In the framework of hierarchical structure formation ellipticals can form from merging of smaller disk galaxies. The nearby interacting 'Antennae' galaxy pair (NGC 4038/39) is one of the best-studied local systems of merging spirals, thus presenting us with an ideal laboratory for the study of galaxy evolution models. The Antennae are believed to be in a state prior to their final encounter with rapid subsequent merging, which puts them in the first position of the Toomre (1977) merger sequence. Here we present first numerical high-resolution, self-consistent, smoothed particle hydrodynamics (SPH) simulations of the Antennae system, including star formation and stellar feedback, and compare our results to VLA HI observations by Hibbard et al. (2001). We are able to obtain a close, but not yet perfect match to the observed morphology and kinematics of the system.
Large numbers of young stars are formed in merging galaxies, such as the Antennae galaxies. Most of these stars are formed in compact star clusters (i.e., super star clusters), which have been the focus of a large number of studies. However, an increasing number of projects are beginning to focus on the individual stars as well. In this contribution, we examine a few results relevant to the triggering of star and star cluster formation; ask what fraction of stars form in the field rather than in clusters; and begin to explore the demographics of both the massive stars and star clusters in the …
The study of young star cluster (YSC) systems, preferentially in starburst and merging galaxies, has seen great interest in the recent past, as it provides important input to models of star formation. However, even some basic properties (like the luminosity function [LF]) of YSC systems are still under debate. Here we study the photometric properties of the YSC system in the nearest major merger system, the Antennae galaxies. We find evidence for the existence of a statistically significant turnover in the LF.
We compare mid-infrared images of the Antennae galaxies (NGC 4038/39) from the Infrared Space Observatory, with optical images from the Hubble Space Telescope. The mid-infrared observations show that the most intense starburst in this colliding system of galaxies takes place in an off-nucleus reg ion that is inconspicuous at optical wavelengths. The analyses of the mid-infrar ed spectra indicate that the most massive stars are being formed in an optically obscured knot of 50 pc radius, which produces about 15% of the total luminosity from the Antennae galaxies between 12.5 microns and 18 micro…
The Antennae galaxies are the closest example of an ongoing major galaxy merger, and thereby represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper HST/WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST/ACS imaging that resolves the composite stellar populations, and most importantly, reveals a well-…
The collision of gas-rich galaxies is believed to produce strong shocks between their gas clouds which cause the onset of the observed bursts of extended star formation. However, the so far observed shock signatures in colliding galaxies can be explained essentially by winds from already existing massive stars and supernovae and thus do not give any evidence for an outstanding pre-starburst phase. Either pre-starburst gas shocks are too short-lived to be detected or one has to modify our perception of colliding galaxies. A dedicated analysis of ISOCAM-CVF mid-infrared spectral maps led us to t…
The prominent interacting galaxy pair NGC 4038/9 contains many active star-forming regions and is continuously forming new star clusters. We present a self-consistent n-body model for this system which includes an SPH gas component. The model qualitatively explains the apparent concentration of gas in the so-called overlap region between the two nuclei as a bridge of gas connecting the two galaxies. Projected on the sky, the bridge appears as a dense spot of gas. We discuss some implications for the evolution of Ultra-luminous infrared galaxies.
We determine the mass function of young star clusters in the merging galaxies the ``Antennae'' (NGC 4038/39) from deep images taken with the WFPC2 on the refurbished Hubble Space Telescope (HST). This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age and hence the mass of each cluster. We find that the mass function of the young star clusters (with ages <~ 160 Myr) is well represented by a power law of the form psi(M) ~ M^{-2} over the range 10^4 < M < 10^6 solar masses. This result may h…
We report the results of a deep Chandra ACIS pointing at the merging system NGC 4038/39. We detect an extraordinarily luminous population of X-ray sources, with luminosity well above that of XRBs in M31 and the Milky Way. If these sources are unbeamed XRBs, our observations may point to them being 10-100Mo black hole counterparts. We detect an X-ray bright hot ISM, with features including bright superbubbles associated with the actively star-forming knots, regions where hot and warm (H$α$) ISM intermingle, and a large-scale outflow.
