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Two domains of extended Lyman-alpha emission around galaxies: from local radiation to environmental regulation

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Daria Kozlova, Lutz Wisotzki, John Pharo, Tanya Urrutia, Ramona Augustin, Yucheng Guo, Haruka Kusakabe, Joop Schaye
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Daniil Smirnov, Ismael Pessa

Astrophysics of Galaxies · Submitted 17 Aug 2026 · 137 references

Pith's verdict: First object-by-object look at faint LAE outer halos, with a plausible two-domain picture, but the environmental side rests on one significant bin and a selection subtlety—worth refereeing, with serious revisions.

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arXiv ID2608.16665 (version 1)
Pith ID4G54VCKU, Pith's permanent record of this paper and its review
CitationDaniil Smirnov, Daria Kozlova, Haruka Kusakabe, Ismael Pessa, John Pharo, Joop Schaye, Lutz Wisotzki, Ramona Augustin, Tanya Urrutia, Yucheng Guo. "Two domains of extended Lyman-alpha emission around galaxies: from local radiation to environmental regulation." arXiv:2608.16665 (2026). https://pith.science/paper/4G54VCKU
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The paper

The paper claims

Lyα halos switch from star-formation powering to environmental regulation near one virial radius.

The reading

This paper uses ultradeep integral-field spectroscopy to measure individual Lyman-alpha halos around 21 intrinsically faint galaxies at redshift 3 to 4 out to 50 kpc, and argues that the halos split into two distinct radial domains at about one virial radius (~20 kpc). Inside that radius the surface brightness is driven by the host's star formation rate and is oblivious to the surrounding environment; outside it the correlation flips, with no SFR dependence but a positive correlation with the local density of Lyman-alpha emitters. The paper further argues that neither individually undetected neighboring galaxies nor ultraviolet-background fluorescence can supply the observed outer emission, so the outer halo must be diffuse gas whose amount or excitation is environmentally regulated. If right, this locates the CGM/IGM transition at a concrete radius and redirects theoretical work on outer halos toward accretion and cooling radiation rather than aggregated satellites.

What carries the argument

The load-bearing object is the azimuthally averaged Lyα surface-brightness profile split into six radial apertures from 0 to 50 kpc, with a fixed outer annulus 15-50 kpc used to define 'halo-bright' objects. The argument runs on the contrast between two correlation sequences across those apertures: SB versus SFR and SB versus environmental density $\delta+1$, estimated from a kernel-smoothed redshift distribution of LAEs. The turnover at ~1 $r_{\mathrm{vir}}$ is what carries the two-domain conclusion. The quantitative exclusion of discrete neighbors uses the predicted integrated SB of undetected LAEs, $\mathrm{SB}_{\mathrm{unb}}$, built from the Lyα luminosity function, the survey's detection selection function, and a halo-occupation clustering boost, evaluated object by object; the exclusion of fluorescence uses the optically-thick-cloud Lyα SB relation with a modern UVB photoionization rate and unity covering fraction as an upper limit.

Extended reading notes

Core claim

The central claim is that the radial surface brightness profile of a Lyα halo encodes a regime change rather than a single exponential plus a smooth 'two-halo' addition. For radii below roughly 15-20 kpc the observed SB correlates with the SFR of the central galaxy (Pearson r about 0.56-0.63) and shows no correlation with environmental overdensity $\delta+1$; beyond ~20 kpc the SFR correlation is lost (r about 0.06, -0.04, -0.34 in the outer three annuli, none significant) while a positive correlation with $\delta+1$ emerges in every outer annulus. Since three independent mass estimates place the typical virial radius at ~20 kpc, the authors identify the turnover at ~1 $r_{\mathrm{vir}}$. They then test two external power sources quantitatively: the integrated light from undetected faint LAEs is predicted to be several times below the measured SB inside 37 kpc and only approaches it in the outermost bin, and UVB fluorescence at maximum covering factor contributes at most about 1-$\sigma$. The paper's conclusion is that the outer Lyα halo is mostly diffuse cool gas beyond the virial radius, regulated by the environment, not a superposition of unresolved galaxies.

Load-bearing premise

The argument depends on the halo-bright selection: objects were admitted by their signal-to-noise in the 15-50 kpc annulus, and the environmental correlation is measured on that sample; if selecting on outer surface brightness also selects overdense sightlines, the turnover at ~1 $r_{\mathrm{vir}}$ could be partly a selection effect rather than purely a physical regime change.

