Spatial Distribution of O vi Covering Fractions in the Simulated Circumgalactic Medium

Marra, Rachel; Churchill, Christopher W.; Kacprzak, Glenn G.; Vliet, Rachel Vander; Ceverino, Daniel; Lewis, James; Nielsen, Nikole M.; Muzahid, Sowgat; Charlton, Jane C.

doi:10.3847/1538-4357/abd033

Cited by 6 publications

(8 citation statements)

References 93 publications

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“…The ambiguity of the relative location between the gas and the galaxies does not pose a problem for CGM analyses using large volume hydrodynamic simulations, though zoom-in simulations may underestimate the projection effects. Simulations that reproduce the radial profiles of the column density of LIS gas (Ford et al 2016;Oppenheimer et al 2018a) often underestimate the O VI column density around ∼L * galaxies by about a factor of two (e.g., Hummels et al 2013;Oppenheimer et al 2016;Gutcke et al 2017;Suresh et al 2017;Marra et al 2021). This problem could potentially be resolved by fossil AGN proximity zones (Oppenheimer & Schaye 2013;Oppenheimer et al 2018b), black hole feedback (Nelson et al 2018), by including cosmic ray physics (Ji et al 2020), or by changing the model for the UV background (Appleby et al 2021).…”

Section: Introductionmentioning

confidence: 99%

How identifying circumgalactic gas by line-of-sight velocity instead of the location in 3D space affects O VI measurements

Ho,

Martin,

Schaye

2021

Preprint

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The high incidence rate of the O VI λλ1032, 1038 absorption around low-redshift, ∼L * star-forming galaxies has generated interest in studies of the circumgalactic medium. We use the high-resolution EAGLE cosmological simulation to analyze the circumgalactic O VI gas around z ≈ 0.3 star-forming galaxies. Motivated by the limitation that observations do not reveal where the gas lies along the lineof-sight, we compare the O VI measurements produced by gas within fixed distances around galaxies and by gas selected using line-of-sight velocity cuts commonly adopted by observers. We show that gas selected by a velocity cut of ±300 km s −1 or ±500 km s −1 produces a higher O VI column density, a flatter column density profile, and a higher covering fraction compared to gas within one, two, or three times the virial radius (r vir ) of galaxies. The discrepancy increases with impact parameter and worsens for lower mass galaxies. For example, compared to the gas within 2r vir , identifying the gas using velocity cuts of 200-500 km s −1 increases the O VI column density by 0.2 dex (0.1 dex) at 1r vir to over 0.75 dex (0.7 dex) at ≈ 2r vir for galaxies with stellar masses of 10 9 -10 9.5 M (10 10 -10 10.5 M ). We furthermore estimate that excluding O VI outside r vir decreases the circumgalactic oxygen mass measured by Tumlinson et al. (2011) by over 50%. Our results demonstrate that gas at large lineof-sight separations but selected by conventional velocity windows has significant effects on the O VI measurements and may not be observationally distinguishable from gas near the galaxies.

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Section: Introductionmentioning

confidence: 99%

How identifying circumgalactic gas by line-of-sight velocity instead of the location in 3D space affects O VI measurements

Ho,

Martin,

Schaye

2021

Preprint

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“…In particular, single-phase ionisation modelling does not accurately capture the underlying distribution of metallicities and gas physical conditions (see Haislmaier et al 2021;Sameer et al 2021), even if it might capture the mean values (Marra et al 2021b;Sameer et al 2021). We expect that more complex, multi-phase modelling approaches (e.g., Zahedy et al 2019Zahedy et al , 2021Haislmaier et al 2021;Sameer et al 2021Sameer et al , 2022Nielsen et al 2022), will provide an improved characterisation of CGM gas.…”

Section: Discussionmentioning

confidence: 99%

“…to a spatially isolated iso-thermal cloud. Simulations show the absorbing gas structures are far more complex (Churchill et al 2015;Liang et al 2018;Peeples et al 2019;Marra et al 2021b). For example, Churchill et al (2015) found that Mg absorption and the bulk of H absorption arise in "cloud-like" structures that are confined within contiguous gas cells over small spatial scales.…”

Section: Introductionmentioning

confidence: 99%

Examining quasar absorption-line analysis methods: the tension between simulations and observational assumptions key to modelling clouds

