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Enhanced mass-to-light ratios in ultracompact dwarf galaxies through tidal interaction with the centre of the host galaxy
A recent study of ultracompact dwarf galaxies (UCDs) in the Virgocluster revealed that some of them show faint envelopes and havemeasured mass-to-light ratios (M/Ls) of 5 and larger, which cannot beexplained by simple population-synthesis models. It is believed thatthis proves that some of the UCDs must possess a dark matter halo andmay therefore be the stripped nuclei of dwarf ellipticals rather thanmerged star cluster complexes.Using an efficient N-body method, we investigate if a close passage of aUCD through the central region of the host galaxy is able to enhance themeasured M/L by tidal forces leaving the satellite slightly out ofvirial equilibrium and thereby leading to an overestimation of itsvirial mass.We find this to be possible and discuss the general problem of measuringdynamical masses for objects that are probably interacting with theirhosts.
| A data-driven Bayesian approach for finding young stellar populations in early-type galaxies from their ultraviolet-optical spectra
Efficient predictive models and data analysis techniques for theanalysis of photometric and spectroscopic observations of galaxies arenot only desirable, but also required, in view of the overwhelmingquantities of data becoming available. We present the results of a novelapplication of Bayesian latent variable modelling techniques, where wehave formulated a data-driven algorithm that allows one to explore thestellar populations of a large sample of galaxies from their spectra,without the application of detailed physical models. Our only assumptionis that the galaxy spectrum can be expressed as a linear superpositionof a small number of independent factors, each a spectrum of a stellarsubpopulation that cannot be individually observed. A probabilisticlatent variable architecture that explicitly encodes this assumption isthen formulated, and a rigorous Bayesian methodology is employed forsolving the inverse modelling problem from the available data. Apowerful aspect of this method is that it formulates a density model ofthe spectra, based on which we can handle observational errors. Further,we can recover missing data both from the original set of spectra whichmight have incomplete spectral coverage of each galaxy, or frompreviously unseen spectra of the same kind.We apply this method to a sample of 21 ultraviolet-optical spectra ofwell-studied early-type galaxies, for which we also derive detailedphysical models of star formation history (i.e. age, metallicity andrelative mass fraction of the component stellar populations). We alsoapply it to synthetic spectra made up of two stellar populations,spanning a large range of parameters. We apply four different datamodels, starting from a formulation of principal component analysis(PCA), which has been widely used. We explore alternative factor models,relaxing the physically unrealistic assumption of Gaussian factors, aswell as constraining the possibility of negative flux values that areallowed in PCA, and show that other models perform equally well orbetter, while yielding more physically acceptable results. Inparticular, the more physically motivated assumptions of our rectifiedfactor analysis enable it to perform better than PCA, and to recoverphysically meaningful results.We find that our data-driven Bayesian modelling allows us to identifythose early-type galaxies that contain a significant stellar populationthat is <~1-Gyr old. This experiment also concludes that our sampleof early-type spectra showed no evidence of more than two major stellarpopulations differing significantly in age and metallicity. This methodwill help us to search for such young populations in a large ensemble ofspectra of early-type galaxies, without fitting detailed models, andthereby to study the underlying physical processes governing theformation and evolution of early-type galaxies, particularly thoseleading to the suppression of star formation in dense environments. Inparticular, this method would be a very useful tool for automaticallydiscovering various interesting subclasses of galaxies, for example,post-starburst or E+A galaxies.
| Two Populations of Young Massive Star Clusters in Arp 220
We present new optical observations of young massive star clusters inArp 220, the nearest ultraluminous infrared galaxy, taken in UBVI withthe Hubble Space Telescope ACS HRC camera. We find a total of 206probable clusters whose spatial distribution is centrally concentratedtoward the nucleus of Arp 220. We use model star cluster tracks todetermine ages, luminosities, and masses for 14 clusters with completeUBVI indices or previously published near-infrared data. We estimaterough masses for 24 additional clusters with I<24 mag from BVIindices alone. The clusters with useful ages fall into two distinctgroups: a ``young'' population (<10 Myr) and an intermediate-agepopulation (~=300 Myr). There are many clusters with masses clearlyabove 106 Msolar and possibly even above107 Msolar in the most extreme instances. Thesemasses are high enough that the clusters being formed in the Arp 220starburst can be considered to be genuine young globular clusters. Inaddition, this study allows us to extend the observed correlationbetween global star formation rate and maximum cluster luminosity bymore than 1 order of magnitude in star formation rate.
| Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters
We present new (B, I) photometry for the globular cluster systems ineight brightest cluster galaxies (BCGs), obtained with the ACS/WFCcamera on the Hubble Space Telescope. In the very rich cluster systemsthat reside within these giant galaxies, we find that all have stronglybimodal color distributions that are clearly resolved by themetallicity-sensitive (B-I) index. Furthermore, the mean colors andinternal color range of the blue subpopulation are remarkably similarfrom one galaxy to the next, to well within the +/-0.02-0.03 maguncertainties in the foreground reddenings and photometric zero points.By contrast, the mean color and internal color range for the redsubpopulation differ from one galaxy to the next by twice as much as theblue population. All the BCGs show population gradients, with muchhigher relative numbers of red clusters within 5 kpc of their centers,consistent with their having formed at later times than the blue,metal-poor population. A striking new feature of the color distributionsemerging from our data is that for the brightest clusters(MI<-10.5) the color distribution becomes broad and lessobviously bimodal. This effect was first noticed by Ostrov et al. andDirsch et al. for the Fornax giant NGC 1399; our data suggest that itmay be a characteristic of many BCGs and perhaps other large galaxies.Our data indicate that the blue (metal-poor) clusters brighter thanMI~=-10 become progressively redder with increasingluminosity, following a mass/metallicity scaling relationZ~M0.55. A basically similar relation has been found for M87by Strader et al. (2005). We argue that these GCS characteristics areconsistent with a hierarchical-merging galaxy formation picture in whichthe metal-poor clusters formed in protogalactic clouds or densestarburst complexes with gas masses in the range107-1010 Msolar, but where the moremassive clusters on average formed in bigger clouds with deeperpotential wells where more preenrichment could occur.
| The relation between mergers and AGN activity. Results from radio galaxy and luminous infrared galaxy studies
There is morphological evidence that the activity in powerful radiogalaxies could be triggered by mergers and galaxy interactions. However,nothing is known about the timescales, order of events, and the type ofinteraction involved. It is not yet known whether there exists anevolutionary link between radio galaxies and other merger systems suchas very luminous and ultraluminous infrared galaxies (VLIRGs andULIRGs). Here, we report preliminary results obtained from the analysisof near-ultraviolet and optical spectroscopic observations of samples ofRadio Galaxies and VLIRGs-ULIRGs to investigate, through age-dating oftheir young stellar population, whether an evolutionary link existsbetween VLIRGs/ULIRGs-Radio Galaxies-normal elliptical galaxies. Theseresults will help to understand the genesis events that lead to theformation of radio jet and quasar activity, and they will allow us toplace radio galaxies in the context of hierarchical evolution models forthe population of giant elliptical galaxies.
| Neutral hydrogen in radio galaxies: Results from nearby, importance for far away
The study of neutral hydrogen emission and absorption in radio galaxiesis giving new and important insights on a variety of phenomena observedin these objects. Such observations are helping to understand the originof the host galaxy, the effects of the interaction between the radio jetand the ISM, the presence of fast gaseous outflows as well asjet-induced star formation. Recent results obtained on these phenomenaare summarized in this review. Although the {H I observationsconcentrate on nearby radio galaxies, the results also have relevancefor the high-z objects as all these phenomena are important, and likelyeven more common, in high-redshift radio sources.
| Spectroscopic Metallicities for Fornax Ultracompact Dwarf Galaxies, Globular Clusters, and Nucleated Dwarf Elliptical Galaxies
Various formation channels for the puzzling ultracompact dwarf galaxies(UCDs) have been proposed in the last few years. To better judge some ofthe competing scenarios, we present spectroscopic [Fe/H] estimates for asample of 26 compact objects in the central region of the FornaxCluster, covering the magnitude range of UCDs and bright globularclusters (18 mag-11 mag. This metallicity break is accompanied by achange in the size-luminosity relation for compact objects, as deducedfrom Hubble Space Telescope imaging: for MV<-11 mag,rh scales with luminosity, while for MV>-11mag, rh is almost luminosity-independent. In our study wetherefore assume a limiting absolute magnitude of MV=-11 magbetween UCDs and globular clusters. The mean metallicity of five Fornaxnucleated dwarf elliptical galaxy (dE,N) nuclei included in our study isabout 0.8 dex lower than that of the UCDs, a difference