Editorial extensions

If this is right

  • Outer-halo surface brightness beyond ~1 $r_{\mathrm{vir}}$ is not a star-formation indicator; it should be interpreted as an environmental tracer.
  • Stacked profiles that flatten near 30-50 kpc should be read as the onset of the environmentally regulated outer domain rather than as domination by unresolved satellites.
  • Radially resolved spectra should show systematically different Lyα kinematics, such as bluer emission, outside ~1 $r_{\mathrm{vir}}$ if the outer halo is inflow- or cooling-driven.
  • Environmental density becomes a predictor of outer-halo extent, allowing surveys to map gas reservoirs around low-mass galaxies without detecting the gas directly.
  • Simulations of the CGM/IGM interface now have a fixed physical radius, ~1 $r_{\mathrm{vir}}$, against which to calibrate the transition from stellar-powered to environment-regulated Lyα emission.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The turnover radius may scale with halo mass; if so, studying halos in units of $r/r_{\mathrm{vir}}$ rather than kiloparsecs should unify low- and high-mass LAE populations.
  • A testable extension would compare halo-bright objects with a volume-limited, environment-blind sample to determine how much of the reported correlation is selection-driven.
  • If the outer halo is accretion or cooling radiation, its ratio to the inner SFR-powered halo should increase with halo mass, predicting brighter outer halos around more massive emitters such as Lyman-break galaxies and AGN.
  • The lack of an internal correlation inside 15 kpc suggests that environment acts on the gas reservoir rather than on ongoing star formation, which could explain part of the scatter in Lyα escape fractions without invoking radiative-transfer complexity.
Original abstract

We examine the relation between extended Ly$\alpha$ halos around high-redshift galaxies and the main factors responsible for driving the emission in such halos, in particular at distances around and beyond one virial radius $r_\mathrm{vir}$. To reach the required surface brightness sensitivity we take advantage of the MUSE eXtremely Deep Field (MXDF) survey, allowing us to probe levels as faint as $\sim 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ in individual Ly$\alpha$ halos. Our sample consists of the 21 apparently core- and halo-brightest (yet intrinsically low luminosity $\log_{10}$L$_{\mathrm{Ly}\alpha} < 42.3$ erg s$^{-1}$) Ly$\alpha$ emitters (LAEs) in the MXDF at $3<z<4$, with typical virial radii around 20 kpc. We measure their radial surface brightness profiles out to 50 kpc (more than $2r_{\mathrm{vir}}$) and investigate the correlations between surface brightness and internal (star formation rates of the host galaxies, SFR) or external influences (environmental density, $\delta+1$). We find a clear break in these correlations at radii around or just below $1r_{\mathrm{vir}}$. Below this break the emission correlates tightly with SFR (as expected) and not at all with $\delta+1$. Beyond $\sim 1r_\mathrm{vir}$(20 kpc) we observe the opposite trend with no dependence on SFR, but an emerging correlation with $\delta+1$. We compare our measurements with the expected integrated surface brightness from ultrafaint, individually undetected LAEs and find that the latter is insufficient to drive the observed correlation. We conclude that Ly$\alpha$ emission from the outer halos is regulated by the surrounding environment, but originates mostly from diffuse gas rather than discrete sources.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,386 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.49 · human participants 0.01 · controlled comparison 0.16 · randomized assignment 0.01 · states quantitative result 0.97 · compares with prior results 0.73 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We examine the relation between extended Ly$\alpha$ halos around high-redshift galaxies and the main factors responsible for driving the emission in such halos, in particular at distances around and beyond one virial radius $r_\mathrm{vir}$. To reach the required surface brightness sensitivity we take advantage of the MUSE eXtremely Deep Field (MXDF) survey, allowing us to probe levels as faint as $\sim 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ in individual Ly$\alpha$ halos. Our sample consists of the 21 apparently core- and halo-brightest (yet intrinsically low luminosity $\log_{10}$L$_{\mathrm{Ly}\alpha} < 42.3$ erg s$^{-1}$) Ly$\alpha$ emitters (LAEs) in the MXDF at $3<z<4$, with typical virial radii around 20 kpc. We measure their radial surface brightness profiles out to 50 kpc (more than $2r_{\mathrm{vir}}$) and investigate the correlations between surface brightness and internal (star formation rates of the host galaxies, SFR) or external influences (environmental density, $\delta+1$). We find a clear break in these correlations at radii around or just below $1r_{\mathrm{vir}}$. Below this break the emission correlates tightly with SFR (as expected) and not at all with $\delta+1$. Beyond $\sim 1r_\mathrm{vir}$(20 kpc) we observe the opposite trend with no dependence on SFR, but an emerging correlation with $\delta+1$. We compare our measurements with the expected integrated surface brightness from ultrafaint, individually undetected LAEs and find that the latter is insufficient to drive the observed correlation. We conclude that Ly$\alpha$ emission from the outer halos is regulated by the surrounding environment, but originates mostly from diffuse gas rather than discrete sources.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We examine the relation between extended Ly$\alpha$ halos around high-redshift galaxies and the main factors responsible for driving the emission in such halos, in particular at distances around and beyond one virial radius $r_\mathrm{vir}$. contribution 0.84 0.77 0.03 unclear 0.70
s1 To reach the required surface brightness sensitivity we take advantage of the MUSE eXtremely Deep Field (MXDF) survey, allowing us to probe levels as faint as $\sim 10^{-20}$ erg cm$^{-2}$ s$^{-1}$ arcsec$^{-2}$ in individual Ly$\alpha$ halos. method 1.00 0.85 0.01 asserted 0.97
s2 Our sample consists of the 21 apparently core- and halo-brightest (yet intrinsically low luminosity $\log_{10}$L$_{\mathrm{Ly}\alpha} < 42.3$ erg s$^{-1}$) Ly$\alpha$ emitters (LAEs) in the MXDF at $3<z<4$, with typical virial radii around 20 kpc. method 0.80 0.77 0.01 asserted 1.00
s3 We measure their radial surface brightness profiles out to 50 kpc (more than $2r_{\mathrm{vir}}$) and investigate the correlations between surface brightness and internal (star formation rates of the host galaxies, SFR) or external influences (environmental density, $\delta+1$). method 0.80 0.97 0.02 asserted 0.98
s4 We find a clear break in these correlations at radii around or just below $1r_{\mathrm{vir}}$. result 1.00 0.97 0.09 asserted 1.00
s5 Below this break the emission correlates tightly with SFR (as expected) and not at all with $\delta+1$. result 1.00 0.96 0.09 asserted 1.00
s6 Beyond $\sim 1r_\mathrm{vir}$(20 kpc) we observe the opposite trend with no dependence on SFR, but an emerging correlation with $\delta+1$. result 1.00 0.97 0.20 asserted 1.00
s7 We compare our measurements with the expected integrated surface brightness from ultrafaint, individually undetected LAEs and find that the latter is insufficient to drive the observed correlation. result 0.95 0.93 0.05 asserted 1.00
s8 We conclude that Ly$\alpha$ emission from the outer halos is regulated by the surrounding environment, but originates mostly from diffuse gas rather than discrete sources. interpretation 1.00 0.88 0.97 asserted 0.98

