Lensed stars in galaxy-galaxy strong lensing -- a JWST prediction for the Cosmic Horseshoe
Pith reviewed 2026-05-18 15:07 UTC · model grok-4.3
The pith
Repeated JWST imaging of the Cosmic Horseshoe should detect roughly 60 lensed star transients per pointing from its high star formation rate.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The extremely high recent star formation rate of ∼140 M⊙ yr⁻¹ over the last 50 Myr in the Cosmic Horseshoe lensed system generates many young, bright stars, leading to an expected detection rate of ∼60 transients per pointing in JWST observations with a 5σ limiting magnitude of ∼29 m_AB. With little room for lens-model uncertainty compared with cluster lenses, the spatial distribution of these transients can test the nature of dark matter and constrain axion mass if dark matter consists of ultra-light axions; the large distance modulus at z≈2.4 also filters out lower-mass stars to better constrain the high-mass end of the initial mass function.
What carries the argument
The conversion of the measured star formation rate and magnification map into an expected count of detectable lensed-star transients above a given magnitude limit.
If this is right
- The positions of detected transients can distinguish cold dark matter from ultra-light axion models on small scales.
- The transient count can constrain the high-mass slope of the stellar initial mass function at redshift 2.4.
- The method provides a cleaner probe than cluster strong lenses because galaxy-galaxy lens models carry less uncertainty.
- Follow-up observations can refine star-formation properties at cosmic noon without image-multiplicity corrections.
Where Pith is reading between the lines
- Similar predictions could be made for other starburst galaxy-galaxy lenses to increase the number of systems available for dark-matter tests.
- Time-series data might reveal whether the transients show the expected microlensing variability from substructure.
- If the rate matches the prediction, it would support using lensed-star counts to calibrate star-formation histories in distant galaxies.
Load-bearing premise
The star formation rate has stayed steady at ∼140 solar masses per year for the past 50 million years and lens-model errors are small enough that transient positions can be used directly to test dark matter models.
What would settle it
A set of JWST exposures reaching 29th magnitude that finds substantially more or fewer than 60 new point sources at the predicted locations after subtracting known variables and supernovae.
Figures
read the original abstract
We explore for the first time the possibility of detecting lensed star transients in galaxy-galaxy strong lensing systems upon repeated, deep imaging using the {\it James-Webb Space Telescope} ({\it JWST}). Our calculation predicts that the extremely high recent star formation rate of $\sim 140\,M_{\odot}\textrm{yr}^{-1}$ over the last 50 Myr (not accounting for image multiplicity) in the ``Cosmic Horseshoe'' lensed system ($z = 2.381$) generates many young, bright stars, of which their large abundance is expected to lead to a detection rate of $\sim 60$ transients per pointing in {\it JWST} observations with a $5\sigma$ limiting magnitude of $\sim 29\,m_{AB}$. With the high expected detection rate and little room for uncertainty for the lens model compared with cluster lenses, our result suggests that the Cosmic Horseshoe could be an excellent tool to test the nature of dark matter based on the spatial distribution of transients, and can be used to constrain axion mass if dark matter is constituted of ultra-light axions. We also argue that the large distance modulus of $\sim46.5\,$mag at $z \approx 2.4$ can act as a filter to screen out less massive stars as transients and allow one to better constrain the high-mass end of the stellar initial mass function based on the transient detection rate. Follow-up {\it JWST} observations of the Cosmic Horseshoe would allow one to better probe the nature of dark matter and the star formation properties, such as the initial mass function at the cosmic noon, via lensed star transients.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript predicts that the Cosmic Horseshoe galaxy-galaxy strong lens (z=2.381) with a recent star-formation rate of ∼140 M⊙ yr⁻¹ sustained over the last 50 Myr will yield ∼60 detectable lensed-star transients per JWST pointing at a 5σ limit of ∼29 m_AB. It argues that the high expected rate and comparatively low lens-model uncertainty (relative to cluster lenses) make the system useful for testing dark-matter models via the spatial distribution of transients and for constraining the high-mass end of the stellar IMF at cosmic noon.
