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Discovery of a very extended X-ray halo around a quiescent spiral galaxy The “missing link” of galaxy formation
Hot gaseous haloes surrounding galaxies and extending well beyond thedistribution of stars are a ubiquitous prediction of galaxy formationscenarios. The haloes are believed to consist of gravitationally trappedgas with a temperature of millions of Kelvin. The existence of such hothaloes around massive elliptical galaxies has been established throughtheir X-ray emission. While gas out-flowing from starburst spiralgalaxies has been detected, searches for hot haloes around normal,quiescent spiral galaxies have so far failed, casting doubts on thefundamental physics in galaxy formation models. Here we present thefirst detection of a hot, large-scale gaseous halo surrounding a normal,quiescent spiral galaxy, NGC 5746, alleviating a long-standing problemfor galaxy formation models. In contrast to starburst galaxies, wherethe X-ray halo can be powered by the supernova energy, there is no suchpower source in NGC 5746. The only compelling explanation is that we arehere witnessing a galaxy forming from gradually in-flowing hot anddilute halo gas.
| Where Are the ``Missing'' Galactic Baryons?
Based on 19 high-resolution N-body/gasdynamical galaxy formationsimulations in the ΛCDM cosmology, it is shown that for a galaxylike the Milky Way, in addition to the baryonic mass of the galaxyitself, about 70% extra baryonic mass should reside around the galaxy(inside of the virial radius), chiefly in the form of hot gas. Averagingover the entire field galaxy population, this ``external'' componentamounts to 64%-85% of the baryonic mass of the population itself. Theseresults are supported by the recent detection of very extended, softX-ray emission from the halo of the quiescent, massive disk galaxy NGC5746. Some of the hot gas may, by thermal instability, have condensedinto mainly pressure-supported, warm clouds, similar to the Galactichigh-velocity clouds (HVCs). Based on an ultra-high-resolutioncosmological test simulation of a Milky Way-like galaxy (with a gasparticle mass and gravity softening length of only7.6×103 h-1 Msolar and 83h-1 pc, respectively), it is argued that the hot gas phasedominates over the warm gas phase, in the halo. Finally, an origin ofHVCs as ``leftovers'' from filamentary, ``cold'' accretion events,mainly occurring early in the history of galaxies, is proposed.
| Bar Diagnostics in Edge-On Spiral Galaxies. III. N-Body Simulations of Disks
Present in over 45% of local spirals, boxy and peanut-shaped bulges aregenerally interpreted as edge-on bars and may represent a key phase inbar evolution. Aiming to test such claims, the kinematic properties ofself-consistent three-dimensional N-body simulations of bar-unstabledisks are studied. Using Gauss-Hermite polynomials to describe themajor-axis stellar kinematics, a number of characteristic bar signaturesare identified in edge-on disks: (1) a major-axis light profile with aquasi-exponential central peak and a plateau at moderate radii (Freemantype II profile); (2) a ``double-hump'' rotation curve; (3) a sometimesflat central velocity dispersion peak with a plateau at moderate radiiand occasional local central minimum and secondary peak; and (4) anh3-V correlation over the projected bar length. All of thesekinematic features are spatially correlated and can easily be understoodfrom the orbital structure of barred disks. They thus provide a reliableand easy-to-use tool to identify edge-on bars. Interestingly, they areall produced without dissipation and are increasingly realized to becommon in spirals, lending support to bar-driven evolution scenarios forbulge formation. So called ``figure-of-eight'' position-velocitydiagrams are never observed, as expected for realistic orbitalconfigurations. Although not uniquely related to triaxiality,line-of-sight velocity distributions with a high-velocity tail (i.e., anh3-V correlation) appear as particularly promising tracers ofbars. The stellar kinematic features identified grow in strength as thebar evolves and vary only slightly for small inclination variations.Many can be used to trace the bar length. Comparisons with observationsare encouraging and support the view that boxy and peanut-shaped bulgesare simply thick bars viewed edge-on.
| On the alignment between binary spiral galaxies
We show some significance against the null hypothesis of randominteractions of binary spiral galaxies, and in favour of the alternativethat more interactions than expected occur for axes either nearlyparallel (spins being parallel or anti-parallel) or nearly orthogonal.We discuss this in the context of similar prior studies, using adifferent statistical focus in such a way that we are able toincorporate additional data.
