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Showing papers on "Star formation published in 2002"


Journal ArticleDOI
TL;DR: In this paper, the authors developed a new method to constrain the star formation histories, dust attenuation and stellar masses of galaxies based on two stellar absorption line indices, the 4000 Angstrom break strength and the Balmer absorption line index Hdelta_A.
Abstract: We develop a new method to constrain the star formation histories, dust attenuation and stellar masses of galaxies. It is based on two stellar absorption line indices, the 4000 Angstrom break strength and the Balmer absorption line index Hdelta_A. Together, these indices allow us to constrain the mean stellar ages of galaxies and the fractional stellar mass formed in bursts over the past few Gyr. A comparison with broad band photometry then yields estimates of dust attenuation and of stellar mass. We generate a large library of Monte Carlo realizations of different star formation histories, including starbursts of varying strength and a range of metallicities. We use this library to generate median likelihood estimates of burst mass fractions, dust attenuation strengths, stellar masses and stellar mass-to-light ratios for a sample of 122,208 galaxies drawn from the Sloan Digital Sky Survey. The typical 95% confidence range in our estimated stellar masses is +-40%. We study how the stellar mass-to-light ratios of galaxies vary as a function of absolute magnitude, concentration index and photometric pass-band and how dust attenuation varies as a function of absolute magnitude and 4000 Angstrom break strength. We also calculate how the total stellar mass of the present Universe is distributed over galaxies as a function of their mass, size, concentration, colour, burst mass fraction and surface mass density. We find that most of the stellar mass in the local Universe resides in galaxies that have stellar masses \~5\times 10^10 M_sol, half light radii ~3 kpc, and half- light surface mass densities ~10^9 M_sol/kpc^2. The distribution of D(4000) is strongly bimodal, showing a clear division between galaxies dominated by old stellar populations and galaxies with more recent star formation.

2,042 citations


Journal ArticleDOI
Pavel Kroupa1
04 Jan 2002-Science
TL;DR: Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF, which appears to hold for populations including present-day star formation in small molecular clouds.
Abstract: The distribution of stellar masses that form in one star formation event in a given volume of space is called the initial mass function (IMF). The IMF has been estimated from low-mass brown dwarfs to very massive stars. Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF. This general insight appears to hold for populations including present-day star formation in small molecular clouds, rich and dense massive star-clusters forming in giant clouds, through to ancient and metal-poor exotic stellar populations that may be dominated by dark matter. This apparent universality of the IMF is a challenge for star formation theory, because elementary considerations suggest that the IMF ought to systematically vary with star-forming conditions.

1,733 citations


Journal ArticleDOI
TL;DR: In this article, a model for star formation and supernova feedback that describes the multi-phase structure of star forming gas on scales that are typically not resolved in cosmological simulations is presented.
Abstract: We present a model for star formation and supernova feedback that describes the multi-phase structure of star forming gas on scales that are typically not resolved in cosmological simulations Our approach includes radiative heating and cooling, the growth of cold clouds embedded in an ambient hot medium, star formation in these clouds, feedback from supernovae in the form of thermal heating and cloud evaporation, galactic winds and outflows, and metal enrichment Implemented using SPH, our scheme is a significantly modified and extended version of the grid-based method of Yepes et al (1997), and enables us to achieve high dynamic range in simulations of structure formation We discuss properties of the feedback model in detail and show that it predicts a self-regulated, quiescent mode of star formation, which, in particular, stabilises the star forming gaseous layers of disk galaxies The parameterisation of this mode can be reduced to a single free quantity which determines the overall timescale for star formation We fix this parameter to match the observed rates of star formation in local disk galaxies When normalised in this manner, cosmological simulations nevertheless overproduce the observed cosmic abundance of stellar material We are thus motivated to extend our feedback model to include galactic winds associated with star formation Using small-scale simulations of individual star-forming disk galaxies, we show that these winds produce either galactic fountains or outflows, depending on the depth of the gravitational potential Moreover, outflows from galaxies in these simulations drive chemical enrichment of the intergalactic medium, in principle accounting for the presence of metals in the Lyman alpha forest (abridged)

1,713 citations


Journal ArticleDOI
04 Jan 2002-Science
TL;DR: It is concluded that at most one massive metal-free star forms per pregalactic halo, consistent with recent abundance measurements of metal-poor galactic halo stars.
Abstract: We describe results from a fully self-consistent three-dimensional hydrodynamical simulation of the formation of one of the first stars in the Universe. In current models of structure formation, dark matter initially dominates, and pregalactic objects form because of gravitational instability from small initial density perturbations. As they assemble via hierarchical merging, primordial gas cools through ro-vibrational lines of hydrogen molecules and sinks to the center of the dark matter potential well. The high-redshift analog of a molecular cloud is formed. As the dense, central parts of the cold gas cloud become self-gravitating, a dense core of approximately 100 M (where M is the mass of the Sun) undergoes rapid contraction. At particle number densities greater than 10(9) per cubic centimeter, a 1 M protostellar core becomes fully molecular as a result of three-body H2 formation. Contrary to analytical expectations, this process does not lead to renewed fragmentation and only one star is formed. The calculation is stopped when optical depth effects become important, leaving the final mass of the fully formed star somewhat uncertain. At this stage the protostar is accreting material very rapidly (approximately 10(-2) M year-1). Radiative feedback from the star will not only halt its growth but also inhibit the formation of other stars in the same pregalactic object (at least until the first star ends its life, presumably as a supernova). We conclude that at most one massive (M 1 M) metal-free star forms per pregalactic halo, consistent with recent abundance measurements of metal-poor galactic halo stars.

