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A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations
This paper is the second in a series exploring the properties of 51optically selected, single-nuclei merger remnants. Spectroscopic datahave been obtained for a subsample of 38 mergers and combined withpreviously obtained infrared photometry to test whether mergers exhibitthe same correlations as elliptical galaxies among parameters such asstellar luminosity and distribution, central stellar velocity dispersion(σ0), and metallicity. Paramount to the study is totest whether mergers lie on the fundamental plane. Measurements ofσ0 have been made using the Ca triplet absorption lineat 8500 Å for all 38 mergers in the subsample. Additionalmeasurements of σ0 were made for two of the mergers inthe subsample using the CO absorption line at 2.29 μm. The resultsindicate that mergers show a strong correlation among the parameters ofthe fundamental plane but fail to show a strong correlation betweenσ0 and metallicity (Mg2). In contrast toearlier studies, the σ0 values of the mergers areconsistent with objects that lie somewhere between intermediate-mass andluminous giant elliptical galaxies. However, the discrepancies withearlier studies appear to correlate with whether the Ca triplet or COabsorption lines are used to derive σ0, with the latteralmost always producing smaller values. Finally, the photometric andkinematic data are used to demonstrate for the first time that thecentral phase-space densities of mergers are equivalent to those inelliptical galaxies. This resolves a long-standing criticism of themerger hypothesis.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Cold and warm dust along a merging galaxy sequence
We investigate the cold and warm dust properties during galaxyinteractions using a merging galaxy sample ordered into a chronologicalsequence from pre- to post-mergers. Our sample comprises a total of 29merging systems selected to have far-infrared and submillimetreobservations. The submillimetre data are mainly culled from theliterature, while for five galaxies (NGC 3597, 3690, 6090, 6670 and7252) the submillimetre observations are presented here for the firsttime. We use the 100- to 850-μm flux density ratio,f100/f850, as a proxy for the mass fraction of thewarm and cold dust in these systems. We find evidence for an increase inf100/f850 along the merging sequence from early toadvanced mergers, and interpret this trend as an increase of the warmrelative to the cold dust mass. We argue that the two key parametersaffecting the f100/f850 flux ratio is the starformation rate and the dust content of individual systems relative tothe stars. Using a sophisticated model for the absorption andre-emission of the stellar ultraviolet radiation by dust, we show thatthese parameters can indeed explain both the increase and the observedscatter in f100/f850 along the merging galaxysequence. We also discuss our results under the hypothesis thatelliptical galaxies are formed via disc galaxy mergers.
| Formation of Globular Clusters in Galaxy Mergers
We present a high-resolution simulation of globular cluster formation ina galaxy merger. For the first time in such a simulation, individualstar clusters are directly identified and followed on their orbits. Wequantitatively compare star formation in the merger to that in theunperturbed galaxies. The merging galaxies show a strong starburst, insharp contrast to their isolated progenitors. Most star clusters form inthe tidal features. With a mass range of5×105-5×106 Msolar, theyare identified as globular clusters. The merger remnant is an ellipticalgalaxy. Clusters with different masses or ages have different radialdistributions in the galaxy. Our results show that the high specificfrequency and bimodal distribution of metallicity observed in ellipticalgalaxies are natural products of gas-rich mergers, supporting a mergerorigin for the ellipticals and their globular cluster systems.
