correction,'' i.e., thechange in appearance of a galaxy with rest-frame wavelength. Whereasearly-type galaxies (E-S0) appear about the same at all wavelengthslongward of the Balmer break, there is a mild but significantlydetermined wavelength dependence of the CAS parameters for galaxy typeslater than S0, which generally become less concentrated and moreasymmetric and clumpy toward shorter wavelengths. Also, as a mergerprogresses from premerger via major-merger to merger-remnant stages, itevolves through the CAS parameter space, becoming first lessconcentrated and more asymmetric and clumpy, and then returning towardthe ``locus'' of normal galaxies. The final merger products are, onaverage, much more concentrated than normal spiral galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated with HSTprograms 8645 and 9124.
| X-Ray Thermal Coronae of Galaxies in Hot Clusters: Ubiquity of Embedded Mini-Cooling Cores
We present a systematic investigation of X-ray thermal coronae in 157early-type and 22 late-type galaxies from a survey of 25 hot (kT>3keV), nearby (z<0.05) clusters, based on Chandra archival data. Coolgalactic coronae (kT=0.5-1.1 keV generally) have been found to becommon, >60% in LKs>2L* galaxies.These embedded coronae in hot clusters are generally smaller, lessluminous, and less massive than coronae in poor environments,demonstrating the negative effects of hot cluster environments ongalactic coronae. Nevertheless, these coronae still manage to surviveICM stripping, evaporation, rapid cooling, and powerful AGN outflows,making them a rich source of information about gas stripping,microscopic transport, and feedback processes in the clusterenvironment. Heat conduction across the boundary of the coronae has tobe suppressed by a factor of >~100, which implies that the X-ray gasin early-type galaxies is magnetized and the magnetic field plays animportant role in energy transfer. The luminous, embedded coronae, withhigh central density (0.1-0.4 cm-3), are miniversions ofgroup and cluster cooling cores. As the prevalence of coronae of massivegalaxies implies a long lifetime (>~several Gyr), there must be aheat source inside coronae to offset cooling. While we argue that AGNheating may not generally be the heat source, we conclude that SNheating can be enough as long as the kinetic energy of SNe can beefficiently dissipated. Diffuse thermal coronae have also been detectedin at least 8 of 22 late-type galaxies in our sample. The fraction ofluminous X-ray AGNs (>1041 ergs s-1) is notsmall (~5%) in our sample.
| Enhanced mergers of galaxies in low-redshift clusters
An ensemble cluster has been formed from a data set comprising acomplete magnitude-limited sample of 680 giant galaxies(M0B <~ -19) in eight low-redshift clusters,normalized by the velocity dispersions and virial radii for theearly-type cluster populations. Distinct galaxy populations have beenidentified, including an infall population. A majority (50-70 per centor greater) of the infall population are found to be in interacting ormerging systems characterized by slow gravitational encounters. Theobserved enhancement of galaxy-galaxy encounters in the infallpopulation compared to the field can be explained by gravitationalshocking. It is shown that disc galaxy mergers in the infall populationintegrated over the estimated lifetime of the cluster (~10 Gyr) canreadily account for the present cluster S0 population.
| Witnessing galaxy preprocessing in the local Universe: the case of a star-bursting group falling into Abell 1367
We present a multiwavelength analysis of a compact group of galaxiesinfalling at high speed into the dynamically young cluster Abell 1367.Peculiar morphologies and unusually high Hα emission areassociated with two giant galaxies and at least ten dwarfs/extragalacticHII regions, making this group the region with the highest density ofstar formation activity ever observed in the local clusters. MoreoverHα imaging observations reveal extraordinary complex trails ofionized gas behind the galaxies, with projected lengths exceeding 150kpc. These unique cometary trails mark the gaseous trajectory ofgalaxies, witnessing their dive into the hot cluster intergalacticmedium. Under the combined action of tidal forces among group membersand the ram-pressure by the cluster ambient medium, the group galaxieswere fragmented and the ionized gas was blown out. The properties ofthis group suggest that environmental effects within infalling groupsmay have represented a preprocessing step of the galaxy evolution duringthe high redshift cluster assembly phase.
| Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| H I observations of galaxies. II. The Coma Supercluster
High sensitivity 21-cm H i line observations with an rms noise level of~0.5 mJy were made of 35 spiral galaxies in the Coma Supercluster, usingthe refurbished Arecibo telescope, leading to detection of 25 objects.These data, combined with the measurements available in the literature,provide the set of H i data for 94% of all late-type galaxies in theComa Supercluster with an apparent photographic magnitude mp≤ 15.7 mag. We confirm that the typical scale of H i deficiencyaround the Coma cluster is 2 Mpc, i.e. one virial radius. Comparing theH i mass function (HIMF) of cluster with non-cluster members of the ComaSupercluster we detected a shortage of high H i mass galaxies amongcluster members that can be attributed to the pattern of H i deficiencyfound in rich clusters.
| UBVR and Hubble Space Telescope Mid-Ultraviolet and Near-Infrared Surface Photometry and Radial Color Gradients of Late-Type, Irregular, and Peculiar Galaxies
We introduce a data set of 142 mostly late-type spiral, irregular, andpeculiar (interacting or merging) nearby galaxies observed in UBVR atthe Vatican Advanced Technology Telescope (VATT), and we present ananalysis of their radial color gradients. We confirm that nearbyelliptical and early- to mid-type spiral galaxies show either no or onlysmall color gradients, becoming slightly bluer with radius. In contrast,we find that late-type spiral, irregular, peculiar, and merging galaxiesbecome on average redder with increasing distance from the center. Thescatter in radial color gradient trends increases toward later Hubbletype. As a preliminary analysis of a larger data set obtained with theHubble Space Telescope (HST), we also analyze the color gradients of sixnearby galaxies observed with NICMOS in the near-IR (H) and with WFPC2in the mid-UV (F300W) and red (F814W). We discuss the possibleimplications of these results on galaxy formation and compare our nearbygalaxy color gradients to those at high redshift. We present examples ofimages and UBVR radial surface brightness and color profiles, as well asof the tables of measurements; the full atlas and tables are publishedin the electronic edition only.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 8645, 9124, and 9824.
| 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.
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| Radio and Far-Infrared Emission as Tracers of Star Formation and Active Galactic Nuclei in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114galaxies in the seven nearest clusters (<100 Mpc) with prominentX-ray emission to investigate the impact of the cluster environment onthe star formation and active galactic nucleus (AGN) activity in themember galaxies. The X-ray selection criterion is adopted to focus onthe most massive and dynamically relaxed clusters. A large majority ofcluster galaxies show an excess in radio emission over that predictedfrom the radio-FIR correlation, the fraction of sources with radioexcess increases toward cluster cores, and the radial gradient in theFIR/radio flux ratio is a result of radio enhancement. Of theradio-excess sources, 70% are early-type galaxies, and the same fractionhost an AGN. The galaxy density drops by a factor of 10 from thecomposite cluster center out to 1.5 Mpc, yet galaxies show no change inFIR properties over this region and show no indication of masssegregation. We have examined in detail the physical mechanisms thatmight impact the FIR and radio emission of cluster galaxies. Whilecollisional heating of dust may be important for galaxies in clustercenters, it appears to have a negligible effect on the observed FIRemission for our sample galaxies. The correlations between radio and FIRluminosity and radius could be explained by magnetic compression fromthermal intracluster medium pressure. We also find that simple delayedharassment cannot fully account for the observed radio, FIR, and mid-IRproperties of cluster galaxies.
