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The connection between Lymanalpha absorption in low-redshift
quasars and intervening galaxies
Tutor:
Johen
Liske
Alizera Aghaee, Begona Ascaso Angles, Ryan Houghton, Yuri Bialetski
What are quasars ?
• The first quasar (3C273) was discovered in 1963
• Quasars are small, compact, powerful and
extremely remote objects
• Observed luminosity is approximately 10 - 1000
times that of a normal galaxy
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
What sort of data do we need ?
Large homogeneous
spectroscopic database
of low-redshift QSO’s
HST, Faint Object Spectrograph (FOS)
Large homogeneous
database of
galaxies
Sloan Digital Sky Survey
What are the
options?
We can use …
1. RAW data
2. Reduced data
3. Analysed (higher-level) data
Go to the Archive …
… and get High-Level data !!!
Method Overview
1. Search for homogenous archive of QSOs
• Online archive of HST (FOS) QSO spectra
compiled by Bechtold et al. 2002
2. Search for large galaxy database
• SDSS
3. Analyse & Cross-correlate absorbers and
galaxies
Quasar archive
1.
2.
3.
4.
Locate
335
Download
Select useful data
Summarize properties
•
•
273
Table of Quasar properties
Table of ‘clean’ Ly-a absorption
properties
Galaxy archive
1. Locate which quasars positions in SDSS
2. Search for spectral observations taken
‘near’ these positions (SQL search)
• 2o x 2o bounding box
• Galaxies ONLY!
3. Extract galaxy properties from SDSS
database
58
Analysis and Cross-correlation
1. For each quasar calculate distance between
each galaxy and each Lyman Alpha
absorber (to find nearest neighbour galaxy)
Can’t be related
Conclusions
1. We found 6 galaxies which are likely to be
associated with Lyman Alpha absorbers
2. For these six: EW ~ r  -0.64 ± 0.1 (consistent with
literature)
3. Relation holds up to r  ~ 1 Mpc !!
4. However 4 of our pair are derived from one
quasar spectrum (Q1226+0219) which is also
used in Penton et al.
5. We need wider, deeper galaxy surveys to match
the depth of the quasar archive (SDSS not
finished)
Summary
1. Compiled list of ‘clean’ Ly-a absorption lines in
QSO spectra
2. Locate galaxies near QSO coordinates and
extract their properties
3. Cross correlate and analyse:
•
•
Find nearest galaxy to each Ly-a absorption line
Study correlation between absorber and galaxy
properties
4. EW ~ r  -0.64 ± 0.1