233 research outputs found
QUASII: QUery-Aware Spatial Incremental Index.
With large-scale simulations of increasingly detailed models and improvement of data acquisition technologies, massive amounts of data are easily and quickly created and collected. Traditional systems require indexes to be built before analytic queries can be executed efficiently. Such an indexing step requires substantial computing resources and introduces a considerable and growing data-to-insight gap where scientists need to wait before they can perform any analysis. Moreover, scientists often only use a small fraction of the data - the parts containing interesting phenomena - and indexing it fully does not always pay off. In this paper we develop a novel incremental index for the exploration of spatial data. Our approach, QUASII, builds a data-oriented index as a side-effect of query execution. QUASII distributes the cost of indexing across all queries, while building the index structure only for the subset of data queried. It reduces data-to-insight time and curbs the cost of incremental indexing by gradually and partially sorting the data, while producing a data-oriented hierarchical structure at the same time. As our experiments show, QUASII reduces the data-to-insight time by up to a factor of 11.4x, while its performance converges to that of the state-of-the-art static indexes
Ultraviolet to infrared emission of z>1 galaxies: Can we derive reliable star formation rates and stellar masses?
We seek to derive star formation rates (SFR) and stellar masses (M_star) in
distant galaxies and to quantify the main uncertainties affecting their
measurement. We explore the impact of the assumptions made in their derivation
with standard calibrations or through a fitting process, as well as the impact
of the available data, focusing on the role of IR emission originating from
dust. We build a sample of galaxies with z>1, all observed from the UV to the
IR (rest frame). The data are fitted with the code CIGALE, which is also used
to build and analyse a catalogue of mock galaxies. Models with different SFHs
are introduced. We define different set of data, with or without a good
sampling of the UV range, NIR, and thermal IR data. The impact of these
different cases on the determination of M_star and SFR are analysed.
Exponentially decreasing models with a redshift formation of the stellar
population z ~8 cannot fit the data correctly. The other models fit the data
correctly at the price of unrealistically young ages when the age of the single
stellar population is taken to be a free parameter. The best fits are obtained
with two stellar populations. As long as one measurement of the dust emission
continuum is available, SFR are robustly estimated whatever the chosen model
is, including standard recipes. M_star measurement is more subject to
uncertainty, depending on the chosen model and the presence of NIR data, with
an impact on the SFR-M_star scatter plot. Conversely, when thermal IR data from
dust emission are missing, the uncertainty on SFR measurements largely exceeds
that of stellar mass. Among all physical properties investigated here, the
stellar ages are found to be the most difficult to constrain and this
uncertainty acts as a second parameter in SFR measurements and as the most
important parameter for M_star measurements.Comment: 14 pages, 14 figures, accepted for publication A&
The spectral energy distribution of galaxies at z > 2.5: Implications from the Herschel/SPIRE color-color diagram
We use the Herschel SPIRE color-color diagram to study the spectral energy
distribution (SED) and the redshift estimation of high-z galaxies. We compiled
a sample of 57 galaxies with spectroscopically confirmed redshifts and SPIRE
detections in all three bands at , and compared their average SPIRE
colors with SED templates from local and high-z libraries. We find that local
SEDs are inconsistent with high-z observations. The local calibrations of the
parameters need to be adjusted to describe the average colors of high-z
galaxies. For high-z libraries, the templates with an evolution from z=0 to 3
can well describe the average colors of the observations at high redshift.
Using these templates, we defined color cuts to divide the SPIRE color-color
diagram into different regions with different mean redshifts. We tested this
method and two other color cut methods using a large sample of 783
Herschel-selected galaxies, and find that although these methods can separate
the sample into populations with different mean redshifts, the dispersion of
redshifts in each population is considerably large. Additional information is
needed for better sampling.Comment: 17 pages, 14 figures, accepted for publication in A&
Space odyssey: efficient exploration of scientific data.
Advances in data acquisition---through more powerful supercomputers for simulation or sensors with better resolution---help scientists tremendously to understand natural phenomena. At the same time, however, it leaves them with a plethora of data and the challenge of analysing it. Ingesting all the data in a database or indexing it for an efficient analysis is unlikely to pay off because scientists rarely need to analyse all data. Not knowing a priori what parts of the datasets need to be analysed makes the problem challenging. Tools and methods to analyse only subsets of this data are rather rare. In this paper we therefore present Space Odyssey, a novel approach enabling scientists to efficiently explore multiple spatial datasets of massive size. Without any prior information, Space Odyssey incrementally indexes the datasets and optimizes the access to datasets frequently queried together. As our experiments show, through incrementally indexing and changing the data layout on disk, Space Odyssey accelerates exploratory analysis of spatial data by substantially reducing query-to-insight time compared to the state of the art
AKARI/IRC Broadband Mid-infrared data as an indicator of Star Formation Rate
AKARI/Infrared Camera (IRC) Point Source Catalog provides a large amount of
flux data at {\it S9W} () and {\it L18W} ()
bands. With the goal of constructing Star-Formation Rate(SFR) calculations
using IRC data, we analyzed an IR selected
GALEX-SDSS-2MASS-AKARI(IRC/Far-Infrared Surveyor) sample of 153 nearby
galaxies. The far-infrared fluxes were obtained from AKARI diffuse maps to
correct the underestimation for extended sources raised by the point-spread
function photometry. SFRs of these galaxies were derived by the spectral energy
distribution fitting program CIGALE. In spite of complicated features contained
in these bands, both the {\it S9W} and {\it L18W} emission correlate with the