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.58.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Lyα central (1 Kron radius) versus annular (15 < r < 50 kpc) signal-to-noise ratios of the parent sample (grey diamonds) and of the selected LAEs (orange filled and blue empty diamonds for halo- and core-bright LAEs, respectively). The dashed lines mark our selection thresholds of S/N15−50kpc≥2 and S/NKron ≥ 30. Objects falling into both categories are marked accordingly. The orange triangle indicate ID 8537 with centrally suppressed Lyα emission for which no meaningful cen￾tral S/N value could be measured. to the local UV radiation field. The implications of these results are…
Figure 2. Lyα pseudo-narrowband surface brightness images of all LAEs in the sample, smoothed by a Gaussian kernel with σ = 0.75′′. The color of the box around each ID number indicates the subset to which the object belongs (orange: halo-bright, blue: core-bright). The objects are sorted in descending order by their Lyα signal-to-noise ratios, using S/N15−50kpc for the halo-bright and S/NKron for the core-bright LAEs. The black dotted circles mark radii of 15 kpc and 50 kpc, respectively. The contours (thin black, bold black, yellow) correspond to surface brightness levels of (1, 10, 100) ×…
Figure 3. Surface brightness profiles of the LAEs in our sample (blue dots: core-bright, orange: halo-bright LAEs). The solid lines show two￾component galfit models, with the central (dashed line) and extended components (dotted). The vertical lines mark the physical radius of r = 15 kpc at the redshift of each object. SBobs 1σ upper limits are indicated by triangles. numbers of objects (Wisotzki et al. 2018; Gallego et al. 2021; Lujan Niemeyer et al. 2022; Kikuchihara et al. 2022; Guo et al. 2024b), due to the extremely low SB levels required. Our indi￾vidual LAH measurements reveal a large…
Figure 4. Lyα median-stacked radial surface brightness profiles of the LAEs in the two samples. For comparison, the grey diamonds show also the median profile of 3 < z < 4 LAEs from Wisotzki et al. (2018). All SB profiles are rescaled to match at r = 0. SBobs 1σ upper limits are indicated by triangles. All SB values are re-scaled to z = 3.5 by a factor of [(1 + z)/(1 + 3.5)]4 . Small horizontal shifts between the different sets are applied for visualization purposes only. applied. On the other hand, the imprint of neighbors onto the Lyα emission around a single object depends strongly on the…
Figure 6. Observed Lyα surface brightness versus LAE environmental density δ + 1, estimated as described in the text. Each panel represents a different radial range, as indicated. The straight lines show the best-fit linear regression, the shaded regions the 68% confidence interval). The numbers provide the strength of correlation characterized as as Pearson coefficient r and the probablility p for the null hypothesis of no correlation. The special case of ID 8537 with its central Lyα depression is indicated by the black-edged symbol; it is excluded from the regression in the innermost radial…
Figure 7. Observed Lyα surface brightness versus star formation rate (estimated as described in the text) for all LAEs in our sample with detected UV counterparts in deep HST imaging. As in
Figure 8. Maximum fractional contribution from UVB fluorescence to the Lyα emission at 27 < r < 37 kpc (left) and 37 < r < 50 kpc (right). Each panel shows the ratio of the observed SB to the predicted SB (up￾per limit assuming fc = 1, see text), SBobs/SBUVB, plotted versus envi￾ronmental density. The horizontal dashed black lines represent constant values of 0 and 1, respectively. Objects with nearby luminous AGNs are marked with red edges: thin edge correspond to ionization rate enhance￾ment compared to the UVB of ∼ 2, thick edges of ∼ 10. bars. We have to acknowledge that our individual…
Figure 9. Artistic view of the different domains of LAHs. Four quarters picture four possible combinations of low/high SFR and δ + 1 that then manifest themselves via fainter/brighter inner (orange) and outer (violet) CGM Lyα emission. Blue stars refer to star-forming regions within galaxies, where larger stars indicate higher SFR. The large-scale overdensity is represented by the small orange-violet halos, as well as by a number of faint, individually unresolved LAEs (orange circles with a single star). The blue arrows represent accreting neutral gas from the IGM. dius, beyond which…

