431 research outputs found
Exact solutions for the spatial de Vaucouleurs and Sersic laws and related quantities
Using the Mathematica package, we find exact analytical expressions for the
so-called de-projected De Vaucouleurs and Sersic laws as well as for related
spatial (3D) quantities, such the mass, gravitational potential, the total
energy and the central velocity dispersion, generally involved in astronomical
calculations expressed in terms of the Meijer G functions.Comment: 11 pages, accepted in A
Collisionless evaporation from cluster elliptical galaxies
We describe a particular aspect of the effects of the parent cluster tidal
field (CTF) on stellar orbits inside cluster Elliptical galaxies. In particular
we discuss, with the aid of a simple numerical model, the possibility that
collisionless stellar evaporation from elliptical galaxies is an effective
mechanism for the production of the recently discovered intracluster stellar
populations. A preliminary investigation, based on very idealized galaxy
density profiles (Ferrers density distributions), showed that over an Hubble
time, the amount of stars lost by a representative galaxy may sum up to the 10%
of the initial galaxy mass, a fraction in interesting agreement with
observational data. The effectiveness of this mechanism is due to the fact that
the galaxy oscillation periods near equilibrium configurations in the CTF are
comparable to stellar orbital times in the external galaxy regions. Here we
extend our previous study to more realistic galaxy density profiles, in
particular by adopting a triaxial Hernquist model.Comment: 6 pages, 2 figures. To appear on "Lecture Notes in Physics",
proceedings of the Workshop on "Galaxies and Chaos. Theory and Observations",
Athens (September 16-19, 2002), G. Contopoulos and N. Voglis, ed
X-ray emission and internal kinematics in early-type galaxies. I. Observations
Long slit spectroscopic data for 7 early-type galaxies with X-ray emission
have been analyzed to derive velocity dispersion and radial velocity profiles.
Major axis rotation curves out to are presented. Adding these new
data to those available in the literature, we have built a sample of early-type
galaxies with detected X-ray emission and known kinematics (central velocity
dispersion and maximum rotational velocity ). Using this sample
we investigate from an observational point of view the role of rotation and
flattening on the X-ray emission, particularly with regard to the X-ray
underluminosity of flat systems. The trend between the X-ray to optical ratio
, a measure of the hot gas content of the galaxies, and
is L-shaped, with the X-ray brightest objects confined at ,
both for Es and S0s. Neither for low or intermediate, nor for high ,
there is any clear correlation between X-ray emission and rotational
properties. The trend between and the ellipticity is also
L-shaped: there are no high objects with high . The
existence of a relation between and makes it
difficult to assess whether rotation or flattening is at the basis of the
trends found. Our observational findings are finally discussed in connection
with the effects that rotation and flattening are predicted to have in the
cooling flow and in the wind/outflow/inflow scenarios.Comment: 10 pages, plain tex, plus 7 .ps figures and tex-macro mn.tex --
accepted by MNRAS (main journal
Bipotential precursors of putative fibrous astrocytes and oligodendrocytes in rat cerebellar cultures express distinct surface features and "neuron-like" gamma-aminobutyric acid transport.
Predicting success in the worldwide start-up network
By drawing on large-scale online data we construct and analyze the
time-varying worldwide network of professional relationships among start-ups.
