543 research outputs found
Environment, morphology and stellar populations of bulgeless low surface brightness galaxies
Based on the Sloan Digital Sky Survey DR 7, we investigate the environment,
morphology and stellar population of bulgeless low surface brightness (LSB)
galaxies in a volume-limited sample with redshift ranging from 0.024 to 0.04
and . The local density parameter is used to
trace their environments. We find that, for bulgeless galaxies, the surface
brightness does not depend on the environment. The stellar populations are
compared for bulgeless LSB galaxies in different environments and for bulgeless
LSB galaxies with different morphologies. The stellar populations of LSB
galaxies in low density regions are similar to those of LSB galaxies in high
density regions. Irregular LSB galaxies have more young stars and are more
metal-poor than regular LSB galaxies. These results suggest that the evolution
of LSB galaxies may be driven by their dynamics including mergers rather than
by their large scale environment.Comment: 12 pages, 13 figures, Accepted by A&
Characterizing the red optical sky background fluctuations from narrow-band imaging
The detection and characterization of the physical properties of very distant
galaxies will be one the prominent science case of all future Extremely Large
Telescopes, including the 39m E-ELT. Multi-Object Spectroscopic instruments are
potentially very important tools for studying these objects, and in particular
fiber-based concepts. However, detecting and studying such faint and distant
sources will require subtraction of the sky background signal (i.e., between OH
airglow lines) with an accuracy of ~1%. This requires a precise and accurate
knowledge of the sky background temporal and spatial fluctuations. Using FORS2
narrow-band filter imaging data, we are currently investigating what are the
fluctuations of the sky background at ~9000A. We present preliminary results of
sky background fluctuations from this study over spatial scales reaching ~4
arcmin, as well as first glimpses into the temporal variations of such
fluctuations over timescales of the order of the hour. This study (and other
complementary on-going studies) will be essential in designing the
next-generation fiber-fed instruments for the E-ELT.Comment: To be published in Proc SPIE 8446: Ground-based & Airborne
Instrumentation for Astronomy IV; 12 pages, 3 tables, 8 figure
Erbium fibre laser pumped nanosecond optical parametric oscillator
We report a periodically-poled lithium niobate optical parametric oscillator pumped by a frequency doubled Q-switched erbium-fibre laser. Pump tuning of the OPO signal with thresholds below 10µJ was demonstrated. Signal tuning range was 0.99-1.45µm
Permeability and conductivity of platelet-reinforced membranes and composites
We present large scale simulations of the diffusion constant of a random
composite consisting of aligned platelets with aspect ratio in a
matrix (with diffusion constant ) and find that , where and is the platelet volume fraction. We
demonstrate that for large aspect ratio platelets the pair term ()
dominates suggesting large property enhancements for these materials. However a
small amount of face-to-face ordering of the platelets markedly degrades the
efficiency of platelet reinforcement.Comment: RevTeX, 5 pages, 4 figures, submitted to PR
The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies
[Abridged]We compare both the Milky Way and M31 galaxies to local external
disk galaxies within the same mass range, using their relative locations in the
planes formed by V_flat versus M_K, j_disk, and the average Fe abundance of
stars in the galaxy outskirts. We find, for all relationships, that the MW is
systematically offset by ~ 1 sigma, showing a significant deficiency in stellar
mass, in angular momentum, in disk radius and [Fe/H] in the stars in its
outskirts at a given V_flat. On the basis of their location in the M_K, V_flat,
and R_d volume, the fraction of spirals like the MW is 7+/-1%, while M31
appears to be a "typical'' spiral. Our Galaxy appears to have escaped any
significant merger over the last ~10 Gyrs which may explain why it is deficient
by a factor 2 to 3 in stellar mass, angular momentum and outskirts metallicity
and then, unrepresentative of the typical spiral. As with M31, most local
spirals show evidence for a history shaped mainly by relatively recent merging.
We conclude that the standard scenario of secular evolution is generally unable
to reproduce the properties of most (if not all) spiral galaxies. However, the
so-called "spiral rebuilding'' scenario proposed by Hammer et al. 2005 is
consistent with the properties of both distant galaxies and of their
descendants - the local spirals.Comment: 14 pages, 6 figures, to appear in Ap
Theory of itinerant-electron ferromagnetism
A theory of Kondo lattices or a expansion theory, with spatial
dimensionality, is applied to studying itinerant-electron ferromagnetism. Two
relevant multi-band models are examined: a band-edge model where the chemical
potential is at one of band-edges, the top or bottom of bands, and a flat-band
model where one of bands is almost flat or dispersionless and the chemical
potential is at the flat band. In both the models, a novel ferromagnetic
exchange interaction arises from the virtual exchange of pair excitations of
quasiparticles; it has two novel properties such as its strength is in
proportion to the effective Fermi energy of quasiparticles and its temperature
dependence is responsible for the Curie-Weiss law. When the Hund coupling
is strong enough, the superexchange interaction, which arises from the virtual
exchange of pair excitations of electrons across the Mott-Hubbard gap, is
ferromagnetic. In particular, it is definitely ferromagnetic for any nonzero
in the large limit of band multiplicity. Ferromagnetic instability
occurs, when the sum of the two exchange interactions is ferromagnetic and it
overcomes the quenching of magnetic moments by the Kondo effect or local
quantum spin fluctuations and the suppression of magnetic instability by the
mode-mode coupling among intersite spin fluctuations.Comment: 14 pages, 4 figure
The Tully-Fisher Relation for 25,000 SDSS Galaxies as Function of Environment
We construct Tully-Fisher relationships (TFRs) in the , , , and
bands and stellar mass TFRs (smTFRs) for a sample of late spiral
type galaxies (with ) from the Sloan Digital Sky Survey (SDSS)
and study the effects of environment on the relation. We use SDSS-measured
Balmer emission line widths, , as a proxy for disc circular
velocity, . A priori it is not clear whether we can construct
accurate TFRs given the small diameter of the fibres used for SDSS
spectroscopic measurements. However, we show by modelling the H
emission profile as observed through a aperture that for galaxies at
appropriate redshifts () the fibres sample enough of the disc to
obtain a linear relationship between and ,
allowing us to obtain a TFR and to investigate dependence on other variables.
