Intergalactic Globular Clusters Research Articles

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 PIPER Survey. I. An Initial Look at the Intergalactic Globular Cluster Population in the Perseus Cluster

Abstract We describe the goals and first results of the Program for Imaging of the PERseus cluster of galaxies (PIPER). The first phase of the program builds on imaging of fields obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS)/WFC and WFC3/UVIS cameras. Our PIPER target fields with HST include major early-type galaxies including the active central giant NGC 1275, known ultra-diffuse galaxies, and the intracluster medium. The resulting two-color photometry in F475W and F814W reaches deep enough to resolve and measure the globular cluster (GC) populations in the Perseus member galaxies. Here we present initial results for eight pairs of outer fields that confirm the presence of intergalactic GCs (IGCs) in fields as distant as 740 kpc from the Perseus center (40% of the virial radius of the cluster). Roughly 90% of these IGCs are identifiably blue (metal-poor) but there is a clear trace of a red (metal-rich) component as well, even at these very remote distances.

No sign (yet) of intergalactic globular clusters in the Local Group

Abstract We present Gemini Multi-Object Spectrograph (GMOS) imaging of 12 candidate intergalactic globular clusters (IGCs) in the Local Group, identified in a recent survey of the Sloan Digital Sky Survey (SDSS) footprint by di Tullio Zinn & Zinn. Our image quality is sufficiently high, at ∼0.4–0.7 arcsec, that we are able to unambiguously classify all 12 targets as distant galaxies. To reinforce this conclusion we use GMOS images of globular clusters in the M31 halo, taken under very similar conditions, to show that any genuine clusters in the putative IGC sample would be straightforward to distinguish. Based on the stated sensitivity of the di Tullio Zinn & Zinn search algorithm, we conclude that there cannot be a significant number of IGCs with MV ≤ −6 lying unseen in the SDSS area if their properties mirror those of globular clusters in the outskirts of M31 – even a population of 4 would have only a ≈1 per cent chance of non-detection.

A SEARCH FOR INTERGALACTIC GLOBULAR CLUSTERS IN THE LOCAL GROUP

The whole Sloan Digital Sky Survey (SDSS, ) has been searched for intergalactic globular clusters (IGCs) in the Local Group (LG). Using optical, infrared, and ultraviolet photometric selection criteria and photometric redshifts, the objects in the SDSS Galaxy Catalog were reduced to only 183,791 brighter than that might be GCs. Visual examination of their SDSS images recovered 84% of the confirmed GCs in M31 and M33 and yielded 17 new GC candidates, 5 of them of high confidence, which we could confirm as GCs in MegaPrime images from the Canada–France–Hawaii Telescope. These 5 GCs are within M31's halo, but the other 12 candidates are not close to LG galaxies or galaxies within 3 Mpc of the LG. Even though this search covers only one-third of the sky and some GCs could have been missed, it suggests that the LG does not contain a large population of IGCs more luminous than . In the direction of the M81 Group, the search yielded five candidate GCs, probable members of that group.

A SEARCH FOR GLOBULAR CLUSTERS IN MORE REMOTE AREAS AROUND M31 AND M33 II

Using the Sloan Digital Sky Survey (SDSS), ~900 deg2 of the sky surrounding M31 and M33 have been searched for globular clusters (GCs) that through galaxy interaction have become unbound from their parent systems and M31 (hence, intergalactic globular clusters, IGCs). This search reached a maximum of ~500 kpc in projected galactocentric distance (R gc) from M31. Visual examination of 283,871 SDSS cutout images and of 1143 fits images yielded 320 candidates. This sample was reduced to six GCs and one likely candidate by excluding galaxies on the basis of combinations of their optical, ultraviolet, and infrared colors from the SDSS, the Galaxy Evolution Explorer satellite, and the Wide-field Infrared Survey Explorer satellite, as well as their photometric redshifts from the SDSS. Since these seven objects have 14 kpc ≤ R gc ≤ 137 kpc, they are more likely to be GCs in the halo of M31 than IGCs. They are all classical as opposed to extended GCs, and they provide further evidence that the remote halo of M31 (R gc ≥ 50 kpc) contains more GCs of all types and, in particular, far more classical ones than the remote halo of the Milky Way.