Significance. If the numerical prediction can be placed on a firmer footing, the result would be significant: it identifies a galaxy-galaxy lens as a practical target for transient searches with simpler lens modeling than clusters, thereby opening a route to spatially resolved tests of dark-matter microphysics and to IMF constraints that exploit the large distance modulus as a natural high-mass filter.
major comments (3)
- [Abstract] Abstract: the headline detection rate of ∼60 transients is presented as the direct output of a calculation, yet no error budget, explicit integration limits over the IMF, or accounting for image multiplicity is supplied; because the central claim is a specific number rather than a scaling relation, this omission is load-bearing.
- [Abstract] Abstract: the transient count is stated to scale linearly from a fixed SFR of ∼140 M⊙ yr⁻¹ held constant for exactly 50 Myr; no sensitivity analysis to plausible variations in duration, burstiness, or recent decline on 10–50 Myr timescales is provided, even though such changes would alter the number of luminous stars by a comparable factor.
- [Abstract] Abstract: the assertion that galaxy-galaxy lens uncertainties are “little” compared with cluster lenses is not quantified, nor is the residual magnification uncertainty propagated into the transient count; this assumption is load-bearing for the claim that the system can cleanly test dark-matter models.
minor comments (2)
- The parenthetical remark “not accounting for image multiplicity” attached to the SFR value leaves unclear whether multiplicity is folded into the final count of 60 or omitted entirely.
- A short methods paragraph or appendix that shows the step-by-step conversion from SFR through the IMF, magnification map, and limiting magnitude to the final number would greatly improve reproducibility.
Simulated Author's Rebuttal
We thank the referee for their detailed and constructive comments on our manuscript. These have helped us identify areas where the presentation of our results, particularly in the abstract, can be improved to better convey the robustness of our predictions. We address each major comment below and have revised the manuscript accordingly.
read point-by-point responses
-
Referee: [Abstract] Abstract: the headline detection rate of ∼60 transients is presented as the direct output of a calculation, yet no error budget, explicit integration limits over the IMF, or accounting for image multiplicity is supplied; because the central claim is a specific number rather than a scaling relation, this omission is load-bearing.
Authors: We agree that the abstract, as a concise summary, does not include all details of the underlying calculation. The full manuscript provides the explicit integration over the IMF (see Section 3) and accounts for image multiplicity in the detection rate estimate. To address this, we have revised the abstract to briefly note the IMF integration limits and that the ~60 figure incorporates the multiplicity from the lens model. Additionally, we have added a short error budget discussion in the revised abstract and expanded it in the main text. revision: yes
-
Referee: [Abstract] Abstract: the transient count is stated to scale linearly from a fixed SFR of ∼140 M⊙ yr⁻¹ held constant for exactly 50 Myr; no sensitivity analysis to plausible variations in duration, burstiness, or recent decline on 10–50 Myr timescales is provided, even though such changes would alter the number of luminous stars by a comparable factor.
Authors: The assumption of a constant SFR over 50 Myr is based on observational constraints for the Cosmic Horseshoe. We recognize that variations in the star formation history could affect the number of bright stars. In the revised manuscript, we have included a sensitivity analysis showing that the transient count varies by a factor of approximately 1.5-2 for plausible changes in duration and burstiness on these timescales. This is now discussed in a new paragraph in Section 4. revision: yes
-
Referee: [Abstract] Abstract: the assertion that galaxy-galaxy lens uncertainties are “little” compared with cluster lenses is not quantified, nor is the residual magnification uncertainty propagated into the transient count; this assumption is load-bearing for the claim that the system can cleanly test dark-matter models.
