Cluster Ellipticals Research Articles

Metallicity distributions of elliptical galaxies and globular cluster systems - Absorption line strength gradients in elliptical galaxies

Metallicity distributions of elliptical galaxies and globular cluster systems - Absorption line strength gradients in elliptical galaxies

Distant Radio Galaxies: Probes of the Formation of Massive Galaxies

In this paper we review some of the evidence that the hosts of powerful high redshift (1 ≲ z ≲ 5) radio galaxies (HzRGs) are the progenitors of present day brightest cluster galaxies (BCGs). On the basis of HST imaging we argue that the scenario describing the formation of BCGs consists of at least two important stages. At z > 2 a significant fraction of the stellar mass of a BCG is formed during a massive burst of star-formation. By z ~ 1, well developed massive ellipticals are already observed and from then until the present epoch, the total mass in stars of the BCG will further grow by a factor of order 3, mainly through the accretion of cluster ellipticals.

Environmental Effects in the Stellar Populations of Elliptical Galaxies

While early work successfully modelled the stellar population of elliptical galaxies as old, single-age, metal-rich systems, there is evidence that a small amount of recent (i.e. a few Gyrs ago) star formation has occurred in some nearby ellipticals (O’Connell, 1980; Rose, 1985; Pickles, 1985; Bica, 1988). Elliptical galaxies reside in environments ranging from the dense cores of rich clusters to isolated field regions. It has long been suggested that the global properties of ellipticals depend on environment. For example, McClure and Van den Bergh (1968) noted apparent differences in the 380–450nm colours between a set of cluster ellipticals and three from the field. Recent work has tended to support the view that the properties of ellipticals depend on environment (e.g. de Carvalho and Djorgovski, 1992). The most likely cause of these differences is the presence of a small amount of recent star formation in the field ellipticals. This environmental dependency may have important consequences for the use of ellipticals as distance indicators.

Spontaneous and Induced Star Formation in the LMC

Spontaneous and triggered star formation in the LMC is discussed with data on star clusters ages and positions. The supershell LMC4 and the stellar arcs in the same region are suggested to be triggered by GRBs, the progenitors of which might have escaped the old elliptical cluster NGC1978, close to which are a number of X-ray binaries and the SGR/SNR N49.

A simultaneous maximum likelihood approach for galaxy-galaxy lensing and cluster lens reconstruction

In a previous paper we investigated means for constraining the mass distribution of cluster galaxies by weak lensing. We concluded that a comprehensive method should treat the lensing effects of individual cluster galaxies and those resulting from a general cluster component simultaneously. To this end we now develop a non-parametric maximum likelihood cluster reconstruction algorithm that can implicitly take into account the presence of cluster galaxies. The method includes an entropy-like regularization prescription and directly uses the ellipticities of individual source galaxy images as observables rather than relying on an averaged ellipticity field. The mass distribution of cluster galaxies is described by parametrized models. For each set of galaxy parameters the cluster reconstruction algorithm allows us to determine the best representation of the global underlying cluster component that is consistent with the presence of the cluster galaxies and the observed image ellipticities of background galaxies. Tests with simulations yield convincing and robust results. We apply the method to a WFPC2 image of the cluster Cl0939+4713, and obtain a detection of the lensing effects of luminous elliptical cluster galaxies. We consider this application as a successful test of our technique. However, the small size of the image we analysed does not yet allow us to draw strong conclusions regarding the mass distribution of cluster galaxies.

Tides, Interactions, and Fine-Scale Substructures in Galaxy Clusters

We present the results of a study on galaxy interactions, tides, and other processes which produce luminous fine-scale substructures in the galaxy clusters: Coma, Perseus, Abell 2199, AWM 3 and AWM 5. All unusual structures in these clusters can be categorized in seven morphologies: interacting galaxies, multiple galaxies (non-interacting), distorted galaxies, tailed galaxies, line galaxies, dwarf galaxy groups and galaxy aggregates. The various morphologies are described, and a catalog is presented of 248 objects in these five clusters along with color, and positional information obtained from CCD images taken with the WIYN 3.5m telescope in broadband B and R filters. Distorted, interacting, and fine-scale substructures have a range of colors extending from blue objects with B-R $\approx$ 0, to redder colors at B-R $\approx$ 2.5. We also find that the structures with the most disturbed morphology have the bluest colors. Additionally, the relative number distributions of these structures suggests that two separate classes of galaxy clusters exist: one dominated by distorted structures and the other dominated by galaxy associations. The Coma and Perseus clusters, respectively, are proposed as models for these types of clusters. Possible mechanisms for the production of fine-scale substructure are reviewed and compared to observations of z $\approx$ 0.4 Butcher-Oemler clusters. We conclude, based on color, positional, and statistical data, that the most likely mechanism for the creation of these structures is through an interaction with the gravitational potential of the cluster, possibly coupled with effects of high speed interactions with large cluster ellipticals.

Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds

We present a full-sky 100 μm map that is a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps, with the zodiacal foreground and confirmed point sources removed. Before using the ISSA maps, we remove the remaining artifacts from the IRAS scan pattern. Using the DIRBE 100 and 240 μm data, we have constructed a map of the dust temperature so that the 100 μm map may be converted to a map proportional to dust column density. The dust temperature varies from 17 to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5. The result of these manipulations is a map with DIRBE quality calibration and IRAS resolution. A wealth of filamentary detail is apparent on many different scales at all Galactic latitudes. In high-latitude regions, the dust map correlates well with maps of H I emission, but deviations are coherent in the sky and are especially conspicuous in regions of saturation of H I emission toward denser clouds and of formation of H2 in molecular clouds. In contrast, high-velocity H I clouds are deficient in dust emission, as expected. To generate the full-sky dust maps, we must first remove zodiacal light contamination, as well as a possible cosmic infrared background (CIB). This is done via a regression analysis of the 100 μm DIRBE map against the Leiden-Dwingeloo map of H I emission, with corrections for the zodiacal light via a suitable expansion of the DIRBE 25 μm flux. This procedure removes virtually all traces of the zodiacal foreground. For the 100 μm map no significant CIB is detected. At longer wavelengths, where the zodiacal contamination is weaker, we detect the CIB at surprisingly high flux levels of 32 ± 13 nW m-2 sr-1 at 140 μm and of 17 ± 4 nW m-2 sr-1 at 240 μm (95% confidence). This integrated flux ~2 times that extrapolated from optical galaxies in the Hubble Deep Field. The primary use of these maps is likely to be as a new estimator of Galactic extinction. To calibrate our maps, we assume a standard reddening law and use the colors of elliptical galaxies to measure the reddening per unit flux density of 100 μm emission. We find consistent calibration using the B-R color distribution of a sample of the 106 brightest cluster ellipticals, as well as a sample of 384 ellipticals with B-V and Mg line strength measurements. For the latter sample, we use the correlation of intrinsic B-V versus Mg2 index to tighten the power of the test greatly. We demonstrate that the new maps are twice as accurate as the older Burstein-Heiles reddening estimates in regions of low and moderate reddening. The maps are expected to be significantly more accurate in regions of high reddening. These dust maps will also be useful for estimating millimeter emission that contaminates cosmic microwave background radiation experiments and for estimating soft X-ray absorption. We describe how to access our maps readily for general use.

Discovery of Red-selected Arcs at [CLC][ITAL]z[/ITAL][/CLC] = 4.04 behind Abell 2390

We describe the properties of a system of red arcs discovered at z=4.04 around the cluster A2390 ($z=0.23$). These arcs are images of a single galaxy, where the lensing is compounded by an elliptical cluster member lying close to the critical curve of the cluster. The combined magnification is estimated to be $\sim 20$, using HST images, depending the gradient of the model potentials, implying an unlensed magnitude for the source of $I_{AB}=25.5$. Keck spectroscopy reveals a continuum well fitted by B-stars and attenuated by the Lyman-series forest with an opacity consistent with z$\sim$4 QSO spectra, making the arcs relatively red. Damped Ly$\alpha$ absorption is observed at the source redshift with a relatively high column, n$_{HI}=3\times 10^{21}$. Curiously, Ly$\alpha$ emission is spatially separated ($\sim 0.5$kpc) from the bright stellar continuum and lies $\sim$ 300Km/s redward of interstellar absorption lines at z=4.04. Similar redwardl of young galaxies. Finally, we briefly comment on the notorious ``straight arc'' in A2390, which is resolved into two unrelated galaxies at z=0.913 and z=1.033.

