45 research outputs found
Recent structural evolution of early-type galaxies : size growth from z=1 to z=0
Strong size and internal density evolution of early-type galaxies between z similar to 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z similar to 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (sigma) measurements are taken from the literature for 50 morphologically selected 0.8 < z < 1.2 field and cluster early-type galaxies with typical masses M-dyn = 2 x 10(11) M-circle dot. Sizes (R-eff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby (0.04 < z < 0.08) early-type galaxies extracted from the Sloan Digital Sky Survey for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant difference between the sigma-R-eff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in R-eff at fixed mass between z = 1 and the present is a factor of 1.97 +/- 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z similar to 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations
The Spiderweb galaxy: a forming massive cluster galaxy at z~2
We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letter
A Wide-Field Study of the z~0.8 Cluster RX J0152.7-1357: the Role of Environment in the Formation of the Red-Sequence
[ABRIDGED] We present the first results from the largest spectroscopic survey
to date of an intermediate redshift galaxy cluster, the z=0.834 cluster RX
J0152.7-1357. We use the colors of galaxies, assembled from a D~12 Mpc region
centered on the cluster, to investigate the properties of the red-sequence as a
function of density and clustercentric radius. Our wide-field multi-slit survey
with a low-dispersion prism in the IMACS spectrograph at Magellan allowed us to
identify 475 new members of the cluster and its surrounding large scale
structure with a redshift accuracy of dz/(1+z)~1% and a contamination rate of
~2% for galaxies with i<23.75 mag. We combine these new members with the 279
previously known spectroscopic members to give a total of 754 galaxies from
which we obtain a mass-limited sample of 300 galaxies with stellar masses
M>4x10^{10} M_sun. We find that the red galaxy fraction is 93+/-3% in the two
merging cores of the cluster and declines to a level of 64+/-3% at projected
clustercentric radii R>~3 Mpc. At these large projected distances, the
correlation between clustercentric radius and local density is nonexistent.
This allows an assessment of the influence of the local environment on galaxy
evolution, as opposed to mechanisms that operate on cluster scales. Even beyond
R>3 Mpc we find an increasing fraction of red galaxies with increasing local
density. The red fraction at the highest local densities in two groups at R>3
Mpc matches the red fraction found in the two cores. Strikingly, galaxies at
intermediate densities at R>3 Mpc, that are not group members, also show signs
of an enhanced red fraction. Our results point to such intermediate density
regions and the groups in the outskirts of the cluster, as sites where the
local environment influences the transition of galaxies onto the red-sequence.Comment: 15 pages, 10 figures, 1 table, accepted for publication in ApJ,
expanded introduction and additional references adde
Discovery of Two Distant Type Ia Supernovae in the Hubble Deep Field North with the Advanced Camera for Surveys
We present observations of the first two supernovae discovered with the
recently installed Advanced Camera for Surveys (ACS) on the Hubble Space
Telescope. The supernovae were found in Wide Field Camera images of the Hubble
Deep Field North taken with the F775W, F850LP, and G800L optical elements as
part of the ACS guaranteed time observation program. Spectra extracted from the
ACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNe
Ia) at redshifts z=0.47 and z=0.95. Follow-up HST observations have been
conducted with ACS in F775W and F850LP and with NICMOS in the near-infrared
F110W bandpass, yielding a total of 9 flux measurements in the 3 bandpasses
over a period of 50 days in the observed frame. We discuss many of the
important issues in doing accurate photometry with the ACS. We analyze the
multi-band light curves using two different fitting methods to calibrate the
supernovae luminosities and place them on the SNe Ia Hubble diagram. The
resulting distances are consistent with the redshift-distance relation of the
accelerating universe model, although evolving intergalactic grey dust remains
as a less likely possibility. The relative ease with which these SNe Ia were
found, confirmed, and monitored demonstrates the potential ACS holds for
revolutionizing the field of high-redshift SNe Ia, and therefore of testing the
accelerating universe cosmology and constraining the "epoch of deceleration".Comment: 11 pages, 8 embedded figures. Accepted for publication in Ap
Evolution of the Color-Magnitude Relation in Galaxy Clusters at z ~1 from the ACS Intermediate Redshift Cluster Survey
We apply detailed observations of the Color-Magnitude Relation (CMR) with the
ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red
sequence is well defined and elliptical and lenticular galaxies lie on similar
CMRs. We analyze CMR parameters as a function of redshift, galaxy properties
and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the
elliptical population in cluster cores is smaller than that of the S0
population, although the two become similar at faint magnitudes. While the
