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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.
| 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.
| 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.
| The Second Byurakan Survey. General Catalogue
The Second Byurakan Survey (SBS) General Catalogue is presented. TheSBS, a continuation of the Markarian survey reaching fainter limitingmagnitudes, is the first survey which combines the search of galaxiesand QSOs. A total area of 991OS#square;degrees of the Northern sky wascovered with the use of three objective prisms in combination withSchott filters. The limited magnitude on the best plates reached B ~19.5.The General Catalogue consists of 3563 objects presented in two parts: aCatalogue of galaxies (1863 objects) and one of stellar objects (1700objects). The Catalogue of SBS AGN consists of 761 objects (155 SyG, 596QSOs, and 10 BLLac). Multi-wavelength data are presented for 1438 SBSobjects identified with X-ray, IRAS and FIRST sources.Spectrophotometric observations obtained over 26 years are available for3132 objects. Redshifts were measured for ~ 2100 extragalactic objects.Spectral classification is presented for ~ 2970 objects. The majority ofthe data is presented here for the first time. The Catalogue presentsnew large homogeneous deep representative complete samples of brightQSOs, AGNs, and faint UVX galaxies in the Northern sky. The SBS sampleis found to be complete at 70% for galaxies and ~ 85% for AGN/QSOs withB ≤ 17.5.
| 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).
| The XMM-Newton Needles in the Haystack Survey: the local X-ray luminosity function of `normal' galaxies
In this paper we estimate the local (z < 0.22) X-ray luminosityfunction of `normal' galaxies derived from the XMM-Newton Needles in theHaystack Survey. This is an on-going project that aims to identifyX-ray-selected normal galaxies (i.e. non-AGN dominated) in the localUniverse. We are using a total of 70 XMM-Newton fields covering an areaof 11 deg2 which overlap with the Sloan Digital Sky SurveyData Release 2. Normal galaxies are selected on the basis of theirresolved optical light profile, their low X-ray-to-optical flux ratio[log(fx/fo) < - 2] and soft X-ray colours. Wefind a total of 28 candidate normal galaxies to the 0.5-8keV band fluxlimit of ~2 × 10-15ergcm-2s-1.Optical spectra are available for most sources in our sample (82 percent). These provide additional evidence that our sources are bona fidenormal galaxies with X-ray emission coming from diffuse hot gas emissionand/or X-ray binaries rather than a supermassive black hole. 16 of ourgalaxies have narrow emission lines or a late-type spectral energydistribution (SED) while the remaining 12 present only absorption linesor an early-type SED. Combining our XMM-Newton sample with 18 local (z< 0.22) galaxies from the Chandra Deep Field North and South surveys,we construct the local X-ray luminosity function of normal galaxies.This can be represented with a Schechter form with a break atL*~ 3+1.4-1.0×1041ergs-1 and a slope of α~ 1.78 +/- 0.12.Using this luminosity function and assuming pure luminosity evolution ofthe form ~(1 +z)3.3 we estimate a contribution to the X-raybackground from normal galaxies of ~10-20 per cent (0.5-8keV). Finally,we derive, for the first time, the luminosity functions for early- andlate-type systems separately.
| 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.
| 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.
| 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).
| Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation
We present the results of a meta-analysis of Keck spectra ofextragalactic globular clusters (GCs) in a sample of eight galaxies,ranging from dwarf galaxies to massive elliptical galaxies. We inferages for the metal-poor and metal-rich GCs in these galaxies throughcomparisons to Galactic GCs. Both subpopulations appear to be no youngerthan their Galactic counterparts, with ages >~10 Gyr. This is thelargest sample of galaxies for which ages have been constrainedspectroscopically. Our results support the formation of most GCs inmassive galaxies at high redshift. We propose a scenario for theformation of GC subpopulations that synthesizes aspects of bothaccretion and in situ approaches in the context of galaxy formationthrough hierarchical merging.
| Study of DDO 68: nearest candidate for a young galaxy?
