126 research outputs found
The GHOSTS survey. II. The diversity of Halo Color and Metallicity Profiles of Massive Disk Galaxies
We study the stellar halo color properties of six nearby massive highly
inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3 observations in both F606W and F814W filters from the
GHOSTS survey. The observed fields, placed both along the minor and major axis
of each galaxy, probe the stellar outskirts out to projected distances of ~
50-70 kpc from their galactic centre along the minor axis. The 50% completeness
levels of the color magnitude diagrams are typically at two mag below the tip
of the red giant branch. We find that all galaxies have extended stellar halos
out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color
distribution and color profile for each galaxy using the median colors of stars
in the RGB. Within each galaxy we find variations in the median colors as a
function of radius which likely indicates population variations, reflecting
that their outskirts were built from several small accreted objects. We find
that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear
negative color gradient, reflecting a declining metallicity in their halos; the
other have no significant color or population gradient. In addition,
notwithstanding the modest sample size of galaxies, there is no strong
correlation between their halo color/metallicity or gradient with galaxy's
properties such as rotational velocity or stellar mass. The diversity in halo
color profiles observed in the GHOSTS galaxies qualitatively supports the
predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of
the stochasticity inherent in the assembling history of galaxies.Comment: Accepted by MNRAS, 31 pages, 18 figures. Appendix added and some
editions to match accepted version. Conclusions unchange
A Classification of Decision Automation and Delegation in Digital Investment Management Systems
Digital investment management systems, commonly known as robo-advisors, provide new alternatives to traditional human services, offering competitive investment returns at lower cost and customer effort. However, users must give up control over their investments and rely on automated decision-making. Because humans display aversion to high levels of automation and delegation, it is important to understand the interplay of these two aspects. This study proposes a taxonomy of digital investment management systems based on their levels of decision automation and delegation along the investment management process. We find that the degree of automation depends on the frequency and urgency of decisions as well as the accuracy of algorithms. Notably, most providers only invest in a subset of funds pre-selected by humans, potentially limiting efficiency gains. Based on our taxonomy, we identify archetypical system designs, which facilitate further research on perception and adoption of digital investment management systems
Extragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies
We study the individual evolution histories of three nearby low-mass edge-on
galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations,
we constructed star count density maps for populations of different ages and
analyzed the change of structural parameters with stellar age within each
galaxy.
We do not detect a separate thick disk in any of the three galaxies, even
though our observations cover a wider range in equivalent surface brightness
than any integrated light study. While scale heights increase with age, each
population can be well described by a single disk. Two of the galaxies contain
a very weak additional component, which we identify as the faint halo. The mass
of these faint halos is lower than 1% of the mass of the disk. The three
galaxies show low vertical heating rates, which are much lower than the heating
rate of the Milky Way. This indicates that heating agents, such as giant
molecular clouds and spiral structure, are weak in low-mass galaxies. All
populations in the three galaxies exhibit no or only little flaring. While this
finding is consistent with previous integrated light studies, it poses strong
constraints on galaxy simulations, where strong flaring is often found as a
result of interactions or radial migration.Comment: Accepted for publication in A&
A probabilistic approach to hybrid role mining
Role mining algorithms address an important access con-trol problem: configuring a role-based access control sys-tem. Given a direct assignment of users to permissions, role mining discovers a set of roles together with an assignment of users to roles. The results should closely agree with the direct assignment. Moreover, the roles should be under-standable from the business perspective in that they reflect functional roles within the enterprise. This requires hybrid role mining methods that work with both direct assignments and business information from the enterprise. In this paper, we provide statistical measures to analyze the relevance of different kinds of business information for defining roles. We then present an approach that incor-porates relevant business information into a probabilistic model with an associated algorithm for hybrid role mining. Experiments on actual enterprise data show that our algo-rithm yields roles that both explain the given user-permission assignments and are meaningful from the business perspec-tive
Testing galaxy formation models with the GHOSTS survey: The color profile of M81's stellar halo
We study the properties of the stellar populations in M81's outermost part,
which hereafter we will term the stellar halo, using HST ACS/WFC observations
of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo
out to a projected distance of ~ 50 kpc from the galactic center. Each field
was observed in both F606W and F814W filters. The 50% completeness levels of
the color magnitude diagrams (CMDs) are typically at 2 mag below the tip of the
red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence
of disk-dominated populations whereas fields at larger distances are mostly
populated by halo stars. The RGB of the M81's halo CMDs is well matched with
isochrones of ~ 10 Gyr and metallicities [Fe/H] ~ -1.2 dex, suggesting that the
dominant stellar population of M81's halo has a similar age and metallicity.