| E+A Galaxies and the Formation of Early-Type Galaxies at z~0
E+A galaxies, whose spectra have deep Balmer absorption lines but nosignificant [O II] emission, are the best candidates for an evolutionarylink between star-forming, gas-rich galaxies and quiescent, gas-poorgalaxies. However, their current morphologies are not well known. Wepresent Hubble Space Telescope WFPC2 observations of the five bluest E+Agalaxies (z~0.1) in the sample of Zabludoff and coworkers to studywhether their detailed morphologies are consistent with late- toearly-type evolution and to determine what drives that evolution. Themorphologies of four galaxies are disturbed, indicating that agalaxy-galaxy merger is at least one mechanism that leads to the E+Aphase. Two-dimensional image fitting shows that the E+As are generallybulge-dominated systems, even though at least two E+As may haveunderlying disks. In the fundamental plane, E+As stand apart from theE/S0 galaxies mainly because of their high effective surface brightness.Fading of the young stellar population and the corresponding increase intheir effective radii will cause these galaxies to migrate toward thelocus of E/S0 galaxies. E+As have profiles qualitatively like those ofnormal power-law early-type galaxies but have higher surfacebrightnesses. This result provides the first direct evidence supportingthe hypothesis that power-law elliptical galaxies form via gas-richmergers. In total, at least four E+As are morphologically consistentwith early-type galaxies. We detect compact sources, possibly young starclusters, associated with the galaxies. These sources are much brighter(MR~-13) than Galactic globular clusters, have luminositiesconsistent with the brightest clusters in nearby starburst galaxies, andhave blue colors consistent with the ages estimated from the E+A galaxyspectra (several 108 yr). Further study of such young starcluster candidates might provide the elusive chronometer needed to breakthe age/burst-strength degeneracy for these postmerger galaxies.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| A Nursery of Young Objects: Intergalactic H II Regions in Stephan's Quintet
We have discovered four intergalactic H II regions in Stephan's quintet,which is more than a 25 kpc projected distance from the center of thenearest group galaxy, with no apparent optical connection to it. Theyhave MB ranging from -11.9 to -12.5 mag, colors B-R=0.7-1.1mag, radial velocities from 6565 to 6651 km s-1, and they aresuperposed onto the H I tail east of NGC 7319, with a mean radialvelocity of 6610 km s-1. In addition, they have metallicitiesof the order of 12+log(O/H)=8.58+/-0.25, which suggests that they wereformed from preenriched material. We derive a mean age of 4.6+/-0.6 Myrand a mean stellar mass of (2.9+/-1.4)×104Msolar for the four objects. The masses, ages, colors,velocities, metallicities, and location of the objects suggest that theyare H II regions that were formed far away from the galaxies throughcompression of the intergalactic H I gas by galaxy collisions.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).
| A Deep K-Band Photometric Survey of Merger Remnants
We present K-band photometry for 51 candidate merger remnants to assessthe viability of whether spiral-spiral mergers can produce bona fideelliptical galaxies. Using both the de Vaucouleurs r1/4 andSérsic r1/n fitting laws, it is found that the stellarcomponent in a majority of the galaxies in the sample has undergoneviolent relaxation. However, the sample shows evidence for incompletephase mixing. The analysis also indicates the presence of ``excesslight'' in the surface brightness profiles of nearly one-third of themerger remnants. Circumstantial evidence suggests that this is due tothe effects of a starburst induced by the dissipative collapse of thegas. The integrated light of the galaxies also shows that mergers canmake L* elliptical galaxies, in contrast to earlier infrared studies.The isophotal shapes and related structural parameters are alsodiscussed, including the fact that 70% of the sample show evidence fordisky isophotes. The data and results presented are part of a largerphotometric and spectroscopic campaign to thoroughly investigate a largesample of mergers in the local universe.
| Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921
Low-resolution, ultraviolet-to-visual spectra of two candidate globularclusters in the merger remnant NGC 3921 (czLG=6021 kms-1) are presented. These two clusters of apparent magnitudeV~22.2 (MV~-12.5) lie at projected distances of about 5 kpc(0.9 Reff) from the center and move with halo-type radialvelocities relative to the local galaxy background. Their spectra showstrong Balmer absorption lines [EW(Hβ-Hδ)=11-13 Å]indicative of main-sequence turnoffs dominated by A-type stars.Comparisons with model-cluster spectra computed by Bruzual & Charlotand others yield cluster ages in the range of 200-530 Myr, andmetallicities about solar to within a factor of three. Given their smallhalf-light radii (Reff<~5 pc) and ages corresponding to~102 core-crossing times, these clusters are gravitationallybound and hence indeed young globulars. Assuming that they hadChabrier-type initial mass functions, their estimated current masses are(2.3+/-0.1)×106Msolar and(1.5+/-0.1)×106Msolar, respectively, orroughly half the mass of ω Cen. Since NGC 3921 itself shows manysigns of being a 0.7+/-0.3 Gyr old protoelliptical, these two youngglobulars of roughly solar metallicity and their many counterpartsobserved with the Hubble Space Telescope provide supporting evidencethat, in the process of forming elliptical-like remnants, major mergersof gas-rich disks can also increase the number of metal-rich globularclusters.
