No Arabic abstract
We have observed the canonical gigahertz-peaked spectrum source 0108+388 with the VLBA at a range of frequencies above and below the spectral peak. The activity that dominates the radio emission from 0108+388, which is also classified as a Compact Symmetric Object, is thought to be less than 1000 years old. We present strong evidence that the spectral turnover in 0108+388 results from free-free absorption by non-uniform gas, possibly in the form of a disk in the central tens of parsecs.
We describe radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set of a sample of 31 objects that have well-measured radio spectra up to at least 23 GHz. The radio spectra of these objects below 1.4 GHz show a great variety of forms and are rarely a simple power-law extrapolation of the synchrotron spectra at higher frequencies. Most objects of this class have spectral turn-overs or bends in their radio spectra. We interpret these spectra in terms of free-free absorption in the starburst environment. Several objects show radio spectra with two components having free-free turn-overs at different frequencies (including Arp 220 and Arp 299), indicating that synchrotron emission originates from regions with very different emission measures. In these sources, using a simple model for the supernova rate, we estimate the time for which synchrotron emission is subject to strong free-free absorption by ionized gas, and compare this to expected HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII regions. We discuss the implications of this result and argue that those sources which have a significant radio component with strong free-free absorption are those in which the star formation rate is still increasing with time. We note that if ionization losses modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.
The radio emission of normal galaxies may become opaque at low radio frequencies due to thermal ionized gas. We performed modelling of the free-free absorption to reproduce the ocal spectrum of SgrA Complex and the global spectrum of the starburst galaxy M82. We show the importance of resolution of radio observations and the value of filling factor of the absorbing gas for correct modelling of the absorption.
We address the issue of anomalous image flux ratios seen in the double-image gravitational lens JVAS B0218+357. From the multi-frequency observations presented in a recent study (Mittal et al. 2006) and several previous observations made by other authors, the anomaly is well-established in that the image flux-density ratio (A/B) decreases from 3.9 to 2.0 over the observed frequency range from 15 GHz to 1.65 GHz. In Mittal et al. (2006), the authors investigated whether an interplay between a frequency-dependent structure of the background radio-source and a gradient in the relative image-magnification can explain away the anomaly. Insufficient shifts in the image centroids with frequency led them to discard the above effect as the cause of the anomaly. In this paper, we first take this analysis further by evaluating the combined effect of the background source extension and magnification gradients in the lens plane in more detail. This is done by making a direct use of the observed VLBI flux-distributions for each image to estimate the image flux-density ratios at different frequencies from a lens-model. As a result of this investigation, this mechanism does not account for the anomaly. Following this, we analyze the effects of mechanisms which are non-gravitational in nature on the image flux ratios in B0218+357. These are free-free absorption and scattering, and are assumed to occur under the hypothesis of a molecular cloud residing in the lens galaxy along the line-of-sight to image A. We show that free-free absorption due to an H II region covering the entire structure of image A at 1.65 GHz can explain the image flux ratio anomaly. We also discuss whether H II regions with physical parameters as derived from our analysis are consistent with those observed in Galactic and extragalactic H II regions.
We study the free-fall of a quantum particle in the context of noncommutative quantum mechanics (NCQM). Assuming noncommutativity of the canonical type between the coordinates of a two-dimensional configuration space, we consider a neutral particle trapped in a gravitational well and exactly solve the energy eigenvalue problem. By resorting to experimental data from the GRANIT experiment, in which the first energy levels of freely falling quantum ultracold neutrons were determined, we impose an upper-bound on the noncommutativity parameter. We also investigate the time of flight of a quantum particle moving in a uniform gravitational field in NCQM. This is related to the weak equivalence principle. As we consider stationary, energy eigenstates, i.e., delocalized states, the time of flight must be measured by a quantum clock, suitably coupled to the particle. By considering the clock as a small perturbation, we solve the (stationary) scattering problem associated and show that the time of flight is equal to the classical result, when the measurement is made far from the turning point. This result is interpreted as an extension of the equivalence principle to the realm of NCQM.
Using analytical models and cosmological N-body simulations, we study the free-free radio emission from ionized gas in clusters and groups of galaxies. The results obtained with the simulations are compared with analytical predictions based on the mass function and scaling relations. Earlier works based on analytical models have shown that the average free-free signal from small haloes (galaxies) during and after the reionization time could be detected with future experiments as a distortion of the CMB spectrum at low frequencies ($ u <$ 5 GHz). We focus on the period after the reionization time (from redshift $z=0$ up to $z=7$) and on haloes that are more massive than in previous works (groups and clusters). We show how the average signal from haloes with $M > 10^{13} h^{-1} M_{odot}$ is less than 10% the signal from the more abundant and colder smaller mass haloes. However, the individual signal from the massive haloes could be detected with future experiments opening the door for a new window to study the intracluster medium.