Cluster Luminosity Function Research Articles

The Hα Luminosity Functions of Nearby Clusters of Galaxies

We present a deep wide field Hα imaging survey of the central regions of the two nearby clusters of galaxies Coma and Abell 1367, taken with the WFC at the INT 2.5m telescope. We determine for the first time the Schechter parameters of the Hα luminosity function (LF) of cluster galaxies. The Hα LFs of Abell 1367 and Coma are compared with each other. Typical parameters of φ* ≈ 100.00±0.07 Mpc−3, L* ≈ 1041.25±0.05 erg sec−1 and α ≈ −0.70 ± 0.10 are found for both clusters.

Dynamical Evolution of Rich Star Clusters in the LMC

We combine the results of a large HST study of rich LMC clusters with state of the art N-body simulations. We summarise recent highlights from our project: the estimation of age spreads in young LMC clusters, the measurement of the binary fraction of NGC1818, and the determination of deep cluster luminosity functions (LF). We also present preliminary results from an investigation of the core radius versus age relation evident among LMC clusters.

The Formation and Evolution of Young Star Clusters in the Antennae

Five populations of young, massive, compact star clusters have been identified in the “Antennae galaxies”, the nearest and youngest example of a prototypical merging galaxy. The brightest of these clusters have all the attributes expected of young globular clusters, hence allowing us to study the formation and evolution of globular clusters in the local universe. Comparisons between the different populations and a variety of multi-wavelength observations are providing new insights into the formation of the clusters. For example, the very red clusters originally identified by Whitmore and Schweizer (1995) appear to be the youngest population, just emerging from their dust cocoon. The cluster luminosity functions for a wide variety of galaxies (i.e., mergers, starbursts, barred galaxies, spirals) appear to follow a “universal” power law, with index ≈ −2. The primary difference between the different galaxies is the normalization, with roughly a tenfold increase in the number of clusters in merging and starbursting galaxies. Hence, the fact that the brightest clusters are in mergers may be largely a statistical result. Simulations are now showing how the initial power law distribution for the clusters will evolve toward the peaked distribution found for old globular clusters, via a combination of processes including two-body evaporation, disk shocking, and stellar mass loss.

New Insights from a Study of the Globular Cluster Systems of 60 Early-Type Galaxies

We present the results of our detailed WFPC2-based photometric study of the globular cluster systems (GCS) of over 60 elliptical and SO galaxies. Approximately 50% of the GCSs of ellipticals, and at least 15–20% of S0s reveal bimodality in the color distribution. We trace the variation in GCS properties with Hubble type and discuss the implications on the various models of galaxy (and cluster system) formation. We also provide evidence that the globular cluster luminosity function is an excellent distance indicator with an accuracy comparable to the surface brightness fluctuation method.

Globular Clusters in Dwarf Elliptical Galaxies

We present preliminary results on the shape of the globular cluster luminosity function and the colors and inferred metallicities of the clusters in dwarf elliptical galaxies imaged with HST. The luminosity function (LF) of the GC candidates is consistent with a Gaussian-shaped LF similar to that in giant ellipticals. Also, with a mean color of (V - I) = 0.94, most of the GCs appear to be old and metal-poor ([Fe/H] = −1.4) like GCs in the Galaxy and in nearby giant ellipticals. This suggests that the bulk of the clusters were formed more than 10 Gyr ago.

The Globular Cluster System of NGC 1316

We have studied the Globular Cluster System of the merger galaxy NGC 1316 in Fornax, using CCD BV I photometry. Dividing the sample into red (presumably metal-rich) and blue (metal-poor) subpopulations at B - I = 1.75, we find that they follow strikingly different angular distributions. The red clusters show a strong correlation with the galaxy elongation, but the blue ones are circularly distributed. An astonishingly low specific frequency for NGC 1316 of only SN = 0.9±0.2 is derived, which confirms with a larger field a previous finding by Grillmair et al. (1999). Assuming a “normal” SN of ∼ 4 for early-type galaxies, we use stellar population synthesis models to estimate the merger age to about 2 Gyr, if an intermediate-age population were to explain the low SN we observe. By fitting t5 functions to the Globular Cluster Luminosity Function (GCLF), we derive the following turnover magnitudes: and . They support that NGC 1316, in spite of its outlying location, is at the same distance as the core of the Fornax cluster.

