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K-band observations of boxy bulges - I. Morphology and surface brightness profiles
In this first paper of a series on the structure of boxy andpeanut-shaped (B/PS) bulges, Kn-band observations of a sample of 30edge-on spiral galaxies are described and discussed. Kn-bandobservations best trace the dominant luminous galactic mass and areminimally affected by dust. Images, unsharp-masked images, as well asmajor-axis and vertically summed surface brightness profiles arepresented and discussed. Galaxies with a B/PS bulge tend to have a morecomplex morphology than galaxies with other bulge types, more oftenshowing centred or off-centred X structures, secondary maxima along themajor-axis and spiral-like structures. While probably not uniquelyrelated to bars, those features are observed in three-dimensional N-bodysimulations of barred discs and may trace the main bar orbit families.The surface brightness profiles of galaxies with a B/PS bulge are alsomore complex, typically containing three or more clearly separatedregions, including a shallow or flat intermediate region (Freeman TypeII profiles). The breaks in the profiles offer evidence for bar-driventransfer of angular momentum and radial redistribution of material. Theprofiles further suggest a rapid variation of the scaleheight of thedisc material, contrary to conventional wisdom but again as expectedfrom the vertical resonances and instabilities present in barred discs.Interestingly, the steep inner region of the surface brightness profilesis often shorter than the isophotally thick part of the galaxies, itselfalways shorter than the flat intermediate region of the profiles. Thesteep inner region is also much more prominent along the major-axis thanin the vertically summed profiles. Similarly to other recent work butcontrary to the standard `bulge + disc' model (where the bulge is boththick and steep), we thus propose that galaxies with a B/PS bulge arecomposed of a thin concentrated disc (a disc-like bulge) containedwithin a partially thick bar (the B/PS bulge), itself contained within athin outer disc. The inner disc likely formed secularly throughbar-driven processes and is responsible for the steep inner region ofthe surface brightness profiles, traditionally associated with a classicbulge, while the bar is responsible for the flat intermediate region ofthe surface brightness profiles and the thick complex morphologicalstructures observed. Those components are strongly coupled dynamicallyand are formed mostly of the same (disc) material, shaped by the weakbut relentless action of the bar resonances. Any competing formationscenario for galaxies with a B/PS bulge, which represent at least 45 percent of the local disc galaxy population, must explain equally well andself-consistently the above morphological and photometric properties,the complex gas and stellar kinematics observed, and the correlationsbetween them.
| 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.
| 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.
| The Tully-Fisher Relation for Hickson Compact Groups
We investigate the properties of the B-band Tully-Fisher (TF) relationfor 25 compact group galaxies, using Vmax derived from 2-Dvelocity maps. Our main result is that the majority of the HicksonCompact Group (HCG) galaxies lie on the TF relation, although with largescatter. However, 20% of the galaxies, including the lowest-masssystems, seem to have higher B luminosities, for a given mass, oralternatively, a mass which is too low for their luminosities. We favourthe scenario of brightening of the outliers due to either enhanced starformation or merging, rather than truncation of the dark halo due tointeractions, to explain the position of the outliers on the TFrelation.
| The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies
Using a sample of 91 galaxies distributed over 27 compact groups (CGs)of galaxies, we define an index that allows us to quantify their levelof activity due to an active galactic nucleus (AGN) or star formation.By combining the mean activity index with the mean morphological type ofthe galaxies in a group, we are able to quantify the evolutionary stateof the groups. We find that they span an evolutionary sequence thatcorrelates with the spatial configuration of the galaxies in the CG. Wedistinguish three main configuration types: A, B, and C. Type A CGs showpredominantly low velocity dispersions and are rich in late-type spiralsthat show active star formation or harbor an AGN. Type B groups haveintermediate velocity dispersions and contain a large fraction ofinteracting or merging galaxies. Type C comprises CGs with high velocitydispersions, which are dominated by elliptical galaxies that show noactivity. We suggest that evolution proceeds A==>B==>C. Mappingthe groups with different evolution levels in a diagram of radius versusvelocity dispersion does not reveal the pattern expected based on theconventional fast merger model for CGs, which predicts a direct relationbetween these two parameters. Instead, we observe a trend contrary toexpectation: the evolutionary state of a group increases with velocitydispersion. This trend seems to be related to the masses of thestructures in which CGs are embedded. In general, the evolutionary stateof a group increases with the mass of the structure. This suggestseither that galaxies evolve more rapidly in massive structures or thatthe formation of CGs embedded in massive structures predated theformation of CGs associated with lower mass systems. Our observationsare consistent with the structure formation predicted by the CDM model(or ΛCDM), only if the formation of galaxies is a biased process.
