Total Number Of Globular Clusters Research Articles

The Next Generation Virgo Cluster Survey (NGVS). XXVII. The Size and Structure of Globular Cluster Systems and Their Connection to Dark Matter Halos

We study the size and structure of globular cluster (GC) systems of 118 early-type galaxies from the NGVS, MATLAS, and ACSVCS surveys. Fitting Sérsic profiles, we investigate the relationship between effective radii of GC systems (R e,gc) and galaxy properties. GC systems are 2–4 times more extended than host galaxies across the entire stellar mass range of our sample (108.3 M ⊙ < M * < 1011.6 M ⊙). The relationship between R e,gc and galaxy stellar mass exhibits a characteristic “knee” at a stellar mass of M p ≃ 1010.8, similar to the galaxy R e –stellar mass relationship. We present a new characterization of the traditional blue and red GC color subpopulations, describing them with respect to host galaxy (g′−i′) color (Δgi): GCs with similar colors to their hosts have a “red” Δgi, and those significantly bluer GCs have a “blue” Δgi. The GC populations with red Δgi, even in dwarf galaxies, are twice as extended as the stars, suggesting that formation or survival mechanisms favor the outer regions. We find a tight correlation between R e,gc and the total number of GCs, with intrinsic scatter ≲0.1 dex spanning two and three orders of magnitude in size and number, respectively. This holds for both red and blue subpopulations, albeit with different slopes. Assuming that N GC,Total correlates with M 200, we find that the red GC systems have effective radii of roughly 1%–5% R 200, while the blue GC systems in massive galaxies can have sizes as large as ∼10% R 200. Environmental dependence on R e,gc is also found, with lower-density environments exhibiting more extended GC systems at fixed mass.

The Relation between Globular Cluster Systems and Supermassive Black Holes in Spiral Galaxies. III. The Link to the M •–M * Correlation

We continue to explore the relationship between the total number of globular clusters (GCs), N GC, and the central black hole mass, M •, in spiral galaxies. We present here results for the Sab galaxies NGC 3368, NGC 4736 (M94), and NGC 4826 (M64), and the Sm galaxy NGC 4395. The GC candidate selection is based on the (u* – ) versus ( – K s ) color–color diagram, and -band shape parameters. We determine the M • versus N GC correlation for these spirals, plus NGC 4258, NGC 253, M104, M81, M31, and the Milky Way. We also redetermine the correlation for the elliptical sample in Harris et al., with updated galaxy types from Sahu et al. Additionally, we derive the total stellar galaxy mass, M *, from its two-slope correlation with N GC, and fit M • versus M * for both spirals and ellipticals. We obtain log M • ∝ (1.01 ± 0.13) log N GC for ellipticals, and log M • ∝ (1.64 ± 0.24) log N GC for late-type galaxies (LTGs). The linear M • versus N GC correlation in ellipticals could be due to statistical convergence through mergers, unlike the much steeper correlation for LTGs. However, in the M • versus total stellar mass (M *) parameter space, with M * derived from its correlation with N GC, M • ∝ (1.48 ± 0.18) log M * for ellipticals, and M • ∝ (1.21 ± 0.16) log M * for LTGs. The observed agreement between ellipticals and LTGs in this parameter space may imply that black holes and galaxies coevolve through calm accretion, active galactic nuclei feedback, and other secular processes.

Tracing the Giant Outer Halo of the Mysterious Massive Disk Galaxy M104. I. Photometry of the Extended Globular Cluster Systems

