177 research outputs found
The Dark Halo - Spheroid Conspiracy and the Origin of Elliptical Galaxies
Dynamical modeling and strong lensing data indicate that the total density
profiles of early-type galaxies are close to isothermal, i.e., rho_tot ~
r^gamma with gamma approx -2. To understand the origin of this universal slope
we study a set of simulated spheroids formed in isolated binary mergers as well
as the formation within the cosmological framework. The total stellar plus dark
matter density profiles can always be described by a power law with an index of
gamma approx -2.1 with a tendency toward steeper slopes for more compact,
lower-mass ellipticals. In the binary mergers the amount of gas involved in the
merger determines the precise steepness of the slope. This agrees with results
from the cosmological simulations where ellipticals with steeper slopes have a
higher fraction of stars formed in situ. Each gas-poor merger event evolves the
slope toward gamma ~ -2, once this slope is reached further merger events do
not change it anymore. All our ellipticals have flat intrinsic combined stellar
and dark matter velocity dispersion profiles. We conclude that flat velocity
dispersion profiles and total density distributions with a slope of gamma ~ -2
for the combined system of stars and dark matter act as a natural attractor.
The variety of complex formation histories as present in cosmological
simulations, including major as well as minor merger events, is essential to
generate the full range of observed density slopes seen for present-day
elliptical galaxies.Comment: Accepted by the Astrophysical Journal, 17 pages, 12 figure
Short-lived star-forming giant clumps in cosmological simulations of z~2 disks
Many observed massive star-forming z\approx2 galaxies are large disks that
exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We
present the largest sample to date of high-resolution cosmological SPH
simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun
galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong
stellar feedback parameterized as momentum-driven galactic winds. This model
reproduces many characteristic features of this observed class of galaxies,
such as their clumpy morphologies, smooth and monotonic velocity gradients,
high gas fractions (f_g\sim50%) and high specific star-formation rates
(\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps
(Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities.
However, the galactic winds are critical for their subsequent evolution. The
giant clumps we obtain are short-lived and are disrupted by wind-driven mass
loss. They do not virialise or migrate to the galaxy centers as suggested in
recent work neglecting strong winds. By phenomenologically implementing the
winds that are observed from high-redshift galaxies and in particular from
individual clumps, our simulations reproduce well new observational constraints
on clump kinematics and clump ages. In particular, the observation that older
clumps appear closer to their galaxy centers is reproduced in our simulations,
as a result of inside-out formation of the disks rather than inward clump
migration.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the
Astrophysical Journa
Forming Early-Type Galaxies in LambdaCDM Simulations -I. Assembly histories
We present a sample of nine high resolution cosmological simulations in the
mass range of M_vir=7x10^11-4x10^12 M_sun starting from LambdaCDM initial
conditions. Our simulations include primordial radiative cooling,
photoionization, star formation, supernova II feedback, but exclude supernova
driven winds and AGN feedback. The simulated galaxies assemble in two phases,
with the initial growth dominated by compact (r<r_eff) in situ star formation
fueled by cold, low entropy gas streams resulting in a very similar mean
assembly redshift of z_{f,ins}~2.5 for the in situ stellar component in all
galaxies. The late growth is dominated by accretion of old stars formed in
subunits outside the main galaxy (r>r_eff) resulting in an assembly redshift of
z_{f,acc}~0.5-1.5 with much larger scatter. We find a positive correlation
between the fraction of accreted stars and the final mass of our galaxies. We
show that gravitational feedback strongly suppresses late star formation in
massive galaxies contributing to the observed galaxy color bimodality. The
accretion of stellar material is also responsible for the observed size growth
of early-type galaxies. In addition, we find that the dark matter fractions
within the stellar half-mass radii continuously increase towards lower redshift
from about f_DM~0.05 at z~3 to f_DM~0.1-0.3 at z=0. Furthermore, the
logarithmic slope of the total density profile is nearly isothermal at the
present-day (gamma'~1.9-2.2). Finally, the input of gravitational heating
lowers the central dark matter densities in the galaxies, with the effect being
smaller compared to simulations without supernova feedback.Comment: 23 pages, 16 figures, accepted for publication in Ap
The Two Phases of Galaxy Formation
Cosmological simulations of galaxy formation appear to show a two-phase
character with a rapid early phase at z>2 during which in-situ stars are formed
within the galaxy from infalling cold gas followed by an extended phase since
z<3 during which ex-situ stars are primarily accreted. In the latter phase
massive systems grow considerably in mass and radius by accretion of smaller
satellite stellar systems formed at quite early times (z>3) outside of the
virial radius of the forming central galaxy. These tentative conclusions are
obtained from high resolution re-simulations of 39 individual galaxies in a
full cosmological context with present-day virial halo masses ranging from 7e11
M_sun h^-1 < M_vir < 2.7e13 M_sun h^-1 and central galaxy masses between 4.5e10
M_sun h^-1 < M_* < 3.6e11 M_sun h^-1. The simulations include the effects of a