| 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.
| Dynamical Effects of Interactions and the Tully-Fisher Relation for Hickson Compact Groups
We investigate the properties of the B-band Tully-Fisher (T-F) relationfor 25 compact group galaxies, using Vmax derived fromtwo-dimensional velocity maps. Our main result is that the majority ofthe Hickson Compact Group galaxies lie on the T-F relation. However,about 20% of the galaxies, including the lowest-mass systems, havehigher B luminosities for a given mass, or alternatively, a mass that istoo low for their luminosities. We favor a scenario in which outliershave been brightened because of either enhanced star formation ormerging. Alternatively, the T-F outliers may have undergone truncationof their dark halo due to interactions. It is possible that in somecases both effects contribute. The fact that the B-band T-F relation issimilar for compact group and field galaxies tells us that thesegalaxies show common mass-to-size relations and that the halos ofcompact group galaxies have not been significantly stripped insideR25. We find that 75% of the compact group galaxies studied(22 out of 29) have highly peculiar velocity fields. Nevertheless, acareful choice of inclination, position angle, and center, obtained fromthe velocity field, and an average of the velocities over a large sectorof the galaxy enabled the determination of fairly well-behaved rotationcurves for the galaxies. However, two of the compact group galaxies HCG91a and HCG 96a, which are the most massive members in M51-like pairs,have very asymmetric rotation curves, with one arm rising and the otherone falling, indicating most probably a recent perturbation by the smallclose companions.
| Gas kinematics of a sample of five Hickson Compact Groups. The data
We present Fabry Perot observations of galaxies in five Hickson CompactGroups: HCG 10, HCG 19, HCG 87, HCG 91 and HCG 96. We observed a totalof 15 galaxies and we have detected ionized gas for 13 of them. We wereable to derive 2D velocity, monochromatic, continuum maps and rotationcurves for the 13 objects. Almost all galaxies in this study arelate-type systems; only HCG 19a is an elliptical galaxy. We can see atrend of kinematic evolution within a group and among different groups -from strongly interacting systems to galaxies with no signs ofinteraction - HCG 10 exhibits at least one galaxy strongly disturbed;HCG 19 seems to be quite evolved; HCG 87 does not show any sign ofmerging but a possible gas exchange between two galaxies; HCG 91 couldbe accreting a new intruder; HCG 96 exhibits past and presentinteractions.Based on observations collected at the European Southern Observatory, LaSilla, Chile and at the Canada-France-Hawaii Observatory, Hawaii, USA.Tables 1 and 2, Figs. 1, 3, 5, 7, 9, 11, 13, 15, 23 are only availablein electronic form at http://www.edpsciences.org
| Observaciones CCD de Grupos Compactos de Hickson
Not Available
| A Dynamical Study of Galaxies in the Hickson Compact Groups
To investigate dynamical properties of spiral galaxies in the Hicksoncompact groups (HCGs), we present rotation curves of 30 galaxies in 20HCGs. We found as follows: (1) There is no significant relation betweendynamical peculiarity and morphological peculiarity in HCG spiralgalaxies. (2) There is no significant relation between the dynamicalproperties and the frequency distribution of nuclear activities in HCGspiral galaxies. (3) There are no significant correlations between thedynamical properties of HCG spiral galaxies and any group properties(i.e., size, velocity dispersion, galaxy number density, and crossingtime). (4) Asymmetric and peculiar rotation curves are more frequentlyseen in the HCG spiral galaxies than in field spiral galaxies or incluster ones. However, this tendency is more obviously seen in late-typeHCG spiral galaxies. These results suggest that the dynamical propertiesof HCG spiral galaxies do not strongly correlate with the morphology,the nuclear activity, and the group properties. Our results also suggestthat more frequent galaxy collisions occur in the HCGs than in the fieldand in the clusters.
| 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.}
| 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
| Southern Isolated Galaxy Triplets
Seventy-six isolated triple systems of galaxies with declinatiosnδ<-3° were selected using ESO/SERC and POSS-I sky surveydata. The equatorial coordinates, configuration types, angular sizes,component angular separations, component morphological types, totalmagnitudes, and other parameters are reported for each triplet.Radial-velocity estimates are available for all components in 33 of the76 triplets. The median values of the main dynamicalparametersradial-velocity dispersion, mean harmonic radius,absolute magnitudes of member galaxies, and mass-to-luminosityratiosare similar to those obtained earlier for 83 isolatedtriple systems with δ>-3°.
