[Paper Review] Metal Absorption Systems in Spectra of Pairs of QSOs

David Tytler, Mark Gleed+7 more|arXiv (Cornell University)|Nov 15, 2007

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy4 references18 citations

TL;DR

This study presents the first large-scale survey of metal absorption systems in paired quasar (QSO) sightlines separated by ~1 Mpc at z ≈ 2, revealing strong spatial clustering of absorbers on 0.5 Mpc scales. It finds that absorption in one line of sight implies a 50% probability of detecting absorption within 500 km s⁻¹ in the paired sightline at <100 kpc, indicating absorbers trace galaxy-scale metal distributions rather than fast winds, with evidence against strong anisotropic UV feedback from QSOs.

ABSTRACT

We present the first large sample of absorption systems in paired QSOs consisting of 691 absorption systems in the spectra of 310 QSOs including 170 pairings. All these absorption systems have metal lines, usually C IV or Mg II. We see 17 cases of absorption in one line-of-sight within 200 km/s (1 Mpc) of absorption in the paired line-of-sight with the probability at least approx 50% at 100kpc, declining rapidly to 23% at 100 - 200 kpc. We detect clustering on 0.5Mpc scales and see a hint of the "fingers of God" redshift-space distortion. The distribution matches absorbers arising in galaxies at z=2 with a normal correlation function and systematic infall velocities but unusually low random pair-wise velocity differences. Absorption in gas flowing out from galaxies at a mean velocity of 250 km/s would produce vastly more elongation than we see. The UV absorption from fast winds that Adelberger et al. 2005 see in spectra of LBGs is not representative of the absorption that we see. Either the winds are confined to LBGs, or they can not extend to 40 kpc with large velocities, while continuing to make UV absorption we see, implying most metals were in place in the IGM long before z=2. Separately, when we examine the absorption seen when a sight line passes a second QSO, we see 19 absorbers within 400 km/s of the partner QSO. The probability of seeing absorption is approximately constant for impact parameters 0.1 - 1.5 Mpc. Perhaps we do not see a rapid rise in the probability at small impact parameters because the UV from QSOs destroys some absorbers near to the QSOs. The 3D distribution of 64 absorbers around 313 QSOs is to first order isotropic, with just a hint of the anisotropy expected if the QSO UV emission is beamed, or alternatively QSOs might emit UV isotropically but for a surprisingly short time of only 0.3Myr.

Motivation & Objective

• To investigate the spatial correlation of metal absorption systems in paired QSO sightlines separated by ~1 Mpc at z ≈ 2.
• To determine whether absorbers are associated with galaxy-scale structures or fast outflows from QSOs.
• To assess the role of QSO UV radiation in suppressing absorbers at small impact parameters.
• To test for anisotropy in the distribution of absorbers around QSOs, probing whether UV emission is beamed or isotropic.

Proposed method

• Analysis of 310 QSOs in 170 pairings with sightline separations of ~1 Mpc at z ≈ 2 using moderate-resolution spectroscopy (FWHM 100–250 km s⁻¹).
• Identification of absorption systems via rest-frame equivalent width and ion species (e.g., C IV, Mg II) in spectra.
• Cross-correlation of absorption redshifts between paired sightlines to measure velocity offsets and infer spatial proximity.
• Use of redshift-space distortions and clustering statistics to infer large-scale structure and peculiar velocities.
• Comparison of transverse absorber probabilities at varying impact parameters (0.1–1.5 Mpc) to test for suppression near QSOs.
• Statistical modeling of absorber distribution anisotropy, considering beam geometry, UV destruction timescales, and redshift error effects.

Experimental results

Research questions

• RQ1 What is the spatial correlation length of metal absorption systems in paired QSO sightlines at z ≈ 2?
• RQ2 Do absorbers near QSOs show signs of anisotropic distribution due to beamed or isotropic UV emission?
• RQ3 Is the observed clustering consistent with absorbers arising in galaxies or in fast outflows from QSOs?
• RQ4 Does the probability of detecting transverse absorbers increase sharply at small impact parameters, or is it suppressed by QSO UV radiation?
• RQ5 What is the role of peculiar velocities and redshift-space distortions in shaping the observed absorption clustering?

Key findings

• The probability of detecting absorption within 500 km s⁻¹ in a paired sightline is ≥50% at <100 kpc, declining to 23% at 100–200 kpc and 0.7% at 1–2 Mpc, indicating strong clustering on ~0.5 Mpc scales.
• Absorber-absorber correlation is inconsistent with fast winds; a mean outflow velocity of 250 km s⁻¹ would produce much stronger redshift elongation than observed.
• Transverse absorbers near QSOs are more tightly clustered around the QSO redshift than typical associated absorbers, with 19 within ±400 km s⁻¹ and 30 within ±1000 km s⁻¹.
• The probability of detecting absorbers at impact parameters 0.1–1.5 Mpc from QSOs is approximately constant, suggesting suppression of absorbers near QSOs, possibly due to UV destruction.
• The 3D distribution of 64 absorbers around 313 QSOs is nearly isotropic, with only a weak hint of anisotropy consistent with a 20° half-apex-angle UV beam or a short QSO lifetime of ~0.3 Myr.
• The lack of a strong proximity effect in H I data and the absence of a rapid rise in transverse absorber probability at small impact parameters suggest QSO UV radiation may destroy nearby absorbers, obscuring anisotropy.