| Capturing a Star Formation Burst in Galaxies Infalling onto the Cluster A1367
The discovery of a striking astrophysical laboratory in the cluster ofgalaxies A1367 by Sakai and coworkers is confirmed with independentimaging and spectroscopic observations and further investigated in thepresent analysis. Two giant and 10 dwarf/H II galaxies, members of agroup, are simultaneously undergoing a burst of star formation. Redshiftmeasurements suggest that the group galaxies are in the process offalling into the cluster at very high speed. We explore two possiblemechanisms that could have triggered the short-lived stellar burst thatwe are witnessing: the first, internal to the group itself, via tidalinteractions among its members, the hypothesis favored by Sakai et al.;the second associated with the high-velocity infall of the groupgalaxies into the cluster intergalactic medium. We present evidence infavor of and against the two hypotheses.
| CAIRNS: The Cluster and Infall Region Nearby Survey. I. Redshifts and Mass Profiles
The CAIRNS (Cluster and Infall Region Nearby Survey) project is aspectroscopic survey of the infall regions surrounding eight nearby,rich, X-ray-luminous clusters of galaxies. We have collected 15,654redshifts (3471 new or remeasured) within ~5-10 h-1 Mpc ofthe centers of the clusters, making it the largest study of the infallregions of clusters. We determine cluster membership and the massprofiles of the clusters based on the phase-space distribution of thegalaxies. All of the clusters display decreasing velocity dispersionprofiles. The mass profiles are fitted well by functional forms based onnumerical simulations but exclude an isothermal sphere. Specifically,NFW and Hernquist models provide good descriptions of cluster massprofiles to their turnaround radii. Our sample shows that the predictedinfall pattern is ubiquitous in rich, X-ray-luminous clusters over alarge mass range. The caustic mass estimates are in excellent agreementwith independent X-ray estimates at small radii and with virialestimates at intermediate radii. The mean ratio of the caustic mass tothe X-ray mass is 1.03+/-0.11, and the mean ratio of the caustic mass tothe virial mass (when corrected for the surface pressure term) is0.93+/-0.07. We further demonstrate that the caustic technique providesreasonable mass estimates even in merging clusters.
| Hα surface photometry of galaxies in the Virgo cluster. IV. The current star formation in nearby clusters of galaxies
Hα +[NII] imaging observations of 369 late-type (spiral) galaxiesin the Virgo cluster and in the Coma/A1367 supercluster are analyzed,covering 3 rich nearby clusters (A1367, Coma and Virgo) and nearlyisolated galaxies in the Great-Wall. They constitute an opticallyselected sample (mp<16.0) observed with ~ 60 %completeness. These observations provide us with the current(T<107 yrs) star formation properties of galaxies that westudy as a function of the clustercentric projected distances (Theta ).The expected decrease of the star formation rate (SFR), as traced by theHα EW, with decreasing Theta is found only when galaxies brighterthan Mp ~ -19.5 are considered. Fainter objects show no orreverse trends. We also include in our analysis Near Infrared data,providing information on the old (T>109 yrs) stars. Puttogether, the young and the old stellar indicators give the ratio ofcurrently formed stars over the stars formed in the past, or``birthrate'' parameter b. For the considered galaxies we also determinethe ``global gas content'' combining HI with CO observations. We definethe ``gas deficiency'' parameter as the logarithmic difference betweenthe gas content of isolated galaxies of a given Hubble type and themeasured gas content. For the isolated objects we find that b decreaseswith increasing NIR luminosity. In other words less massive galaxies arecurrently forming stars at a higher rate than their giant counterpartswhich experienced most of their star formation activity at earliercosmological epochs. The gas-deficient objects, primarily members of theVirgo cluster, have a birthrate significantly lower than the isolatedobjects with normal gas content and of similar NIR luminosity. Thisindicates that the current star formation is regulated by the gaseouscontent of spirals. Whatever mechanism (most plausibly ram-pressurestripping) is responsible for the pattern of gas deficiency observed inspiral galaxies members of rich clusters, it also produces the observedquenching of the current star formation. A significant