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The effect of galaxy interactions on star formation rates in the COLIBRE simulations

T0 review · 3 major / 2 minor · reviewed 2026-08-16 · deepseek-v4-flash

Pith's one-line read Using the COLIBRE simulations, the paper shows that galaxies with a close companion have mean specific star formation rates enhanced by up to a factor of about 1.7 at 5-10 kpc separations, with a weaker enhancement out to about 200 kpc.

desk verdict A careful and honest first look at interaction-driven sSFR enhancement in COLIBRE, but the headline factor is inflated by an uncorrected stellar-mass stripping effect and an over-generous abstract. read the letter →

arxiv 2608.12132 v1 pith:HKXJSD6X submitted 2026-08-12 astro-ph.GA

classification astro-ph.GA
keywords galaxyinteractionsstarformationenhancementspecificratehydrodynamicalsimulationsCOLIBREmatchedcontrolsamplesSDSScomparisoncosmicdensity
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using the COLIBRE cosmological hydrodynamical simulations, this paper asks whether galaxies with a close companion form stars faster than otherwise equivalent isolated galaxies. It builds large samples of interacting galaxies and controls matched in stellar mass, redshift, local environment, and isolation, and finds that the mean specific star formation rate is enhanced by up to a factor of about 1.7 at separations of 5-10 kpc, with a smaller but significant enhancement out to roughly 200 kpc. The enhancement is strongest in the central regions, larger for lower-mass galaxies and for more equal-mass pairs, and grows with numerical resolution. Comparison with SDSS observations shows the same separation dependence but a simulation normalisation about a factor of two lower. The paper concludes that resolved pre-merger interactions contribute about 2.1 per cent of the $z\approx0$ cosmic star formation rate density.

What carries the argument

The central object is the sSFR enhancement ratio $Q(\mathrm{sSFR})=\langle \mathrm{sSFR}_{\rm interacting}\rangle/\langle \mathrm{sSFR}_{\rm control}\rangle$ computed in bins of pair separation. The machinery is the COLIBRE simulation model, which resolves a cold, molecular interstellar medium and forms stars in gravitationally unstable gas, together with the Patton et al. (2016) matching algorithm that pairs each interacting galaxy with isolated controls having the same redshift, stellar mass within 0.05 dex, local density within 10 per cent, and isolation within 10 per cent. The ratio isolates the effect of the companion by construction, and the same samples, with projected separations and fibre-like apertures, are used for the SDSS comparison.

What would settle it

Re-run the L200m6 analysis using controls matched on the local density within 0.8 Mpc instead of 2 Mpc, restricted to galaxies with $M_\ast>10^{10}\,\mathrm{M_\odot}$; if the small-separation enhancement drops well below $Q\approx1.7$ or the large-separation plateau persists, part of the claimed interaction effect is environmental rather than caused by the companion.

Watch

Extended reading notes

Core claim

The central claim is that, in the COLIBRE simulations at $z\approx0$, the mean specific star formation rate of a star-forming galaxy with a companion whose stellar mass is at least one tenth of its own is enhanced relative to a matched isolated control. The enhancement reaches $Q(\mathrm{sSFR})\approx1.7$ at a three-dimensional separation of roughly 5 kpc, declines monotonically with separation, and remains statistically significant out to about 200 kpc before converging to unity. The enhancement is stronger for lower-mass galaxies (peaking near $M_\ast\approx10^9\,\mathrm{M_\odot}$), for pairs with more equal stellar masses, and when the star formation rate is measured within smaller apertures, reaching about 2.6 within 1 kpc. The COLIBRE prediction has the same radial shape as the SDSS data but is lower in normalisation by about a factor of two, and the paper estimates that the pre-merger excess contributes about 2.1 per cent of the cosmic star formation rate density at $z\approx0$.

Load-bearing premise

The load-bearing premise is that the matched control galaxies are statistically identical to interacting galaxies in every star-formation-relevant property except the presence of a close companion; Appendix B shows this is only approximately achieved, with residual differences in small-scale environment for low-mass galaxies.

