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Star formation history in early-type galaxies - I. The line absorption indices diagnostics
To unravel the formation mechanism and the evolutionary history ofelliptical galaxies (EGs) is one of the goals of modern astrophysics. Ina simplified picture of the issue, the question to be answered iswhether they have formed by hierarchical merging of pre-existingsubstructures (maybe disc galaxies) made of stars and gas, with eachmerging event probably accompanied by strong star formation, orconversely, whether they originated from the early aggregation of lumpsof gas turned into stars in the remote past via a burst-like episodeever since followed by quiescence so as to mimic a sort of monolithicprocess. Even if the two alternatives seem to oppose each other,actually they may both contribute to shaping the final properties of EGsas seen today. Are there distinct signatures of the underlying dominantprocess in the observational data? To this aim we have examined the lineabsorption indices on the Lick system of the normal, field EGs of Tragerand the interacting EGs (pair- and shell-objects) of Longhetti et al.The data show that both normal, field and interacting galaxies have thesame scattered but smooth distribution in the Hβ versus [MgFe]plane even if the interacting ones show a more pronounced tail towardhigh Hβ values. This may suggest that a common physical cause is atthe origin of their distribution. There are two straightforwardinterpretations of increasing complexity. (i) EGs span true large rangesof ages and metallicities. A young age is the signature of theaggregation mechanism, each event accompanied by metal enrichment. Thissimple scheme cannot, however, explain other spectro-photometricproperties of EGs and has to be discarded. (ii) The bulk population ofstars is old but subsequent episodes of star formation scatter the EGsin the diagnostic planes. However, this scheme would predict anoutstanding clump at low Hβ values, contrary to what is observed.The model can be cured by supposing that the primary star formationactivity lasted for a significant fraction of the Hubble time (5<=T<= 13 Gyr) accompanied by global metal enrichment. The`younger' galaxies are more metal-rich. The later burst of starformation should be small otherwise too many high-Hβ objects wouldbe observed. Therefore, the distribution of normal, pair- andshell-galaxies in the Hβ versus [MgFe] plane is due to global metalenrichment. Even though the above schemes provide a formal explanation,they seem to be too demanding because of the many ad hoc ingredientsthat have to be introduced. Furthermore, they neglect theobservationally grounded hint that the stellar content of EGs is likelyto be enhanced in α-elements with [α/Fe] ranging from 0.1 to0.4 dex. Here we propose a new scheme, in which the bulk dispersion ofgalaxies in the Hβ versus [MgFe] plane is caused by a differentmean degree of enhancement. In this model, neither the large age rangesnor the universal enrichment law for the old component are required andthe observed distribution along Hβ is naturally recovered.Furthermore, later bursts of stellar activity are a rare event,involving only those galaxies with very high Hβ (roughly >2.5).Finally, simulations of the scatter in broad-band colours of EGs seem toconfirm that the bulk stars have formed in the remote past, and thatmergers and companion star formation in a recent past are not likely,unless the intensity of the secondary activity is very small.
| A catalogue and analysis of local galaxy ages and metallicities
We have assembled a catalogue of relative ages, metallicities andabundance ratios for about 150 local galaxies in field, group andcluster environments. The galaxies span morphological types from cD andellipticals, to late-type spirals. Ages and metallicities were estimatedfrom high-quality published spectral line indices using Worthey &Ottaviani (1997) single stellar population evolutionary models. Theidentification of galaxy age as a fourth parameter in the fundamentalplane (Forbes, Ponman & Brown 1998) is confirmed by our largersample of ages. We investigate trends between age and metallicity, andwith other physical parameters of the galaxies, such as ellipticity,luminosity and kinematic anisotropy. We demonstrate the existence of agalaxy age-metallicity relation similar to that seen for local galacticdisc stars, whereby young galaxies have high metallicity, while oldgalaxies span a large range in metallicities. We also investigate theinfluence of environment and morphology on the galaxy age andmetallicity, especially the predictions made by semi-analytichierarchical clustering models (HCM). We confirm that non-clusterellipticals are indeed younger on average than cluster ellipticals aspredicted by the HCM models. However we also find a trend for the moreluminous galaxies to have a higher [Mg/Fe] ratio than the lowerluminosity galaxies, which is opposite to the expectation from HCMmodels.
