Cluster Luminosity Function Research Articles

The galaxy luminosity function in clusters and the field

We present the results from a CCD survey of the B-band luminosity function of nine clusters of galaxies, and compare them to published photographic luminosity functions of nearby poor clusters like Virgo and Fornax, and also to the field luminosity function. We derive a composite luminosity function by taking the weighted mean of all the individual cluster luminosity functions; this composite luminosity function is steep at bright and faint magnitudes and is shallow in-between. All clusters have luminosity functions consistent with this single composite function. This is true both for rich clusters like Coma and for poor clusters like Virgo. This same composite function is also individually consistent with the deep field luminosity functions found by Cowie et al. and Ellis et al., and also with the faint end of the Las Campanas Redshift Survey R-band luminosity function, shifted by 1.5 mag. A comparison with the Loveday et al. field luminosity function, which is well determined at the bright end, shows that the composite function, which fits the field data well fainter than MB=−19, drops too steeply between MB=−19 and −22 to fit the field data there.

Schmidt plate survey in the Galactic centre and anticentre direction. Luminosity and mass functions of open clusters

AbstractThe proper motions and stellar magnitudes for more than 20 000 stars distributed in areas around eight open clusters in the directions of the Sagittarius‐Carina and Perseus spiral arms were used to determine the cluster membership. Both spatial‐kinematical and statistical criteria were used for member seletion. The luminosity functions of cluster members were constructed and used for the further data improvement. The safest portions of the luminosity function free both from evolved stars and faint field star contamination were used for mass function construction. The composite mass function was derived from the data on five young clusters with log t < 8.2. A dependence of the mass function slope on the adopted input parameters (mass‐luminosity relation and distance scale) and on member selection procedure was tested. The slope was found to vary in a range from 1.36 to 1.80 for lg M/M⊙ between 0.3 and 1.2. A possible relation between the individual MF slopes and cluster ages was indicated. The results are in good agreement with published data.

Recent results from the SHARC Survey

AbstractWe review recent results from the SHARC Survey for distant X‐ray clusters of galaxies. We first discuss the SHARC South project which finds no evidence for evolution in the faint end of the X‐ray cluster luminosity function out to a redshift of z ≲ 0.7. We then give an overview of the Bright SHARC project; this project will provide an independent measurement of the evolution of bright (Lx[0.5 ‐ 2keV] > 3 × 1044 erg s−1) X‐ray clusters and allow us to put strong constraints on the value of ω0.

The Warps X-Ray Survey of Galaxies, Groups, and Clusters

We have embarked on a survey of ROSAT PSPC archival data searching for all detected surface brightness enhancements due to sources in the innermost R ≤ 15′ of the PSPC field of view in the energy band 0.5–2.0 keV. This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. Accurate measurements of the high redshift XLF would allow the form of the XLF evolution to be determined via the position of the Schechter function break. This would help discriminate between luminosity and density evolution, and discriminate between different hierarchical models, e.g., those including a different mix of fundamental particles, a flat power spectrum of the initial fluctuations, and reheating of the intracluster gas at high redshifts.

The Luminosity Function of the Globular Cluster NGC 6397 near the Limit of Hydrogen Burning

Second-epoch Hubble Space Telescope observations of NGC 6397 have led to the measurement of proper motions accurate enough to separate the faintest cluster stars from the field, thus extending the luminosity function of this globular cluster far enough to approach the limit of hydrogen burning on the main sequence. We isolate a sample of 1385 main-sequence stars, from just below the turnoff down to I = 24.5 (MI 12.5), which corresponds to a mass of less than 0.1 m☉ for the metallicity of this cluster. Below I = 22 (MI 10), the main-sequence luminosity function drops rapidly, in a manner similar to that predicted by theoretical models of low-mass stars near the hydrogen-burning limit.