Editorial extensions

If this is right

  • At separations below about 50 kpc, the sSFR enhancement is large and rises steeply, so close pairs dominate the interaction-driven excess at $z\approx0$.
  • The enhancement is concentrated in galaxy centres: measuring within 1 kpc apertures roughly doubles the effect compared with 10 kpc apertures.
  • Lower stellar mass and more equal-mass pairs show stronger enhancement, while the most massive galaxies show almost no mean enhancement.
  • COLIBRE matches the observed radial dependence of the SDSS enhancement but predicts about half the amplitude, and the gap narrows at higher resolution.
  • Pre-merger galaxy interactions contribute only about 2.1 per cent of the cosmic SFR density at $z\approx0$, so they are not the main driver of cosmic star formation at this epoch.

Reading between the lines

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

  • I infer that if the resolution trend from m7 to m6 continues to m5, the small-separation normalisation would rise toward the SDSS value, making much of the reported factor-of-two deficit a numerical-resolution effect rather than a physics disagreement.
  • The residual environment mismatch shown in Appendix B suggests that part of the apparent enhancement in the lowest stellar mass bin is environmental; a direct test would be to adopt sub-Mpc density matching for all mass bins and check whether the large-separation plateau disappears.
  • Because the paper counts only the pre-merger phase, including post-merger remnants and very low mass-ratio companions would likely push the cosmic contribution above the quoted 2.1 per cent.
  • The strong aperture dependence predicts that fibre or IFU observations of galaxy centres should find larger interaction-induced sSFR excesses than integrated galaxy-wide measurements.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 2 minor

Summary. The paper uses the COLIBRE cosmological hydrodynamical simulations at z≈0 to measure the mean specific star formation rate enhancement Q(sSFR) of interacting galaxies relative to mass-, environment-, and redshift-matched isolated controls. It reports Q≈1.7 at separations of ≈5 kpc for galaxies with M*>1e10 M_sun, enhancement remaining significant out to ≈200 kpc, stronger enhancement for lower-mass galaxies and smaller apertures, a resolution-dependent normalization, a comparison with SDSS showing a similar separation dependence but a normalization lower by about a factor of two, and a ≈2.1 per cent contribution of pre-merger interactions to the z≈0 cosmic SFR density.

Significance. If the results hold, they provide a quantitative benchmark for merger-driven star formation in a modern simulation with a multiphase interstellar medium. The SDSS comparison is a genuine prediction, because the COLIBRE model was calibrated to the stellar mass function and size-mass relation rather than to interaction-induced sSFR enhancement. The bootstrap uncertainties and the box-size/resolution convergence tests in Section 4.2 support the statistical claims. The main qualification is the denominator confound from stellar mass stripping, which directly affects the headline amplitude of Q(sSFR), and the need to align the abstract with the fiducial 10 kpc aperture numbers.

