Euclid Quick Data Release (Q1). Quenching precedes bulge formation in dense environments but follows it in the field

TL;DR

The paper analyzes the first Euclid Q1 data to investigate how galaxy quenching and morphological transformation depend on stellar mass and local environment. By classifying ~10^6 galaxies into four populations (star-forming discs, quiescent spheroids, star-forming spheroids, quiescent discs) using rest-frame colours and Sérsic indices, and mapping their abundances in the $M_ot{*}$–density plane via a tomographic $$log$_{10}(1+elta)$ density field, the authors reveal distinct evolutionary channels in field vs dense environments. In the field, bulge growth occurs largely on the main sequence followed by internal quenching, while in high-density regions quenching precedes morphological transformation, producing quiescent discs that later become bulge-dominated quiescent galaxies. Comparison with the EuclidFlagship-2 simulation supports a picture where halo mass and central/satellite status modulate the observed trends, linking environmental processes to quenching and structural evolution across cosmic time. These findings establish a baseline for environment-dependent galaxy evolution with Euclid data and set the stage for deeper analyses with future releases.

Abstract

(Abridged) The bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: the galaxy quenching and the morphological transformation. In this paper, we aim to understand the link between these 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 deg2 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 $M_\ast>10^{9.5} M_\odot$. We divide the sample into four sub-populations of galaxies, based on their star-formation activity and morphology. We then analyse the physical properties of these populations and their relative abundances in the stellar mass vs. 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. At the same time, star-forming bulge-dominated galaxies are more common in field regions, 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.

Figures (7)

• Figure 1: (First three panels:) Density field as estimated in Sect. \ref{['sec:densty']} for the three EDFs. The three panels show the density contrast parameter as a function of the redshift in each field. The horizontal solid, dashed, and dotted lines represent the median value of $\log_{10}(1+\delta)$, the $3$$\sigma$, and the $5$$\sigma$ levels, respectively. They separate the field, intermediate-density regions, and in high-density ones. (Last panel:) Distribution of the $\log(1+\delta)$ in the three EDFs. The dashed lines indicate the same levels as in the previous panels. The shaded area reports the area excluded from the analysis.
• Figure 2: Selection of star-forming and quiescent galaxies following the rest-frame $\mathrm{NUV}$--$r^{+}$--$J$ colour selection by Ilbert_10, with the quiescent galaxies located in the upper left part of the plot. The left panel reports the number of discy galaxies belonging to the two selected families, the right panel the number of spheroidal galaxies. The numbers in the two plots report how many galaxies belong to each of the four sub-populations.
• Figure 3: Some examples of galaxies selected according to the criteria presented in Sect. \ref{['sec:sub-populations']}. Starting from the upper left row and proceeding clockwise, the figure shows star-forming discs, quiescent spheroids, star-forming spheroids, and quiescent discs. All the cutouts have a 5 arcsec side and are realized by combining the images in the , , and filters through the algorithm by Lupton_04.
• Figure 4: The four sub-populations of galaxies in the SFR versus stellar mass plane. Each pixel reports the relative abundance of each population through the colour-code reported in the colour-bar (which is different for each panel). The white solid line reports the location of the main sequence of star-forming galaxies as parametrised by Q1-SP031, while the shaded area reports its intrinsic scatter of $\sigma\sim0.3\,\mathrm{dex}$.
• Figure 5: Relative abundances of the four sub-populations of galaxies analysed in this paper as a function of stellar mass and local density contrast $\log_{10}(1+\delta)$. Each bin has a fixed size of $0.175\,\mathrm{dex}$ on both axes. The three rows report the results in three redshift bins with a fixed size of $0.25$ in the range $0.25<z<1$. The dashed lines separate the different density regions (field, intermediate, and high) defined in Sect. \ref{['sec:densty']} and Fig. \ref{['fig:density_field']}.