fraction of gas``healthy'' (i.e. with a gas deficiency parameter less than 0.4) andcurrently star forming galaxies is unexpectedly found projected near thecenter of the Virgo cluster. Their average Tully-Fisher distance isfound approximately one magnitude further away (muo = 31.77)than the distance of their gas-deficient counterparts (muo =30.85), suggesting that the gas healthy objects belong to a cloudprojected onto the cluster center, but in fact lying a few Mpc behindVirgo, thus unaffected by the dense IGM of the cluster. Based onobservations taken at the Observatorio Astronómico Nacional(Mexico), the OHP (France), Calar Alto and NOT (Spain) observatories.Table \ref{tab4} is only available in electronic form athttp://www.edpsciences.org
| A Hubble Space Telescope Survey of the Mid-Ultraviolet Morphology of Nearby Galaxies
We present a systematic imaging survey of 37 nearby galaxies observedwith the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2(WFPC2) in the mid-UV F300W filter, centered at 2930 Å, as well asin the I-band (F814W) filter at 8230 Å. Eleven of these galaxieswere also imaged in the F255W filter, centered at 2550 Å. Oursample is carefully selected to include galaxies of sufficiently smallradius and high predicted mid-UV surface brightness to be detectablewith WFPC2 in one orbit and covers a wide range of Hubble types andinclinations. The mid-UV (2000-3200 Å) spans the gap betweenground-based UBVR(IJHK) images, which are available or were acquired forthe current study, and far-UV images available from the Astro/UITmissions for 15 galaxies in our sample. The first qualitative resultsfrom our study are as follows:1. Early-type galaxies show a significantdecrease in surface brightness going from the red to the mid-UV,reflecting the absence of a dominant young stellar population and insome cases the presence of significant (central) dust lanes. Galaxiesthat are early types in the optical show a variety of morphologies inthe mid-UV that can lead to a different morphological classification,although not necessarily as later type. Some early-type galaxies becomedominated by a blue nuclear feature or a point source in the mid-UV,e.g., as a result of the presence of a Seyfert nucleus or a LINER. Thisis in part due to our mid-UV surface brightness selection, but it alsosuggests that part of the strong apparent evolution of weak AGNs inearly-type galaxies may be due to surface brightness dimming of theirUV-faint stellar population, which renders the early-type host galaxiesinvisible at intermediate to higher redshifts.2. About half of themid-type spiral and star-forming galaxies appear as a latermorphological type in the mid-UV, as Astro/UIT also found primarily inthe far-UV. Sometimes these differences are dramatic (e.g., NGC 6782shows a spectacular ring of hot stars in the mid-UV). However, not allmid-type spiral galaxies look significantly different in the mid-UV.Their mid-UV images show a considerable range in the scale and surfacebrightness of individual star-forming regions. Almost without exception,the mid-type spirals in our sample have their small bulges bisected by adust lane, which often appears to be connected to the inner spiral armstructure.3. The majority of the heterogeneous subset of late-type,irregular, peculiar, and merging galaxies display F300W morphologiesthat are similar to those seen in F814W, but with important differencesdue to recognizable dust features absorbing the bluer light and to hotstars, star clusters, and star formation ``ridges'' that are bright inthe mid-UV. Less than one-third of the galaxies classified as late typein the optical appear sufficiently different in the mid-UV to result ina different classification.Our HST mid-UV survey of nearby galaxiesshows that, when observed in the rest-frame mid-UV, early- to mid-typegalaxies are more likely to be misclassified as later types thanlate-type galaxies are to be misclassified as earlier types. This isbecause the later type galaxies are dominated by the same young and hotstars in all filters from the mid-UV to the red and so have a smaller``morphological K-correction'' than true earlier type galaxies. Themorphological K-correction can thus explain part, but certainly not all,of the excess faint blue late-type galaxies seen in deep HST fields.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555. Also based in part onobservations made with the Vatican Advanced Technology Telescope: theAlice P. Lennon Telescope and the Thomas J. Bannan AstrophysicsFacility.