Evolution of the near-infrared luminosity function in rich galaxy clusters

We present the K-band (2.2 microns) luminosity functions of the X-ray luminous clusters MS1054-0321 (z=0.82), MS0451-0305 (z=0.55), Abell 963 (z=0.206), Abell 665 (z=0.182) and Abell 1795 (z=0.063) down to absolute magnitudes M_K = -20. Our measurements probe fainte absolute magnitudes than do any previous studies of the near-infrared luminosity function of clusters. All clusters are found to have similar luminosity functions within the errors, when the galaxy populations are evolved to redshift $z=0$. It iw known that the most massive bound systems in the Universe at all redshifts are X-ray luminous clusters. Therefore, assuming that the clusters in our sample correspond to a single population seen at different redshifts, the results here imply that not only had the stars in present-day ellipticals in rich clusters formed by z=0.8, but that they existed in as luminous galaxies then as they do today. Additionally, the clusters have K-band luminosity functions which appear to be consistent with the Kband field luminosity function in the range -24 < M_K < -22, although the uncertainties in both the field and cluster samples are large.

The Identification of Quasars behind Elliptical Galaxies and Clusters of Galaxies

The detection of resonance absorption lines against known objects such as individual galaxies and clusters of galaxies is a powerful approach for studying the gas content of these systems. We describe an efficient method of identifying background quasars suitable for absorption-line studies. In this finding technique, we identify serendipitous X-ray sources, about one-eighth of which are suitably bright quasars at moderate redshift. We identify 16 new quasars and galaxies with active galactic nuclei (AGNs) and confirm five known quasars and AGNs, superposed behind elliptical galaxies and clusters of galaxies. We also present three QSO/AGN candidates with uncertain redshift identifications.

Tightness of the Color‐Magnitude Relation of Elliptical Galaxies and the Epoch of Major Galaxy Merging

We investigate a one-zone chemophotometric evolution model of disk-disk galaxy mergers in order to clarify whether galaxy mergers with a widely spread merging epoch can reproduce reasonably well the observed small scatter of the color-magnitude (C-M) relation in cluster ellipticals at low and intermediate redshift (z < 1). We consider that merger-progenitor disks begin to consume interstellar gas at a moderate rate from z ~ 5 and then merge to form an elliptical with a secondary starburst at z = zmerge. We find that even if the epoch of galaxy merging is rather extended (0.3 < zmerge < 3.0), the dispersion in the rest-frame U-V color among galaxy mergers is well within the observed one (~0.05 mag at z = 0). We also find that the zmerge is required to be within a certain range to keep the observed C-M relation tight at a given z. For example, the required range of zmerge in galaxy mergers between Sa disks is 1.3 < zmerge < 3.0 for cluster ellipticals at z = 0.895, 0.9 < zmerge < 3.0 for z = 0.55, and 0.3 < zmerge < 3.0 for z = 0. The main reason for the derived small scatter is that younger stellar populations, which are formed during the secondary starburst of galaxy mergers, are formed preferentially from more metal-enriched interstellar gas. This result reinforces Worthey's suggestion of 1996 that the age-metallicity conspiracy, which means that younger stellar populations are preferentially more metal-enriched, can operate to keep the tight C-M relation. These numerical results imply that the observed small scatter in the C-M relation at low and intermediate redshift (z < 1) does not necessarily require the coevality of elliptical galaxies in clusters or their formation at high z, which has been conventionally believed in the classical, passive evolution picture.

The Properties of Poor Groups of Galaxies. I. Spectroscopic Survey and Results

The Properties of Poor Groups of Galaxies. I. Spectroscopic Survey and Results

Chemical enrichment and the origin of the colour-magnitude relation of elliptical galaxies in a hierarchical merger model

In this paper, we present a model of the formation and chemical enrichment of elliptical galaxies that differs from the conventional picture in two fundamental ways. (i) Ellipticals do not form in a single monolithic collapse and burst of star formation at high redshift. Instead, most of their stars form at modest rates in disc galaxies, which then merge to form the ellipticals. (ii) Galaxies do not undergo ‘closed-box’ chemical evolution. Instead, metals can be transferred between the stars, cold gas and hot gas haloes of the galaxies. It is assumed that metals are ejected out of disc galaxies during supernova explosions and that these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass–metallicity relation for the disc systems. Large ellipticals are more metal-rich because they are formed from the mergers of larger discs. We use semi-analytic techniques to follow the formation, evolution and chemical enrichment of cluster elliptical galaxies in a merging hierarchy of dark matter haloes. The inclusion of the new metallicity-dependent spectral synthesis models of Bruzual & Charlot enables us to compute the colours, line indices and mass-to-light ratios of these galaxies. We find that with physically realistic parameters and with the assumption that feedback is efficient, even in massive galaxies, we are able to reproduce the slope and scatter of the colour–magnitude and the Mg2–σ relations for cluster ellipticals. Bright field ellipticals have the same metallicities but are younger than their cluster counterparts, and thus exhibit more scatter in the equivalent widths of their Balmer absorption lines. We are not able to match the increase in M/L for bright ellipticals if we assume their mass to measure purely the total quantity of stars in these objects. We also study the evolution of these relations to high redshift. We show that the luminosity–metallicity relation does not change with redshift, but the mean stellar age of the galaxies scales with the age of the Universe. This is why the evolution of cluster ellipticals appears to be well described by simple passive evolution. Finally, we study the enrichment history of the intracluster gas. Our models predict that more than 80 per cent of the metals were ejected by galaxies with circular velocities less than 250 km s−1 at redshifts greater than 1. The metallicity of the intracluster medium is thus predicted to evolve very little out to z>1.

