565 research outputs found
The search for Population III stars
Population III stars, the first generation of stars formed from primordial
Big Bang material with a top-heavy IMF, should contribute substantially to the
Universe reionization and they are crucial for understanding the early metal
enrichment of galaxies. Therefore it is very important that these objects,
foreseen by theories, are detected by observations. However PopIII stars,
searched through the HeII 1640A line signature, have remained elusive. We
report about the search for the HeII line in a galaxy at z=6.5, which is a very
promising candidate. Unfortunately we are not yet able to show the results of
this search. However we call attention to the possible detection of PopIII
stars in a lensed HII dwarf galaxy at z=3.4, which appeared in the literature
some years ago, but has been overlooked.Comment: Contribution for the Proceedings of the IAU Symposium No. 255 on
"Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies
Probing Outflows in z= 1~2 Galaxies through FeII/FeII* Multiplets
We report on a study of the 2300-2600\AA FeII/FeII* multiplets in the rest-UV
spectra of star-forming galaxies at 1.0<z<2.6 as probes of galactic-scale
outflows. We extracted a mass-limited sample of 97 galaxies at z~1.0-2.6 from
ultra-deep spectra obtained during the GMASS spetroscopic survey in the GOODS
South field with the VLT and FORS2. We obtain robust measures of the rest
equivalent width of the FeII absorption lines down to a limit of W_r>1.5 \AA
and of the FeII* emission lines to W_r>0.5 \AA. Whenever we can measure the
systemic redshift of the galaxies from the [OII] emission line, we find that
both the FeII and MgII absorption lines are blueshifted, indicative that both
species trace gaseous outflows. We also find, however, that the FeII gas has
generally lower outflow velocity relative to that of MgII. We investigate the
variation of FeII line profiles as a function of the radiative transfer
properties of the lines, and find that transitions with higher oscillator
strengths are more blueshifted in terms of both line centroids and line wings.
We discuss the possibility that FeII lines are suppressed by stellar
absorptions. The lower velocities of the FeII lines relative to the MgII
doublet, as well as the absence of spatially extended FeII* emission in 2D
stacked spectra, suggest that most clouds responsible for the FeII absorption
lie close (3~4 kpc) to the disks of galaxies. We show that the FeII/FeII*
multiplets offer unique probes of the kinematic structure of galactic outflows.Comment: 53 pages, 22 Figures, accepted for publication in ApJ, revised
according to referee comment
Investigation of Surface Topography Effects on Metal Flow Under Lubricated Hot Compression of Aluminum
An investigation was conducted to study the effects of die surface topography, specifically surface roughness and lay, on metal flow and the friction factor under lubricated hot compression. 6061-T6 aluminum rings and square bar stock specimens were compressed on H-13 tool steel platens machined with a unidirectional lay pattern to six different roughnesses between a R 10 and 240 μin. A lab based hydraulic press mounted with an experimental die set was used for all testing. Repeated trials were conducted using high temperature vegetable oil and boron nitride lubricants. Metal flow was quantified as a function of surface roughness, lay orientation, and die temperature. Approximate plane strain cigar test specimens were compressed at platen temperatures of 300 °F and 400 °F and at orientations of 0°, 45°, and 90° between the longitudinal axis and unidirectional platen surface lay. The friction factor was assessed using the ring compression test under varying platen roughness conditions and die temperatures between 250 °F and 400 °F. Results indicate metal flow is optimized at low platen roughnesses and orientations parallel to the surface lay of the platen. Die temperature was not found to influence metal flow within the temperature range investigated. The friction factor was observed to be minimized at lower die temperatures and platen roughnesses
A vestige low metallicity gas shell surrounding the radio galaxy 0943-242 at z=2.92
Observations are presented showing the doublet CIV 1550 absorption lines
superimposed on the CIV emission in the radio galaxy 0943-242. Within the
errors, the redshift of the absorption system that has a column density of
N_CIV = 10^{14.5 +- 0.1} cm-2 coincides with that of the deep Ly-alpha
absorption trough observed by Rottgering et al. (1995). The gas seen in
absorption has a resolved spatial extent of at least 13 kpc (the size of the
extended emission line region). We first model the absorption and emission gas
as co-spatial components with the same metallicity and degree of excitation.
Using the information provided by the emission and absorption line ratios of
CIV and Ly-alpha, we find that the observed quantities are incompatible with
photoionization or collisional ionization of cloudlets with uniform properties.
We therefore reject the possibility that the absorption and emission phases are
co-spatial and favour the explanation that the absorption gas has low
metallicity and is located further away from the host galaxy (than the emission
line gas). The estimated low metallicity for the absorption gas in 0943-242 (Z
\~ 1% solar) and its proposed location -outer halo outside the radio cocoon-
suggest that its existence preceeds the observed AGN phase and is a vestige of
the initial starburst at the onset of formation of the parent galaxy.Comment: 11 pages,5 figures, A&A accepte
The galaxy environment of a QSO at z ~ 5.7
High-redshift quasars are believed to reside in massive halos in the early
universe and should therefore be located in fields with overdensities of
galaxies, which are thought to evolve into galaxy clusters seen in the local
universe. However, despite many efforts, the relationship between galaxy
overdensities and z~6 quasars is ambiguous. This can possibly be attributed to
the difficulty of finding galaxies with accurate redshifts in the vicinity of
z~6 quasars. So far, overdensity searches around z~6 quasars have been based on
studies of Lyman break galaxies (LBGs), which probe a redshift range of Delta z
~ 1. This range is large enough to select galaxies that may not be physically
related to the quasar. We use deep narrow- and broadband imaging to study the
environment of the z=5.72 quasar ULAS J0203+0012. The redshift range probed by
our narrow-band selection of Lyman alpha emitters (LAEs) is Delta z ~ 0.1,
which is significantly narrower than the LBG searches. This is the first time
that LAEs were searched for near a z~6 quasar, in an effort to provide clues
about the environments of quasars at the end of the epoch of reionization. We
find no enhancement of LAEs in the surroundings of ULAS J0203+0012 in
comparison with blank fields. We explore different explanations and
interpretations for this non-detection of a galaxy overdensity, including that
(1) the strong ionization from the quasar may prevent galaxy formation in its
immediate vicinity and (2) high-redshift quasars may not reside in the center
of the most massive dark matter halos.Comment: text updated to match published versio
A Taxonomy of Symbiotic tenant relationships in Web-based malls and communities
An increasingly common implementation of electronic commerce over the World Wide Web is based on the virtual mall or community concept (Gaffin, 1995). Such virtual communities operate as a assortment of individual electronic storefronts that offer a variety of goods and services to the on-line shopper. (Hoffman, Novak and Chatterjee, 1996). The most visible electronic commerce transactions are the ones between the electronic tenants and their potential customers. A less obvious type of electronic commerce relationships but also as important, is the one linking individual tenants of electronic communities. In this paper we propose a taxonomy of such symbiotic relationships and present an agenda for further researc
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