| The Nuclear Activity of Galaxies in the Hickson Compact Groups
In order to investigate the nuclear activity of galaxies residing incompact groups of galaxies, we present results of our opticalspectroscopic program made at Okayama Astrophysical Observatory. We haveperformed optical spectroscopy of 69 galaxies belonging to 31 Hicksoncompact groups (HCGs) of galaxies. Among them, three galaxies havediscordant redshifts and, moreover, spectral quality is too poor toclassify another three galaxies. Therefore, we describe our results forthe remaining 63 galaxies. Our main results are summarized as follows:(1) We have found in our sample 28 active galactic nuclei (AGNs), 16 HII nuclei, and 19 normal galaxies showing no emission line. We used thisHCG sample for statistical analyses. (2) Comparing the frequencydistributions of activity types between the HCGs and the field galaxieswhose data are taken from Ho, Filippenko, & Sargent (382 fieldgalaxies), we find that the frequency of H II nuclei in the HCGs issignificantly less than that in the field. However, this difference maybe due to selection bias to the effect that our HCG sample contains moreearly-type galaxies than the field, because it is known that H II nucleiare rarer in early-type galaxies than in later ones. (3) Applying acorrection to this morphological bias to the HCG sample, we find thatthere is no statistically significant difference in the frequency ofoccurrence of emission-line galaxies between the HCGs and the field.This implies that the dense galaxy environment in the HCGs does notaffect the triggering of either the AGN activity and the nuclearstarburst. We discuss some implications on the nuclear activity in theHCG galaxies.
| The Nature of Boxy/Peanut-Shaped Bulges in Spiral Galaxies
We present a systematic observational study of the relationship betweenbars and boxy/peanut-shaped (B/PS) bulges. We first review and discussproposed mechanisms for their formation, focusing on accretion andbar-buckling scenarios. Using new methods relying on the kinematics ofedge-on disks, we then look for bars in a large sample of edge-on spiralgalaxies with a B/PS bulge and in a smaller control sample of edge-onspirals with more spheroidal bulges. We present position-velocitydiagrams of the ionized gas obtained from optical long-slitspectroscopy. We show that almost all B/PS bulges are due to a thick barviewed edge-on, while only a few extreme cases may be due to theaccretion of external material. This strongly supports the bar-bucklingmechanism for the formation of B/PS bulges. None of the galaxies in thecontrol sample show evidence for a bar, which suggests conversely thatbars are generally B/PS.We consider the effects of dust in the disk ofthe galaxies but conclude that it does not significantly affect ourresults. Unusual emission-line ratios correlating with kinematicalstructures are observed in many objects, and we argue that this isconsistent with the presence of strong bars in the disk of the galaxies.As expected from N-body simulations, the boxy-peanut transition appearsto be related to the viewing angle, but more work is required to derivethe precise orientation of the bars in the bulges. The reliableidentification of bars in edge-on spiral galaxies opens up for the firsttime the possibility of studying observationally the vertical structureof bars.
| The Evolution of Galaxies in Compact Groups
We present an analysis of the spectra of 62 galaxies in 15 compactgroups. The galaxies are classified into four activity classes: galaxieswithout emission, starburst galaxies, luminous AGNs (Seyfert andLINERs), and low-luminosity AGNs (LLAGNs). The star formation in theHickson compact group (HCG) starbursts is more intense than in normalspirals, but comparable to that observed in starburst-nucleus galaxies(SBNGs) in the field. In general, the HCG starbursts have mean solar gasmetallicity and do not follow the metallicity-luminosity relation tracedby the early-type SBNGs in the field, suggesting that most of them arelate-type SBNGs. This morphology preference, coupled with theobservation that the HCG starbursts are predominantly located in thehalos of the groups, is consistent with the idea that compact groups areembedded in sparser structures. The stellar metallicities of thenonstarburst galaxies are comparable to those observed in normalgalaxies with similar morphologies, but are relatively high for theirluminosities. In these galaxies, the metal absorption line equivalentwidths are slightly narrower than normal, while the Balmer absorptionlines are relatively strong. All these observations suggest the presenceof a population of intermediate-age stars. These galaxies could bepoststarburst, but at a very advanced stage of evolution, the lastbursts having happened more than 2 Gyr in the past. Our observationssupport a scenario in which the cores of the groups are slowlycollapsing evolved systems embedded in more extended structures. In thecores of the groups, the interactions were more frequent and thegalaxies evolved at a more rapid rate than in their halos.