The halo of M81 is characterized by a color distribution of width ~ 0.4 mag and
an approximately constant median value of (F606W - F814W) ~ 1 mag measured
using stars within the magnitude range 23.7 < F814W < 25.5. When considering
only fields located at galactocentric radius R > 15 kpc, we detect no color
gradient in the stellar halo of M81. We place a limit of 0.03+/-0.11 mag
difference between the median color of RGB M81 halo stars at ~ 15 and at 50
kpc, corresponding to a metallicity difference of 0.08+/-0.35 dex over that
radial range for an assumed constant age of 10 Gyr. We compare these results
with model predictions for the colors of stellar halos formed purely via
accretion of satellite galaxies. When we analyze the cosmologically motivated
models in the same way as the HST data, we find that they predict no color
gradient for the stellar halos, in good agreement with the observations.Comment: 16 pages, 7 figures. Accepted to Ap
Constraining the age of the NGC 4565 HI Disk Warp: Determining the Origin of Gas Warps
We have mapped the distribution of young and old stars in the gaseous HI warp
of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the
warp, but no coincident old stars (>1 Gyr), which places an upper limit on the
age of the structure. The formation rate of the young stars, which increased
~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5
M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp,
similar to the timescales calculated for the outer HI disks of nearby spiral
galaxies. While some stars associated with the warp fall into the asymptotic
giant branch (AGB) region of the color magnitude diagram, where stars could be
as old as 1 Gyr, further investigation suggests that they may be interlopers
rather than real AGB stars. We discuss the implications of these age
constraints for the formation of HI warps, and the gas fueling of disk
galaxies.Comment: 12 pages, 9 figures. Accepted for publication in Ap
Estimation of Attendance at a Large Outdoor Event
Accurately estimating program attendance in large, undefined areas is difficult. Yet attendance is an important factor in effective impact assessment and accountability reporting. A simple method, consisting of a combined activity count and exit poll, can be used to produce reasonable results with a measurable assurance of accuracy. A case study application of this method at a major university campus horticulture event is included to summarize the method
GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc
We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS
data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick
disk, and Star clusters) fields. Its detected individual stars populate an
approximately one magnitude range of its luminosity function (LF). Using
synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we
find that the colors and magnitudes of GHOSTS I's individual stars are most
consistent with being young helium-burning and asymptotic giant branch stars at
a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively
forming stars, so we tentatively classify it as a dwarf irregular (dIrr)
galaxy, although future HST observations deep enough to resolve a larger
magnitude range in its LF are required to make a more secure classification.
GHOSTS I's absolute magnitude is , making it one
of the least luminous dIrr galaxies known, and its metallicity is lower than
[Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its
ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at
comparable luminosity. There are no luminous massive galaxies or galaxy
clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host,
making it a very isolated dwarf galaxy in the Local Universe.Comment: 8 pages, 7 figures. Accepted for publication in Ap
Rapid design and manufacture tools in architecture
The continuing development of rapid prototyping technologies and the introduction of concept modelling technologies means that their use is expanding into a greater range of applications. The primary aim of this paper is to give the reader an overview of the current state of the art in layered manufacturing (LM) technology and its applicability in the field of architecture. The paper reports on the findings of a benchmarking study, conducted by the Rapid Design and Manufacturing (RDM) Group in Glasgow [G.J. Ryder, A. McGown, W. Ion, G. Green, D. Harrison, B. Wood, Rapid prototyping feasibility report, Rapid Prototyping Group, Glasgow School of Art, 1998.], which identified that the applicability of LM technologies in any application can be governed by a series of critical process and application specific issues. A further survey carried out by the RDM group investigated current model making practice, current 3D CAD use and current use of LM technologies within the field of architecture. The findings are then compared with the capabilities of LM technologies. Future research needs in this area are identified and briefly outlined
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