We present an analysis of the properties of the hot interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed using the deep, coadded ~411 ks Chandra ACIS-S data set. These deep X-ray observations and Chandra's high angular resolution allow us to investigate the properties of the hot ISM with unprecedented spatial and spectral resolution. Through a spatially resolved spectral analysis, we find a variety of temperatures (from 0.2 to 0.7 keV) and Nh (from Galactic to 2x10^21 cm^-2). Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughou…
We derive the X-ray luminosity function (XLF) of the X-ray source population detected in the Chandra observation of NGC4038/39 (the Antennae). We explicitly include photon counting and spectral parameter uncertainties in our calculations. The cumulative XLF is well represented by a flat power law ($α=-0.47$), similar to those describing the XLFs of other star-forming systems (e.g. M82, the disk of M81), but different from those of early type galaxies. This result associates the X-ray source population in the Antennae with young High Mass X-ray Binaries. In comparison with less actively star-fo…
We report velocity-resolved spectroscopy of infrared hydrogen recombination lines in the interacting region of the Antennae galaxies (NGC 4038/4039). It generally has been assumed that the active star formation found there is due to the interaction of the disks of the two galaxies and indeed two molecular cloud complexes, separated in velocity by $\sim$100 km s$^{-1}$, have been observed in the southern part of this region. Our measurements imply that the two cloud complexes are moving away from each other. This result poses interesting questions about the physical mechanisms leading to starbu…
We report the results of the analysis of the extended soft emission discovered in the Chandra ACIS pointing at the merging system NGC 4038/39 (the Antennae). We present a `multi-color' X-ray image that suggests both extensive absorption by the dust in this system, peaking in the contact region, as well as variations in the temperature of different emitting regions of the hot interstellar medium (ISM). Spectral fits to multi-component thermal emission models confirm this picture and give a first evaluation of the parameters of the hot plasma. We compare the diffuse X-ray emission with radio con…
We report the results of spectral fits to four bright ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observed for 41 ksec with XMM-Newton. Although emission regions are not resolved as well as in prior Chandra observations, at least four ULXs (X-11, X-16, X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently bright and well-separated with XMM-Newton that reliable extractions and spectral analyses are possible. We find that the single component multi-color disk blackbody models cannot describe any of the spectra. Sources X-11 and X-16 are acceptably fit…
We report the detailed analysis of the X-ray properties of the discrete sources detected in a long (72 ks) \chandra ACIS-S observation of the Antennae galaxies. We detect 49 sources, down to a detection limit of ~ 10^{38} erg\s; 18 sources have Lx >10^{39} erg\s (Ultra Luminous X-ray sources; ULXs). Six of the 49 sources have an extended component. Two sources show evidence for variability during this observation, and three sources exhibit long term variability in timescales of a few years. The spectra of the discrete sources are diverse, suggesting different emission mechanisms, broadly corre…
We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/dτ\propto τ^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationall…
To study the properties of the interstellar medium in the prototypical merging system of the Antennae galaxies (NGC 4038 and NGC 4039), we have obtained CO(1-0), (2-1) and (3-2) line maps, as well as a map of the 870 micron continuum emission. Our results are analysed in conjunction with data from X-ray to radio wavelengths. In order to distinguish between exact coincidence and merely close correspondence of emission features, we compare the morphological structure of the different emission components at the highest available angular resolution. To constrain the physical state of the molecular…
We present deep near-infrared images of the Antennae galaxies, taken with the Palomar Wide-Field Infrared Camera WIRC. The images cover a 4.33' x 4.33' (24.7kpc x 24.7kpc) area around the galaxy interaction zone. We derive J and K_s band photometric fluxes for 172 infrared star clusters, and discuss details of the two galactic nuclei and the overlap region. We also discuss the properties of a subset of 27 sources which have been detected with WIRC, HST and the VLA. The sources in common are young clusters of less than 10 Myr, which show no correlation between their infrared colors and 6 cm rad…
We report on a multi-wavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the past several years. Quantitative comparisons are made between the positions of the star clusters (broken into three age groups) and the properties of the interstellar medium by calculating the two-point …
