Cluster Ellipticals Research Articles

Spatial distribution of kimberlite in the Slave craton, Canada: a geometrical approach

Exploration within the Slave craton has revealed clusters of kimberlite intrusions, commonly with internally consistent geochemical and temporal characteristics. Translation diagrams (“Fry analysis”) allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database. Recurrent patterns are visually accentuated due to the square function in data density. Circular histograms quantify the azimuthal density of kimberlite at various distances. For this study, the database comprises the geographic position of 212 kimberlite occurrences of which 70% are from the Lac de Gras field (LDG). Analyses are presented separately for the LDG data and for all non-LDG data in order to test for regional variations and to avoid overwhelming the craton-scale studies by the high density of LDG data. Empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences. Clusters within the western part of the craton are elongate to the north–northeast and align within a narrow zone (“Western Corridor”). Elsewhere, the clusters are elongate to the northwest or west–northwest and appear to be arranged en echelon within a poorly defined north–northwest trending zone (“Central Corridor”). Geometrical spatial analyses of kimberlite locations highlight the craton-scale pattern of emplacement within the two main corridors. At regional and local scales, individual intrusions are preferentially located towards the west–northwest (ca. 280°) and north–northeast (ca. 015°) of other intrusions, and these orientations are interpreted to reflect upper mantle trends in magma generation. At local scales (10–25 km), kimberlite of the central and southern craton tends to be located to the northeast (ca. 045°), and possibly weakly to the east–northeast (ca. 070°), of other intrusions, and these orientations correspond to major crustal fractures systems. It is proposed that kimberlite emplacement is controlled primarily by the interaction of elongate 280° and 015° source regions with near-surface deviations influenced by crustal fracture systems. The 015° trend evident at craton, regional, and local scales is parallel to a swarm of alkaline diabase dykes that are concentrated in a ca. 30-km-wide corridor passing through Lac de Gras. A profound spatial association between significantly diamondiferous kimberlite and the margins of the dyke corridor suggests the corridor is the surface expression of a mantle-depth structure. It remains unclear whether the proposed mantle structure coincides with a diamond-rich zone near the base of the lithosphere, or delineates pathways favorable for diamond preservation during emplacement. The linear array of kimberlite within the western craton forms a parallel corridor that may be an analogous mantle structure, but which to date has failed to yield economic diamond concentrations.

Spatial distribution of kimberlite in the Slave craton, Canada: a geometrical approach

Exploration within the Slave craton has revealed clusters of kimberlite intrusions, commonly with internally consistent geochemical and temporal characteristics. Translation diagrams (“Fry analysis”) allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database. Recurrent patterns are visually accentuated due to the square function in data density. Circular histograms quantify the azimuthal density of kimberlite at various distances. For this study, the database comprises the geographic position of 212 kimberlite occurrences of which 70% are from the Lac de Gras field (LDG). Analyses are presented separately for the LDG data and for all non-LDG data in order to test for regional variations and to avoid overwhelming the craton-scale studies by the high density of LDG data. Empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences. Clusters within the western part of the craton are elongate to the north–northeast and align within a narrow zone (“Western Corridor”). Elsewhere, the clusters are elongate to the northwest or west–northwest and appear to be arranged en echelon within a poorly defined north–northwest trending zone (“Central Corridor”). Geometrical spatial analyses of kimberlite locations highlight the craton-scale pattern of emplacement within the two main corridors. At regional and local scales, individual intrusions are preferentially located towards the west–northwest (ca. 280°) and north–northeast (ca. 015°) of other intrusions, and these orientations are interpreted to reflect upper mantle trends in magma generation. At local scales (10–25 km), kimberlite of the central and southern craton tends to be located to the northeast (ca. 045°), and possibly weakly to the east–northeast (ca. 070°), of other intrusions, and these orientations correspond to major crustal fractures systems. It is proposed that kimberlite emplacement is controlled primarily by the interaction of elongate 280° and 015° source regions with near-surface deviations influenced by crustal fracture systems. The 015° trend evident at craton, regional, and local scales is parallel to a swarm of alkaline diabase dykes that are concentrated in a ca. 30-km-wide corridor passing through Lac de Gras. A profound spatial association between significantly diamondiferous kimberlite and the margins of the dyke corridor suggests the corridor is the surface expression of a mantle-depth structure. It remains unclear whether the proposed mantle structure coincides with a diamond-rich zone near the base of the lithosphere, or delineates pathways favorable for diamond preservation during emplacement. The linear array of kimberlite within the western craton forms a parallel corridor that may be an analogous mantle structure, but which to date has failed to yield economic diamond concentrations.

