1,250 research outputs found
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
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
Beyond Elementary: Examining Conceptual Demands of Division of Fractions in Current US Curricula
The Common Core State Standards of Mathematics (CCSSM), a set of US educational standards which has recently been adopted by 45 states, creates a more rigorous and coherent set of standards for American students, making elementary math anything but elementary. The adoption of these new standards formulates the research questions for this study: How well do current curricula match the CCSSM and how well do current curricula support teacher knowledge to implement the standards? In this study, three diverse curricula used in the United States, Prentice Hall, Singapore Math, and CK-12, are examined with three evaluation tools. These tools measure (a) the cognitive demands of the mathematical tasks in each curricula, (b) the mathematical coherency of an instructional unit, and (c) the resources in each curricula that support teachers’ understanding of mathematics. Division of fractions is the topic of analysis because of its frequent occurrence in algebra which is the foundation for higher-level math. I find that Singapore Math’s problems reach higher-level cognitive demands more often than Prentice Hall and CK-12. Prentice Hall and CK-12’s reliance on using the standard division algorithm inhibits conceptual thinking for both students and teachers. From a Curriculum Review Tool, which focuses on teacher knowledge, I find that Singapore Math is the closest to reach the division of fraction CCSSM when compared to Prentice Hall and CK-12. Resource tools for teachers can be developed that better support students’ learning by combining characteristics from each curriculum such as word problems, manipulatives/pictures, and samples of students’ work
Dissipative Entanglement of Quantum Spin Fluctuations
We consider two non-interacting infinite quantum spin chains immersed in a
common thermal environment and undergoing a local dissipative dynamics of
Lindblad type. We study the time evolution of collective mesoscopic quantum
spin fluctuations that, unlike macroscopic mean-field observables, retain a
quantum character in the thermodynamical limit. We show that the microscopic
dissipative dynamics is able to entangle these mesoscopic degrees of freedom,
through a purely mixing mechanism. Further, the behaviour of the dissipatively
generated quantum correlations between the two chains is studied as a function
of temperature and dissipation strength.Comment: 54 pages, 8 figure
Large stellar disks in small elliptical galaxies
We present the rotation velocities V and velocity dispersions sigma along the
principal axes of seven elliptical galaxies less luminous than M_B= -19.5.
These kinematics extend beyond the half-light radii for all systems in this
photometrically selected sample. At large radii the kinematics not only confirm
that rotation and "diskiness" are important in faint ellipticals, as was
previously known, but also demonstrate that in most sample galaxies the stars
at large galactocentric distances have (V/sigma)_max of about 2, similar to the
disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar
motion in all sample galaxies with numerical simulations of dissipationless
mergers argues against mergers with mass ratios <=3:1 as an important mechanism
in the final shaping of low-luminosity ellipticals, and favors instead the
dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure
Dark matter in elliptical galaxies
We present measurements of the shape of the stellar line-of-sight velocity
distribution out to two effective radii along the major axes of the four
elliptical galaxies NGC 2434, 2663, 3706, and 5018. The velocity dispersion
profiles are flat or decline gently with radius. We compare the data to the
predictions of f=f(E,L_z) axisymmetric models with and without dark matter.
Strong tangential anisotropy is ruled out at large radii. We conclude from our
measurements that massive dark halos must be present in three of the four
galaxies, while for the fourth galaxy (NGC 2663) the case is inconclusive.Comment: 15 pages, uuencoded compressed PostScript, includes 3 figure
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