We present high-resolution (R ~ 24,600) near-IR spectroscopy of the youngest super star clusters (SSCs) in the prototypical starburst merger, the Antennae Galaxies. These SSCs are young (3-7 Myr old) and massive (10^5 - 10^7 M_sun for a Kroupa IMF) and their spectra are characterized by broad, extended Br-gamma emission, so we refer to them as emission-line clusters (ELCs) to distinguish them from older SSCs. The Brgamma lines of most ELCs have supersonic widths (60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters' escape velocities. This high-velocity unbound gas is …
The temporal and spectral properties of ultraluminous X-ray sources (ULXs, L>2x10^39 ergs/s) and bright X-ray sources (L>3x10^38 ergs/s) are examined and compared in two extremely different host environments: the old elliptical galaxy NGC 1399 and the young, starforming Antennae galaxies (NGC 4038/4039). ULXs in NGC 1399 show little variability on either long or short time scales. Only 1 of 8 ULXs and 10 of 63 bright sources in NGC 1399 are variable at a confidence level of 90%. On long timescales, the NGC 1399 sources are steadier than most Galactic black hole X-ray binaries, but similar to G…
A reexamination of the correspondence between 6 cm radio continuum sources and young star clusters in the Antennae galaxies indicates that 85 % of the strong thermal sources have optical counterparts, once the optical image is shifted 1.2 arcsec to the southwest. A sample of 37 radio-optical matches are studied in detail showing correlations between radio properties and a variety of optical characteristics. There is a strong correlation between the radio flux and the intrinsic optical brightness. In particular, the brightest radio source is also the intrinsically brightest optical cluster (WS8…
Imaging of the Antennae galaxies (NGC 4038/4039) with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope reveals large concentrations of star forming activity away from both nuclei of the two merging galaxies. These images confirm earlier findings based on ISO data with lower angular resolution. The short wavelength emission shows numerous compact sources identified as stellar clusters. At the longer wavelengths, bright, more amorphous and filamentary features correlate well with the known distributions of denser gas, warm dust, and HII regions. There are also fainter, more di…
For the Antennae interacting galaxy pair, we have obtained high quality, fully sampled $^{12}$CO J=1-0, 2-1 and 3-2 maps of the regions surrounding the nuclei and the area of overlap between the two galaxies, with an angular resolution of 15" or 1.5 kpc. The $^{12}$CO J=1-0, 2-1, 3-2 emission all peak in the overlap region, which contains ~ 4 x 10^9 solar mass of molecular gas mass if the conventional X factor is used. Line emission at $^{13}$CO J=2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the ^{12}CO/^{13}CO J=2-1 and 3-2 integrated intensity rat…
We have analyzed Chandra archival observations of the Antennae galaxies to study the distribution and physical properties of its hot interstellar gas. Eleven distinct diffuse X-ray emission regions are selected according to their underlying interstellar structures and star formation activity. The X-ray spectra of these regions are used to determine their thermal energy contents and cooling timescales. Young star clusters in these regions are also identified and their photometric measurements are compared to evolutionary stellar population synthesis models to assess their masses and ages. The c…
Recently, mid-infrared instruments have become available on several large ground-based telescopes, resulting in data sets with unprecedented spatial resolution at these long wavelengths. In this paper we examine 'ground-based-only' diagnostics, which can be used in the study of star-forming regions in starburst galaxies. By combining output from the stellar population synthesis code Starburst 99 with the photoionization code Mappings, we model stellar clusters and their surrounding interstellar medium, focusing on the evolution of emission lines in the N- and Q-band atmospheric windows (8-13 a…
We present a framework for understanding the demographics of star cluster systems, and develop a toy model which incorporates a universal initial power law mass function, selected formation histories, selected disruption laws, and a convolution with common artifacts and selection effects found in observational data. The model confirms that the observed correlation between the brightest young cluster in a galaxy and the total number of young clusters can be understood as a statistical size-of-sample effect, rather than a difference in the physical process responsible for the formation of the cl…
We present deep, photometrically calibrated BVRJHK images of the nearby interacting galaxy pair NGC 4038/39 (``The Antennae''). Color maps of the images are derived, and those using the B, V, and K-bands are analyzed with techniques developed for examining the colors of stars. From these data we derive pixel-by-pixel maps of the distributions of stellar populations and dust extinction for the galaxies. Analysis of the stellar population map reveals two distinct episodes of recent star formation: one currently in progress and a second that occurred ~600 Myr ago. A roughly 15 Gyr-old population …