| The AGB phase-transition outside the local group: K-band observations of young star clusters in NGC 7252
We have extended the study of the young star clusters observed in themerger remnant galaxy NGC 7252 by obtaining K band photometry for theseclusters. Our K band data significantly complement the opticalphotometry and spectroscopy in the literature: K band data arefundamental to study the Asymptotic Giant Branch (AGB) population ofthese clusters, since the AGB phase transition (occuring between the ageof ~ 200 Myr and ~ 1 Gyr) causes abrupt changes in the near-infraredluminosity of the clusters while producing only small changes in theoptical. Therefore, the e.g. V-K colour is ideal to study thisevolutionary phase of stellar populations. For the present analysis wepresent models for Simple Stellar Populations which include thecontribution of the AGB stellar phase, calibrated with the young andintermediate age star clusters of the Magellanic Clouds. The comparisonwith the colour distribution of the NGC 7252 star clusters shows thatthey are indeed intermediate age clusters undergoing the AGB phasetransition. The AGB phase transition is observed for the first timeoutside the Local Group. Most of the studied clusters span the verynarrow age range 300-500 Myr, and likely have metallicities 0.5-1Zsun. A very important exception is the cluster W32, whichhas already completed its AGB epoch, its colours being consistent withan age of ~ 1-2 Gyr. This impacts on the duration of the merger-inducedstarburst. The strengths of the magnesium and iron lines in the spectrumof the best observed cluster W3, and in the spectrum of the diffusecentral light of NGC 7252, do not show an overabundance in alpha-elements, in contrast to the bulk stellar population of ellipticalgalaxies.
| 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.
| Seeing Galaxies through Thick and Thin. I. Optical Opacity Measures in Overlapping Galaxies
We describe the use of partially overlapping galaxies to provide directmeasurements of the effective absorption in galaxy disks, independent ofassumptions about internal disk structure. The nonoverlapping parts ofthe galaxies and symmetry considerations are used to reconstruct, viadifferential photometry, how much background galaxy light is lost inpassing through the foreground disks. Extensive catalog searches andfollow-up imaging yield ~15-25 nearby galaxy pairs suitable for varyingdegrees of our analysis; 11 of the best such examples are presentedhere. From these pairs, we find that interarm extinction is modest,declining from AB~1 mag at 0.3RB25 toessentially zero by RB25; the interarm dust has ascale length consistent with that of the disk starlight. In contrast,dust in spiral arms and resonance rings may be optically thick(AB>2) at virtually any radius. Some disks have flatterextinction curves than the Galaxy, with AB/AI~1.6this is probably the signature of clumpy dust distributions. Even thoughtypical spirals are not optically thick throughout their disks, wherethey are optically thick is correlated with where they are mostluminous: in spiral arms and inner disks. This correlation betweenabsorption and emission regions may account for their apparent surfacebrightness being only mildly dependent on inclination, erroneouslyindicating that spirals are generally optically thick. Taken as anensemble, the opacities of spiral galaxies may be just great enough tosignificantly affect QSO counts, though not enough to cause theirhigh-redshift cutoff. Based in part on archival observations with theNASA/ESA Hubble Space Telescope (HST) obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555.
| Spectroscopic Observations of Merging Galaxies
In this paper we describe the spectroscopic and infrared properties of asample of 25 merging galaxy pairs, selected from the catalog of Arp& Madore, and we compare them with those observed in a similarsample of interacting galaxies (Donzelli & Pastoriza). It is notedthat mergers as well as interacting systems comprise a wide range ofspectral types, going from those corresponding to well-evolved stellarpopulations (older than 200 Myr) to those that show clear signatures ofH II regions with stellar populations younger than 8 Myr. However,merger galaxies show on average more excited spectra than interactingpairs, which could be attributed to lower gas metallicity. From theemission lines we also found that merging systems show on average higher(about a factor of 2) star formation rates than interacting galaxies.Classical diagnostic diagrams show that only three of 50 of the galaxies(6%) present some form of nuclear activity: two Seyfert galaxies and oneLINER. However, through a detailed analysis of the pure emission-linespectra, we conclude that this fraction may raise up to 23% of themergers if we consider that some galaxies host a low-luminosity activenucleus surrounded by strong star-forming regions. This latterassumption is also supported by the infrared colors of the galaxies.Regarding to the total infrared luminosities, the merging galaxies showon average an IR luminosity, log(Lir)=10.7, lower than thatof interacting systems, log(Lir)=10.9. We find that onlythree mergers of the sample (12%) can be classified as luminous infraredgalaxies, while this fraction increases to 24% in the interactingsample. Based on observations made at CASLEO. ComplejoAstronómico El Leoncito is operated under agreement between theConsejo Nacional de Investigaciones Científicas y Técnicasde la República Argentina and the National Universities of LaPlata, Córdoba and San Juan.
| Star formation history of early-type galaxies in low density environments. IV. What do we learn from nuclear line-strength indices?