| Effects of Interaction-induced Activities in Hickson Compact Groups: CO and Far-Infrared Study
A study of 2.6 mm CO J = 1 --> 0 and far-infrared (FIR) emission in adistance-limited (z < 0.03) complete sample of Hickson compact group(HCG) galaxies was conducted in order to examine the effects of theirunique environment on the interstellar medium of component galaxies andto search for a possible enhancement of star formation and nuclearactivity. Ubiquitous tidal interactions in these dense groups wouldpredict enhanced activities among the HCG galaxies compared to isolatedgalaxies. Instead, their CO and FIR properties (thus, "star formationefficiency") are surprisingly similar to isolated spirals. The CO datafor 80 HCG galaxies presented here (including 10 obtained from theliterature) indicate that the spirals globally show the same H2 contentas the isolated comparison sample, although 20% are deficient in COemission. Because of their large optical luminosity, low metallicity isnot likely the main cause for the low CO luminosity. The CO deficiencyappears linked with the group evolution, and gas exhaustion through paststar formation and removal of the external gas reserve by tidalstripping of the outer H I disk offer a possible explanation. The IRASdata for the entire redshift-limited complete sample of 161 HCG galaxieswere reanalyzed using ADDSCAN/SCANPI, improving the sensitivity by afactor of 3-5 over the existing Point Source Catalog (PSC) and resolvingbetter the contribution from individual galaxies. The new analysis ofthe IRAS data confirms the previous suggestion that FIR emission in HCGgalaxies is similar to isolated, Virgo Cluster, and weakly interactinggalaxies. Their H2 and FIR characteristics yield a star formationefficiency that is similar to that of these comparison samples. A factor2 enhancement in the 25-100 mu m flux ratio among the HCG spirals isfound, which suggests intense localized nuclear starburst activitysimilar to that of H II galaxies. A number of early-type galaxies inHCGs are detected in CO and FIR, lending further support to the ideathat tidal interactions and tidally induced evolution of the groups andmember galaxies are important in our sample.
| Structural and Dynamical Analysis of the Hickson Compact Groups
Based on the spectroscopic survey of de Carvalho et al., we analyze thestructural and dynamical properties of 17 Hickson compact groups. Thisanalysis probes a region of 0.dg5 x 0.dg5 around each group and showsthat most of them are part of larger structures. Our results alsosuggest that the Hickson sample is composed of different dynamicalstages of the groups" evolution. Specifically, we identify threepossible evolutionary phases among groups in the sample: loose groups,core + halo systems, and compact groups, each one presenting a distinctsurface density profile. This sequence is consistent with thereplenishment scenario for the formation and evolution of compact groupswithin larger and less dense systems.
| The Nature of the Activity in Hickson Compact Groups of Galaxies
We present the results of the spectral classification of the 82brightest galaxies in a sample of 17 compact groups. We verify that theactive galactic nuclei (AGNs) are preferentially located in the mostearly-type and luminous galaxies of the groups, as is usually observedin the field. But these AGNs also appear to be systematicallyconcentrated toward the central parts of the groups. Our observationssuggest a correlation between activity types, morphologies, anddensities of galaxies in the compact groups. This is consistent with ascenario in which galaxies of compact groups evolve by interacting withtheir environment and are currently in a quiet phase of their activity.
| Redshift Survey of Galaxies around a Selected Sample of Compact Groups
We report the results of a spectroscopic survey of faint galaxies in theregions surrounding Hickson compact groups. Our sample is composed of 17groups within 9000 km s-1. The spectra were taken at the prime focus ofthe Tololo 4 m telescope, using the ARGUS fiber-fed spectrograph. Fromthese observations, redshifts were determined for the faint galaxiespreviously identified by de Carvalho, Ribeiro, & Zepf in thesurroundings of the groups. Statistical methods were applied to theresultant catalog in order to determine the kinematical structure ofeach group. This analysis confirms the idea that the Hickson sample ofcompact groups contains a wide variety of projection and dynamicalconfigurations. Our results demonstrate the necessity of newspectroscopic surveys around compact groups in order to assess theircomplete velocity distribution.
| The molecular gas content of spiral galaxies in compact groups.
