Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z 0.75

arXiv:2606.02687 · astro-ph.GA · Submitted 2026-06-01 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z 0.75".

Jocelyn: Two massive, star-forming galaxies located in strong over-densities at intermediate redshift are discovered using Euclid data,

Vera: First, who's behind it and why it matters.

Title and authors: Vera: So, we're looking at this paper today, "Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z zero point seven five." It’s really interesting because the title itself points to some very unusual things happening in these specific massive galaxies.

Jocelyn: I agree, Vera; the focus on "disky titans" suggests we're looking at objects that defy what we typically expect for their environment and redshift. The authors are clearly pointing towards a phenomenon where these galaxies are behaving differently than the standard picture suggests for massive systems at that time.

Subrahmanyan: From a theoretical standpoint, I think the title immediately signals a tension between galaxy quenching and rejuvenation processes, which is something we've been grappling with in simulations for quite some time.

Vera: Exactly, Subrahmanyan; it sets up this interesting dichotomy right from the start about star formation activity and gas content in these group galaxies.

Jocelyn: And the authors are using Euclid data to highlight these specific targets, which gives us a strong observational anchor for what they're claiming is happening.

Subrahmanyan: I wonder if they are suggesting that the environment itself isn't the sole driver of their current star formation state, perhaps something else is at play.

The paper's summary: Vera: So, this paper explains that these two galaxies, DT-one and DT-two are massive and star-forming even though they’re in quite a dense environment at redshift zero point seven five. They found surprisingly high star formation efficiency and significant cold gas reservoirs within them.

Jocelyn: That’s the core finding, Vera; it means these galaxies aren't just passively evolving or quenched despite being in a massive group structure, which is pretty surprising given what we usually see in those environments.

Subrahmanyan: The summary suggests that this high star formation activity and gas content might be the result of a merger-induced rejuvenation episode, where the most massive galaxy briefly restarts its star formation by accreting cold gas from another member.

Vera: That merger scenario is what really catches my attention; it explains how they can maintain such a lot of molecular gas even with high halo masses.

Jocelyn: And they’ve characterized the physical properties quite well, showing that DT-two has an SFE of about one while DT-one is actually located above the Schmidt–Kennicutt relation, which implies a lower depletion time and thus higher star formation efficiency.

Subrahmanyan: That finding about the SFE being compatible with star-bursting galaxies really ties into the big picture of how galaxy assembly works over cosmic time.

The paper's improvements: Vera: The authors highlight several aspects of their methodology and data processing that make this study particularly insightful, especially when we think about how we can tackle these complex problems in the future.

Jocelyn: They describe using a combination of Euclid imaging, ground-based photometry, and millimetre follow-up with NOEMA to get this multi-wavelength characterization. That synergy is key for seeing what’s happening across different physical scales.

Subrahmanyan: From a modeling perspective, the paper points out that they are analyzing these targets within massive groups, with halo masses estimated at log10(Mh/M⊙) ∼ thirteen point eight M⊙ and density contrasts of one point four and one point zero above the median.

Vera: They also use morphological analysis, specifically double Sérsic profiles across different bands like IE and HE to separate the contribution of a massive passive bulge from the star-forming disc, which gives them a much clearer picture than just looking at one image.

Jocelyn: And they emphasize that their selection criteria were quite specific, filtering for galaxies with NUV − r and r − J colours that were within the intrinsic scatter of the relation, which helps reduce selection bias.

Subrahmanyan: It’s a good point they make about how this approach allows them to disentangle quenching from morphological transformation, as mentioned in the introductory material.

Conclusion: Vera: So, looking at the end of "Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z zero point seven five," the main implication is that we need to reconsider how we model galaxy evolution when a galaxy is simultaneously massive and active in an over-dense region.

Jocelyn: It suggests that the simple picture of environmental quenching might not be sufficient to explain all cases, and merger-induced rejuvenation could be a necessary component of the evolutionary cycle for some galaxies.

Subrahmanyan: I think this pushes us to refine our cosmological models to include more complex feedback mechanisms that allow for these temporary bursts of star formation triggered by mergers before the galaxy settles into its final state.