uniform UV background, radiative cooling, star formation and energetic feedback
from SNII. The importance of stellar accretion increases with galaxy mass and
towards lower redshift. In our simulations lower mass galaxies (M_* > 1.7e11 M_sun h^-1) assembly is dominated by accretion and
merging with about 80 per cent of the stars added by the present-day. In
general the simulated galaxies approximately double their mass since z=1. For
massive systems this mass growth is not accompanied by significant star
formation. The majority of the in-situ created stars is formed at z>2,
primarily out of cold gas flows. We recover the observational result of
archaeological downsizing, where the most massive galaxies harbor the oldest
stars. We find that this is not in contradiction with hierarchical structure
formation. Most stars in the massive galaxies are formed early on in smaller
structures, the galaxies themselves are assembled late.Comment: 13 pages, 13 figures, accepted for publication in Ap
Assembly of the Red Sequence in Infrared-Selected Galaxy Clusters from the IRAC Shallow Cluster Survey
We present results for the assembly and star formation histories of massive
(~L*) red sequence galaxies in 11 spectroscopically confirmed,
infrared-selected galaxy clusters at 1.0 < z < 1.5, the precursors to
present-day massive clusters with M ~ 10^15 M_sun. Using rest-frame optical
photometry, we investigate evolution in the color and scatter of the red
sequence galaxy population, comparing with models of possible star formation
histories. In contrast to studies of central cluster galaxies at lower redshift
(z < 1), these data are clearly inconsistent with the continued evolution of
stars formed and assembled primarily at a single, much-earlier time.
Specifically, we find that the colors of massive cluster galaxies at z = 1.5
imply that the bulk of star formation occurred at z ~ 3, whereas by z = 1 their
colors imply formation at z ~ 2; therefore these galaxies exhibit approximately
the same luminosity-weighted stellar age at 1 < z < 1.5. This likely reflects
star formation that occurs over an extended period, the effects of significant
progenitor bias, or both. Our results generally indicate that massive cluster
galaxy populations began forming a significant mass of stars at z >~ 4,
contained some red spheroids by z ~ 1.5, and were actively assembling much of
their final mass during 1 < z < 2 in the form of younger stars. Qualitatively,
the slopes of the cluster color-magnitude relations are consistent with no
significant evolution relative to local clusters.Comment: 24 pages, 9 figures, accepted to Ap
The cosmological size and velocity dispersion evolution of massive early-type galaxies
We analyze 40 cosmological re-simulations of individual massive galaxies with
present-day stellar masses of in order
to investigate the physical origin of the observed strong increase in galaxy
sizes and the decrease of the stellar velocity dispersions since redshift . At present 25 out of 40 galaxies are quiescent with structural
parameters (sizes and velocity dispersions) in agreement with local early type
galaxies. At z=2 all simulated galaxies with (11
out of 40) at z=2 are compact with projected half-mass radii of 0.77
(0.24) kpc and line-of-sight velocity dispersions within the projected
half-mass radius of 262 (28) kms (3 out of 11 are already
quiescent). Similar to observed compact early-type galaxies at high redshift
the simulated galaxies are clearly offset from the local mass-size and
mass-velocity dispersion relations. Towards redshift zero the sizes increase by
a factor of , following with for quiescent galaxies ( for all galaxies). The
velocity dispersions drop by about one-third since , following
with for the quiescent
galaxies ( for all galaxies). The simulated size and dispersion
evolution is in good agreement with observations and results from the
subsequent accretion and merging of stellar systems at which is a
natural consequence of the hierarchical structure formation. A significant
number of the simulated massive galaxies (7 out of 40) experience no merger
more massive than 1:4 (usually considered as major mergers). On average, the
dominant accretion mode is stellar minor mergers with a mass-weighted
mass-ratio of 1:5. (abridged)Comment: 9 pages, 6 figures, accepted for publication in Ap
CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62
We discuss the structural and morphological properties of galaxies in a
z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope
Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology
relation: galaxies with colors of quiescent stellar populations generally have
morphologies consistent with spheroids, and galaxies with colors consistent
with ongoing star formation have disk-like and irregular morphologies. The size
distribution of the quiescent cluster galaxies shows a deficit of compact (<
1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a
result the cluster quiescent galaxies have larger average effective sizes
compared to field galaxies at fixed mass at greater than 90% significance.
Combined with data from the literature, the size evolution of quiescent cluster
galaxies is relatively slow from z~1.6 to the present, growing as
(1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that
physical processes associated with the denser cluster region seems to have
caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower
subsequent growth at z<1.6 compared to galaxies in the lower density field. The
quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower
redshift samples at fixed mass, and their surface-brightness profiles suggest
that they contain extended stellar disks. We argue the cluster galaxies require
dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk
material and to match the relations for ellipticity, stellar mass, size, and
color of early-type galaxies in z<1 clusters.Comment: Accepted for publication in ApJ. 14 pages in emulateapj format.
