IUCAA Preprints
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Item Limits on the time variation of the electromagnetic fine-structure constant in the low energy limit from absorption lines in the spectra of distant quasars(2011-07-06) Srianand, R.; Chand, Hum; Petitjean, Patrick; et al.Most of the successful physical theories rely on the constancy of few fundamental quantities (such as the speed of light, c, the fine-structure constant, α, the proton to electron mass ratio, µ, etc), and constraining the possible time variations of these fundamental quantities is an important step toward a complete physical theory. Time variation of α can be accurately probed using absorption lines seen in the spectra of distant quasars. Here, we present the results of a detailed many-multiplet analysis performed on a new sample of Mg ii systems observed in high quality quasar spectra obtained using the Very Large Telescope. The weighted mean value of the variation in α derived from our analysis over the redshift range 0.4 ≤ z ≤ 2.3 is ∆α/α = (−0.06 ± 0.06) × 10−5. The median redshift of our sample (z≃1.55) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. This gives a 3σ limit, −2.5 × 10−16 yr −1 ≤ (∆α/α∆t) ≤ +1.2 × 10−16 yr −1, for the time variation of α, that forms the strongest constraint obtained based on high redshift quasar absorption line systems.Item Density structure around quasars from optical depth statistics(2005-05-01) Rollinde, Emmanuel; Srianand, R.; Chand, Hum; et al.We present a method for studying the proximity effect and the density structure around redshift z=2-3 quasars. It is based on the probability distribution of Lyman-α pixel optical depths and its evolution with redshift. We validate the method using mock spectra obtained from hydrodynamical simulations, and then apply it to a sample of 12 bright quasars at redshifts 2-3 observed with UVES at the VLT-UT2 Kueyen ESO telescope. These quasars do not show signatures of associated absorption and have a mean monochromatic luminosity of 5.4 × 1031 h−2 erg s−1 Hz−1 at the Lyman limit. The observed distribution of optical depth within 10 h−1Mpc from the QSO is statistically different from that measured in the general intergalacticmedium at the same redshift. Such a change will result from the combined effects of the increase in photoionisation rate above the mean UV-background due to the extra ionizing photons from the quasar radiation (proximity effect), and the higher density of the IGM if the quasars reside in overdense regions (as expected from biased galaxy formation). The first factor decreases the optical depth whereas the second one increases the optical depth, but our measurement cannot distinguish a high background from a low overdensity. An overdensity of the order of a few is required if we use the amplitude of the UV-background inferred from the mean Lyman-α opacity. If no overdensity is present, then we require the UV-background to be higher, and consistent with the existing measurements based on standard analysis of the proximity effect.