major comments (3)
  1. [Section 4.4, Appendix D, Eq. (2)] The paper measures sSFR within a 10 kpc aperture and interprets Q(sSFR) as the interaction-induced enhancement of star formation, but it does not test whether the enhancement is driven by SFR or by a decrease in the stellar-mass denominator. Equation (2) defines Q(sSFR) as the ratio of mean sSFRs, and Appendix D (Fig. D1) shows that the mean stellar mass ratio M_now/M_max decreases by up to ≈20 per cent at r_sep≈1 due to combined physical and numerical stripping. Section 4.4 declines to apply the Patton et al. (2020) correction and does not present Q(SFR). For a galaxy with 20 per cent mass loss, an observed Q(sSFR)≈1.7 would correspond to Q(SFR)≈1.36 if the SFR were unchanged. The authors should compute the SFR-only ratio Q(SFR), or apply a stripping correction, or at least quantify the maximum effect of stripping on the smallest-separation bin before claiming an sSFR enhancement of the quoted amplitude.
  2. [Abstract, §3.1.1, §3.2.1, Figs. 2 and 8] The abstract states that the average sSFR of interacting galaxies is enhanced by up to a factor of ≈2 for separations of ≈10 kpc, and §3.2.1 says the fiducial 3D sample reaches Q≈2. However, the fiducial result for M*>1e10 M_sun with the standard 10 kpc aperture is Q≈1.7 at r3D≈5 kpc (Fig. 2 and Fig. 8), and Q values near 2 are obtained only for lower stellar masses (Fig. 4) or for apertures of ≤3 kpc (Fig. 8). The abstract and Section 3.2.1 should be harmonized with the fiducial numbers, or explicitly state the mass and aperture conditions under which Q≈2 is reached.
  3. [Appendix A, Eq. (A1)] Equation (A1) as printed, w_xi=|x-x_i|/x_tol, is 0 for a perfect match and 1 at the edge of the tolerance range, which is the opposite of the prose statement that a perfect match has weight 1 and the edge has weight 0. Since the text then says the control with the largest weight is selected, the printed formula would select the worst-matched control. This is either a typographical error (likely missing '1 -' before the ratio) or an inconsistency with the actual matching code. The formula and the selection rule must be corrected and brought into agreement.
minor comments (2)
  1. [Section 3.2.1, Fig. 9] The text says the fiducial 3D sample reaches Q≈2, but Fig. 2 and Section 3.1.1 report Q≈1.7 for the M*>1e10 sample; please specify which stellar mass range and aperture are used in Fig. 9, or change the wording to avoid confusion.
  2. [Abstract and text throughout] There are minor typesetting issues with missing spaces, e.g. 'colibresimulations' in the abstract; a careful proofread would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the interaction-driven sSFR enhancement in COLIBRE is a direct simulation comparison, not a fit or a self-citation chain.

full rationale

The central quantity Q(sSFR) is defined by Eq. (2) as the ratio of the mean sSFRs of interacting and matched control galaxies, both measured directly from the COLIBRE output; no parameter is fitted to Q, and the interacting/control classification is made by the Patton et al. (2016) matching algorithm, which does not use Q as an input. COLIBRE's calibration (Section 2.1.1) targets the z=0 stellar mass function, the size–stellar mass relation, and black-hole masses; interaction-induced sSFR enhancement is not among the calibrated targets, so the enhancement is a genuine prediction. The SDSS comparison in Section 3.2 is an external benchmark, and the paper reports that COLIBRE underpredicts the observed normalization by about a factor of two, which is a falsifiable, non-circular result. The acknowledged caveats in Appendix B (residual environmental differences in the lowest-mass bin) and Appendix D (stellar-mass stripping at close separations) are correctness/interpretation limitations, not reductions of the derivation to its inputs: they ask whether the measured ratio is unbiased, not whether Q was constructed from itself. Self-citations to COLIBRE model papers and to Patton et al. are normal tool citations and are not load-bearing for the central claim.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central claim depends on the COLIBRE subgrid model, whose parameters (star formation efficiency, feedback strengths) were calibrated in prior work to reproduce the observed stellar mass function, size-mass relation, and BH masses, not to the sSFR enhancement. The paper introduces no new free parameters of its own, but inherits the model calibration. The matching tolerances and aperture choices are hand-chosen analysis settings. No invented physical entities are introduced.

