Resolving the Internal Structure of Circum-Galactic Medium using Gravitationally Lensed Quasars

TL;DR

This study uses gravitationally lensed quasar spectra to reveal the small-scale internal structure of the circum-galactic medium, distinguishing high- and low-ionization regions and their spatial distributions.

Contribution

It provides the first detailed model of the CGM's internal structure based on a large sample of lensed quasar spectra, highlighting differences between high- and low-ionization absorbers.

Findings

High-ion absorbers are more homogeneously distributed than low-ion absorbers.

The CGM extends at least 500 kpc in size.

Low-ion absorbers are composed of small clouds (~1 kpc or smaller).

Abstract

We study the internal structure of the Circum-Galactic Medium (CGM), using 29 spectra of 13 gravitationally lensed quasars with image separation angles of a few arcseconds, which correspond to 100 pc to 10 kpc in physical distances. After separating metal absorption lines detected in the spectra into high-ions with ionization parameter (IP) $>$ 40 eV and low-ions with IP $<$ 20 eV, we find that i) the fraction of absorption lines that are detected in only one of the lensed images is larger for low-ions ($\sim$16%) than high-ions ($\sim$2%), ii) the fractional difference of equivalent widths ($EW$s) between the lensed images is almost same (${\rm d}EW$ $\sim$ 0.2) for both groups although the low-ions have a slightly larger variation, and iii) weak low-ion absorbers tend to have larger ${\rm d}EW$ compared to weak high-ion absorbers. We construct simple models to reproduce these observed properties and investigate the distribution of physical quantities such as size and location of absorbers, using some free parameters. Our best models for absorbers with high-ions and low-ions suggest that i) an overall size of the CGM is at least $\sim$ 500 kpc, ii) a size of spherical clumpy cloud is $\sim$ 1 kpc or smaller, and iii) only high-ion absorbers can have diffusely distributed homogeneous component throughout the CGM. We infer that a high ionization absorber distributes almost homogeneously with a small-scale internal fluctuation, while a low ionization absorber consists of a large number of small-scale clouds in the diffusely distributed higher ionized region. This is the first result to investigate the internal small-scale structure of the CGM, based on the large number of gravitationally lensed quasar spectra.