significant atthe 4.5 σ level. This difference is marginally higher thanexpected from a comparison of their (V-I) colors, indicating that UCDsare younger than or at most coeval to dE,N nuclei. Because of the largemetallicity discrepancy between UCDs and nuclei, we disfavor thehypothesis that most of the Fornax UCDs are the remnant nuclei oftidally stripped dE,Ns. Our metallicity estimates for UCDs are closer tobut slightly below those derived for young massive clusters (YMCs) ofcomparable masses. We therefore favor a scenario in which most UCDs inFornax are successors of merged YMCs produced in the course of violentgalaxy-galaxy mergers. It is noted that, in contrast, the properties ofVirgo UCDs are more consistent with the stripping scenario, suggestingthat different UCD formation channels may dominate in either cluster.Based partly on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster
We have discovered nine ultracompact dwarf galaxies (UCDs) in the VirgoCluster, extending samples of these objects outside the Fornax Cluster.Using the Two Degree Field (2dF) multifiber spectrograph on theAnglo-Australian Telescope, the new Virgo members were found among 1500color-selected, starlike targets with 16.0
| A Spectroscopic Study of the Star-Forming Properties of the Center of NGC 4194
We have obtained Space Telescope Imaging Spectrograph long-slit spectraof the central region of the advanced merger NGC 4194. The spectra coverthe wavelength ranges 1150-1750 Å in the UV and 2900-10270 Åin the visible. Results from the study of the properties of 14star-forming regions (knots) are presented. If the [N II] contributionis 40% of the combined Hα + [N II] flux, then the averageE(B-V)=0.7 mag. The metal abundances are approximately solar, withindividual knot abundances ranging from log(O/H)+12=8.1+/-0.5 to8.9+/-0.4. The Hα luminosities of the 14 observed knots yield atotal star formation rate (SFR) of ~46 Msolaryr-1. The sizes of the H II regions associated with the knotswere determined from L(Hβ) and range from ~28 to ~119 pc when afilling factor of 0.1 is assumed. The sizes are a factor of ~2.15smaller for a filling factor of 1. Using Starburst99, the EW(Hα +[N II]), and EW(Hβ), we estimate the ages of the star-formingregions to be 5.5-10.5 Myr. From ground-based spectra the effectivetemperatures of the H II regions are found to be ~11,000 K, and theelectron densities are determined to be ~530 cm-3. We findthat eight of the knots probably formed with a Salpeter initial massfunction truncated at an upper mass of 30 Msolar, and one ofthe knots likely formed with a standard Salpeter initial mass function.We suggest that the knots in our sample are the precursors of globularclusters. Two of the knots are in a region of flowing gas and are amongthe most massive, are the largest in radius, have the highest SFR, andare among the youngest of the knots.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| 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.
| The impact of starbursts and post-starbursts on the photometric evolution of high redshift galaxies
We present evolutionary synthesis models for galaxies of spectral typesSa through Sd with starbursts of various strengths triggered at variousredshifts and study their photometric evolution before, during, andafter their bursts in a cosmological context. We find that bursts athigh redshift, even very strong ones, only cause a small blueing oftheir intrinsically blue young parent galaxies. At lower redshift, incontrast, even small bursts cause a significant blueing of theirintrinsically redder galaxies. While the burst phase is generally short,typically a few hundred Myr in normal-mass galaxies, the postburst stagewith its red colors and, in particular the very red ones for earlybursts at high redshift, lasts much longer, on the order of several Gyr.We find that, even without any dust, which in the postburst stage is notexpected to play an important role anyway, models easily reach thecolors of EROs in the redshift range z 2 through z 0.5 afterstarbursts at redshifts between 2 and 4. We therefore propose a thirdalternative for the ERO galaxies beyond the two established ones ofpassive galaxies vs. dusty starbursts: the dust-freepost-(strong-)starbursts. A very first comparison of our models to HDFdata with photometric redshifts shows that almost all of the outliersthat could not be described with our chemically consistent models forundisturbed normal galaxy types E through Sd can now be explained verywell. Galaxies in the redshift range from z 2.5 to z 0.5 thatare redder, and in some cases much redder, than our reddest undisturbedmodel for a high-metallicity classical elliptical are well described bypost-starburst models after starbursts at redshifts between 2 and 4.Galaxies bluer than our bluest low metallicity Sd model, most of whichhave redshifts lower than 1, are well explained by ongoing starbursts.