In this paper we analyze the line-strength indices in the Lick-systemmeasured by Longhetti et al. (1998a, b) for a sample of 51 early-typegalaxies located in low density environments (LDE) and showingsignatures of fine structures and/or interactions. The sample contains21 shell-galaxies and 30 members of interacting pairs. Firstly weperform a preliminary comparison between three different sources ofcalibrations of the line strength indices, namely Buzzoni et al. (1992,1994), Worthey (1992), Worthey et al. (1994) and Idiart et al. (1995),derived from stars with different effective temperature, gravity, andmetallicity. Looking at the three indices in common, i.e. Mg2, Fe5270,and Hβ , the calibrations by Buzzoni et al. (1992, 1994), Worthey(1992) and Worthey et al. (1994) lead to mutually consistent results.The calibration of Hβ by Idiart et al. (1995) can be compared withthe previous ones only for a limited range of ages, in which goodagreement is found. Mg2 and Mgb indices predicted by the Idiart's et al.(1995) fitting functions result to be systematically lower than thoseobtained from using Worthey (1992) calibrations. Secondly, we discussthe properties of the galaxies in our sample by comparing them both withtheoretical Single Stellar Populations (SSPs) and the normal galaxies ofthe González (1993: G93) sample. The analysis is performed bymeans of several diagnostic planes. In the sigma , Mg2, Fe5270 andFe5335 space, normal, shell- and pair-galaxies have a differentbehavior. First of all, normal and pair-galaxies follow the universalsigma vs. Mg2 relation, whereas shell-galaxies lie above it; secondlythe Fe versus Mg2 relation of normal, shell- and pair-galaxies isflatter than the theoretical expectation. This fact hints forenhancement of alpha -elements with respect to solar partition ingalaxies with strong Fe indices and/or high velocity dispersion, massand luminosity in turn. In the sigma vs. Hβ plane normal galaxiesseem to follow a nice relation suggesting that objects with shallowgravitational potential have strong Hβ values (youth signature?),whereas shell- and pair-galaxies scatter all over the plane. A group ofgalaxies with deep gravitational potential and strong Hβ is found.Is this a signature of recent star formation? In the Hβ vs. [MgFe]plane, which is perhaps best suited to infer the age of the stellarpopulations, the peculiar galaxies in our sample show nearly the samedistribution of the normal galaxies in the G93 sample. There is howevera number of peculiar galaxies with much stronger Hβ . Does thismean that the scatter in the Hβ vs. [MgFe] plane, of normal, shell-and pair-galaxies has a common origin, perhaps a secondary episode ofstar formation? We suggest that, owing to their apparent youth, shell-and pair-galaxies should have experienced at least one interaction eventafter their formation. The explanation comes natural for shell- andpair-galaxies where the signatures of interactions are evident. It ismore intrigued in normal galaxies (perhaps other causes may concur).Noteworthy, the distribution in the Hβ vs. [MgFe] plane of normal,shell- and pair-galaxies is confined within a narrow strip that runssignificantly steeper than the path followed by aging SSPs. This featureis explained as due to metal enrichment always accompanying starformation. Shell-galaxies encompass the whole range of ages inferredfrom the Hβ vs. [MgFe] plane, indicating that among them recent andold interaction/acquisition events are equally probable. If shells areformed at the same time at which the rejuvenating event took place,shells ought to be long lasting phenomena. [MgFe] = sqrt { xMgb}, = (Fe5270 + Fe5335)/2 }
| Star formation history of early-type galaxies in low density environments. V. Blue line-strength indices for the nuclear region
