729 research outputs found
Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust Content of Selected Galaxies
The presence of spectroscopically confirmed Balmer breaks in galaxy spectral
energy distributions (SEDs) at provides one of the best probes of the
assembly history of the first generations of stars in our Universe. Recent
observations of the gravitationally lensed source, MACS 1149_JD1 (JD1),
indicate that significant amounts of star formation likely occurred at
redshifts as high as . The inferred stellar mass, dust mass, and
assembly history of JD1, or any other galaxy at these redshifts that exhibits a
strong Balmer break, can provide a strong test of our best theoretical models
from high-resolution cosmological simulations. In this work, we present the
results from a cosmological radiation-hydrodynamics simulation of the region
surrounding a massive Lyman-break galaxy. For two of our most massive systems,
we show that dust preferentially resides in the vicinity of the young stars
thereby increasing the strength of the measured Balmer break such that the
simulated SEDs are consistent with the photometry of JD1 and two other
systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high
redshift. We find strong variations in the shape and luminosity of the SEDs of
galaxies with nearly identical stellar and halo masses, indicating the
importance of morphology, assembly history, and dust distribution in making
inferences on the properties of individual galaxies at high redshifts. Our
results stress the importance that dust may play in modulating the observable
properties of galaxies, even at the extreme redshifts of .Comment: 16 pages, 13 Figures, Accepted to MNRA
The unusual multi-wavelength SED of two optical dropout galaxies
We have used deep optical, near-IR, and IR observations from VLT, Spitzer, Herschel, and LABOCA in strong lensing clusters to study distant galaxies. In searches for optical-dropout galaxies (i.e. for z ≳ 7 candidates) we have found several galaxies with very unusual SEDs characterised by a strong spectral break, presumably indicative of high-z, although the objects are detected even in the Herschel bands between 160 and 500 μm and at 870 μm. The latter indicates, from simple estimates of the bolometric luminosity and from the IR SED, that these ob jects are most likely at z ~ 2-2.5. The resulting SEDs imply very high IR/UV ratios, indicative of very large attenuation. Despite this, the large spectral break observed between the optical and near-IR data is difficult to understand with currently know spectral templates from galaxies, EROs, SMGs, and others, both empirical and theoretical one
Fluorescent C II* 1335A emission spectroscopically resolved in a galaxy at z = 5.754
We report the discovery of the first spectroscopically resolved C II /C II*
1334, 1335A doublet in the Lyman-break galaxy J0215-0555 at z = 5.754. The
separation of the resonant and fluorescent emission channels was possible
thanks to the large redshift of the source and long integration time, as well
as the small velocity width of the feature, 0.6 +- 0.2A. We model this emission
and find that at least two components are required to reproduce the combination
of morphologies of C II* emission, C II absorption and emission, and
Lyman-alpha emission from the object. We suggest that the close alignment
between the fluorescence and Lyman-alpha emission could indicate an ionisation
escape channel within the object. While the faintness of such a C II /C II*
doublet makes it prohibitively difficult to pursue for similar systems with
current facilities, we suggest it can become a valuable porosity diagnostic in
the era of JWST and the upcoming generations of ELTs.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter
Recherche et étude des premières galaxies
L'évolution des galaxies est relativement bien contrainte jusqu'au premier milliard d'années de l'Univers. Au delà de cette limite et compte-tenu du faible nombre de sources confirmées à z>6.0, il est difficile de déterminer le rôle joué par les premières galaxies à cette époque. L'objectif de ce travail de thèse était de déterminer l'évolution de la fonction de luminosité des galaxies au cours du premier milliard d'années en se basant sur un échantillon représentatif d'objets à grand redshift (z>4.5) suffisamment brillant pour être observé par les spectrographes actuels. Dans ce but, nous avons réalisé une sélection photométrique ciblée de galaxies à cassure de Lyman dans deux champs de vue : un premier autour de l'amas de galaxies d'Abell 2667 et un second dans