| Secular Evolution and the Formation of Pseudobulges in Disk Galaxies
The Universe is in transition. At early times, galactic evolution wasdominated by hierarchical clustering and merging, processes that areviolent and rapid. In the far future, evolution will mostly be secularthe slow rearrangement of energy and mass that results from interactionsinvolving collective phenomena such as bars, oval disks, spiralstructure, and triaxial dark halos. Both processes are important now.This review discusses internal secular evolution, concentrating on oneimportant consequence, the buildup of dense central components in diskgalaxies that look like classical, merger-built bulges but that weremade slowly out of disk gas. We call these pseudobulges.
| Mid-Infrared Galaxy Morphology along the Hubble Sequence
The mid-infrared emission from 18 nearby galaxies were imaged with theInfrared Array Camera (IRAC) on the Spitzer Space Telescope, samplingthe spatial distributions of the reddening-free stellar photosphericemission and the warm dust in the interstellar medium. These twocomponents provide a new framework for galaxy morphology classificationin which the presence of spiral arms and their emission strengthrelative to the starlight can be measured directly and with highcontrast. Four mid-infrared classification methods are explored, threeof which are based on quantitative global parameters (colors andbulge-to-disk ratio) that are similar to those used in the past foroptical studies; in this limited sample, all correlate well withtraditional B-band classification. We suggest reasons why infraredclassification may be superior to optical classification.
| The Rotation Curves of Dwarf Galaxies: A Problem for Cold Dark Matter?
We address the issue of accuracy in recovering density profiles fromobservations of rotation curves of galaxies. We ``observe'' and analyzeour models in much the same way as observers do the real galaxies. Ourmodels include stellar disks, disks with bars, and small bulges. We findthat the tilted-ring model analysis produces an underestimate of thecentral rotational velocity. In some cases the galaxy halo densityprofile seems to have a flat core, while in reality it does not. Weidentify three effects that explain the systematic biases: inclination,small bulge, and bar. Inclination effects are due to the finitethickness of the disk, bar, or bulge. Admixture of a nonrotating bulgecomponent reduces the rotational velocity. A small (200-500 pc) bulgemay be overlooked, leading to systematic bias even on relatively large(~1 kpc) distances. In the case of a disk with a bar, the underestimateof the circular velocity is larger because of a combination ofnoncircular motions and random velocities. The effect of the bar dependson the angle that the bar makes with the line of sight. Signatures ofbars can be difficult to detect in the surface brightness profiles ofthe model galaxies. The variations of inclination angle and isophoteposition angle with radius are more reliable indicators of bar presencethan the surface brightness profiles. The systematic biases in thecentral ~1 kpc of galaxies are not large. Each effect separately givestypically a few km s-1 error, but the effects add up. In somecases the error in circular velocity was a factor of 2, but typically weget about a 20% effect. The result is the false inference that thedensity profile of the halo flattens in the central parts. Ourobservations of real galaxies show that for a large fraction of galaxiesthe velocity of gas rotation (as measured by emission lines) is veryclose to the rotation of the stellar component (as measured byabsorption lines). This implies that the systematic effects discussed inthis paper are also applicable both for the stars and emission-line gas.
| Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
| A New Nonparametric Approach to Galaxy Morphological Classification
We present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local ``normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05
| Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies
To understand the nuclear stellar populations and star formationhistories of the nuclei of spiral galaxies, we have obtained K-bandnuclear spectra for 41 galaxies and H-band spectra for 20 galaxies inthe Infrared Space Observatory's Atlas of Bright Spiral Galaxies. In thevast majority of the subsample (80%), the near-infrared spectra suggestthat evolved red stars completely dominate the nuclear stellarpopulations and that hot young stars are virtually nonexistent. Thesignatures of recent star formation activity are only found in 20% ofthe subsample, even though older red stars still dominate the stellarpopulations in these galaxies. Given the dominance of evolved stars inmost galaxy nuclei and the nature of the emission lines in the galaxieswhere they were detected, we suggest that nuclear star formationproceeds in the form of instantaneous bursts. The stars produced bythese bursts comprise only ~2% of the total nuclear stellar mass inthese galaxies, but we demonstrate how the nuclear stellar populationsof normal spiral galaxies can be built up through a series of thesebursts. The bursts were detected only in Sbc galaxies and later, andboth bars and interactions appeared to be sufficient, but not necessary,triggers for the nuclear star formation activity. The vast majority ofgalaxies with nuclear star formation were classified as H II galaxies.With one exception, LINERs and transition objects were dominated byolder red stars, which suggested that star formation was not responsiblefor generating these galaxies' optical line emission.
