The young star cluster system of the Antennae galaxies
Peter Anders, Uta Fritze, Richard de Grijs
备注：
4 pages, 3 figures, to appear in "Young Massive Star Clusters - Initial Conditions and Environments", 2008, Astrophysics & Space Science, eds. E. Perez, R. de Grijs, R. M. Gonzalez Delgado
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.
The Antennae is a pair of late type spirals in the course of merging. The interaction triggered an ongoing strong burst of star formation that also produced a large number of Young Star Clusters (YSCs), many of which seem to be young Globular Clusters (GCs). The observed Luminosity Function of the YSC system is a power-law. We use evolutionary synthesis models for star clusters in comparison with HST WFPC observations of the YSC system in the Antennae to analyse the mass function of this ${\rm \sim 2 \cdot 10^8}$ yr young star cluster system. Properly accounting for age spread effects by indiv…
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 have used previously published observations of the CO emission from the Antennae (NGC 4038/39) to study the detailed properties of the super giant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5E6 to 9E8 solar masses, the molecular 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 in molecular clouds and cloud cores in the Galaxy. Compared to the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Ant…
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 …
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 …
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…
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 (…
The luminosity functions (LFs) of star cluster systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of star cluster formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterised by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged by studies of galaxy merger remnants and post-starburst galaxies. In this paper we re-evaluate the young ($\lta$ few hundreds of Myrs, with the majority $\lta$ few tens of Myrs) star cluster syst…
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 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 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 …
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…
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 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…
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 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…
In this letter we present ground-based subarcsecond mid-infrared imaging and spectroscopy of young super star clusters in the overlap region of the merging galaxies NGC4038/4039 (the Antennae) obtained with the VLT Imager and Spectrometer for mid-Infrared (VISIR). With its unprecedented spatial resolution VISIR begins to resolve the HII/PDR complexes around the star-forming regions for the first time. In the N-band spectra of two young star clusters unexpectedly low polycyclic aromatic hydrocarbon (PAH) emission is observed, compared to what is seen with the Infrared Space Observatory (ISO) an…
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…
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…
We analyse the population of bright star clusters in the interacting galaxy pair NGC 4038/39 detected with HST WFPC1 by Whitmore & Schweizer (1995) using our spectrophotometric evolutionary synthesis models for various initial metallicities. We derive individual ages for all clusters with V-I colour and follow the time evolution of their luminosity function (LF). The age distribution clearly reveals the presence of a number of old Globular Clusters (GCs) from the parent galaxies. Their effective radii do not significantly differ from those of the young star cluster population. We confirm Meure…
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…
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…
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…
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 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…
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…
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…
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.…
(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,…
Gravitational tides are widely understood to strip and destroy galactic substructures. In the course of a galaxy merger, however, transient totally compressive tides may develop and prevent star forming regions from dissolving, after they condensed to form clusters of stars. We study the statistics of such compressive modes in an N-body model of the galaxy merger NGC 4038/39 (the Antennae) and show that ~15% of the disc material undergoes compressive tides at pericentre. The spatial distribution of observed young clusters in the overlap and nuclear regions of the Antennae matches surprisingly …
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 new aperture synthesis CO maps of the Antennae (NGC 4038/39) obtained with the Caltech Millimeter Array. These sensitive images show molecular emission associated with the two nuclei and a partial ring of star formation to the west of NGC 4038, as well as revealing the large extent of the extra-nuclear region of star formation (the ``overlap region''), which dominates the CO emission from this system. The largest molecular complexes have masses of 3-6x10^8 M_sun, typically an order of magnitude larger than the largest structures seen to date in more quiescent galaxy disks. The extre…