un très grand champ vide (le relevé WUDS). L'étude des données prises autour d'Abell 2667 a montré un taux de contamination d'environ 80% de notre échantillon d'objets à z≈. Nous avons mis en évidence l'existence d'une population atypique de galaxies fortement rougies par la poussière à z≈2, et qui ne peuvent être écartées des échantillons qu'en ajoutant des contraintes dans l'IR lointain. Les observations spectroscopiques ont permis d'identifier un nouveau type de contaminant à bas redshift combinant deux populations stellaires d'âges très différents, et demandant une profondeur photométrique extrême afin de les exclure des échantillons actuels. A partir des échantillons dégagés nous avons pu apporter des contraintes fortes et indépendantes sur la partie brillante de la fonction de luminosité, et ainsi en déduire de façon homogène son évolution au cours du premier milliard d'années de l'Univers.Galaxy evolution is well constrained up to the first billion years of the Universe. Beyond this limit and considering the few number of sources with z >6.0 confirmed, it appears difficult to constrain the role played by the first galaxies during this period. One of the main goal of this thesis was to study the evolution of the luminosity function for star forming galaxies during the first billion years by using a representative sample of high-redshift objects (z >4.5) bright enough to be observed by spectroscopy. Thus, we have analyzed two different fields of view: one behind the lensing cluster A2667 and another one behind a large blank field (the WUDS survey). The analysis of the A2667 field showed a contamination rate of our sample of about 80%. We combined our observations with far IR data. Using these new data, we highlighted the existence of an extreme type of galaxies strongly affected by dust, which can only be removed from high-z samples by adding data in the far infra-red domain. Using spectroscopic follow up, we have detected another kind of contaminant at low-z combining two different stellar populations with different ages, and which cannot be removed from our sample excepted by using very deep images in the optical bands beyond the limits of current observations. From our samples we were able to add strong constraints on the bright-end of the luminosity function, and then we deduced its evolution during the first billion years of the Universe. Evolution of the LF allows us to deduce the UV density produced by these galaxies during the cosmic reionization, and to show that the relatively bright galaxies currently detected can not be the sole contributors
A spectroscopic search for AGN activity in the reionization era
The ubiquity of Lyman alpha (Ly) emission in a sample of four bright
[O III]-strong star-forming galaxies with redshifts above 7 has led to the
suggestion that such luminous sources represent a distinct population compared
to their fainter, more numerous, counterparts. The presence of Ly
emission within the reionization era could indicate that these sources created
early ionized bubbles due to their unusually strong radiation, possibly due to
the presence of active galactic nuclei. To test this hypothesis we have secured
long integration spectra with XSHOOTER on the VLT for three 7 sources
selected to have similar luminosities and prominent excess fluxes in the IRAC
3.6 or 4.5m band, usually attributed to strong [O III] emission. We
secured additional spectroscopy for one of these galaxies at =7.15 using
MOSFIRE at the Keck telescope. For this, the most well-studied source in our
sample with the strongest IRAC excess, we detect prominent nebular emission
from He II and NV indicative of a non-thermal source. For the other two sources
at =6.81 and =6.85, for which no previous optical/near infrared
spectroscopy was initially available, Ly is seen in one and CIII]
emission in the other. Although a modest sample, our results further support
the hypothesis that the phenomenon of intense [O III] emission is associated
preferentially with sources lying in early ionized bubbles. However, even
though one of our sources at =7.15 clearly indicates the presence of
non-thermal radiation, such ionized bubbles may not uniquely arise in this
manner. We discuss the unique advantages of extending such challenging
diagnostic studies with JWST.Comment: 11 pages, 7 figures, submitted to Ap
The Mean Absorption Line Spectra of a Selection of Luminous z~6 Lyman Break Galaxies
We examine the absorption line spectra of a sample of 31 luminous (M_UV=-23)
Lyman break galaxies at redshift z~6 using data taken with the FOCAS and OSIRIS