Marra¹,

Churchill²,

Kacprzak³

et al. 2022

Preprint

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A key assumption in quasar absorption line studies of the circumgalactic medium (CGM) is that each absorption component maps to a spatially isolated "cloud" structure that has single valued properties (e.g. density, temperature, metallicity). We aim to assess and quantify the degree of accuracy underlying this assumption. We used adaptive mesh refinement hydrodynamic cosmological simulations of two 𝑧 = 1 dwarf galaxies and generated synthetic quasar absorption-line spectra of their CGM. For the Si 𝜆1260 transition, and the C 𝜆𝜆1548, 1550 and O 𝜆𝜆1031, 1037 fine-structure doublets, we objectively determined which gas cells along a line-of-sight (LOS) contribute to detected absorption. We implemented a fast, efficient, and objective method to define individual absorption components in each absorption profile. For each absorption component, we quantified the spatial distribution of the absorbing gas. We studied a total of 1,302 absorption systems containing a total of 7,755 absorption components. 48% of Si , 68% of C , and 72% of O absorption components arise from two or more spatially isolated "cloud" structures along the LOS. Spatially isolated "cloud" structures were most likely to have cloud-cloud LOS separations of 0.03𝑅 𝑣𝑖𝑟 , 0.11𝑅 𝑣𝑖𝑟 , and 0.13𝑅 𝑣𝑖𝑟 for Si , C , and O , respectively. There can be very little overlap between multi-phase gas structures giving rise to absorption components. If our results reflect the underlying reality of how absorption lines record CGM gas, they place tension on current observational analysis methods as they suggest that component-by-component absorption line formation is more complex than is assumed and applied for chemical-ionisation modelling.

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“…The H code is well suited for studying the low-density CGM (log 𝑛 H /cm −3 < −1) as it gives the best results for optically thin, low density gas. 2 Previous studies (Churchill et al 2012;Kacprzak et al 2012;Churchill et al 2015;Kacprzak et al 2019;Marra et al 2021) have used H…”

Section: Synthetic Absorption Line Generationmentioning

confidence: 99%

“…Though simulations are commonly touted as a powerful tool for providing deeper insights into the interpretation of quasar absorption line data, and many forays in this direction have been undertaken (e.g., Churchill et al 2015;Liang et al 2018;Kacprzak et al 2019;Peeples et al 2019;Péroux et al 2020;Marra et al 2021;Strawn et al 2021), no experiment has been performed as to how the standard spectral analysis plus chemical-ionisation modelling methods employed by observers hold for simulated quasar absorption lines, i.e., do we recover the intrinsic mean physical properties of the absorbing gas using standard VP fitting to the spectra (see Churchill et al 2020, and references therein) followed by C (Ferland et al 2017) ionisation modelling constrained by the Voigt profile column densities?…”

Section: Introductionmentioning

confidence: 99%

Using cosmological simulations and synthetic absorption spectra to assess the accuracy of observationally derived CGM metallicities

Marra,

Churchill,

Doughty

et al. 2021

Preprint

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We used adaptive mesh refinement hydrodynamic cosmological simulations of a 𝑧 = 1 Milky Way-type galaxy and a 𝑧 = 0 Dwarf galaxy and generated synthetic quasar absorption-line spectra of their circumgalactic medium (CGM). Our goal is to assess whether standard observational spectroscopic analysis methods accurately reproduce intrinsic column densities, metallicities [Si/H], and hydrogen densities 𝑛 H , in simulated absorption-line systems. Without knowledge of the intrinsic simulated properties (blind study), we analysed synthetic COS and HIRES spectra with fixed 𝑆/𝑁 = 30 to determine the column densities, metallicity, and 𝑛 H , using Voigt profile fitting combined with Markov-Chain Monte-Carlo single-phase C modelling techniques. To quantify the intrinsic simulated absorbing gas properties, we objectively determined which gas cells along a line of sight (LOS) contribute to detected absorption in the spectra and adopt the unweighted geometric mean of these gas cell properties. For this pilot study, we performed this experiment for five LOS in the Milky-Way galaxy and five LOS in the Dwarf galaxy. We found an average agreement between the "observed" and intrinsic metallicity overestimated within 0.8𝜎 or 0.2 dex for the "Milky-Way" CGM and overestimated within 1.4𝜎 or 0.2 dex for the Dwarf galaxy CGM. We found that the spectroscopically-derived 𝑛 H are underestimated within 0.8𝜎 or 0.4 dex of the intrinsic 𝑛 H for the "Milky-Way" CGM and overestimated within 0.3𝜎 or 0.3 dex for the Dwarf galaxy CGM. The overall agreement suggests that, for single-phase ionisation modelling of systems where there is substantial spread in gas properties, global metallicity measurements from quasar absorption line studies are capturing the average metallicity and ionisation parameters in a given astrophysical environment.

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Spatial Distribution of O vi Covering Fractions in the Simulated Circumgalactic Medium

Cited by 6 publications

References 93 publications

How identifying circumgalactic gas by line-of-sight velocity instead of the location in 3D space affects O VI measurements

How identifying circumgalactic gas by line-of-sight velocity instead of the location in 3D space affects O VI measurements

Examining quasar absorption-line analysis methods: the tension between simulations and observational assumptions key to modelling clouds

Using cosmological simulations and synthetic absorption spectra to assess the accuracy of observationally derived CGM metallicities

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