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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.
| 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.
| 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.
| Redshift-Distance Survey of Early-Type Galaxies. I. The ENEARc Cluster Sample
This paper presents data on the ENEARc subsample of the larger ENEARsurvey of nearby early-type galaxies. The ENEARc galaxies belong toclusters and were specifically chosen to be used for the construction ofa Dn-σ template. The ENEARc sample includes newmeasurements of spectroscopic and photometric parameters (redshift,velocity dispersion, line index Mg2, and the angular diameterdn), as well as data from the literature. New spectroscopicdata are given for 229 cluster early-type galaxies, and new photometryis presented for 348 objects. Repeat and overlap observations withexternal data sets are used to construct a final merged catalogconsisting of 640 early-type galaxies in 28 clusters. Objectivecriteria, based on catalogs of groups of galaxies derived from completeredshift surveys of the nearby universe, are used to assign galaxies toclusters. In a companion paper, these data are used to construct thetemplate Dn-σ distance relation for early-typegalaxies, which has been used to estimate galaxy distances and derivepeculiar velocities for the ENEAR all-sky sample. Based on observationsat Complejo Astronomico El Leoncito, operated under agreement betweenthe Consejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; Cerro Tololo Inter-American Observatory,National Optical Astronomical Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation; the EuropeanSouthern Observatory (ESO), partially under the ESO-ON agreement; theFred Lawrence Whipple Observatory; the Observatório do Pico dosDias, operated by the Laboratório Nacional de Astrofísicaand the MDM Observatory at Kitt Peak.
| 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.
| 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 }
| 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
| Southern Galaxy Catalogue.
Not Available
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