631 research outputs found
The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1
We present initial results of a program to obtain and analyze HST WFPC2
images of galaxies identified in an imaging and spectroscopic survey of faint
galaxies in fields of HST spectroscopic target QSOs. We measure properties of
87 galaxies, of which 33 are associated with corresponding \lya absorption
systems and 24 do not produce corresponding \lya absorption lines to within
sensitive upper limits. Considering only galaxy and absorber pairs that are
likely to be physically associated and excluding galaxy and absorber pairs
within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and
seven galaxies that do not produce corresponding \lya absorption lines to
within sensitive upper limits. Redshifts of the galaxy and absorber pairs range
from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations
of the galaxy and absorber pairs range from 12.4 to kpc with a
median of kpc. The primary result of the analysis is that the
amount of gas encountered along the line of sight depends on the galaxy impact
parameter and B-band luminosity but does not depend strongly on the galaxy
average surface brightness, disk-to-bulge ratio, or redshift. This result
confirms and improves upon the anti-correlation between \lya absorption
equivalent width and galaxy impact parameter found previously by Lanzetta et
al. (1995). There is no evidence that galaxy interactions play an important
role in distributing tenuous gas around galaxies in most cases. Galaxies might
account for all \lya absorption systems with \AA, but this depends on
the unknown luminosity function and gaseous cross sections of low-luminosity
galaxies as well as on the uncertainties of the observed number density of \lya
absorption systems.Comment: Minor changes. Figure 1 stays intact and is available at
ftp://ftp.ess.sunysb.edu/pub/lanzetta/wfpc
The VIMOS VLT Deep Survey. The different assembly history of passive and star-forming L_B >= L*_B galaxies in the group environment at z < 1
We use the VIMOS VLT Deep Survey to study the close environment of galaxies
in groups at 0.2 = L*_B galaxies (Me_B =
M_B + 1.1z <= -20) are identified with Me_B <= -18.25 and within a relative
distance 5h^-1 kpc <= rp <= 100h^-1 kpc and relative velocity Delta v <= 500
km/s . The richness N of a group is defined as the number of Me_B <= -18.25
galaxies belonging to that group. We split our principal sample into red,
passive galaxies with NUV - r >= 4.25 and blue, star-forming galaxies with NUV
- r < 4.25. We find that blue galaxies with a close companion are primarily
located in poor groups, while the red ones are in rich groups. The number of
close neighbours per red galaxy increases with N, with n_red being proportional
to 0.11N, while that of blue galaxies does not depend on N and is roughly
constant. In addition, these trends are found to be independent of redshift,
and only the average n_blue evolves, decreasing with cosmic time. Our results
support the following assembly history of L_B >= L*_B galaxies in the group
environment: red, massive galaxies were formed in or accreted by the dark
matter halo of the group at early times (z >= 1), therefore their number of
neighbours provides a fossil record of the stellar mass assembly of groups,
traced by their richness N. On the other hand, blue, less massive galaxies have
recently been accreted by the group potential and are still in their parent
dark matter halo, having the same number of neighbours irrespective of N. As
time goes by, these blue galaxies settle in the group potential and turn red
and/or fainter, thus becoming satellite galaxies in the group. With a toy
quenching model, we estimate an infall rate of field galaxies into the group
environment of R_infall = 0.9 - 1.5 x 10^-4 Mpc^-3 Gyr^-1 at z ~ 0.7.Comment: Astronomy and Astrophysics, in press. 11 pages, 11 figures, 4 tables.
Minor changes with respect to the first versio
The VIMOS Ultra Deep Survey. Luminosity and stellar mass dependence of galaxy clustering at z~3
We present the study of the dependence of galaxy clustering on luminosity and
stellar mass in the redshift range 2z3.5 using 3236 galaxies with robust
spectroscopic redshifts from the VIMOS Ultra Deep Survey (VUDS). We measure the
two-point real-space correlation function for four volume-limited
stellar mass and four luminosity, M absolute magnitude selected,
sub-samples. We find that the scale dependent clustering amplitude
significantly increases with increasing luminosity and stellar mass indicating
a strong galaxy clustering dependence on these properties. This corresponds to
a strong relative bias between these two sub-samples of b/b=0.43.