| Formalism and quality of a proper motion link with extragalactic objects for astrometric satellite missions
The accuracy of the link of the proper motion system of astrometricsatellite missions like AMEX and GAIA is discussed. Monte-Carlo methodswere used to simulate catalogues of positions and proper motions ofquasars and galaxies to test the link. The main conclusion is, thatfuture satellite missions like GAIA may be ``self-calibrated'' by theirmeasurements of QSOs, while additional measurements from radio stars orHST-data are needed to calibrate the less deep reaching astrometricsatellite missions of AMEX type.
| On the formation of star clusters in the merger NGC 6240
We identified star clusters in archived Hubble Space Telescope/WideField Planetary Camera 2 (HST/WFPC2) images of the merger andultraluminous infrared galaxy NGC 6240, with the aim of investigatingwhether star cluster properties (luminosity, age and mass) in such anextreme environment differ from those of clusters in less luminousstarburst galaxies. We found 54 star clusters in all of the F450W, F547Mand F814W exposures, of which 41 are located in the main body of NGC6240 and 13 are located in the galactic tails. Given that only twocolours are available to derive two independent variables (clusterreddening and age), we adopted an ad hoc procedure to derive clusterparameters statistically under the assumption that the clustermetallicity is like that in the Large Magellanic Cloud. The colours ofeach cluster are fitted to STARBURST99 models of fixed mass and variableages and reddenings. All cluster reddening and age solutions withχ2 < 1 are considered to be consistent with the data.Masses are derived by scaling the luminosity of the models withbest-fitting χ2 < 1 by the observed V luminosity,after correction for reddening and distance. Therefore, each cluster isdescribed by a range of reddening values, ages and masses; for each ofthese parameters, we derive probability functions. Thus we infer thatthe most probable age of the observed clusters is between 5 and 13 Myrand their most probable mass is about (1-2) × 105Msolar. A low probability exists for clusters as massive as108 Msolar, as well as for the trend that the meancluster mass increases towards the double nuclei of NGC 6240. Comparisonwith star clusters in starburst galaxies seems to point to more massiveclusters being formed in more massive galaxies and gas-rich mergers,while the overall cluster mass distribution might be relativelyindependent of the details of the associated starburst where dense,massive clusters preferentially form.
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| Near-infrared imaging of ellipticals: surface brightness profiles and photometry
We present near-infrared K-band imaging of a large sample of candidatemerger remnant galaxies and Hickson Compact Group ellipticals. We derivelight profile indices, effective radii and surface brightnesses, as wellas total K-band magnitudes. We find that the light distributions of themerger remnant candidates are consistent with those of `normal'ellipticals, and scatter around a mean profile index of (1/n) = 0.20.Many of our sample galaxies have surface brightness profiles that arenot well described by a de Vaucouleurs law (1/n= 0.25), and we discussthe implications of this on the derived total magnitudes. Comparing thetotal K magnitudes calculated by extrapolating a de Vaucouleurs profileand those derived using a generalized Sérsic form, we find that asignificant bias is introduced if the de Vaucouleurs law is not a gooddescription of the actual light profile.
| FLASH redshift survey - I. Observations and catalogue
The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of4613 galaxies brighter than bJ= 16.7 (corrected for Galacticextinction) over a 700-deg2 region of sky in the generaldirection of the Local Group motion. The survey region is a70°× 10° strip spanning the sky from the ShapleySupercluster to the Hydra cluster, and contains 3141 galaxies withmeasured redshifts. Designed to explore the effect of the galaxyconcentrations in this direction (in particular the Supergalactic planeand the Shapley Supercluster) upon the Local Group motion, the 68 percent completeness allows us to sample the large-scale structure betterthan similar sparsely-sampled surveys. The survey region does notoverlap with the areas covered by ongoing wide-angle (Sloan or 2dF)complete redshift surveys. In this paper, the first in a series, wedescribe the observation and data reduction procedures, the analysis forthe redshift errors and survey completeness, and present the surveydata.
| 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.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| 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.