The evolution of the luminosity function of young open clusters

The luminosity function of a star cluster evolves markedly during the pre-main sequence phase. With an assumed initial mass function and pre-main sequence tracks, we calculate a set of monochromatic luminosity functions which, when compared with observations, can be used to infer the age and star formation history (coeval versus intermittent) of a star cluster. Applied to the Trapezium cluster, our model suggests an age close to a million years, whereas in IC 348 the age estimate yields 4–6 million years and continual bursts of star formation seem to have occurred in this cluster. CCD imaging observations in the I-band are presented for NGC 663, for which an age of 10–30 million years is inferred. The initial mass function for NGC 663 in the range 2–7.1 M ⊚ has a slope of −0.77 ± 0.20, much shallower than that for the solar neighborhood field stars. We interpret this being due to mass segregation in the cluster. We interpret this as being due to mass segregation in the cluster.

The Role of Cooling Flows in the Star Formation History of Central Cluster Galaxies

Using a recent determination of the X-ray luminosity function of galaxy clusters, accurate mass deposition rates and LX in cooling flow clusters from the literature, and reliable constraints on the star fomation episodes in cooling flows obtained by our group, we quantify the relevance of such star formation in comparison with the SFR in the local universe. We conclude that the SFR density in CF is ≲ 0.1% the total local reference.

Contribution of White Dwarfs to Cluster Masses

The study of cluster white dwarfs (WDs) has been invigorated recently bythe Hubble Space Telescope (HST). Recent WD studies have been motivated by the new and independent cluster distance (Renzini et al. 1996), age (von Hippel et al. 1995; Richer et al. 1997), and stellar evolution (Koester & Reimers 1996) information that cluster WDs can provide. An important byproduct of these studies has been an estimate of the WD mass contribution in open and globular clusters. The cluster WD mass fraction is of importance for understanding the dynamical state and history of star clusters. It also bears an important connection to the WD mass fractions of the Galactic disk and halo. Current evidence indicates that the open clusters (e.g. von Hippel et al. 1996; Reid this volume) have essentially the same luminosity function (LF) as the solar neighborhood population. The case for the halo is less clear, despite the number of very good globular cluster LFs down to nearly 0.1 solar masses (e.g. Cool et al. 1996; Piotto, this volume), as the field halo LF is poorly known. For most clusters dynamical evolution should cause evaporation of the lowest mass members, biasing clusters to have flatter present-day mass functions (PDMFs) than the disk and halo field populations. Dynamical evolution should also allow cluster WDs to escape, though not in the same numbers as the much lower mass main sequence stars. The detailed connection between cluster PDMFs and the field IMF awaits elucidation from observations and the new combined N-body and stellar evolution models (Tout, this volume). Nevertheless, the WD mass fraction of clusters already provides an estimate for the WD mass fraction of the disk and halo field populations. A literature search to collect cluster WDs and a simple interpretive model follow. This is a work in progress and the full details of the literature search and the model will be published elsewhere.

The Star Cluster System of the Merger Remnant NGC 7252

view Abstract Citations (208) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Star Cluster System of the Merger Remnant NGC 7252 Miller, Bryan W. ; Whitmore, Bradley C. ; Schweizer, Francois ; Fall, S. Michael Abstract We present new U, B, V, and I images of the prototypical merger remnant NGC 7252 obtained with the WFPC2 instrument of the Hubble Space Telescope. The photometry reaches about 3 mag deeper than the previous observations with WF/PC1 and we detect 499 cluster candidates, most of them previously undiscovered. We can distinguish three populations of star clusters. We confirm the existence of a very luminous, blue population of clusters with a narrow range in color. Comparisons with Bruzual & Charlot (1996, in preparation) models incorporating a Salpeter stellar IMF show that the mean ages of these clusters are 650 Myr for [Fe/H]= 0.0 and 750 Myr if [Fe/H]= -0.3. Therefore, these are the clusters that formed during the merger event. The upper limit to the effective radii of these objects is 4.8±0.4 pc (for D=64.4 Mpc, H0=75), suggesting that they have physical properties like Galactic globular clusters. The second population of clusters is associated with the inner disk. Their (U-B) colors and reddening-free Q values indicate that their light is dominated by 0 stars and that they have ages less than 10 Myr. However, with (Reff)=83±0.6 pc, these objects may be more like stellar associations and they may not survive for a significant time. Finally, there is also evidence for the presence of the most luminous of the old, metal-poor globular clusters that belonged to the progenitor galaxies. The cluster luminosity function is a single power law with slope α≍-1.8 down to a limiting magnitude of V=26. The current specific cluster frequency is SN=0.6±0.3 for old clusters and young clusters with M>105 Msun,but after the remnant has faded for about 15 Gyr SN will have increased to about 2.5. The central disk is now resolved into much finer detail than in earlier WF/PC1 observations and the light profile of the nuclear region is consistent with a single power law with slope y=-1.26±0.03. After both the main body and the cluster system of NGC 7252 have aged for several Gyr it may have the properties of a field elliptical. Publication: The Astronomical Journal Pub Date: December 1997 DOI: 10.1086/118655 Bibcode: 1997AJ....114.2381M Keywords: GALAXIES: STAR CLUSTERS; GALAXIES: SPIRAL; GALAXIES: FORMATION; GALAXIES: INTERACTIONS full text sources ADS | data products SIMBAD (92) NED (2) MAST (1) ESA (1) CDS (1)

