Fraction Of Early-type Galaxies Research Articles

Galaxy groups in the 2dF Galaxy Redshift Survey: effects of environment on star formation

We estimate the fraction of star-forming galaxies in a catalogue of groups, constructed from the 2dF Galaxy Redshift Survey by Merchan & Zandivarez. We use the η spectral type parameter of galaxies and subdivide the sample of galaxies in groups into four types depending on the values of the η parameter following Madgwick et al. We obtain a strong correlation between the relative fraction of galaxies with high star formation and the parent group virial mass. We find that even in the environment of groups with low virial mass M∼1013 M⊙ the star formation of their member galaxies is significantly suppressed. The relation between the fraction of early-type galaxies and the group virial mass obeys a simple power law spanning over three orders of magnitude in virial mass. Our results show quantitatively the way that the presence of galaxies with high star formation rates is inhibited in massive galaxy systems.

Morphological Evolution and the Ages of Early‐Type Galaxies in Clusters

Morphological and spectroscopic studies of high-redshift clusters indicate that a significant fraction of present-day, early-type galaxies was transformed from star-forming galaxies at z < 1. On the other hand, the slow luminosity evolution of early-type galaxies and the low scatter in their color-magnitude relation indicate a high formation redshift of their stars. In this paper we construct models that reconcile these apparently contradictory lines of evidence, and we quantify the effects of morphological evolution on the observed photometric properties of early-type galaxies in distant clusters. We show that in the case of strong morphological evolution the apparent luminosity and color evolution of early-type galaxies are similar to those of a single-age stellar population formed at z = ∞, irrespective of the true star formation history of the galaxies. Furthermore, the scatter in age, and hence the scatter in color and luminosity, is approximately constant with redshift. These results are consequences of the bias: the progenitors of the youngest low-redshift, early-type galaxies drop out of the sample at high redshift. We construct models that reproduce the observed evolution of the number fraction of early-type galaxies in rich clusters and their color and luminosity evolution simultaneously. Our modeling indicates that ~50% of early-type galaxies were transformed from other galaxy types at z < 1, and their progenitor galaxies may have had roughly constant star formation rates prior to morphological transformation. The effect of the progenitor bias on the evolution of the mean M/L ratio and color can be estimated. The progenitor bias is a linear function of the scatter in the color-magnitude relation produced by age variations and is maximal if the observed scatter is entirely due to age differences. After correcting the observed evolution of the mean M/LB ratio for the maximum progenitor bias, we find that the mean luminosity-weighted formation redshift of stars in early-type galaxies z* = 3.0 for Ωm = 0.3 and ΩΛ = 0 and z* = 2.0 for Ωm = 0.3 and ΩΛ = 0.7. Our analysis places the star formation epoch of early-type galaxies later than previous studies, which ignored the effects of progenitor bias. The results are consistent with the idea that (some) Lyman break galaxies are star-forming building blocks of massive early-type galaxies in clusters.

The Galaxy Population of Cluster RX J0848+4453 at [CLC][ITAL]z[/ITAL][/CLC] = 1.27

We present a study of the galaxy population in the IR-selected cluster RX J0848+4453 at z = 1.27, using deep Hubble Space Telescope (HST) NICMOS HF160W and WFPC2 IF814W images of the cluster core. We morphologically classify all galaxies to Ks = 20.6 that are covered by the HST imaging and determine photometric redshifts using deep ground-based BRIzJKs photometry. Of 22 likely cluster members with morphological classifications, 11 (50%) are classified as early-type galaxies, nine (41%) as spiral galaxies, and two (9%) as merger/peculiar. At HST resolution, the second brightest cluster galaxy is resolved into a spectacular merger between three red galaxies of similar luminosity, separated from each other by ≈6 h kpc, with an integrated magnitude K = 17.6 (~3L* at z = 1.27). The two most luminous early-type galaxies also show evidence for recent or ongoing interactions. Mergers and interactions between galaxies are possible because RX J0848+4453 is not yet relaxed. The fraction of early-type galaxies in our sample is similar to that in clusters at 0.5 < z < 1 and consistent with a gradual decrease of the number of early-type galaxies in clusters from z = 0 to z ≈ 1.3. We find evidence that the color-magnitude relation of the early-type galaxies is less steep than in the nearby Coma Cluster. This may indicate that the brightest early-type galaxies have young stellar populations at z = 1.27 but is also consistent with predictions of single-age monolithic models with a galactic wind. The scatter in the color-magnitude relation is ≈0.04 in rest-frame U-V, similar to that in clusters at 0 < z < 1. Taken together, these results show that luminous early-type galaxies exist in clusters at z ≈ 1.3 but that their number density may be smaller than in the local universe. Additional observations are needed to determine whether the brightest early-type galaxies harbor young stellar populations.