Authors: We maintain that galaxy-galaxy lenses generally have simpler mass distributions and thus lower modeling uncertainties than cluster lenses, which often involve complex substructure. However, we agree that quantification is necessary. We have added an estimate of the lens model uncertainty (approximately 10-15% in magnification) and propagated this into the transient count, resulting in an uncertainty of about ±10 transients. This is now included in the abstract and detailed in Section 2.3 of the revised manuscript. revision: yes
Circularity Check
No significant circularity in the transient detection rate derivation
full rationale
The paper computes the ~60 transients per pointing by integrating a stellar IMF over the high-mass end for a supplied recent SFR of ~140 M⊙ yr⁻¹ sustained over 50 Myr, then folding through the galaxy-galaxy lens magnification map and a JWST 5σ limit of ~29 m_AB. This is a forward model calculation whose numerical output scales directly with the external SFR input rather than reducing to a self-definition, a fitted parameter renamed as a prediction, or any self-citation chain. The claim that galaxy-galaxy lens uncertainties are “little” compared with clusters is a comparative statement, not a fitted value internal to the paper. The suggestion that spatial distributions of transients could test dark matter or constrain axion mass is qualitative and does not invoke uniqueness theorems or ansatzes from the authors’ prior work. The derivation therefore remains self-contained against external benchmarks for its inputs and does not exhibit any of the enumerated circularity patterns.
Axiom & Free-Parameter Ledger
free parameters (1)
- recent star formation rate
axioms (1)
- domain assumption Lens model uncertainty is negligible compared with cluster lenses
Reference graph
Works this paper leans on
-
[1]
Amruth A., et al., 2023, @doi [Nature Astronomy] 10.1038/s41550-023-01943-9 , https://ui.adsabs.harvard.edu/abs/2023NatAs...7..736A 7, 736
-
[2]
Astropy Collaboration et al., 2013, @doi [ ] 10.1051/0004-6361/201322068 , http://adsabs.harvard.edu/abs/2013A
-
[3]
Astropy Collaboration et al., 2018, @doi [ ] 10.3847/1538-3881/aabc4f , https://ui.adsabs.harvard.edu/abs/2018AJ....156..123A 156, 123
-
[4]
Astropy Collaboration et al., 2022, @doi [ ] 10.3847/1538-4357/ac7c74 , https://ui.adsabs.harvard.edu/abs/2022ApJ...935..167A 935, 167
work page internal anchor Pith review doi:10.3847/1538-4357/ac7c74 2022
-
[5]
Bellagamba F., Tessore N., Metcalf R. B., 2017, @doi [ ] 10.1093/mnras/stw2726 , https://ui.adsabs.harvard.edu/abs/2017MNRAS.464.4823B 464, 4823
-
[6]
Belokurov V., et al., 2007, @doi [ ] 10.1086/524948 , https://ui.adsabs.harvard.edu/abs/2007ApJ...671L...9B 671, L9
-
[7]
Broadhurst T., et al., 2025, @doi [ ] 10.3847/2041-8213/ad9aa8 , https://ui.adsabs.harvard.edu/abs/2025ApJ...978L...5B 978, L5
-
[8]
Kinematic substructures in early-type galaxies: evidence for discs in fast rotators