A possible explanation for the Arp-Burbidge paradox

The Arp-Burbidge paradox (the enhanced density of distant quasars in the vicinity of some nearby galaxies) can potentially be explained as an effect of gravitational lensing within the existing cosmological paradigm. Distant, rich X-ray clusters, or even superclusters, of galaxies are observed along the lines of sight toward many of the objects on which this “paradox” is based. Such clusters can act as complex, transparent gravitational lenses, which can change the observed surface density of background objects due to the gravitational fields of both the cluster as a whole and of massive member galaxies, as well as of possible intergalactic globular clusters that may contain an appreciable fraction (∼10%) of the dark matter in galaxy clusters. We have verified the statistical basis for the supposedly “paradoxical” observational facts using data from the SDSS catalog, and used the minimal spanning tree method to search for inhomogeneities in the surface distributions of 32 800 quasars in one region of the celestial sphere (α = 120°–260°, δ = 20°–70°).

A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 - II. Colour and spatial distribution

We have performed a wide-field imaging survey of the globular cluster (GC) populations around M87 with Suprime-Cam on the 8.2-m Subaru Telescope. A 2° x 0.°5 field extending from the centre of M87 out to a radius of ∼0.5 Mpc was observed through the BVI filters. By investigating the GC colour distribution as a function of distance from M87 and NGC 4552, another luminous Virgo elliptical in our survey field, it is found that clear bimodality [(V - I) peak ∼ 1-0 and 1.2] is seen only in the inner regions (≤10 kpc) of the host galaxies and that it becomes weaker with radius due to the decreasing contribution of the red GC (V - I > 1.1) subpopulation. It is also found (both around M87 and around NGC 4552) that while the spatial distribution of the red GCs is as centrally concentrated as the host galaxy halo light distribution, the distribution of the blue GCs (V - I ≤ 1.1) tends to be more extended. However, the blue GC distribution around M87 is not as extended as the Virgo cluster mass density profile. Based on these facts, we argue that the contribution of GCs associated with the Virgo cluster [e.g. intergalactic GCs (i-GCs)] is not significant around M87 and is not the primary reason for the high S N value of M87. Instead, most of the blue GCs around luminous ellipticals, as well as the red GCs, are presumed to be associated with the host galaxy. We model the radial profile of GC surface densities out to ∼0.5 Mpc from M87 by a superposition of the GC populations associated with M87 and with NGC 4552. It is found that there are some regions where the GC surface densities are larger than that which is expected from this model, suggesting the existence of an additional i-GC population independent of the luminous ellipticals. By comparing the estimated i-GC surface density with the luminosity density of the intracluster stellar population inferred from the intergalactic planetary nebulae in the Virgo cluster, we obtain a crude estimate of i-GC specific frequency S N = 2.9 +4.2 -1.5 . If this S N value represents the stellar population tidally stripped by a massive central galaxy from other (less-luminous) galaxies, the contribution of tidally captured GCs in the GC population of M87 would need to be low to be consistent with the high S N value of M87.

Intergalactic Globular Clusters

AbstractWe confirm and extend our previous detection of a population of intergalactic globular clusters in Abell 1185, and report the first discovery of an intergalactic globular cluster in the nearby Virgo cluster of galaxies. The numbers, colors and luminosities of these objects can place constraints on their origin, which in turn may yield new insights to the evolution of galaxies in dense environments.

A Point-Source Excess in Abell 1185: Intergalactic Globular Clusters?