| Discovery of a Group of Star-forming Dwarf Galaxies in A1367
We describe the properties of a remarkable group of activelystar-forming dwarf galaxies and H II galaxies in the A1367 cluster,which were discovered in a large-scale Hα imaging survey of thecluster. Approximately 30 Hα-emitting knots were identified in aregion approximately 150 kpc across, in the vicinity of the spiralgalaxies NGC 3860, CGCG 97-125, and CGCG 97-114. Follow-up imaging andspectroscopy reveal that some of the knots are associated withpreviously uncataloged dwarf galaxies (MB=-15.8 to -16.5),while others appear to be isolated H II galaxies or intergalactic H IIregions. Radial velocities obtained for several of the knots show thatthey are physically associated with a small group or subcluster,including CGCG 97-114 and CGCG 97-125. No comparable concentration ofemission-line objects has been found elsewhere in any of the eightnorthern Abell clusters surveyed to date. The strong Hα emissionin the objects and their high spatial density argue against this being agroup of normal, unperturbed dwarf galaxies. Emission-line spectra ofseveral of the knots also show some to be anomalously metal-richrelative to their luminosities. The results suggest that many of theseobjects were formed or triggered by tidal interactions or mergersinvolving CGCG 97-125 and other members of the group. A WesterborkSynthesis Radio Telescope H I map of the region shows direct evidencefor tidal interactions in the group. These objects may be related to thetidal dwarf galaxies found in some interacting galaxy pairs, mergerremnants, and compact groups. They could also represent evolutionaryprecursors to the class of isolated ultracompact dwarf galaxies thathave been identified in the Fornax Cluster.
| Chandra View of the Dynamically Young Cluster of Galaxies A1367. II. Point Sources
A 40 ks Chandra ACIS-S observation of the dynamically young clusterA1367 yields new insights on X-ray emission from cluster membergalaxies. We detect 59 pointlike sources in the ACIS field, of whicheight are identified with known cluster member galaxies. Thus, in total10 member galaxies are detected in X-rays when three galaxies discussedin Paper I (NGC 3860 is discussed in both papers) are included. Thesuperior spatial resolution and good spectroscopy capability of Chandraallow us to constrain the emission nature of these galaxies. Centralnuclei, thermal halos, and stellar components are revealed in theirspectra. Two new low-luminosity active galactic nuclei (LLAGNs) arefound, including an absorbed one (NGC 3861). Besides these two for sure,two new LLAGN candidates are also found. This discovery makes theLLAGN/AGN content in this part of A1367 very high (>~12%). Thermalhalos with temperatures around 0.5-0.8 keV are revealed in the spectraof NGC 3842 and NGC 3837, which suggests that galactic coronae cansurvive in clusters and that heat conduction must be suppressed. TheX-ray spectrum of NGC 3862 (3C 264) resembles a BL Lac object with aphoton index of ~2.5. We also present an analysis of other point sourcesin the field and discuss the apparent source excess (~2.5 σ) inthe central field.
| Chandra View of the Dynamically Young Cluster of Galaxies A1367. I. Small-Scale Structures
The 40 ks Chandra ACIS-S observation of A1367 provides new insights intosmall-scale structures and point sources in this dynamically youngcluster. Here we concentrate on small-scale extended structures. Aridgelike structure around the center (``the ridge'') is significant inthe Chandra image. The ridge, with a projected length of ~8' (or 300h-10.5 kpc), is elongated from northwest tosoutheast, as is the X-ray surface brightness distribution on muchlarger scales (~2 h-10.5 Mpc). The ridge is coolerthan its western and southern surroundings, while the differences fromits eastern and northern surroundings are small. We also searched forsmall-scale structures with sizes of about an arcminute. Nine extendedfeatures, with sizes from ~0.5′ to 1.5′, were detected atsignificance levels above 4 σ. Five of the nine features arelocated in the ridge and form local crests. The nine extended featurescan be divided into two types. Those associated with galaxies (NGC3860B, NGC 3860, and UGC 6697) are significantly cooler than theirsurroundings (0.3-0.9 keV vs. 3-4.5 keV). The masses of their hostgalaxies are sufficient to bind the extended gas. These extendedfeatures are probably related to thermal halos or galactic superwinds oftheir host galaxies. The existence of these relatively cold halosimplies that galaxy coronae can survive in a cluster environment (e.g.,Vikhlinin and coworkers in 2001). Features of the second type are notapparently associated with galaxies. Their temperatures may not besignificantly different from those of their surroundings. This class ofextended features may be related to the ridge. We consider severalpossibilities for the ridge and the second type of extended features.The merging scenario is preferred.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| 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.