Galaxy Morphologies in the Cluster Cl 1358+62 atz = 0.33

We describe the morphological composition of a sample of 518 galaxies in the field of Cl 1358+62 at z = 0.33, drawn from a large Hubble Space Telescope mosaic covering 53 square arcminutes. The sample is complete to I = 22, corresponding to MV = -18.5 in the rest frame. The galaxy morphologies have been independently classified by the authors of this paper and by Alan Dressler. Dressler's classifications place our work in context with the previous MORPHS study and allow us to estimate the scatter between different sets of visual classifications. We restrict most of our analysis to the brighter part of the sample, I < 21 (MV < -19.5), where the scatter between the two sets of classifications is ~1 in morphological type. The scatter doubles at I = 22, presumably owing to the lower signal-to-noise and poorer sampling of faint, small galaxy images. To I = 21 the two sets of classifiers agree on the fraction of early type galaxies (elliptical + S0): 72%. We conclude that Cl 1358+62 does not contain the large population of spiral galaxies found in other studies of clusters at z ~ 0.3 and that there is probably a significant spread in the degree of cluster evolution at intermediate redshift. The two groups of classifiers differ on the relative fraction of S0 and elliptical galaxies. We show that the distributions of ellipticities and bulge/total light cannot resolve this discrepancy. Nonetheless, we can derive significant constraints on physical models for the evolution of the galaxy population in Cl 1358+62. The higher ratio of S0 to elliptical galaxies (1.6) found by DF/MF/PvD requires that the evolution preserve the relative fraction of elliptical, S0 and spiral galaxies. Alternately, the lower ratio (1.1) found by Dressler requires that the evolution preserve the early-type to spiral ratio while increasing the S0 to elliptical ratio. In the latter case, a possible evolutionary mechanism is accretion of galaxies that predominantly evolve to S0 galaxies between z = 0.33 and the present. We use our large body of spectra to make the correspondence between spectral and morphological type. Our data follow the pattern seen in the field at low redshift: emission-line spectra are more prevalent among the later morphological types. The 11 identified k+a galaxies (absorption-line spectra with strong Balmer lines) have S0-Sb morphologies.

Star Formation in Las Campanas Compact Groups

Compact groups (CGs) of galaxies offer an exceptional laboratory for the study of dense galactic environments, where interactions, tidally induced activity, and mergers are expected to be at their highest rate of occurrence. Here we present first results from a new catalog of compact groups, one based on the Las Campanas Redshift Survey (LCRS). Using the equivalent width of [O II] λ3727, we have studied the star formation activity in LCRS CGs: we find strong evidence of depressed star formation in CGs relative to that in loose groups or the field. Although much of this effect can be ascribed to a morphological mix (CGs contain a high fraction of early-type galaxies), there is some evidence that the star formation rate in late-type galaxies is particularly deficient—perhaps only one-half to one-third that of field spiral galaxies. We conclude that gas-stripping mechanisms may play a role in CG environments.

A New Look at Poor Groups of Galaxies

We use a new fiber spectroscopic survey of 12 nearby, poor groups of galaxies to examine the dynamics and evolution of galaxies in these common, but poorly studied, environments. Some of our conclusions are: (1) The nine groups in our sample with diffuse X-ray emission are in fact bound systems with at least 20-50 group members with absolute magnitudes as faint as MB ~ −14 + 5 log10h. (2) Galaxies in each X-ray-detected group have not all merged together because a significant fraction of the group mass lies outside of the galaxies and in a common halo, thereby reducing the rate of galaxy-galaxy interactions. (3) The similarity of the recent star formation histories and the fraction of early type galaxies in some poor groups to those in rich clusters suggest that cluster-specific environmental effects may not play a dominant role in the recent evolution of cluster galaxies. The evolution of group and cluster members may be driven instead by galaxy-galaxy interactions, which are likely to occur with equal frequency in field groups and in groups that have recently fallen into clusters (i.e., subclusters).

Compact Groups of Galaxies

This paper reviews some of the outstanding questions concerning compact groups of galaxies. These relate to the physical nature and dynamical status of the groups, their formation and evolution, and their role in galaxy evolution. The picture that emerges is that compact groups are generally physically dense systems, although often contaminated by optical projections. Their evolution is likely a continuous process of infall, interaction and merging. As new galaxies are added, and previous ones merge, the membership of the group evolves. I suggest that while the size of the group changes little, other physical properties such as total mass, gas mass, velocity dispersion, fraction of early-type galaxies increase with time. This picture is at least qualitatively consistent with observations and provides a natural explanation for the strongest correlations found in compact group samples.

