Louis Quilley

We present a detailed visual morphology catalogue for the Euclid Quick Release 1 (Q1). Our catalogue includes galaxy features such as bars, spiral arms, and ongoing mergers, for the 378 000 bright ( I _E < 20.5) or extended (area ≥700 pixels) galaxies in Q1. The catalogue was created by finetuning the Zoobot galaxy foundation models on annotations from an intensive one-month campaign by Galaxy Zoo volunteers. Our measurements are fully automated, and hence fully scaleable. This catalogue is the first 0.4% of the approximately 100 million galaxies where Euclid will ultimately resolve detailed morphology.

The well-known bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: galaxy quenching, causing the strong reduction of star formation, and morphological transformation, causing the transition from disc-dominated structures to bulge-dominated ones. In this paper, we aim to understand the link between these two processes and their relation with the stellar mass of galaxies and their local environment. Taking advantage of the first data released by the Euclid Collaboration, covering more than 60 deg ² with space-based imaging and photometry, we analyse a mass-complete sample of nearly one million galaxies in the range 0.25 < z < 1 with M _* > 10 ^9.5 M _⊙ , using a combination of photometric and spectroscopic redshifts. We divide the sample into four sub-populations of galaxies, based on their star-formation activity (star-forming and quiescent) and morphology (disc-dominated and bulge-dominated). We then analyse the physical properties of these populations and their relative abundances in the stellar mass versus local density plane. Together with confirming the passivity-density relation and the morphology-density relation, we find that quiescent discy galaxies are more abundant in the low-mass regime of high-density environment where log ₁₀ (1 + δ ) > 1.3. At the same time, star-forming bulge-dominated galaxies are more common in field regions with log ₁₀ (1 + δ ) < 0.8, preferentially at high masses. Building on these results and interpreting them through comparison with simulations, we propose a scenario where the evolution of galaxies in the field significantly differs from that in higher-density environments. The morphological transformation in the majority of field galaxies takes place before the onset of quenching and is mainly driven by secular processes taking place within the main sequence, leading to the formation of star-forming bulge-dominated galaxies as intermediate-stage galaxies. Conversely, quenching of star formation precedes morphological transformation for most galaxies in higher-density environments. This causes the formation of quiescent disc-dominated galaxies before their transition into bulge-dominated ones.

Context. One of the most outstanding questions in contemporary astrophysics is: What is the significance of galaxy morphology? What physical processes underlie the morphologies we observe and is a galaxy’s internal structure shaping its evolution (nature) or is it a sign of the external processes which drive galaxy evolution (nurture)? Aims. We aim to understand the color dichotomy and gradients in bulges and disks along the Hubble sequence. Methods. We fit Sérsic functions to the 2D light distributions in the g , r , and i bands to bulges and disks of the large EFIGI (Extraction de Formes Idéalisées de Galaxies en Imagerie) sample of galaxies with high-quality morphological classifications. Results. In early-type galaxies, bulges and disks have similarly red and nearly uniform colors. Disks become significantly bluer with increasing lateness of their types and bulges get slightly redder because of patchy dust. Disks have increasingly blue colors with increasing radius, whereas dust extinction and scattering leads to smaller effective radii of the bulges and lower steepness of the best-fit Sérsic functions in g versus i . This impact depends on Hubble type, with the bulges of intermediate-type spirals (Sb-Sc) having the reddest mean colors, the largest scatter in their colors, and the largest mean and scatter in their color gradients. Similarly to bulges, disks of the intermediate-type galaxies show the strongest color gradients. The variations in bulge gradients appear to result from the change in the Hubble type rather than in the total galaxy stellar mass. Conclusions. We interpret these properties of the bulges and disks of intermediate type spirals as being due to dust extinction and scattering, which we hypothesize to be an indicator of the gas content and inflow of gas. If early-type galaxies do evolve from massive spiral galaxies, typically intermediate-type spirals, these color gradients are signs of in situ stellar growth within their bulges, likely driven by morphological structure in their disks. These results favor secular evolution (nature) as the primary driver of galaxy evolution in the local Universe.