| Stellar Kinematics of Boxy Bulges: Large-Scale Bars and Inner Disks
Long-slit stellar kinematic observations were obtained along the majoraxis of 30 edge-on spiral galaxies, 24 with a boxy or peanut-shaped(B/PS) bulge and six with other bulge types for comparison. Such B/PSbulges are identified in at least 45% of highly inclined systems, and agrowing body of theoretical and observational work suggests that theyare the edge-on projection of thickened bars. Profiles of the meanstellar velocity V, the velocity dispersion σ, as well as theasymmetric (h3) and symmetric (h4) deviations froma pure Gaussian are presented for all objects. Comparing these profileswith stellar kinematic bar diagnostics developed from N-bodysimulations, we find bar signatures in 24 of our sample galaxies (80%).Galaxies with a B/PS bulge typically show a double-humped rotation curvewith an intermediate dip or plateau. They also frequently show a ratherflat central velocity dispersion profile accompanied by a secondary peakor plateau, and numerous galaxies have a local central σ minimum(>~40%). The h3 profiles display up to three slopereversals. Most importantly, h3 is normally correlated with Vover the presumed bar length, contrary to expectations from axisymmetricdisks. These characteristic bar signatures strengthen the case for aclose relationship between B/PS bulges and bars and leave little roomfor other explanations of the bulges' shape. We also find thath3 is anticorrelated with V in the very center of mostgalaxies (>~60%), indicating that these objects additionally harborcold and dense decoupled (quasi-) axisymmetric central stellar disks,which may be related to the central light peaks. These central diskscoincide with previously identified star-forming ionized-gas disks(nuclear spirals) in gas-rich systems, and we argue that they formed outof gas accumulated by the bar at its center through inflow. As suggestedby N-body models, the asymmetry of the velocity profile (h3)appears to be a reliable tracer of asymmetries in disks, allowing us todiscriminate between axisymmetric and barred disks seen in projection.B/PS bulges (and thus a large fraction of all bulges) appear to be madeup mostly of disk material, which has acquired a large vertical extentthrough bar-driven vertical instabilities. Their formation is thusprobably dominated by secular evolution processes rather than merging.
| Detailed comparison of the structures and kinematics of simulated and observed barred galaxies
We examine the observable properties of simulated barred galaxies,including radial mass profiles, edge-on structure and kinematics, barlengths and pattern speed evolution for detailed comparison to realsystems. We have run several simulations in which bars are createdthrough inherent instabilities in self-consistent simulations of arealistic disc+halo galaxy model with a disc-dominated, flat rotationcurve. These simulations were run at high (N= 20 million particles) andlow (N= 500000 particles) resolution to test numerical convergence. Wedetermine the pattern speeds in simulations directly from the phaseangle of the bar versus time and the Tremaine-Weinberg method.Fundamental dynamics do not change between the high and low resolution,suggesting that convergence has been reached in this case. We find thatthe higher resolution is needed to simulate structural and kinematicproperties accurately. The edge-on view of the higher-resolution systemshows the bending instability and formation of a peanut-shaped bulgeclearly. We determined bar lengths by different means to determine thatthe simulated bar is fast, with a corotation to bar length ratio ofunder 1.5. Simulated bars in these models form with pattern speedsslower than those observed and slow-down during their evolution.Dynamical friction between the bar and dark halo is responsible for thisdeceleration, as revealed by the transfer of angular momentum betweenthe disc and the halo. However, even though the pattern speed is reducedat later times, the instantaneous scalelength of the disc has grownsufficiently for the bar motion to agree with many observations. Byusing a different model and simulation technique than other authors, weare able to compare the robustness of these methods. An animation of theface-on and edge-on views of the 20-million-particle simulation isavailable at http://www.astro.utoronto.ca/~oneill.
