12,437 research outputs found
The sigma - L correlation in Nearby Early-Type Galaxies
Early-type galaxy velocity dispersions and luminosities are correlated. The
correlation estimated in local samples (< 100 Mpc) differs from that measured
more recently in the SDSS. This is true even when systematics in the SDSS
photometric and spectroscopic parameters have been accounted-for. We show that
this is also true for the ENEAR sample if galaxy luminosities are estimated
using distances which have been corrected for peculiar motions. We then show
that, because the estimate of the `true' distance is derived from a correlation
with velocity dispersion, in this case the D_n-sigma relation, using it in the
sigma-L relation leads to an artificially tight relation with a biased slope.
Making no correction for peculiar velocities results in a sigma-L relation
which is very similar to that of the SDSS, although with larger scatter. We
also measure the sigma-L correlation in a mock ENEAR catalog, in which the
underlying galaxy sample has the same sigma-L correlation as seen in the SDSS.
The mock catalog produces the same D_n-sigma relation as the data, the same
biased slope when D_n-sigma distances are used to estimate luminosities, and
good agreement with the input sigma-L relation when redshift is used as the
distance indicator. This provides further evidence that the true sigma-L
relation of ENEAR galaxies is indeed very similar to that of SDSS early-types.
Our results suggest that local sigma-L relations which are based on Fundamental
Plane distances should also be re-evaluated. Our findings also have important
implications for black hole demographics; the best direct estimates of the
masses of supermassive black holes come from local galaxies, so estimates of
the black hole mass function are more safely made by working with the Mbh-sigma
correlation than with Mbh-L.Comment: 9 pages, 9 figures. Accepted by AJ. A new appendix describes
systematics effects we found in the SDSS velocity dispersion measurements
(sigmas < 150 km/s are biased towards larger values; this bias was not
present in the Bernardi et al. 2003 sample) and luminosity measurement
Separation probabilities for products of permutations
We study the mixing properties of permutations obtained as a product of two
uniformly random permutations of fixed cycle types. For instance, we give an
exact formula for the probability that elements are in distinct
cycles of the random permutation of obtained as product of two
uniformly random -cycles
The effect of the environment on the Faber Jackson relation
We investigate the effect of the environment on the Faber Jackson (FJ)
relation, using a sample of 384 nearby elliptical galaxies and estimating
objectively their environment on the typical scale of galaxy clusters. We show
that the intrinsic scatter of the FJ is significantly reduced when ellipticals
in high density environments are compared to ellipticals in low density ones.
This result, which holds on a limited range of overdensities, is likely to
provide an important observational link between scaling relations and formation
mechanisms in galaxies.Comment: accepted by Ap
Sum rules of codon usage probabilities
In the crystal basis model of the genetic code, it is deduced that the sum of
usage probabilities of the codons with C and A in the third position for the
quartets and/or sextets is independent of the biological species for
vertebrates. A comparison with experimental data shows that the prediction is
satisfied within about 5 %.Comment: 7 page
A Statistical Semi-Empirical Model: Satellite galaxies in Groups and Clusters
We present STEEL a STatistical sEmi-Empirical modeL designed to probe the
distribution of satellite galaxies in groups and clusters. Our fast statistical
methodology relies on tracing the abundances of central and satellite haloes
via their mass functions at all cosmic epochs with virtually no limitation on
cosmic volume and mass resolution. From mean halo accretion histories and
subhalo mass functions the satellite mass function is progressively built in
time via abundance matching techniques constrained by number densities of
centrals in the local Universe. By enforcing dynamical merging timescales as
predicted by high-resolution N-body simulations, we obtain satellite
distributions as a function of stellar mass and halo mass consistent with
current data. We show that stellar stripping, star formation, and quenching
play all a secondary role in setting the number densities of massive satellites
above . We further show that observed
star formation rates used in our empirical model over predict low-mass
satellites below , whereas, star
formation rates derived from a continuity equation approach yield the correct
abundances similar to previous results for centrals.Comment: 21 pages, 17 Figures. MNRAS, in pres
The inner structure of very massive elliptical galaxies: implications for the inside-out formation mechanism of z~2 galaxies
We analyze a sample of 23 supermassive elliptical galaxies (central velocity
dispersion larger than 330 km s-1), drawn from the SDSS. For each object, we
estimate the dynamical mass from the light profile and central velocity
dispersion, and compare it with the stellar mass derived from stellar
population models. We show that these galaxies are dominated by luminous matter
within the radius for which the velocity dispersion is measured. We find that
