SQuIGG𝐿E Sample: Massive Post-Starburst Galaxies

• SQuIGG𝐿E Sample is a curated collection of massive, intermediate-redshift post-starburst galaxies characterized by strong Balmer absorption features and high stellar masses.
• It employs multiwavelength, IFU spectroscopy and ALMA CO(2–1) observations to precisely measure stellar population gradients, quenching uniformity, and cold gas reservoirs.
• Findings challenge classical quenching models by revealing spatially uniform shutdown of star formation and rapid molecular gas depletion despite substantial gas reserves.

The SQuIGG$\vec{L}$E Sample refers to a well-defined collection of intermediate-redshift (primarily $z\sim0.6$–0.7), massive post-starburst galaxies selected and studied in a series of observational astrophysics projects. The sample is constructed to enable precise investigations of galaxy quenching physics, cold gas reservoirs, stellar population gradients, and environmental context by leveraging multiwavelength spectroscopy, integral-field spectroscopy, and deep imaging. SQuIGG$\vec{L}$E survey galaxies are characterized by their dominant A-star populations, extremely strong Balmer absorption features, and high stellar mass ($M_\star \gtrsim 10^{11}\,M_\odot$), all indicating a very recent and rapid cessation of star formation. Collectively, SQuIGG$\vec{L}$E investigations provide key constraints on the mechanisms, timescales, and consequences of galaxy-wide quenching across substantial cosmic volume.

1. Construction and Selection of the SQuIGG$\vec{L}$E Sample

The SQuIGG$\vec{L}$E Sample is assembled by applying strict spectroscopic and photometric criteria to the Sloan Digital Sky Survey (SDSS) DR14 database. Candidate post-starburst galaxies are identified based on rest-frame color selections—specifically [U–B] $>$ 0.975 and $-0.25<$ [B–V] $<$ 0.45—which effectively isolate strong Balmer breaks and blue colors redward of the break. While optical selection does not explicitly require Balmer absorption, the overwhelming majority of SQuIGG$\vec{L}$E candidates have H$\delta_A>4$ Å, with a median of 7.12 Å, confirming their post-starburst nature. The sample is restricted to the most massive galaxies ($M_\star \geq 10^{11}\,M_\odot$), and intermediate redshift ($z\sim0.6-0.7$), focusing on the regime where massive galaxies undergo dramatic structural and star-formation transitions (Setton et al., 2020).

Follow-up campaigns utilize Gemini GMOS-IFU spectroscopy, deep Hyper Suprime-Cam i-band imaging, and (crucially) ALMA CO(2–1) observations. Data reduction steps include standard bias, flat, wavelength, sky, and cosmic ray processing, with precise spatial binning (Voronoi/adaptive annular schemes) applied to boost S/N for spatially resolved analyses.

2. Stellar Population Gradients and Quenching Uniformity

Spatially resolved spectroscopy reveals that SQuIGG$\vec{L}$E sample galaxies exhibit remarkably flat H$\delta_A$ gradients—values often exceeding 7 Å out to $\geq 5$ kpc (Setton et al., 2020). High H$\delta_A$ levels trace the dominance of A-type stars and thus probe light-weighted ages; two-component burst population models indicate ages $\sim 600$ Myr at all radii.

To interpret the radial H$\delta_A$ trends, a two-burst, two-top-hat toy model is implemented: one burst (extended and old) corresponds to bulk population assembled at $z\sim2$, while a second, recent burst generates the post-starburst phase. By modeling both spatial and mass-fraction parameters, the analysis finds that observed flat profiles cannot be reproduced by unresolved nuclear bursts unless the recent episode forms a majority (f$_{burst} \gtrsim 0.5-0.66$) of the galaxy's stellar mass. Thus, quenching appears to be spatially uniform, with no strong evidence for centrally concentrated starburst-driven cessation (Setton et al., 2020).

3. Structural Properties: Compactness and Central Densities

Wide-depth HSC i-band imaging and Sérsic profile fitting demonstrate that SQuIGG$\vec{L}$E post-starburst galaxies possess highly concentrated stellar distributions. Their effective radii $r_e$ lie $0.1$ dex below the mass-size relation for coeval quiescent galaxies, i.e., $\sim25\%$ smaller at a given mass. Sérsic indices frequently reach the upper constraint ($n=6$), highlighting strong central concentration, which is corroborated by high Gini coefficients and generalized concentration metrics (Setton et al., 2022).

Central mass surface densities within 1 kpc are similar between post-starburst and control quiescent galaxies, showing that compactness is not simply a consequence of a secondary nuclear burst but may reflect a prior phase of structural transformation. The lack of positive correlation between $r_e$ and time since quenching disfavors a scenario where fading of a central burst “inflates” the envelope. Instead, rapid quenching likely follows the achievement of a threshold central density, with envelope growth possible via later dry mergers (Setton et al., 2022).