bright S0 population consistently shows larger scatter than the ellipticals,
the scatter of the latter increases in the peripheral cluster regions. If we
interpret these results as due to age differences, bright elliptical galaxies
in cluster cores are on average older than S0 galaxies and peripheral
elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in
the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3
nor significant dependence on cluster mass. Two of our clusters display CMR
zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our
sample. We also analyze the fraction of morphological early-type and late-type
galaxies on the red sequence. We find that, while in the majority of the
clusters most (80% to 90%) of the CMR population is composed of early-type
galaxies, in the highest redshift, low mass cluster of our sample, the CMR
late-type/early-type fractions are similar (~50%), with most of the late-type
population composed of galaxies classified as S0/a. This trend is not
correlated with the cluster's X-ray luminosity, nor with its velocity
dispersion, and could be a real evolution with redshift.Comment: ApJ, in press, 27 pages, 22 figure
Strong Lensing Analysis of A1689 from Deep Advanced Camera Images
We analyse deep multi-colour Advanced Camera images of the largest known
gravitational lens, A1689. Radial and tangential arcs delineate the critical
curves in unprecedented detail and many small counter-images are found near the
center of mass. We construct a flexible light deflection field to predict the
appearance and positions of counter-images. The model is refined as new
counter-images are identified and incorporated to improve the model, yielding a
total of 106 images of 30 multiply lensed background galaxies, spanning a wide
redshift range, 1.0z5.5. The resulting mass map is more circular in
projection than the clumpy distribution of cluster galaxies and the light is
more concentrated than the mass within . The projected mass profile
flattens steadily towards the center with a shallow mean slope of
, over the observed range,
r, matching well an NFW profile, but with a relatively high
concentration, . A softened isothermal profile
(\arcs) is not conclusively excluded, illustrating that
lensing constrains only projected quantities. Regarding cosmology, we clearly
detect the purely geometric increase of bend-angles with redshift. The
dependence on the cosmological parameters is weak due to the proximity of
A1689, , constraining the locus, .
This consistency with standard cosmology provides independent support for our
model, because the redshift information is not required to derive an accurate
mass map. Similarly, the relative fluxes of the multiple images are reproduced
well by our best fitting lens model.Comment: Accepted by ApJ. For high quality figures see
http://wise-obs.tau.ac.il/~kerens/A168
Role of the AP2 β-Appendage Hub in Recruiting Partners for Clathrin-Coated Vesicle Assembly
Adaptor protein complex 2 α and β-appendage domains act as hubs for the assembly of accessory protein networks involved in clathrin-coated vesicle formation. We identify a large repertoire of β-appendage interactors by mass spectrometry. These interact with two distinct ligand interaction sites on the β-appendage (the “top” and “side” sites) that bind motifs distinct from those previously identified on the α-appendage. We solved the structure of the β-appendage with a peptide from the accessory protein Eps15 bound to the side site and with a peptide from the accessory cargo adaptor β-arrestin bound to the top site. We show that accessory proteins can bind simultaneously to multiple appendages, allowing these to cooperate in enhancing ligand avidities that appear to be irreversible in vitro. We now propose that clathrin, which interacts with the β-appendage, achieves ligand displacement in vivo by self-polymerisation as the coated pit matures. This changes the interaction environment from liquid-phase, affinity-driven interactions, to interactions driven by solid-phase stability (“matricity”). Accessory proteins that interact solely with the appendages are thereby displaced to areas of the coated pit where clathrin has not yet polymerised. However, proteins such as β-arrestin (non-visual arrestin) and autosomal recessive hypercholesterolemia protein, which have direct clathrin interactions, will remain in the coated pits with their interacting receptors
Membrane fission by dynamin: what we know and what we need to know
Abstract The large GTPase dynamin is the first protein shown to catalyze membrane fission. Dynamin and its related proteins are essential to many cell functions, from endocytosis to organelle division and fusion, and it plays a critical role in many physiological functions such as synaptic transmission and muscle contraction. Research of the past three decades has focused on understanding how dynamin works. In this review, we present the basis for an emerging consensus on how dynamin functions. Three properties of dynamin are strongly supported by experimental data: first, dynamin oligomerizes into a helical polymer; second, dynamin oligomer constricts in the presence of GTP; and third, dynamin catalyzes membrane fission upon GTP hydrolysis. We present the two current models for fission, essentially diverging in how GTP energy is spent. We further discuss how future research might solve the remaining open questions presently under discussion