The Ages of Dwarf Elliptical Galaxies

We present narrow band photometry of 91 dwarf ellipticals in the Coma and Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs reveals two distinct populations of early-type systems based on integrated colors. The class of dEN galaxies are redder in their continuum colors as compared to bright cluster ellipticals and dE type dwarfs, and their position in multi-color diagrams can only be explained by an older mean age for their underlying stellar populations. By comparison with the narrow band photometry of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are a higher metallicity continuation of the old, metal-poor color sequence of galactic globulars and the blue population of M87 globulars. Bright ellipticals and dE dwarfs, on the other hand, follow the color sequence of the metal-rich, red population of M87 globulars. A comparison to SED models, convolved to a simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The implication is that globulars and dEN galaxies are primordial and have metallicities set by external constraints such as the enrichment of their formation clouds. Bright ellipticals and dE galaxies have metallicities and ages that suggest an extended phase of initial star formation to produce a younger mean age, even if their formation epoch is similar to that of dENs and blue globulars, and an internally driven chemical evolutionary history.

On the ellipticity of galaxy clusters

We analysed 246 scanned ACO clusters and we do not find any dependence of galaxy cluster ellipticity on redshift.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

X-Ray Constraints on Dark Matter in Galaxy Clusters and Elliptical Galaxies: A View from Chandra and XMM

X-ray observations with Chandra and XMM are providing valuable new measurements of the dark matter content of elliptical galaxies and galaxy clusters. I review constraints on the radial density profiles and ellipticities of the dark matter in these systems (with an emphasis on clusters) obtained from recent X-ray observations and discuss their implications, especially for the self-interacting dark matter model.

Galaxy Disruption Caught in the Act

Direct evidence of stellar material from galaxy disruption in the intra-cluster medium (ICM) relies on challenging observations of individual stars, planetary nebulae and diffuse optical light. Here we show that the ultra-compact dwarf galaxies (UCDs) we have discovered in the Fornax Cluster are a new and easy-to-measure probe of disruption in the ICM. We present spectroscopic observations supporting the hypothesis that the UCDs are the remnant nuclei of tidally “threshed” dwarf galaxies. Deep optical imaging of the cluster has revealed a 43-kpc long arc of tidal debris, flanking a nucleated dwarf elliptical (dE,N) cluster member. We may be witnessing galaxy threshing in action.

The Discovery of Globular Clusters in the Protospiral Galaxy NGC 2915: Implications for Hierarchical Galaxy Evolution

We have discovered three globular clusters beyond the Holmberg radius in Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark matter-dominated blue compact dwarf galaxy NGC 2915. The clusters, all of which start to resolve into stars, have MV606 = -8.9 to -9.8 mag, significantly brighter than the peak of the luminosity function of Milky Way globular clusters. Their colors suggest a metallicity [Fe/H] ≈ -1.9 dex, typical of metal-poor Galactic globular clusters. The specific frequency of clusters is at a minimum normal, compared to spiral galaxies. However, since only a small portion of the system has been surveyed, it is more likely that the luminosity and mass normalized cluster content is higher, like that seen in elliptical galaxies and galaxy clusters. This suggests that NGC 2915 resembles a key phase in the early hierarchical assembly of galaxies—the epoch when much of the old stellar population has formed but little of the stellar disk. Depending on the subsequent interaction history, such systems could go on to build up larger elliptical galaxies, evolve into normal spirals, or in rare circumstances remain suspended in their development to become systems like NGC 2915.

