971 research outputs found
Where are the missing cosmic metals ?
The majority of the heavy elements produced by stars 2 billion years after
the Big Bang (redshift z~3) are presently undetected at those epochs. We
propose a solution to this cosmic `missing metals' problem in which such
elements are stored in gaseous halos produced by supernova explosions around
star-forming galaxies. By using data from the ESO/VLT Large Program, we find
that:(i) only 5%-9% of the produced metals reside in the cold phase, the rest
being found in the hot (log T=5.8-6.4) phase; (ii) 1%-6% (3%-30%) of the
observed CIV (OVI) is in the hot phase. We conclude that at z~3 more than 90%
of the metals produced during the star forming history can be placed in a hot
phase of the IGM, without violating any observational constraint. The observed
galaxy mass-metallicity relation, and the intergalactic medium and intracluster
medium metallicity evolution are also naturally explained by this hypothesis.Comment: 9 pages, 2 figures, ApJ Letters, in pres
Passive Scalar Structures in Supersonic Turbulence
We conduct a systematic numerical study of passive scalar structures in
supersonic turbulent flows. We find that the degree of intermittency in the
scalar structures increases only slightly as the flow changes from transonic to
highly supersonic, while the velocity structures become significantly more
intermittent. This difference is due to the absence of shock-like
discontinuities in the scalar field. The structure functions of the scalar
field are well described by the intermittency model of She and L\'{e}v\^{e}que
[Phys. Rev. Lett. 72, 336 (1994)], and the most intense scalar structures are
found to be sheet-like at all Mach numbers.Comment: 4 pages, 3 figures, to appear in PR
Exposure to the Dental Environment and Prevalence of Respiratory Illness in Dental Student Populations
Objective: To determine if the prevalence of respiratory disease among dental students and dental residents varies with their exposure to the clinical dental environment.
Methods: A detailed questionnaire was administered to 817 students at 3 dental schools. The questionnaire sought information concerning demographic characteristics, school year, exposure to the dental environment and dental procedures, and history of respiratory disease. The data obtained were subjected to bivariate and multiple logistic regression analysis.
Results: Respondents reported experiencing the following respiratory conditions during the previous year: asthma (26 cases), bronchitis (11 cases), chronic lung disease (6 cases), pneumonia (5 cases) and streptococcal pharyngitis (50 cases). Bivariate statistical analyses indicated no significant associations between the prevalence of any of the respiratory conditions and year in dental school, except for asthma, for which there was a significantly higher prevalence at 1 school compared to the other 2 schools. When all cases of respiratory disease were combined as a composite variable and subjected to multivariate logistic regression analysis controlling for age, sex, race, dental school, smoking history and alcohol consumption, no statistically significant association was observed between respiratory condition and year in dental school or exposure to the dental environment as a dental patient.
Conclusion: No association was found between the prevalence of respiratory disease and a student\u27s year in dental school or previous exposure to the dental environment as a patient. These results suggest that exposure to the dental environment does not increase the risk for respiratory infection in healthy dental health care workers
How Does Feedback Affect Milky Way Satellite Formation?
We use sub-parsec resolution hydrodynamic resimulations of a Milky Way (MW)
like galaxy at high redshift to investigate the formation of the MW satellite
galaxies. More specifically, we assess the impact of supernova feedback on the
dwarf progenitors of these satellite, and the efficiency of a simple
instantaneous reionisation scenario in suppressing star formation at the
low-mass end of this dwarf distribution. Identifying galaxies in our high
redshift simulation and tracking them to z=0 using a dark matter halo merger
tree, we compare our results to present-day observations and determine the
epoch at which we deem satellite galaxy formation must be completed. We find
that only the low-mass end of the population of luminous subhalos of the
Milky-Way like galaxy is not complete before redshift 8, and that although
supernovae feedback reduces the stellar mass of the low-mass subhalos
(log(M/Msolar) < 9), the number of surviving satellites around the Milky-Way
like galaxy at z = 0 is the same in the run with or without supernova feedback.
If a luminous halo is able to avoid accretion by the Milky-Way progenitor
before redshift 3, then it is likely to survive as a MW satellite to redshift
0.Comment: Oral Presentation, Proceedings of "A Universe of Dwarf Galaxies"
Conference, Lyon 201
The role of tidal interactions in driving galaxy evolution
We carry out a statistical analysis of galaxy pairs selected from chemical
hydrodynamical simulations with the aim at assessing the capability of
hierarchical scenarios to reproduce recent observational results for galaxies
in pairs. Particularly, we analyse the effects of mergers and interactions on
the star formation (SF) activity, the global mean chemical properties and the
colour distribution of interacting galaxies. We also assess the effects of
spurious pairs.Comment: to appear in "Groups of galaxies in the nearby Universe" ESO
Workshop, (Dec 2005) Santiago, Chil
Suppression of Dwarf Galaxy Formation by Cosmic Shocks
We carry out a numerical study of the effects of supernova-driven shocks on
galaxy formation at z=9. These "cosmic explosions" can have a significant
impact on galaxies forming nearby. We study such interactions in two key cases.
In the first case in which the forming galaxy has already virialized, the
impinging shock has only a small effect (< 1% of the gas is removed) and star
formation continues relatively unimpeded. However, in the second case in which
the nearby forming galaxy is at the more diffuse turn-around stage, a large
fraction (~70%) of the gas is stripped away from the host dark-matter halo and
ejected into the intergalactic medium. As the time spent near turn-around is
much longer than the interval from virialization to galaxy formation due to
strong radiative losses, we expect the second case to be more representative of
the majority of outflow-galaxy interactions. Thus SN-driven pregalactic
outflows may be an efficient mechanism for inhibiting the formation of
neighbouring galaxies at high redshift. We briefly outline the possible
cosmological consequences of this effect.Comment: 11 pages, 6 figures, accepted for publication in MNRA
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