The background galaxies in Euclid Early Release Observation images of the Perseus cluster make up a remarkable sample for the combination of a 0.57 deg ² area, 25.3 and 23.2 AB mag depth, and angular resolutions in the optical and near-infrared bands of 0 _⋅ ^′′ 1 and 0 _⋅ ^′′ 3, respectively. As part of the effort towards characterising the history of the Hubble sequence, we performed a morphological analysis of 2445 and 12 786 galaxies with I _E ≤ 21 and I _E ≤ 23, respectively. We used single-Sérsic profiles and the sums of a Sérsic bulge and an exponential disc to model these galaxies with SourceXtractor++ and analysed their positional, structural, and flux parameters in order to assess their similarities and differences. The fitted galaxies to I _E ≤ 21 span the various Hubble types with ubiquitous bulge and disc components and a bulge-to-total light ratio ( B/T ) that takes all values from 0 to 1. The effective radius of the single-Sérsic profile is an intermediate estimate of galaxy size (between the bulge and disc effective radii) depending on B/T . The axis ratio of the single-Sérsic profile is higher than the disc axis ratio, and this difference increases with B/T . The choice of model impacts the photometry with −0.08 to 0.01 mag median systematic I _E offsets between single-Sérsic and bulge-disc total magnitudes and a 0.05–0.15 mag dispersion from low to high B/T . We measured a median 0.3 mag bulge-disc colour difference in rest-frame M _g − M _i that originates from the disc-dominated galaxies, whereas bulge-dominated galaxies have median colours similar to those of their components. Remarkably, we also measured redder inside disc colour gradients based on 5 to 10% systematic variations of disc effective radii between the optical and near-infrared bands. This analysis demonstrates the usefulness and limitations of single-Sérsic profile modelling and the power of bulge-disc decomposition for characterising the morphology of lenticulars and spirals in Euclid images. We make available the catalogues of best-fit parameters for the morphological and SED fits.

La structure et l'évolution des galaxies par leurs bulbes et disques dans l'Univers proche: L’astronome Edwin Hubble classa les galaxies selon leurs formes, créant ainsi la séquence de Hubble, qui décrit les classes morphologiques des elliptiques (E), des lenticulaires (S0), des spirales (de Sa à Sm) et des irrégulières (Im). La formation de la séquence de Hubble pose un défi aux théories de formation des galaxies, qui doivent expliquer comment une telle diversité d’objets a pu se former. Répondre à cette question demande de caractériser la morphologie d’échantillons représentatifs de galaxies afin de comprendre leur histoire de formation. Dans cette thèse, je développe avec SourceXtractor++ une procédure de modélisation fiable des profils de luminosité des composantes dominantes des galaxies, les bulbes et les disques, pour les 4458 galaxies proches du catalogue morphologique EFIGI. Avec ZPEG, j'ajuste des spectres sur leurs flux multi-bandes afin de déterminer leurs masses stellaires et leurs taux de formation d’étoiles. Dans le diagramme de couleur absolue NUV−r versus masse, je retrouve la bimodalité en couleur et la zone intermédiaire de la Vallée Verte, et montre qu’elles sont parcourues par les types morphologiques successifs de la séquence de Hubble. Les galaxies situées dans la Vallée Verte subissent un rougissement systématique du disque et une croissance marquée du bulbe par un facteur de 2 à 3, ce qui exclut un scénario d’extinction rapide. En examinant les bulbes, disques, barres, anneaux et bras spiraux, nous dressons un schéma de l’évolution des galaxies suggérant que la dynamique interne, sous l’effet d’interactions ou de fusions, pourrait être la clé du vieillissement des galaxies. La séquence de Hubble peut alors être considérée comme une séquence inverse de l’évolution physique des galaxies. Je réexamine les relations d’échelle entre la taille, la luminosité et la brillance de surface des galaxies, bulbes et disques en fonction de leur morphologie. Je montre que la relation de Kormendy pour les galaxies elliptiques s’étend aux bulbes des types S0 à Sb, tandis que les bulbes moins brillants et plus petits des types plus tardifs s’en écartent, avec une diminution du rapport bulbe-sur-total (B/T) et des indices de Sérsic. Je mets en évidence une transition continue des pseudo-bulbes aux bulbes classiques, plutôt qu’une dichotomie entre ces deux classes. Je dérive de nouvelles relations convexes entre taille et luminosité pour les bulbes et disques des galaxies EFIGI, améliorant l’estimation de leur taille à toutes les magnitudes. Cet aspect est crucial pour générer des images synthétiques réalistes comprenant tous les types de galaxies existants, qui sont nécessaires pour contrôler les effets systématiques liés à la forme des galaxies dans les mesures qui sont déduites des grands relevés de galaxies, tels qu'avec Euclid. De plus, j’obtiens le résultat inédit que les rayons effectifs des bulbes et des disques des spirales ou des lenticulaires augmentent selon des lois de puissance du B/T. L’analyse du catalogue morphologique EFIGI peut être étendue à MorCat, un relevé limité en magnitude apparente à g≤15.5, dont EFIGI est un sous-échantillon de fréquence comparable des types de Hubble, ce qui permet de dériver des distributions des paramètres morphologiques représentatives de l’Univers proche. Cette étude des galaxies proches dresse un scénario d’évolution pour les types de Hubble paramétré par les propriétés des bulbes et des disques. Toutefois, l’analyse directe de la morphologie en fonction du temps cosmique est nécessaire pour comprendre comment la séquence de Hubble s’est mise en place. Pour cela, ma méthodologie peut être adaptée à d’autres relevés de galaxies, comme avec Euclid, ou les champs profonds du CFHT Legacy Survey ou de Hyper-Suprime Cam. Je compte également localiser les types morphologiques parmi les feuillets, filaments et nœuds de la toile cosmique, afin d’explorer le rôle de l’environnement local et à grande échelle dans les transformations du Nuage Bleu vers la Séquence Rouge.