1,498 citations


Journal ArticleDOI
TL;DR: In this article, the relation between the density profiles of dark matter halos and their mass assembly histories was studied using a statistical sample of halos in a high-resolution N-body simulation of the ΛCDM cosmology.
Abstract: We study the relation between the density profiles of dark matter halos and their mass assembly histories using a statistical sample of halos in a high-resolution N-body simulation of the ΛCDM cosmology. For each halo at z = 0, we identify its merger history tree and determine concentration parameters cvir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch ac, defined when the log mass accretion rate d log M/d log a falls below a critical value S. We find that late-forming galaxies tend to be less concentrated, such that cvir "observed" at any epoch ao is strongly correlated with ac via cvir = c1ao/ac. Scatter about this relation is mostly due to measurement errors in cvir and ac, implying that the actual spread in cvir for halos of a given mass can be mostly attributed to scatter in ac. We demonstrate that this relation can also be used to predict the mass and redshift dependence of cvir and the scatter about the median cvir(M, z) using accretion histories derived from the extended Press-Schechter (EPS) formalism, after adjusting for a constant offset between the formation times as predicted by EPS and as measured in the simulations; this new ingredient can thus be easily incorporated into semianalytic models of galaxy formation. The correlation found between halo concentration and mass accretion rate suggests a physical interpretation: for high mass infall rates, the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant, and the halo concentration just grows as Rvir. Because of the direct connection between halo concentration and velocity rotation curves and because of probable connections between halo mass assembly history and star formation history, the tight correlation between these properties provides an essential new ingredient for galaxy formation modeling.

1,213 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the relationship between stellar mass, star formation history, size and internal structure for a complete sample of 122,808 galaxies drawn from the Sloan Digital Sky Survey, and showed that low-redshift galaxies divide into two distinct families at a stellar mass of 3 \times 10^10 M_sol.
Abstract: We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 122,808 galaxies drawn from the Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3 \times 10^10 M_sol. Lower mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of disks. A significant fraction of the lowest mass galaxies in our sample have experienced recent starbursts. At given stellar mass, the sizes of low mass galaxies are log- normally distributed with dispersion sigma(ln R_50) \sim 0.5, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo. Their median stellar surface mass density scales with stellar mass as mu* propto M_*^0.54, suggesting that the stellar mass of a disk galaxy is proprtional to the three halves power of its halo mass. This suggests that the efficiency of the conversion of baryons into stars in low mass galaxies increases in propor- tion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 \times 10^10 M_sol, there is a rapidly increasing frac- tion of galaxies with old stellar populations, high surface mass densities and high concentrations typical of bulges. In this regime, the size distribution is log-normal, but its dispersion decreases rapidly with increasing stellar mass and the median mass surface density is approximately constant. This suggests that the star formation efficiency decreases in the highest mass halos, and that little star formation occurs in massive galaxies once they have assembled.

1,154 citations


Journal ArticleDOI
TL;DR: In this paper, the physics of primordial star formation were investigated by means of three-dimensional simulations of the dark matter and gas components, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation.
Abstract: To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of three-dimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ~ a few 100 K, and the density, n ~ 103-104 cm-3, characterizing the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is MJ ~ 103 M?. The existence of these characteristic values has a robust explanation in the microphysics of H2 cooling, connected to the minimum temperature that can be reached with the H2 coolant, and to the critical density at which the transition takes place between levels being populated according to non-LTE (NLTE), and according to LTE. In all cases, the gas dissipatively settles into an irregular, central configuration that has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ~103 M?, close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M* 100 M?. After a gas element has undergone runaway collapse, and has reached densities in excess of 108 cm-3, a sink particle is created. This procedure allows us to follow the evolution of the overall system beyond the point where the first nonlinear region would otherwise force the calculation to a halt. These later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 104 M?, is merging between clumps. Since the merging rate sensitively depends on the density of the gas, halos with the highest degree of central concentration are able to assemble the most massive clumps. Among these are halos with a low spin (? 0.01), and with DM fluctuations imprinted according to a white-noise spectrum.

1,061 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of plane-parallel non-LTE model atmospheres and comoving frame calculations is presented for massive Population III stars and stellar populations based on a recent stellar evolution tracks and up-to-date evolutionary synthesis models, with the aim to study their spectral properties, including their dependence on age, star formation history, and IMF.
Abstract: We present realistic models for massive Population III stars and stellar populations based on non-LTE model atmospheres, recent stellar evolution tracks and up-to-date evolutionary synthesis models, with the aim to study their spectral properties, including their dependence on age, star formation history, and IMF. A comparison of plane parallel non-LTE model atmospheres and comoving frame calculations shows that even in the presence of some putative weak mass loss, the ionising spectra of metal-free populations differ little or negligibly from those obtained using plane parallel non-LTE models. As already discussed by Tumlinson & Shull ([CITE]), the main salient property of Pop III stars is their increased ionising flux, especially in the He+ continuum (>54 eV). The main result obtained for individual Pop III stars is the following: due to their redward evolution off the zero age main sequence (ZAMS) the spectral hardness measured by the He/H ionising flux is decreased by a factor ~2 when averaged over their lifetime. If such stars would suffer strong mass loss, their spectral appearance could, however, remain similar to that of their ZAMS position. The main results regarding integrated stellar populations are: – for young bursts and the case of a constant SFR, nebular continuous emission – neglected in previous studies – dominates the spectrum redward of Lyman-α if the escape fraction of ionising photons out of the considered region is small or negligible. In consequence predicted emission line equivalent widths are considerably smaller than found in earlier studies, whereas the detection of the continuum is eased. Nebular line and continuous emission strongly affect the broad band photometric properties of Pop III objects; – due to the redward stellar evolution and short lifetimes of the most massive stars, the hardness of the ionising spectrum decreases rapidly, leading to the disappearance of the characteristic He ii recombination lines after ~3 Myr in instantaneous bursts; – He ii λ 1640, Hα (and other) line luminosities usable as indicators of the star formation rate are given for the case of a constant SFR. For obvious reasons such indicators depend strongly on the IMF; – due to an increased photon production and reduced metal yields, the relative efficiency of ionising photon energy to heavy element rest mass production, η , of metal-poor and metal-free populations is increased by factors of ~4 to 18 with respect to solar metallicity and for “standard” IMFs; – the lowest values of 1.6–2.2% are obtained for IMFs exclusively populated with high mass stars (). If correct, the yields dominated by pair creation SNae then predict large overabundances of O/C and Si/C compared to solar abundance ratios. Detailed results are given in tabular form and as fit formulae for easy implementation in other calculations. The predicted spectra will be used to study the detectability of Pop III galaxies and to derive optimal search strategies for such objects.

