Abstract
Abstract
Metallicity estimates of circumgalactic gas based on absorption-line measurements typically require photoionization modeling to account for unseen ionization states. We explore the impact of uncertainties in the extreme-ultraviolet background (EUVB) radiation on such metallicity determinations for the z < 1 circumgalactic medium. In particular, we study how uncertainties in the power-law slope of the EUV radiation, α
EUVB, from active galactic nuclei affect metallicity estimates in a sample of 34 absorbers with H i column densities between
15.25
<
log
(
N
H
I
/
cm
−
2
)
<
17.25
and measured metal ion column densities. We demonstrate the sensitivity of metallicity estimates to changes in the EUV power-law slope of active galactic nuclei, α
EUVB, at low redshift (z < 1), showing that derived absorber metallicities increase on average by ≈0.3 dex as the EUV slope is hardened from α
EUVB = −2.0 to −1.4. We use Markov Chain Monte Carlo sampling of photoionization models with α
EUVB as a free parameter to derive metallicities for these absorbers. The current sample of absorbers does not provide a robust constraint on the slope α
EUVB itself; we discuss how future analyses may provide stronger constraints. Marginalizing over the uncertainty in the slope of the background, we find that the average uncertainties in the metallicity determinations increase from 0.08 to 0.14 dex when switching from a fixed EUVB slope to one that freely varies. Thus, we demonstrate that EUVB uncertainties can be included in ionization models while still allowing for robust metallicity inferences.
Publisher
American Astronomical Society
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
7 articles.
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