Fitting a 5-parameter HOD model we find that the most luminous and massive
galaxies occupy the most massive dark matter haloes with
M = 10 h M. Similar to the
trends observed at lower redshift, the minimum halo mass M depends on
the luminosity and stellar mass of galaxies and grows from M
=10 hM to M=10 hM
from the faintest to the brightest among our galaxy sample, respectively. We
find the difference between these halo masses to be much more pronounced than
is observed for local galaxies of similar properties. Moreover, at z~3, we
observe that the masses at which a halo hosts, on average, one satellite and
one central galaxy is M4M over all luminosity ranges,
significantly lower than observed at z~0 indicating that the halo satellite
occupation increases with redshift. The luminosity and stellar mass dependence
is also reflected in the measurements of the large scale galaxy bias, which we
model as b(L)=1.92+25.36(L/L). We conclude our study
with measurements of the stellar-to-halo mass ratio (SHMR).Comment: 20 pages, 11 figures, A&A in press, v2. revised discussion in sec.
5.5, changed Fig. 4 and Fig. 11, added reference
The extended epoch of galaxy formation: age dating of ~3600 galaxies with 2<z<6.5 in the VIMOS Ultra-Deep Survey
We aim at improving constraints on the epoch of galaxy formation by measuring
the ages of 3597 galaxies with spectroscopic redshifts 2<z<6.5 in the VIMOS
Ultra Deep Survey (VUDS). We derive ages and other physical parameters from the
simultaneous fitting with the GOSSIP+ software of observed UV rest-frame
spectra and photometric data from the u-band up to 4.5 microns using composite
stellar population models. We conclude from extensive simulations that at z>2
the joint analysis of spectroscopy and photometry combined with restricted age
possibilities when taking into account the age of the Universe substantially
reduces systematic uncertainties and degeneracies in the age derivation. We
find galaxy ages ranging from very young with a few tens of million years to
substantially evolved with ages up to ~1.5-2 Gyr. The formation redshifts z_f
derived from the measured ages indicate that galaxies may have started forming
stars as early as z_f~15. We produce the formation redshift function (FzF), the
number of galaxies per unit volume formed at a redshift z_f, and compare the
FzF in increasing redshift bins finding a remarkably constant 'universal' FzF.
The FzF is parametrized with (1+z)^\zeta, with \zeta~0.58+/-0.06, indicating a
smooth 2 dex increase from z~15 to z~2. Remarkably this observed increase is of
the same order as the observed rise in the star formation rate density (SFRD).
The ratio of the SFRD with the FzF gives an average SFR per galaxy of
~7-17Msun/yr at z~4-6, in agreement with the measured SFR for galaxies at these
redshifts. From the smooth rise in the FzF we infer that the period of galaxy
formation extends from the highest possible redshifts that we can probe at z~15
down to redshifts z~2. This indicates that galaxy formation is a continuous
process over cosmic time, with a higher number of galaxies forming at the peak
in SFRD at z~2 than at earlier epochs. (Abridged)Comment: Submitted to A&A, 24 page
Degree Distribution of Human Brain Functional Connectivity is Generalized Pareto: A Multi-Scale Analysis
Are degree distributions of human brain functional connectivity networks heavy-tailed? Initial claims based on least-square fitting suggested that brain functional connectivity networks obey power law scaling in their degree distributions. This interpretation has been challenged on methodological grounds. Subsequently, estimators based on maximum-likelihood and non-parametric tests involving surrogate data have been proposed. No clear consensus has emerged as results especially depended on data resolution. To identify the underlying topological distribution of brain functional connectivity calls for a closer examination of the relationship between resolution and statistics of model fitting. In this study, we analyze high-resolution functional magnetic resonance imaging (fMRI) data from the Human Connectome Project to assess its degree distribution across resolutions. We consider resolutions from one thousand to eighty thousand regions of interest (ROIs) and test whether they follow a heavy or short-tailed distribution. We analyze power law, exponential, truncated power law, log-normal, Weibull and generalized Pareto probability distributions. Notably, the Generalized Pareto distribution is of particular interest since it interpolates between heavy-tailed and short-tailed distributions, and it provides a handle on estimating the tail's heaviness or shortness directly from the data. Our results show that the statistics support the short-tailed limit of the generalized Pareto distribution, rather than a power law or any other heavy-tailed distribution. Working across resolutions of the data and performing cross-model comparisons, we further establish the overall robustness of the generalized Pareto model in explaining the data. Moreover, we account for earlier ambiguities by showing that down-sampling the data systematically affects statistical results. At lower resolutions models cannot easily be differentiated on statistical grounds while their plausibility consistently increases up to an upper bound. Indeed, more power law distributions are reported at low resolutions (5K) than at higher ones (50K or 80K). However, we show that these positive identifications at low resolutions fail cross-model comparisons and that down-sampling data introduces the risk of detecting spurious heavy-tailed distributions. This dependence of the statistics of degree distributions on sampling resolution has broader implications for neuroinformatic methodology, especially, when several analyses rely on down-sampled data, for instance, due to a choice of anatomical parcellations or measurement technique. Our findings that node degrees of human brain functional networks follow a short-tailed distribution have important implications for claims of brain organization and function. Our findings do not support common simplistic representations of the brain as a generic complex system with optimally efficient architecture and function, modeled with simple growth mechanisms. Instead these findings reflect a more nuanced picture of a biological system that has been shaped by longstanding and pervasive developmental and architectural constraints, including wiring-cost constraints on the centrality architecture of individual nodes
HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?
