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Boxy/peanut `bulges': comparing the structure of galaxies with the underlying families of periodic orbits
The vertical profiles of disc galaxies are built by the material trappedaround stable periodic orbits, which form their `skeletons'. Therefore,knowledge of the stability of the main families of periodic orbits inappropriate 3D models enables one to predict possible morphologies foredge-on disc galaxies. In a pilot survey we compare the orbitalstructures that lead to the appearance of `peanut'- and `X'-likefeatures with the edge-on profiles of three disc galaxies (IC 2531, NGC4013 and UGC 2048). The subtraction from the images of a modelrepresenting the axisymmetric component of the galaxies reveals thecontribution of the non-axisymmetric terms. We find a directcorrespondence between the orbital profiles of 3D bars in models and theobserved main morphological features of the residuals. We also apply asimple unsharp masking technique in order to study the sharpest featuresof the images. Our basic conclusion is that the morphology of the boxy`bulges' of these galaxies can be explained by considering disc materialtrapped around stable 3D periodic orbits. In most models, thesebuilding-block periodic orbits are bifurcated from the planar centralfamily of a non-axisymmetric component, usually a bar, at low-ordervertical resonances. In such a case, the boxy `bulges' are parts of barsseen edge-on. For the three galaxies we study, the families associatedwith the `peanut' or `X'-shape morphology are probably bifurcations atthe vertical 2/1 or 4/1 resonance.
| Discovery of counter-rotating gas in the galaxies NGC 1596 and 3203 and the incidence of gas counter-rotation in S0 galaxies
We have identified two new galaxies with gas counter-rotation (NGC 1596and 3203) and have confirmed similar behaviour in another one (NGC 128),this using results from separate studies of the ionized-gas and stellarkinematics of a well-defined sample of 30 edge-on disc galaxies. Gascounter-rotators thus represent 10 +/- 5 per cent of our sample, but thefraction climbs to 21 +/- 11 per cent when only lenticular (S0) galaxiesare considered and to 27 +/- 13 per cent for S0 galaxies with detectedionized gas only. Those fractions are consistent with but slightlyhigher than previous studies. A compilation from well-defined studies ofS0 galaxies in the literature yields fractions of 15 +/- 4 and 23 +/- 5per cent, respectively. Although mainly based on circumstantialevidence, we argue that the counter-rotating gas originates primarilyfrom minor mergers and tidally induced transfer of material from nearbyobjects. Assuming isotropic accretion, twice those fractions of objectsmust have undergone similar processes, underlining the importance of(minor) accretion for galaxy evolution. Applications of gascounter-rotators to barred galaxy dynamics are also discussed.
| Gas distribution, kinematics and star formation in faint dwarf galaxies
We compare the gas distribution, kinematics and the current starformation in a sample of 10 very faint (-13.37 < MB <-9.55) dwarf galaxies. For five of these galaxies we present fresh,high-sensitivity, Giant Metrewave Radio Telescope HI 21-cm observations.We find that the large-scale HI distribution in the galaxies istypically irregular and clumpy, with the peak gas density rarelyoccurring at the geometric centre. We also find that the velocity fieldsfor all the galaxies have an ordered component, although in general, thepatterns seen do not fit that expected from a rotating disc. For all ourgalaxies we construct maps of the HI column density at a constant linearresolution of ~300 pc; this forms an excellent data set to check for thepresence of a threshold column density for star formation. We find thatwhile current star formation (as traced by Hα emission) isconfined to regions with relatively large [NHI > (0.4-1.7)× 1021cm-2] HI column density, themorphology of the Hα emission is in general not correlated withthat of the high HI column density gas. Thus, while high column densitygas may be necessary for star formation, in this sample at least, it isnot sufficient to ensure that star formation does in fact occur. Weexamine the line profiles of the HI emission, but do not find a simplerelation between regions with complex line profiles and those withongoing star formation. Our sample includes examples of regions wherethere is ongoing star formation, but the profiles are well fitted by asingle Gaussian, as well as regions where there is no star formation butthe line profiles are complex. Finally, we examine the very fine scale(~20-100 pc) distribution of the HI gas, and find that at these scalesthe emission exhibits a variety of shell-like, clumpy and filamentaryfeatures. The Hα emission is sometimes associated withhigh-density HI clumps, sometimes the Hα emission lies inside ahigh-density shell, and sometimes there is no correspondence between theHα emission and the HI clumps. In summary, the interplay betweenstar formation and gas density in these galaxies does not seem to showthe simple large-scale patterns observed in brighter galaxies.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
| On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations
Objects designated as bulges in disc galaxies do not form a homogeneousclass. I distinguish three types: the classical bulges, the propertiesof which are similar to those of ellipticals and which form by collapseor merging; boxy and peanut bulges, which are seen in near-edge-ongalaxies and which are in fact just a part of the bar seen edge-on; and,finally, disc-like bulges, which result from the inflow of (mainly) gasto the centre-most parts, and subsequent star formation. I make adetailed comparison of the properties of boxy and peanut bulges withthose of N-body bars seen edge-on, and answer previously voicedobjections about the links between the two. I also present and analysesimulations where a boxy/peanut feature is present at the same time as aclassical spheroidal bulge, and compare them with observations. Finally,I propose a nomenclature that can help to distinguish between the threetypes of bulges and avoid considerable confusion.