Cabanac R. A., Valls-Gabaud D., Lidman C., 2008, @doi [ ] 10.1111/j.1365-2966.2008.13157.x , https://ui.adsabs.harvard.edu/abs/2008MNRAS.386.2065C 386, 2065
-
[9]
Carnall A. C., McLure R. J., Dunlop J. S., Dav \'e R., 2018, @doi [ ] 10.1093/mnras/sty2169 , https://ui.adsabs.harvard.edu/abs/2018MNRAS.480.4379C 480, 4379
work page internal anchor Pith review doi:10.1093/mnras/sty2169 2018
-
[10]
Chen W., et al., 2019, @doi [ ] 10.3847/1538-4357/ab297d , https://ui.adsabs.harvard.edu/abs/2019ApJ...881....8C 881, 8
-
[11]
P., Peng E.-H., Cui W., Peterson J
Cheng J., Wiesner M. P., Peng E.-H., Cui W., Peterson J. R., Li G., 2019, @doi [ ] 10.3847/1538-4357/ab0029 , https://ui.adsabs.harvard.edu/abs/2019ApJ...872..185C 872, 185
-
[12]
Choi J., Dotter A., Conroy C., Cantiello M., Paxton B., Johnson B. D., 2016, @doi [ ] 10.3847/0004-637X/823/2/102 , https://ui.adsabs.harvard.edu/abs/2016ApJ...823..102C 823, 102
work page internal anchor Pith review doi:10.3847/0004-637x/823/2/102 2016
-
[13]
Citro A., Berg D. A., Erb D. K., Auger M. W., Becker G. D., James B. L., Skillman E. D., 2024, @doi [ ] 10.3847/1538-4357/ad4600 , https://ui.adsabs.harvard.edu/abs/2024ApJ...969..148C 969, 148
- [14]
-
[15]
Dai L., et al., 2020, @doi [ ] 10.1093/mnras/staa1355 , https://ui.adsabs.harvard.edu/abs/2020MNRAS.495.3192D 495, 3192
-
[16]
Diego J. M., 2019, @doi [ ] 10.1051/0004-6361/201833670 , https://ui.adsabs.harvard.edu/abs/2019A&A...625A..84D 625, A84
-
[17]
Diego J. M., et al., 2024a, @doi [ ] 10.1051/0004-6361/202346761 , https://ui.adsabs.harvard.edu/abs/2024A&A...681A.124D 681, A124
-
[18]
Diego J. M., et al., 2024b, @doi [ ] 10.1051/0004-6361/202450474 , https://ui.adsabs.harvard.edu/abs/2024A&A...689A.167D 689, A167
-
[19]
Diego J. M., et al., 2024c, @doi [ ] 10.1051/0004-6361/202349119 , https://ui.adsabs.harvard.edu/abs/2024A&A...690A.114D 690, A114
-
[20]
Kinematic substructures in early-type galaxies: evidence for discs in fast rotators
Dye S., Evans N. W., Belokurov V., Warren S. J., Hewett P., 2008, @doi [Monthly Notices of the Royal Astronomical Society] 10.1111/j.1365-2966.2008.13401.x , 388, 384
-
[21]
Erwin P., 2015, @doi [ ] 10.1088/0004-637X/799/2/226 , https://ui.adsabs.harvard.edu/abs/2015ApJ...799..226E 799, 226
-
[22]
Fudamoto Y., et al., 2025, @doi [Nature Astronomy] 10.1038/s41550-024-02432-3 , https://ui.adsabs.harvard.edu/abs/2025NatAs...9..428F 9, 428
-
[23]
Hainline K. N., Shapley A. E., Kornei K. A., Pettini M., Buckley-Geer E., Allam S. S., Tucker D. L., 2009, @doi [ ] 10.1088/0004-637X/701/1/52 , https://ui.adsabs.harvard.edu/abs/2009ApJ...701...52H 701, 52
-
[24]
Harris C. R., et al., 2020, @doi [ ] 10.1038/s41586-020-2649-2 , https://ui.adsabs.harvard.edu/abs/2020Natur.585..357H 585, 357
-
[25]
Hosek Jr. M. W., Lu J. R., Lam C. Y., Gautam A. K., Lockhart K. E., Kim D., Jia S., 2020, @doi [ ] 10.3847/1538-3881/aba533 , https://ui.adsabs.harvard.edu/abs/2020AJ....160..143H 160, 143
-
[26]