854 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the relationship between H I, H 2, and the star formation rate (SFR) using azimuthally averaged data for seven CO-bright spiral galaxies.
Abstract: We investigate the relationship between H I, H2, and the star formation rate (SFR) using azimuthally averaged data for seven CO-bright spiral galaxies. Contrary to some earlier studies based on global fluxes, we find that ΣSFR exhibits a much stronger correlation with Σ than with Σ, as Σ saturates at a value of ~ 10 M☉ pc-2 or even declines for large ΣSFR. Hence, the good correlation between ΣSFR and the total (H I+H2) gas surface density Σgas is driven by the molecular component in these galaxies, with the observed relation between ΣSFR and Σ following a Schmidt-type law of index nmol ≈ 0.8 if a uniform extinction correction is applied or nmol ≈ 1.4 for a radially varying correction dependent on gas density. The corresponding Schmidt law indices for Σgas versus ΣSFR are 1.1 and 1.7 for the two extinction models, in rough agreement with previous studies of the disk-averaged star formation law. An alternative to the Schmidt law, in which the gas depletion timescale is proportional to the orbital timescale, is also consistent with the data if radially varying extinction corrections are applied. We find no clear evidence for a link between the gravitational instability parameter for the gas disk (Qg) and the SFR, and we suggest that Qg be considered a measure of the gas fraction. This implies that for a state of marginal gravitational stability to exist in galaxies with low gas fractions, it must be enforced by the stellar component. In regions where we have both H I and CO measurements, the ratio of H I to H2 surface density scales with radius as roughly R1.5, and we suggest that the balance between H I and H2 is determined primarily by the midplane interstellar pressure. These results favor a "law" of star formation in quiescent disks in which the ambient pressure and metallicity control the formation of molecular clouds from H I, with star formation then occurring at a roughly constant rate per unit H2 mass.

748 citations


Journal ArticleDOI
TL;DR: In this paper, a linear relation between HMXB number and the star formation rate has been shown to exist in the high-mass X-ray binary (HMXB) populations in the Milky Way and Magellanic Clouds.
Abstract: Based on CHANDRA observations of nearby starburst galaxies and RXTE/ASM, ASCA and MIR-KVANT/TTM studies of high mass X-ray binary (HMXB) populations in the Milky Way and Magellanic Clouds, we propose that the number and/or the collective X-ray luminosity of HMXBs can be used to measure the star formation rate (SFR) of a galaxy. We show that, within the accuracy of the presently available data, a linear relation between HMXB number and the star formation rate exists. The relation between SFR and collective luminosity of HMXBs is non-linear in the low SFR regime, $L_X\propto \SFR^{\approx 1.7}$, and becomes linear only for sufficiently high star formation rate, when the total number of HMXB sources becomes sufficiently large. Such behaviour is caused by the fact, that we measure collective luminosity of a population of the discrete sources. Although more subtle SFR dependent effects are likely to exist, the data are broadly consistent with the existence of a universal luminosity function of HMXBs which can be roughly described as a power law with a differential slope of $\sim 1.6$, a cutoff at $L_X \sim few \times 10^{40}$ erg/sec and a normalisation proportional to the star formation rate. We apply our results to (spatially unresolved) starburst galaxies observed by CHANDRA at redshifts up to $z\sim 1$ in the Hubble Deep Field North and show that the calibration of the collective luminosity of HMXBs as a SFR indicator based on the local sample agrees well with the SFR estimates obtained for these distant galaxies with conventional methods.

635 citations


Journal ArticleDOI
TL;DR: In this article, the locations of gamma-ray bursts relative to their host galaxies were measured using ground-based images from Palomar and Keck and space-based imagery from the Hubble Space Telescope (HST).
Abstract: We present a comprehensive study to measure the locations of gamma-ray bursts (GRBs) relative to their host galaxies. In total, we find the offsets of 20 long-duration GRBs from their apparent host galaxy centers by utilizing ground-based images from Palomar and Keck and space-based images from the Hubble Space Telescope (HST). We discuss in detail how a host galaxy is assigned to an individual GRB and the robustness of the assignment process. The median projected angular (physical) offset is 017 (1.3 kpc). The median offset normalized by the individual host half-light radii is 0.98, suggesting a strong connection of GRB locations with the UV light of their hosts. This provides strong observational evidence for the connection of GRBs to star formation. We further compare the observed offset distribution with the predicted burst locations of leading stellar-mass progenitor models. In particular, we compare the observed offset distribution with an exponential disk, a model for the location of collapsars and promptly bursting binaries (e.g., helium star–black hole binaries). The statistical comparison shows good agreement, given the simplicity of the model, with the Kolmogorov-Smirnov probability that the observed offsets derive from the model distribution of PKS = 0.45. We also compare the observed GRB offsets with the expected offset distribution of delayed merging remnant progenitors (black hole–neutron star and neutron star–neutron star binaries). We find that delayed merging remnant progenitors, insofar as the predicted offset distributions from population synthesis studies are representative, can be ruled out at the 2 × 10-3 level. This is arguably the strongest observational constraint yet against delayed merging remnants as the progenitors of long-duration GRBs. In the course of this study, we have also discovered the putative host galaxies of GRB 990510 and GRB 990308 in archival HST data.