| 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.
| Nearby early-type galaxies with ionized gas. I. Line-strength indices of the underlying stellar population
With the aim of building a data-set of spectral properties of wellstudied early-type galaxies showing emission lines, we presentintermediate resolution spectra of 50 galaxies in the nearby Universe.The sample, which covers several of the E and S0 morphologicalsub-classes, is biased toward objects that might be expected to haveongoing and recent star formation, at least in small amounts, because ofthe presence of the emission lines. The emission is expected to comefrom the combination of active galactic nuclei and star formationregions within the galaxies. Sample galaxies are located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. Our long-slit spectra coverthe 3700-7250 Å wavelength range with a spectral resolution of≈7.6 Å at 5550 Å. The specific aim of this paper, and ourfirst step in the investigation, is to map the underlying galaxy stellarpopulation by measuring, along the slit positioned along the galaxymajor axis, line-strength indices at several, homogeneousgalacto-centric distances. For each object we extracted 7luminosity-weighted apertures (with radii 1.5´´,2.5´´, 10´´, r_e/10, r_e/8, r_e/4 and r_e/2)corrected for the galaxy ellipticity and 4 gradients (0 ≤ r ≤r_e/16, r_e/16 ≤ r ≤ r_e/8, r_e/8 ≤ r ≤ r_e/4 and r_e/4≤ r ≤ r_e/2). For each aperture and gradient we measured 25line-strength indices: 21 of the set defined by the Lick-IDS“standard” system (Trager et al. [CITE], ApJS, 116, 1) and 4introduced by Worthey & Ottaviani ([CITE], ApJS, 111, 377).Line-strength indices have been transformed to the Lick-IDS system.Indices derived then include Hβ, Mg1, Mg2, Mgb, MgFe, Fe5270,Fe5335 commonly used in classic index-index diagrams. The paperintroduces the sample, presents the observations, describes the datareduction procedures, the extraction of apertures and gradients, thedetermination and correction of the line-strength indices, the procedureadopted to transform them into the Lick-IDS System and the proceduresadopted for the emission correction. We finally discuss the comparisonsbetween our dataset and line-strength indices available in theliterature. A significant fraction, about 60%, of galaxies in thepresent sample has one previous measurement in the Lick-IDS system butbasically restricted within the r_e/8 region. Line-strength measuresobtained both from apertures and gradients outside this area and withinthe r_e/8 region, with the present radial mapping, are completely new.Full appendix and Figs. 8 to 13 are only available in electronic form athttp://www.edpsciences.org Full Tables 6, 7, 9 and 10 are only availableat the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/497 Based onobservations obtained at the European Southern Observatory, La Silla,Chile (Programs Nr. 60.A-0647 and 61.A-0406).
| 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.