The Butcher‐Oemler Effect in Abell 2317

This paper presents deep narrowband photometry of the cluster A2317 (z = 0.211) carried out using the KPNO 4 m and Steward 2.3 m telescopes. Using rest frame Stromgren photometry, it is determined that A2317 has an unusually high fraction of blue galaxies (the Butcher-Oemler effect) for its redshift (fB = 0.35). We demonstrate that the ratio of blue to red galaxies has a strong dependence on absolute magnitude such that blue galaxies dominate the top of the luminosity function. Spectrophotometric classification shows that a majority of the red galaxies are E/S0's, with a small number of reddened starburst galaxies. Butcher-Oemler galaxies are shown to be galaxies with star formation rates typical of late-type spirals and to be irregular. Starburst systems were typically found to be on the lower end of the cluster luminosity function. In addition, blue galaxies are preferentially found in the outer edges of the cluster, whereas the red galaxies are concentrated in the cluster core.

The Globular Cluster Luminosity Function as a Distance Indicator: Dynamical Effects

The dynamical evolution of the globular cluster systems in galaxies is predicted, based on the standard dynamical theory normalized to the example of the Milky Way. The major processes varying with the galactocentric distance are the tidal shocks and dynamical friction. Our simple model explains, on a quantitative basis, the observed differences of the inner and outer populations of globular clusters. We can thus calculate corrections for dynamical evolution for the luminosity function of globular clusters with the assumption that the initial luminosity function is identical in all galaxies (and we can test this assumption as well, in certain cases). Then we can compute the expected distribution of absolute magnitudes and compare it with the observed distribution of apparent magnitudes to estimate the distance moduli for M31 and M87. Using this new method we find dm(M31)=24.05 +- 0.23, dm(M87)=30.83 +- 0.17, as compared to current best estimates using other methods of dm(M31)=24.30 +- 0.20, dm(M87)=31.0 +- 0.1. As a check on the method we compute, and compare with observations, the differences between the inner and outer globular clusters in all three galaxies. This new method, coupled with HST observations, promises to provide an independent method of estimating distances to galaxies with recession velocities < 10,000 km/s, or D < 100 Mpc.

On the Stringent Constraint on Massive Dark Clusters in the Galactic Halo

This Letter revises the recent claims of a dynamical constraint on massive dark clusters from the Galactic globular cluster luminosity function (GCLF). We point out that this argument is invalid for two reasons. The first point is that with a proper sample of globular clusters, the GCLF presents a significant trend with Galactocentric position, and second, the disruption timescale of globular clusters by encounters with these massive objects depends strongly on many uncertain parameters. For the usual halo model of Cadwell & Ostriker, it may be smaller by only a factor of 2 for distant globular clusters. Therefore, the existence of massive dark clusters in the halo could be possible. This provides a scenario in which the formation of dark clusters and globular clusters may have a similar origin.