Deep Wide-Field Photometry of the NGC 5128 Globular Cluster System: A Progress Report

We have obtained CMR photometry for a roughly 1° square region centered on NGC 5128. Preliminary results indicate that the limiting magnitude of the images is ≳ 1 magnitude fainter than the peak of the globular cluster luminosity function (GCLF) at R ⋍ 21.

Mass segregation in young compact star clusters in the Large Magellanic Cloud - I. Data and luminosity functions

We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the ESO New Technology Telescope (NTT) of two young (∼10–25 Myr) compact star clusters in the LMC, NGC 1805 and 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terms of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and 1818 the LF slopes reach a stable level well beyond the core of the clusters or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner ∼4Rhl. The global cluster LF, although strongly non-linear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most probably massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as ∼25 Myr.

The star cluster system of the 3-Gyr-old merger remnant NGC 1316: clues from optical and near-infrared photometry

The giant merger remnant galaxy NGC 1316 (Fornax A) is an ideal probe for studying the long-term effects of a past major merger on star cluster systems, given its spectroscopically derived merger age of � 3 Gyr which we reported in a recent paper (Goudfrooij et al. 2001). Here we present new ground-based, large-area optical and near-IR imaging of star clusters in NGC 1316. The ground-based photometry is complemented with deep Hubble Space Telescope WFPC2 imaging, constituting an excellent combination for studying globular cluster systems. We find that the optical-near-IR colours and lumino sities of the brightest � 10 clusters in NGC 1316 are consistent with those of intermediate-age (2 ‐ 3 Gyr) populations. In particular, the near-IR data preclude ages 4 Gyr for those clusters. Unlike ‘normal’ giant ellipticals, the B I colour distribution of clusters in NGC 1316 is not clearly bimodal. However, the luminosity functions (LFs) of the blue and red parts of the cluster colour distribution are different: The red cluster LF is well represented by a power law, �(L)dL / L 1.2±0.3 dL, extending to about 1.5 mag brighter (in B) than those of typical giant ellipticals. In contrast, the shape of the blue cluster LF is consistent with that of ‘normal’ spiral and elliptical galaxies. We conclude that t he star cluster system of NGC 1316 is a combination of a population of age � 3 Gyr having roughly solar metallicity and a population of old, metal-poor clusters which probably belonged to the pre-merger galaxies. After the 3 Gyr old, metal-rich clusters fade to an age of 10 Gyr, they will form a red ‘peak’ in a bimodal cluster colour distribution. This ‘red peak’ will have a colour consistent with that found in ‘normal, old’ giant ellipticals of the same galaxy l uminosity (taking age dimming into account). The surface density profile of clusters in the innermost regions is lower than that of the integrated light of the galaxy, presumably due to the collective effect of extended star formation in the inner regions of NGC 1316 and tidal shocking of the inner clusters. Outside the core, the surface density profile of clusters is cons istent with that of the underlying starlight, suggesting that the cluster system originally e xperienced the same violent relaxation as did the main body of the merger remnant. The specific cluste r frequency is presently SN = 1.7 ± 0.4 down to the 50 per cent completeness limit of the WFPC2 photometry, and will increase to SN > 2.0 as the merger-induced stellar (and star cluster) popula tion fades to an age of � 10 Gyr (barring further merger events), consistent with specific frequencies of typical giant ellipticals in the field and in poor groups. These f eatures of the star cluster system of NGC 1316 are fully consistent with scenarios for forming ‘normal’ giant elliptical galaxies through gas-rich mergers at look-back times > 10 Gyr.

Photometry and membership for low mass stars in the young open cluster NGC 2516

We present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades. Our survey of cluster members is complete to and is used to select a preliminary catalogue of 1254 low mass () cluster candidates, of which about 70-80 percent are expected to be genuine. After applying corrections for contamination by non-members and adding data for higher mass stars from the literature, we investigate the cluster binarity, luminosity and mass function, mass segregation and total mass. We find a binary fraction of percent, for A to M-type systems with mass ratios between 0.6 and 1, which is very similar to the Pleiades. The mass function is metallicity and evolutionary-model dependent, but consistent with a Salpeter-like law (, or for the solar and half-solar metallicity models of Siess et al. [CITE], and for the solar metallicity models of D'Antona & Mazzitelli [CITE]), for . At lower masses () there is a sharp fall in the mass function, with or (for the solar and half-solar metallicity models of Siess et al.), and (for the solar metallicity models of D'Antona & Mazzitelli). The true stellar mass function might have α values up to 0.4 larger if account were taken of low mass stars in unresolved binary systems with mass ratios less than 0.6. The falling mass function of NGC 2516 at lower masses seems inconsistent with the much flatter mass functions derived from comparable data in the Pleiades and field populations. This deficit of lower mass, fainter stars is also seen in the observed luminosity function. We rule out incompleteness as the cause of this discrepancy, but demonstrate that mass segregation is clearly present in NGC 2516, with more than half the low-mass ( ) stars included. Taking this into account, it is probable that the whole-cluster mass functions for NGC 2516 and the Pleiades are similar down to 0.3. The mass of NGC 2516 stars with inside our survey is , depending on metallicity and what corrections are applied for unresolved binarity. Correcting for mass segregation increases this to ~, about twice the total mass of the Pleiades. If NGC 2516 and the Pleiades do have similar mass functions, then less massive stars and brown dwarfs contribute about a further 15 percent to the mass of NGC 2516 and we predict a cluster population of about 360-440 brown dwarfs with .

