Starburst BCGs: Rejuvenation in Clusters

• Starburst BCGs are rare, cluster-central galaxies combining a dominant old stellar population with bursts of recent star formation identified via UV, optical, and IR diagnostics.
• Observational studies reveal star formation rates ranging from under 1 to several hundred M☉/yr, influenced by low-entropy cool cores and, at high redshift, by gas-rich mergers.
• Empirical links between ICM thermodynamics and BCG star formation support feedback-regulated cooling, where AGN activity and cooling flows drive intermittent, localized starbursts.

Searching arXiv for the provided BCG starburst and post-starburst literature to ground the article in the cited papers. Starburst brightest cluster galaxies (BCGs) are the uncommon subset of cluster-central galaxies in which substantial recent or ongoing star formation is superposed on the dominant old stellar population that otherwise characterizes most BCGs. Across low and intermediate redshift samples, BCGs are described as “mostly elliptical galaxies” that “very rarely have prominent star formation,” and even when activity is present it is often confined to a small minority of systems (Liu et al., 2010). The starburst class is therefore defined less by morphology than by multi-wavelength diagnostics: ultraviolet excess, nebular emission lines, mid- and far-infrared luminosity, blue filamentary structure, and, in some systems, post-starburst Balmer absorption after rapid quenching. The observational literature further shows that these episodes are closely linked to the thermodynamical state of the intracluster medium (ICM), especially in low-entropy or cool-core clusters, while a separate high-redshift channel appears associated with gas-rich interactions and multi-object mergers (Donahue et al., 2010).

1. Definition and observational scope

In the observational literature, starburst BCGs occupy the extreme tail of the BCG population. A statistically complete WISE study of 245 X-ray selected nearby BCGs found that “99 ± 0.6% of the 245 BCGs have inferred SFRs below 10 $M_\odot\,\mathrm{yr}^{-1}$,” with Perseus A and Cygnus A the only galaxies in that sample above $40\,M_\odot\,\mathrm{yr}^{-1}$ (Fraser-McKelvie et al., 2014). In a large optically selected SDSS sample, 120 early-type BCGs with significant ongoing star formation constituted “$\simeq 0.5\%$ of the optically selected early-type BCG population,” again emphasizing rarity (Liu et al., 2012). A systematic search for post-starburst signatures in 8,812 SDSS BCGs identified only five E+A systems, corresponding to a fraction of “$\simeq 0.06$ percent” (Liu et al., 2010).

The term encompasses several related observational states. Some systems are vigorously star-forming, with extinction-corrected star formation rates ranging from “below $0.1\,M_\odot\,\mathrm{yr}^{-1}$ up to several hundred $M_\odot\,\mathrm{yr}^{-1}$” in the CLASH sample, and as high as $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$ in the Phoenix cluster BCG (Fogarty et al., 2015). Others are post-starburst BCGs, defined by strong H$\delta$ absorption together with the absence of [O II] and H$\alpha$ emission, indicating that star formation occurred recently but has already been rapidly quenched (Liu et al., 2010). This suggests that the topic is best understood as a spectrum of cluster-central rejuvenation phenomena rather than a single phenomenological category.

At the same time, the broader BCG literature establishes the baseline against which starburst systems are judged. Large structural studies of nearby BCGs describe them primarily as massive early-type systems whose evolution is dominated by dynamical assembly, envelope growth, and their location within the cluster potential, rather than by current star formation (Lauer et al., 2014). The starburst BCG is therefore exceptional precisely because it interrupts the canonical “red and dead” state.

2. Diagnostic criteria and spectral signatures

Starburst BCGs are identified through a combination of UV, optical, IR, and sometimes radio diagnostics. In the REXCESS sample, inactive BCGs have $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$ AB, whereas star-forming BCGs exhibit a UV excess with $40\,M_\odot\,\mathrm{yr}^{-1}$0 AB or $40\,M_\odot\,\mathrm{yr}^{-1}$1 AB (Donahue et al., 2010). In the ACCEPT analysis, a “quiescent baseline” of $40\,M_\odot\,\mathrm{yr}^{-1}$2 AB mag was established, and BCGs were flagged as UV-active when $40\,M_\odot\,\mathrm{yr}^{-1}$3 (Hoffer et al., 2012). These color diagnostics isolate unobscured recent star formation from the old stellar population expected in quiescent ellipticals.

