184 research outputs found
Secular evolution of galactic discs: constraints on phase-space density
It was argued in the past that bulges of galaxies cannot be formed through
collisionless secular evolution because that would violate constraints on the
phase-space density: the phase-space density in bulges is several times larger
than in the inner parts of discs. We show that these arguments against secular
evolution are not correct. Observations give estimates of the coarsely grained
phase-space densities of galaxies, f'=rho_s/(sigma_R sigma_phi sigma_z), where
rho_s is stellar density and sigma_R, sigma_phi, sigma_z are the radial,
tangential, and vertical rms velocities of stars. Using high-resolution N-body
simulations, we study the evolution of f' in discs of Galaxy-size models.
During the secular evolution, the discs, which are embedded in live CDM haloes,
form a bar and then a thick, dynamically hot, central mass concentration. In
the course of evolution f' declines at all radii, not just in the central
region. However, the decline is different in different parts of the disc. In
the inner disc, f'(R) develops a valley with a minimum around the end of the
central mass concentration. The final result is that the values of f' in the
central regions are significantly larger than those in the inner disc. The
minimum, which gets deeper with time, seems to be due to a large phase mixing
produced by the outer bar. We find that the shape and the amplitude of f'(R)
for different simulations agree qualitatively with the observed f'(R) in our
Galaxy. Curiously enough, the fact that the coarsely grained phase-space
density of the bulge is significantly larger than the one of the inner disc
turns out to be an argument in favor of secular formation of bulges, not
against it.Comment: 9 pages, 5 figures included. Accepted for publication in MNRAS. Minor
changes after referee's report. Two figures added (possition-velocity
diagrams) to show (i) the agreement in the mass distribution of one of our
models with that of the Galaxy, and (ii) the (minor) influence of gas on this
distributio
The Ursinus Weekly, May 8, 1975
From the cluttered desk of the U.S.G.A. President • Band finishes • B.C. to A.D. • Record review: Straight shooter - Bad Company • Letters to the editor • Parents\u27 Day plea: Donations for care • Spring Parents\u27 Day events scheduled • Track team takes fourth • Lantern elects • Placement Office active for students • Award to Noar • Telethon • Night school • How to Succeed • Suds abound in Shampoo • Baseball drops two • Girls winhttps://digitalcommons.ursinus.edu/weekly/1038/thumbnail.jp
The Formation of a Disk Galaxy within a Growing Dark Halo
We present a dynamical model for the formation and evolution of a massive
disk galaxy, within a growing dark halo whose mass evolves according to
cosmological simulations of structure formation. The galactic evolution is
simulated with a new 3D chemo-dynamical code, including dark matter, stars and
a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark
halo in a LCDM universe and we follow the evolution until the present epoch.
The energy release by massive stars and SNe prevents a rapid collapse of the
baryonic matter and delays the maximum star formation until z=1. The galaxy
forms radially from inside-out and vertically from halo to disk. The first
galactic component that forms is the halo, followed by the bulge, the disk-halo
transition region, and the disk. At z=1, a bar begins to form which later turns
into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk
stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge
and inner disk (G-dwarf problem). The mean rotation and the distribution of
orbital eccentricities for all stars as a function of metallicity are not very
different from those observed in the solar neighbourhood, showing that
homogeneous collapse models are oversimplified. The approach presented here
provides a detailed description of the formation and evolution of an isolated
disk galaxy in a LCDM universe, yielding new information about the kinematical
and chemical history of the stars and the ISM, but also about the evolution of
the luminosity, the colours and the morphology of disk galaxies.Comment: 23 pages, LaTeX, 18 figures, A&A accepted, a high resolution version
of the paper can be found at http://www.astro.unibas.ch/leute/ms.shtm
Ancient heat flow, crustal thickness, and lithospheric mantle rheology in the Amenthes region, Mars
Surface heat flow calculations for the Amenthes region of Mars can be independently performed using the depth to the brittle–ductile
transition and the effective elastic thickness of the lithosphere estimated for the Late Noachian/Early Hesperian (equivalent to an estimated absolute
age of ~3.6–3.8 Ga). This, along with crustal heat production rates estimated from heat-producing elements abundances, permits us to put constraints,
for that particular place and time, on both the thermal and mechanical properties of the lithosphere and the crustal thickness. The depth to the brittle–
ductile transition deduced from modeling of the topography of Amenthes Rupes is 27–35 km, and the associated surface heat flow is 26–37 mWm−2.