| The Relationship between Stellar Light Distributions of Galaxies and Their Formation Histories
A major problem in extragalactic astronomy is the inability todistinguish in a robust, physical, and model-independent way how galaxypopulations are physically related to each other and to their formationhistories. A similar, but distinct, and also long-standing question iswhether the structural appearances of galaxies, as seen through theirstellar light distributions, contain enough physical information tooffer this classification. We argue through the use of 240 images ofnearby galaxies that three model-independent parameters measured on asingle galaxy image reveal its major ongoing and past formation modesand can be used as a robust classification system. These parametersquantitatively measure: the concentration (C), asymmetry (A), andclumpiness (S) of a galaxy's stellar light distribution. When combinedinto a three-dimensional ``CAS'' volume all major classes of galaxies invarious phases of evolution are cleanly distinguished. We argue thatthese three parameters correlate with important modes of galaxyevolution: star formation and major merging activity. This is arguedthrough the strong correlation of Hα equivalent width andbroadband colors with the clumpiness parameter S, the uniquely largeasymmetries of 66 galaxies undergoing mergers, and the correlation ofbulge to total light ratios, and stellar masses, with the concentrationindex. As an obvious goal is to use this system at high redshifts totrace evolution, we demonstrate that these parameters can be measured,within a reasonable and quantifiable uncertainty with available data outto z~3 using the Hubble Space Telescope GOODS ACS and Hubble Deep Fieldimages.
| The Near-Infrared Ca II Triplet-σ Relation for Bulges of Spiral Galaxies
We present measurements of the near-infrared Ca II triplet (CaT, CaT*),Paschen (PaT), and magnesium (Mg I) indices for a well-studied sample of19 bulges of early to intermediate spiral galaxies. We find that boththe CaT* and CaT indices decrease with central velocity dispersionσ with small scatter. This dependence is similar to that recentlyfound by Cenarro for elliptical galaxies, implying a uniformCaT*-σ relation that applies to galaxies from ellipticals tointermediate-type spirals. The decrease of CaT and CaT* with σcontrasts with the well-known increase of another α-element index,Mg2, with σ. We discuss the role of Ca underabundance([Ca/Fe]<0) and initial mass function variations in the onset of theobserved relations.
| A Search for ``Dwarf'' Seyfert Nuclei. VI. Properties of Emission-Line Nuclei in Nearby Galaxies
We use the database from Paper III to quantify the global and nuclearproperties of emission-line nuclei in the Palomar spectroscopic surveyof nearby galaxies. We show that the host galaxies of Seyferts, LINERs,and transition objects share remarkably similar large-scale propertiesand local environments. The distinguishing traits emerge on nuclearscales. Compared with LINERs, Seyfert nuclei are an order of magnitudemore luminous and exhibit higher electron densities and internalextinction. We suggest that Seyfert galaxies possess characteristicallymore gas-rich circumnuclear regions and hence a more abundant fuelreservoir and plausibly higher accretion rates. The differences betweenthe ionization states of the narrow emission-line regions of Seyfertsand LINERs can be partly explained by the differences in their nebularproperties. Transition-type objects are consistent with being composite(LINER/H II) systems. With very few exceptions, the stellar populationwithin the central few hundred parsecs of the host galaxies is uniformlyold, a finding that presents a serious challenge to starburst orpost-starburst models for these objects. Seyferts and LINERs havevirtually indistinguishable velocity fields as inferred from their linewidths and line asymmetries. Transition nuclei tend to have narrowerlines and more ambiguous evidence for line asymmetries. All threeclasses of objects obey a strong correlation between line width and lineluminosity. We argue that the angular momentum content of circumnucleargas may be an important factor in determining whether a nucleus becomesactive. Finally, we discuss some possible complications for theunification model of Seyfert galaxies posed by our observations.
| The 2MASS Large Galaxy Atlas
We present the largest galaxies as seen in the near-infrared (1-2μm), imaged with the Two Micron All Sky Survey (2MASS), ranging inangular size from 1' to 1.5d. We highlight the 100 largest in thesample. The galaxies span all Hubble morphological types, includingelliptical galaxies, normal and barred spirals, and dwarf and peculiarclasses. The 2MASS Large Galaxy Atlas provides the necessary sensitivityand angular resolution to examine in detail morphologies in thenear-infrared, which may be radically different from those in theoptical. Internal structures such as spirals, bulges, warps, rings,bars, and star formation regions are resolved by 2MASS. In addition tolarge mosaic images, the atlas includes astrometric, photometric, andshape global measurements for each galaxy. A comparison of fundamentalmeasures (e.g., surface brightness, Hubble type) is carried out for thesample and compared with the Third Reference Catalogue. We furthershowcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality anddepth of the data. The atlas represents the first uniform, all-sky,dust-penetrated view of galaxies of every type, as seen in thenear-infrared wavelength window that is most sensitive to the dominantmass component of galaxies. The images and catalogs are availablethrough the NASA/IPAC Extragalactic Database and Infrared ScienceArchive and are part of the 2MASS Extended Source Catalog.