the sizes and stellar masses are tightly correlated, with Re ~ M*^{1.1}$,
making the mean density within the de Vaucouleurs radius a steeply declining
function of M*: rho_e ~ M*^{-2.2}. These scalings are easily derived from the
virial theorem if one recalls that this sample has essentially fixed (but
large) sigma_0. In contrast, the mean density within 1 kpc is almost
independent of M*, at a value that is in good agreement with recent studies of
z ~ 2 galaxies. The fact that the mass within 1 kpc has remained approximately
unchanged suggests assembly histories that were dominated by minor mergers --
but we discuss why this is not the unique way to achieve this. Moreover, the
total stellar mass of the objects in our sample is typically a factor of ~ 5
larger than that in the high redshift (z ~ 2) sample, an amount which seems
difficult to achieve. If our galaxies are the evolved objects of the recent
high redshift studies, then we suggest that major mergers were required at z >
1.5, and that minor mergers become the dominant growth mechanism for massive
galaxies at z < 1.5.Comment: 11 pages, 8 figures, accepted in MNRA
Doctor of Philosophy
dissertationPower generation in a nanoscale-gap thermophotovoltaic (nano-TPV) device can be enhanced, compared to conventional thermophotovoltaic (TPV) systems, due to radiative heat transfer exceeding the blackbody limit. TPV power generation refers to direct thermal-to-electrical energy conversion of near infrared and infrared radiation emitted by a terrestrial source. By separating the radiator and the cell by a gap smaller than the peak emitted wavelength, radiative heat transfer can exceed the blackbody predictions by a few orders of magnitude due to energy transport by waves evanescently confined to the surface of the radiator. This enhanced energy transfer can lead to a significant increase in TPV power generation. This dissertation is divided into two main parts. First, a numerical model is presented which demonstrates increased power generation in nano-TPV devices when compared to conventional TPV systems. The model incorporates near-field radiation, heat and charge transport while accounting for radiative, electrical and thermal losses in the cell. The devices analyzed consist of GaSb cells illuminated by a broadband tungsten and a quasi-monochromatic Drude emitter at 2000 K. Results show an increase in power generation by a factor of 4.7 with a tungsten emitter and a 100-nm-thick gap. Furthermore, it is shown that nano-TPV power generators may perform better with broadband emitters where radiative heat transfer is dominated by frustrated modes rather than surface modes. The second part of this dissertation is devoted to the experimental demonstration of radiative heat transfer exceeding the blackbody limit, which is the fundamental phenomenon underlying enhanced power generation in nano-TPV systems. A MEMS-based experimental device has been fabricated for radiative heat flux measurements between 5 5 mm2 planar intrinsic silicon surfaces separated by a variable gap as small as 150 nm. The separation gap is maintained via rigid spacers and a compliant membrane allows for variation of the gap size via mechanical forces. Results agree well with predictions based on fluctuational electrodynamics. At a gap size of 150 nm, the blackbody limit is exceeded by a factor of 8.4. This is the largest value ever recorded between macroscale planar surfaces at non-cryogenic temperatures
Results for a turbulent system with unbounded viscosities: weak formulations, existence of solutions, boundedness, smoothness'
We consider a circulation system arising in turbulence modelling in fluid
dynamics with unbounded eddy viscosities. Various notions of weak solutions are
considered and compared. We establish existence and regularity results. In
particular we study the boundedness of weak solutions. We also establish an
existence result for a classical solutio
Kelvin probe characterization of buried graphitic microchannels in single-crystal diamond
In this work, we present an investigation by Kelvin Probe Microscopy (KPM) of
buried graphitic microchannels fabricated in single-crystal diamond by direct
MeV ion microbeam writing. Metal deposition of variable-thickness masks was
adopted to implant channels with emerging endpoints and high temperature
annealing was performed in order to induce the graphitization of the
highly-damaged buried region. When an electrical current was flowing through
the biased buried channel, the structure was clearly evidenced by KPM maps of
the electrical potential of the surface region overlying the channel at
increasing distances from the grounded electrode. The KPM profiling shows
regions of opposite contrast located at different distances from the endpoints
of the channel. This effect is attributed to the different electrical
conduction properties of the surface and of the buried graphitic layer. The
model adopted to interpret these KPM maps and profiles proved to be suitable
for the electronic characterization of buried conductive channels, providing a
non-invasive method to measure the local resistivity with a micrometer
resolution. The results demonstrate the potential of the technique as a
powerful diagnostic tool to monitor the functionality of all-carbon
graphite/diamond devices to be fabricated by MeV ion beam lithography.Comment: 21 pages, 5 figure
- …