On the other hand, the effective elastic thickness in this region is between 19 and 35 km: the surface heat flow deduced by considering crustal and
lithospheric mantle contributions to the total lithospheric strength, as well as wet or dry olivine for lithospheric mantle rheology, is 31–49mWm−2. The
relatively limited overlap among Te- andzBDT-based heat flowvalues implies a surface heat flowof 31–36mWm−2 (with a high fraction originated from
crustal heat sources) and awet mantle rheology. The so obtained local crustal thickness is 43–74 km,which suggests an average thickness of~40–75 km
for the Martian crust; for the frequently used crustal density of 2900 kgm−3, our results suggest a crustal thickness of 50–63km for theAmenthes region,
and an average crustal thickness of ~45–65 km for Mars
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
X-ray, lensing and Sunyaev Zel'dovich triaxial analysis of Abell 1835 out to R_{200}
Measuring the intrinsic shape and orientation of dark matter (DM) and
intracluster (IC) gas in galaxy clusters is crucial to constraining their
formation and evolution, and for enhancing the use of clusters as more precise
cosmological probes. Extending our previous works, we present for the first
time results from a triaxial joint analysis of the galaxy cluster Abell 1835,
by means of X-ray, strong lensing (SL) and Sunyaev Zel'dovich (SZ) data. We
parametrically reconstruct the full three-dimensional structure (triaxial shape
and principal axis orientation) of both the DM and the IC gas, and the level of
non-thermal pressure of the IC gas. We find that the intermediate-major and
minor-major axis ratios of the DM are 0.71+/-0.08 and 0.59+/-0.05,
respectively, and the major axis of the DM halo is inclined with respect to the
line of sight at 18.3+/-5.2 deg. We present the first observational measurement
of the non-thermal pressure out to R_{200}, which has been evaluated to be a
few percent of the total energy budget in the internal regions, while reaching
approximately 20% in the outer volumes. We discuss the implications of our
method for the viability of the CDM scenario, focusing on the concentration
parameter C and the inner slope of the DM gamma in order to test the cold dark
matter (CDM) paradigm for structure formation: we measure gamma=1.01+/-0.06 and
C=4.32+/-0.44, values which are close to the predictions of the CDM model. The
combination of X-ray/SL data at high spatial resolution, capable of resolving
the cluster core, with the SZ data, which are more sensitive to the cluster
outer volume, allows us to characterize the level and the gradient of the gas
entropy distribution and non-thermal pressure out to R_{200}, breaking the
degeneracy among the physical models describing the thermal history of the ICM.Comment: MNRAS in press. arXiv admin note: substantial text overlap with
arXiv:1108.076
Melt propagation and volcanism in mantle convection simulations, with applications for Martian volcanic and atmospheric evolution
The population of barred galaxies in the local universe I. Detection and characterisation of bars
(Abridge) Bars are very common in the centre of the disc galaxies, and they
drive the evolution of their structure. A volume-limited sample of 2106 disc
galaxies extracted from the Sloan Digital Sky Survey Data Release 5 was studied
to derive the bar fraction, length, and strength as a function of the
morphology, size, local galaxy density, light concentration, and colour of the
host galaxy. The bars were detected using the ellipse fitting method and
Fourier analysis method. The ellipse fitting method was found to be more
efficient in detecting bars in spiral galaxies. The fraction of barred galaxies
turned out to be 45%. A bar was found in 29% of the lenticular galaxies, in 55%
and 54% of the early- and late-type spirals, respectively. The bar length
(normalised by the galaxy size) of late-type spirals is shorter than in
early-type or lenticular ones. A correlation between the bar length and galaxy
size was found with longer bars hosted by larger galaxies. The bars of the
lenticular galaxies are weaker than those in spirals. Moreover, the unimodal
distribution of the bar strength found for all the galaxy types argues against
a quick transition between the barred and unbarred statues. There is no
difference between the local galaxy density of barred and unbarred galaxies.
Besides, neither the length nor strength of the bars are correlated with the
local density of the galaxy neighbourhoods. In contrast, a statistical
significant difference between the central light concentration and colour of
barred and unbarred galaxies was found. Bars are mostly located in less
concentrated and bluer galaxies. These results indicate that the properties of
bars are strongly related to those of their host galaxies, but do not depend on
the local environment.Comment: 15 pages, 13 figures. Accepted for publication in A&
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