Spiral-to-S0 Evolution

Updated 10 February 2026

Spiral-to-S0 transition is the process where spiral galaxies lose their distinct arms and active star formation, evolving into lenticular systems with older stellar populations.
Observations and simulations demonstrate that environmental mechanisms like ram-pressure stripping and tidal interactions, along with mergers and secular evolution, drive measurable changes in morphology and kinematics.
Quantitative analyses reveal systematic offsets in Tully-Fisher and Faber-Jackson relations, highlighting mass-dependent evolutionary pathways and multi-Gyr transformation timescales.

Spiral-to-S0 Transition

The spiral-to-S0 transition refers to the physical and evolutionary processes by which star-forming spiral galaxies (characterized by prominent arms and ongoing star formation) are transformed into lenticular (S0) galaxies, which possess bulge+disk structures similar to spirals but lack spiral features and show older stellar populations. S0s represent an intermediate morphology between spirals and ellipticals, and their formation and prevalence show strong dependence on cosmic time, environment, and galaxy mass. Quantitative studies incorporating stellar populations, dynamics, and structural parameters across surveys have constrained both the timescales and dominant mechanisms involved in this transformation, revealing multiple evolutionary channels.

1. Demographics and Structural Bimodality

The rise of S0 galaxies since $z \sim 1$ is empirically well established, with the S0 fraction (f_S0) in clusters increasing by factors of 3–4 since $z \sim 0.8$ and overtaking the spiral population below $z\sim0.3$ (Cavanagh et al., 2023, D'Onofrio et al., 2015). Deep-learning classifications in COSMOS indicate that high-mass S0s (above $M_*\sim10^{10}\,M_\odot$ ) become prevalent earlier, with $f_{\rm S0}$ rising sharply from $z\sim0.9$ ; low-mass S0s show a delayed increase, most pronounced below $z\sim0.6$ .

Stellar population studies reveal a marked bimodality in S0 properties: high-mass S0s are old and metal-rich, usually with bulges older than disks; low-mass S0s are younger and metal-poor, with bulge ages comparable to or younger than their disks. This dichotomy underpins two main evolutionary tracks: (a) merger-driven formation for high-mass S0s, and (b) environmental quenching and disk fading for low-mass S0s (Fraser-McKelvie et al., 2018, Cavanagh et al., 2023). The proportion of S0s hosting pseudobulges is low ( $\sim 15\%$ ) compared to spirals ( $\sim 75\%$ ), with pseudobulge S0s predominantly derived from early-type spiral progenitors (Vaghmare et al., 2015).

2. Observational Morphology–Kinematic Evolution

Kinematic and photometric analyses establish that S0s differ from spirals in both angular momentum and central concentration. Integral-field spectroscopy from surveys such as SAMI and MaNGA show that S0s have systematically lower stellar spin ( $\lambda_R$ ) and higher concentration ( $C$ or $R_{90}/R_{50}$ ) than spirals of equal mass (Croom et al., 2021, Querejeta et al., 2015). The offset is not fully accounted for by passive disk fading. For example, in SAMI, disk fading models yield $\langle\Delta\lambda_R\rangle \sim -0.055$ and $\langle\Delta C\rangle\sim+0.08$ after 5 Gyr, explaining only 42% of the $\lambda_R$ and 20% of the $C$ gap to S0s; observed differences are $\Delta\lambda_R=-0.13$ , $\Delta C=+0.42$ (Croom et al., 2021). EAGLE simulations indicate progenitor bias moves the data further from fading predictions.

Kinematic decomposition using planetary nebulae reveals S0 disks are rotationally supported but 'hotter' (lower $V_*/\sigma_\phi$ , $3.3-5.3$, mean $4.2\pm0.8$ at $3R_d$ ) than spiral disks ( $V/\sigma_\phi\sim8-12$ ), inconsistent with pure passive quenching (Cortesi et al., 2013). S0s are underluminous by $\sim1$ mag on the Tully-Fisher (TF) relation compared to spirals at fixed $V_c$ , and their spheroids are overluminous by $\sim1$ mag on the Faber-Jackson relation relative to ellipticals. These TF offsets are quantitatively reproduced by abrupt quenching models: in Coma, offsets are $\Delta M_g=1.10\pm0.18$ , $\Delta M_i=0.86\pm0.19$ , $\Delta M_{Ks}=0.83\pm0.19$ mag at a constant $V_{\max}$ (Rawle et al., 2013).

3. Physical Mechanisms: Environmental and Internal Drivers

Environmental Channels: Clusters and Groups

Observational and model constraints show that in clusters, ram-pressure stripping (RPS) can quench star formation in spirals on timescales $t_s$ of $0.1$–$1$ Gyr, with a strong anti-correlation between $t_s$ and galaxy-to-cluster mass ratio (Marasco et al., 5 Feb 2026). However, the morphological transformation occurs on a longer timescale, $t_m\sim1$ –$5$ Gyr, broadly independent of mass. This ordering ( $t_m>t_s$ ) and absolute scale matches the spectrophotometric aging channel, in which fading of spiral structure and disk smoothness leads to an S0 morphology in several Gyr (Marasco et al., 5 Feb 2026). Wolf et al. identify optically passive (“red spiral”) galaxies in cluster infall regions as a transient, slowly quenched phase (SFR suppressed by a factor $\sim4$ , $1$–$3$ Gyr duration), bridging star-forming spirals and S0s (0906.0306).