| Photometry of three gamma-ray-loud quasars and implications for supermassive black holes*
The results of photometry of three gamma-ray-loud quasars, PKS 1510-089,3C 279 and OJ248, during 2001 in the B, V and R bands and implicationsfor the supermassive black hole (SMBH) model in these objects arepresented. The results show that the three quasars had significant, butirregular, rapid outbursts with a typical time-scale of about 40-60minand a typical amplitude of about Δm~=0.60-1.26mag during theobservations. For 3C 279 and OJ248 the upper limits of the masses ofSMBHs are estimated from the observed minimum optical variabilitytime-scale, and the lower limits are also estimated by a method proposedby Dermer & Gehrels. The masses derived from both methods arereasonable. For PKS 1510-089 a new brightness minima was also observed.Analysing the observations from 1999 to 2001, three minima wereobserved. Although their light curves are poorly sampled, it isintriguing to note that these minimum at intervals of about 43min seemto indicate the possibility of a periodic phenomenon. So a hypothesis isproposed to interpret the phenomenon, suggesting that there is a binaryblack hole system at the centre. The parameters of the system areestimated by a very simple method. The periodicity is about 336+/-14d inthe observed frame; the masses of the primary and secondary black holesare 108.0 and 106.2Msolar,respectively; the distance between two black holes is1015.81cm (378Rg of the primary black hole), andthe lifetime of the system is 106.0yr. These results seem toindicate that PKS 1510-089 is another possible candidate binary blackhole system. It should be monitored in future to obtain more data forfurther analysis to test the hypothesis.
| Star Formation Rates in Interacting Starburst Galaxies
By narrowband imaging in Hα and in the adjacent red stellarcontinuum we have studied the rate and distribution of star formation in43 systems of luminous and ultraluminous IR galaxies currentlyundergoing interaction and merging. These galaxies are amongst the mostluminous at 60 μm and range in distance from ~50 up to 100 Mpc. Herewe present the Hα and the adjacent red-continuum narrowbandimages, and we compare the star formation rates derived from Hαwith those estimated from the IR luminosity. We find clear evidence forsubstantial extinction and obscuration of star-forming regions in theoptical. Without correction for reddening in the host galaxy orcorrection for [N II] contamination, the star formation rates derivedfor Hα are typically 0.5-1.0 dex lower than those estimated fromthe IR flux, and the scatter in the correlation is very large. However,an unexpected result is that when spectroscopic data are used toeliminate objects dominated by an active nucleus, to determine thegalaxian extinction, and to correct the Hα flux for both reddeningand for the contamination by the [N II] emission, a remarkably goodcorrelation emerges between the star formation rates estimated from theHα flux and those derived from the FIR continuum. In addition, astrong correlation is found between the extinction in the line-emittingregion, AHα, and the rate of star formation. Ourresults invalidate the use of Hα imaging as a reliable indicatorof star formation in starburst galaxies unless spectroscopic data arealso available. This has important implications for the determination ofstar formation rates in high-redshift galaxies. Finally, we find nocorrelation between the measured star formation rates, and theinteraction class, suggesting that the enhanced star formation ratestriggered by the interaction continue throughout the whole of themerging sequence.
| First spectroscopic evidence for carbon stars outside the local group: Properties of a massive star cluster in NGC 7252
We present near-infrared [1-2.3 mu m] spectroscopy of the massiveintermediate age star cluster W3 in the merger remnant andproto-elliptical galaxy NGC 7252, obtained with the NTT telescope. Thiscluster has an age when the integrated near-infrared properties of astellar population are dominated by the cool and luminous AsymptoticGiant Branch (AGB). We compare the data with instantaneous burst modelpredictions from new evolutionary synthesis models that include: (i) thecomputation of the evolution through the thermally pulsing AGB (TP-AGB)for low- and intermediate-massive stars, with the initial mass andmetallicity dependent formation of carbon stars; (ii) spectroscopic datafrom a new stellar library in which differences between static redgiants, variable oxygen rich TP-AGB stars and carbon stars are accountedfor. The new evolutionary model predicts that the contribution of carbonrich stars to the luminosities in the near-IR passbands is a strongfunction of metallicity. The comparison of the data to the modelsclearly shows that carbon stars are present: for the first time, carbonrich star spectral features are thus detected directly outside the LocalGroup galaxies. Good fits to the available optical/near-IR photometryand the near-IR spectrum of NGC 7252-W3 are found for an age of 300-400Myr and AV =~ 0.6-0.8. The models show that these parametersdepend weakly on the model metallicity in the range ofZ/Zsun=0.4-1, with higher likelihood for solar metallicitymodels. At solar metallicity, a mixture of carbon rich and oxygen richstars is predicted. The strength of the near-IR molecular bands thatoriginated from oxygen rich AGB stars can be used to constrain theabsolute Teff scale of these objects, i.e. a relation betweencolour and Teff. We found that, in the framework of our setof evolutionary tracks, the data are more consistent with thetemperature scale calibrated on Long Period Variables than on giantstars. At a given colour, variable AGB stars have a lowerTeff than static (or quasi-static) M giants. Based onobservations obtained at the NTT 3.5 m of the European SouthernObservatory, Chile.