Journal ArticleDOI
TL;DR: In this paper, a comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution.
Abstract: A comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution. Among these are the need for a complete sampling of the synthetic CMD, the use of proper statistics for dealing with Poisson-distributed data (and a demonstration of why Χ 2 must not be used), measuring full uncertainties in all reported parameters, quantifying the goodness-of-fit, and questions of binning the CMD and incorporating outside information. Several example star formation history measurements are given. Two examples involve synthetic data, in which the input and recovered parameters can be compared to locate possible flaws in the methodology (none were apparent) and measure the accuracy with which ages, metallicities and star formation rates can be recovered. Solutions of the histories of seven Galactic dwarf spheroidal companions (Carina, Draco, Leo I, Leo II, Sagittarius, Sculptor and Ursa Minor) illustrate the ability to measure star formation histories given a variety of conditions -numbers of stars, complexity of star formation history and amount of foreground contamination. Significant measurements of ancient >8 Gyr star formation are made in all seven galaxies. Sculptor, Draco and Ursa Minor appear entirely ancient, while the other systems show varying amounts of younger stars.

Journal ArticleDOI
TL;DR: In this paper, the authors study the growth rate of stars via stellar collisions in dense star clusters, calibrating their analytic calculations with direct N-body simulations of up to 65,536 stars, performed on the GRAPE family of specialpurpose computers.
Abstract: We study the growth rate of stars via stellar collisions in dense star clusters, calibrating our analytic calculations with direct N-body simulations of up to 65,536 stars, performed on the GRAPE family of special-purpose computers. We find that star clusters with initial half-mass relaxation times 25 Myr are dominated by stellar collisions, the first collisions occurring at or near the point of core collapse, which is driven by the segregation of the most massive stars to the cluster center, where they end up in hard binaries. The majority of collisions occur with the same star, resulting in the runaway growth of a supermassive object. This object can grow up to ~0.1% of the mass of the entire star cluster and could manifest itself as an intermediate-mass black hole (IMBH). The phase of runaway growth lasts until mass loss by stellar evolution arrests core collapse. Star clusters older than about 5 Myr and with present-day half-mass relaxation times 100 Myr are expected to contain an IMBH.

Journal ArticleDOI
TL;DR: In this article, a detailed 1.2 mm continuum and CS spectral line study of a large sample of 69 massive star forming regions in very early stages of evolution, most of them prior to building up an ultracompact H II region is presented.
Abstract: We present a detailed 1.2 mm continuum and CS spectral line study of a large sample of 69 massive star forming regions in very early stages of evolution, most of them prior to building up an ultracompact H II region. The continuum data show a zoo of different morphologies and give detailed information on the spatial distributions, masses, column densities, and average densities of the whole sample. Fitting the radial intensity profiles shows that three parameters are needed to describe the spatial distribution of the sources: constant emission from the center out to a few arcseconds radius followed by a first power-law intensity distribution, which steepens farther outside into a second power-law distribution. The inner flat region is possibly caused by fragmentation of the large-scale cores into smaller subsources, whereas the steeper outer power-law distributions indicate finite sizes of the cores. Separating the sources into subsamples suggests that in the earliest stages prior to the onset of massive star formation, the intensity radial distributions are rather flat, resembling the structure of intensity peaks in more quiescent molecular clouds. Then in the subsequent collapse and accretion phase the intensity distributions become centrally peaked, with steep power-law indices. In this evolutionary stage the sources show also the broadest C34S line width. During the following phase, when ultracompact H II regions evolve, the intensity power-law radial distributions flatten out again. This is probably caused by the ignited massive stars in the center which disrupt the surrounding cores. The mean inner power-law intensity index mi (I ~ r) is 1.2, corresponding to density indices p (n ~ r-p) of 1.6. In total, the density distributions of our massive star formation sites seem to be not too different from their low-mass counterparts, but we show that setting tight constrains on the density indices is very difficult and subject to many possible errors. The local densities we derive from CS calculations are higher (up to 1 order of magnitude) than the mean densities we find via the millimeter continuum. Such inhomogeneous density distribution reflects most likely the ubiquitous phenomenon of clumping and fragmentation in molecular clouds. Line width-mass relations show a departure from virial equilibrium in the stages of strongly collapsing cores.

Journal ArticleDOI
TL;DR: In this article, the history of cosmic star formation from the "dark ages" at redshift z~20 to the present is studied, including radiative heating and cooling of gas, star formation, supernova feedback, and galactic winds.
Abstract: Employing hydrodynamic simulations of structure formation in a LCDM cosmology, we study the history of cosmic star formation from the "dark ages" at redshift z~20 to the present. In addition to gravity and ordinary hydrodynamics, our model includes radiative heating and cooling of gas, star formation, supernova feedback, and galactic winds. By making use of a comprehensive set of simulations on interlocking scales and epochs, we demonstrate numerical convergence of our results on all relevant halo mass scales, ranging from 10^8 to 10^15 Msun/h. The predicted density of cosmic star formation is broadly consistent with measurements, given observational uncertainty. From the present epoch, it gradually rises by about a factor of ten to a peak at z~5-6, which is beyond the redshift range where it has been estimated observationally. 50% of the stars are predicted to have formed by redshift z~2.1, and are thus older than 10.4 Gyr, while only 25% form at redshifts lower than z~1. The mean age of all stars at the present is about 9 Gyr. Our model predicts a total stellar density at z=0 of Omega_*=0.004, corresponding to about 10% of all baryons being locked up in long-lived stars, in agreement with recent determinations of the luminosity density of the Universe. We determine the "multiplicity function of cosmic star formation" as a function of redshift; i.e. the distribution of star formation with respect to halo mass. We also briefly examine possible implications of our predicted star formation history for reionisation of hydrogen in the Universe. We find that the star formation rate predicted by the simulations is sufficient to account for hydrogen reionisation by z~6, but only if a high escape fraction close to unity is assumed. (abridged)

