1,757 research outputs found
Superfluidity in the interior-gap states
We investigate superfluidity in the interior-gap states proposed by Liu and
Wilczek. At weak coupling, we find the {\em gapless} interior-gap state
unstable in physically accessible regimes of the parameter space, where the
superfluid density is shown to be always negative. We therefore conclude that
the spatially-uniform interior-gap phase is extremely unstable unless it is
fully gapped; in this case, however, the state is rather similar to
conventional BCS states.Comment: To appear in Physical Review
Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV
We derive stellar masses, ages and star formation histories of massive
early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them
with those measured in a similarly mass-selected sample of field contemporaries
drawn from the GOODS South Field. Robust estimates of these parameters are
obtained by comparing a large grid of composite stellar population models with
8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the
entire relevant domain of emission of the different stellar populations.
Additionally, we present new, deep -band photometry of both fields, giving
access to the critical FUV rest-frame, in order to constrain empirically the
dependence on the environment of the most recent star formation processes. We
find that early-type galaxies, both in the cluster and in the field, show
analogous optical morphologies, follow comparable mass vs. size relation, have
congruent average surface stellar mass densities and lie on the same Kormendy
relation. We also that a fraction of early-type galaxies in the field employ
longer timescales, , to assemble their mass than their cluster
contemporaries. Hence we conclude that, while the formation epoch of early-type
only depends on their mass, the environment does regulate the timescales of
their star formation histories. Our deep -band imaging strongly supports
this conclusions. It shows that cluster galaxies are at least 0.5 mag fainter
than their field contemporaries of similar mass and optical-to-infrared colors,
implying that the last episode of star formation must have happened more
recently in the field than in the cluster.Comment: 20pages, 10 figures. to appear on Ap
Chandra and XMM-Newton Observations of RDCS1252.9-2927, A Massive Cluster at z=1.24
We present deep Chandra and XMM obervations of the galaxy cluster
RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS)
and confirmed by extensive spectroscopy with the VLT at redshift z=1.237. With
the Chandra data, the X-ray emission from the intra-cluster medium is well
resolved and traced out to 500 kpc, thus allowing a measurement of the physical
properties of the gas with unprecedented accuracy at this redshift. We detect a
clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within
1% of the spectroscopic one. By augmenting our spectroscopic analysis with the
XMM data (MOS detectors only), we significantly narrow down the 1 sigma error
bar to 10% for the temperature and 30% for the metallicity, with best fit
values kT = 6.0(+0.7,-0.5) keV, Z = 0.36(+0.12,-0.10) Z_sun. In the likely
hypothesis of hydrostatic equilibrium, we measure a total mass of M_{500} =
(1.9+-0.3)10^14 h_70^{-1}M_sun within R_{Delta=500} = 536 kpc. Overall, these
observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely
the most massive bona-fide cluster discovered to date at z>1. When combined
with current samples of distant clusters, these data lend further support to a
mild evolution of the cluster scaling relations, as well the metallicity of the
intra-cluster gas. Inspection of the cluster mass function in the current
cosmological concordance model (h,Omega_m,Omega_Lambda)=(0.7,0.3,0.7) and
sigma_8=0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z=1.24, in
keeping with number density expectations in the RDCS survey volume.Comment: 9 pages, 1 color figure (fig6.jpg). The Astronomical Journal in press
(Jan 2004). Full resolution preprint available at
http://www.eso.org/~prosati/RDCS1252
The Possible z=0.83 Precursors of z=0 M* Early-type Cluster Galaxies
We examine the distribution of stellar masses of galaxies in MS 1054-03 and
RX J0152.7-1357, two X-ray selected clusters of galaxies at z=0.83. Our stellar
mass estimates, from spectral energy distribution fitting, reproduce the
dynamical masses as measured from velocity dispersions and half-light radii
with a scatter of 0.2 dex in the mass for early-type galaxies. When we restrict
our sample of members to high stellar masses, > 1e11.1 Msun (M* in the
Schechter mass function for cluster galaxies), we find that the fraction of
early-type galaxies is 79 +/- 6% at z=0.83 and 87 +/- 6% at z=0.023 for the
Coma cluster, consistent with no evolution. Previous work with
luminosity-selected samples finds that the early-type fraction in rich clusters
declines from =~80% at z=0 to =~60% at z=0.8. The observed evolution in the
early-type fraction from luminosity-selected samples must predominately occur
among sub-M* galaxies. As M* for field and group galaxies, especially
late-types, is below M* for clusters galaxies, infall could explain most of the
recent early-type fraction growth. Future surveys could determine the
morphological distributions of lower mass systems which will confirm or refute
this explanation.Comment: 5 pages in emulate ApJ format with three color figures. Accepted for
publication in ApJ Letters, v642n2. Updated to correct grammatical and
typographic errors found by the journa
Resonant control of elastic collisions in an optically trapped Fermi gas of atoms
We have loaded an ultracold gas of fermionic atoms into a far off resonance
optical dipole trap and precisely controlled the spin composition of the
trapped gas. We have measured a magnetic-field Feshbach resonance between atoms
in the two lowest energy spin-states, |9/2, -9/2> and |9/2, -7/2>. The
resonance peaks at a magnetic field of 201.5 plus or minus 1.4 G and has a
width of 8.0 plus or minus 1.1 G. Using this resonance we have changed the
elastic collision cross section in the gas by nearly 3 orders of magnitude.Comment: 4 pages, 3 figure
Manipulating the critical temperature for the superfluid phase transition in trapped atomic Fermi gases
We examine the effect of the trapping potential on the critical temperature,
, for the BCS transition to a superfluid state in trapped atomic gases of
fermions. for an arbitrary power law trap is calculated in the
Thomas-Fermi approximation. For anharmonic traps, can be increased by
several orders of magnitude in comparison to a harmonic trap. Our theoretical
results indicate that, in practice, one could manipulate the critical
temperature for the BCS phase transition by shaping the traps confining the
atomic Fermi gases.Comment: 4 page
Mass-Selection and the Evolution of the Morphology-Density Relation from z=0.8 to z=0
We examined the morphology-density relations for galaxy samples selected by
luminosity and by mass in each of five massive X-ray clusters from z=0.023 to
0.83 for 674 spectroscopically-confirmed members. Rest-frame optical colors and
visual morphologies were obtained primarily from Hubble Space Telescope images.