| BUDDA: A New Two-dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies
We present BUDDA (Bulge/Disk Decomposition Analysis), a new code devotedto perform a two-dimensional bulge/disk decomposition directly from theimages of galaxies. The bulge component is fitted with a generalizedSérsic profile, whereas disks have an exponential profile. Noother components are included. Bars and other substructures, likelenses, rings, inner bars, and inner disks, are studied with theresidual images obtained through the subtraction of bulges and disksfrom the original images. This means that a detailed structural analysisof galaxies may be performed with a small number of parameters, andsubstructures may be directly studied with no a priori assumptions. Ashas been already shown by several studies, two-dimensional fitting ismuch more reliable than one-dimensional profile fitting. Moreover, ourcode has been thoroughly tested with artificial data, and we demonstrateit to be an accurate tool for determining structural parameters ofgalaxies. We also show that our code is useful in various kinds ofstudies, including galaxies of, e.g., different morphological types, andinclinations, which also may be observed at different spatialresolutions. Thus, the code has a broader range of potentialapplications than most of the previous codes, which are developed totackle specific problems. To illustrate its usefulness, we present theresults obtained with a sample of 51 mostly early-type galaxies (butcovering the whole Hubble sequence). These results show some of theapplications in which the code may be used: the determination ofparameters for fundamental plane and structural studies, quantitativemorphological classification of galaxies, and the identification andstudy of hidden substructures. We have determined the structuralparameters of the galaxies in our sample and found many examples ofhidden inner disks in ellipticals, secondary bars, nuclear rings anddust lanes in lenticulars and spirals, and also wrong morphologicalclassification cases. We now make BUDDA generally available to theastronomical community.Based on observations made at the Pico dos Dias Observatory(PDO/LNA-CNPq), Brazil.
| 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.
| Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
| Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
| Evidence for a large stellar bar in the Low Surface Brightness galaxy UGC 7321
Late-type spiral galaxies are thought to be the dynamically simplesttype of disk galaxy and our understanding of their properties plays akey role in galaxy formation and evolution scenarios. The low surfacebrightness (LSB) galaxy UGC 7321, a nearby, isolated, ``superthin''edge-on galaxy, is an ideal object to study these purely disk-dominatedbulge-less galaxies. Although late type spirals are believed to exhibitthe simplest possible structure, even prior observations showeddeviations from a pure single component exponential disk in the case ofUGC 7321. We present for the first time photometric evidence forpeanut-shaped outer isophotes from a deep optical (R-band) image of UGC7321. Observations and dynamical modeling suggest thatboxy/peanut-shaped (b/p) bulges in general form through the bucklinginstability in bars of the parent galaxy disks. Together with recent HIobservations supporting the presence of a stellar bar in UGC 7321, thiscould be the earliest known case of the buckling process during theevolutionary life of a LSB galaxy, whereby material in the disk-bar hasstarted to be pumped up above the disk, but a genuine bulge has not yetformed.Based on observations obtained at the German-Spanish Astronomical Center(DSAZ), Calar Alto, jointly operated by the Max-Planck-Institut fürAstronomie Heidelberg and the Spanish National Commission for Astronomy.
| Orbital dynamics of three-dimensional bars - III. Boxy/peanut edge-on profiles
We present families, and sets of families, of periodic orbits thatprovide building blocks for boxy and peanut (hereafter b/p) edge-onprofiles. We find cases where the b/p profile is confined to the centralparts of the model and cases where a major fraction of the barparticipates in this morphology. A b/p feature can be built either by 3Dfamilies associated with 3D bifurcations of the x1 family, or, in somemodels, even by families related with the z-axis orbits and existingover large energy intervals. The `X'feature observed inside the boxy bulges of several edge-on galaxies canbe attributed to the peaks of successive x1v1 orbits, provided theirstability allows it. However in general, the x1v1 family has to overcomethe obstacle of a S ->Δ-> S transition in order to supportthe structure of a b/p feature. Other families that can be the backbonesof b/p features are x1v4 and z3.1s. The morphology and the size of theboxy or peanut-shaped structures we find in our models are determined bythe presence and stability of the families that support b/p features.The present study favours the idea that the observed edge-on profilesare the imprints of families of periodic orbits that can be found inappropriately chosen Hamiltonian systems, describing the potential ofthe bar.
| 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.
| 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
| A catalogue and analysis of X-ray luminosities of early-type galaxies
We present a catalogue of X-ray luminosities for 401 early-typegalaxies, of which 136 are based on newly analysed ROSAT PSPC pointedobservations. The remaining luminosities are taken from the literatureand converted to a common energy band, spectral model and distancescale. Using this sample we fit the LX:LB relationfor early-type galaxies and find a best-fit slope for the catalogue of~2.2. We demonstrate the influence of group-dominant galaxies on the fitand present evidence that the relation is not well modelled by a singlepower-law fit. We also derive estimates of the contribution to galaxyX-ray luminosities from discrete-sources and conclude that they provideLdscr/LB~=29.5ergs-1LBsolar-1. Wecompare this result with luminosities from our catalogue. Lastly, weexamine the influence of environment on galaxy X-ray luminosity and onthe form of the LX:LB relation. We conclude thatalthough environment undoubtedly affects the X-ray properties ofindividual galaxies, particularly those in the centres of groups andclusters, it does not change the nature of whole populations.
| Arp 220: A Circumnuclear Polar Ring as an Alternative to a Double Nucleus?
