525 research outputs found
Fluorescence and absorption properties of chromophoric dissolved organic matter (CDOM) in coastal surface waters of the northwestern Mediterranean Sea, influence of the RhĂŽne River
International audienceSeawater samples were collected monthly in surface waters (2 and 5 m depths) of the Bay of Marseilles (northwestern Mediterranean Sea; 5 âą 17 30 E, 43 âą 14 30 N) during one year from November 2007 to December 2008 and studied for total organic carbon (TOC) as well as chro-mophoric dissolved organic matter (CDOM) optical properties (absorbance and fluorescence). The annual mean value of surface CDOM absorption coefficient at 350 nm [a CDOM (350)] was very low (0.10 ± 0.02 m â1) in comparison to values usually found in coastal waters, and no significant seasonal trend in a CDOM (350) could be determined. By contrast, the spectral slope of CDOM absorption (S CDOM) was significantly higher (0.023 ± 0.003 nm â1) in summer than in fall and winter periods (0.017 ± 0.002 nm â1), reflecting either CDOM photobleaching or production in surface waters during stratified sunny periods. The CDOM fluorescence, assessed through excitation emission matrices (EEMs), was dominated by protein-like component (peak T; 1.30-21.94 QSU) and marine humic-like component (peak M; 0.55-5.82 QSU), while terrestrial humic-like fluores-cence (peak C; 0.34-2.99 QSU) remained very low. This reflected a dominance of relatively fresh material from biological origin within the CDOM fluorescent pool. At the end of summer, surface CDOM fluorescence was very low and strongly blue shifted, reinforcing the hypothesis of CDOM photobleaching. Our results suggested that unusual RhĂŽne Correspondence to: R. SempĂ©rĂ© ([email protected]) River plume eastward intrusion events might reach Mar-seilles Bay within 2-3 days and induce local phytoplank-ton blooms and subsequent fluorescent CDOM production (peaks M and T) without adding terrestrial fluorescence signatures (peaks C and A). Besides RhĂŽne River plumes, mixing events of the entire water column injected relative aged (peaks C and M) CDOM from the bottom into the surface and thus appeared also as an important source of CDOM in surface waters of the Marseilles Bay. Therefore, the assessment of CDOM optical properties, within the hydrological context, pointed out several biotic (in situ biological production , biological production within RhĂŽne River plumes) and abiotic (photobleaching, mixing) factors controlling CDOM transport, production and removal in this highly urbanized coastal area
Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization
We present the final results from our ultra-deep spectroscopic campaign with
FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates
selected from our VLT/Hawk-I imaging survey over three independent fields. In
particular we report on two newly discovered galaxies at redshift ~6.7 in the
NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs
of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present
the results of ultra-deep observations of a sample of i-dropout galaxies, from
which we set a solid upper limit on the fraction of interlopers. Out of the 20
z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is
systematically below the expectations drawn on the basis of lower redshift
observations: in particular there is a significant lack of objects with
intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for
the fraction of Ly-alpha emission in LBGs that is constantly increasing from
z~3 to z~6 is most probably reversed from z~6 to z~7.
Explaining the observed rapid change in the LAE fraction among the drop-out
population with reionization requires a fast evolution of the neutral fraction
of hydrogen in the Universe. Assuming that the Universe is completely ionized
at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we
find that our data require a change of the neutral hydrogen fraction of the
order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape
fraction does not increase dramatically over the same redshift interval.Comment: Submitted to Ap
The bright end of the z ~ 7 UV Luminosity Function from a wide and deep HAWK-I survey
(Abridged) We present here the second half of an ESO Large Programme, which
exploits the unique combination of area and sensitivity provided in the near-IR
by the camera Hawk-I at the VLT. We have obtained - 30 observing hours with
Hawk-I in the Y-band of two high galactic latitude fields. We combined the
Y-band data with deep J and K Hawk-I observations, and with FORS1/FORS2 U, B,
V, R, I, and Z observations to select z-drop galaxies having Z - Y > 1, no
optical detection and flat Y - J and Y - K colour terms. We detect 8
high-quality candidates in the magnitude range Y = 25.5 - 26.5 that we add to
the z-drop candidates selected in two Hawk-I pointings over the GOODS-South
field. We use this full sample of 15 objects found in -161 arcmin^2 of our
survey to constrain the average physical properties and the evolution of the
number density of z ~ 7 LBGs. A stacking analysis yields a best-fit SED with
photometric redshift z= 6.85 +0.20 -0.15 and an E(B-V)=0.05 +0.15 -0.05. We
compute a binned estimate of the z ~ 7 LF and explore the effects of
photometric scatter and model uncertainties on the statistical constraints.