The Perseus field captured by Euclid as part of its Early Release Observations provides a unique opportunity to study cluster environment ranging from outskirts to dense regions. Leveraging unprecedented optical and near-infrared depths, we investigate the stellar structure of massive disc galaxies in this field. This study focuses on outer disc profiles, including simple exponential (Type I), down-bending break (Type II) and up-bending break (Type III) profiles, and their associated colour gradients, to trace late assembly processes across various environments. Type II profiles, though relatively rare in high dense environments, appear stabilised by internal mechanisms like bars and resonances, even within dense cluster cores. Simulations suggest that in dense environments, Type II profiles tend to evolve into Type I profiles over time. Type III profiles often exhibit small colour gradients beyond the break, hinting at older stellar populations, potentially due to radial migration or accretion events. We analyse correlations between galaxy mass, morphology, and profile types. Mass distributions show weak trends of decreasing mass from the centre to the outskirts of the Perseus cluster. Type III profiles become more prevalent, while Type I profiles decrease in lower-mass galaxies with cluster centric distance. Type I profiles dominate in spiral galaxies, while Type III profiles are more common in S0 galaxies. Type II profiles are consistently observed across all morphological types. While the limited sample size restricts statistical power, our findings shed light on the mechanisms shaping galaxy profiles in cluster environments. Future work should extend observations to the cluster outskirts to enhance statistical significance and explore looser environments. Additionally, 3D velocity maps are needed to achieve a non-projected view of galaxy positions, offering deeper insights into spatial distribution and dynamics.