We analyze the star formation properties of a sample of 21 shellgalaxies and 30 early-type galaxies members of interacting pairs,located in low density environments (Longhetti et al. 1998a, 1998b). Thestudy is based on new models developed to interpret the informationcoming from `blue' Hdelta /FeI, H+K(CaII) and Delta 4000 line-strengthindices proposed by Rose (1984; 1985) and Hamilton (1985). We find thatthe last star forming event that occurred in the nuclear region of shellgalaxies is statistically old (from 0.1 up to several Gyr) with respectto the corresponding one in the sub-sample of pair galaxies (<0.1 Gyror even ongoing star formation). If the stellar activity is somehowrelated to the formation of shells, as predicted by several dynamicalmodels of galaxy interaction, shells have to be considered long lastingstructures. Since pair members show evidence of very recent starformation, we suggest that either large reservoirs of gas have to bepresent to maintain active star formation, if these galaxies are onperiodic orbits, or most of the pair members in the present sample areexperiencing unbound encounters. Table~2 is available in electronic formonly, at CDS: via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| The Durham/UKST Galaxy Redshift Survey - V. The catalogue
We present the radial velocities and blue, optical magnitudes for all ofthe galaxies within the Durham/UKST Galaxy Redshift Survey. Thiscatalogue consists of ~2500 galaxy redshifts to a limiting apparentmagnitude of B_J⋍17 mag, covering a ~1500-deg^2 area around theSouth Galactic Pole. The galaxies in this survey were selected from theEdinburgh/Durham Southern Galaxy Catalogue and were sampled, in order ofapparent magnitude, at a rate of one galaxy in every three. Thespectroscopy was performed at the 1.2-m UK Schmidt Telescope inAustralia using the FLAIR multi-object spectrograph. We show that ourradial velocity measurements made with this instrument have an empiricalaccuracy of +/-150 km s^-1. The observational techniques and datareduction procedures used in the construction of this survey are alsodiscussed. This survey demonstrates that the UKST can be used to make athree-dimensional map of the large-scale galaxy distribution, via aredshift survey to b_J⋍17 mag, over a wide area of the sky.
| The Southern Sky Redshift Survey
We report redshifts, magnitudes, and morphological classifications for5369 galaxies with m_B <= 15.5 and for 57 galaxies fainter than thislimit, in two regions covering a total of 1.70 sr in the southerncelestial hemisphere. The galaxy catalog is drawn primarily from thelist of nonstellar objects identified in the Hubble Space TelescopeGuide Star Catalog (GSC). The galaxies have positions accurate to ~1"and magnitudes with an rms scatter of ~0.3 mag. We compute magnitudes(m_SSRS2) from the relation between instrumental GSC magnitudes and thephotometry by Lauberts & Valentijn. From a comparison with CCDphotometry, we find that our system is homogeneous across the sky andcorresponds to magnitudes measured at the isophotal level ~26 magarcsec^-2. The precision of the radial velocities is ~40 km s^-1, andthe redshift survey is more than 99% complete to the m_SSRS2 = 15.5 maglimit. This sample is in the direction opposite that of the CfA2; incombination the two surveys provide an important database for studies ofthe properties of galaxies and their large-scale distribution in thenearby universe. Based on observations obtained at Cerro TololoInter-American Observatory, National Optical Astronomy Observatories,operated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation;Complejo Astronomico El Leoncito, operated under agreement between theConsejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; the European Southern Observatory, LaSilla, Chile, partially under the bilateral ESO-ObservatórioNacional agreement; Fred Lawrence Whipple Observatory;Laboratório Nacional de Astrofísica, Brazil; and the SouthAfrican Astronomical Observatory.
| A catalogue of Mg_2 indices of galaxies and globular clusters
We present a catalogue of published absorption-line Mg_2 indices ofgalaxies and globular clusters. The catalogue is maintained up-to-datein the HYPERCAT database. The measurements are listed together with thereferences to the articles where the data were published. A codeddescription of the observations is provided. The catalogue gathers 3541measurements for 1491 objects (galaxies or globular clusters) from 55datasets. Compiled raw data for 1060 galaxies are zero-point correctedand transformed to a homogeneous system. Tables 1, 3, and 4 areavailable in electronic form only at the CDS, Strasbourg, via anonymousftp 130.79.128.5. Table 2 is available both in text and electronic form.