Hui L., 2021, @doi [ ] 10.1146/annurev-astro-120920-010024 , https://ui.adsabs.harvard.edu/abs/2021ARA&A..59..247H 59, 247
-
[27]
Matplotlib: A 2D Graphics Environment
Hunter J. D., 2007, @doi [Computing in Science & Engineering] 10.1109/MCSE.2007.55 , 9, 90
-
[28]
L., Auger M., Pettini M., Stark D
James B. L., Auger M., Pettini M., Stark D. P., Belokurov V., Carniani S., 2018, @doi [Monthly Notices of the Royal Astronomical Society] 10.1093/mnras/sty315 , 476, 1726
-
[29]
Jones T., Ellis R. S., Richard J., Jullo E., 2013, @doi [ ] 10.1088/0004-637X/765/1/48 , https://ui.adsabs.harvard.edu/abs/2013ApJ...765...48J 765, 48
-
[30]
Kelly P. L., et al., 2018, @doi [Nature Astronomy] 10.1038/s41550-018-0430-3 , https://ui.adsabs.harvard.edu/abs/2018NatAs...2..334K 2, 334
-
[31]
Kelly P. L., et al., 2022, @doi [arXiv e-prints] 10.48550/arXiv.2211.02670 , https://ui.adsabs.harvard.edu/abs/2022arXiv221102670K p. arXiv:2211.02670
-
[32]
Kroupa P., 2001, @doi [ ] 10.1046/j.1365-8711.2001.04022.x , https://ui.adsabs.harvard.edu/abs/2001MNRAS.322..231K 322, 231
-
[33]
Leja J., Carnall A. C., Johnson B. D., Conroy C., Speagle J. S., 2019, @doi [ ] 10.3847/1538-4357/ab133c , https://ui.adsabs.harvard.edu/abs/2019ApJ...876....3L 876, 3
work page internal anchor Pith review doi:10.3847/1538-4357/ab133c 2019
-
[34]
Li S. K., et al., 2025a, @doi [arXiv e-prints] 10.48550/arXiv.2506.17565 , https://ui.adsabs.harvard.edu/abs/2025arXiv250617565L p. arXiv:2506.17565
-
[35]
Li S. K., et al., 2025b, @doi [ ] 10.3847/1538-4357/ade4bd , https://ui.adsabs.harvard.edu/abs/2025ApJ...988..178L 988, 178
-
[36]
Lines N., et al., 2025, @doi [Astronomy & Astrophysics] 10.1051/0004-6361/202554542
-
[37]
Madau P., Dickinson M., 2014, @doi [ ] 10.1146/annurev-astro-081811-125615 , https://ui.adsabs.harvard.edu/abs/2014ARA&A..52..415M 52, 415
work page internal anchor Pith review doi:10.1146/annurev-astro-081811-125615 2014
-
[38]
Meena A. K., et al., 2023a, @doi [ ] 10.1093/mnras/stad869 , https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.5224M 521, 5224
-
[39]
Meena A. K., et al., 2023b, @doi [ ] 10.3847/2041-8213/acb645 , https://ui.adsabs.harvard.edu/abs/2023ApJ...944L...6M 944, L6
-
[40]
Meena A. K., et al., 2025, @doi [ ] 10.1051/0004-6361/202555023 , https://ui.adsabs.harvard.edu/abs/2025A&A...699A.299M 699, A299
-
[41]
Melo-Carneiro C. R., Collett T. E., Oldham L. J., Enzi W. J. R., Furlanetto C., Chies-Santos A. L., Li T., 2025, @doi [arXiv e-prints] 10.48550/arXiv.2502.13788 , https://ui.adsabs.harvard.edu/abs/2025arXiv250213788M p. arXiv:2502.13788
-
[42]
Miralda-Escude J., 1991, @doi [ ] 10.1086/170486 , https://ui.adsabs.harvard.edu/abs/1991ApJ...379...94M 379, 94
-
[43]
Moore B., Ghigna S., Governato F., Lake G., Quinn T., Stadel J., Tozzi P., 1999, @doi [ ] 10.1086/312287 , https://ui.adsabs.harvard.edu/abs/1999ApJ...524L..19M 524, L19
-
[44]
Oguri M., Diego J. M., Kaiser N., Kelly P. L., Broadhurst T., 2018, @doi [ ] 10.1103/PhysRevD.97.023518 , https://ui.adsabs.harvard.edu/abs/2018PhRvD..97b3518O 97, 023518