Journal ArticleDOI
TL;DR: In this paper, the role of massive outflows in high-mass star formation was investigated and a strong correlation between the outflow mass and the core mass over many orders of magnitude was found.
Abstract: With the aim of understanding the role of massive outflows in high-mass star formation, we mapped in the 12 CO transition 26 high-mass star-forming regions at very early stages of their evolution. At a spatial resolution of bipolar molecular outflows were found in 21 of them. The other five sources show confusing morphology but strong line wings. This high detection rate of bipolar structure proves that outflows common in low-mass sources are also ubiquitous phenomena in the formation process of massive stars. The flows are large, very massive and energetic, and the data indicate stronger collimation than previously thought. The dynamical timescales of the flows correspond well to the free-fall timescales of the associated cores. Comparing with correlations known for low-mass flows, we find continuity up to the high-mass regime suggesting similar flow-formation scenarios for all masses and luminosities. Accretion rate estimates in the range are around yr-1 , higher than required for low-mass star formation, but consistent with high-mass star formation scenarios. Additionally, we find a tight correlation between the outflow mass and the core mass over many orders of magnitude. The strong correlation between those two quantities implies that the product of the accretion efficiency and (the ratio between jet mass loss rate and accretion rate), which equals the ratio between jet and core mass (), is roughly constant for all core masses. This again indicates that the flow-formation processes are similar over a large range of masses. Additionally, we estimate median and values of approximately 0.2 and 0.01, respectively, which is consistent with current jet-entrainment models. To summarize, the analysis of the bipolar outflow data strongly supports theories which explain massive star formation by scaled up, but otherwise similar physical processes – mainly accretion – to their low-mass counterparts.

Journal ArticleDOI
TL;DR: In this paper, spectral properties of the ionising continua, the Lyman-break, and the Ly-a and HeII 1640 recombination lines in starbursts were examined.
Abstract: We examine spectral properties of the ionising continua, the Lyman-break, and the Ly-a and HeII 1640 recombination lines in starbursts. The transition from primordial galaxies to currently observed metallicities, is examined. For the average properties of starbursts, the main findings are: 1) The Lyman continuum flux increases with decreasing metallicity. For a universal Salpeter IMF from 1-100 Msun the enhancement reaches typically a factor of ~3 between solar metallicity and Pop III objects. 2) While for metallicities Z >~ 1/50 Zsun the amplitude of the Lyman-break depends little on Z, a reduction by a factor ~2 is found at lower Z, due to the strong increase of the average stellar temperature. 3) Using theoretical models and empirical constraints we discuss the expected evolution of the hardness of He+ to H ionising photons with metallicity and possible uncertainties. We also provide a simple estimate of the possible impact of hot WR like stars on Q(He+)/Q(H) at very low metallicities. 4) Calibrations for star formation rate determinations from various recombination lines at all metallicities and for various IMFs are derived. For young bursts the maximum Ly-a equivalent width is shown to increase strongly with decreasing metallicity for the same IMF. However, for well known reasons, the Ly-a emission predicted likely represents an upper limit. Non-negligible HeII 1640 emission due to stellar photoionisation appears to be limited to very low metallicities and Population III objects.(abridged abstract)

Journal ArticleDOI
31 Oct 2002-Nature
TL;DR: This work reports the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value, which suggests that population III stars could still exist and that the first generation of stars also contained long-livedLow-mass objects.
Abstract: The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts1,2; if totally metal-free (‘population III’) stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun3,4, leading to the suggestion5,6 that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist—that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.

Journal ArticleDOI
TL;DR: In this paper, a review of the evolution of the population of galaxy clusters in the X-ray band is presented, focusing on observations with the ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX.
Abstract: ■ Abstract Considerable progress has been made over the past decade in the study of the evolutionary trends of the population of galaxy clusters in the Universe. In this review we focus on observations in the X-ray band. X-ray surveys with the ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX, have allowed an assessment of the evolution of the space density of clusters out to z 1 and the evolution of the physical properties of the intracluster medium out to z 0.5. With the advent of Chandra and Newton-XMM and their unprecedented sensitivity and angular resolution, these studies have been extended beyond redshift unity and have revealed the complexity of the thermodynamical structure of clusters. The properties of the intracluster gas are significantly affected by nongravitational processes including star formation and Active Galactic Nuclei (AGN) activity. Convincing evidence has emerged for modest evolution of both the bulk of the X-ray cluster population and their thermodynamical properties since redshift unity. Such an observational scenario is consistent with hierarchical models of structure formation in a flat low-density