Optical emission lines provide a second, widely used diagnostic. In REXCESS, detection of H$40\,M_\odot\,\mathrm{yr}^{-1}$4 equivalent width $40\,M_\odot\,\mathrm{yr}^{-1}$5 was used to define emission-line BCGs, and seven of 31 systems showed H$40\,M_\odot\,\mathrm{yr}^{-1}$6 plus [N II], [S II], and occasionally O I. In the SDSS-based study of early-type BCGs, selection required all five lines [N II] $40\,M_\odot\,\mathrm{yr}^{-1}$7, H$40\,M_\odot\,\mathrm{yr}^{-1}$8, [O III] $40\,M_\odot\,\mathrm{yr}^{-1}$9, H$\simeq 0.5\%$0, and [O II] $\simeq 0.5\%$1 detected at $\simeq 0.5\%$2, with $\simeq 0.5\%$3 and $\simeq 0.5\%$4, followed by BPT selection of pure star-forming or composite objects (Liu et al., 2012). In CLASH, blue-line and BPT diagnostics placed most UV-bright star-forming BCGs in the “composite” or star-forming regions, with weaker line emitters often showing modest LINER-like contributions (Fogarty et al., 2015).

Infrared diagnostics are critical where dust obscuration is significant. The WISE analysis used the color cut $\simeq 0.5\%$5 mag to identify significant IR excess attributable to star formation (Fraser-McKelvie et al., 2014). In MACS J1931.8-2634, Herschel $\simeq 0.5\%$6–$\simeq 0.5\%$7 photometry yielded $\simeq 0.5\%$8, while AGN-starburst decomposition returned a total $\simeq 0.5\%$9 split roughly $\simeq 0.06$0 from the AGN and $\simeq 0.06$1 from star formation (Santos et al., 2015). In the ACCEPT sample, the warm-dust excess criterion $\simeq 0.06$2-band flux ratio $\simeq 0.06$3 identified 43% of cool-core BCGs as mid-IR active (Hoffer et al., 2012).

Post-starburst BCGs are defined differently. Liu et al. required $\simeq 0.06$4, $\simeq 0.06$5, and $\simeq 0.06$6, with positive equivalent widths denoting absorption and negative equivalent widths denoting emission (Liu et al., 2010). In these systems, strong H$\simeq 0.06$7 absorption indicates a significant A-star population with lifetimes of order $\simeq 0.06$8–$\simeq 0.06$9 Gyr, while the absence of [O II] and H$0.1\,M_\odot\,\mathrm{yr}^{-1}$0 implies that the burst was rapidly quenched (Liu et al., 2010).

3. Incidence, demographics, and star-formation rates

The incidence of star formation in BCGs depends strongly on sample selection and cluster environment. The REXCESS survey found H$0.1\,M_\odot\,\mathrm{yr}^{-1}$1 and forbidden-line emission in 7 of 32 BCGs, giving an overall emission-line fraction of $0.1\,M_\odot\,\mathrm{yr}^{-1}$2, but among cool-core clusters the incidence was $0.1\,M_\odot\,\mathrm{yr}^{-1}$3 (Donahue et al., 2010). The ACCEPT study similarly found that 38% of low-$0.1\,M_\odot\,\mathrm{yr}^{-1}$4 systems had UV excess and 43% showed warm-dust excess, whereas high-entropy systems showed none of these signatures (Hoffer et al., 2012). By contrast, in the optically selected SDSS BCG population, genuinely star-forming early-type BCGs accounted for only about $0.1\,M_\odot\,\mathrm{yr}^{-1}$5 of systems (Liu et al., 2012). This contrast indicates that X-ray selection preferentially identifies the cool-core environments in which BCG star formation is enabled.