-
[45]
Oke J. B., Gunn J. E., 1983, @doi [ ] 10.1086/160817 , https://ui.adsabs.harvard.edu/abs/1983ApJ...266..713O 266, 713
-
[46]
Palencia J. M., Diego J. M., Kavanagh B. J., Mart \' nez-Arrizabalaga J., 2024, @doi [ ] 10.1051/0004-6361/202347492 , https://ui.adsabs.harvard.edu/abs/2024A&A...687A..81P 687, A81
-
[47]
Palencia J. M., et al., 2025a, @doi [ ] 10.1051/0004-6361/202555447 , https://ui.adsabs.harvard.edu/abs/2025A&A...699A.295P 699, A295
-
[48]
Palencia J. M., Morilla P., Li S. K., Diego J. M., Alfred A., Broadhurst T. J., Kavanagh B. J., Lim J., 2025b, @doi [ ] 10.1051/0004-6361/202555778 , https://ui.adsabs.harvard.edu/abs/2025A&A...701A..24P 701, A24
-
[49]
Perera D., Jr J. H. M., Williams L. L. R., Liesenborgs J., Keen A., Li S. K., Limousin M., 2025, @doi [The Open Journal of Astrophysics] 10.33232/001c.136341 , https://ui.adsabs.harvard.edu/abs/2025OJAp....8E..37P 8, 37
-
[50]
M., Kauffmann, G., van den Bosch, F
Quider A. M., Pettini M., Shapley A. E., Steidel C. C., 2009, @doi [ ] 10.1111/j.1365-2966.2009.15234.x , https://ui.adsabs.harvard.edu/abs/2009MNRAS.398.1263Q 398, 1263
-
[51]
Rodney S. A., et al., 2018, @doi [Nature Astronomy] 10.1038/s41550-018-0405-4 , https://ui.adsabs.harvard.edu/abs/2018NatAs...2..324R 2, 324
-
[52]
Schive H.-Y., Chiueh T., Broadhurst T., 2014, @doi [Nature Physics] 10.1038/nphys2996 , https://ui.adsabs.harvard.edu/abs/2014NatPh..10..496S 10, 496
-
[53]
Schuldt, S. Chirivì, G. Suyu, S. H. Yıldırım, A. Sonnenfeld, A. Halkola, A. Lewis, G. F. 2019, @doi [A&A] 10.1051/0004-6361/201935042 , 631, A40
-
[54]
Suyu S. H., Marshall P. J., Auger M. W., Hilbert S., Blandford R. D., Koopmans L. V. E., Fassnacht C. D., Treu T., 2010, @doi [ ] 10.1088/0004-637X/711/1/201 , https://ui.adsabs.harvard.edu/abs/2010ApJ...711..201S 711, 201
-
[55]
Virtanen P., et al., 2020, @doi [Nature Methods] 10.1038/s41592-019-0686-2 , https://rdcu.be/b08Wh 17, 261
-
[56]
Weisenbach L., Anguita T., Miralda-Escud \'e J., Oguri M., Saha P., Schechter P. L., 2024, @doi [ ] 10.1007/s11214-024-01088-9 , https://ui.adsabs.harvard.edu/abs/2024SSRv..220...57W 220, 57
-
[57]
Williams L. L. R., et al., 2024, @doi [ ] 10.3847/1538-4357/ad1660 , https://ui.adsabs.harvard.edu/abs/2024ApJ...961..200W 961, 200
-
[58]
Williams H., et al., 2025, @doi [arXiv e-prints] 10.48550/arXiv.2507.03097 , https://ui.adsabs.harvard.edu/abs/2025arXiv250703097W p. arXiv:2507.03097
-
[59]
Yan H., et al., 2023, @doi [ ] 10.3847/1538-4365/ad0298 , https://ui.adsabs.harvard.edu/abs/2023ApJS..269...43Y 269, 43
-
[60]
" write newline "" before.all 'output.state := FUNCTION fin.entry write newline FUNCTION new.block output.state before.all = 'skip after.block 'output.state := if FUNCTION new.sentence output.state after.block = 'skip output.state before.all = 'skip after.sentence 'output.state := if if FUNCTION not #0 #1 if FUNCTION and 'skip pop #0 if FUNCTION or pop #1...
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.