287 research outputs found
Rigidification of connective comodules
We show that we can rigidify homotopy coherent comodules in connective
modules over the Eilenberg-Mac Lane spectrum of a field, or more generally of a
finite product of fields . That is, the -category of
homotopy coherent comodules is represented by a model category of strict
comodules in non-negative chain complexes over or in simplicial
-modules. These comodules are over a coalgebra that is strict and
simply connected. The rigidification result allows us to derive the notion of
cotensor product of comodules and endows the -category of comodules
with a symmetric monoidal structure. We lift the usual Dold-Kan correspondance
to these model categories of comodules. We also show a rigidification result
for comodules in the non-connective case when the coalgebra is dualizable. To
prove these results, we introduce Postnikov towers of comodules that dualize
the cellularity of combinatorial model categories. Moreover, we define the
notion of symmetric comonoidal Quillen model categories and weak comonoidal
Quillen equivalences in order to describe the homotopical behavior of our
derived cotensor product of comodules.Comment: 60 page
Classical and MgII-selected Damped Lyman-alpha Absorbers: impact on Omega_HI at z<1.7
The Damped Lyman-alpha systems (DLAs), seen in absorption in the spectrum of
quasars, are believed to contain a large fraction of the neutral gas in the
Universe. Paradoxically, these systems are more difficult to observe at
z_abs<1.7, since they are rare and their HI feature then falls in UV spectra.
Rao & Turnshek (2000) pioneered a method based on MgII-selected DLAs, that is
absorbers discovered thanks to our knowledge of their MgII feature in optical
spectra. We use new observations undertaken at the TNG as well as a careful
literature & archival search to build samples of low redshift absorbers
classified according to the technique used for their discovery. We successfully
recover N(HI) and equivalent widths of FeII 2600, MgII 2796, MgII 2803 and MgII
2852 for a sample of 36 absorbers, 21 of which are MgII-selected. We find that
the MgII-selected sample contains a marginally larger fraction of absorbers
with log N(HI)>21.0 than seen otherwise at low redshift. If confirmed, this
property will in turn affect estimates of Omega_HI which is dominated by the
highest HI column densities. We find that log N(HI) does not correlate
significantly with metal equivalent widths. Similarly, we find no evidence that
gravitational lensing, the fraction of associated systems or redshift evolution
affect the absorber samples in a different way. We conclude that the hint of
discrepancies in N(HI) distributions most likely arises from small number
statistics. Therefore, further observations are required to better clarify the
impact of this discrepancy on estimates of Omega_HI at low redshift.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
Revisiting the origin of the high metallicities of sub-damped Lyman-alpha systems
Sub-damped Lyman-alpha systems (sub-DLAs) have previously been found to
exhibit a steeper metallicity evolution than the classical damped Lyman-alpha
systems (DLAs), evolving to close to solar metallicity by z~1. From new
high-resolution spectra of 17 sub-DLAs we have increased the number of
measurements of [Fe/H] at z<1.7 by 25% and compiled the most complete
literature sample of sub-DLA and DLA abundances to date. We find that sub-DLAs
are indeed significantly more metal-rich than DLAs, but only at z<1.7; the
metallicity distributions of sub-DLAs and DLAs at z>1.7 are statistically
consistent. We also present the first evidence that sub-DLAs follow a velocity
width-metallicity correlation over the same velocity range as DLAs, but the
relation is offset to higher metallicities than the DLA relation. On the basis
of these results, we revisit the previous explanation that the systematically
higher metallicities observed in sub-DLAs are indicative of higher host galaxy
masses. We discuss the various problems that this interpretation encounters and
conclude that in general sub-DLAs are not uniquely synonymous with massive
galaxies. We rule out physically related sources of bias (dust, environment,
ionization effects) and examine systematics associated with the selection and
analysis of low-redshift sub-DLAs. We propose that the high metallicities of
sub-DLAs at z<1.7 that drives an apparently steep evolution may be due to the
selection of most low-redshift sub-DLAs based on their high MgII equivalent
widths.Comment: 6 pages, 3 figures, MNRAS, in pres
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
Statistical Properties of DLAs and sub-DLAs
Quasar absorbers provide a powerful observational tool with which to probe
both galaxies and the intergalactic medium up to high redshift. We present a
study of the evolution of the column density distribution, f(N,z), and total
neutral hydrogen mass in high-column density quasar absorbers using data from a
recent high-redshift survey for damped Lyman-alpha (DLA) and Lyman limit system
(LLS) absorbers. Whilst in the redshift range 2 to 3.5, ~90% of the neutral HI
mass is in DLAs, we find that at z>3.5 this fraction drops to only 55% and that
the remaining 'missing' mass fraction of the neutral gas lies in sub-DLAs with
N(HI) 10^{19} - 2 * 10^{20} cm^{-2}.Comment: 6 pages, 4 figures, in "Chemical Enrichment of Intracluster and
Intergalactic medium", Proceedings of the Vulcano Workshop, May 14-18, 200
Coinductive control of inductive data types
We combine the theory of inductive data types with the theory of universal
measurings. By doing so, we find that many categories of algebras of
endofunctors are actually enriched in the corresponding category of coalgebras
of the same endofunctor. The enrichment captures all possible partial algebra
homomorphisms, defined by measuring coalgebras. Thus this enriched category
carries more information than the usual category of algebras which captures
only total algebra homomorphisms. We specify new algebras besides the initial
one using a generalization of the notion of initial algebra.Comment: 21 page
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