The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain
the source of the hard ionizing continuum that powers the HeII emission. We
have assembled a sample of 277 galaxies with a high quality spectroscopic
redshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640A
emitters. We study their spectral properties, measuring the fluxes, equivalent
widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show
HeII in emission, with rest frame equivalent widths EW0~1-7A, equally
distributed between galaxies with Lya in emission or in absorption. We find 11
high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13
high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and
an additional 12 possible HeII emitters. The properties of the individual broad
emitters are in agreement with expectations from a W-R model. On the contrary,
the properties of the narrow emitters are not compatible with such model,
neither with predictions of gravitational cooling radiation produced by gas
accretion. Rather, we find that the EW of the narrow HeII line emitters are in
agreement with expectations for a PopIII star formation, if the episode of star
formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1
only is enough to sustain the observed HeII flux. We conclude that narrow HeII
emitters are either powered by the ionizing flux from a stellar population rare
at z~0 but much more common at z~3, or by PopIII star formation. As proposed by
Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of
pristine gas at the periphery of galaxies from which PopIII may form, even down
to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in
PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value
predicted by Tornatore et al. (2007).Comment: accepted for publication in A&
The infrared luminosity function of galaxies at redshifts z=1 and z~2 in the GOODS fields
We present the rest-frame 8 micron luminosity function (LF) at redshifts z=1
and ~2, computed from Spitzer 24 micron-selected galaxies in the GOODS fields
over an area of 291 sq. arcmin. Using classification criteria based on X-ray
data and IRAC colours, we identify the AGN in our sample. The rest-frame 8
micron LF for star-forming galaxies at redshifts z=1 and ~2 have the same shape
as at z~0, but with a strong positive luminosity evolution. The number density
of star-forming galaxies with log_{10}(nu L_nu(8 micron))>11 increases by a
factor >250 from redshift z~0 to 1, and is basically the same at z=1 and ~2.
The resulting rest-frame 8 micron luminosity densities associated with star
formation at z=1 and ~2 are more than four and two times larger than at z~0,
respectively. We also compute the total rest-frame 8 micron LF for star-forming
galaxies and AGN at z~2 and show that AGN dominate its bright end, which is
well-described by a power-law. Using a new calibration based on Spitzer
star-forming galaxies at 0<z<0.6 and validated at higher redshifts through
stacking analysis, we compute the bolometric infrared (IR) LF for star-forming
galaxies at z=1 and ~2. We find that the respective bolometric IR luminosity
densities are (1.2+/-0.2) x 10^9 and (6.6^{+1.2}_{-1.0}) x 10^8 L_sun Mpc^{-3},
in agreement with previous studies within the error bars. At z~2, around 90% of
the IR luminosity density associated with star formation is produced by
luminous and ultraluminous IR galaxies (LIRG and ULIRG), with the two
populations contributing in roughly similar amounts. Finally, we discuss the
consistency of our findings with other existing observational results on galaxy
evolution.Comment: Accepted for publication in the ApJ. 33 pages, 15 figures. Uses
emulateap
SUSTAINING MOROCCAN OASIS AGRICULTURAL SYSTEM through SMALL MECHANIZATION INPUTS
Saharan oases are a highly artificial ecosystem that has proved to be economicallyviable for centuries, despiteitsâfragile equilibrium. The pillar of this ecosystem is the good management of the palm grove, which allows the establishment of a three-layer structure and of a microclimate favourable for agriculture. Nowadays oases ecosystem is at risk, endangered by climate changes and economic and social factors and the poor management of the palm grove,caused by lack of human and material resources,is the main cause of itsâ collapsing. The introduction of small mechanization can provide tools to invigorate this farming system and attract young people in the agricultural sector
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