The Edinburgh-Durham Southern Galaxy Catalogue -- VIII. The cluster galaxy luminosity function

We have re-examined the nature of the cluster galaxy luminosity function using the data from the Edinburgh-Durham Southern Galaxy Catalogue and the Edinburgh-Milano Redshift Survey. We derive a best-fitting luminosity function (LF) over the range −18 to − 21 in M(bj), for a composite sample of 22 of the richer clusters that has M(bj)⋆= −20.16 ±0.02 and α= −1.22 ± 0.04. The dominant error in these values results from the choice of background subtraction method. From extensive simulations we can show that when the LF is fitted over this narrow range, it is difficult to discriminate against bright values of M ⋆ in the single cluster fits, but that faint values provide a strong test of the universality of the luminosity function. We find that all the individual cluster data are well-fitted by a Schechter function with α fixed at −1.25, and that ≤ 10 per cent of these have fitted values of M ⋆ that disagree from the average at the 99 per cent confidence level. We further show that fitting only a single parameter Schechter function to composite subsets of the data can give erroneous results for the derived M⋆, as might be expected from the known tight correlation between M⋆ and α. By considering two parameter fits, the results of Monte Carlo simulations and direct two-sample χ2 tests, we conclude that there is only weak evidence for differences between the data when broken down into subsets based on physical properties (Bautz-Morgan class, richness, velocity dispersion): from our simulations, only the evidence for a difference between subsets based on velocity dispersion may in fact be significant. However, we find no evidence at all that a Schechter function is not a good model for the intrinsic cluster luminosity function over this absolute magnitude range. Models that invoke strong evolution of galaxy luminosity of all galaxies within clusters are inconsistent with our results.

Near-infrared luminosity function and colours of dwarf galaxies in the Coma cluster

We present K-band observations of the low-luminosity galaxies in the Coma cluster, which are responsible for the steep upturn in the optical luminosity function at M_R ~ -16, discovered recently. The main results of this study are (i) The optical$-$near-infrared colours of these galaxies imply that they are dwarf spheroidals. The median M-K colour for galaxies with -19.3 < M_K < -16.3 is 3.6 mag. (ii) The K-band luminosity function in the Coma cluster at the faint-end is not wee constrained, because of the uncertainties due to the field-to-field variance of the background. However, within the estimate large errors, it is consistent with the R-band luminosity function, shifted by $\sim3$ magnitudes. (iii) Many of the cluster dwarfs lie in a region of the B-K vs. B-R colour-colour diagram where background galaxies are rare Local dwarf spheroidal galaxies lie in this region too. This suggests that a better measurement of the K-band cluster luminosity function can be made if the field-to-field variance of the background can be measured as a function of colour. (iv) If we assume that none of the galaxies in the region of the B-K vs. B-R plane given in (iii) in our cluster fields are background, and that all the cluster galaxies with $15.5 < K < 18.5$ lie in this region of the plane, then we measure alpha = -1.41 +/- 0.35 for -19.3 < M_K < -16.3, where alpha is the logarithmic slope of the luminosity function.

THE SEARCH FOR MAIN SEQUENCE BINARY STARS IN GALACTIC GLOBULAR CLUSTERS

ABSTRACT Binary stars should be present in relatively large numbers in globular clusters (GCs). While individual binary systems have been previously observed, until now, no large population had been detected in a dense GC. As part of this thesis, we developed a new technique to determine the fraction of main-sequence stars which are binaries. We analyzed the redward spread of the main-sequence ridge-line of the post-core-collapse GC NGC~6752. These data, obtained with post-refurbishment HST data, indicate that in the inner core-radius, the 3-σ lower and upper limits on the binary fraction are 15% and 38%, respectively. The region outside the core has a 3-σ upper limit on the binary fraction of 16%, and is consistent with no binaries. This rapid change in binary fraction is clear evidence of mass segregation operating within the inner 3 core-radii. We also find that the binary fraction increases at fainter magnitudes; mass segregation would also account for such behavior. We discuss the likely effects on the cluster's luminosity function (LF). In particular, we find that if the contribution of binaries to the LF was not removed, the apparent number of low mass stars would be over-estimated. I also examined the time-series data and discovered that one of the blue stragglers in NGC~6752 is an SX Phe pulsator. Its pulsation mass is 1-1.2 M⊙, consistent with the mass of the best fit stellar models from the Yale Isochrones, and also consistent with any of the blue straggler formation scenarios. No contact or eclipsing binaries were unambiguously detected. However, many low-amplitude variable candidates were observed at the limit of our ability to detect variability. The putative period of these candidate objects is consistent with these objects being produced via tidal capture events. In a time-series study of NGC~6397, we found a contact binary. This W~UMa system probably has a mass greater than that of main sequence turn-off stars implying that when this system coalesces due to angular momentum loss, it will become a blue straggler star. The frequency of W~UMas in this cluster is not statistically different from that of other GCs where ∼0.1% of the main-sequence stars are contact binarie.