Deep imaging with WFPC2 and the Hubble Space Telescope has been used to search for a population of intergalactic globular clusters (GCs) belonging to Abell 1185, a richness I cluster at cz = 9800 km/s. The field is noteworthy in that it contains no bright galaxies and yet is centered on the peak of the cluster's X-ray surface brightess distribution. We detect a total of 99 point sources in this field to a limiting magnitude I_F814W ~ 26. An identical analysis of the Hubble Deep Field North, which serves as our control field, reveals a total of 12 objects in the same magnitude range. We discuss possible explanations for this discrepancy, and conclude that the point source excess is likely due to the presence of GCs within A1185. The number and spatial distribution of these GCs are consistent with their being intergalactic in nature, although we cannot rule out the possibility that some of the GCs may be associated with neighboring bright galaxies. Deeper imaging with the Advanced Camera for Surveys may resolve this ambiguity.

Intergalactic Globular Clusters and the Faint End of the Galaxy Number Counts in A1656 (Coma)

The existence of an intergalactic globular cluster population in the Coma cluster of galaxies has been tested using surface brightness fluctuations. The main result is that the intergalactic globular cluster surface density (NIGC) does not correlate with the distance to the center of Coma and hence with the environment. Furthermore, comparing these results with different Coma mass distribution model predictions, it is suggested that NIGC must in fact be zero all over Coma. On the other hand, the results for NIGC and the faint end of the galaxy number counts (beyond mR = 23.5) are connected. So NIGC = 0 settles the slope of this function, which turns out to be γ = 0.36 ± 0.01 down to mR = 26.5. The fact that NIGC = 0 all over Coma suggests that globular clusters were formed only, or almost only, from protogalactic clouds. None, or perhaps very few, could have formed in isolated regions. It also seems inappropriate to advocate a relationship between intergalactic globular clusters and dark matter distributions, although it is true that the relationship could still exist but not be strong enough to have been detected. Finally, since our conclusion is that intergalactic globular clusters do not exist in Coma, accretion of intergalactic globular clusters might not be significant in galaxy formation and evolutionary processes in the Coma galaxies.

Dynamical limits on galactic winds, halo MACHOs and intergalactic globular clusters

We argue that any violent galactic winds following early epoch of star bursts would significantly weaken the potentials of galaxies, and leave lasting signatures such as a lowered dark halo density and preferentially radial/escaping orbits for halo tracers such as globular clusters. A galaxy is disintegrated if more than half of its dynamical mass is blown off. The presence of dense halos in galaxies and the absence of intergalactic/escaping globulars should imply an upper limit on the amount of baryons lost in galactic winds around 4% of the total mass of the galaxy. This translates to limits on the baryons participating the early star bursts and baryons locked in stellar remnents, such as white dwarfs. The amount of halo white dwarfs claimed in recent proper motion searches and microlensing observations in the Galactic halo are too high to be consistent with our dynamical upper limits. Similar arguments also imply upper limits for the amount of neutron stars and stellar black holes, in galaxy halos. Nevertheless, a milder outflow is desirable, especially in dwarf galaxies, both for lowering their cold dark matter central density and for injecting metals to the intergalactic medium.

M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation

We have used the high-resolution camera at the Canada-France-Hawaii Telescope to obtain VRI photometry of the globular clusters in the innermost 140'' of the M87 halo. The results are used to discuss several issues concerning the formation and evolution of globular cluster systems in supergiant elliptical galaxies like M87. Our principal results are as follows: (1) From our deep R-band photometry of the cluster population, we find no significant change in the globular cluster luminosity function (GCLF) with galactocentric radius, for cluster masses M 105 M☉. This result places constraints on current theoretical predictions of the rate of cluster evolution by tidal shocking and evaporation, indicating that the main effects of dynamical evolution may be felt only on lower mass clusters (105 M☉) that are below the faint limit of most current observations. (2) Combining our V - I color indices with other data in the literature, we derive the metallicity gradient and mean metallicity of the clusters from r = 9'' out to r 500'' (~0.7–35 kpc). Within the core radius rc = 1' of the globular cluster system, the metallicity distribution is uniform, but at larger radii the mean metallicity declines steadily as Z/Z☉ ~ r-0.9. (3) The various options for explaining the existence of high specific frequency galaxies like M87 are evaluated. We argue that alternatives involving secondary evolution (such as the formation of many globular clusters during mergers, or the existence of a population of intergalactic globular clusters centered on the same location as the central elliptical galaxy) are capable of modifying the characteristics of the globular cluster system in distinctive ways, but are not likely to be the primary explanation for high-SN galaxies. (4) We offer a new explanation for the large observed SN range among the brightest cluster elliptical galaxies (BCGs). It is suggested that these central supergiant E galaxies formed in an exceptionally turbulent or high-density environment which favored a very rapid initial star formation burst. As a result, a much higher than average fraction of the protogalactic gas was driven out in a galactic wind not long after the first globular clusters were formed, thus artificially boosting the specific frequency of the remaining galaxy. From a total sample of 30 BCGs, we derive empirical scaling relations that relate to this hypothesis. Our analysis favors the view that BCGs began forming at redshifts z 5, distinctly earlier than most other galaxies.