The DEEP Groth Strip Survey. IX. Evolution of the Fundamental Plane of Field Galaxies

Fundamental plane studies provide an excellent means of understanding the evolutionary history of early-type galaxies. Using the Low Resolution Imaging Spectrograph on the Keck Telescope, we obtained internal stellar kinematic information for 36 field galaxies in the Groth Strip, 21 early-type and 15 disk galaxies. Their redshifts range from 0.3 to 1.0, with a median redshift of 0.8. The slope of the relation shows no difference compared with the local slope. However, there is significant evolution in the zero-point offset; an offset due to evolution in magnitude requires a 2.4 mag luminosity brightening at z = 1. We see little differences of the offset with bulge fraction, which is a good surrogate for galaxy type. Correcting for the luminosity evolution reduces the orthogonal scatter in the fundamental plane to 8%, consistent with the local scatter. This scatter is measured for our sample and does not include results from other studies, which may have different selection effects. The difference in the degree of evolution between our field sample and published cluster galaxies suggests a more recent formation epoch, around z = 1.5 for field galaxies compared to z > 2.0 for cluster galaxies. The magnitude difference implies that the field early-type galaxies are about 2 Gyr younger than the cluster ellipticals using standard single-burst models. However, the same models imply a significant change in the rest-frame U-B color from then to the present, which is not seen in our sample. Continuous low-level star formation, however, would serve to explain the constant colors over this large magnitude change. A consistent model has 7% of the stellar mass created after the initial burst, using an exponentially decaying star formation rate with an e-folding time of 5 Gyr.

Dynamical evolution of globular cluster systems in clusters of galaxies - I. The case of NGC 1404 in the Fornax cluster

We investigate, via numerical simulations, the tidal stripping and accretion of globular clusters (GCs). In particular, we focus on creating models that simulate the situation for the GC systems of NGC 1404 and NGC 1399 in the Fornax cluster, which have poor (specific frequency $S_{\rm N}$ $\sim$ 2) and rich ($S_{\rm N}$ $\sim$ 10) GC systems respectively. We initially assign NGC 1404 in our simulation a typical $S_{\rm N}$ ($\sim$ 5) for cluster ellipticals, and find that its GC system can only be reduced through stripping to the presently observed value, if its orbit is highly eccentric (with orbital eccentricity of $>$ 0.5) and if the initial scale length of the GCs system is about twice as large as the effective radius of NGC 1404 itself. These stripped GCs can be said to have formed a `tidal stream' of intracluster globular clusters (ICGCs) orbiting the centre of Fornax cluster (many of which would be assigned to NGC 1399 in an imaging study). The physical properties of these GCs (e.g., number, radial distribution) depend on the orbit and initial distribution of GCs in NGC 1404. Our simulations also predict a trend for $S_{\rm N}$ to rise with increasing clustercentric distance - a trend for which there is some observational support in the Fornax cluster.

Prospects for the Determination ofH0through Observation of Multiply Imaged Supernovae in Galaxy Cluster Fields

We assess the possibility of determining the Hubble constant H0 by measuring time delays between multiple images of supernovae gravitationally lensed by rich clusters of galaxies and combining these delay measurements with detailed cluster potential models based on other lensing constraints. Such a lensing determination of H0 would be complementary to those obtained from galaxy-quasar lensing studies and could potentially be better calibrated. We show that relatively low-redshift (z ~ 0.2), significantly elliptical clusters have appreciable lensing cross sections for observable image pairings with tractable time delays on the order of a few years despite large lensing mass scales. We find that a targeted search for such image pairs would be a significant undertaking for current observatories but that it would be appropriate for a facility such as the proposed Large-Aperture Synoptic Survey Telescope.

Circulation Flows: Cooling Flows with Bubble Return

The failure of the XMM-Newton and Chandra X-ray telescopes to detect cooling gas in elliptical galaxies and clusters of galaxies has led many to adopt the position that the gas is not cooling at all and that heating by an active nucleus in the central E or cD galaxy is sufficient to offset radiative cooling. In this paper we explore an idealized limiting example of this point of view in which hot, buoyant bubbles formed near the center return the inflowing, radiatively cooling gas to distant regions in the flow. We show that idealized steady state, centrally heated non-cooling flows can indeed be constructed. In addition, the emission-weighted temperature profiles in these circulating flows resemble those of normal cooling flows. However, these solutions are valid only (1) for a range of bubble parameters for which there is no independent justification, (2) for a limited spatial region in the cooling flow, and (3) for a limited period of time after which cooling seems inevitable. Our exploration of non-cooling flows is set in the context of galaxy/group flows.