Published star formation rates span a large dynamic range. In REXCESS, UV- and H$0.1\,M_\odot\,\mathrm{yr}^{-1}$6-derived values were consistent and ranged from $0.1\,M_\odot\,\mathrm{yr}^{-1}$7–$0.1\,M_\odot\,\mathrm{yr}^{-1}$8 and $0.1\,M_\odot\,\mathrm{yr}^{-1}$9–$M_\odot\,\mathrm{yr}^{-1}$0, respectively (Donahue et al., 2010). In the SDSS early-type BCG sample, H$M_\odot\,\mathrm{yr}^{-1}$1-derived SFRs ranged from $M_\odot\,\mathrm{yr}^{-1}$2 to $M_\odot\,\mathrm{yr}^{-1}$3, with $M_\odot\,\mathrm{yr}^{-1}$4, while [O II]-derived estimates ranged from $M_\odot\,\mathrm{yr}^{-1}$5 to $M_\odot\,\mathrm{yr}^{-1}$6 (Liu et al., 2012). The ACCEPT compilation extended the obscured SFR range to “tens or even hundreds of $M_\odot\,\mathrm{yr}^{-1}$7” in extreme cool cores such as Abell 1835 and Abell 1068 (Hoffer et al., 2012). In the CLASH sample, BCG SFRs span $M_\odot\,\mathrm{yr}^{-1}$8–$M_\odot\,\mathrm{yr}^{-1}$9 (Fogarty et al., 2017).

Several clusters define the high-SFR end of the distribution. The Phoenix cluster BCG has an extinction-corrected $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$0, based on UV imaging and reddening corrections (McDonald et al., 2012). The BCG in MACS J1931.8-2634 has $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$1 after removal of the AGN contribution (Santos et al., 2015). The Abell 2667 BCG has $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$2, distributed in blue filaments and clumps (Iani et al., 2019). At higher redshift, the central BCG-associated starburst in SpARCS104922.6+564032.5 has $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$3, while SPT-CL J2215-3537 shows $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$4 and $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$5 (Webb et al., 2015).

These rates coexist with the result that most BCG stellar mass is old. In the SDSS spectral-synthesis analysis, the current or recent starburst contributed on average only $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$6 of the total stellar mass, superposed on a dominant population older than $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$7 Gyr (Liu et al., 2012). In the post-starburst sample, the H$798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$8 equivalent widths were weaker than in field E+A galaxies, leading to the interpretation that recent star formation in these BCGs was historically “not very violent” (Liu et al., 2010). This suggests that even conspicuous BCG starbursts often represent rejuvenation rather than wholesale galaxy formation.

4. Relation to cool cores, entropy, and precipitation

A central result of the literature is that starburst BCGs are strongly associated with dense, rapidly cooling cluster cores. In REXCESS, all seven emission-line BCGs inhabit clusters classified as cool cores with central cooling time $798 \pm 42\,M_\odot\,\mathrm{yr}^{-1}$9 Gyr, and survival-analysis tests gave $\delta$0 for the correlation between $\delta$1 and central electron density or cooling time (Donahue et al., 2010). The ACCEPT study defined entropy as $\delta$2 and found that cool cores with $\delta$3 host the UV and IR excesses, while high-entropy systems “almost never show BCG star-formation signatures” (Hoffer et al., 2012). In CLASH, all BCGs with $\delta$4 had $\delta$5 (Fogarty et al., 2015).

The CLASH thermodynamic analysis provided a tighter formulation by relating star formation directly to the cooling-to-freefall ratio. Using

$\delta$6

and

$\delta$7

measured at $\delta$8, Donahue et al. found

$\delta$9

with intrinsic scatter $\alpha$0 dex (Fogarty et al., 2017). This is a direct empirical link between ICM thermodynamics and BCG star formation intensity. The same study found that burst durations $\alpha$1 may correlate with $\alpha$2, and that when $\alpha$3 reaches the Gyr regime it approaches the cooling time (Fogarty et al., 2017).