We present ^12^CO(J=1-0) line observations of 15 late-type opticallyselected galaxies in compact groups. In order to study the effects ofenvironment on the molecular content of compact group galaxies we usethe CO(1-0) emission to estimate an indicative molecular hydrogencontent of the target galaxies adopting a standard conversion factorX=N(H_2_)/I(CO). We compare the frequency distribution of the normalizedhybrid molecular gas surface density to the one obtained for anoptically selected reference sample of isolated galaxies. Despite theirhigh-density environment, compact group galaxies have, on average,similar CO emission compared to isolated galaxies of same sizes andluminosities. However there are some compact group galaxies withperturbed morphologies and high far-infrared fluxes which have anenhanced CO emissivity.
| Quantifying the Fragility of Galactic Disks in Minor Mergers
We perform fully self-consistent stellar dynamical simulations of theaccretion of a companion ("satellite") galaxy by a large disk galaxy toinvestigate the interaction between the disk, halo, and satellitecomponents of the system during a merger. Our fiducial encounter beginswith a satellite in a prograde, circular orbit inclined 30^deg^ withrespect to the disk plane at a galactocentric distance of 6 disk scalelengths. The satellite's mass is 10% of the disk's mass, and its half-mass radius is ~1.3 kpc. The system is modeled with 500,000 particles,which is sufficient to mitigate numerical relaxation noise over themerging time. The satellite sinks in only ~1 Gyr and a core containing45% of its initial mass reaches the center of the disk. With so much ofthe satellite's mass remaining intact, the disk sustains significantdamage as the satellite passes through. At the solar circle we find thatthe disk thickens 60%, the velocity dispersions increase by{DELTA}σ ~ (10,8,8) km s^-1^ to (σ_R_,σphi_, σ_z_) ~ (48,42,38) km s^-1^, and theasymmetric drift is unchanged at ~18 km s^-1^. Although the disk is notdestroyed by these events (hence "minor" mergers), its final stateresembles a disk galaxy of an earlier Hubble type than its initialstate-thicker and hotter, with the satellite's core enhancing the bulge.Thus minor mergers continue to be a promising mechanism for drivinggalaxy evolution.
| A Merger Origin for X Structures in S0 Galaxies
Using numerical simulation, we study the response of a disk galaxy to amerger involving a low-mass satellite companion. During a progradesatellite accretion, the disk galaxy forms a strong bar in response tothe perturbation of the companion. After the accretion event is over,the bar buckles vertically due to a bending instability, sending diskmaterial well out of the disk plane. The material forms into an X-shapedfeature when seen edge-on, morphologically similar to X structuresobserved in several S0/Sa galaxies. The mechanism described here unitespreviously suggested accretion and bar instability hypotheses forforming X structures into a self-consistent scenario for merger-inducedformation and evolution of S0 galaxies. To complement these models, wealso present observations of the peculiar S0 galaxy Hickson 87a. Thethick disk, isophotal warping, and strong X structure described in themerger model are all evident in Hickson 87a, suggesting this galaxy maybe an excellent example of such merger-induced galaxy evolution.
| Forming X-shapes in Disk Galaxies Through Minor Mergers
We have run fully self-consistent stellar dynamical simulations of theaccretion of a small galaxy by a large disk galaxy and found that anX-shaped structure forms in the disk after the merger is complete. Wewish to emphasize that this structure consists mostly of disk materialrather than accreted material. The result is morphologically similar tosuch peculiar S0 galaxies as NGC 128, NGC 7332, Hickson 87A, and IC4767, an assertion quantitatively supported by the ``a_4'' (cos 4 theta) coefficient. A possible evolutionary link between spiral and S0galaxies is suggested.
| Morphology of galaxies in compact groups
We present the results of an isophotal analysis of 140 early-typegalaxies and a visual inspection of images of an additional 202 galaxiesin compact groups. This is essentially the entire sample of galaxies inthe subset of 92 Hickson compact groups which have at least threeaccordant members. About 12% of the elliptical galaxies have largercharacteristic radii and shallower surface brightness profiles thangalaxies of the same luminosity in less dense environments. The averageellipticity of elliptical galaxies in compact groups is a slowlyincreasing function of the metric radius, as it is for field andloose-group galaxies. No alignment is found among the major axes of thegalaxies and the major axis of the group. When combined with previouslypublished morphological, kinematic, radio, infrared, and colorinformation on the same galaxies, our data show that 43% of the galaxiesin the compact group sample show morphological and/or kinematicaldistortions indicative of interactions and/or mergers. About 32% of thegroups have three or more galaxies which show some sign of interaction.This is a lower limit, since for the great majority of the galaxies inthe groups, only imaging and low-resolution spectra are available. Forthe subsample of 16 groups for which published detailed kinematical dataare also available, the fraction of groups with three or more galaxiesin interaction is 75%. No correlation is found between the number ofinteracting galaxies in a group and the group velocity dispersion orcrossing time. These observations strongly support the view that compactgroups are systems of physically associated galaxies and not chancealignments of field, loose-group, or cluster galaxies. They also confirmthe importance of compact groups for studies of interactions and galaxyevolution. While the lack of a good control sample makes it difficult tomake quantitative comparisons for some aspects of this study, it isclear that the fraction of galaxies showing evidence of interactions ismuch higher in compact groups than in other environments.