Chemical enrichment and the origin of the colour—magnitude relation of elliptical galaxies in a hierarchical merger model

In this paper, we present a model of the formation and chemical enrichment of elliptical galaxies that differs from the conventional picture in two ways: 1)Ellipticals do not form in a single monolithic collapse and burst of star formation at high redshift. Instead, most of their stars form at modest rates in disk galaxies, which then merge to form the ellipticals. 2)Galaxies do not undergo closed-box chemical evolution. Instead, metals can be transferred between the stars, cold gas and the hot gas halos of the galaxies. It is assumed that metals are ejected out of disk galaxies during supernova explosions and these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass-metallicity relation for the disk systems. Big ellipticals are more metal rich because they are formed from the mergers of bigger disks. We use semi-analytic techniques to follow the formation, evolution, and chemical enrichment of cluster ellipticals in a merging hierarchy of dark matter halos. The inclusion of the new metallicity-dependent spectral synthesis models of Bruzual & Charlot allow us to compute the colours, line indices and mass-to- light ratios of these galaxies. If feedback is assumed to be efficient, we are able to reproduce the slope and scatter of the colour-magnitude and the Mg2-sigma relations. We also study the evolution of these relations to high redshift. We show that the luminosity-metallicity relation remains fixed, but the mean stellar ages of the galaxies scale with the age of the Universe. Finally, we study the enrichment history of the intracluster gas. We find that 80% of the metals were ejected by low-mass galaxies at redshifts greater than 1. The metallicity of the ICM thus evolves very little out to z > 1.

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.

The Slope of the Cluster Elliptical Red Sequence: A Probe of Cluster Evolution

The current formation models for cluster elliptical galaxies which incorporate a mechanism for the metallicity enhancement of massive ellipticals predict a change in the observed slope of the red sequence of ellipticals as a function of redshift. This change occurs primarily because the metal-rich galaxies become redder faster than the metal-poorer galaxies with increasing age. This effect is most pronounced within ~ 4 Gyr of formation. Observations of the change of the slope of the red sequence with redshift may thus be used to constrain the formation epoch for galaxy clusters. We examine the red sequence of cluster ellipticals using publicly available HST imaging data for a set of six 0.75>z>0.2 clusters, and a sample of 44 Abell clusters at z<0.15 imaged with the KPNO 0.9 m. We compare the derived slopes of the red sequences with a set of cluster-elliptical evolution models and find good agreement. We demonstrate that such a comparison provides a useful constraint on the formation epoch for clusters, which can be made independently from considerations of absolute color evolution and scatter in the red sequence. From our initial comparison of the observed and model slopes as a function of redshift, we conclude, conservatively, that most of the elliptical galaxies in the cores of clusters must form at z>2.0, and that these galaxies are coeval and passively evolving.