| Star formation history of early-type galaxies in low density environments. II. Kinematics
The present paper is a companion of two others dedicated one to themeasurement of the line-strength indices \cite[(Longhetti et al.1997a)]{Lo7a} and the second to trace back the star formation history ofa sample of early-type galaxies by comparing observed indices to thepredictions of new spectro-photometric models \cite[(Longhetti et al.1997b).]{Lo7b} The sample of 51 early-type galaxies in low densityenvironments is composed of two sub-sets of galaxies: 21 shell galaxiesfrom the \cite[Malin & Carter (1983)]{Ma3} catalogue (one of whichshows double nucleous and has been considered as two separate objects)and 30 members of isolated interacting pairs from the \cite[Reduzzi& Rampazzo (1995)]{Re5} catalogue. Most of the objects show finestructures. The paper collects nuclear kinematic data together with thevelocity and velocity dispersion curves of the stellar and gaseouscomponents as a function of the distance from the galaxies centres. Thegalaxies heliocentric systemic velocity compares within -1+/- 32 kms(-1) with RC3 data, while their central velocity dispersion compareswithin 9+/- 9 km s(-1) , 10+/- 27 km s(-1) and 2+/- 33 km s(-1) with\cite[Gonzalez (1993),]{Go3} \cite[Davies et al. (1987)]{Da7} and\cite[Carter et al. (1988)]{Ca8} respectively. The detailed comparisonbetween our velocity and velocity dispersion curves and those fromseveral authors is discussed. 9 out of 22 shell galaxies nuclei showemission lines, 4 of which, using data in the literature, have lineratios characteristic of LINERs. 10 members of pairs out of 30 showemission lines. RR 331a has a Seyfert like nucleus, while for theremaining galaxies the ([O III] lambda 5007)/Hβ ratio ischaracteristic of low ionization regions. In a small fraction of theobjects the emission component is detectable outside the central value.None of the objects in the sample shows counter-rotation of the gaseousversus the stellar component. The two components appear associated,although, in two cases there is evidence that gas and stars lie ondifferent planes. This latter phenomenon could be associated toaccretion events. Emission lines in the central part of the RR 331a showa secondary component in the emission lines profile. E 2400100 has twonuclei embedded in the main body of the galaxy. The U-shape profile ofthe stellar velocity profile shows the ongoing interaction of the twonuclei. V/sigma profile of shell galaxies is, finally, discussed inrelation to the hypothesis of the accretion/merging origin of thesegalaxies. Based on observations obtained at ESO, La Silla, Chile. Dataand and kinematical profiles are available at CDS.
| Star formation history of early-type galaxies in low density environments. I. Nuclear line-strength indices
This paper is the first of a series \cite[(Longhetti et al.1997a,b)]{lon97} dedicated to the study of the star formation history inearly-type galaxies which show fine structures and/or signatures ofinteraction. It presents nuclear line-strength indices for a samplecomposed of 21 shell galaxies, from the \cite[Malin & Carter(1983)]{mal83} southern survey, and 30 members of isolated interactingpairs, from the \cite[Reduzzi & Rampazzo (1995)]{red95} catalogue,located in low density environments. The spectral range covers 3700Angstroms < lambda < 5700 Angstroms at 2.1 Angstroms FWHMresolution. We measure 16 red (lambda > 4200 Angstroms) indicesdefined by the Lick Group. Measures have been transformed into theLick-IDS ``standard'' system. The procedure has been tested on a set of5 elliptical galaxies selected from the \cite[Gonzalez (1993)]{gon93}sample. We derive also three blue (lambda < 4200) indices, namelyDelta (4000 Angstroms) defined by \cite[Hamilton (1985)]{ham85},H+K(CaII) and Hdelta /FeI defined by \cite[Rose (1984, 1985)]{ros84}.Blue indices are correlated to the age of the last starburst occurred ina galaxy \cite[(Leonardi & Rose 1996)]{leo96}. The determination ofthese indices, the estimate of the measurement errors and the correctionfor the galaxies velocity dispersions are discussed in detail. In theAppendix A we present the indices for a set of hot stars (T> 10000 K)which may be used for extending W92 fitting functions toward hightemperatures. Based on observations obtained at ESO, La Silla, Chile.Tables 1-8 are also available in electronic form at CDS and Tables 9-15are only available in electronic form at CDS: via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Surface photometry of binary galaxies. I. A multicolour study of morphologies due to the interaction.