After accounting for the expected incompleteness through MonteCarlo
simulations, we strengthen our previous finding that a Schechter luminosity
function constant from z=6 to z=7 is ruled out at a >99% confidence level, even
including the effects of cosmic variance. For galaxies brighter than M_1500=
-19.0, we derive a luminosity density rho_UV = 1.5^{+2.1}{-0.8} x 10^25
erg/s/Hz/Mpc^3, implying a decrease by a factor 3.5 from z=6 to z=6.8. We find
that, under standard assumptions, the emission rate of ionizing photons coming
from UV bright galaxies is lower by at least a factor of two than the value
required for reionization. Finally, we exploit deep Hawk-I J and K band
observations to derive an upper limit on the number density of M1500<~ -22.0
LBGs at z-8 (Y-dropouts).Comment: 12 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysic
Searching for z~7.7 Lyman Alpha Emitters in the COSMOS Field with NEWFIRM
The study of Ly-alpha emission in the high-redshift universe is a useful
probe of the epoch of reionization, as the Ly-alpha line should be attenuated
by the intergalactic medium (IGM) at low to moderate neutral hydrogen
fractions. Here we present the results of a deep and wide imaging search for
Ly-alpha emitters in the COSMOS field. We have used two ultra-narrowband
filters (filter width of ~8-9 {\deg}A) on the NEWFIRM camera, installed on the
Mayall 4m telescope at Kitt Peak National Observatory, in order to isolate
Ly-alpha emitters at z = 7.7; such ultra-narrowband imaging searches have
proved to be excellent at detecting Ly-alpha emitters. We found 5-sigma
detections of four candidate Ly-alpha emitters in a survey volume of 2.8 x 10^4
Mpc^3 (total survey area ~760 arcmin^2). Each candidate has a line flux greater
than 8 x 10^-18 erg s^-1 cm^-2. Using these results to construct a luminosity
function and comparing to previously established Ly-alpha luminosity functions
at z = 5.7 and z = 6.5, we find no conclusive evidence for evolution of the
luminosity function between z = 5.7 and z = 7.7. Statistical Monte Carlo
simulations suggest that half of these candidates are real z = 7.7 targets, and
spectroscopic follow-up will be required to verify the redshift of these
candidates. However, our results are consistent with no strong evolution in the
neutral hydrogen fraction of the IGM between z = 5.7 and z = 7.7, even if only
one or two of the z = 7.7 candidates are spectroscopically confirmed.Comment: 29 pages, 5 figures, accepted to ApJ (12/11
A Search for z=7.3 Ly{\alpha} Emitters behind Gravitationally Lensing Clusters
We searched for z=7.3 Lya emitters (LAEs) behind two lensing clusters, Abell
2390 and CL 0024, with the Subaru Telescope Suprime-Cam and a narrowband NB1006
(FWHM ~ 21 nm centered at 1005 nm). We investigated if there exist objects
consistent with the color of z=7.3 LAEs behind the clusters but could not
detect any LAEs to the unlensed line limit F(Lya) ~ 6.9 x 10^{-18} erg/s/cm^2.
Using several z=7 Lya luminosity functions (LFs) from the literature, we
estimated and compared the expected detection numbers of z ~ 7 LAEs in lensing
and blank field surveys in the case of using an 8m class ground based
telescope. Given the steep bright-end slope of the LFs, when the detector
field-of view (FOV) is comparable to the angular extent of a massive lensing
cluster, imaging cluster(s) is more efficient in detecting z ~ 7 LAEs than
imaging a blank field. However, the gain is expected to be modest, a factor of
two at most and likely much less depending on the adopted LFs. The main
advantage of lensing-cluster survey, therefore, remains to be the gain in depth
and not necessarily in detection efficiency. For much larger detectors, the
lensing effect becomes negligible and the efficiency of LAE detection is
proportional to the instrumental FOV. We also inspected NB1006 images of three
z ~ 7 z-dropouts previously detected in Abell 2390 and found that none of them
are detected in NB1006. Two of them are consistent with predictions from the
previous studies that they would be at lower redshifts. The other one has a
photometric redshift of z ~ 7.3, and if it is at z=7.3, its unlensed Lya line
flux would be very faint: F(Lya) < 4.4 x 10^{-18} erg/s/cm^2 (1 sigma upper
limit) or rest frame equivalent width of W(Lya) < 26A. Its Lya emission might
be attenuated by neutral hydrogen, as recent studies show that the fraction of
Lyman break galaxies displaying strong Lya emission is lower at z ~ 7 than at z
<~ 6.Comment: Abstract has been replaced; accepted for publication in Monthly
Notices of the Royal Astronomical Society on April 11, 201
Constraining the nature of the most distant Gamma-Ray Burst host galaxies
Long duration gamma-ray bursts (GRBs) allow us to explore the distant
Universe, and are potentially the most effective tracer of the most distant
objects. Our current knowledge of the properties of GRB host galaxies at
redshifts >5 is very scarce. We propose to improve this situation by obtaining
more observations of high-redshift hosts to better understand their properties
and help enable us to use GRBs as probes of the high-redshift universe. We
performed very deep photometric observations of three high-redshift GRB host
galaxies, GRB 080913 at z =6.7, GRB 060927 at z =5.5 and GRB 060522 at z =5.1.