| Measuring Sizes of Marginally Resolved Young Globular Clusters with the Hubble Space Telescope
We present a method for deriving sizes of marginally resolved starclusters from Hubble Space Telescope/WFPC2 observations by fitting Kingmodels to observations. We describe results both on simulated images andon observations of young compact clusters in NGC 3597 and NGC 1275. Fromthe simulations, we find that we can measure King model concentrations(c) to an accuracy of about a factor of 2 for all combinations of c andKing radius (r0) of interest if the data have highsignal-to-noise ratio (S/N) (>~500 for the integrated brightness). Ifthe concentration is accurately measured, we can measure the King radiusaccurately. For lower S/N, marginally resolved King profiles suffer froma degeneracy; different values of the concentration give differentr0 but have comparable reduced χ2 values. Inthis case, neither the core radius nor the concentration can beconstrained individually, but the half-light radius can be recoveredaccurately. In NGC 3597, we can differentiate between concentrationsonly for the very brightest clusters; these suggest a concentration of~2. Assuming a concentration of 2 for the rest of the objects, we findan average King radius for the clusters in NGC 3597 of 0.7 pc, while theclusters in NGC 1275 have an average radius of 1.1 pc. These are similarto the average core radii for Galactic globular clusters, 0.92 pc. Wefind average half-light radii of 5.4 and 6.2 pc for the young clustersin NGC 3597 and NGC 1275, respectively, while the average half-lightradii of Galactic globular clusters is 3.4 pc. The spread in the derivedradial parameters in each cluster system is comparable to that observedin the Galactic globular cluster system. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, operated by the Association of Universities for Research inAstronomy, Inc., under contract to NASA.
| Astrophysics in 2000
It was a year in which some topics selected themselves as importantthrough the sheer numbers of papers published. These include theconnection(s) between galaxies with active central engines and galaxieswith starbursts, the transition from asymptotic giant branch stars towhite dwarfs, gamma-ray bursters, solar data from three major satellitemissions, and the cosmological parameters, including dark matter andvery large scale structure. Several sections are oriented aroundprocesses-accretion, collimation, mergers, and disruptions-shared by anumber of kinds of stars and galaxies. And, of course, there are theusual frivolities of errors, omissions, exceptions, and inventories.
| Supernovae in the nuclear regions of starburst galaxies
The feasibility of using near-infrared observations to discoversupernovae in the nuclear and circumnuclear regions of nearby starburstgalaxies is investigated. We provide updated estimates of the intrinsiccore-collapse supernova rates in these regions. We discuss the problemof extinction, and present new estimates of the extinction towards 33supernova remnants in the starburst galaxy M 82. This is done using Hiand H2 column density measurements. We estimate the molecularto atomic hydrogen mass ratio to be 7.4+/-1.0 in M 82. We have assemblednear-infrared photometric data for a total of 13 core-collapsesupernovae, some unpublished hitherto. This constitutes the largestdatabase of infrared light curves for such events. We show that theinfrared light curves fall into two classes, `ordinary' and `slowlydeclining'. Template JHKL light curves are derived for both classes. Forordinary core-collapse supernovae, the average peak JHKL absolutemagnitudes are -18.4, -18.6, -18.6 and -19.0 respectively. The slowlydeclining core-collapse supernovae are found to be significantly moreluminous than the ordinary events, even at early times, having averagepeak JHKL absolute magnitudes of -19.9, -20.0, -20.0 and -20.4respectively. We investigate the efficiency of a computerized imagesubtraction method in supernova detection. We then carry out a MonteCarlo simulation of a supernova search using K-band images of NGC 5962.The effects of extinction and observing strategy are discussed. Weconclude that a modest observational programme will be able to discovera number of nuclear supernovae.