We present the results of optical spectroscopy and imaging with the SAO6 m telescope for the dwarf galaxy DDO 68 (UGC 5340 = VV 542), fallinginto the region of very low density of luminous (L > L*)galaxies (Lynx-Cancer void). Its deep images in V,R bands and in thenarrow Hα-filter show that this galaxy has the very irregularmorphology, with a long curved tail on the South and a ring-likestructure at the Northern edge. The latter consists of 5 separateregions, in three of which we could measure O/H by the classicalTe method. Their weighted mean oxygen abundance correspondsto 12+log (O/H)=7.21 ± 0.03, coincident within uncertainties withthose for I Zw 18. The (V-R) colour of DDO 68 is rather blue all overthe galaxy, indicating the youth of its stellar populations. Comparingthe (V-R)0 colour of the underlying exponential disk of0.12m±0.04 with the PEGASE.2 models for the evolving stellarclusters, we give the first estimate of the ages of the oldest stellarpopulation, which needs confirmation by the other colours and thephotometry of resolved stars. These ages are in the range of 200-900 Myrfor continuous star formation law, and ~100-115 Myr for theinstantaneous starburst. We discuss the properties and the possibleyouth of this nearby object (~2.3 times closer than the famous younggalaxy I Zw 18) in the context of its atypical environment.
| 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.
| XMM-Newton observations of the merger-remnant galaxies NGC 3921 and 7252
Using the high sensitivity of XMM-Newton, we have studied the X-rayemission of the two prototypical late-stage merger remnants, NGC 3921and 7252. In the case of NGC 7252, this is complemented by archivalChandra data. We investigate the nature of the discrete X-ray pointsource populations and the hot diffuse gas components in these twogalaxies, and compare them in the light of their different merger agesand histories.We detect three candidate ultraluminous X-ray point sources (ULXs) inNGC 3921 and at least six in NGC 7252, for which we have high spatialresolution Chandra data. These have luminosities ranging from ~1.4× 1039-1040 erg s-1 (forH0= 75 km s-1 Mpc-1). We expect theseULXs to be high-mass X-ray binaries, associated with the recent starformation in these two galaxies.Extended hot gas is observed in both galaxies. We have sufficient countsin the XMM-Newton data to fit two-component hot plasma models to theirX-ray spectra, and estimate the X-ray luminosities of the hot diffusegas components to be 2.75 × 1040 erg s-1 and2.09 × 1040 erg s-1 in NGC 3921 and 7252,respectively. These luminosities are low compared with the luminositiesobserved in typical mature elliptical galaxies (LX~1041-1042 erg s-1), into which thesemerger remnants are expected to evolve. We do not see evidence that theX-ray haloes of these galaxies are currently being regenerated to themasses and luminosities seen in typical elliptical galaxies. The mass ofatomic gas available to fall back into the main bodies of these galaxiesand shock-heat to X-ray temperatures is insufficient for this to be thesole halo regeneration mechanism. We conclude that halo regeneration ismost likely a long-term (>10 Gyr) process, occurring predominantlyvia mass loss from evolving stars, in a subsonic outflow stagecommencing ~2 Gyr after the merging event.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations
We investigate the detailed response of gas to the formation oftransient and long-lived dynamical structures induced in the earlystages of a disk-disk collision and identify observational signatures ofradial gas inflow through a detailed examination of the collisionsimulation of an equal-mass bulge-dominated galaxy. Our analysis anddiscussion mainly focuses on the evolution of the diffuse and dense gasin the early stages of the collision, when the two disks are interactingbut have not yet merged. Stars respond to the tidal interaction byforming both transient arms and long-lived m=2 bars, but the gasresponse is more transient, flowing directly toward the central regionswithin about 108 yr after the initial collision. The rate ofinflow declines when more than half of the total gas supply reaches theinner few kiloparsecs, where the gas forms a dense nuclear ring insidethe stellar bar. The average gas inflow rate to the central 1.8 kpc is~7 Msolar yr-1 with a peak rate of 17Msolar yr-1. Gas with high volume density is foundin the inner parts of the postcollision disks at size scales close tothe spatial resolution of the simulations, and this may be a directresult of shocks traced by the discontinuity in the gas velocity field.The evolution of gas in a bulgeless progenitor galaxy is also discussed,and a possible link to the ``chain galaxy'' population observed at highredshifts is inferred. The evolution of the structural parameters suchas asymmetry and concentration of both stars and gas are studied indetail. Further, a new structure parameter (the compactness parameter K)that traces the evolution of the size scale of the gas relative to thestellar disk is introduced, and this may be a useful tracer to determinethe merger chronology of colliding systems. Noncircular gas kinematicsdriven by the perturbation of the nonaxisymmetric structure can producedistinct emission features in the ``forbidden velocity quadrants'' ofthe position-velocity diagram (PVD). The dynamical mass calculated usingthe rotation curve derived from fitting the emission envelope of the PVDcan determine the true mass to within 20%-40%. The evolution of themolecular fraction(MH2/MH2+HI) is a potentialtracer to quantitatively assign the age of the interaction, but theapplication to real systems may require additional observationaldiagnostics to properly assess the exact chronology of the mergerevolution.
| 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.
| 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.
| Early-Type Galaxies in Extremely Isolated Environments: Typical Ellipticals?