spectrographs on the Subaru and GTC telescopes. For two of these sources we
present longer exposure data taken at higher spectral resolution from ESO's
X-shooter spectrograph. Using these data, we demonstrate the practicality of
stacking our lower resolution data to measure the depth of various interstellar
and stellar absorption lines to probe the covering fraction of low ionization
gas and the gas-phase and stellar metallicities near the end of the era of
cosmic reionization. From maximum absorption line depths of SiII1260 and
CII1334, we infer a mean covering fraction of >0.85+/-0.16 for our sample. This
is larger than that determined using similar methods for lower luminosity
galaxies at slightly lower redshifts, suggesting that the most luminous
galaxies appear to have a lower escape fraction than fainter galaxies, and
therefore may not play a prominent role in concluding reionization. Using
various interstellar absorption lines we deduce gas-phase metallicities close
to solar indicative of substantial early enrichment. Using selected stellar
absorption lines, we model our spectra with a range of metallicities using
techniques successfully employed at lower redshift and deduce a stellar
metallicity of 0.4 +0.3/-0.1 solar, consistent with the stellar mass - stellar
metallicity relation recently found at z~3-5. We discuss the implications of
these metallicity estimates for the typical ages of our luminous galaxies and
conclude our results imply initial star formation at redshifts z~10, consistent
with independent analyses of earlier objects.Comment: 13 pages, 10 figures, 3 tables, Accepted for Publication in Ap
Integrating workload smoothing and inventory reduction in three intermodal logistics platforms of a European car manufacturer
We consider the optimization of container loading at three intermodal logistics platforms (ILP) of a large European car manufacturer (ECM). The decisions focus both on the loading day of each container and on its filling with the products in inventory, which are gradually received over the week from inland suppliers. The objective is either to reduce the largest inventory level needed in the ILP, or to smooth the weekly workload. We develop a solution methodology that allows the handling of complex loading constraints related to dimensions and weight of the products. We model the problem as a mixed integer linear program and we develop a decomposition heuristic to solve it. We perform extensive computation tests on real instances provided by ECM. Compared with current industrial practices, our solutions yield an average improvement of 46.8% for the inventory reduction and of 25.8% for the smoothing of the workload. Our results highlight the benefit of jointly optimizing container loading and operations scheduling.</p
Spectroscopic Constraints on UV Metal Line Emission at z~6-9: The Nature of Lyman-alpha Emitting Galaxies in the Reionization-Era
Recent studies have revealed intense UV metal emission lines in a modest
sample of z>7 Lyman-alpha emitters, indicating a hard ionizing spectrum is
present. If such high ionization features are shown to be common, it may
indicate that extreme radiation fields play a role in regulating the visibility
of Lyman-alpha in the reionization era. Here we present deep near-infrared
spectra of seven galaxies with Lyman-alpha emission at 5.4<z<8.7 (including a
newly-confirmed lensed galaxy at z=6.031) and three bright z>7 photometric
targets. In nine sources we do not detect UV metal lines. However in the
z=8.683 galaxy EGSY8p7, we detect a 4.6 sigma emission line in the narrow
spectral window expected for NV 1243. The feature is unresolved (FWHM<90 km/s)
and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals
non-detections of CIV, He II, and OIII]. The presence of NV requires a
substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum
powered by an AGN or fast radiative shocks. Regardless of its origin, the
intense radiation field of EGSY8p7 may aid the transmission of Lyman-alpha
through what is likely a partially neutral IGM. With this new detection, five
of thirteen known Lyman-alpha emitters at z>7 have now been shown to have
intense UV line emission, suggesting that extreme radiation fields are
commonplace among the Lyman-alpha population. Future observations with JWST
will eventually clarify the origin of these features and explain their role in
the visibility of Lyman-alpha in the reionization era.Comment: 15 pages, 3 tables and 7 figures. Accepted for publication in MNRA
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