The nodes of this network represent companies, while the links model the flow
of employees and the associated transfer of know-how across companies. We use
network centrality measures to assess, at an early stage, the likelihood of the
long-term positive performance of a start-up, showing that the start-up network
has predictive power and provides valuable recommendations doubling the current
state of the art performance of venture funds. Our network-based approach not
only offers an effective alternative to the labour-intensive screening
processes of venture capital firms, but can also enable entrepreneurs and
policy-makers to conduct a more objective assessment of the long-term
potentials of innovation ecosystems and to target interventions accordingly
A More Fundamental Plane
We combine strong-lensing masses with SDSS stellar velocity dispersions and
HST-ACS effective (half-light) radii for 36 lens galaxies from the Sloan Lens
ACS (SLACS) Survey to study the mass dependence of mass-dynamical structure in
early-type galaxies. We find that over a 180--390 km/s range in velocity
dispersion, structure is independent of lensing mass to within 5%. This result
suggests a systematic variation in the total (i.e., luminous plus dark matter)
mass-to-light ratio as the origin of the tilt of the fundamental plane (FP)
scaling relationship between galaxy size, velocity dispersion, and surface
brightness. We construct the FP of the lens sample, which we find to be
consistent with the FP of the parent SDSS early-type galaxy population, and
present the first observational correlation between mass-to-light ratio and
residuals about the FP. Finally, we re-formulate the FP in terms of surface
mass density rather than surface brightness. By removing the complexities of
stellar-population effects, this mass-plane formulation will facilitate
comparison to numerical simulations and possible use as a cosmological distance
indicator.Comment: 4+epsilon pages, 1 figure, emulateapj. Revised version accepted for
publication in the ApJ Letter
Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68
We present chemical abundances and radial velocities of six HII regions in
the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived
from deep spectra in the wavelength range 3500 - 10,000 {\AA}, acquired with
the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope
(LBT). In the three regions where the [O III]4363 {\AA} line was
detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the
"direct" method. We also derived the oxygen abundances of all the six regions
adopting indirect method calibrations. We confirm that DDO 68 is an extremely
metal-poor galaxy, and a strong outlier in the luminosity - metallicity
relation defined by star-forming galaxies. With the direct-method we find
indeed an oxygen abundance of 12+log(O/H)=7.140.07 in the northernmost
region of the galaxy and, although with large uncertainties, an even lower
12+log(O/H)=6.960.09 in the "tail". This is, at face value, the most
metal-poor direct abundance detection of any galaxy known. We derive a radial
oxygen gradient of -0.060.03 dex/kpc (or -0.30 dex ) with the
direct method, and a steeper gradient of -0.120.03 dex/kpc (or -0.59 dex
) from the indirect method. For the -element to oxygen
ratios we obtain values in agreement with those found in other metal-poor
star-forming dwarfs. For nitrogen, instead, we infer much higher values,
leading to log(N/O), at variance with the suggested existence of a
tight plateau at in extremely metal poor dwarfs. The derived helium mass
fraction ranges from Y=0.2400.005 to Y=0.250.02, compatible with
standard big bang nucleosynthesis. Finally, we measured HII region radial
velocities in the range 479522 km/s from the tail to the head of the
"comet", consistent with the rotation derived in the HI.Comment: Accepted for publication in MNRA
The internal structure and formation of early-type galaxies: the gravitational--lens system MG2016+112 at z=1.004
[Abridged] We combine our measurements of the velocity dispersion and the
surface brightness profile of the lens galaxy D in the system MG2016+112
(z=1.004) with constraints from gravitational lensing to study its internal
mass distribution. We find that: (i) dark matter accounts for >50% of the total
mass within the Einstein radius (99% CL), excluding at the 8-sigma level that
mass follows light inside the Einstein radius with a constant mass-to-light
ratio (M/L). (ii) the total mass distribution inside the Einstein radius is
well-described by a density profile ~r^-gamma' with an effective slope
gamma'=2.0+-0.1+-0.1, including random and systematic uncertainties. (iii) The
offset of galaxy D from the local Fundamental Plane independently constrains
the stellar M/L, and matches the range derived from our models, leading to a
more stringent lower limit of >60% on the fraction of dark matter within the
Einstein radius (99%CL).
Under the assumption of adiabatic contraction, the inner slope of the dark
matter halo before the baryons collapsed is gamma_i<1.4 (68 CL), marginally
consistent with the highest-resolution cold dark matter simulations that
indicate gamma_i~1.5. This might indicate that either adiabatic contraction is
a poor description of E/S0 formation or that additional processes play a role
as well. Indeed, the apparently isothermal density distribution inside the
Einstein radius, is not a natural outcome of adiabatic contraction models,
where it appears to be a mere coincidence. By contrast, we argue that
isothermality might be the result of a stronger coupling between luminous and
dark-matter, possibly the result of (incomplete) violent relaxation processes.
Hence, we conclude that galaxy D appears already relaxed 8 Gyr ago.Comment: 8 pages, 4 figures, ApJ, in press, minor change
The MOND Fundamental Plane
Modified Newtonian Dynamics (MOND) has been shown to be able to fit spiral
galaxy rotation curves as well as giving a theoretical foundation for
empirically determined scaling relations, such as the Tully - Fisher law,
without the need for a dark matter halo. As a complementary analysis, one
should investigate whether MOND can also reproduce the dynamics of early - type
galaxies (ETGs) without dark matter. As a first step, we here show that MOND
can indeed fit the observed central velocity dispersion of a large
sample of ETGs assuming a simple MOND interpolating functions and constant
anisotropy. We also show that, under some assumptions on the luminosity
dependence of the Sersic n parameter and the stellar M/L ratio, MOND predicts a
fundamental plane for ETGs : a log - linear relation among the effective radius
, and the mean effective intensity .
However, we predict a tilt between the observed and the MOND fundamental
planes.Comment: 16 pages, 2 figures, 2 tables, accepted for publication on MNRA
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