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a semianalytical scheme for augmenting existing evolutionary tracks of pre-main-sequence protostars by including the effects of accretion.
Abstract: We calculate numerically the collapse of slowly rotating, nonmagnetic, massive molecular clumps of masses 30,60, and 120 Stellar Mass, which conceivably could lead to the formation of massive stars. Because radiative acceleration on dust grains plays a critical role in the clump's dynamical evolution, we have improved the module for continuum radiation transfer in an existing two-dimensional (axial symmetry assumed) radiation hydrodynamic code. In particular, rather than using "gray" dust opacities and "gray" radiation transfer, we calculate the dust's wavelength-dependent absorption and emission simultaneously with the radiation density at each wavelength and the equilibrium temperatures of three grain components: amorphous carbon particles. silicates, and " dirty ice " -coated silicates. Because our simulations cannot spatially resolve the innermost regions of the molecular clump, however, we cannot distinguish between the formation of a dense central cluster or a single massive object. Furthermore, we cannot exclude significant mass loss from the central object(s) that may interact with the inflow into the central grid cell. Thus, with our basic assumption that all material in the innermost grid cell accretes onto a single object. we are able to provide only an upper limit to the mass of stars that could possibly be formed. We introduce a semianalytical scheme for augmenting existing evolutionary tracks of pre-main-sequence protostars by including the effects of accretion. By considering an open outermost boundary, an arbitrary amount of material could, in principal, be accreted onto this central star. However, for the three cases considered (30, 60, and 120 Stellar Mass originally within the computation grid), radiation acceleration limited the final masses to 3 1.6, 33.6, and 42.9 Stellar Mass, respectively, for wavelength-dependent radiation transfer and to 19.1, 20.1, and 22.9 Stellar Mass. for the corresponding simulations with gray radiation transfer. Our calculations demonstrate that massive stars can in principle be formed via accretion through a disk. The accretion rate onto the central source increases rapidly after one initial free-fall time and decreases monotonically afterward. By enhancing the nonisotropic character of the radiation field, the accretion disk reduces the effects of radiative acceleration in the radial direction - a process we call the "flashlight effect." The flashlight effect is further amplified in our case by including the effects of frequency-dependent radiation transfer. We conclude with the warning that a careful treatment of radiation transfer is a mandatory requirement for realistic simulations of the formation of massive stars.

Journal ArticleDOI
TL;DR: In this article, the authors describe a systematic program aimed at identifying and characterizing candidate high-mass protostellar objects (HMPOs) using far-infrared, radio continuum, and molecular line data.
Abstract: We describe a systematic program aimed at identifying and characterizing candidate high-mass protostellar objects (HMPOs). Our candidate sample consists of 69 objects selected by criteria based on those established by Ramesh & Sridharan using far-infrared, radio continuum, and molecular line data. IRAS and Midcourse Space Experiment data were used to study the larger scale environments of the candidate sources and determine their total luminosities and dust temperatures. To derive the physical and chemical properties of our target regions, we observed continuum and spectral line radiation at millimeter and radio wavelengths. We imaged the free-free and dust continuum emission at wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H2O and CH3OH maser emission, and observed the CO J = 2 → 1 line and several NH3 lines toward all sources in our sample. Other molecular tracers were observed in a subsample. While dust continuum emission was detected in all sources, most of them show only weak or no emission at 3.6 cm. Where detected, the centimeter emission is frequently found to be offset from the millimeter emission, indicating that the free-free and dust emissions arise from different subsources possibly belonging to the same (proto)cluster. A comparison of the luminosities derived from the centimeter emission with bolometric luminosities calculated from the IRAS far-infrared fluxes shows that the centimeter emission very likely traces the most massive source, whereas the whole cluster contributes to the far-infrared luminosity. Estimates of the accretion luminosity indicate that a significant fraction of the bolometric luminosity is still due to accretion processes. The earliest stages of HMPO evolution we seek to identify are represented by dust cores without radio emission. Line wings due to outflow activity are nearly omnipresent in the CO observations, and the molecular line data indicate the presence of hot cores for several sources, where the abundances of various molecular species are elevated because of evaporation of icy grain mantles. Kinetic gas temperatures of 40 sources are derived from NH3 (1, 1) and (2, 2) data, and we compare the results with the dust temperatures obtained from the IRAS data. Comparing the amount of dust, and hence the gas, associated with the HMPOs and with ultracompact H II (UCH II) regions, we find that the two types of sources are clearly separated in mass-luminosity diagrams: for the same dust masses, the UCH II regions have higher bolometric luminosities than HMPOs. We suggest that this is an evolutionary trend, with the HMPOs being younger and reprocessing less (stellar) radiation in the IR than the more evolved UCH II regions. These results indicate that a substantial fraction of our sample harbors HMPOs in a pre-UCH II region phase, the earliest known stage in the high-mass star formation process.

Journal ArticleDOI
TL;DR: In this article, a tight linear relation holds between the 2-10 keV X-ray luminosity and both the radio and far infrared luminosities of star forming galaxies.
Abstract: Radio and far infrared luminosities of star forming galaxies follow a tight linear relation. Making use of BeppoSAX and ASCA observations of a well-defined sample of star forming galaxies, we argue that a tight linear relation holds between the 2-10 keV X-ray luminosity and both the radio and far infrared ones. It is suggested that the hard X-ray emission is directly related to the Star Formation Rate. Preliminary results obtained from deep Chandra and radio observations of the Hubble Deep Field North show that a similar relation might hold also at high (0.2

Journal ArticleDOI
TL;DR: In this article, the authors analyzed IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 and derived the large-scale distribution of stars and interstellar matter in the Nuclear Bulge.
Abstract: We analyse IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 of the central 500 pc of the Galaxy and derive the large-scale distribution of stars and interstellar matter in the Nuclear Bulge. Models of the Galactic Disk and Bulge are developed in order to correctly decompose the total surface brightness maps of the inner Galaxy and to apply proper extinction corrections. The Nuclear Bulge appears as a distinct, massive disk-like complex of stars and molecular clouds which is, on a large scale, symmetric with respect to the Galactic Centre. It is distinguished from the Galactic Bulge by its flat disk-like morphology, very high density of stars and molecular gas, and ongoing star formation. The Nuclear Bulge consists of an R^(-2) Nuclear Stellar Cluster at the centre, a large Nuclear Stellar Disk with radius 230 ± 20 pc and scale height 45 ± 5 pc, and the Nuclear Molecular Disk of same size. The total stellar mass and luminosity of the Nuclear Bulge are 1.4 ± 0.6 x 10^9 and 2.5 ± 1 x 10^9, respectively. About 70% of the luminosity is due to optical and UV radiation from young massive Main-Sequence stars which are most abundant in the Nuclear Stellar Cluster. For the first time, we derive a photometric mass distribution for the central 500 pc of the Galaxy which is fully consistent with the kinematic mass distribution. We find that the often cited R^(-2) distribution holds only for the central ~30 pc and that at larger radii the mass distribution is dominated by the Nuclear Stellar Disk which has a flatter density profile. The total interstellar hydrogen mass in the Nuclear Bulge is M_H = 2 ± 0.3 x 10^7, distributed in a warm inner disk with R = 110 ± 20 pc and a massive, cold outer torus which contains more than 80% of this mass. Interstellar matter in the Nuclear Bulge is very clumpy with ~90% of the mass contained in dense and massive molecular clouds with a volume filling factor of only a few per cent. This extreme clumpiness, probably caused by the tidal stability limit in the gravitational potential of the Nuclear Bulge, enables the strong interstellar radiation field to penetrate the entire Nuclear Bulge and explains the relatively low average extinction towards the Galactic Centre. In addition, we find 3 x 10^7 of cold and dense material outside the Nuclear Bulge at positive longitudes and 1 x 10^7 at negative longitudes. This material is not heated by the stars in the Nuclear Bulge and gives rise to the observed asymmetry in the distribution of interstellar matter in the Central Molecular Zone.