We have conducted a BVR imaging survey of nine early-type galaxiespreviously verified to exist in extremely isolated environments. Ourgoals are to establish a baseline of morphological and photometricproperties for spheroidal systems evolving in extremely low-densityenvironments and to compare these properties with signatures predictedfor merged galaxy groups. We find that these isolated systems areunderluminous by at least a magnitude compared with objects identifiedas merged group remnants in other studies. Image processing techniquessensitive to shell features produced no detections, a result in strongcontrast to the high frequency of such structures found in otherisolated elliptical galaxies. Two objects, KIG 164 and KIG 870, appearto be merger remnants, as indicated by their disturbed morphology,apparent tidal features, and blue colors. KIG 164 exhibits an asymmetricnuclear morphology and a low surface brightness ``bridge'' between itand a possible dwarf satellite. KIG 870 shows both fan-shaped emissionat large radii and a possible double nucleus. Two other galaxies, KIG412 and KIG 792, are also blue, but without any morphologicalpeculiarities, suggesting that these systems are advanced mergers, olderthan KIG 164 and KIG 870. Two systems appear to be isolated lenticulargalaxies with no evidence of a merger history. Based on their redcolors, good fit to a R1/4-law light distribution, and thelack of morphological peculiarities, two other galaxies, KIG 557 and KIG824, are found to be excellent candidates for passively evolvingprimordial elliptical galaxies formed early in cosmic time. Optical datawere obtained with the 2.1 m Otto Struve telescope at McDonaldObservatory, which is operated by the University of Texas at Austin.
| Starbursts in barred spiral galaxies. VI. HI observations and the K-band Tully-Fisher relation
This paper reports a study of the effect of a bar on the neutralhydrogen (HI) content of starburst and Seyfert galaxies. We also makecomparisons with a sample of ``normal'' galaxies and investigate howwell starburst and Seyfert galaxies follow the fundamental scalingTully-Fisher (TF) relation defined for normal galaxies. 111 Markarian(Mrk) IRAS galaxies were observed with the Nançay radiotelescope,and HI data were obtained for 80 galaxies, of which 64 are newdetections. We determined the (20 and 50%) linewidths, the maximumvelocity of rotation and total HI flux for each galaxy. Thesemeasurements are complemented by data from the literature to form asample of Mrk IRAS (74% starburst, 23% Seyfert and 3% unknown) galaxiescontaining 105 unbarred and 113 barred ones. Barred galaxies have lowertotal and bias-corrected HI masses than unbarred galaxies, and this istrue for both Mrk IRAS and normal galaxies. This robust result suggeststhat bars funnel the HI gas toward the center of the galaxy where itbecomes molecular before forming new stars. The Mrk IRAS galaxies havehigher bias-corrected HI masses than normal galaxies. They also showsignificant departures from the TF relation, both in the B and K bands.The most deviant points from the TF relation tend to have a strongfar-infrared luminosity and a low oxygen abundance. These resultssuggest that a fraction of our Mrk IRAS galaxies are still in theprocess of formation, and that their neutral HI gas, partly of externalorigin, has not yet reached a stationary state.Based on observations obtained at the large radiotelescope ofObservatoire de Nançay, operated by Observatoire de Paris.Tables 5 and 6 are only (and Table 4 also) available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/515
| Secondary Globular Cluster populations
This study is motivated by two facts: 1. The formation of populous starcluster systems is widely observed to accompany violent star formationepisodes in gas-rich galaxies as e.g. those triggered by stronginteractions or merging. 