| Kinematics, ages and metallicities of star clusters in NGC 1316: a 3-Gyr-old merger remnant
We report on multi-object spectroscopy in the red spectral region of 37candidate star clusters in an ~8×8arcmin2 field centredon the giant early-type radio galaxy NGC 1316 (Fornax A), the brightestgalaxy in the Fornax cluster. Out of this sample, 24 targets are foundto be genuine star clusters associated with NGC 1316, and 13 targets areGalactic foreground stars. For the star cluster sample, we measure amean heliocentric velocity vhel=1698+/-46kms-1 anda velocity dispersion σ=227+/-33kms-1 within agalactocentric radius of 24kpc. Partly responsible for the velocitydispersion is a significant rotation in the star cluster system, with amean velocity of ~175+/-70kms-1 along a position angle of~6°+/-18°. Using the projected mass estimator and assumingisotropic orbits, the estimated total mass is(6.6+/-1.7)×1011Msolar within a radius of24kpc. The mass is uncertain by about a factor of 2, depending on theorbital assumptions. The implied M/LB ratio is in the range3-6. Four star clusters in our sample are exceptionally luminous(MV<-12.3). This means that (1) at least this manyclusters in NGC 1316 are up to an order of magnitude more luminous thanthe most luminous star cluster in our Galaxy or M31, and that (2) theS/N ratio of their spectra allows us to measure line strengths with goodaccuracy. By comparing the measured colours and equivalent widths ofHα and the Caii triplet (λλ 8498, 8542, 8662Å)absorption lines for those bright star clusters in our sample with thoseof single-burst population models (the Bruzual & Charlot models), wefind that they are coeval with an age of 3.0+/-0.5Gyr. Theirmetallicities are found to be solar to within +/-0.15dex. We discuss theproperties of the main body of NGC 1316, and conclude that they areconsistent with having hosted a major merger 3Gyr ago as well. Thepresence of intermediate-age globular clusters in NGC 1316 shows onceagain that globular clusters with near-solar metallicity do form duringgalactic mergers, and, moreover, that they can survive disruptionprocesses taking place during the merger (e.g., dynamical friction,tidal disruption), as well as evaporation. In this respect, NGC 1316provides a hitherto `missing' evolutionary link between young mergerremnants of age ~0.5Gyr such as NGC 3597, 3921 and 7252 on one side andolder giant ellipticals featuring bimodal colour distributions on theother side.
| An Ultraviolet through Infrared Look at Star Formation and Super Star Clusters in Two Circumnuclear Starburst Rings
We present broadband (U, V, I, and H) and narrowband (Hα+[N II]and Paα) images of the circumnuclear starburst rings in two nearbyspiral galaxies, NGC 1512 and NGC 5248, obtained with the WFPC2 andNICMOS cameras on HST. Combined with previously published ultraviolet(UV) HST images at 2300 Å, these data provide a particularly widewavelength range with which to study the properties of the stellarpopulations, the gas, and the dust in the rings. The young star clustersand the line-emitting gas have different spatial distributions, withsome large (50 pc scale) line-emitting regions that have littleassociated continuum emission, but a Paα equivalent widthindicating an embedded stellar population a few megayears old. Theobserved Hα/Paα intensity ratios suggest the gas is mixedwith dust, making it effective at completely obscuring some of the youngclusters. We identify the major (about 500 in each galaxy) compactcontinuum sources (super star clusters and individual stars) and analyzetheir spectral energy distributions (SEDs) from 0.2 to 1.6 μm byfitting them with a grid of spectral synthesis models with a range ofages and dust extinction. Most of the visible clusters are only mildlyreddened, with AV=0 to 1 mag, suggesting that the processesthat clear out the gas and dust of the stellar birth clouds areefficient and fast. The patchiness of the dust distribution makes itdifficult to reliably estimate the star formation rate, based on UVcontinuum slope or hydrogen emission-line ratios, in starbursts such asthese. The cluster SEDs are consistent with a range in ages, from 1 to300 Myr, but with only a minority older than a few tens of megayears. Wepoint out an age bias, the result of the steep luminosity function ofthe clusters combined with the fading of clusters as they age, whichcauses young clusters to be overrepresented at any luminosity.Accounting for this bias, the fraction of old clusters is consistentwith