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

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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

In short

Euclid data discovered two massive, star-forming galaxies in strong over-densities at redshift 0.75 with surprisingly high star formation efficiency and cold gas reserves. These 'disky titans' challenge models by suggesting they are rejuvenated after a merger, allowing them to restart intense star formation despite their large halo masses.

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 used across episodes

This episode discusses

The paper

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

F. Gentile, E. Daddi, D. Elbaz, A. Enia, F. Vito, P.-A. Duc, M. Franco, H. Fu, R. Giuffrida, D. Roberts6, F., S., L., O., G., C., D.', , M.', , B., , , G. Papini, L. Pozzetti, V. Sangalli, J. G. Sorce, L. Spinoglio22, V., Strazzullo15, M., Tarrasse1, G., Toni23, B., Altieri28, S., Andreon20, N. Auricchio, C. Baccigalupi26, M. Baldi30, S. Bardelli2, P. Battaglia2, A., Biviano15, E., Branchini32, M., Brescia34, S., Camera36, V., Capobianco38, C., Carbone39, J. Carretero40, M. Castellano42, S. Cavuoti35, A. Cimatti44, C. Colodro-Conde13, G., Congedo45, L. Conversi46, Y., Copin47, F. Courbin48, H. M. Courtois51, M. Cropper52, H. Degaudenzi53, H., Dole25, F., Dubath53, X., Dupac28, M., Farina22, R. Farinelli2, F. Faustini42, S., Ferriol47, S. Fotopoulou55, M. Frailis15, E., Franceschi2, M. Fumana39, S., Galeotta15, K. George56, B. Gillis45, C., Giocoli2, J., Gracia-Carpio57, A. Grazian12, F., Grupp57, S. Gwyn59, S. V. H., H. Hoekstra10, W. Holmes61, I. M. Hook62, F., Hormuth63, A., Hornstrup64, K. Jahnke66, M., Jhabvala67, B., Joachimi68, S., Kermiche69, A. Kiessling61, B. Kubik47, M., Kümmel58, M. Kunz70, H. Kurki-Suonio71, 72, A. M. C., Le Brun73, S., Ligori38, P., Lilje60, V., Lindholm71, 72, I. Lloro74, G., Mainetti75, O. Mansutti15, Marggraf76, M., Martinelli42, N. Martinet78, F. Marulli23, R., Massey79, E. Medinaceli2, S. Mei80, M. Melchior82, M., Meneghetti2, E., Merlin42, G. Meylan83, A., Mora84, M., Moresco23, L., Moscardini23, Nakajima76, C. Neissner85, S.-M. Niemi86, C., Padilla85, S. Paltani53, F., Pasian15, K. Pedersen87, W. J. Percival88, V., Pettorino86, G. Polenta54, M., Poncet91, L. A., Popa92, F., Raison57, A., Renzi93, J., Rhodes61, G. Riccio35, E. Romelli15, M. Roncarelli2, H. J. A. Rottgering10, B., Rusholme95, R., Saglia58, Z., Sakr96, 97, 98, D. Sapone99, P., Schneider76, Tschrabback100, A. Secroun69, E. Sihvola101, P., Simon76, C., Sirignano93, J., Skottfelt102, L., Stanco94, P., Tallada-Crespí40, A. N. Taylor45, H. I. Teplitz103, I., Tereno104, S., Toft106, Toledo-Moreo108, F., Torradeflot41, I., Tutusaus110, J., V. Valiviita71, T., Vassallo15, Verdoes Kleijn112, A. Veropalumbo20, Y. Wang95, J., Weller58, F. M. Zerbi20, E. Zucca2, M. Bolzonella2, M. Huertas-Company13, M., Sereno2, M.

Euclid Consortium · NOEMA

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.

Transcript

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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