Identifying intervening absorption systems for use as cosmological probes

Raghunathan, Srinivasan (2009) Identifying intervening absorption systems for use as cosmological probes. Masters thesis, University of Central Lancashire.

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Quasars being one of the most luminous objects in the sky can act as cosmological probes for analyzing not only the large-scale structures (LSSs) in the universe, but also to trace the metal absorption systems by probing the gas content (Zibetti
et at., 2005; Ménard et at., 2005) that lie on the line-of-sight (LOS) of the background quasars. This study will generate a database of absorption systems, primarily the MgII, CIV doublets using the spectra of tens of thousands of quasars available from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) and possibly 2dF Quasar Redhshift (2QZ) survey, covering a redshift range of 0 .c z < 2.3 and detected at 3.5cr level.
Quasars, because of their brightness, can be easily detected at redshifts z > 1, compared to galaxies. Also, the intrinsic low volume densities of quasars compared to galaxies (Williger et at., 2002) allow them to be used as tracers to outline the structures that extend up to few hundreds of megaparsecs. But, the mass perturbations traced by these quasars put a limit to the information they reveal about the lower mass objects in the LSSs. This accounts for the use of metal absorbers to probe the detailed characteristics of LSSs (Williger et at., 2002; Bouché, Murphy, and Peroux. 2004). The prime reason for choosing MgII doublet over other metal absorbers, \\ 2796 , 2803 A is because MgII doublets can be detected easily from ground-based sky surveys to redshifts as low as z = 0.11 (Bouche, Murphy, and Péroux, 2004; Nestor, Turnshek, and Rao, .2005; Ménard et at., 2008; Narayanan, 2008).
As an approach to finding the MgTI and CIV doublets, continua derived from a slight modification to the Thompson-Martin digital filter were first fitted to the quasar spectra. This was followed by the generation of noise spectrum using a median filter algorithm to measure the noise level. Doublets are then searched using an automated search algorithm in the continuum subtracted spectra. The absorption features that lie above the 3.5a level and which satisfy the wavelength and doublet ratio of the ideal MgII or CIV, are classified as MgII or CIV absorber and are entered in the catalogue.
The database contains information about the signal-to-noise ratios, line-strengths, equivalent widths, and redshifts of the absorbers detected using the SDSS DRT survey. While generating the list of absorbers from 2QZ survey will be the next
step, this study will also complement the further research in this field, which is to use these absorption systems as cosmological probes to delineate the LSSs ranging from 50 Mpc to 300 Mpc in size using single linkage hierarchical clustering.

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