1,808 research outputs found
Young and intermediate-age massive star clusters
An overview of our current understanding of the formation and evolution of
star clusters is given, with main emphasis on high-mass clusters. Clusters form
deeply embedded within dense clouds of molecular gas. Left-over gas is cleared
within a few million years and, depending on the efficiency of star formation,
the clusters may disperse almost immediately or remain gravitationally bound.
Current evidence suggests that a few percent of star formation occurs in
clusters that remain bound, although it is not yet clear if this fraction is
truly universal. Internal two-body relaxation and external shocks will lead to
further, gradual dissolution on timescales of up to a few hundred million years
for low-mass open clusters in the Milky Way, while the most massive clusters (>
10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe.
The low-mass end of the initial cluster mass function is well approximated by a
power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that
quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5
Msun. In starburst galaxies and old globular cluster systems, this limit
appears to be higher, at least several x 10^6 Msun. The difference is likely
related to the higher gas densities and pressures in starburst galaxies, which
allow denser, more massive giant molecular clouds to form. Low-mass clusters
may thus trace star formation quite universally, while the more long-lived,
massive clusters appear to form preferentially in the context of violent star
formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
PDFLaTeX, requires rspublic.cls style fil
Adaptive Regret Minimization in Bounded-Memory Games
Online learning algorithms that minimize regret provide strong guarantees in
situations that involve repeatedly making decisions in an uncertain
environment, e.g. a driver deciding what route to drive to work every day.
While regret minimization has been extensively studied in repeated games, we
study regret minimization for a richer class of games called bounded memory
games. In each round of a two-player bounded memory-m game, both players
simultaneously play an action, observe an outcome and receive a reward. The
reward may depend on the last m outcomes as well as the actions of the players
in the current round. The standard notion of regret for repeated games is no
longer suitable because actions and rewards can depend on the history of play.
To account for this generality, we introduce the notion of k-adaptive regret,
which compares the reward obtained by playing actions prescribed by the
algorithm against a hypothetical k-adaptive adversary with the reward obtained
by the best expert in hindsight against the same adversary. Roughly, a
hypothetical k-adaptive adversary adapts her strategy to the defender's actions
exactly as the real adversary would within each window of k rounds. Our
definition is parametrized by a set of experts, which can include both fixed
and adaptive defender strategies.
We investigate the inherent complexity of and design algorithms for adaptive
regret minimization in bounded memory games of perfect and imperfect
information. We prove a hardness result showing that, with imperfect
information, any k-adaptive regret minimizing algorithm (with fixed strategies
as experts) must be inefficient unless NP=RP even when playing against an
oblivious adversary. In contrast, for bounded memory games of perfect and
imperfect information we present approximate 0-adaptive regret minimization
algorithms against an oblivious adversary running in time n^{O(1)}.Comment: Full Version. GameSec 2013 (Invited Paper
A Candidate Brightest Proto-Cluster Galaxy at z = 3.03
We report the discovery of a very bright (m_R = 22.2) Lyman break galaxy at z
= 3.03 that appears to be a massive system in a late stage of merging. Deep
imaging reveals multiple peaks in the brightness profile with angular
separations of ~0.''8 (~25 h^-1 kpc comoving). In addition, high
signal-to-noise ratio rest-frame UV spectroscopy shows evidence for ~5
components based on stellar photospheric and ISM absorption lines with a
velocity dispersion of sigma ~460 km s^-1 for the three strongest components.