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| Galaxy classification using fractal signature
Fractal geometry is becoming increasingly important in the study ofimage characteristics. For recognition of regions and objects in naturalscenes, there is always a need for features that are invariant and theyprovide a good set of descriptive values for the region. There are manyfractal features that can be generated from an image. In this paper,fractal signatures of nearby galaxies are studied with the aim ofclassifying them. The fractal signature over a range of scales proved tobe an efficient feature set with good discriminating power. Classifierswere designed using nearest neighbour method and neural networktechnique. Using the nearest distance approach, classification rate wasfound to be 92%. By the neural network method it has been found toincrease to 95%.
| Minor axis kinematics of 19 S0-Sbc bulges
We present minor axis kinematic profiles for a well-studied sample of 19early- to intermediate-type disk galaxies. We introduce, for the firsttime, the use of single-burst stellar population (SSP) models to obtainstellar velocities, velocity dispersions and higher order Gauss-Hermitemoments (h3, h4) from galaxy spectra in thenear-infrared Ca II triplet region. SSP models, which employs thesynthetic spectra of \citet{vazdekis03}, provide a means to address thetemplate-mismatch problem, and are shown to provide as good or betterfits as traditional stellar templates. We anticipate the technique to beof particular use for high-redshift galaxy kinematics. We give themeasurement of a recently defined CaT* index\citep{cenarro01}, and describe the global properties of the bulgekinematics as derived from the kinematic profiles. We detectsmall-amplitude minor-axis rotation, generally due to inner isophotaltwists as a result of slightly triaxial bulges or misaligned innerdisks; such inner features do not show peculiar colors or distinctCaT* index values. Velocity dispersion profiles, which extendwell into the disk region, show a wide range of slopes. Flattened bulgestend to have shallower velocity dispersion profiles. The inferredsimilarity of bulge and disk radial velocity dispersions supports theinterpretation of these bulges as thickened disks.Appendix B is only available in electronic form athttp://www.edpsciences.org
| Edge-on boxy profiles in non-barred disc galaxies
Boxy edge-on profiles can be accounted for not only in models of barredgalaxies, but also in models of normal (non-barred) galaxies. Thus thepresence of a bar is not a sine qua non condition for the appearance ofthis feature, as often assumed. We show that a `boxy' or a `peanut'structure in the central parts of a model is due to the presence ofvertical resonances at which stable families of periodic orbitsbifurcate from the planar x1 family. The orbits of these families reachin their projections on the equatorial plane a maximum distance from thecentre, beyond which they increase their mean radii by increasing onlytheir deviations from the equatorial plane. The resulting orbitalprofiles are `stair-type' and constitute the backbone for the observedboxy structures in edge-on views of N-body models and, we believe, inedge-on views of disc galaxies. Since the existence of verticalresonances is independent of barred or spiral perturbations in the disc,`boxy' profiles may appear also in almost axisymmetric cases.
| Bulges on the Fundamental Plane of early-type galaxies
In an ongoing effort to study the formation and evolution of galacticbulges, we have investigated the position of 19 bulges of type S0-Sbc onthe Fundamental Plane (FP). We find that bulges, in both B and K band,lie close to but slightly below the FP defined by ellipticals and S0s,i.e. are slightly brighter. There are hints that bulges of latermorphological type are situated below the other bulges in our sample.The FP results are consistent with the picture, obtained from our recentanalysis of HST colours, that bulges are old, except for the Sbcgalaxies. The fact that bulges lie so close to the FP of ellipticals andS0s implies that their formation epoch must have been similar to, atmost about 2.5 Gyr earlier than, cluster Es and S0s, and that thesurrounding disc does not significantly affect their structure.