Showing 8 of the paper's 9 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2608.16665 by the authors.

References
How the literature uses it

What this paper does with the arXiv papers it cites

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

What it cites · 19 sentences without a confident label

Uses the method or tool of 5 papers
  • [ref]) and applying the $M⋆/M_vir$ relation from [ref] ; (ii) converting the Ly$α$ luminosities into clustering-inferred virial masses using Fig. 8 of [ref] ; (iii) by applying the [ref] relation, updated for high redshifts by [ref] with resulting conversion factor of $0.02±0.01$ between stellar half-light radius and virial radius.

    The size - virial radius relation of galaxies · Inner Ly$α$ halos are powered only by star formation · 0.83

  • AMUSED provides SFR estimates derived with two different modeling codes, Magphys [ref] and Prospector [ref] , but we checked that the choice between these two has negligble impact on our findings.

    A simple model to interpret the ultraviolet, optical and infrared emission from galaxies · Local UV radiation field · 0.93

  • We employ the LSDCat code [ref] , specifically the routine, to perform adaptive aperture photometry in the NB images, where the apertures are determined from the data as multiples of 3-dimensional Kron radii (see [ref] for the precise definition of these radii).

    LSDCat: Detection and cataloguing of emission-line sources in integral-field spectroscopy datacubes · Core-bright Ly$α$ emitters · 1.00

Cites as support 2 papers
  • We therefore conclude that out to about $1 r_vir$ the observed circumgalactic Ly$α$ emission is mostly powered by massive stars, either as recombination radiation in the Hii regions in the galaxy that is then scattered outwards by neutral gas in the CGM [ref] , or by direct recombination following in-situ photoionization due to Lyman continuum photons leaking into the CGM [ref] .

    Tracing the simulated high-redshift circum-galactic medium with Lyman alpha emission · Inner Ly$α$ halos are powered only by star formation · 0.96

  • This interpretation is supported by cosmological simulations with Ly$α$ radiative transfer [ref] as well as semianalytic models [ref] which typically find that the two halo term becomes highly relevant at about such distances.

    Emulating extended Lyman-alpha haloes around star-forming galaxies · Introduction · 0.97

Cites as background 10 papers

Discussion

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