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…
HST WFPC1 observations revealed 738 bright star clusters spread over the inner 12 kpc of the interacting galaxy pair NGC 4038/39. If the young star clusters (YSCs) formed in the global starburst are (proto-) globular or open clusters has far-reaching consequences for the formation of elliptical galaxies. Effective radii of the YSCs are difficult to measure on WFPC1 images, their present luminosity function (LF) looks exponential. Evolutionary synthesis models for star clusters of various metallicities are used to individually age-date these clusters and describe their future luminosity and col…
We present observations of NGC 4038/39 in the [CII] 158 micron fine structure line taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the KAO with 55" resolution. The total [CII] luminosity of the Antennae is 3.7x10^8 L(sun), which is about 1% of the FIR luminosity. The main part of the [CII] emission probably arises from photodissociation regions (PDRs). Up to 1/3 of the observed [CII] emission may originate from the CNM for higher temperatures and densities. From PDR models we derive densities and FUV intensities for the nuclei and the interaction zone. The mini…
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…
The infrared regime contains a number of unique diagnostic features for probing the astrophysics of starburst galaxies. After a brief summary of the most important tracers, we focus in detail on the use of emission lines to probe the upper part of the main sequence in a young superstarcluster in the Antennae, and the compact, dusty starburst in the nucleus of the nearby ultraluminous infrared galaxy Arp220
Massive clusters are now seen to form easily in interacting and merging galaxies, making these excellent environments for studying the properties of young clusters. New observations of the Antennae (NGC 4038/39) show that the most luminous young clusters do not have a measurable tidal radius. Most observations suggest that the luminosity function (LF) and mass functions of young clusters are single power laws. However, there are many uncertainties at the faint end of the LF. For example, contamination from massive stars may be important. The shape and evolution of the LF, and more fundamentall…
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…
With the advent of efficient near-IR spectrometers on 10m-class telescopes, exploiting the new generation of low readout noise, large format detectors, OH avoidance and sub-arcsecond seeing, 1-2.4 micron spectroscopy is becoming a key means of obtaining detailed galaxy dynamics and for studies of high-z galaxies. In the following we present the results of three recent IR spectroscopy studies on the dynamics of ULIRG mergers, super star clusters in the Antennae, and on the properties of the rotation curves of z~1 disk galaxies, carried out with ISAAC on the VLT and NIRSPEC on the Keck.
We present a comparison of X-ray point source luminosity functions of 3 starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity functions of the starbursts are flatter than those of the spiral galaxies; the starbursts have relatively more sources at high luminosities. This trend extends to early-type galaxies which have steeper luminosity functions than spirals. We show that the luminosity function slope is correlated with 60 micron luminosity, a measure of star formation. We suggest that the di…
Preliminary results from spectrophotometric observations of galaxies with very young starbursts are presented. Starburst galaxies with an age of the burst in the range between 3 and 6 Myr have been observed and new detections of Wolf-Rayet galaxies are reported. We discuss the origin of high excitation nebular lines observed in these galaxies and their possible link with the population of Wolf-Rayet stars.
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…
We have conducted a $UBVRI$ and $H_α$ CCD photometry of 5 barred galaxies, NGC 2523, NGC 2950, NGC 3412, NGC 3945 and NGC 5383, along with SPH simulations, in order to understand the origin of young stellar populations in the nuclei of barred galaxies. The $H_α$ emissions which are thought to be emitted by young stellar populations are either absent or strongly concentrated on the nuclei of early type galaxies (NGC 2950, NGC 3412, NGC 3945), while they are observed in the nuclei and circumnuclear regions of intermediate type galaxies with strong bars (NGC 2523, NGC 5383). SPH simulations of re…
We present some highlights from the mid-infrared (5-16 micron) images of mergers of massive galaxies obtained with the Infrared Space Observatory (ISO). We have observed: 1) ultraluminous infrared nuclei, 2) luminous dust-enshrouded extranuclear starbursts, and 3) active galaxy nuclei (AGNs). In this contribution we discuss the observations of Arp 299, a prototype for very luminous infrared galaxies, the Antennae which is a prototype of mergers, and Centaurus A which is the closest AGN to Earth. From these observations we conclude the following: 1) the most intense starbursts in colliding syst…
Young star clusters with masses well in excess of 100.000 Msun have been observed not only in merger galaxies and large-scale starbursts, but also in fairly normal, undisturbed spiral and irregular galaxies. Here we present virial mass estimates for a sample of 7 such clusters and show that the derived mass-to-light ratios are consistent with "normal" Kroupa-type stellar mass distributions.