ITAL]H[/ITAL]-Band and Spectroscopic Properties of A1644

We discuss H-band (1.65 μm) near-infrared photometry of the central 9 h-2 Mpc2 of Abell 1644 (A1644) to a limiting MH ~ M*H + 3 (throughout this paper H0 = 100 h km s-1 Mpc-1). There are 861 galaxies in the photometric survey region. We also measured radial velocities of 155 galaxies; 141 of these are cluster members within 2.44 h-1 Mpc of the cluster center. The completeness limit of the spectroscopic survey is H ~ 13 (MH ~ M). The cluster velocity dispersion of σ ~ 1000 km s-1 remains constant out to the limiting radius. We find no evidence for substructure in the cluster. The cluster mass within R = 2.4 h-1 Mpc is 7.6 ± 1.3 × 1014 h -1 M⊙. We compute the cluster luminosity function; the Schechter parameters α = -1.14 ± 0.08 and M = -24.3 ± 0.2 (with h = 0.5) agree well with other H-band luminosity functions. From the virial theorem and the caustic method we compute one of the first mass-to-light ratios at H; the result is M/LH = 82–127 h M⊙/L⊙ within 1.5 h-1 Mpc. This ratio corresponds to 374–579 h M⊙/L⊙ at R. The agreement of our IR measurement with previous M/L determinations indicates that at low redshift dust and young stellar populations may produce only negligible systematic errors in optical mass-to-light ratios.

New Insights from [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Studies of Globular Cluster Systems. II. Analysis of 29 S0 Systems

We examine the globular cluster systems (GCSs) of a sample of 34 S0 galaxies from a WFPC2 survey in the V and I bands. Of these 34 galaxies, 29 have measurable globular cluster systems. The mean color of the GCSs of individual galaxies vary from V-I = 0.85 mag to V-I = 1.17 mag. The average color of GCSs in all 29 S0 galaxies, V-I = 1.00 ± 0.07 mag, is similar to the value of V-I = 1.04 ± 0.04 derived for ellipticals in a companion paper. The mean metallicity of a cluster system is correlated to the luminosity (or mass) of the host galaxy, but it is not dependent on the Hubble type. Our measurements of the local specific frequency, on the other hand, confirm that the cluster formation efficiency is a function of Hubble type. The mean local specific frequency of our sample within the WFPC2 field of view is 1.0 ± 0.6, lower than SN(Local) = 2.4 ± 1.8 derived for ellipticals in a similar analysis. Although we are able to confirm a bimodal color distribution in only one galaxy from the shallow snapshot images analyzed in this paper, statistical tests suggest that 10%–20% of S0s are bimodal at the present level of photometric accuracy. There are no significant trends in GCS properties with membership or location of the S0 host in a galaxy cluster. We have measured the turnover luminosity of the globular cluster luminosity function (GCLF) for the richest few GCSs and find the GCLF distances to be in agreement with other estimates. The globular clusters in S0 galaxies have average half-light radii of ≈2.6 pc, which is similar to that of other galaxies, including our own.

The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions

We investigate the dependence of the galaxy infrared luminosity function (LF) and the associated stellar mass function (SMF) on environment and spectral type using photometry from the Two Micron All Sky Survey and redshifts from the Las Campanas Redshift Survey for galaxies brighter than MJ < -19 + 5 log h. In the field environment, the LFs of galaxies with emission lines have much steeper faint-end slopes (?J = -1.39) than those of galaxies without emission lines (?J = -0.59). In the cluster environment, however, even the non-emission-line galaxies have a steep faint-end LF (?J = -1.22). There is also a significant (95%) difference between the overall cluster and field LFs, ??J = -0.34 and ?M*J = -0.54. All these variations are more pronounced in the SMFs, which we compute by relating the strength of the 4000 ? break in the optical spectra to a stellar mass-to-light ratio.

