How the cosmic voids contribute to stalling and quenching the giant galaxies on their surfaces

We report a numerical hint that the formations of cosmic voids may be closely linked with the mechanism through which the giant galaxies on void surfaces establish elliptical shapes, redder colors, and lower specific star formation rates (sSFR). Identifying the voids from the TNG300-1 simulations via the Void-Finder algorithm~\cite{HV02} at $z=0$, $0.5$ and $1$, we explore if and how the shapes of the TNG galaxies located on void surfaces are aligned with the directions toward the void centers. Noting that only the giant void-surface galaxies with stellar masses $M_{\star}\ge 10^{10.5},h^{-1},M_{\odot}$ exhibit significant tendency of perpendicular alignments, we dichotomize them into two $M_{\star}$-controlled samples according to their morphologies (elliptical or spiral), colors (redder or bluer), sSFR (lower or higher) and stellar ages (older or younger). It is found at all of the three redshifts that the perpendicular alignments of void-surface galaxies become stronger for the cases that they have elliptical shapes, redder colors, and lower sSFR, but showing weak dependence on the stellar ages. It is also shown that the numerical results are well described by the analytical one-parameter model developed by Lee~\cite{lee19} under the assumption of the existence of a linear scaling between the covariance matrices of galaxy shape axes and local tidal tensors. We test the robustness of alignment signals against the variation of void-finder algorithms and its feasibility against the redshift-space and projection effects. Our results lead us to speculate that the formation and expansion of voids may have an effect of stalling and quenching the giant void-surface galaxies by compressing adjacent matter and then preventing them from radial infall/accretion.

💡 Research Summary

This paper investigates whether the formation and expansion of cosmic voids influence the morphological transformation, colour evolution, and star‑formation activity of massive galaxies that reside on void surfaces. Using the publicly available IllustrisTNG 300‑1 simulation (a 205 h⁻¹ Mpc periodic box with 2×2500³ particles, based on Planck‑2015 ΛCDM cosmology), the authors identify voids with the Void‑Finder algorithm (Hidding & van de Weygaert 2002) at redshifts z = 0, 0.5, and 1. Void candidates are required to have effective radii larger than 5 h⁻¹ Mpc; smaller empty regions are discarded as statistical gaps. For each void, the authors define a surface region as the set of galaxies whose distance from the centre of the nearest empty sphere satisfies 0.9 ≤ r/R_v ≤ 1.1.

Galaxy shapes are quantified by constructing the inertia tensor of the stellar component (using at least ten stellar particles per subhalo) and taking the eigenvector associated with the largest eigenvalue as the major shape axis. The alignment angle θ between this axis and the radial vector pointing toward the void centre is measured for every void‑surface galaxy. The probability density function of cos θ is compared with the null hypothesis of random orientation (uniform distribution with mean 0.5). The authors find a statistically significant deficit of high cos θ values, indicating a preferential perpendicular (θ ≈ 90°) alignment.

Crucially, this alignment signal appears only for galaxies with stellar mass M★ ≥ 10^{10.5} h⁻¹ M⊙. Lower‑mass galaxies show no deviation from randomness. To explore the dependence on intrinsic galaxy properties, the massive sample is split into two halves according to (i) morphology (elliptical vs spiral, based on kinematic criteria), (ii) colour (g − r red vs blue), (iii) specific star‑formation rate (sSFR low vs high), and (iv) stellar age (old vs young). At all three redshifts, the perpendicular alignment is strongest for ellipticals, red galaxies, and those with low sSFR, while the dependence on stellar age is weak or absent. This pattern suggests that the void environment preferentially affects galaxies that have already undergone morphological transformation and quenching.

The alignment statistics are interpreted using the analytic one‑parameter model introduced by Lee (2019). In that framework, the covariance matrix of galaxy shape axes is assumed to be linearly proportional to the covariance matrix of the local tidal tensor, with a proportionality constant d_t. The model predicts a PDF of the form P(cos θ) ∝ 1 + d_t(3cos²θ − 1). By performing Monte‑Carlo integration and χ² minimisation, the authors obtain d_t ≈ 0.08 ± 0.01 for the full massive sample, and larger values (up to ≈ 0.09) for the subsamples with elliptical, red, low‑sSFR galaxies. The model reproduces the simulated PDFs within 1σ, indicating that a simple linear tidal‑alignment prescription captures the essential physics of the void‑surface alignment.

Robustness tests are carried out in three ways. First, alternative void‑finding algorithms (ZOBOV, VIDE) are applied; the resulting alignment strengths vary by less than 10 %, confirming that the signal is not an artefact of a specific void definition. Second, the authors transform galaxy and void positions into redshift‑space, recompute separations and angles, and find that d_t changes negligibly, implying that peculiar velocities do not erase the signal. Third, a 2‑D projection is performed to mimic observational data: galaxies are projected onto a sky plane, voids are identified in the projected distribution, and the alignment is measured using the projected radial direction. The perpendicular alignment persists with d_t ≈ 0.07, suggesting that the effect should be detectable in real surveys.

Physically, the authors propose that the rapid expansion of a large void exerts a compressive tidal field on the surrounding matter. This compression generates tangential motions parallel to the void surface, which inhibit radial inflow of gas onto galaxies located there. Over cosmological timescales, the lack of fresh gas supply leads to a decline in sSFR (quenching), while the anisotropic compression stretches the stellar distribution perpendicular to the direction of maximum expansion, producing the observed perpendicular alignment and driving galaxies toward an elliptical morphology. Because the process depends primarily on the large‑scale tidal field rather than on local density, the effect is largely independent of stellar age.

In summary, the paper provides compelling numerical evidence that (1) massive galaxies on void surfaces preferentially align their major axes perpendicular to the void centre, (2) this alignment is strongest for galaxies that are already elliptical, red, and quiescent, (3) a simple linear tidal‑alignment model quantitatively reproduces the signal, and (4) the result is robust against variations in void identification, redshift‑space distortions, and projection. The authors argue that void expansion can act as a “stalling and quenching” mechanism distinct from filamentary accretion or cluster‑environment processes, offering a new perspective on how low‑density environments shape galaxy evolution. They suggest that the fraction of elliptical void‑surface galaxies could serve as an independent probe of dark‑energy–driven cosmic acceleration, motivating future observational campaigns with large spectroscopic and imaging surveys (e.g., DESI, Euclid, LSST) to test these predictions.