Recent millimeter interferometer observations of Arp 220 have yielded~0.5" resolution images showing two strong concentrations of millimeterradio continuum and CO line emission embedded in a larger molecular gasdisk (r~1 kpc). The interferometer observations also revealed a complexvelocity field with steep velocity gradients across each of the fluxconcentrations of Δv~500 km s-1 within r=0.3". Thedirections of these gradients are not aligned with each other or withthat of the outer gas disk. This led to the conclusion that the twoemission peaks represent either double nuclei with their own gas disks(r~100 pc), which are counterrotating with respect to each other androtate around the dynamical center of the system, or that they are twohot spots within a nuclear molecular gas disk. The overall structure ofthe molecular gas distribution and the corresponding complex velocityfield, however, are highly symmetric, except for the unequalbrightnesses of the two flux concentrations. This fact and similaritiesto the distribution and kinematics of the molecular gas in the central2'' of NGC 1068 motivated us to try describing the Arp 220nucleus as a warped molecular gas disk in which the peaks in the fluxand line width distributions arise as a result of crowding of inclinedcircular orbits. Hubble Space Telescope (HST) images of the dust lanesflaring toward larger radii provide further support for a warped gasdisk. The final model is surprisingly successful in explaining the COline flux distribution, the velocity field, and position-velocitydiagrams across the central flux concentrations. The model represents astrong 90deg warp of the gas orbits out of the principalplane forming a circumnuclear polar ring and reproduces well thestructures in HST near-IR color and continuum maps. The large number ofyoung stars recently formed out of the warped, polar-ring gas impliesthat those stars will have kinematic properties similar to that of themolecular gas. We suggest that the red near-IR source at the center ofthe polar ring is the true nucleus of Arp 220. The success of the modelimplies that the two flux concentrations are not necessarilycounterrotating nuclei, but rather the result of a combination of hotspots and orbit crowding due to the warp. The warp could be a directconsequence of the recent merger event indicated by tidal tails inoptical images.
| The gas content of peculiar galaxies: Counterrotators and polar rings
This paper studies the global ISM content in a sample of 104 accretinggalaxies, including counterrotators and polar rings, which spans theentire Hubble sequence. The molecular, atomic and hot gas content ofaccretors is compared to a newly compiled sample of normal galaxies. Wepresent results of a small survey of the J=1-0 line of 12COwith the 15 m SEST telescope on a sample of 11 accretors (10counterrotators and 1 polar ring). The SEST sample is enlarged withpublished data from 48 galaxies, for which observational evidence ofcounterrotation in the gas and/or the stars has been found. Furthermore,the available data on a sample of 46 polar ring galaxies has beencompiled. In order to explore the existence of an evolutionary pathlinking the two families of accretors, the gas content ofcounterrotators and polar rings is compared. It was found that thenormalized content of cold gas (Mgas/LB) in polarrings is ~ 1 order of magnitude higher than the reference value derivedfor normal galaxies. The inferred gas masses are sufficient to stabilizepolar rings through self-gravity. In contrast, it was found that thecold gas content of counterrotators is close to normal for all galaxytypes. Although counterrotators and polar rings probably share a commonorigin, the gas masses estimated here confirm that light gas ringsaccreted by future counterrotators may have evolved faster than theself-gravitating structures of polar rings. In this scenario, thetransformation of atomic into molecular gas could be enhanced near thetransition region between the prograde and the retrograde disks,especially in late-type accretors characterized by a high content ofprimordial gas. This is tentatively confirmed in this work: the measuredH2/HI ratio seems larger in counterrotators than in normal orpolar ring galaxies for types later than S0s. Based on observationscollected at SEST telescope, European Southern Observatory, La Silla,Chile. Table 1 is only available in electronic form athttp://www.edpsciences.org
| The visible environment of galaxies with counterrotation
In this paper we present a statistical study of the environments of 49galaxies in which there is gas- or stellar-counterrotation. The numberof possible companions in the field (to apparent magnitude 22), theirsize and concentration were considered. All the statistical parameterswere analysed by means of Kolgomorov-Smirnov tests, using a controlsample of 43 galaxies without counterrotation. From our data, nosignificant differences between the counter-rotating and control samplesappear. This is different to Seyfert or radio-loud galaxies which lie inenvironments with a higher density of companions. On the contrary, if aweak tendency exists, for galaxies with gas counterrotation only, it isdiscovered in regions of space where the large scale density of galaxiesis smaller. Our results tend to disprove the hypothesis thatcounterrotation and polar rings derive from a recent interaction with asmall satellite or a galaxy of similar size. To a first approximation,they seem to follow the idea that all galaxies are born through a mergerprocess of smaller objects occurring very early in their life, or thatthey derive from a continuous, non-traumatic infall of gas that formedstars later. Whatever the special machinery is which producescounterrotation or polar rings instead of a co-planar, co-rotatingdistribution of gas and stars, it seems not to be connected to thepresent galaxy density of their environments.