M104 (NGC 4594, the Sombrero galaxy) is a mysterious massive early-type galaxy that shows a dominant bulge and a prominent disk. However, the presence of a halo in M104 has been elusive, and it is not yet known how M104 has acquired such a peculiar structure. Using wide (∼2 deg2) and deep ugi images of M104 obtained with the CFHT/MegaCam, we detect a large number of globular clusters (GCs) found out to (∼100 kpc). The color distribution of these GCs shows two subpopulations: a blue (metal-poor) system and a red (metal-rich) system. The total number of GCs is estimated to be N GC = 1610 ± 30 and the specific frequency to be S N = 1.8 ± 0.1. The radial number density profile of the GCs is steep in the inner region at and becomes shallow in the outer region at . The outer region is dominated by blue GCs and is extended out to . This shows clearly the existence of a giant metal-poor halo in M104. The inner region is composed of a bulge hosting a disk, corresponding to a metal-rich halo as seen in early-type galaxies. At least two clumps of blue GCs are found in the outer region. One clump is overlapped with a faint stellar stream located in the southwest, indicating that it may be a remnant of a disrupted dwarf galaxy. Our results imply that the metal-rich inner halo of M104 formed first via major mergers, and the metal-poor outer halo grew via numerous minor mergers.

Light from the Darkness: Detecting Ultra-diffuse Galaxies in the Perseus Cluster through Over-densities of Globular Clusters with a Log-Gaussian Cox Process

We introduce a new method for detecting ultra-diffuse galaxies by searching for over-densities in intergalactic globular cluster populations. Our approach is based on an application of the log-Gaussian Cox process, which is a commonly used model in the spatial statistics literature but rarely used in astronomy. This method is applied to the globular cluster data obtained from the PIPER survey, a Hubble Space Telescope imaging program targeting the Perseus cluster. We successfully detect all confirmed ultra-diffuse galaxies with known globular cluster populations in the survey. We also identify a potential galaxy that has no detected diffuse stellar content. Preliminary analysis shows that it is unlikely to be merely an accidental clump of globular clusters or other objects. If confirmed, this system would be the first of its kind. Simulations are used to assess how the physical parameters of the globular cluster systems within ultra-diffuse galaxies affect their detectability using our method. We quantify the correlation of the detection probability with the total number of globular clusters in the galaxy and the anticorrelation with increasing half-number radius of the globular cluster system. The Sérsic index of the globular cluster distribution has little impact on detectability.

The first detection of ultra-diffuse galaxies in the Hydra I cluster from the VEGAS survey

In this paper, we report the discovery of 27 low-surface brightness galaxies, of which 12 are candidates for ultra-diffuse galaxies (UDG) in the Hydra I cluster, based on deep observations taken as part of the VST Early-type Galaxy Survey (VEGAS). This first sample of UDG candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed UDGs and, through study of statistically relevant samples, constrain the nature and formation of UDGs. This study presents the main properties of this class of galaxies in the Hydra I cluster. For all UDGs, we analysed the light and colour distribution, and we provide a census of the globular cluster (GC) systems around them. Given the limitations of a reliable GC selection based on two relatively close optical bands only, we find that half of the UDG candidates have a total GC population consistent with zero. Of the other half, two galaxies have a total population larger than zero at 2σ level. We estimate the stellar mass, the total number of GCs, and the GC specific frequency (SN). Most of the candidates span a range of stellar masses of 107 − 108 M⊙. Based on the GC population of these newly discovered UDGs, we conclude that most of these galaxies have a standard or low dark matter content, with a halo mass of ≤1010 M⊙.

Globular clusters in the outer halo of M 31

In this paper, we present photometry of 53 globular clusters (GCs) in the M 31 outer halo, including the GALEX far-ultraviolet (FUV) and near-ultraviolet (NUV), SDSS ugriz, 15 intermediate-band filters of BATC, and 2MASS JHKs bands. By comparing the multicolour photometry with stellar population synthesis models, we determine the metallicities, ages, and masses for these GCs, aiming to probe the merging/accretion history of M 31. We find no clear trend of metallicity and mass with the de-projected radius. The halo GCs younger than ∼8 Gyr are mostly located at the de-projected radii around 100 kpc, but this may be due to a selection effect. We also find that the halo GCs have consistent metallicities with their spatially associated substructures, which provides further evidence of the physical association between them. Both the disc and halo GCs in M 31 show a bimodal luminosity distribution. However, we should emphasise that there are more faint halo GCs which are not seen in the disc. The bimodal luminosity function of the halo GCs may reflect a different origin or evolution environment in their original hosts. The M 31 halo GCs include one intermediate metallicity group (−1.5 &lt; [Fe/H] &lt; −0.4) and one metal-poor group ([Fe/H] &lt; −1.5), while the disc GCs have one metal-rich group more. There are considerable differences between the halo GCs in M 31 and the Milky Way (MW). The total number of GCs in M 31 is approximately three times greater than in the MW, however M 31 has about six times more halo GCs than the MW. Compared to the halo GCs of M 31, those of the MW are mostly metal-poor. Both the numerous halo GCs and the higher-metallicity component are suggestive of an active merger history of M 31.

