54 research outputs found
Pareto optimality in house allocation problems
We study Pareto optimal matchings in the context of house allocation problems. We present an O(\sqrt{n}m) algorithm, based on Gales Top Trading Cycles Method, for finding a maximum cardinality Pareto optimal matching, where n is the number of agents and m is the total length of the preference lists. By contrast, we show that the problem of finding a minimum cardinality Pareto optimal matching is NP-hard, though approximable within a factor of 2. We then show that there exist Pareto optimal matchings of all sizes between a minimum and maximum cardinality Pareto optimal matching. Finally, we introduce the concept of a signature, which allows us to give a characterization, checkable in linear time, of instances that admit a unique Pareto optimal matching
UV Luminosity Function at z~4, 3, and 2
We use very deep (R_lim=27) UGRI imaging to study the evolution of the faint
end of the UV-selected galaxy luminosity function from z~4 to z~2. We find that
the number of sub-L* galaxies increases from z~4 to z~3 while the number of
bright ones appears to remain constant. We find no evidence for continued
evolution to lower redshift, z~2. If real, this differential evolution of the
luminosity function suggests that differentially comparing key diagnostics of
dust, stellar populations, etc. as a function of z and L may let us isolate the
key mechanisms that drive galaxy evolution at high redshift and we describe
several such studies currently underway.Comment: To appear in proceedings of the conference "Starbursts - from 30
Doradus to Lyman break galaxies" (IoA, Cambridge UK, Sep 2004), Astrophysics
& Space Science Library, eds. de Grijs R., Gonzalez Delgado R.M. (Kluwer:
Dordrecht
Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs. :III. Stellar masses, star formation rates, and metallicities at z > 1
(Abridged) Long gamma-ray bursts (LGRB) have been suggested as promising
tracers of star formation owing to their association with the core-collapse of
massive stars. The goal of this work is to characterise the population of host
galaxies of LGRBs at 1 < z < 2, investigate the conditions in which LGRBs form
at these redshifts and assess their use as tracers of star formation. We
perform a spectro-photometric analysis to determine the stellar mass, star
formation rate, specific star formation rate and metallicity of the complete,
unbiased host galaxy sample of the Swift/BAT6 LGRB sample at 1 < z < 2. We
compare the distribution of these properties to the ones of typical
star-forming galaxies from the MOSDEF and COSMOS2015 Ultra Deep surveys, within
the same redshift range. We find that, similarly to z < 1, LGRBs do not
directly trace star formation at 1 < z < 2, and they tend to avoid high-mass,
high-metallicity host galaxies. We also find evidence for an enhanced fraction
of starbursts among the LGRB host sample with respect to the star-forming
population of galaxies. Nonetheless we demonstrate that the driving factor
ruling the LGRB efficiency is metallicity. The LGRB host distributions can be
reconciled with the ones expected from galaxy surveys by imposing a metallicity
upper limit of 12+logOH ~ 8.55. Metallicity rules the LGRB production
efficiency, which is stifled at Z > 0.7 Zsun. Under this hypothesis we can
expect LGRBs to trace star formation at z > 3, once the bulk of the star
forming galaxy population are characterised by metallicities below this limit.
The moderately high metallicity threshold found is in agreement with the
conditions necessary to rapidly produce a fast-rotating Wolf-Rayet star a in
close binary system, and could be accommodated by single star models under
chemically homogeneous mixing with very rapid rotation and weak magnetic
coupling.Comment: 19 pages, 17 figures, accepted for publication in Astronomy &
Astrophysic
The Great Observatories Origins Deep Survey - VLT/VIMOS Spectroscopy in the GOODS-South Field: Part II
We present the full data set of the VIMOS spectroscopic campaign of the
ESO/GOODS program in the CDFS, which complements the FORS2 ESO/GOODS
spectroscopic campaign. The GOODS/VIMOS spectroscopic campaign is structured in
two separate surveys using two different VIMOS grisms. The VIMOS Low Resolution
Blue (LR-Blue) and Medium Resolution (MR) orange grisms have been used to cover
different redshift ranges. The LR-Blue campaign is aimed at observing galaxies
mainly at 1.8<z<3.5, while the MR campaign mainly aims at galaxies at z<1 and
Lyman Break Galaxies (LBGs) at z>3.5. The full GOODS/VIMOS spectroscopic
campaign consists of 20 VIMOS masks. This release adds 8 new masks to the
previous release (12 masks, Popesso et al. 2009). In total we obtained 5052
spectra, 3634 from the 10 LR-Blue masks and 1418 from the 10 MR masks. A
significant fraction of the extracted spectra comes from serendipitously
observed sources: ~21% in the LR-Blue and ~16% in the MR masks. We obtained
2242 redshifts in the LR-Blue campaign and 976 in the MR campaign for a total
success rate of 62% and 69% respectively, which increases to 66% and 73% if
only primary targets are considered. The typical redshift uncertainty is
estimated to be ~0.0012 (~255 km/s) for the LR-Blue grism and ~0.00040 (~120
km/s) for the MR grism. By complementing our VIMOS spectroscopic catalog with
all existing spectroscopic redshifts publicly available in the CDFS, we
compiled a redshift master catalog with 7332 entries, which we used to
investigate large scale structures out to z~3.7. We produced stacked spectra of
LBGs in a few bins of equivalent width (EW) of the Ly-alpha and found evidence
for a lack of bright LBGs with high EW of the Ly-alpha. Finally, we obtained
new redshifts for 12 X-ray sources of the CDFS and extended-CDFS.Comment: 22 pages, 20 figures, accepted for publication on Astronomy and
Astrophysics, catalogs and data products are available at
http://archive.eso.org/cms/eso-data/data-packages/goods-vimos-spectroscopy-data-release-version-2.0/,
for ESO-GOODS related material consult
http://www.eso.org/sci/activities/projects/goods
Gas filaments of the cosmic web located around active galaxies in a protocluster
Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-alpha emission. Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive proto-clusters
Cosmic histories of star formation and reionization: An analysis with a power-law approximation
With a simple power-law approximation of high-redshift () star
formation history, i.e., , we
investigate the reionization of intergalactic medium (IGM) and the consequent
Thomson scattering optical depth for cosmic microwave background (CMB) photons.
A constraint on the evolution index is derived from the CMB optical
depth measured by the {\it Wilkinson Microwave Anisotropy Probe} (WMAP)
experiment, which reads ,
where the free parameter is the number of the escaped
ionizing ultraviolet photons per baryon. Moreover, the redshift for full
reionization, , can also be expressed as a function of as well as
. By further taking into account the implication of the
Gunn-Peterson trough observations to quasars for the full reionization
redshift, i.e., , we obtain
and .
For a typical number of of ionizing photons released per baryon of
normal stars, the fraction of these photons escaping from the stars, , can be constrained to within the range of .Comment: 10 pages, 4 figures, accepted for publication in JCA
- …