free parameters (4)
  • Star formation efficiency per free-fall time (epsilon) = 0.01
    Adopted from the COLIBRE subgrid model (Schaye et al. 2026; Chaikin et al. 2026a). Not fitted in this paper. The central sSFR enhancement depends on this prescription, which is an input model parameter, not calibrated against the interaction enhancement.
  • Supernova and AGN feedback strengths = Calibrated to z=0 stellar mass function and size-mass relation in Chaikin et al. 2026a
    These subgrid parameters are fitted to external observations in the COLIBRE model papers, not to the sSFR enhancement. They shape the simulated SFRs and therefore the predicted Q(sSFR).
  • AGN feedback coupling efficiency = Calibrated to observed BH masses in Chaikin et al. 2026a
    Separately adjusted to reproduce z=0 BH masses, independent of the sSFR enhancement.
  • Control matching tolerances = 0.05 dex (stellar mass), 10% (local density), 10% (isolation), widened by 50% up to twice if needed
    Hand-chosen analysis parameters following Patton et al. (2016). The results can depend on these tolerances, but they are not fitted to the enhancement.
assumptions (5)
  • domain assumption Lambda-CDM cosmology with parameters from DES '3x2pt + all external constraints' (Abbott et al. 2022)
    The COLIBRE simulations assume this cosmology and the results are conditioned on it. Section 2.1.
  • domain assumption Star-forming gas is identified by a gravitational instability criterion (Nobels et al. 2024, Eq. 1) and SFRs follow the Schmidt law with epsilon=0.01
    The interaction-driven sSFR enhancement is a direct consequence of this subgrid star formation prescription. Section 2.1.1.
  • domain assumption The halo finder HBT-HERONS (Forouhar Moreno et al. 2025) correctly identifies subhaloes, tracks their progenitors, and provides reliable galaxy centres and properties
    All galaxy samples and separations are based on HBT-HERONS output; numerical stripping at small separations (Appendix D) shows this is not perfect. Section 2.1.2.
  • ad hoc to paper The matching procedure of Patton et al. (2016) produces control samples that are statistically indistinguishable from interacting samples except for the presence of the close companion
    This is the key methodological assumption. Appendix B shows residual environmental differences on scales below 2 Mpc for the lowest mass bin, so the assumption is only partially satisfied. Section 2.2.3 and Appendix B.
  • domain assumption The 10 kpc 3D aperture sSFR is not significantly contaminated by aperture overlap or halo-finder misassignment in interacting pairs
    A small aperture is chosen to minimize contamination (Section 2.1.2), but numerical stripping can reduce stellar masses by up to 20 per cent at small separations (Appendix D).

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Pith. "Pith review of The effect of galaxy interactions on star formation rates in the COLIBRE simulations." pith.science (2026). https://pith.science/paper/HKXJSD6X

@misc{pith2026260812132,
  author       = {Pith},
  title        = {Pith review of: The effect of galaxy interactions on star formation rates in the COLIBRE simulations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HKXJSD6X}},
  note         = {Machine review of arXiv:2608.12132}
}
abstract

Observations and theory indicate that galaxy interactions enhance star formation rates (SFRs). However, the degree of enhancement and its dependence on the properties of the interacting galaxies vary across different studies. In this work, we use the COLIBRE simulations of galaxy formation to investigate the effect of interactions on the SFRs of star-forming galaxies at redshift $z\approx0$. The COLIBRE simulations capture the multiphase nature of the interstellar medium and have volumes up to $200^3$ and $400^3$ cMpc$^3$ at m6 (gas and dark-matter particle mass $\sim10^6~\mathrm{M_\odot}$) and m7 ($\sim10^7~\mathrm{M_\odot}$) resolutions, respectively. After constructing samples of interacting galaxies (with mass ratios $>0.1$) and isolated controls, matched in stellar mass, large- and small-scale environment, and redshift, we show that the average specific SFR (sSFR) of interacting galaxies is enhanced by up to a factor of $\approx2$ for separations of $\approx10$ kpc. The enhancement decreases with pair separation but remains significant out to $\approx200$ kpc. The enhancement increases with increasing numerical resolution, is more pronounced in the central regions of galaxies, and decreases with increasing stellar mass at fixed separation. Mergers with higher mass ratios induce stronger sSFR enhancement. We compare our results with observational data from the SDSS, finding good agreement in the dependence of the mean sSFR enhancement on separation, but underpredicting its normalisation by a factor of $\approx2$. Finally, we show that the pre-merger sSFR enhancement of resolved interactions accounts for $\approx2$ per cent of the $z\approx0$ cosmic SFR density.

Figures

Figures reproduced from arXiv: 2608.12132 by the authors.