Several individual clusters exemplify this coupling. In SPT-CL J2215-3537, the cluster core has $\alpha$4, $\alpha$5 Myr, and a maximal ICM cooling rate

$\alpha$6

while the BCG hosts a central starburst with extended blue filaments (Calzadilla et al., 2023). The Phoenix cluster presents an even more extreme case: the star formation rate of $\alpha$7 is interpreted as arising from a cold gas reservoir supplied by a classical cooling flow of $\alpha$8 (McDonald et al., 2012). In Abell 2667, a strong cool core with $\alpha$9 Gyr is explicitly invoked in the chaotic cold accretion interpretation of the observed blue filaments (Iani et al., 2019).

The physical picture advanced in these studies is feedback-regulated cooling or precipitation. In the CLASH interpretation, AGN jets uplift low-entropy ICM, triggering precipitation when $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$0, with condensed cold clouds then fueling both sustained star formation and AGN activity (Fogarty et al., 2017). CLASH imaging of RXJ1532.9+3021 further showed young star-forming filaments aligned with X-ray cavities, supporting a “jet-triggered precipitation” scenario in which AGN outbursts locally drive $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$1 below $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$2–20 (Fogarty et al., 2015). A plausible implication is that the ICM does not merely supply gas passively; in many clusters it is dynamically coupled to the geometry, timescale, and intermittency of the starburst.

5. Morphology, stellar populations, and AGN coexistence

Starburst BCGs often show filamentary, clumpy, or spatially extended structures rather than compact nuclear star formation. The Phoenix BCG displays “narrow, blue filaments extending more than $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$3 ($\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$4 kpc) from the galaxy’s center,” detected in all five HST bands from rest-frame $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$5 to $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$6, while an underlying old stellar population follows a de Vaucouleurs law with half-light radius $\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$7 kpc (McDonald et al., 2012). In Abell 2667, subtraction of a smooth Sérsic component reveals “a network of filamentary ‘arms’ and embedded clumps,” brighter in the blue F450W band and aligned with the galaxy’s major axis (Iani et al., 2019). H$\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$8 kinematics there show predominantly redshifted gas along the filaments and localized blueshifted pockets near the center, while BPT diagrams place the core in the LINER region and the filaments in the “composite” zone (Iani et al., 2019).

The CLASH sample showed similar morphology at scale. Ten BCGs exhibit extended star-forming knots and filaments in rest-frame UV and H$\mathrm{UVW1}-R \simeq 4.7 \pm 0.3$9+[N II] imaging at $40\,M_\odot\,\mathrm{yr}^{-1}$00 significance (Fogarty et al., 2015). In RXJ1532.9+3021, per-pixel SED fitting revealed a more extended $40\,M_\odot\,\mathrm{yr}^{-1}$01–$40\,M_\odot\,\mathrm{yr}^{-1}$02 Gyr starburst together with younger filaments of age $40\,M_\odot\,\mathrm{yr}^{-1}$03–$40\,M_\odot\,\mathrm{yr}^{-1}$04 yr extending up to $40\,M_\odot\,\mathrm{yr}^{-1}$05 kpc (Fogarty et al., 2015). In Abell 2667, the reconstructed mass-assembly history indicates that $40\,M_\odot\,\mathrm{yr}^{-1}$06 of the stellar mass formed $40\,M_\odot\,\mathrm{yr}^{-1}$07 Gyr ago, $40\,M_\odot\,\mathrm{yr}^{-1}$08 between $40\,M_\odot\,\mathrm{yr}^{-1}$09 and $40\,M_\odot\,\mathrm{yr}^{-1}$10 Gyr ago, $40\,M_\odot\,\mathrm{yr}^{-1}$11 between $40\,M_\odot\,\mathrm{yr}^{-1}$12 and $40\,M_\odot\,\mathrm{yr}^{-1}$13 Gyr ago, and only $40\,M_\odot\,\mathrm{yr}^{-1}$14 in the current $40\,M_\odot\,\mathrm{yr}^{-1}$15 Gyr component, whose stars trace the blue clumps (Iani et al., 2019). This reinforces the general conclusion that starburst BCGs are overwhelmingly old stellar systems undergoing limited rejuvenation.

