Galactic disc warps from $z = 2.5$ to modern epoch: ruling out observational effects

A significant fraction of galaxies show warps in their discs, usually noticeable at its periphery. The exact origin of this phenomenon is not fully established, although multiple warp formation mechanisms are proposed. In this study, we create a sample of more than 1000 distant ($z \lesssim 2.5$) edge-on galaxies imaged by HST and JWST. For these galaxies, we measurd characteristics of warps and finally analyse how their parameters and frequency change with time. We focus on our main result that galaxies with strong warps were more prevalent in the past compared to the modern epoch. We check how selection effects and varying image quality between objects in our sample could influence our results and conclude that varying fraction of warped galaxies is not caused by observational effects, but represents a genuine evolution. Such a trend may be consistent with mergers and interactions between galaxies being the primary mechanism of warp formation, as number density of galaxies decreases with time, implying higher rate of mergers and interactions in the past.

💡 Research Summary

This paper investigates the evolution of galactic disc warps from redshift z ≈ 2.5 to the present day, focusing on whether the observed increase in warp frequency at high redshift could be an artifact of observational biases. The authors assemble a sample of 1,027 edge‑on galaxies drawn from two sources: (1) 780 galaxies from the HST COSMOS field observed in the F814W filter, limited to stellar masses M* > 10⁹ M⊙, and (2) 247 galaxies from the JWST Dawn Archive (DJA) observed in the F115W and/or F444W filters. The combined sample spans 0 < z ≲ 2.5 and covers a broad range of stellar masses, as shown in Figure 1 of the paper.

To quantify warps, the authors adopt a skeleton‑based isophote analysis originally described in Reshetnikov et al. (2016). For each galaxy they construct multiple isophotes, generate a one‑pixel‑wide skeleton for each, and extract the longest “thread” as the disc centre‑line. The centre‑line provides the vertical offset ΔH as a function of galactocentric radius r. The warp angle ψₑ is defined as the maximum deviation of this centre‑line from the inner tangent plane, measured at the outermost radius where the centre‑line can be traced. This definition deliberately avoids using a fixed physical radius such as R₂₅, because surface‑brightness dimming, K‑corrections, and band‑shifting make R₂₅ ill‑defined at high redshift. Warps are classified as S‑shaped or U‑shaped based on visual morphology, and a warp is deemed “strong” if ψₑ > 4°.

The main observational result (Figure 3) is that the fraction of strong S‑shaped warps rises dramatically with redshift: only about 10–15 % of low‑z (z ≈ 0) galaxies host such warps, whereas nearly 50 % of galaxies at z ≈ 2 do. By contrast, the fraction of U‑shaped warps remains roughly constant at 10–20 % across the entire redshift range. The average warp amplitude also shows a modest increase toward higher redshift, consistent with the frequency trend.

A substantial part of the paper is devoted to testing whether this trend could be produced by observational effects. Three potential biases are examined in detail:

1. Mass‑selection bias – The high‑z subsample is skewed toward more massive galaxies, yet previous work (e.g., Reshetnikov & Combes 1998) shows that more massive discs tend to have smaller warp angles. Consequently, any mass bias would actually suppress, not enhance, the observed increase, indicating that the trend cannot be explained by mass incompleteness.

2. Image quality and data extent – The authors introduce a parameter dₑ, defined as the outermost radius (in units of the exponential scale length h) at which the centre‑line is measurable. They perform a trilinear regression of ψₑ against redshift (z), log M*, and dₑ for the HST subsample (where reliable h measurements are available). The fitted relation is
   ψₑ = (2.76 ± 0.62) − (0.46 ± 0.21) log M* + (0.27 ± 0.08) dₑ + (5.52 ± 1.88) z.
   The coefficient of z is positive and significant (≈ 2.7 deg per unit redshift), confirming that even after accounting for mass and data‑extent effects, warp angle grows with redshift.

3. Band‑shifting and K‑correction – Because the same observed filter samples different rest‑frame wavelengths at different redshifts, the authors compare ψₑ measured in JWST’s two filters (F115W and F444W) for overlapping galaxies and find no systematic offset. They also simulate “artificial redshifting” of a subset of galaxies by degrading spatial resolution and applying surface‑brightness dimming proportional to (1 + z)⁴, mimicking K‑correction and cosmological dimming. The simulated images sometimes lose the outermost disc, but the authors deliberately exclude images where the warp becomes ambiguous. The overall statistics of ψₑ remain consistent with the original measurements.

Having ruled out these observational systematics, the authors argue that the increase in strong S‑shaped warp frequency is a genuine evolutionary effect. They link this evolution to the well‑established decline in galaxy interaction and merger rates from high to low redshift (e.g., close‑pair statistics, merger counts). Since tidal interactions are thought to preferentially generate S‑shaped warps, a higher interaction rate in the early universe naturally explains the observed trend. The lack of evolution in the U‑shaped warp fraction suggests a different origin, possibly ram‑pressure stripping or other environmental processes that do not evolve strongly with cosmic time.

In the conclusions, the paper emphasizes three methodological advances: (i) the construction of a large, high‑redshift edge‑on sample using both HST and JWST data; (ii) a robust, skeleton‑based warp measurement that avoids fixed‑radius biases; and (iii) a multi‑parameter statistical correction that isolates the redshift dependence of warp amplitude. The authors propose that future deep JWST imaging, combined with high‑resolution hydrodynamical simulations, will allow a more detailed dissection of the physical mechanisms behind warp formation and their dependence on environment, mass, and cosmic epoch.

Overall, the study provides compelling evidence that strong S‑shaped disc warps were far more common in the early universe, supporting the view that galaxy‑galaxy interactions played a dominant role in shaping disc morphology over cosmic time.