| Properties of tidally-triggered vertical disk perturbations
We present a detailed analysis of the properties of warps andtidally-triggered perturbations perpendicular to the plane of 47interacting/merging edge-on spiral galaxies. The derived parameters arecompared with those obtained for a sample of 61 non-interacting edge-onspirals. The entire optical (R-band) sample used for this study waspresented in two previous papers. We find that the scale height of disksin the interacting/merging sample is characterized by perturbations onboth large ( =~ disk cut-off radius) and short ( =~ z0)scales, with amplitudes of the order of 280 pc and 130 pc on average,respectively. The size of these large (short) -scale instabilitiescorresponds to 14% (6%) of the mean disk scale height. This is a factorof 2 (1.5) larger than the value found for non-interacting galaxies. Ahallmark of nearly all tidally distorted disks is a scale height thatincreases systematically with radial distance. The frequent occurrenceand the significantly larger size of these gradients indicate that diskasymmetries on large scales are a common and persistent phenomenon,while local disturbances and bending instabilities decline on shortertimescales. Nearly all (93%) of the interacting/merging and 45% of thenon-interacting galaxies studied are noticeably warped. Warps ofinteracting/merging galaxies are ~ 2.5 times larger on average thanthose observed in the non-interacting sample, with sizes of the order of340 pc and 140 pc, respectively. This indicates that tidal distortionsdo considerably contribute to the formation and size of warps. However,they cannot entirely explain the frequent occurrence of warped disks.Based on observations obtained at the European Southern Observatory(ESO, La Silla, Chile), Calar Alto Observatory operated by the MPIA(DSAZ, Spain), Lowell Observatory (Flagstaff,AZ, USA), and Hoher ListObservatory (Germany).
| 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.
| Box- and peanut-shaped bulges. I. Statistics
We present a classification for bulges of a complete sample of ~ 1350edge-on disk galaxies derived from the RC3 (Third Reference Catalogue ofBright Galaxies, de Vaucouleurs et al. \cite{rc3}). A visualclassification of the bulges using the Digitized Sky Survey (DSS) inthree types of b/p bulges or as an elliptical type is presented andsupported by CCD images. NIR observations reveal that dust extinctiondoes almost not influence the shape of bulges. There is no substantialdifference between the shape of bulges in the optical and in the NIR.Our analysis reveals that 45% of all bulges are box- and peanut-shaped(b/p). The frequency of b/p bulges for all morphological types from S0to Sd is > 40%. In particular, this is for the first time that such alarge frequency of b/p bulges is reported for galaxies as late as Sd.The fraction of the observed b/p bulges is large enough to explain theb/p bulges by bars. Partly based on observations collected at ESO/LaSilla (Chile), DSAZ/Calar Alto (Spain), and Lowell Observatory/Flagstaff(AZ/U.S.A.). Tables 6 and 7 are only available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The influence of interactions and minor mergers on the structure of galactic disks I. Observations and disk models
This paper is the first part in our series on the influence of tidalinteractions and minor mergers on the radial and vertical disk structureof spiral galaxies. We report on the sample selection, our observations,and data reduction. Surface photometry of the optical and near infrareddata of a sample of 110 highly-inclined/edge-on disk galaxies arepresented. This sample consists of two subsamples of 61 non-interactinggalaxies (control sample) and of 49 interacting galaxies/minor mergingcandidates. Additionally, 41 of these galaxies were observed in the nearinfrared. We show that the distribution of morphological types of bothsubsamples is almost indistinguishable, covering the range between 0<= T <= 9. An improved, 3-dimensional disk modelling- and fittingprocedure is described in order to analyze and to compare the diskstructure of our sample galaxies by using characteristic parameters. Wefind that the vertical brightness profiles of galactic disks respondvery sensitive even to small deviations from the perfect edge-onorientation. Hence, projection effects of slightly inclined disks maycause substantial changes in the value of the disk scale height and musttherefore be considered in the subsequent study. Based on observationsobtained at the European Southern Observatory (ESO, La Silla, Chile),Calar Alto Observatory operated by the MPIA (DSAZ, Spain), LowellObservatory (Flagstaff/AZ, USA), and Hoher List Observatory (Germany).