Journal ArticleDOI
TL;DR: In this paper, a detailed study of the interstellar medium (ISM) of MS 1512-cB58, an ~L* Lyman break galaxy at z = 2.7276, based on new spectral observations obtained with the Echelle Spectrograph and Imager on the Keck II telescope at 58 km s-1 resolution.
Abstract: We present the results of a detailed study of the interstellar medium (ISM) of MS 1512-cB58 (cB58 for short), an ~L* Lyman break galaxy at z = 2.7276, based on new spectral observations obtained with the Echelle Spectrograph and Imager on the Keck II telescope at 58 km s-1 resolution. We focus in particular on the chemical abundances and kinematics of the interstellar gas deduced from the analysis of 48 ultraviolet absorption lines, at rest wavelengths between 1134 and 2576 ?, due to elements from H to Zn. Our main findings are as follows. Even at this relatively early epoch, the ISM of this galaxy is already highly enriched in elements released by Type II supernovae; the abundances of O, Mg, Si, P, and S are all ~2/5 of their solar values. In contrast, N and the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of ~3. Based on current ideas of stellar nucleosynthesis, these results can be understood if most of the metal enrichment in cB58 has taken place within the last ~300 Myr, the timescale for the release of N from intermediate-mass stars. Such a young age is consistent with the UV-optical spectral energy distribution. Thus, cB58 seems to be an example of a galaxy in the process of converting its gas into stars on a few dynamical timescales?quite possibly we are witnessing the formation of a galactic bulge or an elliptical galaxy. The energetic star formation activity has stirred the interstellar medium to high velocities; the strongest absorption lines span a velocity interval of ~1000 km s-1. The net effect is a bulk outflow of the ISM at a speed of ~255 km s-1 and at a rate that exceeds the star formation rate. It is unclear whether this gas will be lost or retained by the galaxy. On the one hand, the outflow probably has sufficient energy to escape the potential well of cB58, for which we derive a baryonic mass of ~1010 M?. On the other hand, at least some of the elements manufactured by previous generations of stars must have mixed efficiently with the ambient, neutral, ISM to give the high abundances we measure. We point out that the chemical and kinematic properties of cB58 are markedly different from those of most damped Ly? systems at the same redshift.

Journal ArticleDOI
TL;DR: In this paper, the mass fraction of pair-unstable supernovae (SNs) is estimated to be the dominant sources of the first heavy elements in the early universe.
Abstract: Recent studies suggest that the initial mass function (IMF) of the first stars (Population III) is likely to have been extremely top-heavy, unlike what is observed at present. We propose a scenario to generate fragmentation to lower masses once the first massive stars have formed and derive constraints on the primordial IMF. We estimate the mass fraction of pair-unstable supernovae (SN??), shown to be the dominant sources of the first heavy elements. These metals enrich the surrounding gas up to ?10-4 Z?, when a transition to efficient cooling-driven fragmentation producing 1 M? clumps occurs. We argue that the remaining fraction of the first stars ends up in ?100 M? VMBHs (very massive black holes). If we further assume that all these VMBHs are likely to end up in the centers of galactic nuclei constituting the observed supermassive black holes (SMBHs), then ?6% of the first stars contributed to the initial metal enrichment and the IMF remained top-heavy down to a redshift z ? 18.5%. Interestingly, this is the epoch at which the cool metals detected in the Ly? forest at z ? 3 must have been ejected from galaxies. At the other extreme, if none of these VMBHs has as yet ended up in SMBHs, we expect them to be either (1) en route toward galactic nuclei, thereby accounting for the X-ray-bright off-center sources detected locally by ROSAT, or (2) the dark matter candidate composing the entire baryonic halos of galaxies. For case 1 we expect all but a negligible fraction of the primordial stars to produce metals, causing the transition at the maximum possible redshift of 22.1, and for case 2, ~3 ? 105, a very negligible fraction of the initial stars produce the metals and the transition redshift occurs at zf 5.4. In this paper, we present a framework (albeit one that is not stringently constrained at present) that relates the first episode of star formation to the fate of their remnants at late times. Clearly, further progress in understanding the formation and fragmentation of Population III stars within the cosmological context will provide tighter constraints in the future. We conclude with a discussion of several hitherto unexplored implications of a high-mass-dominated star formation mode in the early universe.

