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, $\alpha$, 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: $8.1\pm1.7$ ppm for the full dataset, versus $9.4\pm2.2$ ppm for the Webb {\it et al.} data alone, both at the $68.3\%$ 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