Star Clusters in the Elliptical Galaxy NGC 4589 Hosting a Calcium-rich SN Ib (SN 2005cz)

Abstract NGC 4589, a bright E2 merger-remnant galaxy, hosts the peculiar fast and faint calcium-rich SN Ib SN 2005cz. The progenitor of Ca-rich SNe Ib has been controversial: it could be (1) a young, massive star with 6–12 M ⊙ in a binary system, or (2) an old, low-mass star in a binary system that was kicked out from the galaxy center. Moreover, previous distance estimates for this galaxy have shown a large spread, ranging from 20 to 60 Mpc. Thus, using archival Hubble Space Telescope/ACS F435W, F555W, and F814W images, we search for star clusters in NGC 4589 in order to help resolve these issues. We find a small population of young star clusters with 25 &lt; V ≤ 27 (−7.1 &lt; M V ≤ −5.1) mag and age &lt;1 Gyr in the central region at R &lt; 0.′5 (&lt;3.8 kpc), thus supporting the massive-star progenitor scenario for SN 2005cz. In addition to young star clusters, we also find a large population of old globular clusters. In contrast to previous results in the literature, we find that the color distribution of the globular clusters is clearly bimodal. The turnover (Vega) magnitude in the V-band luminosity functions of the blue (metal-poor) globular clusters is determined to be V 0(max) = 24.40 ± 0.10 mag. We derive the total number of globular clusters, N GC = 640 ± 50, and the specific frequency, S N = 1.7 ± 0.2. Adopting a calibration for the metal-poor globular clusters, M V (max) = −7.66 ± 0.14 mag, we derive a distance to this galaxy: (m − M)0 = 32.06 ± 0.10(ran) ± 0.15(sys) (d = 25.8 ± 2.2 Mpc).

WHERE ARE MOST OF THE GLOBULAR CLUSTERS IN TODAY’S UNIVERSE?

ABSTRACT The total number of globular clusters (GCs) in a galaxy rises continuously with the galaxy luminosity L, while the relative number of galaxies decreases with L following the Schechter function. The product of these two very nonlinear functions gives the relative number of GCs contained by all galaxies at a given L. It is shown that GCs, in this universal sense, are most commonly found in galaxies within a narrow range around L ⋆. In addition, blue (metal-poor) GCs outnumber the red (metal-richer) ones globally by 4 to 1 when all galaxies are added, pointing to the conclusion that the earliest stages of galaxy formation were especially favorable to forming massive, dense star clusters.

The SLUGGS survey★: the globular cluster systems of three early-type galaxies using wide-field imaging