AGN are common and frequently complicate interpretation. The BCG in MACS J1931.8-2634 hosts a Compton-thick type-II AGN with $40\,M_\odot\,\mathrm{yr}^{-1}$16 and $40\,M_\odot\,\mathrm{yr}^{-1}$17, and its FIR SED requires decomposition because AGN and starburst each contribute roughly half the infrared luminosity (Santos et al., 2015). The Abell 2667 BCG hosts a radio-loud, X-ray-obscured Type 2 AGN with optical features typical of a LINER, and its line emission in the clumps is explicitly described as composite (Iani et al., 2019). SPT-CL J2215-3537 has a weak radio source at $40\,M_\odot\,\mathrm{yr}^{-1}$18 GHz consistent with ongoing AGN feedback, though the implied jet power is less than half the cooling luminosity (Calzadilla et al., 2023). Even in proto-BCG assembly systems, AGN can already be present: in the Euclid “Puddle,” the brightest nucleus at $40\,M_\odot\,\mathrm{yr}^{-1}$19 is AGN-dominated, while SED fitting indicates that the merging BCG experienced a short burst of star formation about $40\,M_\odot\,\mathrm{yr}^{-1}$20 Myr ago (Trudeau et al., 5 Mar 2026).

This coexistence of AGN, old stellar halos, and kpc-scale star-forming filaments is one of the defining features of the class. It also underlies a recurrent observational caution: SFR estimates derived from line emission or IR luminosity must be corrected for AGN contamination wherever possible.

6. Formation channels, quenching, and high-redshift assembly

Two distinct formation channels recur in the literature. At low and intermediate redshift, the dominant interpretation links starburst BCGs to residual cooling, precipitation, or chaotic cold accretion from the ICM. The ACCEPT, REXCESS, and CLASH studies all connect star formation to low entropy, short cooling time, and dense X-ray-bright gas (Hoffer et al., 2012). In Abell 2667, one favored explanation is chaotic cold accretion in which thermal instabilities condense into cold clouds that fuel both AGN and spatially distributed star formation in filaments (Iani et al., 2019). In MACS J1931.8-2634, the fact that the contemporaneous upper limit on the instantaneous mass-deposition rate is $40\,M_\odot\,\mathrm{yr}^{-1}$21, at least a factor of $40\,M_\odot\,\mathrm{yr}^{-1}$22 below the BCG SFR, suggests either fueling by gas cooled at earlier epochs or intermittent cooling on longer timescales than a single cooling-flow episode (Santos et al., 2015).

A second channel is prominent at high redshift: gas-rich interaction or multi-object merger. In SpARCS104922.6+564032.5 at $40\,M_\odot\,\mathrm{yr}^{-1}$23, HST imaging resolves the BCG-associated IR source into a 66 kpc “beads on a string” tidal structure with $40\,M_\odot\,\mathrm{yr}^{-1}$24 compact clumps, and the morphology together with the enormous $40\,M_\odot\,\mathrm{yr}^{-1}$25 argues for a wet merger at the cluster center (Webb et al., 2015). A related radio study found that the compact radio core is too weak to explain the far-infrared star-forming SED and suggested that the star-forming region is extended or clumpy and not located directly within the BCG nucleus (Trudeau et al., 2019). In the z = 4.3 protocluster SPT2349-56, a steep-spectrum radio AGN coincident with the central SMG complex is interpreted as direct evidence of a forming BCG undergoing simultaneous merger-driven starburst and obscured AGN activity (Chapman et al., 2023). The Euclid “Puddle” adds a later-stage example in which six or seven galaxies appear to be assembling into a future BCG with stellar mass $40\,M_\odot\,\mathrm{yr}^{-1}$26 and a burst age of $40\,M_\odot\,\mathrm{yr}^{-1}$27 Gyr (Trudeau et al., 5 Mar 2026).

