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Published October 20, 2025 | Version Published
Journal Article Open

Unions with UNIONS: Using Galaxy–Galaxy Lensing to Probe Galaxy Mergers

Creators

  • 1. ROR icon University of Waterloo
  • 2. ROR icon California Institute of Technology
  • 3. ROR icon Perimeter Institute
  • 4. ROR icon University of Victoria
  • 5. ROR icon University of Hawaii at Manoa
  • 6. ROR icon National Research Council Canada
  • 7. ROR icon University of Paris
  • 8. ROR icon Ruhr University Bochum
  • 9. ROR icon University of British Columbia

Abstract

We use galaxy–galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with 0.012 ≤ z ≤ 0.32 and 10 ≤ log₁₀ ( M⋆/M⊙) ≤ 12 change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey, comprising 1623 postmergers and ∼30,000 nonmerging controls, that live in low-density environments to use as our lenses. These samples are weighted to share the same distributions of stellar mass, redshift, and geometric mean distance to a galaxy’s three nearest neighbors to ensure differences in the lensing signal are due to the merger process itself. We do not detect a statistically significant difference in the excess surface density profile of postmergers and nonmerging controls with current data. Fitting haloes composed of a pointlike stellar mass component and an extended DM structure described by a Navarro–Frenk–White profile to the lensing measurements yields, for both samples, halo masses of Mhalo ∼ 4 × 1012 M⊙ and a moderately negative correlation between Mhalo and concentration c. This allows us to rule out, at the 95% confidence level, merger-induced starbursts in which more than 60% of the stellar mass is formed in the burst. The application of our methods to upcoming surveys that are able to provide samples ∼10× larger than our current catalog is expected to detect the weak-lensing signatures of mergers and further constrain their properties.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We are honored and grateful for the opportunity of observing the Universe from Maunakea and Haleakala, which both have cultural, historical and natural significance in Hawaii. This work is based on data obtained as part of the Canada–France Imaging Survey, using observations obtained with MegaPrime/MegaCam, a joint project of the Canada–France–Hawaii Telescope (CFHT) and CEA Saclay, on the CFHT, which is operated by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This research is based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Pan-STARRS is a project of the Institute for Astronomy of the University of Hawaii, and is supported by the NASA SSO Near Earth Observation Program under grants 80NSSC18K0971, NNX14AM74G, NNX12AR65G, NNX13AQ47G, NNX08AR22G, 80NSSC21K1572, and by the State of Hawaii.

This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. Additionally, we are grateful for the computing resources of the Digital Research Alliance of Canada22 and the CANFAR (Canadian Advanced Network for Astronomical Research) Science Portal23 for enabling the analysis in this paper.

M.J.H. acknowledges support from NSERC through a Discovery Grant. H.H. is supported by a DFG Heisenberg grant (Hi 1495/5-1), the DFG Collaborative Research Center SFB1491, an ERC Consolidator grant (No. 770935), and the DLR project 50QE2305.

We also thank the anonymous referee for their insightful feedback, particularly regarding factors that may contribute to the measured low concentrations.

Contributions

I.C. performed the analysis and wrote the manuscript, with guidance from J.E.-P. and M.J.H. M.J.H. additionally calculated the theoretical SHMRs to compare with our observed ratios. All other co-authors contributed to the various data products used in this paper and/or provided significant feedback to improve this work.

Data Availability

The merger catalog from L. Ferreira et al. (2024a) is publicly available on GitHub24 with redshifts from SDSS DR7 and stellar masses from MPA-JHU (as discussed in the text).

A subset of the raw data underlying the source catalog used in this article is publicly available via the Canadian Astronomy Data Centre.25 The remaining raw data and all processed data are available to members of the Canadian and French communities via reasonable requests to the principal investigators of the Canada–France Imaging Survey, Alan McConnachie and Jean-Charles Cuillandre. All data will be publicly available to the international community at the end of the proprietary period.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2502.00584 (arXiv)
Is supplemented by
Software: https://github.com/astroferreira/MUMMI_UNIONS (URL)
Software: http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/megapipe/ (URL)

Funding

National Aeronautics and Space Administration
80NSSC18K0971
National Aeronautics and Space Administration
NNX14AM74G
National Aeronautics and Space Administration
NNX12AR65G
National Aeronautics and Space Administration
NNX13AQ47G
National Aeronautics and Space Administration
NNX08AR22G
National Aeronautics and Space Administration
80NSSC21K1572
Hawaii State Judiciary
Canadian Space Agency
Natural Sciences and Engineering Research Council
Deutsche Forschungsgemeinschaft
Hi 1495/5-1
Deutsche Forschungsgemeinschaft
SFB1491
European Research Council
770935
Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)
50QE2305

Dates

Accepted
2025-09-03
Available
2025-10-13
Published

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Caltech groups
Division of Physics, Mathematics and Astronomy (PMA)
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Published