3,666 research outputs found
Searches for the Most Metal-Poor Candidates from SDSS and SEGUE
We report on efforts to identify large samples of very and extremely
metal-poor stars based on medium-resolution spectroscopy and ugriz photometry
obtained during the course of the Sloan Digital Sky Survey (SDSS), and its
extension, SDSS-II, which includes the program SEGUE: Sloan Extension for
Galactic Understanding and Exploration. To date, over 8000 stars with [Fe/H] <=
-2.0 and effective temperatures in the range 4500K < T_eff < 7000K have been
found, with the expected numbers in this temperature range to be well over
10,000 once SEGUE is completed. The numbers roughly double when one includes
warmer blue stragglers and Blue Horizontal-Branch (BHB) stars in these counts.
We show the observed low-metallicity tails of the Metallicity Distribution
Functions for the cooler SDSS/SEGUE stars obtained thus far. We also comment on
the confirmation of an inner/outer halo dichotomy in the Milky Way, and on how
this realization may be used to direct searches for even more metal-poor stars
in the near future.Comment: 5 pages, 4 figures, from the conference "First Stars III", held in
July 200
Uhlmann curvature in dissipative phase transitions
We study the mean Uhlmann curvature in fermionic systems undergoing a
dissipative driven phase transition. We consider a paradigmatic class of
lattice fermion systems in non-equilibrium steady-state of an open system with
local reservoirs, which are characterised by a Gaussian fermionic steady state.
In the thermodynamical limit, in systems with translational invariance we show
that a singular behaviour of the Uhlmann curvature represents a sufficient
criterion for criticalities, in the sense of diverging correlation length, and
it is not otherwise sensitive to the closure of the Liouvillian dissipative
gap. In finite size systems, we show that the scaling behaviour of the mean
Uhlmann curvature maps faithfully the phase diagram, and a relation to the
dissipative gap is put forward. We argue that the mean Uhlmann phase can shade
light upon the nature of non equilibrium steady state criticality in particular
with regard to the role played by quantum vs classical fluctuations.Comment: 5 pages, 3 figures with appendix of 10 pages, 1 figur
Old and young bulges in late-type disk galaxies
ABRIDGED: We use HSTACS and NICMOS imaging to study the structure and colors
of a sample of nine late-type spirals. We find: (1) A correlation between bulge
and disks scale-lengths, and a correlation between the colors of the bulges and
those of the inner disks. Our data show a trend for bulges to be more
metal-enriched than their surrounding disks, but otherwise no simple
age-metallicity connection between these systems; (2) A large range in bulge
stellar population properties, and, in particular, in stellar ages.
Specifically, in about a half of the late-type bulges in our sample the bulk of
the stellar mass was produced recently. Thus, in a substantial fraction of the
z=0 disk-dominated bulged galaxies, bulge formation occurs after the
formation/accretion of the disk; (3) In about a half of the late-type bulges in
our sample, however, the bulk of the stellar mass was produced at early epochs;
(4) Even these "old" late-type bulges host a significant fraction of stellar
mass in a young(er) c component; (5) A correlation for bulges between stellar
age and stellar mass, in the sense that more massive late-type bulges are older
than less massive late-type bulges. Since the overall galaxy luminosity (mass)
also correlates with the bulge luminosity (mass), it appears that the galaxy
mass regulates not only what fraction of itself ends up in the bulge component,
but also "when" bulge formation takes place. We show that dynamical friction of
massive clumps in gas-rich disks is a plausible disk-driven mode for the
formation of "old" late-type bulges. If disk evolutionary processes are
responsible for the formation of the entire family of late-type bulges, CDM
simulations need to produce a similar number of initially bulgeless disks in
addition to the disk galaxies that are observed to be bulgeless at z=0.Comment: ApJ in press; paper with high resolution figures available at
http://www.exp-astro.phys.ethz.ch/carollo/carollo1_2006.pdf; B, I, and H
surface brightness profiles published in electronic tabular for
Two-photon vibrational transitions in as probes of variation of the proton-to-electron mass ratio
Vibrational overtones in deeply bound molecules are sensitive probes for
variation of the proton-to-electron mass ratio . In nonpolar molecules,
these overtones may be driven as two-photon transitions. Here, we present
procedures for experiments with , including state-preparation
through photoionization, a two-photon probe, and detection. We calculate
transition dipole moments between all X\,^2\Pi_g vibrational levels and those
of the A\,^2\Pi_u excited electronic state. Using these dipole moments, we
calculate two-photon transition rates and AC-Stark-shift systematics for the
overtones. We estimate other systematic effects and statistical precision.
Two-photon vibrational transitions in provide multiple routes to
improved searches for variation.Comment: 19 pages, 3 figures, supplementary material (v2 fixes an
ancillary-file upload issue
Environment induced entanglement in many-body mesoscopic systems
We show that two, non interacting, infinitely long spin chains can become
globally entangled at the mesoscopic level of their fluctuation operators
through a purely noisy microscopic mechanism induced by the presence of a
common heat bath. By focusing on a suitable class of mesoscopic observables,
the behaviour of the dissipatively generated quantum correlations between the
two chains is studied as a function of the dissipation strength and bath
temperature.Comment: 9 pages, LaTe
The Hubble Sequence in Groups: The Birth of the Early-Type Galaxies
The physical mechanisms and timescales that determine the morphological
signatures and the quenching of star formation of typical (~L*) elliptical
galaxies are not well understood. To address this issue, we have simulated the
formation of a group of galaxies with sufficient resolution to track the
evolution of gas and stars inside about a dozen galaxy group members over
cosmic history. Galaxy groups, which harbor many elliptical galaxies in the
universe, are a particularly promising environment to investigate morphological
transformation and star formation quenching, due to their high galaxy density,
their relatively low velocity dispersion, and the presence of a hot intragroup
medium. Our simulation reproduces galaxies with different Hubble morphologies
and, consequently, enables us to study when and where the morphological
transformation of galaxies takes place. The simulation does not include
feedback from active galactic nuclei showing that it is not an essential
ingredient for producing quiescent, red elliptical galaxies in galaxy groups.
Ellipticals form, as suspected, through galaxy mergers. In contrast with what
has often been speculated, however, these mergers occur at z>1, before the
merging progenitors enter the virial radius of the group and before the group
is fully assembled. The simulation also shows that quenching of star formation
in the still star-forming elliptical galaxies lags behind their morphological
transformation, but, once started, is taking less than a billion years to
complete. As long envisaged the star formation quenching happens as the
galaxies approach and enter the finally assembled group, due to quenching of
gas accretion and (to a lesser degree) stripping. A similar sort is followed by
unmerged, disk galaxies, which, as they join the group, are turned into the
red-and-dead disks that abound in these environments.Comment: 12 pages, 12 Figures, 1 Table, accepted for publication in AP
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