We present the results from a wide-field imaging study of globular cluster (GC) systems in three early-type galaxies. Combinations of Subaru/Suprime-Cam, CFHT/MegaCam and HST/WFPC2/ACS data were used to determine the GC system properties of three highly flattened galaxies NGC 720, NGC 1023 and NGC 2768. This work is the first investigation of the GC system in NGC 720 and NGC 2768 to very large galactocentric radius ($\sim$ 100 kpc). The three galaxies have clear blue and red GC subpopulations. The radial surface densities of the GC systems are fitted with Sersic profiles, and detected out to 15, 8 and 10 galaxy effective radii respectively. The total number of GCs and specific frequency are determined for each GC system. The ellipticity of the red subpopulation is in better agreement with the host galaxy properties than is the blue subpopulation, supporting the traditional view that metal-rich GCs are closely associated with the bulk of their host galaxies' field stars, while metal-poor GCs reflect a distinct stellar halo. With the addition of another 37 literature studied galaxies, we present a new correlation of GC system extent with host galaxy effective radius. We find a dependence of the relative fraction of blue to red GCs with host galaxy environmental density for lenticular galaxies (but not for elliptical or spiral galaxies). We propose that tidal interactions between galaxies in cluster environments might be the reason behind the observed trend for lenticular galaxies.

The SLUGGS Survey: wide field imaging of the globular cluster system of NGC 4278

We use multi-pointing HST ACS and wide field Subaru Suprime-Cam imaging to study the globular cluster system of the L* elliptical galaxy NGC 4278. We have also obtained a handful of new globular cluster spectra with Keck/DEIMOS. We determine the globular cluster surface density profile and use it to calculate the total number of globular clusters, finding the system to be slightly more populous than average for galaxies of its luminosity. We find clear evidence for bimodality in the globular cluster colour distribution and for a colour-magnitude relation in the blue subpopulation (a 'blue tilt'). We also find negative radial colour gradients in both colour subpopulations of equal strength which are similar in strength to those reported in other galaxies. The sizes of NGC 4278's globular clusters decrease with redder colours and increase with galactocentric radius. The ratio of the sizes of blue to red globular clusters is independent of galactocentric radius demonstrating that internal effects are responsible for the size difference with colour.

Isolated ellipticals and their globular cluster systems

The globular cluster (GC) systems of isolated elliptical galaxies have only recently begun to be studied in detail, and may exhibit morphological connections to the evolutionary histories of their hosts. Here we present the first in a series of wide-field analyses of the GC systems of the isolated ellipticals - Washington C and R photometry of NGC 3585 and NGC 5812 down to ~24 mag. The GC systems are characterised, with each system displaying both the "Universal" blue peak at (C-R)~1.3, and a red peak, but each with differing strengths. The total number of GCs in each system, and their specific frequencies, are estimated. The GC colours and specific frequencies are highly indicative that the host galaxy environment plays a role in shaping its GC system. We produce, and subtract, accurate models of each galaxy, revealing interesting underlying features, including the first definitive evidence that NGC 5812 is interacting with a dwarf companion galaxy. From the galaxy models we also determine surface brightness and colour profiles. Both colour profiles appear quite flat and with (C-R)~1.7 and we discuss the apparent youth of NGC 3585 in the context of this work.

The origin of globular cluster systems from cosmological simulations

We investigate the structural, kinematical and chemical properties of globular cluster systems (GCSs) in galaxies of different Hubble types in a self-consistent manner based on high-resolution cosmological N-body simulations combined with semi-analytic models of galaxy and globular cluster (GC) formation. We focus on correlations between the physical properties of GCSs and those of their host galaxies for ∼10 5 simulated galaxies located at the centres of dark matter haloes (i.e. we do not consider satellite galaxies in subhaloes). Our principal results, which can be tested against observations, are as follows. The majority (∼90 per cent) of GCs currently in haloes are formed in low-mass galaxies at redshifts greater than 3 with mean formation redshifts of z = 5.7 (12.7Gyr ago) and 4.3 (12.3Gyr ago) for metal-poor GCs (MPCs) and metal-rich GCs (MRCs), respectively. About 52 per cent of galaxies with GCs show clear bimodality in their metallicity distribution functions, though less luminous galaxies with M B fainter than -17 are much less likely to show bimodality owing to little or no MRCs. The number fraction of MRCs does not depend on Hubble type but is generally smaller for less luminous galaxies. The specific frequencies (S N ) of GCSs are typically higher in ellipticals (S N ∼ 4.0) than in spirals (S N ∼ 1.8), and higher again (S N ∼ 5.0) for galaxies located at the centres of clusters of galaxies. The total number of GCs per unit halo mass does not depend strongly on M B or Hubble type of the host galaxy. The mean metallicities of MPCs and MRCs depend on M B such that they are higher in more luminous galaxies, though the dependence is significantly weaker for MPCs. The spatial distributions of MRCs are more compact than those of MPCs and we find that the half-number radii of MPCs (r e,mpc ) correlate with the halo masses (M h ) such that r e,mpc oc M h 0.18 . There is no significant difference in velocity dispersions between MPCs and MRCs. We qualitatively compare our results to observational data where possible. Finally, we discuss these results in the wider context of galaxy formation and evolution.