Rapid quenching produces a third, transitional state. The five E+A BCGs identified in SDSS have H$40\,M_\odot\,\mathrm{yr}^{-1}$28 absorption but no detectable [O II] or H$40\,M_\odot\,\mathrm{yr}^{-1}$29, indicating that their starbursts ended abruptly (Liu et al., 2010). Compared with field E+A galaxies, these BCGs have larger $40\,M_\odot\,\mathrm{yr}^{-1}$30 and smaller $40\,M_\odot\,\mathrm{yr}^{-1}$31, reflecting older underlying stellar populations and milder bursts; compared with quiescent BCGs of similar mass, they have slightly larger $40\,M_\odot\,\mathrm{yr}^{-1}$32 at the same $40\,M_\odot\,\mathrm{yr}^{-1}$33, consistent with modest rejuvenation (Liu et al., 2010). This suggests that starburst BCGs need not remain in the active phase for long and may transition quickly into weak post-starburst systems.

A common misconception is that all star-forming BCGs are merger remnants. The low-redshift cool-core literature argues against that generalization: the Phoenix BCG lacks tidal features and multiple bulges and is therefore interpreted as being fueled by cooling rather than merger-driven gas delivery (McDonald et al., 2012). Conversely, another misconception is that all BCG starbursts arise from cooling flows. The high-redshift SpARCS104922.6+564032.5 system is explicitly contrasted with low-z cooling-flow filaments because its morphology points directly to galaxy-galaxy dynamics and no X-ray cool core is yet detected (Webb et al., 2015). The current evidence therefore supports multiple pathways whose relative importance evolves with redshift and environment.

7. Extreme systems, structural context, and open issues

Extreme starburst BCGs exist within a broader class of structurally unusual central galaxies. Some BCGs are defined not by star formation but by record-setting depleted cores. A2261-BCG has a cusp radius $40\,M_\odot\,\mathrm{yr}^{-1}$34 kpc and a flat or slightly depressed inner profile, while Holm 15A in Abell 85 has $40\,M_\odot\,\mathrm{yr}^{-1}$35 kpc and a LINER spectrum with a powerful radio source (Postman et al., 2012). These systems are not reported as starbursting BCGs, but they establish that cluster-central galaxies can reach structural extremes through black-hole scouring, recoil, and merger dynamics rather than through current star formation. This structural context is relevant because starburst BCGs are superposed on the same class of massive, dynamically evolved ellipticals.

Dynamical offsets provide further context. Simulations show that a non-negligible fraction of rich clusters can host BCGs that are not at rest at the cluster center owing to recent major mergers, even though the “central galaxy paradigm” often remains approximately valid (Martel et al., 2014). Observationally, large nearby BCG samples likewise show that the BCG’s location relative to the X-ray center and its peculiar velocity are tied to cluster dynamical state and envelope growth (Lauer et al., 2014). A plausible implication is that some ambiguity in the fueling channel of an individual starburst BCG may reflect the fact that cool-core thermodynamics, AGN feedback, and merger history can all operate simultaneously in the cluster center.

Several open issues remain observational rather than conceptual. One is the duty cycle: the WISE result that $40\,M_\odot\,\mathrm{yr}^{-1}$36 of local BCGs have SFRs below $40\,M_\odot\,\mathrm{yr}^{-1}$37 implies that intense starbursts occupy a very short fraction of BCG lifetime (Fraser-McKelvie et al., 2014). Another is the cooling-efficiency problem. In SPT-CL J2215-3537, the global cooling conversion efficiency is $40\,M_\odot\,\mathrm{yr}^{-1}$38, rising to $40\,M_\odot\,\mathrm{yr}^{-1}$39 within the filament radius, much larger than the $40\,M_\odot\,\mathrm{yr}^{-1}$40 typical of low-z cool cores (Calzadilla et al., 2023). MACS J1931.8-2634 presents the opposite tension, where the instantaneous X-ray cooling limit is too low to explain the ongoing SFR (Santos et al., 2015). These systems bracket the regime in which AGN feedback appears either temporarily overwhelmed or temporally decoupled from star formation.

Finally, high-redshift surveys are beginning to reveal assembling BCGs before passive central dominance is established. The Euclid “Puddle” suggests that “multiobject mergers might be a common BCG formation process,” and, assuming a similar density of mergers in the Euclid Wide Survey, approximately 400 assembling BCGs are expected by mission end (Trudeau et al., 5 Mar 2026). This suggests that the starburst BCG is not merely a rare low-redshift anomaly, but also a key phase in early cluster-core assembly.