Formation and Evolution of E and S0 Galaxies from HST and Keck Studies of z~0.3-1 Clusters

We have witnessed a dramatic increase over the last five years in results on distant galaxies, in large part because of the high resolution imaging capability of HST, and the multiobject spectroscopic capability of the Keck telescopes. Our program to obtain wide-field, multi-color WFPC2 mosaics with HST of intermediate redshift clusters, and spectroscopic membership and high S/N spectroscopy with LRIS on Keck, has provided new insights into the nature of elliptical and S0 galaxies in the cluster environment over a wide range of densities. In particular, most ellipticals, and a significant fraction of the S0 population, have large luminosity-weighted ages, suggesting that their stellar populations were formed at redshifts beyond z~2, though the existence of substantial numbers of major mergers in MS 1054-03 at z=0.83 suggests that final assembly of such galaxies may not have occurred until much later for a significant fraction of early-type galaxies.

A Group-Group Merger at a Redshift of [ITAL]z[/ITAL] = 0.84?

We present a dynamical study of the Cl 0023+0423 system at a redshift of z = 0.84. This system consists of two components separated in velocity space by ~2900 km s-1 and on the plane of the sky by ~0.23 h-1 Mpc. A kinematic analysis indicates that the two components are a poor cluster with a velocity dispersion of 415 km s-1 and a mass of ~3?6 ? 1014 h-1 M?, and a less massive group with a velocity dispersion of 158 km s-1 and a mass of ~1013 h-1 M?. The dynamics of galaxy groups at high redshift can provide important insights into the creation of present-day galaxy clusters. Therefore, we have performed a dynamical study on this system in order to determine whether the two groups are infalling. This analysis includes an analytic two-body calculation and N-body simulations. The results of both studies indicate that the system is most likely not bound, but simply a chance projection on the sky; however, within the observational uncertainties, there exist bound solutions where the two galaxy groups are currently moving toward each other and will eventually merge into a larger system of galaxies. We have run 1000 N-body simulations with random initial conditions based on the observed parameters of the Cl 0023+0423 groups. A statistical analysis of these simulations indicates that there is a 20% chance that the two groups will merge. If the Cl 0023+0423 system does merge, it will appear as a cluster on the sky, as well as in velocity space, within 1?2 Gyr. The cluster will evolve dynamically for more than 3 Gyr, appearing during this time more similar to an open, irregular cluster. The final merged system has a velocity dispersion that is consistent with a local Abell richness class 1 cluster. The morphological analysis of the galaxy populations of Cl 0023+0423 suggests that both groups are largely dominated by spiral galaxies. Early-type fractions are 33% or less. These modest early-type fractions have implications for both cluster formation and group evolution. Studies of open clusters at z = 0.31?0.54 indicate that they have early-type fractions between 45%?80%. If the Cl 0023+0423 system is the predecessor of such a cluster, the comparison may suggest that some fraction of early-type galaxies are formed between redshifts of z ~ 0.8 and z ~ 0.3; however, the morphological fractions are still highly uncertain. In addition, the modest early-type fractions in both groups may be inconsistent with the strong correlation between velocity dispersion and early-type fraction observed in nearby groups of galaxies. Both groups apparently have relatively low early-type populations, irrespective of their velocity dispersion. If the groups of Cl 0023+0423 are typical of galaxy groups at high redshift, and if high-redshift groups are the progenitors of local groups, this result may also imply that some early-type formation is occurring at redshifts of z 0.8. These results do not preclude the formation of early-type galaxies at very high redshift, as many observations suggest; however, the observations of the Cl 0023+0423 system may imply that a fraction of the early-type population is forming and/or undergoing significant evolution at redshifts of z < 1.

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

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

Evolution of X-Ray Clusters of Galaxies with Active Protogalaxies

We consider heating due to proto galaxies in the formation process of clusters of galaxies, since much metal is observed in the intracluster gas which must be ejected from protogalaxies vir strong galactic winds and the metal abundance in the intracluster gas correlates with the fraction of early type galaxies in clusters (Arnaud 1993). We also consider radiative cooling. From the difference between the observed LX - TX relation of X-ray cluster (Hatsukade 1989), LX ∝ T2.7-3.3X, and the prediction from the self-similar model (Kaiser 1986), LX ∝ T2X, it is pointed out that physical processes which are not taken into account in the self-similar model, e.g., effect of radiative cooling, and/or effect of proto galaxy heating, play a important role in the formation process (Evrard and Henry 1991, Kaiser 1991).