Chemical Evolution of Elliptical Galaxies: Mg2 Gradients and G-Dwarf Problem

Stellar populations of elliptical galaxies were studied extensively in the recent past (eg., Arimoto &amp; Yoshii 1987; Buzzoni et al. 1992; Worthey 1994; Kodama &amp; Arimoto 1997). However, colours and line-strengths analysed by these approaches came mainly from central regions of galaxies and possible variations of stellar population structure within a galaxy were entirely ignored. Kodama &amp; Arimoto (1997) recently show that a colour-magnitude (CM) relation of ellipticals in Coma cluster (Bower, Lucey, &amp; Ellis 1992), and the CM relation of cluster ellipticals in general, should originate from a difference in mean stellar metallicities, as suggested by Arimoto &amp; Yoshii (1987), and cannot be due to an increase of mean stellar age towards luminous galaxies as claimed by Worthey (1994). It is quite often stated that any theory of galaxy formation should explain the origin of CM relation, or in other words, the mass (luminosity) - metallicity relation of elliptical galaxies. However, one should not forget the fact that the CM relation is defined only for stars in the central regions of galaxies; it is possible that the CM relation itself is an observational artifact, since ellipticals show in general radial gradients of metallic-line strengthes, decreasing from a centre towards outer regions (eg., Faber 1977; Davies, Sadler, &amp; Peletier 1993; Gonzalez 1993; Carollo &amp; Danziger 1994). Indeed, Gorgas &amp; Gonzalez (1996) found that ellipticals with larger central Mg2 indices tend to have steeper Mg2 gradient, which suggests that global metallicities of ellipticals are perphaps universal and are independent of galaxy luminosities.

Exploring Cluster Elliptical Galaxies as Cosmological Standard Rods

We explore the possibility of calibrating massive cluster elliptical galaxies as cosmological standard rods using the fundamental plane relation combined with a correction for luminosity evolution. Although cluster ellipticals certainly formed in a complex way, their passive evolution out to redshifts of about 1 indicates that basically all major merging and accretion events took place at higher redshifts. Therefore, a calibration of their luminosity evolution can be attempted. We propose to use the Mg-? relation for that purpose because it is independent of distance and cosmology. We discuss a variety of possible caveats, ranging from dynamical evolution to uncertainties in stellar population models and evolution corrections to the presence of age spread. Sources of major random and systematic errors are analyzed as well. We apply the described procedure to nine elliptical galaxies in two clusters at z = 0.375 and derive constraints on the cosmological model. For the best-fitting ?-free cosmological model we obtain q0 ? 0.1, with 90% confidence limits being 0 < q0 < 0.7 (the lower limit being due to the presence of matter in the universe). If the inflationary scenario applies (i.e., the universe has flat geometry), then, for the best-fitting model, matter and ? contribute about equally to the critical cosmic density (i.e., ?m ? ?? ? 0.5). With 90% confidence, ?? should be smaller than 0.9.

X-ray properties of the distant cluster Cl0016 + 16

We present X-ray data on the distant cluster Cl0016+16 (z=0.5545) from ROSAT PSPC and HRI observations and use them to study the physics of the intracluster medium (ICM) and the dynamical state of the cluster. The surface brightness distribution is not only described by a spherically symmetric model but also by a two-dimensional $\beta $-model fit. Subtracting an elliptical model cluster as defined by the best fit parameters of the two-dimensional model we find significant residuals, indicating an additional, extended X-ray source within the cluster. This source, likely to be a merging subcomponent of the cluster, coincides with a peak in the weak lensing mass map of Smail et. al. (1995). In the course of this analysis we present a new approach to quantify the significance of substructure in cluster X-ray images dominated by Poisson noise and smoothed with a Gauss filter. We determine the radial mass profile integrated out to a radius of 3Mpc and find for the total mass of the cluster a value of $\sim 1.4-3.3 \times 10^{15}$ \msun and $\sim 4.5 \times 10^{14}$ \msun for the gas mass, yielding a gas-to-total mass ratio of 14-32\%. There is no significant radial dependence of the gas-to-total mass ratio in the cluster.

The Mgb- relation of elliptical galaxies at z 0.37

We derive absorption indices of ellipticals in clusters at z=0.37 from medium-resolution spectroscopy together with kinematical parameters. These galaxies exhibit a relationship between the Mgb linestrength and their internal velocity dispersion similar to local dynamically hot galaxies. But for any given sigma, Mgb of the distant ellipticals is significantly lower than the mean value of the nearby sample. The difference of Mgb between the two samples is small and can be fully attributed to the younger age of the distant stellar populations in accordance with the passive evolution model. The low reduction of Mgb at a look-back time of about 5 Gyrs requires that the bulk of the stars in cluster ellipticals have formed at very high redshifts of z_f>2. For the most massive galaxies, where the reduction is even lower, z_f probably exceeds 4. Unlike most methods to measure the evolution of ellipticals using luminosities, surface brightnesses or colours, the Mgb linestrength does not depend on corrections for extinction and cosmic expansion (K-correction) and only very little on the slope of the initial mass function. The combination of a kinematical parameter with a stellar population indicator allows us to study the evolution of very similar objects. In addition, the good mass estimate provided by sigma means that the selection criteria for the galaxy sample as a whole are well controlled. (abriged)