SFR of galaxies. The SFR calibrations using {\it S9W} and {\it L18W} are
presented for the first time. These calibrations agree well with previous works
based on Spitzer data within the scatters, and should be applicable to
dust-rich galaxies.Comment: PASJ, in pres
The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) III. The Ultraviolet Source Catalogs
In this paper we introduce the deepest and most extensive ultraviolet
extragalactic source catalogs of the Virgo Cluster area to date. Archival and
targeted GALEX imaging is compiled and combined to provide the deepest possible
coverage over ~120 deg^2 in the NUV (lambda_eff=2316 angstroms) and ~40 deg^2
in the FUV (lambda_eff=1539 angstroms) between 180 deg <= R.A. <= 195 deg and 0
deg <= Decl. <= 20 deg. We measure the integrated photometry of 1770 extended
UV sources of all galaxy types and use GALEX pipeline photometry for 1,230,855
point-like sources in the foreground, within, and behind the cluster. Extended
source magnitudes are reliable to m_UV ~22, showing ~0.01 sigma difference from
their asymptotic magnitudes. Point-like source magnitudes have a 1 sigma
standard deviation within ~0.2 mag down to m_uv ~23. The point-like source
catalog is cross-matched with large optical databases and surveys including the
SDSS DR9 (> 1 million Virgo Cluster sources), the Next Generation Virgo Cluster
Survey (NGVS; >13 million Virgo Cluster sources), and the NED (~30,000 sources
in the Virgo Cluster). We find 69% of the entire UV point-like source catalog
has a unique optical counterpart, 11% of which are stars and 129 are Virgo
cluster members neither in the VCC nor part of the bright CGCG galaxy catalog
(i.e., m_pg < 14.5). These data are collected in four catalogs containing the
UV extended sources, the UV point-like sources, and two catalogs each
containing the most relevant optical parameters of UV-optically matched
point-like sources for further studies from SDSS and NGVS. The GUViCS catalogs
provide a unique set of data for future works on UV and multiwavelength studies
in the cluster and background environments.Comment: 35 pages, 24 figures, 15 tables, Accepted for publication in A&
Perceived Realism of Pedestrian Crowds Trajectories in VR
Crowd simulation algorithms play an essential role in populating Virtual Reality (VR) environments with multiple autonomous humanoid agents. The generation of plausible trajectories can be a significant computational cost for real-time graphics engines, especially in untethered and mobile devices such as portable VR devices. Previous research explores the plausibility and realism of crowd simulations on desktop computers but fails to account the impact it has on immersion. This study explores how the realism of crowd trajectories affects the perceived immersion in VR. We do so by running a psychophysical experiment in which participants rate the realism of real/synthetic trajectories data, showing similar level of perceived realism
Services procurement : a systematic literature review of practices and challenges
Organizations are paying greater attention to the potential advantages that can be achieved by adopting a more strategic approach to the procurement of services. Despite services being very different from physical items in many respects, and despite their outsourcing having achieved limited gains, the procurement of services remains under-researched. To address this challenge and develop a strategic platform for new directions in future research in the area, this paper undertakes a systematic literature review of 51 articles published in 21 peer-reviewed academic journals. It reviews the applicability of supply theories to services sourcing, and compares and demonstrates the distinctiveness of services purchasing through problematizing the literature reviewed. A descriptive and thematic analysis concluded that services procurement can be classified into seven research domains: âservice productionâ, âgovernanceâ, âpurchasing approachâ, âsupplier selectionâ, âperformance managementâ, âthe service triadâ and âspecification of requirementsâ. We offer a comparative framework of the services procurement process and emphasize different supply practices. The provided research directions assist scholars in identifying avenues for integrating and expanding existing knowledge
Dust properties of Lyman break galaxies at
We explore from a statistical point of view the far-infrared (far-IR) and
sub-millimeter (sub-mm) properties of a large sample of LBGs (22,000) at z~3 in
the COSMOS field. The large number of galaxies allows us to split it in several
bins as a function of UV luminosity, UV slope, and stellar mass to better
sample their variety. We perform stacking analysis in PACS (100 and 160 um),
SPIRE (250, 350 and 500 um) and AzTEC (1.1 mm) images. Our stacking procedure
corrects the biases induced by galaxy clustering and incompleteness of our
input catalogue in dense regions. We obtain the full IR spectral energy
distributions (SED) of subsamples of LBGs and derive the mean IR luminosity as
a function of UV luminosity, UV slope, and stellar mass. The average IRX is
roughly constant over the UV luminosity range, with a mean of 7.9 (1.8 mag).
However, it is correlated with UV slope, and stellar mass. We investigate using
a statistically-controlled stacking analysis as a function of (stellar mass, UV
slope) the dispersion of the IRX-UVslope and IRX-M* plane. Our results enable
us to study the average relation between star-formation rate (SFR) and stellar
mass, and we show that our LBG sample lies on the main sequence of star
formation at z~3.Comment: Accepted to A&A, 17 Pages, 14 Figures, 2 Table
Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe
We present the first measurements of the angular correlation function of
galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the
GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions.
The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22,
yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm
0.6 Mpc and no significant trend is seen for this value as a function of the
limiting apparent magnitude or between the GALEX bands. This estimate is close
to that found from samples of blue galaxies in the local universe selected in
the visible, and similar to that derived at z ~ 3 for LBGs with similar rest
frame selection criteria. This result supports models that predict anti-biasing
of star forming galaxies at low redshift, and brings an additional clue to the
downsizing of star formation at z<1.Comment: Accepted for publication in GALEX Special ApJs, December 200
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