Morphology-density relations (MDR) are derived in each cluster from a complete,
luminosity-selected sample of 452 galaxies with a magnitude limit M_V <
M^{*}_{V} + 1. The change in the early-type fraction with redshift matches
previous work for massive clusters of galaxies. We performed a similar
analysis, deriving MDRs for complete, mass-selected samples of 441 galaxies
with a mass-limit of 10^{10.6} M_{\sun}. Our mass limit includes faint objects,
the equivalent of =~1 mag below L^{*} for the red cluster galaxies, and
encompasses =~70% of the stellar mass in cluster galaxies. The MDRs in the
mass-selected sample at densities of Sigma > 50 galaxies Mpc^{-2} are similar
to those in the luminosity-selected sample but show larger early-type
fractions. However, the trend with redshift in the fraction of elliptical and
S0 galaxies with masses > 10^{10.6} M_{\sun} differs significantly between the
mass- and luminosity-selected samples. The clear trend seen in the early-type
fraction from z=0 to z=~ 0.8 is not found in mass-selected samples. The
early-type galaxy fraction changes much less, and is consistent with being
constant at 92% +/- 4% at \Sigma> 500 galaxies Mpc^{-2} and 83 +/- 3% at 50 <
\Sigma < 500 galaxies Mpc^{-2}. This suggests that galaxies of mass lower than
> 10^{10.6} M_{\sun} play a significant role in the evolution of the early-type
fraction in luminosity-selected samples. (Abstract abridged)Comment: 18 pages in emulate ApJ format, with 10 color figures, Accepted to
ApJ. Version updated to reflect published version, includes new references
and a correction to table
The Red Sequence of High-Redshift Clusters: a Comparison with Cosmological Galaxy Formation Models
We compare the results from a semi-analytic model of galaxy formation with
spectro-photometric observations of distant galaxy clusters observed in the
range 0.8< z< 1.3. We investigate the properties of their red sequence (RS)
galaxies and compare them with those of the field at the same redshift. In our
model we find that i) a well-defined, narrow RS is obtained already by z= 1.2;
this is found to be more populated than the field RS, analogously to what
observed and predicted at z=0; ii) the predicted U-V rest-frame colors and
scatter of the cluster RS at z=1.2 have average values of 1 and 0.15
respectively, with a cluster-to-cluster variance of 0.2 and 0.06, respectively.
The scatter of the RS of cluster galaxies is around 5 times smaller than the
corresponding field value; iii) when the RS galaxies are considered, the mass
growth histories of field and cluster galaxies at z=1.2 are similar, with 90 %
of the stellar mass of RS galaxies at z=1.2 already formed at cosmic times
t=2.5 Gyr, and 50 % at t=1 Gyr; v) the predicted distribution of stellar ages
of RS galaxies at z=1.2 peaks at 3.7 Gyr for both cluster and field
populations; however, for the latter the distribution is significantly skewed
toward lower ages. When compared with observations, the above findings show an
overall consistency, although the average value 0.07 of the observed cluster RS
scatter (U-V colors) at z=1.2 is smaller than the corresponding model central
value. We discuss the physical origin and the significance of the above results
in the framework of cosmological galaxy formation.Comment: 14 pages, accepted for publication in ApJ. Updated one referenc
VLT and ACS observations of RDCS J1252.9-2927: dynamical structure and galaxy populations in a massive cluster at z=1.237
We present results from an extensive spectroscopic survey, carried out with
VLT FORS, and from an extensive multiwavelength imaging data set from the HST
Advanced Camera for Surveys and ground based facilities, of the cluster of
galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster
members in the redshift range 1.22 < z < 1.25. A cluster median redshift of
z=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s are
obtained. Using the 38 confirmed redshifts, we were able to resolve, for the
first time at z > 1, kinematic structure. The velocity distribution, which is
not Gaussian at the 95% confidence level, is consistent with two groups that
are also responsible for the projected east-west elongation of the cluster. The
groups are composed of 26 and 12 galaxies with velocity dispersions of
486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation is
also seen in the intracluster gas and the dark matter distribution. This leads
us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state.
We extend the analysis of the color-magnitude diagram of spectroscopic members
to more than 1 Mpc from the cluster center. The scatter and slope of
non-[OII]-emitting cluster members in the near-IR red sequence is similar to
that seen in clusters at lower redshift. Furthermore, most of the galaxies with
luminosities greater than ~ K_s*+1.5 do not show any [OII], indicating that
these more luminous, redder galaxies have stopped forming stars earlier than
the fainter, bluer galaxies. Our observations provide detailed dynamical and
spectrophotometric information on galaxies in this exceptional high-redshift
cluster, delivering an in-depth view of structure formation at this epoch only
5 Gyr after the Big Bang.Comment: 29 pages. 16 figures. ApJ accepted. Tables 2,3 and 5, figure 1 and
the full figure 5 will be available in the paper and electronic editions from
ApJ. v2: minor corrections to the abstract and text to match the Journal's
versio
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic Atoms
We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.Comment: 5 pages, 3 figures, 1 tabl
- âŠ