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 um) ISO image presented by Mirabel et al (1998). We obtained high signal-to-noise 2.03-2.45 um spectra of the nucleus and the obscured star cluster at R = 1900. The cl…
Long-slit spectra of several dozen young star clusters have been obtained at three positions in the Antennae galaxies with the Space Telescope Imaging Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line measurements, the average cluster-to-cluster velocity dispersion in 7 different cluster aggregates ("knots") is <10 \kms. The fact that this upper limit is similar to the velocity dispersion of gas in the disks of typical spiral galaxies suggests that the triggering mechanism for the formation of young massive compact clusters ("super star clusters") is unlikely to be high…
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the highest surface brightness peak in the mid-IR (12-18 micron) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 - 2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1…
[abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical HST data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray…
The WFPC2 of the HST has been used to obtain high-resolution images of NGC 4038/4039 that go roughly 3 magnitudes deeper in V than previous observations made during Cycle 2 (-14 < M_V < -6). To first order the luminosity function (LF) is a power law, with exponent α= -2.12 +/- 0.04. However, after decoupling the cluster and stellar LFs, which overlap in the range -9 < M_V < -6, we find an apparent bend in the young cluster LF at approximately M_V = -10.4. The LF has a power law exponent -2.6 +/- 0.2 in the brightward and -1.7 +/- 0.2 in the faintward. The bend corresponds to a mass ~ 10^5 M_{\…
We investigate the physical properties of the interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), using the deep coadded ~411 ks Chandra ACIS-S data set. The method of analysis and some of the main results from the spectral analysis, such as metal abundances and their variations from ~0.2 to ~20-30 times solar, are described in Paper I (Baldi et al. submitted). In the present paper we investigate in detail the physics of the hot emitting gas, deriving measures for the hot-gas mass (~10^ M_sun), cooling times (10^7-10^8 yr), and pressure (3.5x10^-11-2.…
We estimate the dynamical masses of several young (~10 Myr) massive star clusters in two interacting galaxies, NGC 4038/4039 ("The Antennae") and NGC 1487, under the assumption of virial equilibrium. These are compared with photometric mass estimates from K-band photometry and assuming a standard Kroupa IMF. The clusters were selected to have near-infrared colors dominated by red supergiants, and hence to be old enough to have survived the earliest phases of cluster evolution when the interstellar medium is rapidly swept out from the cluster, supported by the fact that there is no obvious Halp…
(abridged) We report here a factor of 5.7 higher total CO flux in Arp~244 (the ``Antennae'' galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5x10$^{10}$ Msun), based on our fully sampled CO(1-0) observations at the NRAO 12m telescope. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) L_{IR}/M(H_2) \approx 4.2 Lsun/Msun,…
Traditionally, the distance to NGC 4038/39 has been derived from the systemic recession velocity, yielding about 20 Mpc for H_0 = 72 km/s/Mpc. Recently, this widely adopted distance has been challenged based on photometry of the presumed tip of the red giant branch (TRGB), which seems to yield a shorter distance of 13.3+-1.0 Mpc and, with it, nearly 1 mag lower luminosities and smaller radii for objects in this prototypical merger. Here we present a new distance estimate based on observations of the Type Ia supernova (SN) 2007sr in the southern tail, made at Las Campanas Observatory as part of…
We present an extensive study of magnetic fields in a system of merging galaxies. We obtained for NGC4038/39 (the Antennae) radio total intensity and polarization maps at 8.44, 4.86 and 1.49GHz using the VLA in the C and D configurations. The radio thermal fraction was found to be about 50% at 10.45GHz, higher than in normal spirals. The mean total magnetic fields in both galaxies are about two times stronger (20microG) than in normal spirals. However,the degree of field regularity is rather low, implying tangling of the regular component in regions with interaction-enhanced star formation. Ou…
We present the integrated 411 ks {\it Chandra} ACIS-S exposure of the Antennae galaxies (NGC 4038/39). Besides a rich population of point-like sources, this spectacular image reveals a spatially and spectrally complex hot diffuse gaseous component. For the first time we detect intense line emission from Fe, Ne, Mg and Si in The Antennae, and obtain a detailed picture of spatially varied metal abundances in the hot interstellar medium (ISM) of a galaxy. In certain regions, the abundances of $α$-elements may be many times solar, while the Fe abundance is sub-solar or near-solar. The differences …