continuous star formation in the rings over the past ~300 Myr.Because of the uncertainties in dating the clusters, we cannot rule outepisodic, ~20 Myr long bursts of star formation, but the presence ofUV-bright rings in about 10% of spiral galaxies argues against thispossibility. Although most of the observed SEDs are well fitted by arange of models, some of the brightest young clusters have excessemission in the IR that is not predicted by the models and may bethermal reradiation by circumstellar dust. The cluster mass functionsfollow a power-law distribution with index -2, similar to that recentlyderived for the starburst in the merging Antennae galaxies, andextending to ~105 Msolar. The lack of a mass scalemeans that subsequent evolution of the mass function is required, ifsome of the SSCs are to evolve into globular clusters. The clusters arespatially unresolved or marginally resolved, corresponding to V-bandGaussian radii of less than a few parsecs, at an assumed distance of 10Mpc. In NGC 5248, we report a previously unknown, 60 pc radius, inneremission-line ring, and in NGC 1512, a peculiar compact (0.1" diameter)source with an Hα+[N II] equivalent width of ~7000 Å, whichmay be a so-called Balmer-dominated supernova remnant. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS 5-26555.
| Cold gas and star formation in a merging galaxy sequence
We explore the evolution of the cold gas (molecular and neutralhydrogen) and star formation activity during galaxy interactions, usinga merging galaxy sequence comprising both pre- and post-mergercandidates. Data for this study come from the literature, but aresupplemented by some new radio observations presented here. First, weconfirm that the ratio of far-infrared luminosity to molecular hydrogenmass (LFIRM(H2); star formation efficiency)increases close to nuclear coalescence. After the merging of the twonuclei there is evidence that the star formation efficiency declinesagain to values typical of ellipticals. This trend can be attributed toM(H2) depletion arising from interaction induced starformation. However, there is significant scatter, likely to arise fromdifferences in the interaction details (e.g., disc-to-bulge ratio,geometry) of individual systems. Secondly, we find that the centralmolecular hydrogen surface density, ΣH2,increases close to the final stages of the merging of the two nuclei.Such a trend, indicating gas inflows caused by gravitationalinstabilities during the interaction, is also predicted by numericalsimulations. Furthermore, there is evidence for a decreasing fraction ofcold gas mass from early interacting systems to merger remnants,attributed to neutral hydrogen conversion into other forms (e.g., stars,hot gas) and molecular hydrogen depletion resulting from ongoing starformation. The evolution of the total-radio to blue-band luminosityratio, reflecting the total (disc and nucleus) star formation activity,is also investigated. Although this ratio is on average higher than thatfor isolated spirals, we find a marginal increase along the mergingsequence, attributed to the relative insensitivity of disc starformation to interactions. However, a similar result is also obtainedfor the nuclear radio emission, although galaxy interactions arebelieved to significantly affect the activity (star formation, AGN) inthe central galaxy regions. Nevertheless, the nuclear-radio to blue-bandluminosity ratio is significantly elevated compared with that forisolated spirals. Finally, we find that the FIR-radio flux ratiodistribution of interacting galaxies is consistent with star formationbeing the main energizing source.
| Imaging of the merging galaxy NGC 3597 and its population of protoglobular clusters
We present wide field-of-view near-infrared imaging from the NTT andvery deep optical imaging from the HST of the young merging galaxy NGC3597. The morphology of the galaxy and the properties of the newlyformed protoglobular clusters (PGCs) are examined. Our K-band datareveal the presence of a second nucleus, which provides further evidencethat NGC 3597 is the result of a recent merger. Combining new K-bandphotometry with optical photometry, we are able for the first time toderive a unique age for the newly formed PGCs of a few Myr. This isconsistent with the galaxy starburst age of <=10Myr. From deep HSTimaging, we are able to probe the luminosity function ~8 magnitudesfainter than normal, old globular clusters, and confirm that the PGCshave a power-law distribution with a slope of ~-2.
