101 research outputs found
Oracle-Based Robust Optimization via Online Learning
Robust optimization is a common framework in optimization under uncertainty
when the problem parameters are not known, but it is rather known that the
parameters belong to some given uncertainty set. In the robust optimization
framework the problem solved is a min-max problem where a solution is judged
according to its performance on the worst possible realization of the
parameters. In many cases, a straightforward solution of the robust
optimization problem of a certain type requires solving an optimization problem
of a more complicated type, and in some cases even NP-hard. For example,
solving a robust conic quadratic program, such as those arising in robust SVM,
ellipsoidal uncertainty leads in general to a semidefinite program. In this
paper we develop a method for approximately solving a robust optimization
problem using tools from online convex optimization, where in every stage a
standard (non-robust) optimization program is solved. Our algorithms find an
approximate robust solution using a number of calls to an oracle that solves
the original (non-robust) problem that is inversely proportional to the square
of the target accuracy
Searching for a Gravitational Heating Signature in Nearby Luminous Ellipticals
We present a new deep optical study of a luminosity limited sample of nearby
elliptical galaxies, attempting to observe the effects of gravitational
interactions on the ISM of these objects. This study is motivated by recent
observations of M86, a nearby elliptical galaxy that shows possible evidence
for gas heating through a recent gravitational interaction. The complete sample
includes luminous ellipticals in clusters, groups and the field. For each of
the galaxies we objectively derive a tidal parameter which measures the
deviation of the stellar body from a smooth, relaxed model and find that 73% of
them show tidal disturbance signatures in their stellar bodies. This is the
first time that such an analysis is done on a statistically complete sample and
it confirms that elliptical galaxies continue to grow and evolve through
gravitational interactions even in the local Universe. Our study of ellipticals
in a wide range of interaction stages, along with available ISM data will
attempt to shed light on this possibly alternative mechanism for maintaining
the observed ISM temperatures of elliptical galaxies.Comment: To appear in proceedings of The Monster's Fiery Breath: Feedback in
Galaxies, Groups, and Clusters (AIP conference series
Mass growth and mergers: direct observations of the luminosity function of LRG satellite galaxies out to z=0.7 from SDSS and BOSS images
We present a statistical study of the luminosity functions of galaxies
surrounding luminous red galaxies (LRGs) at average redshifts =0.34 and
=0.65. The luminosity functions are derived by extracting source photometry
around more than 40,000 LRGs and subtracting foreground and background
contamination using randomly selected control fields. We show that at both
studied redshifts the average luminosity functions of the LRGs and their
satellite galaxies are poorly fitted by a Schechter function due to a
luminosity gap between the centrals and their most luminous satellites. We
utilize a two-component fit of a Schechter function plus a log-normal
distribution to demonstrate that LRGs are typically brighter than their most
luminous satellite by roughly 1.3 magnitudes. This luminosity gap implies that
interactions within LRG environments are typically restricted to minor mergers
with mass ratios of 1:4 or lower. The luminosity functions further imply that
roughly 35% of the mass in the environment is locked in the LRG itself,
supporting the idea that mass growth through major mergers within the
environment is unlikely. Lastly, we show that the luminosity gap may be at
least partially explained by the selection of LRGs as the gap can be reproduced
by sparsely sampling a Schechter function. In that case LRGs may represent only
a small fraction of central galaxies in similar mass halos.Comment: ApJ accepted versio
The faint stellar halos of massive red galaxies from stacks of more than 42000 SDSS LRG images
We study the properties of massive galaxies at an average redshift of z~0.34
through stacking more than 42000 images of Luminous Red Galaxies from the Sloan
Digital Sky Survey. This is the largest dataset ever used for such an analysis
and it allows us to explore the outskirts of massive red galaxies at
unprecedented physical scales. Our image stacks extend farther than 400 kpc,
where the r-band profile surface brightness reaches 30 mag arcsec-2. This
analysis confirms that the stellar bodies of luminous red galaxies follow a
simple Sersic profile out to 100 kpc. At larger radii the profiles deviate from
the best-fit Sersic models and exhibit extra light in the g, r, i and z-band
stacks. This excess light can probably be attributed to unresolved intragroup
or intracluster light or a change in the light profile itself. We further show
that standard analyses of SDSS-depth images typically miss 20% of the total
stellar light and underestimate the size of LRGs by 10% compared to our best
fit r-band Sersic model of n=5.5 and r_e=13.1 kpc. If the excess light at r>100
kpc is considered to be part of the galaxy, the best fit r-band Sersic
parameters are n=5.8 and r_e=13.6 kpc. In addition we study the radially
dependent stack ellipticity and find an increase with radius from e=0.25 at
r=10 kpc to e=0.3 at r=100 kpc. This provides support that the stellar light
that we trace out to at least 100 kpc is physically associated with the