The fine structure of luminosity functions of young open cluster stars

Abstract The luminosity function (LF) of young open cluster stars has been studied for decades to obtain a slope of the initial mass function (IMF), which is important for understanding of star formation. Young open clusters contain a lot of pre-main-sequence (PMS) stars the positions of which in the H—R diagram deviate significantly from the main sequence and consequently the PMS massluminosity relation (MLR) differs from the ZAMS one. Such features of the MLR and related details of young open cluster LFs they produce are the subject of this poster. Three peaks have been studied: (1) the main peak (H-peak)—generated by PMS stars settling on the main sequence; (2) the R-peak—generated by inversion of the radius on the stellar birthline; the D-peak—generated by changes in the MLR and associated with deuterium burning. The evolution of these features with cluster age and possible application for young open cluster study are discussed.

The effect of the Galactic spheroid on globular cluster evolution

We study the combined effects of relaxation, tidal heating and binary heating on globular cluster evolution, exploring the physical consequences of external effects and examining evolutionary trends in the Milky Way population. Our analysis demonstrates that heating on circular and low-eccentricity orbits can dominate cluster evolution. The results also predict rapid evolution on eccentric orbits either due to strong relaxation caused by the high densities needed for tidal limitation or due to efficient bulge shocking of low density clusters. The combination of effects leads to strong evolution of the population as a whole. For example, within the solar circle, tidally-limited 10^5 M_sun clusters lose at least 40% of their mass in 10 Gyr. At high eccentricity most of these clusters evaporate completely. Bulge shocking disrupts clusters within 40 kpc which have less than 80% of their mass within their pericentric inner Lagrange point. Our results are consistent with suggestions that the shape of the cluster luminosity function results from evaporation and disruption of low mass clusters; they further predict that the net velocity dispersion of the cluster system in the inner Galaxy has decreased with time. Preliminary constraints on formation models are also discussed. We conclude that the observed cluster system has largely been shaped by dynamical selection.

Evolution in the X‐Ray Cluster Luminosity Function Revisited

We present new X-ray data taken from the ROSAT Position Sensitive Proportional Counter pointing archive for 21 clusters in the Einstein Extended Medium-Sensitivity Survey (EMSS). We have supplemented these data with new optical follow-up observations found in the literature, and, overall, 32 of the original 67 z > 0.14 EMSS clusters now have new information. Using this revised sample, we find no systematic difference, as a function of X-ray flux, between our measured X-ray cluster fluxes and those in the original EMSS (30% scatter). However, we do detect a marginal correlation between this observed difference in the flux and the redshift of the clusters, with the lower redshift systems having a greater scatter by nearly a factor of 2. We have also determined the X-ray extent of these reobserved EMSS clusters and find that 14 of them have significant extents compared with the ROSAT point-spread function. Combining these data with extended clusters seen in the original EMSS sample, at least 40% of z > 0.14 clusters now have an observed X-ray extent, thus justifying their classification as X-ray clusters. Using our improved EMSS sample, we have redetermined the EMSS X-ray cluster luminosity function as a function of redshift. We have removed potential misclassifications and included our new measurements of the clusters' X-ray luminosities and redshifts. We find similar luminosity functions to those originally presented by Henry et al., albeit with two important differences. First, we show that the original low-redshift EMSS luminosity function is insufficiently constrained. Second, the power-law shape of our new determination of the high-redshift EMSS luminosity function (z = 0.3-0.6) has a shallower slope than that seen by Henry et al. We have compared our new EMSS luminosity functions with those recently derived from a nearby sample of X-ray clusters and find that the overall degree of observed luminosity function evolution is mild at best. This is a result of the shallower slope seen in our EMSS high-redshift luminosity function and a more robust low-redshift determination of the X-ray cluster luminosity function from the literature. We have quantified the degree of evolution seen in the X-ray cluster luminosity by using several statistical tests. The most restrictive analysis indicates that our low- and high-redshift EMSS luminosity functions are statistically different at the 95% level. However, other tests indicate that these low- and high-redshift luminosity functions only differ by as little as 1 σ. These data are, therefore, consistent with no evolution in the X-ray cluster luminosity function out to z 0.5.