| A deep Hα survey of galaxies in the two nearby clusters Abell 1367 and Coma. The Hα luminosity functions
We present a deep wide field Hα imaging survey of the centralregions of the two nearby clusters of galaxies Coma and Abell 1367,taken with the WFC at the INT 2.5 m telescope. We determine for thefirst time the Schechter parameters of the Hα luminosity function(LF) of cluster galaxies. The Hα LFs of Abell 1367 and Coma arecompared with each other and with that of Virgo, estimated using the Bband LF by Sandage et al. (\cite{Sandage85}) and a L(Hα ) vs.MB relation. Typical parameters of phi * ~100.00+/-0.07 Mpc-3, L* ~1041.25+/-0.05 erg s-1 and alpha ~ -0.70+/-0.10are found for the three clusters. The best fitting parameters of thecluster LFs differ from those found for field galaxies, showing flatterslopes and lower scaling luminosities L*. Since, however, ourHα survey is significantly deeper than those of field galaxies,this result must be confirmed on similarly deep measurements of fieldgalaxies. By computing the total SFR per unit volume of clustergalaxies, and taking into account the cluster density in the localUniverse, we estimate that the contribution of clusters like Coma andAbell 1367 is approximately 0.25% of the SFR per unit volume of thelocal Universe. Appendix a is only available in electronic form athttp://www.edpsciences.org
| 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).
| An Ultraviolet/Optical Atlas of Bright Galaxies
We present wide-field imagery and photometry of 43 selected nearbygalaxies of all morphological types at ultraviolet and opticalwavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and2500 Å, were obtained using the Ultraviolet Imaging Telescope(UIT) during the Astro-1 Spacelab mission. The UV images have ~3"resolution, and the comparison sets of ground-based CCD images (in oneor more of B, V, R, and Hα) have pixel scales and fields of viewclosely matching the UV frames. The atlas consists of multiband imagesand plots of UV/optical surface brightness and color profiles. Otherassociated parameters, such as integrated photometry and half-lightradii, are tabulated. In an appendix, we discuss the sensitivity ofdifferent wavebands to a galaxy's star formation history in the form of``history weighting functions'' and emphasize the importance of UVobservations as probes of evolution during the past 10-1000 Myr. We findthat UV galaxy morphologies are usually significantly different fromvisible band morphologies as a consequence of spatially inhomogeneousstellar populations. Differences are quite pronounced for systems in themiddle range of Hubble types, Sa through Sc, but less so for ellipticalsor late-type disks. Normal ellipticals and large spiral bulges arefainter and more compact in the UV. However, they typically exhibitsmooth UV profiles with far-UV/optical color gradients which are largerthan any at optical/IR wavelengths. The far-UV light in these cases isprobably produced by extreme horizontal branch stars and theirdescendants in the dominant, low-mass, metal-rich population. The coolstars in the large bulges of Sa and Sb spirals fade in the UV while hotOB stars in their disks brighten, such that their Hubble classificationsbecome significantly later. In the far-UV, early-type spirals oftenappear as peculiar, ringlike systems. In some spiral disks, UV-brightstructures closely outline the spiral pattern; in others, the disks canbe much more fragmented and chaotic than at optical wavelengths.Contributions by bright active galactic nuclei (AGNs) to the integratedUV light in our sample range from less than 10% to nearly 100%. A numberof systems have unusual UV-bright structures in their inner disks,including rings, compact knots, and starburst nuclei, which could easilydominate the UV light in high-redshift analogs. A significant butvariable fraction of the far-UV light in spiral disks is diffuse ratherthan closely concentrated to star-forming regions. Dust in normal spiraldisks does not control UV morphologies, even in some highly inclineddisk systems. The heaviest extinction is apparently confined to thinlayers and the immediate vicinity of young H II complexes; the UV lightemerges from thicker star distributions, regions evacuated of dust byphotodestruction or winds, or by virtue of strong dust clumpiness. Onlyin cases where the dust layers are disturbed does dust appear to be amajor factor in UV morphology. The UV-bright plume of M82 indicates thatdust scattering of UV photons can be important in some cases. In acompanion paper, we discuss far-UV data from the Astro-2 mission andoptical comparisons for another 35 galaxies, emphasizing face-onspirals.
