986 research outputs found
Welk spoor kiest u? : waarderingskader voor het bevorderen van ondernemerschap
Dit verslag geeft een overzicht van ondernemerschapactiviteiten en projecten die in het onderwijs, onderzoek en bedrijfsleven plaatsgevonden hebben. Om deze activiteiten te kunnen beoordelen op ondernemerschap is een waarderingskader ontwikkeld. Dit kader maakt inzichtelijk in hoeverre ondernemerschap bevorderd wordt. En met behulp van het waarderingskader zijn good practices geïdentificeerd. Het verslag geeft tevens adviezen om ondernemerschap met hulp van het kader verder te ontwikkele
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter
On the Size and Comoving Mass Density Evolution of Early-Type Galaxies
We present a simple, empirically motivated model that simultaneously predicts
the evolution of the mean size and the comoving mass density of massive
early-type galaxies from z=2 to the present. First we demonstrate that some
size evolution of the population can be expected simply due to the continuous
emergence of early-type galaxies. SDSS data reveal that in the present-day
universe more compact early-type galaxies with a given dynamical mass have
older stellar populations. In contrast, at a given stellar velocity dispersion,
SDSS data show that there is no relation between size and age, which implies
that the velocity dispersion can be used to estimate the epoch at which
galaxies stopped forming stars, turning into early-type galaxies. Applying such
a 'formation' criterion to a large sample of nearby early-type galaxies, we
predict the redshift evolution in the size distribution and the comoving mass
density. The resulting evolution in the mean size is roughly half of the
observed evolution. Then we include a prescription for the merger histories of
galaxies between the 'formation' redshift and the present, based on
cosmological simulations of the assembly of dark matter halos. Such mergers
after the transformation into an early-type galaxy are presumably
dissipationless ('dry'), where the increase in size is expected to be
approximately proportional to the increase in mass. This model successfully
reproduces the observed evolution since z~2 in the mean size and in the
comoving mass density of massive early-type galaxies. We conclude that the
recently measured, substantial size evolution of early-type galaxies can be
explained by the combined effect of the continuous emergence of galaxies as
early types and their subsequent growth through dry merging.Comment: Accepted for publication in ApJ (13 pages, 5 figures), small changes
to match journal versio
The Physical Origins of The Morphology-Density Relation: Evidence for Gas Stripping from the SDSS
We provide a physical interpretation and explanation of the
morphology-density relation for galaxies, drawing on stellar masses, star
formation rates, axis ratios and group halo masses from the Sloan Digital Sky
Survey. We first re-cast the classical morphology-density relation in more
quantitative terms, using low star formation rate (quiescence) as a proxy for
early-type morphology and dark matter halo mass from a group catalog as a proxy
for environmental density: for galaxies of a given stellar mass the quiescent
fraction is found to increase with increasing dark matter halo mass. Our novel
result is that - at a given stellar mass - quiescent galaxies are significantly
flatter in dense environments, implying a higher fraction of disk galaxies.
Supposing that the denser environments differ simply by a higher incidence of
quiescent disk galaxies that are structurally similar to star-forming disk
galaxies of similar mass, explains simultaneously and quantitatively these
quiescence -nvironment and shape-environment relations. Our findings add
considerable weight to the slow removal of gas as the main physical driver of
the morphology-density relation, at the expense of other explanations.Comment: published in ApJ: http://adsabs.harvard.edu/abs/2010ApJ...714.1779
The Evolution of Rest-Frame K-band Properties of Early-Type Galaxies from z=1 to the Present
We measure the evolution of the rest-frame K-band Fundamental Plane from z=1
to the present by using IRAC imaging of a sample of early-type galaxies in the
Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We
find that evolves as , which is
slower than in the B-band (). In the B-band
the evolution has been demonstrated to be strongly mass dependent. In the
K-band we find a weaker trend: galaxies more massive than
evolve as ;
less massive galaxies evolve as . As
expected from stellar population models the evolution in is slower than
the evolution in . However, when we make a quantitative comparison, we
find that the single burst Bruzual-Charlot models do not fit the results well,
unless large dust opacities are allowed at z=1. Models with a flat IMF fit
better, Maraston models with a different treatment of AGB stars fit best. These
results show that the interpretation of rest-frame near-IR photometry is
severely hampered by model uncertainties and therefore that the determination
of galaxy masses from rest-frame near-IR photometry may be harder than was
thought before.Comment: 5 pages, 3 figures, Accepted for publication in ApJ
The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
We investigate the stellar structure of massive, quiescent galaxies at z~2,
based on Hubble Space Telescope/WFC3 imaging from the Early Release Science
program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol
and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work,
their half-light radii are <2 kpc, much smaller than equally massive galaxies
in the present-day universe. A significant subset of the sample appears highly
flattened in projection, which implies, considering viewing angle statistics,
that a significant fraction of the galaxies in our sample have pronounced
disks. This is corroborated by two-dimensional surface brightness profile fits.