Both the dynamics and high luminosity, as well as our analysis of a LCDM
numerical simulation, suggest a very massive system with halo mass M ~ 10^13
M_solar. The simulation finds that all halos at z = 3 of this mass contain
sub-halos in agreement with the properties of these observed components and
that such systems typically evolve into M ~ 10^14 M_solar halos in groups and
clusters by z = 0. This discovery provides a rare opportunity to study the
properties and individual components of z ~ 3 systems that are likely to be the
progenitors to brightest cluster galaxies.Comment: 14 pages, 3 figures, submitted to ApJ Letter
The Physical Nature of Rest-UV Galaxy Morphology During the Peak Epoch of Galaxy Formation
Motivated by the irregular and little-understood morphologies of z ~ 2 - 3
galaxies, we use non-parametric coefficents to quantify the morphologies of 216
galaxies which have been spectroscopically confirmed to lie at redshifts z =
1.8 - 3.4 in the GOODS-N field. Using measurements of ultraviolet (UV) and
optical spectral lines, multi-band photometric data, and stellar population
models we statistically assess possible correlations between galaxy morphology
and physical observables such as stellar mass, star formation rate, and the
strength of galaxy-scale outflows. We find evidence that dustier galaxies have
more nebulous UV morphologies and that larger, more luminous galaxies may drive
stronger outflows, but otherwise conclude that UV morphology is either
statistically decoupled from the majority of physical observables or determined
by too complex a combination of physical processes to provide characterizations
with predictive power. Given the absence of strong correlations between UV
morphology and physical parameters such as star formation rates, we are
therefore unable to support the hypothesis that morphologically irregular
galaxies predominantly represent major galaxy mergers. Comparing galaxy
samples, we find that IR-selected BzK galaxies and radio-selected submillimeter
galaxies (SMGs) have UV morphologies similar to the optically selected sample,
while distant red galaxies (DRGs) are more nebulous.Comment: 26 pages. Accepted for publication in the ApJ. Version with full
resolution figures is available at
http://www.astro.caltech.edu/~drlaw/Papers/UVmorph.pd
Spectroscopic Identification of a Proto-Cluster at z=2.300: Environmental Dependence of Galaxy Properties at High Redshift
We have discovered a highly significant over-density of galaxies at
z=2.300+/-0.015 in the course of a redshift survey designed to select
star-forming galaxies in the redshift range z=2.3+/-0.4 in the field of the
bright z=2.72 QSO HS1700+643. The structure has a redshift-space galaxy
over-density of delta_g,z ~= 7 and an estimated matter over-density in real
space of delta_m ~= 1.8, indicating that it will virialize by z~0 with a mass
scale of ~= 1.4x10^15 M_sun, that of a rich galaxy cluster. Detailed modeling
of the spectral energy distribution -- from the rest-far-UV to the rest-near-IR
-- of the 72 spectroscopically confirmed galaxies in this field for which we
have obtained K_s and Spitzer/IRAC photometry, allows for a first direct
comparison of galaxy properties as a function of large-scale environment at
high redshift. We find that galaxies in the proto-cluster environment have mean
stellar masses and inferred ages that are ~2 times larger (at z=2.30) than
identically UV-selected galaxies outside of the structure, and show that this
is consistent with simple theoretical expectations for the acceleration of
structure formation in a region that is over-dense on large scales by the
observed amount. The proto-cluster environment contains a significant number of
galaxies that already appear old, with large stellar masses (>10^11 M_sun), by
z=2.3.Comment: 7 pages including 3 figures. Accepted for publication in ApJ. Typo
correcte
Integral Field Spectroscopy of High-Redshift Star Forming Galaxies with Laser Guided Adaptive Optics: Evidence for Dispersion-Dominated Kinematics
We present early results from an ongoing study of the kinematic structure of
star-forming galaxies at redshift z ~ 2 - 3 using integral-field spectroscopy
of rest-frame optical nebular emission lines in combination with Keck laser
guide star adaptive optics (LGSAO). We show kinematic maps of 3 target galaxies
Q1623-BX453, Q0449-BX93, and DSF2237a-C2 located at redshifts z = 2.1820,
2.0067, and 3.3172 respectively, each of which is well-resolved with a PSF
measuring approximately 0.11 - 0.15 arcsec (~ 900 - 1200 pc at z ~ 2-3) after
cosmetic smoothing. Neither galaxy at z ~ 2 exhibits substantial kinematic
structure on scales >~ 30 km/s; both are instead consistent with largely
dispersion-dominated velocity fields with sigma ~ 80 km/s along any given line
of sight into the galaxy. In contrast, DSF2237a-C2 presents a well-resolved
gradient in velocity over a distance of ~ 4 kpc with peak-to-peak amplitude of
140 km/s. It is unlikely that DSF2237a-C2 represents a dynamically cold
rotating disk of ionized gas as the local velocity dispersion of the galaxy
(sigma = 79 km/s) is comparable to the observed shear. Using extant
multi-wavelength spectroscopy and photometry we relate these kinematic data to
physical properties such as stellar mass, gas fraction, star formation rate,
and outflow kinematics and consider the applicability of current galaxy
formation models.[Abridged]Comment: 19 pages, 10 figures (5 color); accepted for publication in ApJ.