Luminosity distributions within rich clusters — I. A ubiquitous dwarf-rich luminosity function?

From deep CCD observations of the cluster Abell 2554 we have recovered the cluster's luminosity distribution over a wide range of magnitude (-24 < M(R) < -16). We compare the derived A2554 cluster luminosity function (at redshift 0.1) with that of the local Coma Cluster (A1656) and the more distant (z = 0.2) cluster A963. The distribution is remarkably similar for these three clusters of comparable richness and morphology. All show a flat (\alpha = -1.0) luminosity function for the giant galaxies (-24 < M(R) < -19.5) which exhibits a sharp upturn (\alpha = -1.7) at some intermediate magnitude (M(R) = -19) and continues to rise to the limits of existing data. We suggest that such a luminosity function may be ubiquitous among rich clusters and that a similar form may apply for poorer clusters and possibly the field as well. The three cluster dwarf LFs are seen over a range of lookback times covering a quarter of the age of the universe. Therefore the similarity between the three measured LFs seems to rule out strong evolution of the dwarf populations in rich cluster environments, at least out to z = 0.2, unless richness effects conspire to conceal evolutionary changes.

A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas

A universal mechanism for cluster formation in all epochs and environments is found to be consistent with the properties and locations of young and old globular clusters, open clusters and unbound associations, and interstellar clouds. The primary structural differences between various cluster types result from differences in pressure at the time of formation, combined with different ages for subsequent evolution. All clusters begin with a mass distribution similar to that for interstellar clouds, which is approximately n(M)dM M-2 dM. Old halo globulars have a current mass distribution that falls off at low mass because of a Hubble time of cluster destruction. Young globulars have not yet had time for a similar loss, and some old open clusters have survived because of their low densities. The peak in the halo cluster luminosity function depends only on age, and is independent of the host galaxy luminosity, as observed. The peak globular cluster mass is not a characteristic or Jeans mass in the primordial galaxy, as previously suggested. The initial mass distribution functions for young and old globular clusters, open clusters and associations, and interstellar clouds are all power laws with a slope of ~ -2. This distribution could be the result of fractal structure in turbulent gas. New data on clusters in the LMC also follow this power law. The slope is so steep that it implies a significant fraction of star formation occurs in small clusters. Numerous halo field stars should come from the evaporation of small halo clusters, and a high fraction of disk field stars should arise in small unbound disk clusters. This differs significantly from previous suggestions that most disk stars form in large OB associations. Globular clusters of all ages preferentially form in high-pressure regions. This is directly evident today in the form of large kinematic pressures from the densities and relative velocities of member stars. High pressures at the time of globular cluster formation are either the result of a high background virial density in that part of the galaxy (as in dwarf galaxies or galactic nuclei and nuclear rings), turbulence compression (in halo globulars), or large-scale shocks (in interacting galaxies). Massive clusters that form in such high-pressure environments are more likely to be bound than low-mass clusters or clusters of equal mass in low-pressure regions. This is because virialized clouds are more tightly bound at high pressure. A simple model illustrates this effect. One implication of this result is that starburst regions preferentially make globular clusters, in which case some elliptical galaxies could have formed by the violent merger of spiral galaxies.