| Accurate photometry of extended spherically symmetric sources
We present a new method to derive reliable photometry of extendedspherically symmetric sources from HST images (WFPC2, ACS/WFC andNICMOS/NIC2 cameras), extending existing studies of point sources andmarginally resolved sources. We develop a new approach to accuratelydetermine intrinsic sizes of extended spherically symmetric sources,such as star clusters in galaxies beyond the Local Group (at distances20 Mpc), and provide a detailed cookbook to perform aperturephotometry on such sources, by determining size-dependent aperturecorrections (ACs) and taking sky oversubtraction as a function of sourcesize into account. In an extensive Appendix, we provide the parametersof polynomial relations between the FWHM of various input profiles andthose obtained by fitting a Gaussian profile (which we have used forreasons of computational robustness, although the exact model profileused is irrelevant), and between the intrinsic and measured FWHM of thecluster and the derived AC. Both relations are given for a number ofphysically relevant cluster light profiles, intrinsic and observationalparameters. AC relations are provided for a wide range of apertures.Depending on the size of the source and the annuli used for thephotometry, the absolute magnitude of such extended objects can beunderestimated by up to 3 mag, corresponding to an error in mass of afactor of 15. We carefully compare our results to those from the morewidely used DeltaMag method, and find an improvement of a factor of 3-40in both the size determination and the AC.
| The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters
We introduce a method to relate a possible truncation of the starcluster mass function at the high mass end to the shape of the clusterluminosity function (LF). We compare the observed LFs of five galaxiescontaining young star clusters with synthetic cluster population modelswith varying initial conditions. The LF of the SMC, the LMC and NGC 5236are characterized by a power-law behavior N d L ∝L-α d L, with a mean exponent of < α> = 2.0± 0.2. This can be explained by a cluster population formed witha constant cluster formation rate, in which the maximum cluster mass perlogarithmic age bin is determined by the size-of-sample effect andtherefore increases with log (age/yr). The LFs of NGC 6946 and M 51 arebetter described by a double power-law distribution or a Schechterfunction. When a cluster population has a mass function that istruncated below the limit given by the size-of-sample effect, the totalLF shows a bend at the magnitude of the maximum mass, with the age ofthe oldest cluster in the population, typically a few Gyr due todisruption. For NGC 6946 and M 51 this suggests a maximum mass of M_max= 0.5-1×10^6 Mȯ, although the bend is only a 1-2σ detection. Faint-ward of the bend the LF has the same slope asthe underlying initial cluster mass function and bright-ward of the bendit is steeper. This behavior can be well explained by our populationmodel. We compare our results with the only other galaxy for which abend in the LF has been observed, the "Antennae" galaxies (NGC4038/4039). There the bend occurs brighter than in NGC 6946 and M 51,corresponding to a maximum cluster mass of M_max =1.3-2.5×106 Mȯ. Hence, if the maximumcluster mass has a physical limit, then it can vary between differentgalaxies. The fact that we only observe this bend in the LF in the"Antennae" galaxies, NGC 6946 and M 51 is because there are enoughclusters available to reach the limit. In other galaxies there might bea physical limit as well, but the number of clusters formed or observedis so low, that the LF is not sampled up to the luminosity of the bend.The LF can then be approximated with a single power-law distribution,with an index similar to the initial mass function index.
| Star formation and stellar populations in the Wolf-Rayet(?) luminous compact blue galaxy IRAS 08339+6517
Aims.IRAS 08339+6517 is a luminous infrared and Lyα-emittingstarburst galaxy that possesses a dwarf companion object at a projecteddistance of 56 kpc. An H I tidal tail has recently been detected betweenboth galaxies, suggesting that about 70% of the neutral gas has beenejected from them.Methods.We present deep broad-band optical images,together with narrow band Hα CCD images, and opticalintermediate-resolution spectroscopy of both galaxies.Results.The imagesreveal interaction features between both systems and strong Hαemission in the inner part of IRAS 08339+6517. The chemical compositionof the ionized gas of the galaxies is rather similar. The analysis oftheir kinematics also indicates interaction features and reveals anobject that could be a candidate tidal dwarf galaxy or a remnant of anearlier merger. Our data suggest that the H I tail has been mainlyformed from material stripped from the main galaxy. We find weakspectral features that could be attributed to the presence of Wolf-Rayetstars in this starburst galaxy and estimate an age of the most recentburst of around 4-6 Myr. A more evolved underlying stellar population,with a minimal age between 100-200 Myr, is also detected and fits anexponential intensity profile. A model which combines 85% young and 15%old populations can explain both the spectral energy distribution andthe H I Balmer and He I absorption lines presented in our spectrum. Thestar formation rate of the galaxy is consistently derived using severalcalibrations, giving a value of ~9.5 Mȯyr-1.Conclusions.IRAS 08339+6517 does satisfy the criteria ofa luminous compact blue galaxy, rare objects in the local universe butcommon at high redshifts, being a very interesting target for detailedstudies of galaxy evolution and formation.