| Box- and peanut-shaped bulges. II. NIR observations
We have observed 60 edge-on galaxies in the NIR in order to study thestellar distribution in galaxies with box/peanut-shaped bulges. The muchsmaller amount of dust extinction at these wavelengths allows us toidentify in almost all target galaxies with box/peanut-shaped bulges anadditional thin, central component in cuts parallel to the major axis.This structure can be identified with a bar. The length of thisstructure scaled by the length of the bulge correlates with themorphologically classified shape of the bulge. This newly establishedcorrelation is therefore mainly interpreted as the projection of the barat different aspect angles. Galaxies with peanut bulges have a bar seennearly edge-on and the ratio of bar length to thickness, 14 +/- 4, canbe directly measured for the first time. In addition, the correlation ofthe boxiness of bulges with the bar strength indicates that the barcharacteristic could partly explain differences in the bulge shape.Furthermore, a new size relation between the box/peanut structure andthe central bulge is found. Our observations are discussed in comparisonto a N-body simulation for barred galaxies (Pfenniger & Friedli\cite{pfe}). We conclude that the inner region of barred disk galaxiesare build up by three distinct components: the spheroidal bulge, a thinbar, and a b/p structure most likely representing the thick part of thebar. Based on observations collected at ESO/La Silla (61.A-0143),DSAZ/Calar Alto, and TIRGO/Gornergrat.}
| The influence of interactions and minor mergers on the structure of galactic disks. II. Results and interpretations
We present the second part of a detailed statistical study focussed onthe effects of tidal interactions and minor mergers on the radial andvertical disk structure of spiral galaxies. In the first part wereported on the sample selection, observations, and applied disk models.In this paper the results are presented, based on disk parametersderived from a sample of 110 highly-inclined/edge-on galaxies. Thissample consists of two subsamples of 49 interacting/merging and 61non-interacting galaxies. Additionally, 41 of these galaxies wereobserved in the NIR. We find significant changes of the disk structurein vertical direction, resulting in ~ 1.5 times larger scale heights andthus vertical velocity dispersions. The radial disk structure,characterized by the cut-off radius and the scale length, shows nostatistically significant changes. Thus, the ratio of radial to verticalscale parameters, h/z0, is ~ 1.7 times smaller for the sampleof interacting/merging galaxies. The total lack of typical flat diskratios h/z0 > 7 in the latter sample implies that verticaldisk heating is most efficient for (extremely) thin disks. Statisticallynearly all galactic disks in the sample (93%) possess non-isothermalvertical luminosity profiles like the sech (60%) and exp (33%)distribution, independent of the sample and passband investigated. Thisindicates that, in spite of tidal perturbations and disk thickening, theinitial vertical distribution of disk stars is not destroyed byinteractions or minor mergers. Based on observations obtained at theEuropean Southern Observatory (ESO, La Silla, Chile), Calar AltoObservatory operated by the MPIA (DSAZ, Spain), Lowell Observatory(Flagstaff/AZ, USA), and Hoher List Observatory (Germany).
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| The NICMOS Snapshot Survey of Nearby Galaxies
We present ``snapshot'' observations with the Near-Infrared Camera andMulti-Object Spectrometer (NICMOS) on board the Hubble Space Telescope(HST) of 94 nearby galaxies from the Revised Shapley Ames Catalog.Images with 0.2" resolution were obtained in two filters, a broadbandcontinuum filter (F160W, roughly equivalent to the H band) and anarrowband filter centered on the Paα line (F187N or F190N,depending on the galaxy redshift) with the 51^''x51^'' field of view ofthe NICMOS camera 3. A first-order continuum subtraction is performed,and the resulting line maps and integrated Paα line fluxes arepresented. A statistical analysis indicates that the average Paαsurface brightness in the central regions is highest in early-type(Sa-Sb) spirals.
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