We also develop a methodology for distinguishing between astrophysical and
sample bias in the fibre TFR trends. We observe the well-known steepening of
the TFR in redder bands in our sample. We divide the sample of galaxies into
four equal groups using projected neighbour density () quartiles and
find no significant dependence on environment, extending previous work to a
wider range of environments and a much larger sample. Having demonstrated that
we can construct SDSS-based TFRs is very useful for future applications because
of the large sample size available.Comment: accepted for publication in MNRAS. 15 pages, 12 figure
Clustering properties of galaxies selected in stellar mass: Breaking down the link between luminous and dark matter in massive galaxies from z=0 to z=2
We present a study on the clustering of a stellar mass selected sample of
18,482 galaxies with stellar masses M*>10^10M(sun) at redshifts 0.4<z<2.0,
taken from the Palomar Observatory Wide-field Infrared Survey. We examine the
clustering properties of these stellar mass selected samples as a function of
redshift and stellar mass, and discuss the implications of measured clustering
strengths in terms of their likely halo masses. We find that galaxies with high
stellar masses have a progressively higher clustering strength, and amplitude,
than galaxies with lower stellar masses. We also find that galaxies within a
fixed stellar mass range have a higher clustering strength at higher redshifts.
We furthermore use our measured clustering strengths, combined with models from
Mo & White (2002), to determine the average total masses of the dark matter
haloes hosting these galaxies. We conclude that for all galaxies in our sample
the stellar-mass-to-total-mass ratio is always lower than the universal
baryonic mass fraction. Using our results, and a compilation from the
literature, we furthermore show that there is a strong correlation between
stellar-mass-to-total-mass ratio and derived halo masses for central galaxies,
such that more massive haloes contain a lower fraction of their mass in the
form of stars over our entire redshift range. For central galaxies in haloes
with masses M(halo)>10^13M(sun) we find that this ratio is <0.02, much lower
than the universal baryonic mass fraction. We show that the remaining baryonic
mass is included partially in stars within satellite galaxies in these haloes,
and as diffuse hot and warm gas. We also find that, at a fixed stellar mass,
the stellar-to-total-mass ratio increases at lower redshifts. This suggests
that galaxies at a fixed stellar mass form later in lower mass dark matter
haloes, and earlier in massive haloes. We interpret this as a "halo downsizing"
effect, however some of this evolution could be attributed to halo assembly
bias.Comment: Accepted for publication in MNRAS. 19 pages, 8 figures and 3 tables
Nanomechanical investigation of soft biological cell adhesion using atomic force microscopy
Mechanical coupling between living cells is a complex process that is important for a variety of biological processes. In this study the effects of specific biochemical treatment on cell-to-cell adhesion and single cell mechanics were systematically investigated using atomic force microscopy (AFM) single cell force spectroscopy. Functionalised AFM tipless cantilevers were used for attaching single suspended cells that were brought in contact with substrate cells. Cell-to-cell adhesion parameters, such as maximum unbinding force (F max) and work or energy of detachment (W D), were extracted from the retraction force–displacement (F–d) curves. AFM indentation experiments were performed by indenting single cells with a spherical microbead attached to the cantilever. Hertzian contact model was applied to determine the elastic modulus (E) of single cells. Following treatment of the cells with neutralising antibody for epithelial (E)-cadherin, F max was increased by 25%, whereas W D decreased by 11% in response to a 43% increase in E. The results suggest that although the adhesion force between cells was increased after treatment, the energy of adhesion was decreased due to the reduced displacement separation as manifested by the loss of elastic deformation. Conclusively, changes in single cell mechanics are important underlying factors contributing to cell-to-cell adhesion and hence cytomechanical characterization is critical for cell adhesion measurements
Bulge growth through disk instabilities in high-redshift galaxies
The role of disk instabilities, such as bars and spiral arms, and the
associated resonances, in growing bulges in the inner regions of disk galaxies
have long been studied in the low-redshift nearby Universe. There it has long
been probed observationally, in particular through peanut-shaped bulges. This
secular growth of bulges in modern disk galaxies is driven by weak,
non-axisymmetric instabilities: it mostly produces pseudo-bulges at slow rates
and with long star-formation timescales. Disk instabilities at high redshift
(z>1) in moderate-mass to massive galaxies (10^10 to a few 10^11 Msun of stars)
are very different from those found in modern spiral galaxies. High-redshift
disks are globally unstable and fragment into giant clumps containing 10^8-10^9
Msun of gas and stars each, which results in highly irregular galaxy
morphologies. The clumps and other features associated to the violent
instability drive disk evolution and bulge growth through various mechanisms,
on short timescales. The giant clumps can migrate inward and coalesce into the
bulge in a few 10^8 yr. The instability in the very turbulent media drives
intense gas inflows toward the bulge and nuclear region. Thick disks and
supermassive black holes can grow concurrently as a result of the violent
instability. This chapter reviews the properties of high-redshift disk
instabilities, the evolution of giant clumps and other features associated to
the instability, and the resulting growth of bulges and associated sub-galactic
components.Comment: 37 pages, 9 figures. Invited refereed review to appear in "Galactic
Bulges", E. Laurikainen, D. Gadotti, R. Peletier (eds.), Springe
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