The nature of the dwarf population in Abell 868

We present the results of a study of the morphology of the dwarf galaxy population in Abell 868, a rich, intermediate-redshift cluster which has a galaxy luminosity function (LF) with a steep faint-end slope . A statistical background subtraction method is employed to study the colour distribution of the cluster galaxies. This distribution suggests that the galaxies contributing to the faint-end of the measured cluster LF can be split into three populations: dwarf irregular galaxies (dIrrs) with dwarf elliptical galaxies (dEs) with and contaminating background giant ellipticals (gEs) with . The removal of the contribution of the background gEs from the counts only marginally lessens the faint-end slope . However, the removal of the contribution of the dIrrs from the counts produces a flat LF . The dEs and the dIrrs have similar spatial distributions within the cluster, except that the dIrrs appear to be totally absent within a central projected radius of about 0.2 Mpc . The number densities of both dEs and dIrrs appear to fall off beyond a projected radius of ≃ 0.35 Mpc. We suggest that the dE and dIrr populations of A868 have been associated with the cluster for similar time-scales, but evolutionary processes such as ‘galaxy harassment’ tend to fade the dIrr galaxies while having a much smaller effect on the dE galaxies. The harassment would be expected to have the greatest effect on dwarfs residing in the central parts of the cluster.

The globular cluster system of NGC 1316 (Fornax A)

We have studied the Globular Cluster System of the merger galaxy NGC 1316 in Fornax, using CCD BVI photometry. A clear bimodality is not detected from the broadband colours. However, dividing the sample into red (presumably metal- rich) and blue (metal-poor) subpopulations at B-I=1.75, we find that they follow strikingly different angular distributions. The red clusters show a strong correlation with the galaxy elongation, but the blue ones are circularly distributed. No systematic difference is seen in their radial profile and both are equally concentrated. We derive an astonishingly low Specific Frequency for NGC 1316 of only Sn=0.9, which confirms with a larger field a previous finding by Grillmair et al. (1999). Assuming a normal Sn of ~4 for early-type galaxies, we use stellar population synthesis models to estimate in 2 Gyr the age of this galaxy, if an intermediate-age population were to explain the low Sn we observe. This value agrees with the luminosity-weighted mean age of NGC 1316 derived by Kuntschner & Davies (1998) and Mackie & Fabbiano (1998). By fitting t5 functions to the Globular Cluster Luminosity Function (GCLF), we derived the following turnover magnitudes: B=24.69 +/- 0.15, V=23.87 +/- 0.20 and I=22.72 +/- 0.14. They confirm that NGC 1316, in spite of its outlying location, is at the same distance as the core of the Fornax cluster.

New Insights from [ITAL]HST[/ITAL] Studies of Globular Cluster Systems. I. Colors, Distances, and Specific Frequencies of 28 Elliptical Galaxies

We present an analysis of the globular cluster systems of 28 elliptical galaxies using archival WFPC2 images in the V and I-bands. The V-I color distributions of at least 50% of the galaxies appear to be bimodal {\it at the present level of photometric accuracy}. We argue that this is indicative of multiple epochs of cluster formation early in the history of these galaxies, possibly due to mergers. We also present the first evidence of bimodality in low luminosity galaxies and discuss its implication on formation scenarios. The mean color of the 28 cluster systems studied by us is V-I = 1.04$\pm$0.04 (0.01) mag corresponding to a mean metallicity of Fe/H = -1.0$\pm$0.19 (0.04). We find that the turnover magnitudes of the globular cluster luminosity functions (GCLF) of our sample are in excellent agreement with the distance measurements using other methods and conclude that the accuracy of the GCLF is at least as good as the surface brightness fluctuation method. The absolute magnitude of the turnover luminosity of the GCLF is M$_V^0$ = -7.41 (0.03) in V and M$_I^0$ = -8.46 (0.03) in I. The mean local specific frequency of our sample of elliptical galaxies within the WFPC2 field-of-view is 2.4$\pm$1.8 (0.4), considerably higher than the 1.0$\pm$0.6 (0.1) derived for a comparable sample of S0s in a similar analysis. It shows no obvious correlation with metallicity, host galaxy mass or membership in a galaxy cluster. The median size of clusters in all galaxies appears to be remarkably constant at $\sim$2.4 pc. We suggest that in the future it might be possible to use the sizes of clusters in the inner regions of galaxies as a simple geometrical distance indicator.

