Cloud-by-cloud, multiphase, Bayesian modelling: application to four weak, low-ionization absorbers

Abstract

We present a new method aimed at improving the efficiency of component by component ionization modelling of intervening quasar absorption-line systems. We carry out cloud-by-cloud, multiphase modelling making use of cloudy and Bayesian methods to extract physical properties from an ensemble of absorption profiles. Here, as a demonstration of method, we focus on four weak, low-ionization absorbers at low redshift, because they are multiphase but relatively simple to constrain. We place errors on the inferred metallicities and ionization parameters for individual clouds, and show that the values differ from component to component across the absorption profile. Our method requires user input on the number of phases and relies on an optimized transition for each phase, one observed with high resolution and signal-to-noise ratio. The measured Doppler parameter of the optimized transition provides a constraint on the Doppler parameter of H i, thus providing leverage in metallicity measurements even when hydrogen lines are saturated. We present several tests of our methodology, demonstrating that we can recover the input parameters from simulated profiles. We also consider how our model results are affected by which radiative transitions are covered by observations (for example, how many H i transitions) and by uncertainties in the b parameters of optimized transitions. We discuss the successes and limitations of the method, and consider its potential for large statistical studies. This improved methodology will help to establish direct connections between the diverse properties derived from characterizing the absorbers and the multiple physical processes at play in the circumgalactic medium.

The Cosmic Ultraviolet Baryon Survey (CUBS) – III. Physical properties and elemental abundances of Lyman-limit systems at z < 1

“…The example presented in Figure 8 further underscores the importance of observing ionic absorptions from higher ionization species such as O iv, C iv, and Si iv (see e.g., Zahedy et al 2019a;Sameer et al 2021) to reveal and characterize multiphase gas in the CGM. While the gas density and the metallicity of a denser, less ionized gas phase is relatively well constrained by the observed column density ratios of various low ions, additional coverage of these higher ionization species may confirm the presence of more highly ionized gas…”

“…An often-used diagnostic of the origin of CGM/IGM gas is the gas metallicity (e.g., Pei & Fall 1995;Pettini 2001;Schaye et al 2003;Simcoe et al 2004;Lehner et al 2013;Cooke et al 2017;Zahedy et al 2019a;Sameer et al 2021), which quantifies the overall heavy element enrichment level of the gas. Galaxies in the local Universe are known to follow a massmetallicity relation: higher mass galaxies tend to also exhibit both higher stellar and gas-phase metallicities (e.g., Tremonti et al 2004;Gallazzi et al 2005).…”

The Cosmic Ultraviolet Baryon Survey (CUBS) – IV. The complex multiphase circumgalactic medium as revealed by partial Lyman limit systems

“…The two phase solution does not reproduce the measured 𝑁 O VI , which results from a still more highly-ionized phase (see, e.g. Sameer et al 2021) which we do not attempt to model here.…”

“…Most CGM inferences about abundances and densities rely on such assumptions and they may yield bulk metallicity inferences comparable to the average metallicity of the various components that are blended together. However, recent work exploring detailed multicomponent and multiphase photoionization modeling underlines the shortcomings in approaches that use such simplifying assumptions (Zahedy et al 2019(Zahedy et al , 2021Haislmaier et al 2020;Sameer et al 2021). These works demonstrate that gas at a wide range of densities and chemical abundances can be present within the same halo (see also D'Odorico & Petitjean 2001).…”

OUP accepted manuscript

“…The three kinematically distinct components are modeled separately. We adopt the Bayesian approach described in Sameer et al (2021) for the component-wise multiphase modeling of the absorption system. The O was used as the optimizing ion for the low ionization phase of components 1 and 2.…”