On spatial distributions of old globular clusters in clusters of galaxies

We investigate structural properties of old, metal-poor globular clusters (GCs) formed at high redshifts (z>6) and located inside and outside virialized galaxy-scale halos in clusters of galaxies with the total masses of M_CL based on high-resolution cosmological simulations with models of GC formation. We mainly derive the parameter dependences of physical properties of intracluster GCs (ICGCs) based on the results of 14 models. Our principle results are summarized as follows. (1) The projected radial density profiles (Sigma_GC) of ICGCs in clusters with different M_CL can be diverse, though ICGCs have inhomogeneous, asymmetric, and somewhat elongated distributions in most models. If Sigma_GC (R) ~ R^alpha, alpha ranges from -1.5 to -2.5 for GCs in clusters. (2) Although total number of GCs within the central 0.05 Mpc (N_GC,0.05) and 0.2 Mpc (N_GC,0.2) are diverse in different clusters, they can depend weakly on M_CL in such a way that both N_GC,0.05 and N_GC,0.2 are likely to be larger for clusters with larger M_CL. (3) Total number of GCs per cluster masses (specific frequency of GCs for clusters of galaxies) are more likely to be larger in more massive clusters, mainly because a larger number of earlier virialized objects can be located in more massive clusters. (4) Spatial distributions of old GCs in clusters can depend on the truncation epoch of GC formation (z_trun) such that they can be steeper and more compact in the models with higher z_trun. (5) The mean metallicity of ICGCs in a cluster can be smaller than that of GCs within the cluster member galaxy-scale halos by ~ 0.3 in [Fe/H]. Metallicity distribution functions (MDFs) of ICGCs show peak values around [Fe/H] ~ -1.6 and do not have remarkable bimodality.

Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128

We have investigated a number of globular cluster candidates from a recent wide-field study by Harris et al. (2004a, AJ, 128, 712) of the giant elliptical galaxy NGC 5128. We used the Magellan I telescope + MagIC camera under excellent seeing conditions () and obtained very high resolution images for a sample of 44 candidates. Of these, 15 appear to be bonafide globular clusters in NGC 5128 while the rest are either foreground stars or background galaxies. We also serendipitously discovered 18 new cluster candidates in the same fields. Our images allow us to study the light profiles of the likely clusters, all of which are well resolved. This is the first ground-based study of structural parameters for globular clusters outside the Local Group. We compare the psf-deconvolved profiles with King models and derive structural parameters, ellipticities and surface brightnesses. We compare the derived structural properties with those of other well-studied globular cluster systems. In general, our clusters are similar in size, ellipticity, core radius and central surface brightness to their counterparts in other galaxies, in particular those in NGC 5128 observed with HST by Harris et al. (2002, AJ, 124, 1435). However, our clusters extend to higher ellipticities and larger half-light radii than their Galactic counterparts, as do the Harris et al. sample. Combining our results with those of Harris et al. fills in the gaps previously existing in parameter space and indicates that any substantial difference between presumed distinct cluster types in this diagram, including for example the Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the “extended, luminous” M 31 clusters of Huxor et al. (2005, MNRAS, 360, 1007) is now removed and that clusters form a continuum in this diagram. Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. The metal-rich clusters in our sample have half-light radii that are almost twice as large in the mean as their metal-poor counterparts, at odds with the generally accepted trend. The possibility exists that this result could be due in part to contamination by background galaxies. We have carried out additional analysis to quantify this contamination. This shows that, although galaxies cannot be easily told apart from clusters in some of the structural diagrams, the combination of excellent image quality and Washington photometry should limit the contamination to roughly 10% of the population of cluster candidates. Finally, our discovery of a substantial number of new cluster candidates in the relatively distant regions of the NGC 5128 halo suggests that current values of the total number of globular clusters may be underestimates.

