1. 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

2. 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

3. 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

4. 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

5. What are the
options?

6. We can use … 1. RAW data 2. Reduced data
3. Analysed (higher-level) data

7. Go to the Archive …
… and get High-Level data !!!

8. 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

9. Quasar archive 335 273 Locate Download Select useful data
Summarize properties
Table of Quasar properties
Table of ‘clean’ Ly-a absorption properties
335
273

11. 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

12. Analysis and Cross-correlation
For each quasar calculate distance between each galaxy and each Lyman Alpha absorber (to find nearest neighbour galaxy)

13. Can’t be related

18. 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)

19. 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