Version with full-resolution figures is available at
http://www.astro.caltech.edu/~drlaw/Papers/OSIRIS_data1.pd
Flows and Decompositions of Games: Harmonic and Potential Games
In this paper we introduce a novel flow representation for finite games in
strategic form. This representation allows us to develop a canonical direct sum
decomposition of an arbitrary game into three components, which we refer to as
the potential, harmonic and nonstrategic components. We analyze natural classes
of games that are induced by this decomposition, and in particular, focus on
games with no harmonic component and games with no potential component. We show
that the first class corresponds to the well-known potential games. We refer to
the second class of games as harmonic games, and study the structural and
equilibrium properties of this new class of games. Intuitively, the potential
component of a game captures interactions that can equivalently be represented
as a common interest game, while the harmonic part represents the conflicts
between the interests of the players. We make this intuition precise, by
studying the properties of these two classes, and show that indeed they have
quite distinct and remarkable characteristics. For instance, while finite
potential games always have pure Nash equilibria, harmonic games generically
never do. Moreover, we show that the nonstrategic component does not affect the
equilibria of a game, but plays a fundamental role in their efficiency
properties, thus decoupling the location of equilibria and their payoff-related
properties. Exploiting the properties of the decomposition framework, we obtain
explicit expressions for the projections of games onto the subspaces of
potential and harmonic games. This enables an extension of the properties of
potential and harmonic games to "nearby" games. We exemplify this point by
showing that the set of approximate equilibria of an arbitrary game can be
characterized through the equilibria of its projection onto the set of
potential games
Evidence of Substructure in the Cluster of Galaxies A3558
We investigate the dynamical properties of the cluster of galaxies A3558
(Shapley 8). Studying a region of one square degree ( 3 Mpc) centered
on the cluster cD galaxy, we have obtained a statistically complete photometric
catalog with positions and magnitudes of 1421 galaxies (down to a limiting
magnitude of ). This catalog has been matched to the recent velocity
data obtained by Mazure et al. (1997) and from the literature, yielding a
radial velocity catalog containing 322 galaxies. Our analysis shows that the
position/velocity space distribution of galaxies shows significant
substructure. A central bimodal core detected previously in preliminary studies
is confirmed by using the Adaptive Kernel Technique and Wavelet Analysis. We
show that this central bimodal subtructure is nevertheless composed of a
projected feature, kinematically unrelated to the cluster, plus a group of
galaxies probably in its initial merging phase into a relaxed core. The cD
velocity offset with respect to the average cluster redshift, reported earlier
by several authors, is completely eliminated as a result of our dynamical
analysis. The untangling of the relaxed core component also allows a better,
more reliable determination of the central velocity dispersion, which in turn
eliminates the ``-problem'' for A3558. The cluster also shows a
``preferential'' distribution of subclumps coinciding with the direction of the
major axis position angle of the cD galaxy and of the central X-ray emission
ellipsoidal distribution, in agreement with an anisotropic merger scenario.Comment: 35 pages in latex, 17 figures in Postscript, accepted for publication
in the Astrophysical Journa
Interpreting high [O III]/H β ratios with maturing starbursts
Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local (z < 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O III]/Hβ=3.36+0.14−0.04 – comparable to all but the highest ratios seen in star-forming galaxies at z ∼ 2–4. We argue that the stellar population synthesis code BPASS can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ∼100 Myr after a starburst typically have a line ratio [O III]/Hβ ∼ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations
Statistics of Stellar Populations of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud
A comparative analysis of Washington color-magnitude diagrams (CMDs) for 14
star clusters and respective surrounding fields in the Large Magellanic Cloud
(LMC) outer disk is presented. Each CCD frame including field and respective
cluster covers an area of 185 arcmin^2. The stellar population sampled is of
intermediate age and metallicity. CMD radial analysis involving star count
ratios, morphology and integrated light properties are carried out. Luminosity
functions (LFs) are also presented. Two main results are: (i) Within the range
4<R(kpc)<8, the distance from the LMC center is well correlated with the
average age in the sense that inner fields are younger and; (ii) Beyond
approximately 8kpc the outer fields do not show evidence of a significant
intermediate-age component in their stellar populations, as inferred from red
giant clump star counts.Comment: 27 pages, 4 tables, 11 figures; accepted by the A
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