Figure 1
Figure 1. Visual impression of the colibre interacting and control galaxies studied in this work. The definitions of interacting and control galaxies are given in §2.2. Each galaxy image shows the stellar light in HST colours, including dust attenuation, computed using the 1D radiative transfer code Partridge (Huško et al., in preparation). The top two rows show eight interacting galaxies, selected from the colibre L200m6 sim… view at source ↗
Figure 2
Figure 2. Top panel: the mean sSFRs of interacting galaxies (red) and their matched controls (blue) as a function of the 3D separation between the in￾teracting galaxies and their closest companions. The results are shown for galaxies from the colibre L200m6 simulation at redshift 0 ≤ 𝑧 ≤ 0.2. All interacting galaxies have stellar masses 𝑀∗,int > 1010 M⊙ and a closest companion galaxy whose stellar mass is more than 10 per cen… view at source ↗
Figure 3
Figure 3. Top panel: the median sSFR of interacting galaxies and their controls for interacting galaxies with pair separations 𝑟3D < 50 kpc and stellar mass ratios > 0.1, shown as a function of interacting galaxy stellar mass over the range 108 < 𝑀∗,int/M⊙ < 1012. The interacting galaxies are indicated in red and the controls in blue. The shaded areas denote the 16th to 84th percentile scatter. The dotted lines indicate bins … view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4: The sSFR enhancement 𝑄(sSFR) as a function of pair separation, shown in three 1-dex interacting galaxy stellar mass bins spanning the range 108 < 𝑀∗,int/M⊙ < 1011, indicated by colour. On average, 𝑄(sSFR) is larger for interacting galaxies with lower stellar mass. The …
Figure 5
Figure 5. Figure 5: Mean sSFR enhancement of interacting galaxies relative to their controls, 𝑄(sSFR), in logarithmically spaced stellar mass bins, split into five separation bins indicated by colour. At fixed stellar mass, the sSFR enhancement decreases with increasing separation, with t…
Figure 7
Figure 7. Figure 7: shows 𝑄(sSFR) as a function of interacting galaxy stellar mass from 108 to 1011 M⊙, using only interacting galaxies with pair separations 𝑟3D < 50 kpc. The results are shown for three mass ratio bins: 1/10 < 𝑀∗,cc/𝑀∗,int < 1/3 (light-blue), 1/3 < 𝑀∗,cc/𝑀∗,int < 3 (dark…
Figure 9
Figure 9. Figure 9: The mean sSFR enhancement of interacting galaxies with 𝑀∗,int > 109 M⊙ relative to their controls, shown for both the fiducial sample (Sec￾tion 2.2; purple) and the sample using projected separation (Section 2.3; or￾ange) in the colibre L200m6 simulation. The sSFR enha…
Figure 10
Figure 10. Figure 10: The distributions of sSFRs for interacting galaxies (red) and their controls (blue) in the colibre L200m6 simulation and in the SDSS sam￾ple from Patton et al. (2013). The interacting galaxies have stellar masses 109 < 𝑀∗/M⊙ < 1012 and projected separations 𝑟proj < 30…
Figure 11
Figure 11. Figure 11: The mean sSFR enhancement of interacting galaxies relative to their statistical controls predicted by the colibre L200m6 simulation (or￾ange), compared with the observational data from SDSS data (black), shown in equally spaced projected separation bins. In both cases…
Figure 13
Figure 13. Figure 13: Interaction-induced sSFR enhancement in four colibre simula￾tions: L200m6 (orange, solid), L100m6 (orange, dash-dotted), L400m7 (red, solid), and L200m7 (red, dash-dotted). We include only interacting galaxies with stellar masses 𝑀∗,int > 1010 M⊙ and mass ratios > 0.1…
Figure 12
Figure 12. Figure 12: Evolution of interacting galaxies and their controls as a function of lookback time. The interacting galaxies are selected at redshift 𝑧 = 0 to have stellar masses 𝑀∗,int > 1010 M⊙, separations 𝑟3D < 50 kpc, and mass ratios > 0.1. Top panel: the mean sSFRs of interact…
Figure 14
Figure 14. Figure 14: The mean sSFR enhancement of interacting galaxies relative to their controls in the L200m6 (orange) and L200m7 (red) colibre simulations. The results are shown for interacting galaxies with separations 𝑟3D < 50 kpc and their controls. The sSFR enhancement at m7 resolu…

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Pith tools

Reviewed August 16, 2026 · model on record in the stance chip above.