Observing Massive Galaxy Formation*

A major goal of contemporary astrophysics is understanding the origin of the most massive galaxies in the universe, particularly nearby ellipticals and spirals. Theoretical models of galaxy formation have existed for many decades, although observational evidence at both low and high redshifts is only beginning to put constraints on different ideas. We briefly describe these observations and how they are revealing the methods by which galaxies form by contrasting and comparing fiducial rapid collapse and hierarchical formation model predictions. The available data show that cluster ellipticals must have rapidly formed at z > 2 , and that up to 50% of all massive galaxies at z ~ 2.5 are involved in major mergers. While the former is consistent with the monolithic collapse picture, we argue that hierarchical formation is the only model that can reproduce all the available observations.

Old elliptical galaxies at z = 1.5 and the Kormendy relation

Deep spectroscopy of the two mJy radio galaxies LBDS 53W069 and 53W091 has previously shown them to have old (>3 Gyr) stellar populations at z=1.5. Here we present the results of HST observations of these galaxies. 53W069 has an r^1/4 profile in both the F814W & F110W data with an effective radius of 0.30 arcsec (2.7 kpc). The restframe U-B colour gradient requires a stellar population of super-solar (3Z_sun) metallicity that formed on a very short timescale at high redshift (z>5). 53W091 has a regular r^1/4 profile in F110W with an effective radius of 0.32 arcsec (2.9 kpc). The F814W profile is more extended and is consistent with a blue exponential disk that contributes 20% of the flux within r_e. The restframe U-B colour gradient is significantly larger than that observed in field ellipticals at z<1, implying a stellar population of mixed metallicity (1-3Z_sun) that formed in a high-redshift rapid burst. We have compared these two LBDS radio galaxies with the Kormendy relation of ten 3CR radio galaxies at z=0.8. The LBDS galaxies follow the same relation as the more radio-luminous 3CR galaxies, assuming passive evolution of their stellar populations, although they are smaller than the 3CR galaxies whose mean effective radius is 12 kpc. Their sizes and radio luminosities are consistent with scaling relations applied to the 3CR galaxies, in which both radio power and effective radius scale with galaxy mass. Compared with a sample of z=0.4 cluster ellipticals, 53W069 & 53W091 lie well within the scatter of the Kormendy relation. We conclude that the hosts of these mJy radio sources at z=1.5 are passively-evolving elliptical galaxies that will evolve into ordinary L* ellipticals by the present day. (Abridged)

The Incidence of CN‐strong Giants in Globular Clusters

ABSTRACTUsing a database from the published literature, the fraction of CN‐enhanced red giants in globular clusters is correlated against a number of cluster physical properties. Newer data verify the earlier result of Norris that the fraction of CN‐strong stars correlates with cluster ellipticity, suggesting that the production of CN enhancements among globular cluster stars is somehow influenced by the angular momentum content of the parent cluster. This correlation with ellipticity appears to be more pronounced than any dependence on cluster mass or metallicity.

Origin of galactic and extragalactic magnetic fields

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength ${(10}^{\ensuremath{-}7}--{10}^{\ensuremath{-}5}\mathrm{G})$ and huge spatial scale $(\ensuremath{\lesssim}1\mathrm{Mpc}).$ It is generally assumed that magnetic fields in spiral galaxies arise from the combined action of differential rotation and helical turbulence, a process known as the \ensuremath{\alpha}\ensuremath{\omega} dynamo. However, fundamental questions concerning the nature of the dynamo as well as the origin of the seed fields necessary to prime it remain unclear. Moreover, the standard \ensuremath{\alpha}\ensuremath{\omega} dynamo does not explain the existence of magnetic fields in elliptical galaxies and clusters. The author summarizes what is known observationally about magnetic fields in galaxies, clusters, superclusters, and beyond. He then reviews the standard dynamo paradigm, the challenges that have been leveled against it, and several alternative scenarios. He concludes with a discussion of astrophysical and early-Universe candidates for seed fields.