A DISTANT STAR CLUSTER IN HYDRA, AM-4.

ABSTRACT The discovery of a very sparse star cluster in the constellation Hydra in the southern sky is reported. The cluster was found during a search for peculiar galaxies in a plate from the ESO/SRC IIIa-J Southern Sky Survey, observed at a magnification of 20X. Designated AM-4, the cluster contains approximately 50 stars within a 110 arcsec diameter. Assumptions that the brightest stars in the cluster are red giants and that absorption in the direction of the cluster is low lead to the estimate that the cluster is over 200 kpc distant, and so might be intrinsically rich. The chance discovery of the cluster points up the need for more detailed surveys to determine the actual density of extreme halo or intergalactic globular clusters.

Three new faint star clusters

view Abstract Citations (31) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Three new faint star clusters. Madore, B. F. ; Arp, H. C. Abstract Descriptions and photographs are presented for three faint star clusters. The first is extremely distant and may constitute the first identification of a truly intergalactic globular cluster. The second, a low-galactic-latitude object, may also be a globular cluster or a low-mass dwarf member galaxy of the Local Group. Finally, the third object is a far outlying member of the Small Magellanic Cloud, probably an open cluster. Publication: The Astrophysical Journal Pub Date: January 1979 DOI: 10.1086/182876 Bibcode: 1979ApJ...227L.103M Keywords: Globular Clusters; Star Clusters; Astronomical Photography; Intergalactic Media; Magellanic Clouds; Open Clusters; Astronomy; Magellanic Clouds:Open Clusters full text sources ADS | data products SIMBAD (3) NED (3)

Photometry of the Intergalactic Background Light in the Coma Cluster

A very faint intergalactic background light component (to be called IBL) was photographically discovered by Zwicky (1957) in the centre of the Coma Cluster. More recently Welch & Sastry (1971, 1972) have mapped the area using isodensity tracings of photographic plates. In order to provide an accurate calibration of the intensity and to determine the colour of the IBL in the Coma Cluster, photoelectrical observations have been made of a region located ˜2′ east of IC 3949 and ˜12′ south-west of the cluster centre. The observations were made using a 60-cm Ritchey-Chretién telescope at the Metsähovi Observatory near Helsinki. The diaphragm size was 115″. In Table I the results of these observations are given together with three other available photoelectrical values for the IBL intensity in clusters. For comparison, the surface brightness due to visible galaxies has been estimated. The resulting values are: IV = 11.6 S10 (≙ 25.1 mag sec-2), IB = 4.6 S10 (≙ 26.1 mag sec−2), and thus the IBL intensity amounts to 25 percent and 39 percent in V and B, respectively, of the light of visible galaxies. For the colour index of the IBL one obtains using the values in Table I: B-V = 0.54 ± 0.18. Two mechanisms have been hitherto proposed in the literature to explain the IBL in the Coma Cluster: (1) light from dwarf galaxies, intergalactic globular clusters or individual intergalactic stars, and (2) thermal bremsstrahlung from a hot intergalactic gas with Te = 0.5 − 10 × 105 K. A third possible mechanism can still be mentioned, namely the scattering of the light of galaxies by intergalactic dust grains.