In addition, we completed deep spectroscopic observations of the GRB080913 host
galaxy with X-Shooter at the VLT to search for Ly-alpha emission. For the sake
of the discussion, we use published results on another high-redshift GRB host,
GRB 050904 at z = 6.3. The sample of GRB host galaxies studied in this paper
consists of four out of the five spectroscopically confirmed GRBs at z>5.
Despite our presented observations being the deepest ever reported of
high-redshift GRB host galaxies, we do not detect any of the hosts, neither in
photometry nor in spectroscopy in the case of GRB 080913. These observations
indicate that the GRB host galaxies seem to evolve with time and to have lower
SFRs at z >5 than they have at z<1. In addition, the host galaxy of GRB 080913
at z =6.7 does not show Ly-alpha emission. While the measured properties of the
galaxies in our sample agree with the properties of the general galaxy
population at z>5, our observations are not sufficiently sensitive to allow us
to infer further conclusions on whether this specific population is
representative of the general one. The characterization of high-redshift GRB
host galaxies is a very challenging endeavor requiring a lot of telescope time,
but is necessary to improve our understanding of the high-redshift universe.Comment: Accepted to A&
Evolution of the observed Ly-alpha luminosity function from z = 6.5 to z = 7.7: evidence for the epoch of reionization ?
Aims. Ly-alpha emitters (LAEs) can be detected out to very high redshifts
during the epoch of reionization. The evolution of the LAE luminosity function
with redshift is a direct probe of the Ly-alpha transmission of the
intergalactic medium (IGM), and therefore of the IGM neutral-hydrogen fraction.
Measuring the Ly-alpha luminosity function (LF) of LAEs at redshift z = 7.7
therefore allows us to constrain the ionizing state of the Universe at this
redshift.
Methods. We observed three 7.5'x7.5' fields with the HAWK-I instrument at the
VLT with a narrow band filter centred at 1.06 m and targeting LAEs at
redshift z ~ 7.7. The fields were chosen for the availability of
multiwavelength data. One field is a galaxy cluster, the Bullet Cluster, which
allowed us to use gravitational amplification to probe luminosities that are
fainter than in the field. The two other fields are subareas of the GOODS
Chandra Deep Field South and CFHTLS-D4 deep field. We selected z=7.7 LAE
candidates from a variety of colour criteria, in particular from the absence of
detection in the optical bands.
Results. We do not find any LAE candidates at z = 7.7 in ~2.4 x 10^4 Mpc^3
down to a narrow band AB magnitude of ~ 26, which allows us to infer robust
constraints on the Ly-alpha LAE luminosity function at this redshift.
