678 research outputs found
The Evolution of Dust in the Early Universe with Applications to the Galaxy SDSS J1148+5251
Dusty hyperluminous galaxies in the early universe provide unique
environments for studying the role of massive stars in the formation and
destruction of dust. At redshifts above ~ 6, when the universe was less than ~
1 Gyr old, dust could have only condensed in the explosive ejecta of Type II
supernovae (SNe), since most of the progenitors of the AGB stars, the major
alternative source of interstellar dust, did not have time to evolve off the
main sequence since the onset of star formation. In this paper we present
analytical models for the evolution of the gas, dust, and metals in high
redshift galaxies, with a special application to SDSS J1148+5251, a
hyperluminous quasar at z = 6.4. We find that an average supernova must
condense at least 1 Msun of dust to account for the observed dust mass in this
quasar. Observationally, it is in excess of the largest dust yield of ~0.02
Msun found thus far in the ejecta of any SN. If future observations find this
to be a typical supernova dust yield, then additional processes, such as
accretion onto preexisting grains, or condensation around the AGN will need to
be invoked to account for the large amount of dust in this and similar objects.
The galaxy's star formation history is still uncertain, and current
observations of the gas, metal, and dust contents of J1148 can be reproduced by
either an intensive and short burst of star formation (~ 1000 Msun/yr) with a
duration of ~ 100 Myr, or a much lower star formation rate (~ 100 Msun/yr)
occurring over the lifetime of the galaxy.Comment: 35 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
We present the results of a systematic study of mid-IR spectra of Galactic
regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral,
starburst), observed by the satellites ISO and Spitzer. We study the relative
variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially
resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well
as among 90 integrated spectra of 50 objects. Our main results are that the
6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios
between these bands and the 11.3 micron band varies by one order of magnitude.
This implies that the properties of the PAHs are remarkably universal
throughout our sample, and that the relative variations of the band ratios are
mainly controled by the fraction of ionized PAHs. In particular, we show that
we can rule out both the modification of the PAH size distribution, and the
mid-infrared extinction, as an explanation of these variations. Using a few
well-studied Galactic regions (including the spectral image of the Orion Bar),
we give an empirical relation between the I(6.2)/I(11.3) ratio and the
ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful
quantitative diagnostic tool of the physical conditions in the regions where
the PAH emission originates. Finally, we discuss the physical interpretation of
the I(6.2)/I(11.3) ratio, on galactic size scales.Comment: Accepted by the ApJ, 67 pages, 70 figure
The Nature of the Low-Metallicity ISM in the Dwarf Galaxy NGC 1569
We are modeling the spectra of dwarf galaxies from infrared to submillimeter
wavelengths to understand the nature of the various dust components in
low-metallicity environments, which may be comparable to the ISM of galaxies in
their early evolutionary state. The overall nature of the dust in these
environments appears to differ from those of higher metallicity starbursting
systems. Here, we present a study of one of our sample of dwarf galaxies, NGC
1569, which is a nearby, well-studied starbursting dwarf. Using ISOCAM, IRAS,
ISOPHOT and SCUBA data with the Desert et al. (1990) model, we find consistency
with little contribution from PAHs and Very Small Grains and a relative
abundance of bigger colder grains, which dominate the FIR and submillimeter
wavelengths. We are compelled to use 4 dust components, adding a very cold dust
component, to reproduce the submillimetre excess of our observations.Comment: 4 pages, 4 postscript figures. Proceedings of "Infrared and
Submillimeter Astronomy. An International Colloquium to Honor the Memory of
Guy Serra" (2002
Cobalt-Based Electrolytes for Dye-Sensitized Solar Cells: Recent Advances towards Stable Devices
Redox mediators based on cobalt complexes allowed dye-sensitized solar cells (DSCs) to achieve efficiencies exceeding 14%, thus challenging the emerging class of perovskite solar cells. Unfortunately, cobalt-based electrolytes demonstrate much lower long-term stability trends if compared to the traditional iodide/triiodide redox couple. In view of the large-scale commercialization of cobalt-based DSCs, the scientific community has recently proposed various approaches and materials to increase the stability of these devices, which comprise gelling agents, crosslinked polymeric matrices and mixtures of solvents (including water). This review summarizes the most significant advances recently focused towards this direction, also suggesting some intriguing way to fabricate third-generation cobalt-based photoelectrochemical devices stable over time
Hygrothermal Behaviour of Three Internal Retrofit Prototype Solutions
AbstractAlthough the application of internal insulation to existing perimeter walls poses significant challenges in terms of building physics and loss of habitable space, it is sometimes an inevitable choice because of practical or legislative constraints. Innovative solutions are then required to deliver satisfying performances and reduce nuisance to inhabitants of residential buildings in case they are going to remain in their flats during the retrofit works.Three systems for inner thermal retrofitting purposes have been designed and produced as prototypes. Two of them are composed by silica aerogel containing fibrous material: the first one is a rigid flat laminated panel, the second one is a rollable solution with a fabric finishing layer. The third insulating system is a perlite based board with a hydrophobic layer. All the materials composing the retrofit solutions have been characterized by means of laboratory tests in order to measure their main hygrothermal properties. In fact, some parameters are fundamental for determining the hygrothermal performance of the composite systems: thermal conductivity, at dry and wet state (moisture dependant), water vapour diffusion resistance factor, hygroscopic sorption at isotherm condition and water absorption coefficient. All those measured data were necessary for optimizing the solutions, guaranteeing energy efficiency and vapour open layers to systems that are intended for installation on existing walls
Physical conditions in the gas phases of the giant HII region LMC-N11 unveiled by Herschel - I. Diffuse [CII] and [OIII] emission in LMC-N11B
(Abridged) The Magellanic Clouds provide a nearby laboratory for metal-poor
dwarf galaxies. The low dust abundance enhances the penetration of UV photons
into the interstellar medium (ISM), resulting in a relatively larger filling
factor of the ionized gas. Furthermore, there is likely a hidden molecular gas
reservoir probed by the [CII]157um line. We present Herschel/PACS maps in
several tracers, [CII], [OI]63um,145um, [NII]122um, [NIII]57um, and [OIII]88um
in the HII region N11B in the Large Magellanic Cloud. Halpha and [OIII]5007A
images were used as complementary data to investigate the effect of dust
extinction. Observations were interpreted with photoionization models to infer
the gas conditions and estimate the ionized gas contribution to the [CII]
emission. Photodissociation regions (PDRs) are probed through polycyclic
aromatic hydrocarbons (PAHs). We first study the distribution and properties of
the ionized gas. We then constrain the origin of [CII]157um by comparing to
tracers of the low-excitation ionized gas and of PDRs. [OIII] is dominated by
extended emission from the high-excitation diffuse ionized gas; it is the
brightest far-infrared line, ~4 times brighter than [CII]. The extent of the
[OIII] emission suggests that the medium is rather fragmented, allowing far-UV
photons to permeate into the ISM to scales of >30pc. Furthermore, by comparing
[CII] with [NII], we find that 95% of [CII] arises in PDRs, except toward the
stellar cluster for which as much as 15% could arise in the ionized gas. We
find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII]
dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs.
The combination of [CII] and [OI] provides a proxy for the total gas cooling in
PDRs. Our results suggest that PAH emission describes better the PDR gas
heating as compared to the total infrared emission.Comment: Accepted for publication in Astronomy and Astrophysics. Fixed
inverted line ratio in Sect. 5.
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