MusE GAs FLOw and Wind (MEGAFLOW) XIV: Background-Galaxy Absorption Reveals Kiloparsec-Scale Structure in the Cool Circumgalactic Medium
Abstract
The properties of the cool (T∼104~K) gas in the circumgalactic medium (CGM) are closely linked to the physical mechanisms that create and maintain this multiphase medium. The cool CGM is thought to consist of discrete clouds, whose characteristic size is unknown. Here we present a geometric and direct approach to constrain the coherence scale of these cool structures using stacked MgII absorption lines measured against extended background galaxies and effectively point-like background quasars, whose sizes are a few kpc and ≲ 0.01pc, respectively. When the background-source size is smaller than the coherence scale of the foreground clouds, incomplete covering lowers the detection fraction and causes the median stacked absorption to differ from the mean. For stacked MgII absorption against background galaxies, the mean and median equivalent width (EW) profiles are broadly consistent. For stacked MgII absorption against background quasars, by contrast, the median and mean EW profiles differ significantly, reaching a ≈10σ difference beyond 100 kpc. Furthermore, we find that the median and mean EW profiles are broadly consistent for large background galaxies (median half-light radius ≈ 6.6 kpc), but differ at the ≈3σ level for small background galaxies (≈ 1.5 kpc). This suggests that the MgII-bearing gas has an effective transverse coherence scale of order a few kiloparsecs, broadly consistent with the ∼2--7 kpc range probed by the background galaxies. Using a toy model in which the CGM is populated with discrete cool clouds, we show that the observed differences arise naturally from the combination of partial covering and beam averaging. Our results provide a new geometry-based measure of the small-scale structure of cool CGM gas.
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BibTeX
@misc{guo2026muse,
title = {{MusE GAs FLOw and Wind (MEGAFLOW) XIV: Background-Galaxy Absorption Reveals Kiloparsec-Scale Structure in the Cool Circumgalactic Medium}},
author = {Yucheng Guo and Nicolas F. Bouché and Martin Wendt and Timothy Heckman and Joop Schaye and Sanchayeeta Borthakur and Johannes Zabl and Maxime Cherrey and Sowgat Muzahid and Ismael Pessa and Ramona Augustin and Daria Kozlova},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.11089},
eprint = {2606.11089},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.11089},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - MusE GAs FLOw and Wind (MEGAFLOW) XIV: Background-Galaxy Absorption Reveals Kiloparsec-Scale Structure in the Cool Circumgalactic Medium
AU - Yucheng Guo
AU - Nicolas F. Bouché
AU - Martin Wendt
AU - Timothy Heckman
AU - Joop Schaye
AU - Sanchayeeta Borthakur
AU - Johannes Zabl
AU - Maxime Cherrey
AU - Sowgat Muzahid
AU - Ismael Pessa
AU - Ramona Augustin
AU - Daria Kozlova
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2606.11089
UR - https://arxiv.org/abs/2606.11089
AB - The properties of the cool (T∼10^4~K) gas in the circumgalactic medium (CGM) are closely linked to the physical mechanisms that create and maintain this multiphase medium. The cool CGM is thought to consist of discrete clouds, whose characteristic size is unknown. Here we present a geometric and direct approach to constrain the coherence scale of these cool structures using stacked MgII absorption lines measured against extended background galaxies and effectively point-like background quasars, whose sizes are a few kpc and ≲ 0.01pc, respectively. When the background-source size is smaller than the coherence scale of the foreground clouds, incomplete covering lowers the detection fraction and causes the median stacked absorption to differ from the mean. For stacked MgII absorption against background galaxies, the mean and median equivalent width (EW) profiles are broadly consistent. For stacked MgII absorption against background quasars, by contrast, the median and mean EW profiles differ significantly, reaching a ≈10σ difference beyond 100 kpc. Furthermore, we find that the median and mean EW profiles are broadly consistent for large background galaxies (median half-light radius ≈ 6.6 kpc), but differ at the ≈3σ level for small background galaxies (≈ 1.5 kpc). This suggests that the MgII-bearing gas has an effective transverse coherence scale of order a few kiloparsecs, broadly consistent with the ∼2--7 kpc range probed by the background galaxies. Using a toy model in which the CGM is populated with discrete cool clouds, we show that the observed differences arise naturally from the combination of partial covering and beam averaging. Our results provide a new geometry-based measure of the small-scale structure of cool CGM gas.
ER -