Browsing by Author "Chand, Hum"

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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.
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.
On the variation of the fine-structure constant : Very high resolution spectrum of QSO HE 0515-4414
(2006-01-10) Chand, Hum; Srianand, R.; Petitjean, Patrick; et al.
We present a detailed analysis of a very high resolution (R ≈ 112, 000) spectrum of the quasar HE 0515−4414 obtained using the High Accuracy Radial velocity Planet Searcher (HARPS) mounted on the ESO 3.6 m telescope at the La Silla observatory. The main aim is to use HARPS spectrum of very high wavelength calibration accuracy (better than 1 mÅ), to constrain the variation of α = e² /~c and investigate any possible systematic inaccuracies in the wavelength calibration of the UV Echelle Spectrograph (UVES) mounted on the ESO Very Large Telescope (VLT). Methods. A cross-correlation analysis between the Th-Ar lamp spectra obtained with HARPS and UVES is carried out to detect any possible shift between the two spectra. Absolute wavelength calibration accuracies, and how that translate to the uncertainties in ∆ α/α are computed using Gaussian ﬁts for both lamp spectra. The value of ∆α/α at Zabs = 1.1508 is obtained using Many Multiplet method, and simultaneous Voigt proﬁle ﬁts of HARPS and UVES spectra. Results. We ﬁnd the shift between the HARPS and UVES spectra has mean around zero with a dispersion of σ ≃ 1 mÅ. This is shown to be well within the wavelength calibration accuracy of UVES (i.e σ ≃ 4 mÅ). We show that the uncertainties in the wavelength calibration induce an error of about, ∆α/α ≤ 10−⁶ , in the determination of the variation of the ﬁne-structure constant. Thus, the results of non-evolving ∆α/α reported in the literature based on UVES/VLT data should not be heavily inﬂuenced by problems related to wavelength calibration uncertainties. Our higher resolution spectrum of the zabs = 1.1508 Damped Lyman-α system toward HE 0515−4414 reveals more components compared to the UVES spectrum. Using only Fe II lines of zabs = 1.1508 system, we obtain ∆α/α = (0.05 ± 0.24) × 10−⁵ . This result is consistent with the earlier measurement for this system using the UVES spectrum alone.
Probing the cosmological variation of the fine-structure constant:Results on VLT-UVES sample
(2011-07-06) Chand, Hum; Srianand, R.; Petitjean, Patrick; et al.
Abstract. Development of fundamental physics relies on the constancy of various fundamental quantities such as the ﬁne structure constant. Detecting or constraining the possible time variations of these fundamental physical quantities is an important step toward a complete understanding of basic physics. High quality absorption lines seen in the spectra of distant QSOs allow one to probe time variations of several of these quantities. Here we present the results from a detailed many-multiplet analysis, to detect the possible variation of ﬁne-structure constant, performed using high signal-to-noise ratio, (∼70 per pixel), high spectral resolution (R ≥45000) observations of 23 Mg ii systems detected toward 18 QSOs in the redshift range 0.4 ≤ z ≤ 2.3 obtained using UVES at the VLT. We validate our procedure and deﬁne the selection criteria that will avoid possible systematics using detail analysis of simulated data set. The spectra of Mg ii doublets and Fe ii multiplets are generated considering variations in α and speciﬁcations identical to that of our UVES spectra. We show our Voigt proﬁle ﬁtting code recovers the variation in α very accurately when we use single component systems and multiple component systems that are not heavily blended. Spurious detections are frequently seen when we use heavily blended systems or the systems with very weak lines. Thus we avoided heavily blended systems and the systems with Fe ii column density < 2×1012 cm−2 while analysing the UVES data. To make the analysis transparent and accessible to the community for critical scrutiny all the steps involved in the analysis are presented in detail. The weighted mean value of the variation in α obtained 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 is 1.55 and corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. The 3σ upper limit on the time variation of α is −2.5 × 10−16 yr −1 ≤ (∆α/α∆t) ≤ +1.2 × 10−16 yr −1 . To our knowledge this is the strongest constraint from quasar absorption line studies till date.
Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample
(2011-07-06) Chand, Hum; Petitjean, Patrick; Srianand, R.; et al.
We report a new constraint on the variation of the ﬁne-structure constant based on the analysis of 15 Si iv doublets selected from a ESO-UVES sample. We ﬁnd ∆α/α = (+0.15 ± 0.43) × 10−5 over a redshift range of 1.59 ≤ z ≤ 2.92 which is consistent with no variation in α. This result represents a factor of three improvement on the constraint on ∆α/α based on Si iv doublets compared to the published results in the literature. The alkali doublet method used here avoids the implicit assumptions used in the many-multiplet method that chemical and ionization inhomogeneities are negligible and isotopic abundances are close to the terrestrial value.