| Peculiarities and populations in elliptical galaxies. III. Dating the last star formation event
Using 6 colours and 4 Lick line-indices we derive two-component modelsof the populations of ellipticals, involving a "primary" and a"juvenile" population. The first component is defined by the regressionsof indices against the central velocity dispersion found in Papers I andII for the {Nop} sample of non-peculiar objects. The second one isapproximated by an SSP, and the modeling derives its age A, metallicityZ and fractional V-luminosity q_V, the fractional mass qMbeing found therefrom. The model is designed for "blueish" peculiargalaxies, i.e. the {Pec} sample and NGC 2865 family in the terminologyof Paper I. The morphological peculiarities and the population anomalyare then believed to involve the same event, i.e. a merger plusstarburst. It is possible to improve the models in a few cases byintroducing diffuse dust (as suggested by far IR data), and/or by takinginto account the fact that Lick- and colour indices do not relate toidentical galaxy volumes. In most of the cases, the mass ratio of youngstars qM seems too small for the product of a recent majormerger: the events under consideration might be minor mergers bringing"the final touch" to the build-up of the structure of the E-type object.The same modeling has been successfully applied to blueish galaxies ofthe {Nop} sample, without morphological peculiarities however, tosupport the occurence of a distinct perturbing event. A few reddishobjects of the {Pec} sample (NGC 3923 family) and of the {Nop} sampleare also modeled, in terms of an excess of high metallicity stars, ordiffuse dust, or both, but the results are inconclusive.
| Dynamical mass estimates for two luminous star clusters in galactic merger remnants
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
| Observational evidence for a truncation of the star cluster initial mass function at the high mass end
We present the luminosity function (LF) of star clusters in M 51 basedon HST/ACS observations taken as part of the Hubble Heritage project.The clusters are selected based on their size and with the resulting5990 clusters we present one of the largest cluster samples of a singlegalaxy. We find that the LF can be approximated with a double power-lawdistribution with a break around MV = -8.9. On the brightside the index of the power-law distribution is steeper (α = 2.75)than on the faint-side (α = 1.93), similar to what was foundearlier for the "Antennae" galaxies. The location of the bend, however,occurs about 1.6 mag fainter in M 51. We confront the observed LF withthe model for the evolution of integrated properties of clusterpopulations of Gieles et al. (2006, A&A, accepted), which predictsthat a truncated cluster initial mass function would result in a bendin, and a double power-law behaviour of, the integrated LF. Thecombination of the large field-of view and the high star clusterformation rate of M 51 make it possible to detect such a bend in the LF.Hence, we conclude that there exists a fundamental upper limit to themass of star clusters in M 51. Assuming a power-law cluster initial massfunction with exponentional cut-off of the form N d M ∝M-β exp(-M/M_C) d M, we find that MC =105 Mȯ. A direct comparison with the LF ofthe "Antennae" suggests that there MC =4×105 Mȯ.
| Young star cluster complexes in NGC 4038/39. Integral field spectroscopy using VIMOS-VLT
We present the first results of a survey to obtain Integral FieldSpectroscopy of merging galaxies along the Toomre Sequence. In thepresent work, we concentrate on the star cluster complexes in theAntennae galaxies (NGC 4038/39) in the overlap region as well as thenuclear region of NGC 4038. Using optical spectroscopy we derive theextinction, age, metallicity, velocity, velocity dispersion of the gasand star formation rate for each of the eight complexes detected. Wesupplement this study with archival HST-WFPC2 U, B, V, Hα, and Iband imaging. Correcting the observed colours of the star clusterswithin the complexes for extinction, measured through our opticalspectra, we compare the clusters with simple stellar population models,with which we find an excellent agreement, and hence proceed to derivethe ages and masses of the clusters from comparison with the models. Infive of the complexes we detect strong Wolf-Rayet emission features,indicating young ages (3-5 Myr). The ionized gas surrounding thecomplexes is expanding at speeds of 20{-}40 km s-1. This slowexpansion can be understood as a bubble, caused by the stellar winds andsupernovae within the complexes, expanding into the remnant of theprogenitor giant molecular cloud. We also find that the complexesthemselves are grouped, at about the largest scale of which young starclusters are correlated, representing the largest coherent star formingregion. We show that the area normalized star formation rates of thesecomplexes clearly place them in the regime of star forming regions instarburst galaxies, thereby justifying the label of localizedstarbursts. Finally, we estimate the stability of the complexes, andfind that they will probably loose a large fraction of their mass to thesurrounding environment, although the central regions may merge into asingle large star cluster.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| Environmental dependence of the structure of brightest cluster galaxies
We measure the Petrosian structural properties of 33 brightest clustergalaxies (BCGs) at redshifts z<= 0.1 in X-ray selected clusters witha wide range of X-ray luminosities. We find that some BCGs show distinctsignatures in their Petrosian profiles, likely to be due to cD haloes.We also find that BCGs in high X-ray luminosity clusters have shallowersurface brightness profiles than those in low X-ray luminosity clusters.This suggests that the BCGs in high X-ray luminosity clusters haveundergone up to twice as many equal-mass mergers in their past as thosein low X-ray luminosity clusters. This is qualitatively consistent withthe predictions of hierarchical structure formation.