We present the multicolour (B, V, R) surface photometry of 53 doublegalaxies. All the pairs belong to the catalogue of Reduzzi &Rampazzo (1995) containing objects selected according to theKarachentsev (1972) criteria. We comment on the morphological,structural and photometric characteristics of pairs and their members.Different classes of interaction induced phenomena, both among early andlate-type galaxies, are considered. We found that few early-typegalaxies show fine structures. Grand design structure is more frequentlydetected in binary than in field spirals both for barred and non barred,confirming Elmegreen & Elmegreen's (1982) study. The colour of thetails is consistent with the stripping hypothesis since it is similar tothe progenitor galaxy outskirts. Among our objects we have no evidentsign of induced star formation in tails. Rings appear on average bluerthan the disc as a whole.
| Candidates for a southern extension of the Karachentsev catalogue of isolated pairs of galaxies.
Not Available
| A Spectrophotometric Survey of Merging Galaxies
We present long-slit spectrophotometry of 40 merging or stronglyinteracting galaxy systems in the wavelength range 3650-7100 A. Alongwith optically selected objects, the sample includes 10 ultraluminousIRAS galaxies with evidence of ongoing merger activity. The data show awide variety of phenomena, with spectra resembling those of isolatedelliptical galaxies, early and late-type spiral galaxies, activegalactic nuclei starbursts, and poststarburst systems.
| Spectrophotometric Properties of Merging Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...450..547L&db_key=AST
| Fine structure in the early-type components in mixed pairs of galaxies.
Not Available
| General study of group membership. II - Determination of nearby groups
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
| Quantitative morphology of isolated E/S pairs of galaxies
We present a study of the morphology and geometry for 22 southern E+Spair candidates using high resolution CCD frames obtained at the ESO 2.2m telescope. We find a considerable number of true mixed pairs withabout 13/11 elliptical + spiral pairs in the present sample. Theremaining objects include about 3 'disky' pairs (composed of S0 and Smembers) and 3 'early-type' pairs comprising E and S0 members. Weestimate that between 10 and 25 percent of the pairs in any complete(non-hierarchical) sample will be of the E+S type. This suggests thatmorphological concordance is not the rule among paired galaxies. Wesuggest that interaction phenomena and the alterations they produce ingalaxy forms may explain many of the mixed pairs. Eight early-typecomponents show possible shell structure while 78 percent show anisophotal twisting of greater than 10 deg. Three of these objects andtwo additional pairs may be mergers in progress based upon their chaoticmorphologies. We suggest that the early-type components in many pairscould be merger products. These E+S pairs might then be a transientstage in the coalescence of compact groups.
| Observations of binary galaxies at a frequency of 102 MHz
A total of 93 double galaxies from the Karachentsev list was observedwith the large phased array at the Lebedev Physical Institute at 102MHz. The interplanetary oscillation method was used. Two of the galaxieswere found to contain scintillating components with angular dimensionsof less than 1 arcsec and flux densities of more than 1 Jy. Theobservations of double galaxies at low radio frequencies indicate thatgalaxies in pairs are more active than single galaxies.
| Southern Galaxy Catalogue.
Not Available
| The effects of interactions on spiral galaxies. I - Nuclear activity and star formation
When the present results of spectrophotometry for the nuclei of 161(mostly spiral) galaxies with bright companions and emission lineimaging of 63 galaxies were compared to a similarly observed sample ofisolated-spiral nuclei, both samples of interacting galaxies exhibitedsignificant excesses of nuclear emission. The rate of nuclear starformation is significantly above average even in systems withoutnoticeable tidal distortion in the outer disks, suggesting that thenear-nuclear gas is only marginally stable in isolated galaxies. Theresults obtained suggest that nuclear phenomena are triggered by atidally induced influx of gas from the disk into the nuclear regions,rather than gas transfer between the galaxies.
| Recent star formation in interacting galaxies. I - Evidence from JHKL photometry
A survey has been carried out using JHKL photometry to investigaterecent star formation in interacting galaxies. The objective was to lookfor a K-L excess produced by 'warm' dust heated by a putative burst ofstar formation. K-L excesses are found suggesting that interactionsinduce starbursts with an efficiency approaching 100 percent. Theappearance of these inferred starbursts in interacting systems ofdifferent morphological types is qualitatively consistent with dynamicalstudies of galaxy interactions. However, the common occurrence of suchstarbursts shows that interactions have implications for theastrophysics of galaxies well beyond purely morphological effects.
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