4. Cold Molecular Gas Reservoirs and Ongoing Star Formation

ALMA CO(2–1) observations reveal that a substantial fraction of SQuIGG$\vec{L}$E galaxies ($\sim$50%) retain large molecular gas masses ($M_{H_2} \sim 1-5 \times 10^{10} M_\odot$), even in systems classified as “quenched.” Keck/NIRES near-IR spectroscopy enables robust H$\alpha$ SFR measurements, showing that almost all gas-rich systems have low ongoing SFR ($<4.1\,M_\odot\,{\rm yr}^{-1}$), well below what is predicted by the Kennicutt-Schmidt relation, given their gas content (Zhu et al., 27 Jan 2025).

High [N II]/H$\alpha$ ratios observed in all PSBs imply that H$\alpha$ emission is often dominated by non-star-forming mechanisms (AGN, shocks, or evolved stars), and may overestimate true SFRs. Thus, galaxies in SQuIGG$\vec{L}$E can shut down star formation before fully exhausting their molecular fuel—direct evidence for “morphological quenching,” where structural transformations stabilize gas against collapse, and/or feedback processes suppress star formation efficiency (Zhu et al., 27 Jan 2025).

5. Gas Depletion Timescales and the Role of Buried Star Formation

Deep ALMA campaigns targeting 50 SQuIGG$\vec{L}$E galaxies yield a tight anti-correlation between CO(2–1) luminosity and post-starburst age. The observed molecular gas fraction decays on timescales $\lesssim 140$ Myr, an order of magnitude shorter than gas depletion timescales inferred from low SFRs (Setton et al., 29 Aug 2025). This rapid decline implies that gas is removed or “consumed” by processes not directly tracked by current star formation rates—potentially outflows, tidal stripping, or other feedback channels.

Expanded SED fits incorporating mid-/far-IR photometry (WISE, Herschel) test for hidden, dust-obscured star formation. Results show only modest increases in inferred SFR ($\sim$0.5 dex) and no net change in IR luminosity or energy budget. Adopting ULIRG-like $\alpha_{CO}$ factors also fails to reconcile rapid CO depletion with optical/IR SFRs. Thus, neither buried star formation nor conversion factor modifications suffice, and alternative gas-removal processes are favored (Setton et al., 29 Aug 2025).

Some gas-rich SQuIGG$\vec{L}$E galaxies may eventually rejuvenate—that is, transition out of the post-starburst state back onto the star-forming sequence if their cold gas reservoir re-ignites star formation. This possibility introduces nuance into evolutionary models, emphasizing that quenching need not be monotonic.

6. Environmental Context: “Buddy” Galaxies and Typicality

Targeted SQuIGG$\vec{L}$E ALMA fields also reveal that $31\pm6\%$ of massive PSBs host nearby gas-rich companion “buddy galaxies,” typically with $M_\star \gtrsim 10^{10} M_\odot$ and molecular gas comparable to the central PSB, albeit with $0.8$ dex lower stellar mass (Kumar et al., 29 Aug 2025). Phase-space analyses demonstrate that the buddies are gravitationally bound within the host's dark matter halo ($r/r_{200} \lesssim 1$, $\Delta v/\sigma_v$ within virialized bounds).

Comparison with the UniverseMachine cosmological model confirms that SQuIGG$\vec{L}$E environments are not exceptionally overdense: their satellite populations are typical for galaxies at similar mass and redshift. This mitigates previous interpretations that environmental density is a primary driver of quenching in these systems and supports the idea that internal, secular, or satellite interaction processes dominate.

7. Implications for Galaxy Quenching and Evolutionary Pathways

Comprehensive multiwavelength analysis of the SQuIGG$\vec{L}$E sample demonstrates that rapid quenching in massive galaxies is a spatially uniform, structurally transformative process, closely tied to central density thresholds rather than exclusively nuclear events. Gas-rich post-starburst galaxies can persist long after quenching, with potential for rejuvenation depending on their future dynamics.

Environmental studies show that these systems reside in normal massive halos, with typical satellite populations. Together, these results challenge classical quenching paradigms—emphasizing that molecular gas persistence and extended structure are integral, and that feedback and stabilization mechanisms must be revisited in theoretical models.

In summary, the SQuIGG$\vec{L}$E Sample provides an empirically rich, physically diverse laboratory for dissecting the end stages of galaxy assembly and the interplay of stellar population evolution, structure, cold gas, and environment. Its multi-component investigation underscores the complexity of galaxy quenching at the highest masses and intermediate redshifts.