| Orientation and size of the `Z' in X-shaped radio galaxies
Some X-shaped radio galaxies show a Z-symmetric morphology in the lessluminous secondary lobes. Within the scenario of a merger between twogalaxies, each hosting a supermassive black hole in its centre, thisstructure has been explained before. As the smaller galaxy spiralstowards the common centre, it releases gas to the interstellar medium ofthe larger active galaxy. The ram pressure of this streaming gas willbend the lobes of the pre-merger jet into a Z-shape. After the blackholes have merged, the jet propagates in a new direction that is alignedwith the angular momentum of the binary black hole. In this paper wedeproject the pre- and post-merger jets. Taking into account theexpected angles between the jet pairs and with the assumption that theirdirections are uncorrelated, we show that one of three possibleorientations of the jets with respect to the line of sight is morelikely than the others. This actually depends on the distance where thebending occurs. Another result of our deprojection is that the streaminggas bends the jet into a Z-shape in a range between about 30 and 100 kpcdistance to the centre of the primary galaxy. We confirm this finding bycomparing our predictions for the properties of the rotational velocityfield and its radius with observations and numerical simulations ofmerging galaxies. Thus, our results support the merger scenario asexplanation for X- and Z-shaped radio galaxies with the jet pointingalong the former axis of orbital angular momentum of the binary.
| Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies
Evolutionary population synthesis models for a wide range ofmetallicities, ages, star formation histories, initial mass functionsand horizontal branch morphologies, including blue morphologies at highmetallicity, are computed. The model output comprises spectral energydistributions, colours, stellar M/L ratios, bolometric corrections andnear-infrared (IR) spectral line indices. The energetics of the postmain sequence evolutionary phases are evaluated with the fuelconsumption theorem. The impact on the models of the stellarevolutionary tracks (in particular with and without overshooting) isassessed. We find modest differences in synthetic broad-band colours asinduced by the use of different tracks in our code [e.g. Δ(V-K) ~0.08 mag, Δ(B-V) ~ 0.03 mag]. Noticeably, these differences aresubstantially smaller than the scatter among other models in theliterature, even when the latter adopt the same evolutionary tracks. Themodels are calibrated with globular cluster data from the Milky Way forold ages, and the Magellanic clouds plus the merger remnant galaxy NGC7252, both for young ages of ~0.1-2Gyr, in a large wavelength range fromthe U band to the K band. Particular emphasis is put on the contributionfrom the thermally pulsing asymptotic giant branch (TP-AGB) phase. Weshow that this evolutionary phase is crucial for the modelling of youngstellar populations by direct comparison with observed spectral energydistributions of Magellanic cloud clusters, which are characterized byrelatively high fluxes, both blueward and redward of the V band. We findthat the combination of the near-IR spectral indices C2 andH2O can be used to determine the metallicity of ~1 Gyrstellar populations. As an illustrative application, we re-analyse thespectral energy distributions of some of the high-z galaxies (2.4<~z<~ 2.9) observed with the Spitzer Space Telescope by Yan et al.Their high rest-frame near-IR fluxes is reproduced very well with themodels including TP-AGB stars for ages in the range ~0.6-1.5Gyr,suggesting formation redshifts for these objects around z~ 3-6.
| Systematic uncertainties in the analysis of star cluster parameters based on broad-band imaging observations
High-resolution Hubble Space Telescope (HST) imaging observations ofstar cluster systems provide a very interesting and useful alternativeto spectroscopic studies for stellar population analyses with 8-m classtelescopes. Here, we assess the systematic uncertainties in (young)cluster age, mass and (to a lesser extent) extinction and metallicitydeterminations, based on broad-band imaging observations with the HST.Our aim here is to intercompare the results obtained using a variety ofcommonly used modelling techniques, specifically with respect to our ownextensively tested multidimensional approach. Any significantdifferences among the resulting parameters are due to the details of thevarious, independently developed, modelling techniques used, rather thanto the stellar population models themselves. Despite the modeluncertainties and the selection effects inherent to most methods used,we find that the peaks in the relative age and mass distributions of agiven young (<~109 yr) cluster system can be derivedrelatively robustly and consistently, to accuracies ofσt≡Δ<= 0.35 andσM≡Δ<=0.14, respectively, assuming Gaussian distributions in cluster ages andmasses for reasons of simplicity. The peaks in the relative massdistributions can be obtained with a higher degree of confidence thanthose in the relative age distributions, as exemplified by the smallerspread among the peak values of the mass distributions derived. Thisimplies that mass determinations are mostly insensitive to the approachadopted. We reiterate that as extensive a wavelength coverage aspossible is required to obtain robust and internally consistent age andmass estimates for the individual objects, with reasonableuncertainties. Finally, we conclude that the actual filter systems usedfor the observations should be used for constructing model colours,instead of using conversion equations, to achieve more accuratederivations of ages and masses.