2. The Globular Cluster (GC) systems of mostbut not all early-type galaxies show bimodal optical color distributionswith fairly universal blue peaks and somewhat variable red peak colors,yet their Luminosity Functions (LFs) look like simple Gaussians withapparently universal turn-over magnitudes that are used for distancemeasurements and the determination of Ho. Based on a new setof evolutionary synthesis models for Simple (= single burst) StellarPopulations (SSPs) of various metallicities using the latest Padovaisochrones I study the color and luminosity evolution of GC populationsover the wavelength range from U through K, providing an extensive gridof models for comparison with observations. I assume the intrinsicwidths of the color distributions and LFs to be constant in time at thevalues observed today for the Milky Way or M 31 halo GC populations.Taking the color distributions and LFs of the Milky Way or M 31 halo GCpopulation as a reference for old metal-poor GC populations in general,I study for which combinations of age and metallicity a secondary GCpopulation formed in some violent star formation event in the history ofits parent galaxy may or may not be detected in the observed GC colordistributions. I also investigate the effect of these secondary GCs onthe LFs of the total GC system. Significant differences are found amongthe diagnostic efficiencies in various wavelength regions. Inparticular, we predict the NIR to be able to reveal the presence of GCsubpopulations with different age - metallicity combinations that mayperfectly hide within one inconspicuous optical color peak. If theentire manifold of possible age - metallicity combinations is admittedfor a secondary GC population, we find several cases where the resultingLF of the whole GC system is significantly affected and its turn-overcould not serve as a reliable distance indicator. If, on the other hand,we assume some age - metallicity relation for GC populations, the secondpeak of the LFs vanishes and models indicate single-peak GC LFs even inGC systems with bimodal color distributions. A broad but sufficient age- metallicity relation is, for example, obtained if the secondary GCpopulations form in mergers of various spiral galaxy types from the ISMpre-enriched over the redshift range from z 4.4 to z 0. As afirst illustrative example we apply our models to V- and I-band datapresented by Larsen et al. (\cite{Larsen2001}) for blue and red peak GCsin three early-type galaxies. We point out the importance of havingmulti-band information to independently constrain ages and metallicitiesof different GC subpopulations and again stress the diagnostic potentialof K-band data in addition to optical observations. The models presentedhere will be used for the interpretation of multi-wavelength data on GCsystems in galaxies of various types, luminosities and environments aswell as on young star cluster systems in interacting galaxies andmergers. By independently constraining ages and metallicities ofindividual clusters we expect to shed light on both cluster and galaxyformation scenarios.
| 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.
| Star cluster formation and evolution in nearby starburst galaxies - II. Initial conditions
We use the ages, masses and metallicities of the rich young star clustersystems in the nearby starburst galaxies NGC 3310 and 6745 to derivetheir cluster formation histories and subsequent evolution. We furtherexpand our analysis of the systematic uncertainties involved in the useof broad-band observations to derive these parameters (Paper I) byexamining the effects of a priori assumptions on the individual clustermetallicities. The age (and metallicity) distributions of both theclusters in the circumnuclear ring in NGC 3310 and of those outside thering are statistically indistinguishable, but there is a clear andsignificant excess of higher-mass clusters in the ring compared to thenon-ring cluster sample. It is likely that the physical conditions inthe starburst ring may be conducive for the formation of higher-massstar clusters, on average, than in the relatively more quiescentenvironment of the main galactic disc. For the NGC6745 cluster system wederive a median age of ~10 Myr. NGC6745 contains a significantpopulation of high-mass `super star clusters', with masses in the range6.5 <~ log(Mcl/Msolar) <~ 8.0. Thisdetection supports the scenario that such objects form preferentially inthe extreme environments of interacting galaxies. The age of the clusterpopulations in both NGC3310 and 6745 is significantly lower than theirrespective characteristic cluster disruption time-scales, respectivelylog(tdis4/yr) = 8.05 and 7.75, for 104Msolar clusters. This allows us to obtain an independentestimate of the initial cluster mass function slope, α=2.04(+/-0.23)+0.13-0.43 for NGC3310, and1.96(+/-0.15) +/- 0.19 for NGC6745, respectively, for massesMcl>~ 105 Msolar andMcl>~ 4 × 105 Msolar. Thesemass function slopes are consistent with those of other young starcluster systems in interacting and starburst galaxies.