Star clusters in M 33

We have detected 102 star clusters in M 33, from 35 deep Hubble Space Telescope (HST) WFPC2 fields taken from our program and from the HST archive. Twenty-eight fields have V and I band imaging, and an additional seven fields are imaged in a single V filter. Eighty-two of the clusters were previously unknown. Integrated photometry reveals that 25 clusters have () colors typical of those found in Galactic globular clusters (only ten of these objects were previously known). Our discovery of 15 new globular cluster candidates increases previous estimates of the old cluster population in M 33 by ∼60% . An additional eleven objects (only two were previously known) have colors expected for intermediate age clusters. The new clusters have a range of ages from 6 million years to >15 Gyrs, and masses between , although these parameters are estimated from only one color. The number of new clusters detected more than doubles our previous sample (Papers I and II), and extends coverage to older objects and new off-spiral arm locations in M 33. The luminosity function for old M 33 clusters shows a peak at , nearly half a magnitude fainter than found in the Galactic and M 31 globular cluster populations. The luminosity function for intermediate age objects shows no turnover down to . We estimate the total number of globular clusters in M 33 to be . This gives a specific frequency, SN, of -significantly higher than found for other late-type spiral galaxies, but in the range found for ellipticals.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL] Imaging of Globular Clusters in the Edge-on Spiral Galaxies NGC 4565 and NGC 5907

We present a study of the globular cluster systems of two edge-on spiral galaxies, NGC 4565 and NGC 5907, from WFPC2 images in the F450W and F814W filters. The globular cluster systems of both galaxies appear to be similar to the Galactic globular cluster system. In particular, we derive total numbers of globular clusters of NGC(4565) = 204 ± 38 and NGC(5907) = 170 ± 41 (where the first are statistical, the second potential systematic errors) for NGC 4565 and NGC 5907, respectively. This determination is based on a comparison with the Milky Way system, for which we adopt a total number of globular clusters of 180 ± 20. The specific frequency of both galaxies is SN 0.6, indistinguishable from the value for the Milky Way. The similarity in the globular cluster systems of the two galaxies is noteworthy, since they have significantly different thick disks and bulge-to-disk ratios. This would suggest that these two components do not play a major role in the building up of a globular cluster system around late-type galaxies.

The NGC 1399 Globular Cluster System: Washington Photometry Revisited

A new analysis and discussion of Washington CCD photometry for some 500 globular cluster candidates associated with the Fornax cluster cD galaxy NGC 1399 are presented. The image handling in this work includes improved methods for the removal of both the galaxy halo and fringing effects over that employed in a previous work of ours. The analysis of the globular cluster candidates inside an annular region between 05 and 4' from the galaxy center using two-color diagrams shows a clear bimodal distribution for objects fainter than T1 = 21.0 (V ≈ 21.5). The modal colors of the globular clusters belonging to the red population are coincident, within the measuring uncertainties, with those of the galaxy halo. In turn, the average integrated colors of the blue population are comparable to those of the bluest non–star-forming low surface brightness galaxies in Fornax. Red clusters appear more centrally concentrated than the blue clusters. A new determination of the integrated luminosity function turnover (T1 = 23.24) leads to a distance modulus for NGC 1399 of V0 - MV = 31.02 ± 0.2. The total number of globular clusters, combined with our preliminary photometry of the galaxy halo, leads to an overall specific frequency SN = 5.6 ± 1.0, which is considerably lower than previous estimates available in the literature. We note that these last values have been based on an extrapolation of aperture photometry of the inner regions of the galaxy.