The Anomalous Color-Magnitude Diagram of the Remote Globular Cluster NGC 7006

view Abstract Citations (296) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Anomalous Color-Magnitude Diagram of the Remote Globular Cluster NGC 7006 Sandage, Allan ; Wildey, Robert Abstract The color-magnitude diagram for NGC 7006 has been obtained to V = 19.3 by a combination of photographic and photoelectric measurements. The horizontal branch occurs at V = 18.80. Interstellar reddening is small. Adopting Mv = +0.5 for the RR Lyrae variables gives a true modulus of (m - M)0 = 18.3, a distance of 45 kpc from the Sun, and 15 kpc from the galactic plane. The metal abundance is low, as judged by AV = 2.6 mag and by the spectroscopic and photometric criteria of Morgan and van den Bergh, and yet the distribution of stars along the horizontal branch imitates that of much higher metal-abundance clusters. This is the first clear violation of the heretofore unique correlation of metal abundance and density gradient along the horizontal branch. It shows that at least one additional parameter besides Z controls the stellar distribution along the branch. Models of Faulkner and of Faulkner and Iben suggest that the second parameter may be the abundance of helium. The NGC 7006 anomaly is present in many intergalactic globular clusters and in the clusters of the Small Magellanic Cloud, suggesting a difference in the second parameter between objects that have or have not partaken in the chemical evolution of the galaxy. NGC 7006 may not be gravitationally bound to the galaxy, in which case it was formed in the Local Group but outside the galactic halo. If future observations and more extensive model calculations can establish an abundance difference, A Y, between objects inside and outside the Galaxy, then we would be able to conclude that some helium was made in the early history of the galactic system. This additional helium is then added to the pristine helium produced, for example, in the primeval fireball of a Friedman-type universe or in any other event that manufactured helium which preceded the formation of the Local Group of galaxies. Publication: The Astrophysical Journal Pub Date: November 1967 DOI: 10.1086/149350 Bibcode: 1967ApJ...150..469S full text sources ADS | data products SIMBAD (24)

Properties of Two Intergalactic Globular Clusters.

view Abstract Citations (68) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Properties of Two Intergalactic Globular Clusters. Burbidge, E. Margaret ; Sandage, Allan Abstract Data for two very distant globular clusters are given. The 1 1h cluster at R.A. (1950) = 1 Dec. (1950) = + ', has an apparent distance of 1.2 X 10 pc if Mis 0.00 for the horizontal branch. has a diameter of about 90 pc, an integrated absolute magnitude M, = -63, an estimated mass of 1.8 X 10 O, and an estimated total number of stars of 4.3 X 10 . The 10h cluster at R.A. (1950) = Dec. (1950) = + ' has an apparent distance of 1.3 X 10' pc, a diameter of about 80 pc, an integrated M,, of -5.9, an estimated mass of 1.1 X 10 O, and an estimated total number of stars of 2.7 x 10 . The diameters of both clusters are unusually large compared with systems like M3 and Ml3, which are more populous by factors of at least 10. Large diameters are expected if the tidal force of the Galaxy, which limits the radii of most clusters, has always been small. The observed diameters can be explained if the two clusters have never passed closer than 9000 pc to the galactic center. This result requires that the clusters were formed at least 9000 pc from the galactic center. The 1O and 11h clusters could reach their present distances in about 10 years if they are moving in parabolic orbits around the galactic center. If the clusters are escaped members of M31, they could reach their present position in about lOlO years if the clusters have the escape velocity and if we neglect the influence of the Galaxy. The true travel time from M31 will be less than 1010 years because of the attraction of the galactic system. Computation of the correct travel time from M3 1 is given the restricted three-body problem, which is not discussed. Publication: The Astrophysical Journal Pub Date: May 1958 DOI: 10.1086/146484 Bibcode: 1958ApJ...127..527B full text sources ADS | data products SIMBAD (5)