We estimate that 65% +/- 15% of the population of massive, quiescent z~2
galaxies are disk-dominated. The median disk scale length is 1.5 kpc,
substantially smaller than the disks of equally massive galaxies in the
present-day universe. Our results provide strong observational evidence that
the much-discussed ultra-dense high-redshift galaxies should generally be
thought of as disk-like stellar systems with the majority of stars formed from
gas that had time to settle into a disk.Comment: published versio
An Absence of Radio-Loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-Mode Feedback Models
Maintenance-mode feedback from low-accretion rate AGN, manifesting itself
observationally through radio-loudness, is invoked in all cosmological galaxy
formation models as a mechanism that prevents excessive star-formation in
massive galaxies (M 310 M). We
demonstrate that at a fixed mass the incidence of radio-loud AGN (L
10 WHz) identified in the FIRST and NVSS radio surveys among a
large sample of quiescent (non-star forming) galaxies selected from the SDSS is
much higher in geometrically round galaxies than in geometrically flat,
disk-like galaxies. As found previously, the RL AGN fraction increases steeply
with stellar velocity dispersion and stellar mass, but even at a
fixed velocity dispersion of 200-250 kms this fraction increases from
0.3% for flat galaxies (projected axis ratio of q 0.4) to 5% for round
galaxies (q 0.8). We rule out that this strong trend is due to projection
effects in the measured velocity dispersion. The large fraction of radio-loud
AGN in massive, round galaxies is consistent with the hypothesis that such AGN
deposit energy into their hot gaseous halos, preventing cooling and
star-formation. However, the absence of such AGN in disk-like quiescent
galaxies -- most of which are not satellites in massive clusters, raises
important questions: is maintenance-mode feedback a generally valid explanation
for quiescence; and, if so, how does that feedback avoid manifesting at least
occasionally as a radio-loud galaxy?Comment: 7 pages, 5 figures, accepted for publication in ApJ Letter
Low-Frequency Noise Phenomena in Switched MOSFETs
In small-area MOSFETs widely used in analog and RF circuit design, low-frequency (LF) noise behavior is increasingly dominated by single-electron effects. In this paper, the authors review the limitations of current compact noise models which do not model such single-electron effects. The authors present measurement results that illustrate typical LF noise behavior in small-area MOSFETs, and a model based on Shockley-Read-Hall statistics to explain the behavior. Finally, the authors treat practical examples that illustrate the relevance of these effects to analog circuit design. To the analog circuit designer, awareness of these single-electron noise phenomena is crucial if optimal circuits are to be designed, especially since the effects can aid in low-noise circuit design if used properly, while they may be detrimental to performance if inadvertently applie
HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3
We study the structural evolution of massive galaxies by linking progenitors
and descendants at a constant cumulative number density of n_c=1.4x10^{-4}
Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic profiles
to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the
UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given
redshift, we selected the HST band that most closely samples a common
rest-frame wavelength so as to minimize systematics from color gradients in
galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of ~3 from
z~3 to z~0. The size evolution is complex: galaxies appear roughly constant in
size from z~3 to z~2 and then grow rapidly to lower redshifts. The evolution in
the surface mass density profiles indicates that most of the mass at r<2 kpc
was in place by z~2, and that most of the new mass growth occurred at larger
radii. This inside-out mass growth is therefore responsible for the larger
sizes and higher Sersic indices of the descendants toward low redshift. At z<2,
the effective radius evolves with the stellar mass as r_e M^{2.0}, consistent
with scenarios that find dissipationless minor mergers to be a key driver of
size evolution. The progenitors at z~3 were likely star-forming disks with
r_e~2 kpc, based on their low Sersic index of n~1, low median axis ratio of
b/a~0.52, and typical location in the star-forming region of the U-V versus V-J
diagram. By z~1.5, many of these star-forming disks disappeared, giving rise to
compact quiescent galaxies. Toward lower redshifts, these galaxies continued to
assemble mass at larger radii and became the local ellipticals that dominate
the high mass end of the mass function at the present epoch.Comment: 12 pages, 8 figures in main text + appendix. v2 reflects the version
that was accepted to ApJ after addressing the referee repor
Postoperative skeletal stability at the one-year follow-up after splintless Le Fort I osteotomy using patient-specific osteosynthesis versus conventional osteosynthesis:a randomized controlled trial
The purpose of this study was to assess the 1-year skeletal stability of the osteotomized maxilla after Le Fort I surgery, comparing conventional osteosynthesis with patient-specific osteosynthesis. Patients were assigned to a conventional or patient-specific osteosynthesis group using prospective randomization. The primary outcome was the three-dimensional change in postoperative skeletal position of the maxilla between the 2-week and 1-year follow-up cone beam computed tomography scans. Fifty-eight patients completed the protocol for the 2-week postoperative analysis, and 27 patients completed the 1-year follow-up study protocol. Of the 27 patients completing the entire protocol, 13 were in the conventional group and 14 in the patient-specific osteosynthesis group. The three-dimensional translation analysis showed that the use of the patient-specific osteosynthesis resulted in a skeletally stable result, comparable to that of conventional miniplate fixation. For both the patient-specific osteosynthesis and conventional miniplate fixation groups, median translations of less than 1 mm and median rotations of less than 1° were observed, indicating that both methods of fixation resulted in a stable result for the 27 patients examined. For the Le Fort I osteotomy, the choice between patient-specific osteosynthesis and conventional osteosynthesis did not affect the postoperative skeletal stability after 1 year of follow-up
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