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Euclid Consortium · NOEMA

astro-ph.GA

Submitted: 2026-06-01

Updated: 2026-09-28

Comments: Accepted for publication in A&A, 15 pages, 1 appendix, 7(+2) figures, 2 tables

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 76/100

The gist: Two massive, star-forming galaxies located in strong over-densities at intermediate redshift are discovered using Euclid data, revealing a surprisingly high star formation efficiency and significant

Key concepts

Disky Titans: These are massive galaxies (stellar mass > 10¹¹ M⊙) found in strong over-densities at z ~ 0.75 that exhibit high star formation rates and significant cold gas. They are rare because they possess both a clear stellar disc structure and a relatively narrow probability of being found at that specific redshift.
Star Formation Efficiency (SFE): SFE measures how efficiently a galaxy converts its available cold gas into new stars. One target showed an SFE of ~1, which is higher than expected for normal galaxies, suggesting it has a 'lower depletion time' and is undergoing intense starburst activity.
Merger-Induced Rejuvenation: This hypothesis suggests that the high star formation in these massive galaxies resulted from a merger where one galaxy accreted cold gas from another member. This event briefly restarts star formation, allowing the galaxy to rebuild its disc and achieve high SFR levels, similar to what is seen in simulations.
Strong Over-densities: These are regions of the universe where galaxies are clustered more densely than average at a given redshift. The two targets were specifically selected because they resided in groups with density contrasts (log₁₀(1+δ)) significantly above the median, indicating a very crowded environment.

Terminology

AGN — 8× in this paper · explained in 1 episode
Aperture — 5× in this paper · explained in 1 episode
Star Formation Efficiency — 5× in this paper
European Space Agency — 4× in this paper
Abundance — 2× in this paper · explained in 1 episode
Consistency — 2× in this paper · explained in 3 episodes
Velocity Dispersion — 2× in this paper · explained in 2 episodes
Accuracy — 1× in this paper · explained in 1 episode
Aperture Correction — 1× in this paper · explained in 1 episode
Core — 1× in this paper · explained in 2 episodes
Dust Attenuation — 1× in this paper · explained in 2 episodes
Environmental Quenching — 1× in this paper · explained in 2 episodes
Galaxy Quenching — 1× in this paper · explained in 3 episodes
Gauss Centre for Supercomputing — 1× in this paper
Selection Bias — 1× in this paper · explained in 4 episodes
Star Formation Efficiency (SFE) — 1× in this paper · explained in 2 episodes

Summary

Two massive, star-forming galaxies located in strong over-densities at intermediate redshift are discovered using Euclid data, revealing a surprisingly high star formation efficiency and significant cold gas reservoirs within these systems. This finding challenges existing models by suggesting that these disky titans may be the product of a merger-induced rejuvenation episode.

Discovery and Target Selection

The study presents the discovery of two disky titans, defined as massive galaxies with stellar masses greater than 1011 M⊙ and star-forming (SFR ∼ 20 M⊙ yr−1) even in strong over-densities at z ∼ 0.75. These rare sources were identified by selecting galaxies belonging to strong over-densities, specifically those with a density contrast parameter log10(1+δ) above 5σ from the median at their redshift, and subsequently filtering for star-forming galaxies based on their NUV − r and r − J colours being lower than the intrinsic scatter of the relation (σ ∼ 0.3 dex). The final candidates were selected based on the clearer presence of a stellar disc and of a narrower redshift probability function.

Physical Characterization

The characterization involved multi-wavelength data, including Euclid imaging, ground-based ancillary photometry, and millimetre follow-up with NOEMA. Key properties estimated include:

Spectroscopic redshifts: DT-1 at zspec 0.6990 ± 0.0002 and DT-2 at zspec 0.7672 ± 0.0001, derived from CO(2-1) line detections with S/N of 5.2 and 9.1, respectively.
Gas content: The estimated molecular gas masses are MH2 ∼ 1010–1010 M⊙, with a gas fraction µH2 ranging from 0.10 to 0.14 for the two targets, which are compatible with those expected for MS galaxies at z ∼ 0.75.
Star Formation Efficiency (SFE): DT-2 shows an SFE of 1, while DT-1 is located above the Schmidt–Kennicutt relation, indicating a lower depletion time and, consequently, a higher star-formation efficiency (SFE ∼ τ−1dep), compatible with the expectations for star-bursting galaxies.
Morphology: Morphological analysis using double Sérsic profiles showed that both targets present a massive and passive bulge, with the bulge-to-total luminosity ratio increasing from 0.2 and 0.3 in the IE band to 0.5 and 0.9 in HE for the two targets, respectively, with both bulge components having a high Sérsic index (nbulge ∼ 6).

Environmental Context

The two disky titans are situated within massive groups, Group 1 and Group 2, with halo masses estimated at log10(Mh/M⊙) ∼ 13.8 M⊙. These groups are characterized by strong over-densities, with log10(1+δ) of 1.4 and 1.0, corresponding to 6σ and 5σ above the median density at their redshifts. Despite being in these massive structures, both sources are found to be the brightest sources in two massive groupsize structures, and virial-shock heating suggests they are expected to accrete only a negligible amount of cold gas from the surrounding environment, insufficient to sustain significant star formation over long time-scales.