Galaxy formation in pre-heated intergalactic media

We outline a scenario of galaxy formation in which the gas in galaxy-forming regions was pre-heated to high entropy by vigorous energy feedback associated with the formation of stars in old ellipticals and bulges and with active galactic nuclei activity. Such pre-heating probably occurred at redshifts of z∼2–3, and can produce the entropy excess observed today in low-mass clusters of galaxies without destroying the bulk of the Lyα forest. Subsequent galaxy formation is affected by the pre-heating, because the gas no longer follows the dark matter on galaxy scales. The hot gas around galaxy haloes has very shallow profiles and emits only weakly in the X-ray range. Cooling in a pre-heated halo is not inside–out, because the cooling efficiency does not change significantly with radius. Only part of the gas in a protogalaxy region can cool and be accreted into the final galaxy halo by the present time. The accreted gas is probably in diffuse clouds and so does not lose angular momentum to the dark matter. Cluster ellipticals are produced by mergers of stellar systems formed prior to the pre-heating, while large galaxy discs form in low-density environments where gas accretion can continue to the present time.

Recent Dynamical Evolution of Galaxy Clusters

Evidence is presented for a recent evolution of the relaxation processes in clusters of galaxies, using large optical and X-ray cluster samples. The criteria of the cluster relaxation used are the cluster ellipticity, the ICM temperature and X-ray cluster luminosity. We find evidence of varying strength and significance of all three indicators evolving with redshift for z <0.15. This result supports the view that clusters have mostly stopped undergoing mergers and accreting matter, as expected in a low-Omega_matter Universe, and are now in the process of gravitational relaxation, which reduces their flattening, their ICM temperature (shock heated during the merging phase), and their X-ray luminosity. These results support similar recent claims of Melott, Chambers and Miller (2001).

On the formation of globular cluster systems in a hierarchical Universe

ABSTRA C T We have investigated the formation of globular cluster (GC) systems in the fiducial semianalytic model of galaxy formation of Cole et al., by assuming that GCs are formed at high redshifts ﷿z . 5fi in protogalactic fragments, and during the subsequent gas-rich merging of these fragments. Under these assumptions we have simulated the GC systems of 450 elliptical galaxies, and find that the majority (93 per cent) are intrinsically bimodal in metallicity. We find that, in the mean, the metal-rich GC subpopulations are younger than the metal-poor GC subpopulations, with ages of 9 and 12 Gyr respectively, and that the mean ages of the metalrich GCs are dependent upon host galaxy luminosity and environment (halo circular velocity), whereas the metal-poor GCs are not. We find that the continued gaseous merging of the protogalactic fragments leads to significant age-structure amongst the metal-rich GCs. These GCs exhibit a large age-range (5 to 12 Gyr), which increases for low-luminosity galaxies, and for galaxies in low circular velocity haloes. Moreover, the metal-rich GCs associated with low-luminosity field and/or group ellipticals are , 2 Gyr younger than the metal-rich GCs in luminous cluster ellipticals. We find that the total GC populations scale with host galaxy luminosity as NGC / L 1:25 V;gal , a result in agreement with observations of luminous elliptical galaxies. This scaling is due to a systematic increase in the M/L ratios of the galaxy haloes with luminosity for LV;gal . L * galaxies in the model. A comparison between the luminosity growth of the model ellipticals and their GC formation indicates that mergers do not significantly affect SN at z , 2. We find the mean colours of the both the metal-rich and metal-poor GCs exhibit only a weak dependence upon host galaxy luminosity, a result consistent with contemporary observations. We conclude that gaseous merging, the bulk of which occurs at 1 # z # 4i n ourLCDM model, leads to the formation of the metalrich peak of the GC systems of elliptical galaxies. We suggest that the formation and subsequent truncation of the metal-poor GCs in the protogalactic fragments is closely related to the star formation rate in these fragments, which may have been significantly higher at very early times.