Replacement includes improvements from referee report, and updates and
additions to reference
A High Statistics Search for Ultra-High Energy Gamma-Ray Emission from Cygnus X-3 and Hercules X-1
We have carried out a high statistics (2 Billion events) search for
ultra-high energy gamma-ray emission from the X-ray binary sources Cygnus X-3
and Hercules X-1. Using data taken with the CASA-MIA detector over a five year
period (1990-1995), we find no evidence for steady emission from either source
at energies above 115 TeV. The derived upper limits on such emission are more
than two orders of magnitude lower than earlier claimed detections. We also
find no evidence for neutral particle or gamma-ray emission from either source
on time scales of one day and 0.5 hr. For Cygnus X-3, there is no evidence for
emission correlated with the 4.8 hr X-ray periodicity or with the occurrence of
large radio flares. Unless one postulates that these sources were very active
earlier and are now dormant, the limits presented here put into question the
earlier results, and highlight the difficulties that possible future
experiments will have in detecting gamma-ray signals at ultra-high energies.Comment: 26 LaTeX pages, 16 PostScript figures, uses psfig.sty to be published
in Physical Review
Sussing merger trees: the impact of halo merger trees on galaxy properties in a semi-analytic model
A halo merger tree forms the essential backbone of a semi-analytic model for galaxy formation and evolution. Recent studies have pointed out that extracting merger trees from numerical simulations of structure formation is non-trivial; different tree building algorithms can give differing merger histories. These differences should be carefully understood before merger trees are used as input for models of galaxy formation. We investigate the impact of different halo merger trees on a semi-analytic model. We find that the z = 0 galaxy properties in our model show differences between trees when using a common parameter set. The star formation history of the universe and the properties of satellite galaxies can show marked differences between trees with different construction methods. Independently calibrating the semi-analytic model for each tree can reduce the discrepancies between the z = 0 global galaxy properties, at the cost of increasing the differences in the evolutionary histories of galaxies. Furthermore, the underlying physics implied can vary, resulting in key quantities such as the supernova feedback efficiency differing by factors of 2. Such a change alters the regimes where star formation is primarily suppressed by supernovae. Therefore, halo merger trees extracted from a common halo catalogue using different, but reliable, algorithms can result in a difference in the semi-analytic model. Given the uncertainties in galaxy formation physics, however, these differences may not necessarily be viewed as significant
Nck2 promotes human melanoma cell proliferation, migration and invasion in vitro and primary melanoma-derived tumor growth in vivo
<p>Abstract</p> <p>Background</p> <p>Nck1 and Nck2 adaptor proteins are involved in signaling pathways mediating proliferation, cytoskeleton organization and integrated stress response. Overexpression of Nck1 in fibroblasts has been shown to be oncogenic. Through the years this concept has been challenged and the consensus is now that overexpression of either Nck cooperates with strong oncogenes to transform cells. Therefore, variations in Nck expression levels in transformed cells could endorse cancer progression.</p> <p>Methods</p> <p>Expression of Nck1 and Nck2 proteins in various cancer cell lines at different stages of progression were analyzed by western blots. We created human primary melanoma cell lines overexpressing GFP-Nck2 and investigated their ability to proliferate along with metastatic characteristics such as migration and invasion. By western blot analysis, we compared levels of proteins phosphorylated on tyrosine as well as cadherins and integrins in human melanoma cells overexpressing or not Nck2. Finally, in mice we assessed tumor growth rate of human melanoma cells expressing increasing levels of Nck2.</p> <p>Results</p> <p>We found that expression of Nck2 is consistently increased in various metastatic cancer cell lines compared with primary counterparts. Particularly, we observed significant higher levels of Nck2 protein and mRNA, as opposed to no change in Nck1, in human metastatic melanoma cell lines compared with non-metastatic melanoma and normal melanocytes. We demonstrated the involvement of Nck2 in proliferation, migration and invasion in human melanoma cells. Moreover, we discovered that Nck2 overexpression in human primary melanoma cells correlates with higher levels of proteins phosphorylated on tyrosine residues, assembly of Nck2-dependent pY-proteins-containing molecular complexes and downregulation of cadherins and integrins. Importantly, we uncovered that injection of Nck2-overexpressing human primary melanoma cells into mice increases melanoma-derived tumor growth rate.</p> <p>Conclusions</p> <p>Collectively, our data indicate that Nck2 effectively influences human melanoma phenotype progression. At the molecular level, we propose that Nck2 in human primary melanoma promotes the formation of molecular complexes regulating proliferation and actin cytoskeleton dynamics by modulating kinases or phosphatases activities that results in increased levels of proteins phosphorylated on tyrosine residues. This study provides new insights regarding cancer progression that could impact on the therapeutic strategies targeting cancer.</p
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