galaxies themselves and may confirm that the halos of individual LRGs have
higher ellipticities than their central parts. Lastly we show that the
broadband color gradients of the stacked images are flat beyond roughly 40 kpc,
suggesting that the stellar populations do not vary significantly with radius
in the outer parts of massive ellipticals.Comment: Accepted for publication in Ap
The Frequency of Tidal Features Associated with Nearby Luminous Elliptical Galaxies from a Statistically Complete Sample
We present a deep broadband optical imaging study of a complete sample of
luminous elliptical galaxies (M_B<-20) at distances 15 Mpc - 50 Mpc, selected
from the Tully catalog of nearby galaxies. The images are flat to ~0.35% across
the 20' field and reach a V band depth of 27.7 mag arcsec^-2. We derive an
objective tidal interaction parameter for all galaxies and find that 73% of
them show tidal disturbance signatures in their stellar bodies. This is the
first time that such an analysis is done on a statistically complete sample and
it confirms that tidal features in ellipticals are common even in the local
Universe. From the dynamical time of the sample galaxies at the innermost
radius where tidal features are detected we estimate the mass assembly rate of
nearby ellipticals to be dM/M 0.2 per Gyr with large uncertainty. We explore
the relation between gravitational interaction signatures and the galaxy
environment and find that galaxies in clusters are less disturbed than group
and field galaxies. We also study how these interactions affect the broadband
colors of ellipticals and find a moderate correlation, suggesting that the
mergers are not accompanied by significant star-formation. Lastly, we find no
correlation between AGN activity, as measured by 6cm radio emission, and large
scale tidal distortions. This implies that gravitational interactions are not
the only, and perhaps not the most important, trigger of nuclear activity. In
summary, we find that elliptical galaxies in groups and low density
environments continue to grow at the present day through mostly "dry" mergers
involving little star formation.Comment: Accepted for publication in AJ. Appendix and full dataset available
online at: http://www.astro.yale.edu/obe
Determination of the Beta Ray Energy Spectrum from the Absorption Curves of Beta Rays
<p>(a) The decrease in food-discovery time between the first tests on two successive days; (b) the positive correlation between colony size and the number of workers searching (mean values for the first tests on the two successive days are presented). The asterisk indicates a significant difference.</p
DeepSTORM3D: dense three dimensional localization microscopy and point spread function design by deep learning
Localization microscopy is an imaging technique in which the positions of
individual nanoscale point emitters (e.g. fluorescent molecules) are determined
at high precision from their images. This is the key ingredient in
single/multiple-particle-tracking and several super-resolution microscopy
approaches. Localization in three-dimensions (3D) can be performed by modifying
the image that a point-source creates on the camera, namely, the point-spread
function (PSF). The PSF is engineered using additional optical elements to vary
distinctively with the depth of the point-source. However, localizing multiple
adjacent emitters in 3D poses a significant algorithmic challenge, due to the
lateral overlap of their PSFs. Here, we train a neural network to receive an
image containing densely overlapping PSFs of multiple emitters over a large
axial range and output a list of their 3D positions. Furthermore, we then use
the network to design the optimal PSF for the multi-emitter case. We
demonstrate our approach numerically as well as experimentally by 3D STORM
imaging of mitochondria, and volumetric imaging of dozens of
fluorescently-labeled telomeres occupying a mammalian nucleus in a single
snapshot.Comment: main text: 9 pages, 5 figures, supplementary information: 29 pages,
20 figure
A Spectacular H Complex in Virgo: Evidence for a Collision Between M86 and NGC 4438 and Implications for Collisional ISM Heating of Ellipticals
Deep wide-field H+[NII] imaging around the Virgo cluster giant
elliptical galaxy M86 reveals a highly complex and disturbed ISM/ICM. The most
striking feature is a set of H filaments which clearly connect M86 with
the nearby disturbed spiral NGC 4438 (23=120 kpc projected away), providing
strong evidence for a previously unrecognized collision between them.
Spectroscopy of selected regions show a fairly smooth velocity gradient between
M86 and NGC 4438, consistent with the collision scenario. Such a collision
would impart significant energy into the ISM of M86, probably heating the gas
and acting to prevent the gas from cooling to form stars. We propose that cool
gas stripped from NGC 4438 during the collision and deposited in its wake is
heated by shocks, ram pressure drag, or thermal conduction, producing most of
the H filaments. Some H filaments are associated with the
well-known ridge of bright X-ray emission to the NW of the nucleus, suggesting
that the collision is responsible for peculiarities of M86 previously ascribed
to other effects. M86 is radio-quiet, thus AGN heating is unlikely to play a
significant role. The M86 system has implications for understanding the role of
gravitational interactions in the heating of the ISM in ellipticals, and how
collisions in clusters transform galaxies.Comment: 6 pages, 2 figures. For high-resolution images, see
http://www.astro.yale.edu/tal/research/index.htm
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