Probing the physical state of the IGM: Tool of cosmology
Ms. Priyanka Jalan
Date & Time :
Interaction of the photons from the luminous active galactic nuclei (especially quasars) with the matter along the line of sight results in absorption lines in their spectra allowing to probe the high-redshift universe. These absorption lines are a very useful tool to study the environment of quasars from kpc to Mpc scale as well as the physical properties of the Inter-galactic Medium (IGM). One of the astonishing property of the IGM is that only one out of ten thousand Hydrogen is neutral, mainly due to ultraviolet background (UVB) radiation. As a result, it becomes very important to estimate accurately the photoionization rate of the UVB (Gamma_UVB). One of the tools to calculate Gamma_UVB is the proximity effect, which is a deficit of the Lyman-alpha absorption lines due to the extra-ionisation by the quasar's ionizing photons. In this method, the extent of the observed deficit of the Ly-alpha absorption from the quasars (i.e the proximity region), in conjunction to the quasar's luminosity is used to estimate the UVB radiation flux. However, this method assumes the environment of the quasars are similar to the far away IGM which may not be true as quasars may be residing in overdense regions. Overlooking such overdensity may lead to a gross overestimation of Gamma_UVB. Additionally, the assumption of isotropic emission by quasars used to estimate the quasar's luminosity based on their flux along a line of sight, may also not hold, if quasars emission is anisotropic. As a result, it becomes important to probe the quasars environment also in the transverse direction as well. Proximity effect in a closely separated quasars pairs is one of the unique tools to probe the environment of quasars both in the transverse and the longitudinal directions. Therefore we used a large sample of 181 closely separated quasars pairs (with a separation smaller than 1.5 arc-minutes) to study the proximity effect both in the longitudinal as well as in the transverse directions of foreground quasars, by carrying out a statistical comparison of the Lyman-alpha absorption lines in the vicinity of quasars to those seen far away in the general IGM. We have constrained the density profile and found a significant difference in the longitudinal and the transverse directions. Such observed discrepancy among these two directions has many interesting interpretations such as anisotropic radiation, finite episodic light time of AGN etcetera. Among them, our analysis shows that the anisotropic obscuration seems to be the most plausible. Throughout our analysis, any systematic observational biases have been eliminated by using the observed control data sample, while the detail simulation has been used in the validation of our analysis methodology. We also extend this technique on high quality (SNR ~30, R~5100) dataset of 88 quasars at z~4, by studying the longitudinal proximity effect, to constrain their density profile. Furthermore, we also investigated the main parameters which can possibly affect this overdensity profile using high-resolution (R~40000) spectra. In addition to the available archival data, for further improvements, we also used ling slit spectroscopy at 2-4m class telescope to discovered 18 new quasar pairs even at angular separation smaller than 1 arcmin, where both the proximity effect and over-density must be profound. In this search, we have used the available photometric catalogue to list the high confidence probable AGN pairs candidates, and then carry out their long-slit spectroscopy using the 2m class telescope. Moreover, the discovery of gravitationally lensed quasars will be also discussed.
About Speaker :
Priyanka Jalan is SRF at ARIES, Nainital. This is her thesis pre-submission presentation.