| Compact Radio Emission from Warm Infrared Galaxies
In this paper, we present a comparison between the optical spectroscopicdata and the incidence of compact radio emission for a sample of 60 warminfrared galaxies. We find that 80% of optically classified activegalactic nucleus (AGN)-type galaxies contain compact radio sources,while 37% of optically classified starburst galaxies contain compactradio sources. The compact radio luminosity shows a bimodaldistribution, indicating two populations in our sample. The majority ofthe higher radio luminosity class (L>104Lsolar) are AGNs, while the majority of the lower radioluminosity class (L<104 Lsolar) are starbursts.The compact radio emission in the starburst galaxies may be due toeither obscured AGNs or complexes of extremely luminous supernovae suchas that seen in Arp 220. The incidence of optically classified AGNsincreases with increasing far-infrared (FIR) luminosity. Using FIRcolor-color diagrams, we find that globally the energetics of 92% of thegalaxies in our sample are dominated by starburst activity, including60% of galaxies that we find to contain AGNs on the basis of theiroptical classification. The remainder are energetically dominated bytheir AGNs in the infrared. For starburst galaxies, electron densityincreases with dust temperature, consistent with the merger model forinfrared galaxies.
| Kinematics, Ages, and Metallicities of Star Clusters in NGC 1316 (Fornax A):\ A 3 Gyr Old Merger Remnant
We report on multi-object spectroscopy of 37 candidate star clustersaround the giant early-type radio galaxy NGC 1316 (Fornax A), thebrightest galaxy in the Fornax cluster, using the ESO 3.5-m NewTechnology Telescope. For the star cluster sample, we measure a meanheliocentric velocity v hel = 1698 +/- 46 km s-1,a rotation velocity v rot = 175 +/- 70 km s-1 anda velocity dispersion σ = 227 +/- 33 km s-1. Using theprojected mass estimator and assuming isotropic orbits, the estimatedtotal mass is (6.6 +/- 1.7) x 1011 Msun within aradius of 24 kpc. The mass is uncertain by about a factor of two,depending on the orbital assumptions. The impliedM/LB ratio is in the range 3 -- 6. Four starclusters in our sample are exceptionally luminous (MV <-12.3). This means that (1) at least this many clusters in NGC 1316 areup to an order of magnitude more luminous than the most luminous starcluster in our Galaxy or M 31, and (2) that the S/N ratio of theirspectra allows us to measure line strengths with good accuracy. Bycomparing the measured colours and equivalent widths of Hα and theCa ii triplet (λ λ 8498, 8542, 8662 Å) absorptionlines for those bright star clusters in our sample with those ofsingle-burst population models (of Bruzual & Charlot), we find thatthey are coeval with an age of 3.0 +/- 0.5 Gyr. Their metallicities arefound to be solar to within +/- 0.15 dex. We discuss the properties ofthe main body of NGC 1316 and conclude they are consistent with havinghosted a major merger 3 Gyr ago as well. The presence ofintermediate-age globular clusters in NGC 1316 shows once again thatglobular clusters with near-solar metallicity do form during galaxymergers, and, moreover, that they can survive disruption processestaking place during the merger (e.g., dynamical friction, tidaldisruption), as well as evaporation. In this respect, NGC 1316 providesa hitherto missing evolutionary link between young merger remnants ofage ~ 0.5 Gyr such as NGC 3597, NGC 3921 and NGC 7252 on one side, andolder giant ellipticals featuring bimodal colour distributions on theother side.
| Effects of Late Mergers on Stellar Populations in E and S0 Galaxies
Not Available
| Globular Cluster Systems in Other Galaxies
Not Available
| Deep Hubble Space Telescope Observations of Blue Star Clusters in NGC 3597
We have analyzed HST/WFPC2 images of NGC 3597 and find ~700 compactobjects surrounding the galaxy with an average (B-R)_0~0.6. We proposethat the majority of these objects are young globular clusters. Theyhave a spread in colors that is consistent with that expected for apopulation of young clusters with a common age and spread induced byphotometric errors and reddening within NGC 3597. If these objects weresimilar to the Galactic globular cluster system seen at a younger age,we would predict a turnover in the luminosity function at B~23. However,we find that the luminosity function for the clusters is increasing tothe limits of our photometry (B~27).
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