30,393 research outputs found
Updated constraints on spatial variations of the fine-structure constant
Recent work by Webb {\it et al.} has provided indications of spatial
variations of the fine-structure constant, , at a level of a few parts
per million. Using a dataset of 293 archival measurements, they further show
that a dipole provides a statistically good fit to the data, a result
subsequently confirmed by other authors. Here we show that a more recent
dataset of dedicated measurements further constrains these variations: although
there are only 10 such measurements, their uncertainties are considerably
smaller. We find that a dipolar variation is still a good fit to the combined
dataset, but the amplitude of such a dipole must be somewhat smaller:
ppm for the full dataset, versus ppm for the Webb {\it
et al.} data alone, both at the confidence level. Constraints on the
direction on the sky of such a dipole are also significantly improved. On the
other hand the data can't yet discriminate between a pure spatial dipole and
one with an additional redshift dependence.Comment: 10 pages, 4 figure
UV Continuum, Physical Conditions and Filling Factor in Active Galactic Nuclei
The narrow line region of active galaxies is formed by gas clouds surrounded
by a diluted gas. Standard one-dimensional photoionization models are usually
used to model this region in order to reproduce the observed emission lines.
Since the narrow line region is not homogeneous, two major types of models are
used: (a) those assuming a homogeneous gas distribution and a filling factor
less than unity to mimic the presence of the emitting clouds; (b) those based
on a composition of single-cloud models combined in order to obtain the
observed spectra. The first method is largely used but may induce to misleading
conclusions as shown in this paper. The second one is more appropriate, but
requires a large number of observed lines in order to limit the number of
single models used. After discussing the case of an extragalactic HII region,
for which the ionizing radiation spectrum is better known, we show that 1-D
models for the narrow line region with a filling factor less than unit do not
properly mimic the clumpiness, but just simulates an overall lower density.
Multi-cloud models lead to more reliable results. Both models are tested in
this paper, using the emission-line spectra of two well-known Seyfert galaxies,
NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue
bump cannot be excluded by multi-cloud models, although excluded by Alexander
et al. (1999, 2000)using homogeneous models with a filling factor less than
unity.Comment: 23 pages, 7 figures. Accepted for Publication in Ap
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