19,078 research outputs found
The Way We Measure: Comparison of Methods to Derive Radial Surface Brightness Profiles
The breaks and truncations in the luminosity profile of face-on spiral
galaxies offer valuable insights in their formation history. The traditional
method of deriving the surface photometry profile for face-on galaxies is to
use elliptical averaging. In this paper, we explore the question whether
elliptical averaging is the best way to do this. We apply two additional
surface photometry methods, one new: principle axis summation, and one old that
has become seldom used: equivalent profiles. These are compared to elliptically
averaged profiles using a set of 29 face-on galaxies. We find that the
equivalent profiles match extremely well with elliptically averaged profiles,
confirming the validity of using elliptical averaging. The principle axis
summation offers a better comparison to edge-on galaxies.Comment: Accepted for publication by Monthly Notices of the R.A.S. A hi-res
version is available at http://www.astro.rug.nl/~vdkruit/Petersetal-VI.pd
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Split-gate quantum point contacts with tunable channel length
We report on developing split-gate quantum point contacts (QPCs) that have a
tunable length for the transport channel. The QPCs were realized in a
GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below
its surface. The conventional design uses 2 gate fingers on the wafer surface
which deplete the 2DEG underneath when a negative gate voltage is applied, and
this allows for tuning the width of the QPC channel. Our design has 6 gate
fingers and this provides additional control over the form of the electrostatic
potential that defines the channel. Our study is based on electrostatic
simulations and experiments and the results show that we developed QPCs where
the effective channel length can be tuned from about 200 nm to 600 nm.
Length-tunable QPCs are important for studies of electron many-body effects
because these phenomena show a nanoscale dependence on the dimensions of the
QPC channel
Galaxies with Spiral Structure up to z = 0.87 --Limits on M/L and the Stellar Velocity Dispersion
We consider seven distant galaxies with clearly evident spiral structure from
HST images. Three of these were chosen from Vogt et al. (1996) (VFP) and have
measured rotational velocities. Five were chosen from the Medium Deep Survey
and are studied in Sarajedini et al. 1996 (SGGR), and one galaxy is found in
both papers. We place upper limits on their mass-to-light ratios (M/L) by
computing M/L_B for a maximal disk. We find that these galaxies have maximal
disk mass-to-light ratios M/L_B = 1.5 - 3.5 M_sol/L_Bsol at the low end, but
within the range seen in nearby galaxies. The mass-to-light ratios are low
enough to suggest that the galaxies contain a young, rapidly formed stellar
population.
By using a Toomre stability criterion for formation of spiral structure, we
place constraints on the ratio of M/L to the stellar velocity dispersion. If
these galaxies have maximal disks they would have to be nearly unstable so as
to have small enough velocity dispersions that their disks are not
unrealistically thick. This suggests that there is a substantial amount of dark
matter present in the luminous regions of the galaxy.Comment: AAS Latex + PS Figure, accepted for publication in A
Structure of Disk Dominated Galaxies I. Bulge/Disk Parameters, Simulations, and Secular Evolution
(Abridged) A robust analysis of galaxy structural parameters, based on the
modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented
for 121 face-on and moderately inclined late-type spirals. Each surface
brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge
and an exponential disk. The reliability and limitations of our bulge-to-disk
(B/D) decompositions are tested with extensive simulations of galaxy brightness
profiles (1D) and images (2D). Galaxy types are divided into 3 classes
according to their SB profile shapes; Freeman Type-I and Type-II, and a third
``Transition'' class for galaxies whose profiles change from Type-II in the
optical to Type-I in the infrared. We discuss possible interpretations of
Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I
late-type spirals shows a range between n=0.1-2 but, on average, the underlying
surface density profile for the bulge and disk of these galaxies is adequately
described by a double-exponential distribution. We confirm a coupling between
the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or
h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent
N-body simulations of disk formation and models of secular evolution. This
ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types.
The similar scaling relations for early and late-type spirals suggest
comparable formation and/or evolution scenarios for disk galaxies of all Hubble
types.Comment: 78 pages with 23 embedded color figures + tables of galaxy structural
parameters. Accepted for publication in the Astrophysical Journal. The
interested reader is strongly encouraged to ignore some of the low res
figures within; instead, download the high resolution version from
http://www.astro.ubc.ca/people/courteau/public/macarthur02_disks.ps.g
Stabilizing nuclear spins around semiconductor electrons via the interplay of optical coherent population trapping and dynamic nuclear polarization
We experimentally demonstrate how coherent population trapping (CPT) for
donor-bound electron spins in GaAs results in autonomous feedback that prepares
stabilized states for the spin polarization of nuclei around the electrons. CPT
was realized by excitation with two lasers to a bound-exciton state.
Transmission studies of the spectral CPT feature on an ensemble of electrons
directly reveal the statistical distribution of prepared nuclear spin states.
Tuning the laser driving from blue to red detuned drives a transition from one
to two stable states. Our results have importance for ongoing research on
schemes for dynamic nuclear spin polarization, the central spin problem and
control of spin coherence.Comment: 5 pages, 4 figure
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