| A possible formation scenario for the ultramassive cluster W3 in NGC 7252
The intermediate age star cluster W3 (age ~300-500 Myr) in NGC 7252 isthe most luminous star cluster known to date with a dynamical massestimate of 8 +/- 2 × 107Msolar. With aneffective radius of about 17.5 pc and a velocity dispersion of 45kms-1 this object may be viewed as one of the recentlydiscovered ultracompact dwarf galaxies (UCDs). Its intermediate age,however, precludes an origin as a stripped nucleated dwarf ellipticalgalaxy.The galaxy NGC 7252 is a merger remnant from two gas-rich disc galaxies.Interactions between two gas-rich galaxies lead to bursts of intensestar formation. The age of the interaction and the age of W3 are in goodagreement, suggesting that W3 was probably formed in the starburst.We propose a formation scenario for W3. Observations of interactinggalaxies reveal regions of strong star formation forming from dozens upto hundreds of star clusters in confined regions of up to severalhundred parsecs in diameter. The total mass of new stars in theseregions can reach 107 or even108Msolar. By means of numerical simulations wehave shown that the star clusters in these regions merge on shorttime-scales of a few Myr up to a few hundred Myr. We apply this scenarioto W3 and predict properties which could be observable and the futureevolution of this object.This work lends credence to the notion that at least some of the UCDsmay be evolved star cluster complexes formed during early hierarchicalmergers.
| Evolutionary history of the elliptical galaxy NGC 1052
We have obtained Keck spectra for 16 globular clusters (GCs) associatedwith the merger remnant elliptical NGC 1052, as well as a long-slitspectrum of the galaxy. We derive ages, metallicities and abundanceratios from simple stellar population models using the recentlypublished methods of Proctor & Sansom, applied to extragalactic GCsfor the first time. A number of GCs indicate the presence of strong bluehorizontal branches that are not fully accounted for in the currentstellar population models. We find all of the GCs to be ~13 Gyr oldaccording to simple stellar populations, with a large range ofmetallicities. From the galaxy spectrum we find NGC 1052 to have aluminosity-weighted central age of ~2 Gyr and metallicity of[Fe/H]~+0.6. No strong gradients in either age or metallicity were foundto the maximum radius measured (0.3re~= 1 kpc). However, wedo find a strong radial gradient in α-element abundance, whichreaches a very high central value. The young central starburst age isconsistent with the age inferred from the HI tidal tails and infallinggas of ~1 Gyr. Thus, although NGC 1052 shows substantial evidence for arecent merger and an associated starburst, it appears that the mergerdid not induce the formation of new GCs, perhaps suggesting that littlerecent star formation occurred. This interpretation is consistent with`frosting' models for early-type galaxy formation.
| XMM-Newton observations of the interacting galaxy pairs NGC 7771/0 and NGC 2342/1
We present XMM-Newton X-ray observations of the interacting galaxy pairsNGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we areable to resolve the X-ray emission into a bright central source plus twobright (LX > 1040 erg s-1)ultraluminous X-ray sources (ULXs) located either end of the bar. In thebright central source (LX~ 1041 ergs-1), the soft emission is well-modelled by a two-temperaturethermal plasma with kT= 0.4/0.7 keV. The hard emission is modelled witha flat absorbed power-law (Γ~ 1.7, NH~ 1022cm-2), and this together with a low-significance (1.7σ)~ 300 eV equivalent width emission line at ~6 keV are the firstindications that NGC 7771 may host a low-luminosity AGN. For the barULXs, a power-law fit to X-1 is improved at the 2.5σ level withthe addition of a thermal plasma component (kT~ 0.3 keV), while X-2 isimproved only at the 1.3σ level with the addition of a discblackbody component with Tin~ 0.2 keV. Both sources arevariable on short time-scales implying that their emission is dominatedby single accreting X-ray binaries (XRBs). The three remaining galaxies,NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of0.2, 1.8 and 0.9 × 1041 erg s-1,respectively (0.3-10 keV). Their integrated spectra are alsowell-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Γ= 1.8-2.3that are likely to represent the integrated emission of populations ofXRBs as observed in other nearby merger systems. A comparison with otherisolated, interacting and merging systems shows that all four galaxiesfollow the established correlations for starburst galaxies betweenX-ray, far-infrared and radio luminosities, demonstrating that theirX-ray outputs are dominated by their starburst components.
| Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations
CG J1720-67.8 is an ultra compact group of several galaxies with alow-velocity dispersion, and displaying the hallmarks of mutualinteraction and possible tidal dwarf galaxy formation. In hierarchicalmodels, the system is a possible precursor to a massive ellipticalgalaxy. In this paper, we use new optical integral field spectroscopicand radio observations to investigate the evolutionary status of thegroup in more detail: global star formation rates are estimated usingHα and 1.4-GHz radio continuum measurements; HI observationsprovide an upper limit to the global neutral gas content; opticalbroadband colours and spectra provide ages and stellar mass estimatesfor the tidal dwarf candidates; the bi-dimensional Hα velocityfield is used to trace the kinematics of the group and its members,which are compared with numerical simulations of galaxy encounters. Theobservations suggest a model in which multiple interactions haveoccurred, with the latest strong encounter involving at least two majorcomponents within the last 200Myr. Debris from the encounter fuelsongoing star formation at the global level of~20Msolaryr-1, with self-gravity within the tidaltail possibly providing a mechanism to enhance the star formation rateof the tidal dwarf candidates, with bursts of star formation in clumpsof mass ~2 × 107 Msolar appearing within thelast 10Myr. The amount of time required for final merging of all groupcomponents remains uncertain.
| Advanced mergers of galaxies: luminosity profiles and dynamics
We have analyzed the near-IR Ks-band data from the 2MASS archivaldatabase for a large sample of twenty-seven galaxies that show signs ofinteraction but have a single nucleus. Surprisingly, half of theseadvanced mergers of galaxies show luminosity profiles which have anouter exponential fall-off with radius, as in a spiral galaxy (Chitreand Jog 2002). The kinematical data for two of these, Arp 224 and Arp214, available from the HYPERCAT database were used and these showkinematics similar to elliptical galaxies with the random motiondominating the rotation (Jog and Chitre 2002). The origin of the mixedproperties shown by these mergers is a puzzle. To understand thesesystems, we have recently studied the dynamics of mergers via N-bodysimulations for unequal-mass mergers covering a new range of galaxy massratios 4:1-10:1 (Bournaud, Combes, and Jog 2004). We show that suchmergers naturally result in remnants that have the mixed properties aswe have observed from the 2MASS data analysis. The transition betweenelliptical and disk-like remnants is found to occur over a narrow rangeof galaxy mass ratios 3:1-4.5:1 (Bournaud, Jog and Combes 2005).
| Atomic and Molecular Gas in Colliding Galaxy Systems. I. The Data
We present H I and CO (1-0) interferometric observations of 10comparable-mass interacting systems obtained at the Very Large Array(VLA) and the Owens Valley Radio Observatory (OVRO) millimeter array.The primary intent of this study is to investigate the response of coldgas during the early stages of collision of massive disk galaxies. Thesample sources are selected based on their luminosity(MB<=-19), projected separation (5-40 kpc), andsingle-dish CO (1-0) content (SCO>=20 Jy kms-1). These selection criteria result in a sample thatprimarily consists of systems in the early stages of an interaction or amerger. Despite this sample selection, 50% of the systems show long H Itidal tails indicative of a tidal disruption in a prograde orbit. Inaddition, all (4/4) of the infrared luminous pairs (LIRGs) in the sampleshow long H I tails, suggesting that the presence of a long H I tail canbe a possible signature of enhanced star formation activity in acollision of gas-rich galaxies. More than half of the groups show adisplacement of H I peaks from the stellar disks. The CO (1-0)distribution is generally clumpy and widely distributed, unlike in mostIR-selected late stage mergers-in fact, CO peaks are displaced from thestellar nucleus in 20% (4/18) of the galaxies with robust CO detection.H I and CO (1-0) position-velocity diagrams (PVDs) and rotation curvesare also presented, and their comparison with the numerical simulationanalyzed in Paper I show evidence for radial inflow and wide occurrencesof nuclear molecular rings. These results are further quantified byexamining physical and structural parameters derived in comparison withisolated systems in the BIMA SONG sample in our forthcoming paper.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
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