Motivated by the closest major merger, the Antennae Galaxies (NGC4038/4039), we want to improve our genetic algorithm based modeling code Minga (Theis 1999). The aim is to reveal the major interaction and galaxy parameters, e.g. orbital information and halo properties of such an equal mass merger system. Together with the sophisticated search strategy of Minga, one needs fast and reliable models in order to investigate the high dimensional parameter space of this problem. Therefore we use a restricted N-body code which is based on the approach by Toomre & Toomre (1972), however with some refin…
GEMS and GOODS fields were examined to z~1.4 for galaxy interactions and mergers. The basic morphologies are familiar: antennae with long tidal tails, tidal dwarfs, and merged cores; M51-type galaxies with disk spirals and tidal arm companions; early-type galaxies with diffuse plumes; equal-mass grazing-collisions; and thick J-shaped tails beaded with star formation and double cores. One type is not common locally and is apparently a loose assemblage of smaller galaxies. Photometric measurements were made of the tails and clumps, and physical sizes were determined assuming photometric redshift…
Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved `star-like' objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of about 100 pc and absolute B-band magnitudes in the range -14 < M_B < -11.5 mag and lower limits for the central surface brightness mu_B > 23 mag/arcsec^2 (Phillipps et al. 2001), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (Kroupa 1998), which are rich aggregates of…
We present new VLA C+D-array HI observations and optical and NIR imaging of the well known interacting system NGC 4038/9, ``The Antennae''. The radio data reveal a wealth of gaseous sub-structure both within the main bodies of the galaxies and along the tidal tails. In agreement with previous HI studies, we find that the northern tail has HI along its outer length, but none along its base. We suggest that the HI at the base of this tail has been ionized by massive stars in the disk of NGC 4038. The gas in the southern tail has a bifurcated structure, with one filament lying along the optical t…
The tidal field of galaxies is known generally to be disruptive. However, in the case of galaxy mergers, a compressive mode of tidal wave may develop and last long enough to cocoon the formation of star clusters. Using an N-body simulation of the Antennae galaxies, we derive the positions of these compressive regions and the statistics of their duration. Excellent agreement between the spatial distribution of tides and observed young clusters is found, while the characteristic e-folding times of 10 to 30 Myrs derived for the tidal field compare well with cluster formation time-scales.
In this paper we present the results of a 20 ks high resolution Chandra X-ray observation of the peculiar galaxy pair NGC 3395/3396, a system at a very early stage of merging, and less evolved than the famous Antennae and Mice merging systems. Previously unpublished ROSAT HRI data are also presented. The point source population and the hot diffuse gas in this system are investigated, and compared with other merging galaxy pairs. 16 X-ray point sources are detected in Arp 270, 7 of which are classified as ULXs (Lx > 10^39 erg/s). From spectral fits and the age of the system it seems likely that…
I will present a review of some of the many recent developments within the exciting and rapidly evolving field of Interacting and Merging Galaxies. I will touch both on observations and on theory, focussing on those aspects where galaxy evolution modelling may possibly contribute to our understanding. After briefly outlining the basic concepts and a few specific results of stellar- and gasdynamical modelling of galaxy -- galaxy mergers (Sect. 1) I choose three examples of interacting/merged galaxy pairs to discuss different aspects of mergers: tracing back the star formation history of the old…
Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved `star-like' objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of about 100 pc and absolute B-band magnitudes in the range -14 < M_B < -11.5 mag and lower limits for the central surface brightness mu_B >= 23 mag/arcsec^2 (Phillipps et al. 2001, Hilker et al. 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star cl…
The initial conditions of our Universe can be summarized on a single sheet of paper. Yet the Universe is full of complex structures today, such as stars, galaxies and groups of galaxies. In this review I describe the standard theoretical model for how complexity emerged from the simple initial state of the Universe at early cosmic times through the action of gravity. In order to test and inform the related theoretical calculations, large-aperture telescopes and arrays of radio antennae are currently being designed and constructed. The actual transition from simplicity to complexity has not bee…