| Dynamical properties of compact groups of galaxies
Radial velocities are presented for 457 galaxies in the 100 Hicksoncompact groups. More than 84 percent of the galaxies measured havevelocities within 1000 km/s of the median velocity in the group.Ninety-two groups have at least three accordant members, and 69 groupshave at least four. The radial velocities of these groups range from1380 to 42,731 km/s with a median of 8889 km/s, corresponding to amedian distance of 89/h Mpc. The apparent space density of these systemsranges from 300 to as much as 10 exp 8 sq h/sq Mpc, which exceeds thedensities in the centers of rich clusters. The median projectedseparation between galaxies is 39/h kpc, comparable to the sizes of thegalaxies themselves. A significant correlation is found between crossingtime and the fraction of gas-rich galaxies in the groups, and a weakanticorrelation is found between crossing time and the luminositycontrast of the first-ranked galaxy.
| The plane W(Na I) X W(Mg I) - Effects of interstellar Na I in a sample of southern galaxies
Galaxy spectra from a subsample of the Southern Sky Redshift Survey databank were used to study the equivalent width plane for the lines Na Ilambda 5893 A vs Mg I lambda 5175 A. An estimate of how important thecontribution of the interstellar gas for the sodium line is compared tothat of the stellar population. The sample is made up of galaxies withmorphological types from E to Sc and are distributed up to radialvelocities of 25,000 km/s, most of them smaller than 15,000 km/s. Mostearly type galaxies with dust lanes, particularly nearly edge-on So's,present an enhancement of the Na I line. Inclined spiral galaxies tendto present enhanced Na I with respect to face-on spirals. This tendency,previously found in a smaller sample of galaxies limited to V equal toor less than 6000 km/s, is now confirmed for more distant ones. In thelarge velocity sample it shows the global bulge rather than the verynucleus; the persistence of the effect suggests that the scale height ofthe gas layer in the central disk can reach a considerable fraction ofthe bulge radius.
| The luminosity function of compact groups of galaxies
An analysis of the luminosity function of 68 compact groups of galaxiescataloged by Hickson (1982) is presented. The luminosities of compactgroup galaxies are consistent with their being drawn from a Schechterluminosity function. Individual morphological-type luminosity functionsare also determined. Both the total and morphological-type specificluminosity functions of compact group galaxies are significantlydifferent from those of field, loose-group, and cluster galaxies. Inparticular, the luminosity function of HCG elliptical galaxies has amean magnitude which is significantly brighter than the mean magnitudeof Virgo cluster elliptical galaxies. The mean luminosity density ofgalaxies in compact groups is estimated. The obtained result isconsistent with the conventional scenario in which compact groups mergeto form elliptical galaxies on a relatively short time scale.
| Characterising 'box/peanut' galactic bulge morphology
Principal component analyses have been conducted on a large sample ofedge-on galaxies displaying 'box' or 'peanut' shaped bulge distortionsin order to identify the characteristic photometric properties of themorphology. The identified parameter space is strictly two-dimensional,indicating that no single parameter exists by which box/peanutmorphology could be fully characterized. The most significant parametersare measures of bulge dimension, and the bulge-to-total luminosityratio. It is shown that weak boxes and extreme peanuts are merelyextrema in a homogeneous family of isophotal distortions. The resultsalso suggest that the disk component is not an important constraint onthe formation and/or subsequent evolution of box/peanut bulgedistortions.
| A photometric catalog of compact groups of galaxies
The paper presents astrometry, photometry, and morphological types,derived from CCD images, for 463 galaxies in the 100 compact groupsselected by Hickson. Some minor revisions to the membership of theoriginal catalog are made, based on these new images. The completenessof the catalog is considered as a function of group magnitude andGalactic latitude. At high Galactic latitude the catalog is estimated tobe 90 percent complete for groups with total B(T) magnitude 13.0 orless. It is less complete at lower Galactic latitude because ofobscuration and high stellar density.
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