The connection between Lyman-alpha absorption in low-redshift quasars

and intervening galaxies

Alizera Aghaee, Begona Ascaso Angles, Ryan Houghton, Yuri Bialetski

Johen Liske

Tutor:

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

Large homogeneousdatabase of

galaxies

Sloan Digital Sky Survey HST, Faint Object Spectrograph (FOS)

What are the options?

1. RAW data

2. Reduced data

We can use …

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. Locate

2. Download

3. Select useful data

4. Summarize properties• Table of Quasar properties• Table of ‘clean’ Ly-a absorption

properties

335

273

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