| A Comparison of Ultraviolet Imaging Telescope Far-Ultraviolet and Hα Star Formation Rates
We have used archival ultraviolet (UV) imaging of 50 nearby star-forminggalaxies obtained with the Ultraviolet Imaging Telescope (UIT) to deriveintegrated near-UV and far-UV magnitudes, and have combined these datawith Hα, far-infrared, and thermal radio continuum measurements toexplore the consistency of UV and Hα star formation rates (SFRs).In agreement with previous studies, we find that the UV and HαSFRs are qualitatively consistent, even before corrections forextinction are applied. The uncorrected UV SFRs are systematically lowerby a factor of 1.5 (with a factor of 2 scatter) among luminous galaxieswith SFR>~1 Msolar yr-1, indicating a highereffective attenuation of the far-UV radiation. Among less luminousgalaxies there is no significant offset between the Hα and far-UVSFR scales. This behavior is consistent with that of higher redshiftsamples observed by Sullivan et al., Glazebrook et al., and Yan et al.for comparable ranges of galaxy luminosities and absolute SFRs.Far-infrared and thermal radio continuum data available for a subset ofour sample allow us to estimate the attenuation in the UV and atHα independently. The UV and Hα attenuations appear to becorrelated, and confirm systematically higher attenuations in the UV.Although the galaxies in our sample show modest levels of attenuation(with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å),the range across the sample is large, ~4 mag for Hα and >~5 magin the far-UV (1550 Å). This indicates that the application of asingle characteristic extinction correction to Hα or UV SFRs isonly realistic for large, well-defined and well-studied galaxy samples,and that extinction bias may be important for UV oremission-line-selected samples of star-forming galaxies.
| 1.65 μm (H-band) surface photometry of galaxies. V. Profile decomposition of 1157 galaxies
We present near-infrared H-band (1.65 μm) surface brightness profiledecomposition for 1157 galaxies in five nearby clusters of galaxies:Coma, A1367, Virgo, A262 and Cancer, and in the bridge between Coma andA1367 in the ``Great Wall". The optically selected (mpg≤16.0) sample is representative of all Hubble types, from E to Irr+BCD,except dE and of significantly different environments, spanning fromisolated regions to rich clusters of galaxies. We model the surfacebrightness profiles with a de Vaucouleurs r1/4 law (dV), withan exponential disk law (E), or with a combination of the two (B+D).From the fitted quantities we derive the H band effective surfacebrightness (μe) and radius (re) of each component, theasymptotic magnitude HT and the light concentration indexC31. We find that: i) Less than 50% of the Ellipticalgalaxies have pure dV profiles. The majority of E to Sb galaxies is bestrepresented by a B+D profile. All Scd to BCD galaxies have pureexponential profiles. ii) The type of decomposition is a strong functionof the total H band luminosity (mass), independent of the Hubbleclassification: the fraction of pure exponential decompositionsdecreases with increasing luminosity, that of B+D increases withluminosity. Pure dV profiles are absent in the low luminosity rangeLH<1010 L\odot and become dominantabove 1011 L\odot . Based on observations taken atTIRGO, Gornergrat, Switzerland (operated by CAISMI-CNR, Arcetri,Firenze, Italy) and at the Calar Alto Observatory (operated by theMax-Planck-Institut für Astronomie (Heidelberg) jointly with theSpanish National Commission for Astronomy). Table 2 and Figs. 2, 3, 4are available in their entirety only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The second Kiso Survey for ultraviolet-excess galaxies. II.
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