HSTimaging of the globular clusters in the Fornax cluster: NGC 1379

We present B and I photometry for ∼300 globular cluster candidates in NGC 1379, an E0 galaxy in the Fornax cluster. Our data are from both Hubble Space Telescope (HST) and ground-based observations. The HST photometry (B only) is essentially complete and free of foreground/background contamination to ∼2 mag fainter than the peak of the globular cluster luminosity function. Fitting a Gaussian to the luminosity function, we find 〈B〉=24.95±0.30 and σB=1.55??0.21. We estimate the total number of globular clusters to be 436±30. To a radius of 70 arcsec we derive a moderate specific frequency, SN=3.5±0.4. At radii r∼3–6 kpc the surface density profile of the globular cluster system is indistinguishable from that of the underlying galaxy light. At r≲2.5 kpc the profile of the globular cluster system flattens, and at r≲1 kpc, the number density appears to decrease. The (B−I) colour distribution of the globular clusters (from ground-based data) is similar to that for Milky Way globulars, once corrected for background contamination. It shows no evidence for bimodality or for the presence of a population with [Fe/H]≳−0.5. Unlike in the case of larger, centrally located cluster ellipticals, neither mergers nor a multiphase collapse are required to explain the formation of the NGC 1379 globular cluster system. We stress the importance of corrections for background contamination in ground-based samples of this kind: the area covered by a globular cluster system (with radius ∼30 kpc) at the distance of the Virgo or Fornax cluster contains ≳200 background galaxies unresolved from the ground, with magnitudes comparable to brighter globular clusters at that distance. The colour distribution of these galaxies is strongly peaked slightly bluer than the peak of a typical globular cluster distribution. Such contamination can thus create the impression of skewed colour distributions, or even of bimodality, where none exists.

HST imaging of the globular clusters in the Fornax cluster: NGC 1399 and NGC 1404

The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope's Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory's 1.5m telescope. The combination of both data sets provides unique insight on the spatial and colour distribution of globular clusters. From B-I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e. more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N = 5700 +/- 500, S_N = 11.5 +/- 1.0 for NGC 1399 and N = 725 +/- 145, S_N = 2.0 +/- 0.5 for NGC 1404. Our results are compared to the expectations of globular cluster formation scenarios.

Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion

view Abstract Citations (46) References (70) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Clusters Driven to Form by Strong Collisions Between Gas Clouds in High-Velocity Random Motion Fujimoto, Mistuaki ; Kumai, Yasuki Abstract Two topics are discussed about the globular and populous star clusters, based on an idea that they were/are driven to form through strong collisions between massive gas clouds in high-velocity random motion. The first is a global trial to reproduce the approximately-constant specific frequency of globular clusters among normal elliptical galaxies: The ratio of the total number of globular clusters to the luminosity of their host galaxy is obtained to be approximately constant by counting the number of collisions between gas clouds in virialized motion in the gravitational potential of the protogalaxy. The second is presenting a local dynamical model for the formation of binary star clusters such as detected numerously in the Magellanic Clouds. It is stressed that the oblique cloud-cloud collisions occur more frequently than the head-on collisions, and the former collisions yield shear and rotation to the compressed gaseous slab, which works to split a local Jeans gas clump into two objects revolving around each other (fission). In order to examine this dynamical model for the binary star cluster formation, we determine numerical values of the split objects, such as their mass, diameter of revolution, frequency of occurrence, etc., to be compared with those measured in the Large Magellanic Cloud. Publication: The Astronomical Journal Pub Date: January 1997 DOI: 10.1086/118249 Bibcode: 1997AJ....113..249F Keywords: ISM: KINEMATICS AND DYNAMICS; ISM: CLOUDS; GALAXIES: STAR CLUSTERS full text sources ADS | data products SIMBAD (39) NED (1)