Repeated high-speed encounters ("harassment") further heat stellar disks and torque residual gas inward, but are generally not violent enough to destroy the disk (Cortesi et al., 2013, Johnson et al., 2016). In group environments, multiple slow tidal encounters induce disk thickening (vertical dispersion $\sigma_z$ rises from $\sim20$ to $60$ km/s), gas inflows drive central starbursts, and $B/T$ grows by 20–30% over $4$–$6$ Gyr (Bekki et al., 2011). Disks are transformed from thin, arm-rich structures to thick, smooth S0s—often with young, metal-rich bulges and offsets in the TF relation by 0.3–0.5 mag.

Internal/Isolated Mechanisms

A subset of S0s may form in isolation via violent disk instability: if an intrinsically cold, self-gravitating disk (Toomre $Q<0.5$ ) fragments into massive stellar clumps, migration and coalescence of these clumps build a pseudobulge and heat the disk, erasing spiral structure in $<1$ Gyr. This yields end-states with $B/T\sim0.4$ , $n_{\rm bulge}\sim1.3$ , and kinematics (e.g., $v_\varphi/\sigma_\varphi\sim3-4$ ) matching observed field S0s (Saha et al., 2018).

4. Role of Mergers and Secular Evolution

Major mergers provide an alternative, particularly for high-mass S0s. Simulations demonstrate that equal-mass or 3:1 spiral–spiral mergers drive angular momentum loss in the disk (reducing $\lambda_e$ by 0.2–0.4), fuel central starbursts (raising concentration by $\Delta R_{90}/R_{50}\sim0.5-1$ ), and regrow a new disk component around the heated remnant. The resulting S0s populate the same regions of the $\lambda_e$ –concentration plane as observed S0s in CALIFA, a result not achievable by disk fading alone (Querejeta et al., 2015).

Secular processes—including bar-driven gas inflow—can also restructure the bulge/disk through extended star formation, building up pseudobulges. The scaling relations of pseudobulge S0s (bulge effective radius $R_e$ vs. disk scale length $h$ ) are nearly identical to those of their spiral progenitors, except for systematically smaller disks due to gas stripping and fading (Vaghmare et al., 2015). However, such pseudobulge S0s are a minority in the local population.

5. Multiwavelength Structural Evolution

Analysis of V-band surface-brightness profiles reveals that S0s have a higher fraction of pure exponential (Type I, $25\pm5\%$ ) and antitruncated (Type III, $50\pm6\%$ ) disks, and a near absence of truncated (Type II, $<5\%$ ) profiles. Compared to spirals, S0s show a systematic weakening of Type III antitruncations and a threefold increase in bulge-driven outer profiles, consistent with fading of disk star formation and relative bulge growth (Maltby et al., 2014). Bulge-to-disk ratio increases from spirals to S0s, and the enhancement of outer bulge light in S0s supports a scenario in which fading and modest bulge growth (e.g., via minor mergers or gas inflows) dominate the transformation.

Cluster S0s are typically more compact (25% smaller $R_e$ at fixed mass), with higher Sérsic indices and larger central concentrations than their spiral counterparts, further reflecting the predominant role of central mass assembly (via environmental or dynamical processes) in the S0 transition (D'Onofrio et al., 2015).

6. Chemical and Star-Formation Histories

Spectroscopic bulge–disk decomposition in Virgo S0s reveals a two-stage evolutionary sequence: (1) outside-in quenching of disk star formation, typically via ram-pressure stripping or starvation over $\tau_{\rm disc}\sim1-3$ Gyr; and (2) a short, central burst in the bulge (duration $\lesssim0.5$ Gyr, often $t_{\rm bulge}\approx2-5$ Gyr ago), building a younger, more metal-rich, and $\alpha$ -enhanced bulge (Johnston et al., 2014, Johnston et al., 2014). The timing correlation $t_{\rm bulge}\simeq12.5\,\mathrm{Gyr}-1.05\,\tau_{\rm disc}$ links the duration of disk star formation with the epoch of bulge rejuvenation.

The bulge–disk $\alpha$ /Fe ratio is tightly correlated, with a typical offset $\Delta[\alpha/\mathrm{Fe}]\sim0.05-0.1$ dex, indicating that final bulge stars formed from disk-enriched gas. In high-mass S0s, inside-out “morphological quenching” may operate, with bulges older than disks; in low-mass S0s, trends are reversed, allowing for rejuvenation or late compaction (Fraser-McKelvie et al., 2018).

7. Synthesis and Implications

The spiral-to-S0 transition is a composite phenomenon, with the dominant evolutionary channel depending on galaxy mass, environment, and epoch. In clusters and groups, environmental mechanisms—rapid RPS quenching ( $t_s\sim0.1-1$ Gyr), subsequent disk fading and modest dynamical heating ( $t_m\sim1-5$ Gyr), and minor mergers or harassment—produce the bulk of S0s observed today, as evidenced by structural evolution, color–magnitude distributions, and stellar-population gradients (Marasco et al., 5 Feb 2026, D'Onofrio et al., 2015).

Major mergers or violent disk instabilities dominate the formation of high-mass S0s and those found in lower-density environments, explaining the rise of S0s earlier for high-mass systems (Cavanagh et al., 2023, Saha et al., 2018). Secular evolution and gas stripping suffice only for S0s with preexisting massive pseudobulges.

Key empirical trends—systematic TF offsets, decreases in stellar angular momentum and disk fraction, central metallicity and age enhancements, and environmental dependence of the S0 fraction—are all quantitatively reproduced by current models that integrate environmental quenching, merging, and secular processes across cosmic time (Cortesi et al., 2013, Croom et al., 2021, Querejeta et al., 2015). The ladder of transition rates ( $\sim0.07-0.10$ per Gyr in the S0 fraction at $z<1$ ) and multi-Gyr timescales aligns with the observed increase of S0s since $z\sim1$ and supports multiple, mass-dependent formation pathways.