| Star cluster formation and evolution in nearby starburst galaxies - I. Systematic uncertainties
The large majority of extragalactic star cluster studies performed todate have essentially used two- or three-passband aperture photometry,combined with theoretical stellar population synthesis models, to obtainage, mass and extinction estimates, and sometimes also metallicities.The accuracy to which this can be performed depends on the choice of(broad-band) passband combination and, crucially, also on the actualwavelengths and the wavelength range covered by the observations.Understanding the inherent systematic uncertainties (the main aim ofthis paper) is of the utmost importance for a well-balancedinterpretation of the properties of extragalactic star cluster systems.We simultaneously obtain ages, metallicities and extinction values for~300 clusters in the nearby starburst galaxy NGC 3310, based on archivalHubble Space Telescope observations from the ultraviolet (UV) to thenear-infrared (NIR). We show that for ages 6 <~ log(ageyr-1) <~ 9, and if one can only obtain partial coverage ofthe spectral energy distribution (SED), an optical passband combinationof at least four filters including both blue and red passbands resultsin the most representative age distribution, as compared with the betterconstrained ages obtained from the full UV-NIR SED coverage. We findthat while blue-selected passband combinations lead to age distributionsthat are slightly biased towards lower ages due to the well-knownage-metallicity degeneracy, red-dominated passband combinations shouldbe avoided.NGC 3310 underwent a (possibly extended) global burst of clusterformation ~3 × 107 yr ago. This coincides closely withthe last tidal interaction or merger with a low-metallicity galaxy thatprobably induced the formation of the large fraction of clusters with(significantly) subsolar metallicities. The logarithmic slope of theV-band cluster luminosity function, for clusters in the range 17.7 <~F606W <~ 20.2 mag, is αF606W~=-1.8 +/- 0.4. Theobserved cluster system has a median mass of < log (m/Msolar) >~= 5.25 +/- 0.1, obtained from scaling theappropriate model SEDs for known masses to the observed cluster SEDs.
| The Mass Function of Supergiant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/4039)
We have used previously published observations of the CO emission fromthe Antennae (NGC 4038/4039) to study the detailed properties of thesupergiant molecular complexes with the goal of understanding theformation of young massive star clusters. Over a mass range from5×106 to 9×108 Msolar themolecular complexes follow a power-law mass function with a slope of-1.4+/-0.1, which is very similar to the slope seen at lower masses inmolecular clouds and cloud cores in the Galaxy. Compared with the spiralgalaxy M51, which has a similar surface density and total mass ofmolecular gas, the Antennae contain clouds that are an order ofmagnitude more massive. Many of the youngest star clusters lie in thegas-rich overlap region, where extinctions as high as AV~100mag imply that the clusters must lie in front of the gas. Young clustersfound in other regions of the galaxies can be as far as 2 kpc from thenearest massive cloud, which suggests that either young clusters canform occasionally in clouds less massive than 5×106Msolar or that these young clusters have already destroyedtheir parent molecular clouds. Combining data on the young clusters,thermal and nonthermal radio sources, and the molecular gas suggeststhat young massive clusters could have formed at a constant rate in theAntennae over the last 160 Myr and that sufficient gas exists to sustainthis cluster formation rate well into the future. However, thisconclusion requires that a very high fraction of the massive clustersthat form initially in the Antennae do not survive as long as 100 Myr.Furthermore, if most young massive clusters do survive for long periods,the Antennae must be experiencing a relatively short burst of clusterformation to prevent the final merger remnant from exceeding theobserved specific frequency of star clusters in elliptical galaxies by awide margin. Finally, we compare our data with two models for massivestar cluster formation and conclude that the model in which youngmassive star clusters form from dense cores within the observedsupergiant molecular complexes is most consistent with our currentunderstanding of this merging system.
| Azimuthal and Kinematic Segregation of Neutral and Molecular Gas in Arp 118: The Yin-Yang Galaxy NGC 1144
We present new high-resolution H I observations of the disk of thecollisional infrared luminous(LIR=2.2×1011Lsolar) galaxy NGC1144, which reveal an apparent large-scale azimuthal and kinematicsegregation of neutral hydrogen relative to the molecular gasdistribution. Even among violently collisional galaxies, the CO/H Iasymmetry in NGC 1144 is unusual, both in the inner regions and in theouter disk. We suggest that we are observing Arp 118 at a specialmoment, shortly after a high-speed collision between NGC 1144 and itselliptical companion NGC 1143. H I emission with an average molecularfraction fmol<0.5 is observed on one side (northwest) ofthe rotating disk of NGC 1144, while the other side (southeast) isdominated by dense molecular complexes in which fmol isalmost unity. The interface region between the warm- and cool-clouddominated regions lies on a deep spiral-like dust lane that we identifyas a shock wave responsible for the relative shift in the dominance of HI and H2 gas. A strong shock being fed by diffuse H I cloudswith unusually large (>400 km s-1) rotational velocities canexplain (1) the CO/H I asymmetries, (2) a large velocity jump (185 kms-1) across the arm as measured by H I absorption against aradio bright continuum source that straddles the arm, and (3) theasymmetric distribution of star formation and off-nuclear molecular gasresulting from likely streaming motions associated with the strongshock. The new results provide for the first time a coherent picture ofArp 118's many peculiarities and underline the potentially complexchanges in the gas phase that can accompany large gravitationalperturbations of gas-rich galaxies.
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