| A comparison of stellar populations in galaxy spheroids across a wide range of Hubble types
We present line-strengths and kinematics from the central regions of 32galaxies with Hubble types ranging from E to Sbc. Spectral indices,based on the Lick system, are measured in the optical and near-infrared(NIR). The 24 indices measured, in conjunction with models of theeffects of varying abundance ratios, permit the breaking ofage/metallicity degeneracy, and allow estimation of enhancements inspecific light elements (particularly C and Mg). The large range ofHubble types observed allows direct comparison of line-strengths in thecentres of early-type galaxies (E and S0) with those in spiral bulges,free from systematic differences that have plagued comparisons ofresults from different studies. Our sample includes field and Virgocluster galaxies. For early-type galaxies our data are consistent withpreviously reported trends of Mg2 and Mgb with velocitydispersion. In spiral bulges we find trends in all indices with velocitydispersion. We estimate luminosity-weighted ages, metallicities andheavy-element abundance ratios (enhancements) from optical indices.These show that bulges are less enhanced in light (α-capture)elements and have lower average age than early-type galaxies. Trendsinvolving age and metallicity also differ sharply between early and latetypes. An anticorrelation exists between age and metallicity in earlytypes, while, in bulges, metallicity is correlated with velocitydispersion. We consider the implications of these findings for models ofthe formation of these galaxies. We find that primordial collapse modelsof galaxy formation are ruled out by our observations, while severalpredictions of hierarchical clustering (merger) models are confirmed.
| The Radio Properties of Composite LINER/H II Galaxies
Arcsecond-resolution VLA observations-newly obtained as well aspublished-of 40 nearby galaxies are discussed, completing a study of theradio properties of a magnitude-limited sample of nearby galaxies of thecomposite LINER/H II type. Our results reveal an overall detection rateof at least 25% active galactic nucleus (AGN) candidates among thesecomposite sources. The general properties of these AGN candidates, ascompared to non-AGN composite sources and H II galaxies, are discussed.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| An Infrared Space Observatory Atlas of Bright Spiral Galaxies
In this first paper in a series we present an atlas of infrared imagesand photometry from 1.2 to 180 μm for a sample of bright spiralgalaxies. The atlas galaxies are an optically selected,magnitude-limited sample of 77 spiral and S0 galaxies chosen from theRevised Shapley-Ames Catalog (RSA). The sample is a representativesample of spiral galaxies and includes Seyfert galaxies, LINERs,interacting galaxies, and peculiar galaxies. Using the Infrared SpaceObservatory (ISO), we have obtained 12 μm images and photometry at60, 100, and 180 μm for the galaxies. In addition to its imagingcapabilities, ISO provides substantially better angular resolution thanis available in the IRAS survey, and this permits discrimination betweeninfrared activity in the central regions and global infrared emission inthe disks of these galaxies. These ISO data have been supplemented withJHK imaging using ground-based telescopes. The atlas includes 2 and 12μm images. Following an analysis of the properties of the galaxies,we have compared the mid-infrared and far-infrared ISO photometry withIRAS photometry. The systematic differences we find between the IRASFaint Source Catalog and ISO measurements are directly related to thespatial extent of the ISO fluxes, and we discuss the reliability of IRASFaint Source Catalog total flux densities and flux ratios for nearbygalaxies. In our analysis of the 12 μm morphological features we findthat most but not all galaxies have bright nuclear emission. We find 12μm structures such as rings, spiral arm fragments, knotted spiralarms, and bright sources in the disks that are sometimes brighter thanthe nuclei at mid-infrared wavelengths. These features, which arepresumably associated with extranuclear star formation, are common inthe disks of Sb and later galaxies but are relatively unimportant inS0-Sab galaxies. Based on observations with the Infrared SpaceObservatory (ISO), an ESA project with instruments funded by ESA MemberStates (especially the PI countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of ISAS and NASA.
| 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.
| 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
| Supernovae in isolated galaxies, in pairs and in groups of galaxies
In order to investigate the influence of environment on supernova (SN)production, we have performed a statistical investigation of the SNediscovered in isolated galaxies, in pairs and in groups of galaxies. 22SNe in 18 isolated galaxies, 48 SNe in 40 galaxy members of 37 pairs and211 SNe in 170 galaxy members of 116 groups have been selected andstudied. We found that the radial distributions of core-collapse SNe ingalaxies located in different environments are similar, and consistentwith those reported by Bartunov, Makarova & Tsvetkov. SNe discoveredin pairs do not favour a particular direction with respect to thecompanion galaxy. Also, the azimuthal distributions inside the hostmembers of galaxy groups are consistent with being isotropics. The factthat SNe are more frequent in the brighter components of the pairs andgroups is expected from the dependence of the SN rates on the galaxyluminosity. There is an indication that the SN rate is higher in galaxypairs compared with that in groups. This can be related to the enhancedstar formation rate in strongly interacting systems. It is concludedthat, with the possible exception of strongly interacting systems, theparent galaxy environment has no direct influence on SN production.