Quasar Clustering: Evidence for an Increase with Redshift and Implications for the Nature of Active Galactic Nuclei

The evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3 0.0 are excluded at a 0.3% probability level, compared with ~ 0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Ω = 1 cold dark matter in which (1) the difference between the quasar and the galaxy clustering can be explained as a difference in the effective bias and redshift distributions and (2) the quasars, with a lifetime of t ~ 108 yr, sparsely sample halos of mass greater than Mmin ~ 1012-1013 h-1 M☉. We also discuss the possibility that the observed change in the quasar clustering is due to an increase in the fraction of early-type galaxies as quasar hosts at high z.

A Morphological Catalog of Galaxies in the Hubble deep Field

view Abstract Citations (282) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Morphological Catalog of Galaxies in the Hubble deep Field van den Bergh, Sidney ; Abraham, Roberto G. ; Ellis, Richard S. ; Tanvir, Nial R. ; Santiago, Basilio X. ; Glazebrook, Karl G. Abstract We present a catalog of morphological and color data for galaxies with 21 <I_814_ < 25 mag in the Hubble Deep Field (Williams et al. 1996). Galaxies have been inspected and (when possible) independently visually classified on the MDS and DDO systems. Measurements of central concentration and asymmetry are also included in the catalog. The fraction of interacting and merging objects is seen to be significantly higher in the Hubble Deep Field than it is among nearby galaxies. Barred spirals are essentially absent from the deep sample. The fraction of early-type galaxies in the Hubble Deep Field is similar to the fraction of early-types in the Shapley-Ames Catalog, but the fraction of galaxies resembling archetypal grand-design late-type spiral galaxies is dramatically lower in the distant HDF sample. Publication: The Astronomical Journal Pub Date: August 1996 DOI: 10.1086/118020 arXiv: arXiv:astro-ph/9604161 Bibcode: 1996AJ....112..359V Keywords: CATALOGS; GALAXIES: FUNDAMENTAL PARAMETERS; Astrophysics E-Print: 17 pages of text in AAS LaTeX format, plus 6 postscript figures and 6 tables. Accepted for publication in AJ. Two additional figures (Figs. 1 and 8) are large color postscript files, and can be downloaded from the Cambridge HDF webpage at http://www.ast.cam.ac.uk/~abraham/hdf.html full text sources arXiv | ADS | data products NED (271) SIMBAD (270) MAST (1) ESA (1) CDS (1)

Statistical evidence that galaxy companions trigger bars and change the spiral Hubble type

view Abstract Citations (101) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Statistical Evidence That Galaxy Companions Trigger Bars and Change the Spiral Hubble Type Elmegreen, Debra Meloy ; Elmegreen, Bruce G. ; Bellin, Andrew D. Abstract The number of barred (SB-type) galaxies relative to the total number of spiral galaxies in binary and group systems and in the field has been examined. Among field and group galaxies, 0.32 +/- 0.09 and 0.28 +/- 0.02 of all early Hubble-type spirals are SB, whereas in binary systems, 0.50 +/- 0.05 are SB. There is no obvious correlation like this for intermediate and late-type galaxies. This result implies that close galaxy companions are associated with bar formation, but primarily for early Hubble-types. In addition, the relative fraction of early-type galaxies (Sa-Sb) among all types Sa-Scd is examined. Among binary SB galaxies, the early-type fraction is 0.77 +/- 0.05 whereas among all SA types in any environment the early-type fraction is only 0.44 +/- 0.02. Thus binary galaxies tend to be both barred and early type. Considering recent N-body simulations, we suggest that galaxy interactions produce strong spirals, which, because of their torques, lead to bar formation and a significant mass inflow in the disk. For a prolonged encounter, the mass inflow can apparently change a nonbarred galaxy of intermediate Hubble-type, Sbc-Scd, to a barred galaxy of early Hubble-type, SBa-SBb, by making the galaxy density larger in the inner region and by removing a significant amount of the gas mass and, therefore star formation, from the outer disk. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169424 Bibcode: 1990ApJ...364..415E Keywords: Barred Galaxies; Galactic Clusters; Galactic Structure; Interacting Galaxies; Space Density; Statistical Analysis; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: INTERACTIONS; GALAXIES: STRUCTURE full text sources ADS |