From IFS Maps to 3D Structure I: Constraining Geometry in the Circumgalactic Medium
Abstract
The three-dimensional geometric structure of the cool (T~104 K) circumgalactic medium (CGM) -- the line-of-sight depth and the number of discrete clumps -- is poorly constrained. Existing inferences come predominantly from quasar absorption-line statistics, but these pencil-beam probes provide limited spatial and kinematic information. Here we show that spatially resolved emission-line maps from integral field spectrograph (IFS) observations provide complementary, novel constraints on cool CGM geometry when projection effects are accounted for. Observations of a sample of CGM nebulae yield measurements of the line-of-sight and the plane-of-sky velocity dispersions, with ratios σlos/σpos ranging from ~1 to ~2.5. Using direct numerical simulations of isotropic turbulence projected through emitting slabs of varying depth Llos, we show that the ratio σlos/σpos allows us to probe Llos/Lneb, where Lneb is the nebular extent on the plane of the sky. We find that the inferred line-of-sight depth is typically ~0.1-5 Lneb, indicating that these nebulae are broadly comparable in depth and projected extent. Although the cool CGM is intrinsically clumpy, at the current IFS data resolution, each beam aggregates enough clumps that the projected dispersion ratio approaches its volume-filling value. Consequently, the emission kinematics provide little additional information on the detailed clump configuration beyond consistency with the clump incidence rates inferred from absorption-line surveys. These findings serve as a theoretical framework for the turbulence diagnostics developed in the companion paper (Paper II). Together, these papers demonstrate how IFS emission-line kinematics can provide a quantitative probe of the 3D CGM, complementary to absorption-line and emerging FRB approaches.
Related
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BibTeX
@misc{chen2026from,
title = {{From IFS Maps to 3D Structure I: Constraining Geometry in the Circumgalactic Medium}},
author = {Mandy C. Chen and Michael Rauch and Zhijie Qu and Gwen C. Rudie and Hsiao-Wen Chen and James R. Beattie},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.20929},
eprint = {2609.20929},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.20929},
}
RIS
TY - UNPB
TI - From IFS Maps to 3D Structure I: Constraining Geometry in the Circumgalactic Medium
AU - Mandy C. Chen
AU - Michael Rauch
AU - Zhijie Qu
AU - Gwen C. Rudie
AU - Hsiao-Wen Chen
AU - James R. Beattie
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.20929
UR - https://arxiv.org/abs/2609.20929
AB - The three-dimensional geometric structure of the cool (T~10^4 K) circumgalactic medium (CGM) -- the line-of-sight depth and the number of discrete clumps -- is poorly constrained. Existing inferences come predominantly from quasar absorption-line statistics, but these pencil-beam probes provide limited spatial and kinematic information. Here we show that spatially resolved emission-line maps from integral field spectrograph (IFS) observations provide complementary, novel constraints on cool CGM geometry when projection effects are accounted for. Observations of a sample of CGM nebulae yield measurements of the line-of-sight and the plane-of-sky velocity dispersions, with ratios σ_los/σ_pos ranging from ~1 to ~2.5. Using direct numerical simulations of isotropic turbulence projected through emitting slabs of varying depth L_los, we show that the ratio σ_los/σ_pos allows us to probe L_los/L_neb, where L_neb is the nebular extent on the plane of the sky. We find that the inferred line-of-sight depth is typically ~0.1-5 L_neb, indicating that these nebulae are broadly comparable in depth and projected extent. Although the cool CGM is intrinsically clumpy, at the current IFS data resolution, each beam aggregates enough clumps that the projected dispersion ratio approaches its volume-filling value. Consequently, the emission kinematics provide little additional information on the detailed clump configuration beyond consistency with the clump incidence rates inferred from absorption-line surveys. These findings serve as a theoretical framework for the turbulence diagnostics developed in the companion paper (Paper II). Together, these papers demonstrate how IFS emission-line kinematics can provide a quantitative probe of the 3D CGM, complementary to absorption-line and emerging FRB approaches.
ER -