| The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus
We analyze the idea that nuclear activity, either active galactic nuclei(AGNs) or star formation, can be triggered by interactions by studyingthe percentage of active, H II, and quiescent galaxies with companions.Our sample was selected from the Palomar survey and avoids selectionbiases faced by previous studies. This sample was split into fivedifferent groups, Seyfert galaxies, LINERs, transition galaxies, H IIgalaxies, and absorption-line galaxies. The comparison between the localgalaxy density distributions of the different groups showed that in mostcases there is no statistically significant difference among galaxies ofdifferent activity types, with the exception that absorption-linegalaxies are seen in higher density environments, since most of them arein the Virgo Cluster. The comparison of the percentage of galaxies withnearby companions showed that there is a higher percentage of LINERs,transition galaxies, and absorption-line galaxies with companions thanSeyfert and H II galaxies. However, we find that when we consider onlygalaxies of similar morphological types (elliptical or spiral), there isno difference in the percentage of galaxies with companions amongdifferent activity types, indicating that the former result was due tothe morphology-density effect. In addition, only small differences arefound when we consider galaxies with similar Hα luminosities. Thecomparison between H II galaxies of different Hα luminositiesshows that there is a significantly higher percentage of galaxies withcompanions among H II galaxies with L(Hα)>1039 ergss-1 than among those with L(Hα)<=1039ergs s-1, indicating that interactions increase the amount ofcircumnuclear star formation, in agreement with previous results. Thefact that we find that galaxies of different activity types have thesame percentage of companions suggests that interactions betweengalaxies is not a necessary condition to trigger the nuclear activity inAGNs. We compare our results with previous ones and discuss theirimplications.
| Homogenization of the Stellar Population along Late-Type Spiral Galaxies
We present a study of the broadband UBV color profiles for 257 Sbcbarred and nonbarred galaxies, using photoelectric aperture photometrydata from the literature. Using robust statistical methods, we haveestimated the color gradients of the galaxies, as well as the total andbulge mean colors. A comparative photometric study using CCD images wasdone. In our sample, the color gradients are negative (reddish inward)in approximately 59% of the objects, are almost null in 27%, and arepositive in 14%, considering only the face-on galaxies, which representapproximately 51% of the sample. The results do not change, essentially,when we include the edge-on galaxies. As a consequence of this study wehave also found that barred galaxies are overrepresented among theobjects having null or positive gradients, indicating that bars act as amechanism of homogenization of the stellar population. This effect ismore evident in the U-B color index, although it can also be detected inthe B-V color. A correlation between the total and bulge colors wasfound that is a consequence of an underlying correlation between thecolors of bulges and disks found by other authors. Moreover, the meantotal color is the same irrespective of the gradient regime, whilebulges are bluer in galaxies with null or positive gradients, whichindicates an increase of the star formation rate in the central regionsof these objects. We have also made a quantitative evaluation of theamount of extinction in the center of these galaxies. This was doneusing the Wide Field Planetary Camera 2 (WFPC2) and the Near InfraredCamera and Multi-Object Spectrometer (NICMOS) Hubble Space Telescope(HST) archival data, as well as CCD B, V, and I images. We show thatalthough the extinction in the V-band can reach values up to 2 mag inthe central region, it is unlikely that dust plays a fundamental role inglobal color gradients. We found no correlation between color and O/Habundance gradients. This result could suggest that the color gradientsare more sensitive to the age rather than to the metallicity of thestellar population. However, the absence of this correlation may becaused by dust extinction. We discuss this result by considering apicture in which bars are a relatively fast, recurrent phenomenon. Theseresults are not compatible with a pure classical monolithic scenario forbulge and disk formation. On the contrary, they favor a scenario inwhich both these components are evolving in a correlated process inwhich stellar bars play a crucial role. Based partly on observationsmade at the Pico dos Dias Observatory (PDO/LNA-CNPq), Brazil.
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