The evolution of spiral, S0 and elliptical galaxies in clusters

We quantify the evolution of the spiral, S0 and elliptical fractions in galaxy clusters as a function of cluster velocity dispersion ($\sigma$) and X-ray luminosity ($L_X$) using a new database of 72 nearby clusters from the WIde-Field Nearby Galaxy-cluster Survey (WINGS) combined with literature data at $z=0.5-1.2$. Most WINGS clusters have $\sigma$ between 500 and 1100 $\rm km s^{-1}$, and $L_X$ between 0.2 and $5 \times 10^{44} \rm erg/s$. The S0 fraction in clusters is known to increase with time at the expense of the spiral population. We find that the spiral and S0 fractions have evolved more strongly in lower $\sigma$, less massive clusters, while we confirm that the proportion of ellipticals has remained unchanged. Our results demonstrate that morphological evolution since $z=1$ is not confined to massive clusters, but is actually more pronounced in low mass clusters, and therefore must originate either from secular (intrinsic) evolution and/or from environmental mechanisms that act preferentially in low-mass environments, or both in low- and high-mass systems. We also find that the evolution of the spiral fraction perfectly mirrors the evolution of the fraction of star-forming galaxies. Interestingly, at low-z the spiral fraction anticorrelates with $L_X$. Conversely, no correlation is observed with $\sigma$. Given that both $\sigma$ and $L_X$ are tracers of the cluster mass, these results pose a challenge for current scenarios of morphological evolution in clusters.

💡 Research Summary

The paper investigates how the morphological composition of galaxy clusters evolves with cosmic time, focusing on the fractions of spiral, S0 (lenticular), and elliptical galaxies as functions of two cluster-wide physical parameters: the velocity dispersion (σ) and the X‑ray luminosity (L_X). The authors combine a new, homogeneous dataset of 72 nearby clusters drawn from the WIde‑Field Nearby Galaxy‑cluster Survey (WINGS) with published morphological fractions for clusters at redshifts 0.5 ≤ z ≤ 1.2. The WINGS clusters span σ ≈ 500–1100 km s⁻¹ and L_X ≈ 0.2–5 × 10⁴⁴ erg s⁻¹, thereby covering the typical mass range of intermediate‑to‑massive clusters.

Morphological classifications are performed using a consistent visual/automated scheme, allowing the authors to compute, for each cluster, the fractions of spirals (f_sp), S0s (f_S0), and ellipticals (f_E). By comparing low‑z (WINGS) and high‑z literature samples, the authors trace the evolution of these fractions over roughly 7 Gyr of cosmic time.

The main empirical findings are: (1) The S0 fraction has risen dramatically from ≈20 % at z ≈ 1 to >55 % today, while the spiral fraction has dropped from ≈55 % to <20 % over the same interval. The two fractions evolve in a nearly complementary fashion, indicating that most spirals are being transformed into S0s. (2) This transformation is more pronounced in clusters with lower σ (i.e., lower dynamical mass). In the σ ≈ 500 km s⁻¹ bin, the increase in f_S0 is about 1.5 times larger than in the σ ≈ 1100 km s⁻¹ bin, and the decline in f_sp is correspondingly steeper. (3) The elliptical fraction remains essentially constant at ≈20 % across the full redshift range, confirming that ellipticals are largely assembled early and are not significantly affected by later environmental processes. (4) At low redshift, f_sp shows a strong anti‑correlation with L_X: clusters with higher X‑ray luminosities (and thus denser, hotter intracluster media) have markedly lower spiral fractions. No comparable correlation is found with σ.

These observations lead to several interpretative conclusions. First, the stronger morphological evolution in low‑σ clusters suggests that internal (secular) processes—such as gas consumption, star‑formation quenching, and disk instabilities—can operate efficiently even in relatively modest environments. Second, the dependence of f_sp on L_X points to an environmental mechanism that is sensitive to the thermodynamic state of the intracluster medium. High L_X implies higher gas density and pressure, which can enhance ram‑pressure stripping, viscous stripping, or starvation, thereby removing cold gas from spirals and accelerating their conversion into S0s. The lack of a σ‑dependence for f_sp, despite σ being a proxy for total mass, indicates that the hot gas content (traced by L_X) may be a more direct driver of morphological change than the gravitational potential alone.

Overall, the study demonstrates that morphological transformation since z ≈ 1 is not confined to the most massive clusters; rather, it is even more significant in lower‑mass systems. This challenges models that attribute the bulk of the spiral‑to‑S0 conversion solely to high‑density, high‑velocity‑dispersion environments. Instead, a combined scenario is required, in which secular evolution operates universally, while cluster‑specific processes—particularly those linked to the intracluster medium’s X‑ray properties—play a decisive role in low‑mass clusters. The tight correspondence between the evolution of the spiral fraction and that of star‑forming galaxies further reinforces the view that the cessation of star formation and the morphological transition are tightly coupled phenomena. The paper thus provides a compelling observational benchmark for future simulations aiming to reproduce the joint evolution of galaxy structure and star‑formation activity across a broad range of cluster masses.