Interacting galaxies like the famous Antennae (NGC 4038/4039) or Stephan's Quintet (HCG 92) show considerable star forming activity in their tidal arms. High resolution images (e.g. from HST-observations) indicate that these regions consist of up to hundreds of massive stellar clusters or tidal dwarf galaxies (TDG). In this paper we want to investigate the future fate of these clusters of massive star clusters (in this work called super-clusters). We simulate compact super-clusters in the tidal field of a host-galaxy and investigate the influence of orbital and internal parameters on the rate …
We present Gemini optical spectroscopy of 23 young star clusters in NGC3256. We find that the cluster ages range are from few Myr to ~150 Myr. All these clusters are relatively massive (2--40)x 10^{5} \msun$ and appear to be of roughly 1.5 \zo metallicity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may…
We present the first detection of CS in the Antennae galaxies towards the NGC 4038 nucleus, as well as the first detections of two high-J (5-4 and 7-6) CS lines in the center of M 82. The CS(7-6) line in M 82 shows a profile that is surprisingly different to those of other low-J CS transitions we observed. This implies the presence of a separate, denser and warmer molecular gas component. The derived physical properties and the likely location of the CS(7-6) emission suggests an association with the supershell in the centre of M 82.
We use simulations of merging galaxies to explore the sensitivity of the morphology of tidal tails to variations of the halo mass distributions in the parent galaxies. Our goal is to constrain the mass of dark halos in well-known merging pairs. We concentrate on prograde encounters between equal mass galaxies which represent the best cases for creating tidal tails, but also look at systems with different relative orientations, orbital energies and mass ratios. As the mass and extent of the dark halo increase in the model galaxies, the resulting tidal tails become shorter and less massive, even…
We use the previously-identified 15 infrared star-cluster counterparts to X-ray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to study the relationship between total cluster mass and X-ray binary number. This significant population of X-Ray/IR associations allows us to perform, for the first time, a statistical study of X-ray point sources and their environments. We define a quantity, η, relating the fraction of X-ray sources per unit mass as a function of cluster mass in the Antennae. We compute cluster mass by fitting spectral evolutionary models to K_s luminosity. C…
In many different galactic environments the cluster initial mass function (CIMF) is well described by a power-law with index -2. This implies a linear relation between the mass of the most massive cluster (M_max) and the number of clusters. Assuming a constant cluster formation rate and no disruption of the most massive clusters it also means that M_max increases linearly with age when determining M_max in logarithmic age bins. We observe this increase in five out of the seven galaxies in our sample, suggesting that M_max is determined by the size of the sample. It also means that massive clus…
To first order, the initial cluster luminosity function appears to be universal. This means that the brightest young cluster in a galaxy can be predicted from the total number of young clusters based purely on statistics. This suggests that the physical processes responsible for the formation of clusters are similar in a wide variety of galaxies, from mergers to quiescent spirals. One possibility is that conditions for making young massive clusters are globally present in mergers while only locally present in spirals (i.e., in the spiral arms). However, understanding the destruction of cluster…
Using V and I images obtained with WFPC2/HST, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic ``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how the star formation depends upon the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging fro…
We have undertaken several projects with the purpose of determining accurate distances to nearby galaxies to calibrate the extragalactic distance scale. Specifically, I describe the DIRECT project which aims to derive the distance to M31 and M33 directly, using detached eclipsing binaries and the Baade-Wesselink method for Cepheids. I also present a ``hybrid'' method of discovering Cepheids with ground-based telescopes using image subtraction and then following them up with the HST to derive Cepheid period-luminosity distances.
High resolution Fabry-Perot data of six spiral galaxies are presented. Those data extend the previous sample of spiral galaxies studied with high resolution 3D spectroscopy to earlier morphological types. All the galaxies in the sample have available HI data at 21 cm from the VLA or Westerbork. Velocity fields are analyzed and Halpha rotation curves are computed and compared to HI curves. The kinematics of NGC 5055 central regions are looked at more closely. Its peculiar kinematics can be interpreted either as a bipolar outflow or as a counter-rotating disk, possibly hosting a 9 pm 2 10^8 Msol…
