[Paper Review] The cross-correlation of galaxies in absorption with the Lyman $α$ forest

Ignasi Pérez-Ràfols, Matthew M. Pieri+4 more|arXiv (Cornell University)|Oct 6, 2022

Astronomy and Astrophysical Research4 citations

TL;DR

This study presents the first measurement of cross-correlation between Strong Blended Lyman-alpha (SBLA) absorption systems and the Lyman-alpha forest, finding consistent clustering properties—bias $2.329 \pm 0.057$ and redshift-space distortion $\beta = 0.417 \pm 0.010$—with Damped Lyman-alpha absorbers (DLAs), indicating SBLAs trace a similar population of galaxies in the circumgalactic medium. The results demonstrate SBLAs as a promising complementary tracer for Baryon Acoustic Oscillation (BAO) cosmology, with combined probes reducing BAO uncertainty by ~8% in the parallel and ~7% in the perpendicular direction compared to eBOSS limits.

ABSTRACT

We present the first clustering measurement of Strong Blended Lyman $α$ (SBLA) absorption systems by measuring their cross-correlation with the Lyman $α$ forest. SBLAs are a new population of absorbers detected within the Lyman $α$ forest. We find a bias of $2.329\pm0.057$, consistent with that of Damped Lyman $α$ absorbers (DLAs). For DLAs, we recover a bias of $2.331\pm0.057$ larger than previously reported (Pérez-Ràfols et al. 2018b). We also find a redshift space distortion parameter $β=0.417\pm0.010$, also consistent with the recovered value for DLAs ($β=0.416\pm0.010$). This is consistent with SBLA and DLA systems tracing different portions of the circumgalactic medium of a broadly common population of galaxies. Given these common clustering properties, we combined them to perform a cross-correlation of galaxies in absorption with the Ly$α$ forest. We find that the BAO scale uncertainty of this new measurement is $1.75 imes$ that of Ly$α$ auto-correlation and $1.6 imes$ that of the quasar cross-correlation with the Ly$α$ forest. We note that the current preferred metal contamination model for fitting the correlation functions with respect to the Ly$α$ forest is not realistic enough for SBLA systems, likely due to their status as high redshift precision sites of high metal enrichment. Mock spectra including SBLA systems and their associated metal absorption are required to understand this sample fully. We conclude that SBLAs have the potential to complement the standard Ly$α$ cosmological analyses in future surveys.

Motivation & Objective

• To measure the clustering of Strong Blended Lyman-alpha (SBLA) absorption systems via cross-correlation with the Lyman-alpha forest.
• To assess whether SBLA systems trace the same large-scale structure population as Damped Lyman-alpha absorbers (DLAs).
• To evaluate the potential of SBLAs as complementary tracers for Baryon Acoustic Oscillation (BAO) measurements in high-redshift cosmology.
• To identify systematic challenges in modeling metal absorption in SBLA systems and assess the need for improved mock simulations.
• To quantify the precision of BAO measurements using the Lyα × SBLA cross-correlation relative to standard Lyα auto- and quasar cross-correlations.

Proposed method

• Cross-correlation function analysis between SBLA absorption systems and the Lyman-alpha forest flux transmission field using high-resolution quasar spectra from the SDSS.
• Application of the standard cross-power spectrum method with redshift-space distortions modeled via the Kaiser approximation.
• Use of the 'vega' and 'picca' software pipelines for correlation function estimation and systematics mitigation.
• Exclusion of small-scale perpendicular separations ($r_\perp < 30$ h⁻¹ Mpc) where the model breaks down due to metal absorption effects.
• Comparison of SBLA clustering parameters (bias and β) with those of DLAs to assess shared physical origin.
• Combination of Lyα × SBLA, Lyα auto-, and quasar × Lyα cross-correlations to estimate joint BAO uncertainty reduction.

Experimental results

Research questions

• RQ1 Do SBLA absorption systems exhibit clustering properties consistent with those of Damped Lyman-alpha absorbers (DLAs)?
• RQ2 Can SBLA systems serve as a viable and independent tracer for Baryon Acoustic Oscillation (BAO) measurements in the high-redshift universe?
• RQ3 What is the impact of metal absorption on the accuracy of correlation function modeling for SBLA systems?
• RQ4 How does the BAO scale uncertainty of the Lyα × SBLA cross-correlation compare to standard Lyα auto- and quasar × Lyα cross-correlations?
• RQ5 To what extent can combining SBLA, Lyα, and quasar tracers improve the precision of BAO measurements?

Key findings

• The bias of SBLA systems is measured as $2.329 \pm 0.057$, consistent with the DLA bias of $2.331 \pm 0.057$, indicating similar large-scale clustering.
• The redshift-space distortion parameter for SBLAs is $\beta = 0.417 \pm 0.010$, matching the DLA value of $0.416 \pm 0.010$, supporting a common physical origin in the circumgalactic medium.
• The BAO scale uncertainty from the Lyα × SBLA cross-correlation is $1.75\times$ that of the Lyα auto-correlation and $1.6\times$ that of the Lyα × quasar cross-correlation when used independently.
• Combining all three probes (Lyα × SBLA, Lyα auto, and Lyα × quasar) reduces the error bars on the BAO parameters by approximately 8% for $\alpha_\parallel$ and 7% for $\alpha_\perp$ compared to the most recent eBOSS measurements.
• The standard correlation function fitting model breaks down at small perpendicular separations ($r_\perp < 30$ h⁻¹ Mpc), likely due to inadequate modeling of strong metal absorption in SBLA systems.
• Mock simulations including SBLA systems and their associated metal absorption are required to refine the modeling and validate future BAO measurements.