The Euclid Early Release Observations (ERO) programme targeted the Perseus cluster of galaxies, gathering deep data in the central region of the cluster over 0.7 deg², including the cluster core up to 0.25 r₂₀₀. The dataset reaches a point-source depth of I_E = 28.0 (Y_E, J_E, H_E = 25.3), AB magnitudes at 5 σ with a 0′′.16 (0′′.48) full width at half maximum (FWHM), and a surface brightness limit of 30.1 (29.2) mag arcsec⁻² for radially integrated galaxy profiles. The exceptional depth and spatial resolution of this wide-field multi-band data enable simultaneous detection and characterisation of both bright galaxies and low surface brightness ones, along with their globular cluster systems, from the optical to the near-infrared (NIR). Cluster membership was determined using several methods in order to maximise the completeness and minimise the contamination of foreground and background sources. We adopted a catalogue of 1100 dwarf galaxies, detailed in the corresponding ERO paper, that includes their photometric and structural properties. We identified all other sources in the Euclid images and obtained accurate photometric measurements using AutoProf or AstroPhot for 137 bright cluster galaxies and SourceExtractor for half a million compact sources. This study advances beyond previous analyses of the cluster and enables a range of scientific investigations, which are summarised here. We derived the luminosity and stellar mass functions (LF and SMF) of the Perseus cluster in the Euclid I_E band thanks to supplementary u, g, r, i, z, and Hα data from the Canada-France-Hawai’i Telescope (CFHT). Our LF and SMF are the deepest recorded for the Perseus cluster, highlighting the groundbreaking capabilities of the Euclid telescope. We fit the LF and SMF with a Schechter plus Gaussian model. The LF features a dip at M(I_E) ≃ −19 and a faint-end slope of α_S ≃ −1.2 to −1.3. The SMF displays a low-mass-end slope of α_S ≃ −1.2 to −1.35. These observed slopes are flatter than those predicted for dark matter halos in cosmological simulations, offering significant insights for models of galaxy formation and evolution.

The Euclid Early Release Observations (ERO) showcase Euclid’s capabilities in advance of its main mission by targeting 17 astronomical objects, including galaxy clusters, nearby galaxies, globular clusters, and star-forming regions. A total of 24 hours of observing time was allocated in the early months of operation, and the scientific community was engaged through an early public data release. We describe the development of the ERO pipeline to create visually compelling images while simultaneously meeting the scientific demands within months of launch by leveraging a pragmatic data-driven development strategy. The pipeline’s key requirements are to preserve the image quality and to provide flux calibration and photometry for compact and extended sources. The pipeline’s five pillars are removal of instrumental signatures, astrometric calibration, photometric calibration, image stacking, and the production of science-ready catalogues for both the VIS and NISP instruments. We report a point spread function (PSF) with a full width at half maximum of 0ʺ.16 in the optical I_E-band and 0ʺ.49 in the near-infrared (NIR) bands Y_E, J_E, and H_E. Our VIS mean absolute flux calibration is accurate to about 1%, and the accuracy is 10% for NISP due to a limited calibration set; both instruments have considerable colour terms for individual sources. The median depth is 25.3 and 23.2 AB mag with a signal-to-noise ratio (S/N) of ten for galaxies, while it is 27.1 and 24.5 AB mag at an S/N of five for point sources for VIS and NISP, respectively. Euclid’s ability to observe diffuse emission is exceptional due to its extended PSF nearly matching a pure diffraction halo, the best ever achieved by a wide-field high-resolution imaging telescope. Euclid offers unparalleled capabilities for exploring the low-surface brightness (LSB) Universe across all scales, providing high precision within a wide field of view (FoV), and opening a new observational window in the NIR. Median surface-brightness levels of 29.5 and 27.9, AB mag arcsec⁻² are achieved for VIS and NISP, respectively, for detecting a 10ʺ × 10ʺ extended feature at the 1 σ level.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg² of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

We present a detailed visual morphology catalogue for Euclid's Quick Release 1 (Q1). Our catalogue includes galaxy features such as bars, spiral arms, and ongoing mergers, for the 378000 bright ($I_E < 20.5$) or extended (area $\geq 700\,$pixels) galaxies in Q1. The catalogue was created by finetuning the Zoobot galaxy foundation models on annotations from an intensive one month campaign by Galaxy Zoo volunteers. Our measurements are fully automated and hence fully scaleable. This catalogue is the first 0.4% of the approximately 100 million galaxies where Euclid will ultimately resolve detailed morphology.