Evolutionary Scenario: Merger-Induced Rejuvenation

The observed properties are interpreted as evidence that the disky titans are the product of a merger-induced rejuvenation episode, where the most massive galaxy of a group accretes cold gas from another member and briefly restarts star-formation. This scenario explains the presence of significant molecular gas reservoirs despite the high halo masses, suggesting that a wet merger can allow a galaxy to re-build a stellar disc and obtain levels of star formation high enough to bring it in the green valley or in the MS. DT-1 is interpreted as an earlier stage following a close encounter with a satellite, while DT-2 represents a later stage where the SFE returns to standard values and morphology stabilizes. This scenario is supported by comparisons with TNG300 simulations, which show systems spending time in the red sequence before experiencing rejuvenation.

Comparison with Simulations

To verify the plausibility of this scenario, analogues were selected from the IllustrisTNG simulation suite. The selection criteria mimicked those used in the study: galaxies from a snapshot at z = 0.76 with M∗ > 1011 M⊙, within 0.3 dex from the main sequence, and located in over-densities (Mh > 1013 M⊙).

Improvements for AI systems

Here are specific improvements to AI systems that could be derived from the scientific findings in this paper, focusing on areas where current modeling or data processing capabilities are limited:

)Improved AI Systems & Capabilities)

The core capability improved is the ability of an AI system to perform rare object detection and characterization in massive, complex astrophysical environments using multi-wavelength data synergy.
The improved AI system could be a sophisticated Deep Learning pipeline (e.g., a tailored Convolutional Neural Network or Graph Neural Network hybrid) trained on Euclid DR1/Q1 data, combined with NOEMA/XMM-Newton spectral and morphological data.

Specific Improvements & Capabilities:

The AI system can be improved to perform highly accurate, automated selection of rare galaxy populations (like the disky titans).
The system will be capable of identifying galaxies that are massive but simultaneously exhibit star formation rates comparable to Main Sequence (MS) galaxies, even within strong over-densities, by learning the subtle interplay between mass-quenching and merger-induced rejuvenation signatures.
The AI can perform multi-scale morphological decomposition (fitting double Sérsic profiles across different filters) to disentangle the contributions of a massive, passive bulge and a highly efficient star-forming disc, even when faced with noise or lower resolution data (by learning from the IE/HE band dependencies).
The system can be enhanced to predict physical properties (like gas mass, dust content, and depletion time) using only millimeter continuum fluxes (e.g., CO(2-1) and S 2mm), bypassing the need for direct spectroscopic follow-up for every source.
The AI will be able to model complex galaxy evolution scenarios, specifically predicting the transition phases of rejuvenation (merger-induced starburst followed by morphological stabilization), based on observed structural features like tidal tails and asymmetries.
The system can perform automated halo mass estimation by integrating large-scale density field statistics (via tomographic slices) and X-ray data constraints, providing robust estimates that are superior to simple stellar mass-to-halo mass relations alone.

Specific Outputs/Tasks for the Improved AI System:

Automated discovery and classification of galaxies fitting the disky titan criteria (M > 1011 M⊙, SFR > 20 M⊙ yr−1, located in strong over-densities).
Generation of physically constrained parameter estimates (Gas Mass, SFE, Depletion Time) for rare sources using only photometric and millimeter data.
Classification of galaxy evolutionary stages (e.g., distinguishing between a rejuvenated MS galaxy and a quiescent BGG) based on morphological features (bulge-to-total ratios across bands).
Prediction of merger history sequences for galaxies in over-dense regions, identifying the likely timing and morphology of rejuvenation episodes.
Automated assessment of environmental quenching mechanisms (mass vs. environment) by comparing observed gas fractions and accretion rates against theoretical predictions derived from simulations (e.g., TNG300).

Abstract

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive (M>10 11 Msun) star-forming (SFR 20 Msun/yr) discs located in strong over-densities at intermediate redshift (z 0.75). They represent an small fraction of the massive galaxies in over-dense regions (just four candidates in more than 20 deg2 analysed in this study), and their existence is puzzling considering the abundance of passive and bulge-dominated sources commonly found at the centre of groups and clusters at low redshift. Firstly, our analysis shows that these objects are located in massive groups (Mh 10 13.8 Msun), where rapid accretion of cold gas should be prevented from the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs (M h2 10 10.3 Msun) within these sources. Secondly, our morphological analysis shows the presence of a massive and passive bulge in these galaxies, which is expected to stabilise the disc against fragmentation thereby suppressing further star formation. However, these sources lie on the Schmidt-Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy of a group accretes cold gas from another member and briefly restarts star-formation. Such scenario is supported by a comparison with the TNG300 simulation and easily explains the surviving of star-formation activity in massive galaxies in over-dense environments as temporary stages in a more complex evolution. More in general, our study showcases the ability of Euclid to find rare objects thanks to the unprecedented statistics offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.