Journal ArticleDOI
TL;DR: In this article, the authors used a model consistent with the observed FIR/radio correlation to estimate the corresponding star formation rate density within the past τ ~ 3 × 108 yr; it is ρSF(M > 0.1 M ⊙) ≈ 0.018 M⊙ yr-1 Mpc-3.
Abstract: Galaxies from the entire Uppsala Galaxy Catalog (UGC) have been identified with 4583 radio sources stronger than 2.5 mJy at 1.4 GHz from the NRAO VLA Sky Survey (NVSS). The complete sample of 3398 galaxies brighter than mp = 14.5 in the area defined by δ > -2°30' and |b| > 20° yielded the UGC/NVSS sample of 1966 radio sources. Their dominant energy sources were classified as stars (85%) or active galactic nuclei (15%). The luminosity function of star-forming galaxies agrees well with the far-infrared (FIR) luminosity function converted to 1.4 GHz by the FIR/radio correlation. The spectral power density of star-forming galaxies is USF = (1.53 ± 0.07) × 1019 W Hz-1 Mpc-3 (statistical errors only) if H0 = 70 km s-1 Mpc-1. We used a model consistent with the observed FIR/radio correlation to estimate the corresponding star formation rate density within the past τ ~ 3 × 108 yr; it is ρSF(M > 0.1 M⊙) ≈ 0.018 M⊙ yr-1 Mpc-3. The radio sources in star-forming galaxies may be evolving even at moderately low redshifts (z ~ 0.1).

Journal ArticleDOI
07 Mar 2002-Nature
TL;DR: It is shown that t*f is determined by the conditions in the star's natal cloud, and is typically ∼105 yr, which is sufficient to overcome radiation pressure from ∼100M[circdot] protostars, while simultaneously driving intense bipolar gas outflows.
Abstract: Massive stars (with mass m* > 8 solar masses M⊙) are fundamental to the evolution of galaxies, because they produce heavy elements, inject energy into the interstellar medium, and possibly regulate the star formation rate. The individual star formation time, t*f, determines the accretion rate of the star; the value of the former quantity is currently uncertain by many orders of magnitude1,2,3,4,5,6, leading to other astrophysical questions. For example, the variation of t*f with stellar mass dictates whether massive stars can form simultaneously with low-mass stars in clusters. Here we show that t*f is determined by the conditions in the star's natal cloud, and is typically ∼105 yr. The corresponding mass accretion rate depends on the pressure within the cloud—which we relate to the gas surface density—and on both the instantaneous and final stellar masses. Characteristic accretion rates are sufficient to overcome radiation pressure from ∼100M⊙ protostars, while simultaneously driving intense bipolar gas outflows. The weak dependence of t*f on the final mass of the star allows high- and low-mass star formation to occur nearly simultaneously in clusters.

Journal ArticleDOI
TL;DR: In this article, a new grid of ionizing fluxes for O and Wolf-Rayet (W•R) stars is presented for use with evolutionary synthesis codes and single-star H II region analyses.
Abstract: We present a new grid of ionizing fluxes for O and Wolf‐Rayet (W‐R) stars for use with evolutionary synthesis codes and single-star H II region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 Z� ) using the WM-BASIC code of Pauldrach, Hoffmann & Lennon for O stars and the CMFGEN code of Hillier & Miller for W‐R stars. The stellar wind parameters are scaled with metallicity for both O and W‐R stars. We compare the ionizing fluxes of the new models with the CoStar models of Schaerer & de Koter and the pure helium W‐R models of Schmutz, Leitherer & Gruenwald. We find significant differences, particularly above 54 eV, where the emergent flux is determined by the wind density as a function of metallicity. The new models have lower ionizing fluxes in the He I continuum with important implications for nebular line ratios. We incorporate the new models into the evolutionary synthesis code STARBURST99 and compare the ionizing outputs for an instantaneous burst and continuous star formation with the work of Schaerer & Vacca (SV98), and Leitherer et al. The changes in the output ionizing fluxes as a function of age are dramatic. We find that, in contrast to previous studies, nebular He II λ4686 will be at, or just below, the detection limit in low metallicity starbursts during

Journal ArticleDOI
TL;DR: In this article, the authors discuss the evolution to the ultra-compact (UC) state, the properties of UC HII regions and their environments, and their use as probes of Galactic structure.
Abstract: ▪ Abstract This review discusses three main topics: evolution to the ultra-compact (UC) state; the properties of UC HII regions and their environments; and UC HII regions as probes of Galactic structure. The evolution to UC HII regions begins in giant molecular clouds that provide the natal material for prestellar cores that evolve into hot cores, the precursors of UC HII regions. The properties of each evolutionary phase are reviewed, with particular emphasis on those of hot cores. The observed properties of UC HII regions and their environments are summarized with emphasis on the physical processes that may produce the observed properties. The final section summarizes the use of UC HII regions as probes of Galactic structure: in particular, the Galactic population and distribution of newly formed massive stars, the location of spiral arms, and the average galactocentric temperature and abundance gradients.

Journal ArticleDOI
TL;DR: In this article, the authors studied the log(N)-log(S ) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, LX 10 34 10 35 erg s 1, Xray binaries in the Milky Way.
Abstract: We study the Log(N)-Log(S ) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, LX 10 34 10 35 erg s 1 , X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant dierences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge whereas HMXBs tend to avoid the inner 3 4 kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of150 and 410 pc for HMXB and LMXB correspondingly) and show clear signatures of the spiral structure in their spatial distribution. The Log(N)-Log(S ) distributions and the X-ray luminosity functions are also noticeably dierent. LMXB sources have a flatter Log(N)-Log(S ) distribution and luminosity function. The integrated 2-10 keV luminosities of all X-ray binaries in the Galaxy, averaged over 1996-2000, are2 3 10 39 (LMXB) and2 3 10 38 (HMXB) erg s 1 . Normalised to the stellar mass and the star formation rate, respectively, these correspond to5 10 28 erg s 1 M 1 for LMXBs and5 10 37 erg s 1 /(M yr 1 ) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4 M neutron star, at least 12 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.