The Origin, Evolution, and Classification of Field and Cluster Galaxy Populations

In this thesis I address the formation and evolution of lowmass galaxies and investigate their importance in the general galaxy evolution/formation process. Currently favored models of galaxy formation, such as cold dark matter (CDM), suggest that the first galaxies to form in the universe have low masses and sizes, similar to modern dwarf galaxies, that later merge to form more massive systems. The most common galaxies in the nearby universe are dwarf ellipticals in clusters that could be descendants of these early galaxies. This thesis addresses two questions related to these objects: (1) Are the dwarf elliptical galaxies seen in clusters an old primordial population that survived since their initial formation, or are these dwarfs the end product of some evolutionary dynamical process? (2) Do galaxy mergers occur in the universe, and if so, how does this merging evolve through time? To determine whether dwarf elliptical galaxies are an old cluster population, kinematic, structural, and photometric information for these objects was obtained using the WisconsinIndiana-Yale-NOAO 3.5 m telescope. Radial velocities of dwarf elliptical galaxies in the Virgo Cluster suggest that these objects are not an old cluster population but were accreted sometime in the past (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, ApJ, 559, 791). Kinematics also demonstrate that these objects could have originated from a more massive progenitor. Photometry of dwarf elliptical galaxies in the Perseus Cluster further suggests that the stellar populations of these objects are old, with little to no recent star formation or population gradients. The broadband UBR colors of these objects are also consistent with color differences originating from metallicity. Low-mass galaxies in the Perseus Cluster however do not follow the color-magnitude relationship found for the giant elliptical galaxies, with some too red and others too blue for their magnitudes. Through comparisons with LCDM models of galaxy formation, I show that the blue lowmass cluster objects in Perseus are consistent with being old CDM galaxies. Low-mass cluster galaxies that are redder than the color-magnitude relationship are shown to be composed of stars with high metallicities. These red low-mass cluster galaxies must have very high mass-to-light ratios near 250 in solar units or must be the remnants of higher mass galaxies whose stellar material was stripped during high-speed impulsive interactions with other cluster galaxies. To address the issue of galaxy mergers, I develop and use a morphological system to identify robustly, and unambiguously, objects undergoing major mergers (C. J. Conselice, M. A. Bershady, & A. Jangren 2000, ApJ, 529, 886; C. J. Conselice, M. A. Bershady, & J. S. Gallagher III 2000, A&A, 354, L21). This system uses an asymmetry index, such that galaxies undergoing mergers have asymmetry values , with the A 1 0.35 range of possible values being 0–2. When this is applied to the galaxies in the Hubble Deep Field–North, an increase with redshift of galaxies undergoing major mergers is found. Depending on the method of computation, the merger fraction evolution out to can be characterized as , where m m z ∼ 3.5 f p (1 z) ranges from 1 to 2.5. The best fit gives a variable m, such that . m p 2.96 0.32z I also investigate the properties of a nearby massive central cluster galaxy, NGC 1275 in the Perseus Cluster, whose unusual properties and structural aspects can be partially accounted for by recent and past mergers (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, AJ, 122, 2281). Finally, after summarizing the results of this thesis, as well as previous work, I present a new galaxy classification system according to which all galaxies can be fitted based on the three most salient properties that drive galaxy evolution: dissipation, star formation, and galaxy interactions.

Metal-rich and Metal-poor Globular Clusters in Ellipticals: Did we Learn Anything? or Constraints on Galaxy Formation and Evolution from Globular Cluster Sub-populations

A brief review on globular cluster sub-populations in galaxies, and their constraints on galaxy formation and evolution is given. The metal-poor and metal-rich sub-populations are put in a historical context, and their properties, as known to date, are summarized. We review why the study of these sub-populations is extremely useful for the study of galaxy formation and evolution, but highlight a few caveats with the current interpretations. We re-visit the current globular cluster system formation scenarios and show how they boil down to a single scenario for the metal-poor clusters (namely the formation in “universal”, small fragments at high z) and that a hierarchical formation seems favored for the metal-rich clusters.