Effects of Formation Epoch Distribution on X‐Ray Luminosity and Temperature Functions of Galaxy Clusters

We investigate statistical properties of galaxy clusters in the context of a hierarchical clustering scenario, taking into account their formation epoch distribution; this study is motivated by the recent finding by Fujita and Takahara that X-ray clusters form a fundamental plane in which the mass and the formation epoch are regarded as two independent parameters. Using the formalism that discriminates between major mergers and accretion, the epoch of a cluster formation is identified with that of the last major merger. Since tiny mass accretion following formation does not much affect the core structure of clusters, the properties of X-ray emission from clusters are determined by the total mass and density at their formation time. Under these assumptions, we calculate X-ray luminosity and temperature functions of galaxy clusters. We find that the behavior of the luminosity function differs from the model that does not take into account formation epoch distribution; the behavior of the temperature function, however, is not much different. In our model, the luminosity function is shifted to a higher luminosity and shows no significant evolution up to z ~ 1, independent of cosmological models. The clusters are populated on the temperature-luminosity plane, with a finite dispersion. Since the simple scaling model in which the gas temperature is equal to the virial temperature fails to reproduce the observed luminosity-temperature relation, we also consider a model that takes into account the effects of preheating. The preheating model reproduces the observations much more accurately.

Stellar luminosity functions of rich star clusters in the Large Magellanic Cloud

We show the results of deep V and I HST photometry of 6 rich star clusters in the Large Magellanic Cloud with different ages and metallicities. The number of stars with measured magnitudes in each cluster varies from about 3000 to 10 000 stars. We build stellar density and surface brightness profiles for the clusters and extract half-light radii and other structural parameters for each. We also obtain luminosity functions, , down to , and investigate their dependence with distance from the cluster centre well beyond their half-light radius. In all clusters we find a systematic increase in the luminosity functions slope, , with radial distance from the centre. Among the clusters displaying significant mass segregation are the 2 youngest in the sample: NGC 1805, NGC 1818. For these two clusters we obtain present day mass functions. The NGC 1818 mass function is in excellent agreement with that derived by other authors, also using HST data. The young cluster mass function slopes differ, that of NGC 1805 being systematically steeper than NGC 1818. Since these are very young stellar systems ( Myrs), these variations may reflect the initial conditions rather than evolution due to internal dynamics.

Intermediate-band photometry of a rich cluster of galaxies: A2218

This paper presents four color intermediate-band photometry of the cluster A2218 carried out using the Wise Observatory 1 m telescope. A2218 is one of the richest clusters in the Abell catalogue, with richness class 4. We find that A2218 has an unusually low fraction of blue galaxies for its redshift (z=0.171), and is populated mainly by E/S0 galaxies, while starburst systems are very scarce. The few starbursting galaxies present populate the faint end of the cluster luminosity function, close to the limiting magnitude of our observations. The difference between A2218 and other clusters at similar redshifts is very remarkable, underlining the influence of cluster richness on the evolution of its member galaxies.

Deep VLT search for globular clusters in NGC 5128: Color-magnitude diagrams and globular cluster luminosity function

At the distance of NGC 5128 ( Mpc) it is possible to resolve globular clusters with high resolution imaging from the ground, thus allowing the globular cluster candidate selection primarily through their morphological properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT UT1+FORS1 images, including the faintest members () known to date in this galaxy as well as in 5 previously-known clusters. U- and V-band photometry has been measured for all the candidates and the luminosity function, spanning and , constructed. These are the deepest globular cluster luminosity functions in an elliptical galaxy determined so far. The Kolmogorov-Smirnov statistics show that the difference between the globular cluster luminosity functions of NGC 5128 and the MW is not larger than the difference between the ones of M 31 and the MW. The ()0 color histogram shows a bimodal distribution. For 23 globular clusters I obtained K-band images with SOFI at the NTT in La Silla. Their positions in the () vs. () color-color diagram indicate that they are indeed old globular clusters. Assuming that the globular clusters in NGC 5128 span a similar age range as the ones in the Milky Way and adopting a linear fit between the metallicity and ()0 color, the metal-rich clusters peak at dex and the metal-poor ones peak at dex.