# An excess of damped Lyman alpha galaxies near QSOs

**Authors:** David M. Russell, Sara L. Ellison, Chris R. Benn  
**arXiv:** [astro-ph/0512210](https://arxiv.org/abs/astro-ph/0512210)  
**Date:** 2005-12-07  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/astro-ph--0512210v1/

## Key points

- Sample of 33 damped Lyman alpha systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS)
- DLAs have absorption redshifts (z_abs) within 6000 km/s of the systemic redshift (z_sys) of associated quasars (QSOs)
- Based on a sample of 731 non-broad-absorption-line (non-BAL) QSOs with redshifts between 2.5 and 4.5 from SDSS Data Release 3
- Search estimated to be approximately 100% complete for absorbers with column densities of neutral hydrogen (N(HI)) greater than or equal to 2e20 cm^-2
- Derived number density of DLAs per unit redshift, denoted as n(z), within velocity separations less than 6000 km/s is significantly higher compared to intervening absorbers observed in SDSS DR3 data set - almost by a factor of two
- Overdensity of galaxies near QSOs suggests physical motivation for excluding DLAs at small velocity separations in surveys targeting intervening "field" DLAs
- Overdensity is independent of the radio-loudness of the QSOs, indicating similar environments surrounding both radio-loud and radio-quiet QSOs
- Study highlights an excess population of damped Lyman alpha galaxies near quasars and emphasizes the importance of considering proximity effects when studying intervening absorbers in cosmological surveys
- Results contribute to understanding galaxy formation and evolution in relation to quasar environments

## Summary

In their study, David M. Russell, Sara L. Ellison, and Chris R. Benn present a sample of 33 damped Lyman alpha systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS). These DLAs have absorption redshifts (z_abs) that are within 6000 km/s of the systemic redshift (z_sys) of the quasars (QSOs) they are associated with. The authors base their sample on 731 non-broad-absorption-line (non-BAL) QSOs with redshifts between 2.5 and 4.5 from Data Release 3 (DR3) of the SDSS. The researchers estimate that their search is approximately 100% complete for absorbers with column densities of neutral hydrogen (N(HI)) greater than or equal to 2e20 cm^-2. They find that the derived number density of DLAs per unit redshift, denoted as n(z), within velocity separations less than 6000 km/s is significantly higher compared to intervening absorbers observed in the SDSS DR3 data set - almost by a factor of two. This suggests an overdensity of galaxies near the QSOs and provides physical motivation for excluding DLAs at small velocity separations in surveys targeting intervening "field" DLAs. Additionally, it is found that this overdensity is independent of the radio-loudness of the QSOs; indicating similar environments surrounding both radio-loud and radio-quiet QSOs. Overall, this study highlights an excess population of damped Lyman alpha galaxies near quasars and emphasizes the importance of considering proximity effects when studying intervening absorbers in cosmological surveys. The results contribute to our understanding of galaxy formation and evolution in relation to quasar environments.