Conclusions. The predicted mean number of objects at z = 6.5, derived from
somewhat different LFs of Hu et al. (2010), Ouchi et al. (2010), and Kashikawa
et al. (2011) are 2.5, 13.7, and 11.6, respectively. Depending on which of
these LFs we refer to, we exclude a scenario with no evolution from z = 6.5 to
z = 7.7 at 85% confidence without requiring a strong change in the IGM Ly-alpha
transmission, or at 99% confidence with a significant quenching of the IGM
Ly-alpha transmission, possibly from a strong increase in the high
neutral-hydrogen fraction between these two redshifts.Comment: 14 pages, 6 figures, accepted for publication in A&A. Added
references, minor changes applied in text and figures after the first referee
repor
Assimilation séquentielle de données de couleur de l'eau par filtrage de Kalman dans un modÚle couplé physique-biogéochimique en milieu cÎtier
Cette étude porte sur l assimilation de données de couleur de l eau dans modÚle couplé physique biogéochimique appliqué à un écosystÚme cÎtier (Golfe de Fos Méditerranée Nord Occidentale). Le manuscrit est organisé en quatre chapitres retraçant la progression scientifique et chronologique suivie. Le premier chapitre décrit l implémentation technique du filtre de Kalman utilisé pour l assimilation de données de chlorophylle issues de mesures satellites. Des expériences jumelles sont utilisées pour montrer le potentiel de la méthode et l amélioration significative obtenue dans la description de l écosystÚme. Le chapitre suivant traite des résultats obtenus dans une expérience de données satellites réaliste. Le bénéfice en termes de description de la biogéochimie de surface y est souligné alors que les résultats sont plus mitigés dans la partie la plus profonde de la colonne d eau. Cette expérience est alors répétée dans le troisiÚme chapitre sur une période annuelle pour laquelle la dynamique saisonniÚre du systÚme est analysée. Nous présentons alors une méthode pour le calcul des modes propres du systÚme adaptée à une utilisation opérationnelle. Le gain obtenu dans la description de la chlorophylle de surface est substantiel dans les situations d analyses et de prévisions. Finalement le quatriÚme chapitre vise à coupler l assimilation de deux jeux de données différents, chlorophylle satellite et nutriment in situ, afin d étudier le bénéfice apporté par l assimilation de données de terrains. En conclusion, le dernier chapitre discute des principaux résultats de la thÚse et ses perspectives en vue d une océanographie opérationnelleThis study relates to the assimilation of satellite water color data in a coupled physical biogeochemical model, applied to a coastal ecosystem (gulf of Fos - North Western Mediterranean Sea). The manuscript is organized in four chapters representing the scientific and chronological progression followed. The first chapter represents the technical integration of the Kalman filter used for the assimilation of chlorophyll derived satellite data. Twin experiments were used to show the potential of the method and the significant improvement gained in the biogeochemical description of the ecosystem. The following chapter deals with results acquired from the assimilation of a set of in situ independent data. The benefit in terms of a better description of the biogeochemistry in the surface layers is highlighted even if lesser for the deeper part of the water column. This experience is then enlarged in the third chapter over an annual period for which the seasonal dynamics of the system is analysed. We presented there a new method for the calculation of the empirical orthogonal functions applicable to an operational system. The gain obtained in the description of surface chlorophyll is substantial at for both analysis and forecast situation. Finally, the fourth chapter aimed at coupling the assimilation of two sets of data, chlorophyll satellite-derived and in situ nutriment data, to study the benefit acquired from the assimilation process of field data. Conclusively, the finale chapter discusses the stakes of the thesis and its expectations relevant to operational oceanography.AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF
Assimilation of SeaWiFS chlorophyll data into a 3D-coupled physicalâbiogeochemical model applied to a freshwater-influenced coastal zone
International audienceIn order to predict eutrophication events in coastal areas we tested an assimilation scheme based on sequential data assimilation of SeaWiFS chlorophyll data into a coupled 3D physicalâbiogeochemical model. The area investigated is a semi-enclosed estuarine system (Gulf of FosâNorth-western Mediterranean Sea) closely linked to the Rhone River delta. This system is subjected to episodic eutrophication caused by certain hydrodynamic conditions and intermittent nutrient inputs. The 3D hydrodynamic model Symphonie was coupled to the biogeochemical modelling platform Eco3M. Surface chlorophyll concentrations were derived from SeaWiFS data using the OC5 algorithm and were sequentially assimilated using a singular evolutive extended Kalman filter. Assimilation efficiency was evaluated through an independent in situ data set collected during a field survey that took place in May 2001 (ModelFos cruise). An original approach was used in constructing the state vector and the observation vector. By assimilating pseudo-salinity extracted from the model biogeochemical dynamics in both open sea and plume region were respected. We proved that substantial improvements were made in short-term forecasts by integrating such satellite-estimated chlorophyll maps. We showed that missing freshwater inputs could be corrected to a certain extent by the assimilation process. Simulated concentrations of surface chlorophyll and other basic components of the pelagic ecosystem such as nitrates were improved by assimilating surface chlorophyll maps. Finally we showed the coherent spatial behaviour of the filter over the whole modelled domain
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