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The connection between Lyman-alpha absorption in low-redshift quasars and intervening galaxies
Johen Liske Tutor: Alizera Aghaee, Begona Ascaso Angles, Ryan Houghton, Yuri Bialetski
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What are quasars ? The first quasar (3C273) was discovered in 1963
Quasars are small, compact, powerful and extremely remote objects Observed luminosity is approximately times that of a normal galaxy
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Lyman Alpha Absorption
Quasars emit strongly in the Lyman alpha We observe the spectrum shifted to longer wavelengths Light is absorbed by gas located between us and the quasar
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spectroscopic database
What sort of data do we need ? Large homogeneous spectroscopic database of low-redshift QSO’s Large homogeneous database of galaxies HST, Faint Object Spectrograph (FOS) Sloan Digital Sky Survey
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What are the options?
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We can use … 1. RAW data 2. Reduced data
3. Analysed (higher-level) data
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Go to the Archive … … and get High-Level data !!!
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Method Overview Search for homogenous archive of QSOs
Online archive of HST (FOS) QSO spectra compiled by Bechtold et al. 2002 Search for large galaxy database SDSS Analyse & Cross-correlate absorbers and galaxies
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Quasar archive 335 273 Locate Download Select useful data
Summarize properties Table of Quasar properties Table of ‘clean’ Ly-a absorption properties 335 273
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Galaxy archive 58 Locate which quasars positions in SDSS
Search for spectral observations taken ‘near’ these positions (SQL search) 2o x 2o bounding box Galaxies ONLY! Extract galaxy properties from SDSS database
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Analysis and Cross-correlation
For each quasar calculate distance between each galaxy and each Lyman Alpha absorber (to find nearest neighbour galaxy)
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Can’t be related
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Conclusions We found 6 galaxies which are likely to be associated with Lyman Alpha absorbers For these six: EW ~ r ± 0.1 (consistent with literature) Relation holds up to r ~ 1 Mpc !! However 4 of our pair are derived from one quasar spectrum (Q ) which is also used in Penton et al. We need wider, deeper galaxy surveys to match the depth of the quasar archive (SDSS not finished)
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Summary Compiled list of ‘clean’ Ly-a absorption lines in QSO spectra
Locate galaxies near QSO coordinates and extract their properties Cross correlate and analyse: Find nearest galaxy to each Ly-a absorption line Study correlation between absorber and galaxy properties EW ~ r ± 0.1
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