Euclid Quick Data Release (Q1). A probabilistic classification of quenched galaxies

TL;DR

This work develops and validates a probabilistic framework to classify galaxies into ageing, quenched, and retired categories using average specific star-formation rates computed over $10^8$ and $10^9$ year timescales. By combining a posterior-integrated (probabilistic AD) approach with a model-driven method calibrated on IllustrisTNG simulations, the authors quantify how galaxy populations evolve with mass and redshift in the Euclid Q1 data. They find ageing to dominate at low and high masses, with retired galaxies prevalent at high masses and quenched systems peaking at intermediate masses; redshift trends show ageing increasing with time and retirement becoming more common in massive systems. The analysis also links these evolutionary classes to the mass-size-metallicity relation, where ageing galaxies tend to be disc-dominated with low metallicities, retired galaxies are compact and metal-rich, and quenched galaxies occupy an intermediate, more compact and chemically evolved state. Overall, the methods demonstrate Euclid's potential to unravel the physical drivers of quenching across cosmic time using photometric data complemented by simulations.

Abstract

Investigating what drives the quenching of star formation in galaxies is key to understanding their evolution. The Euclid mission will provide rich data from optical to infrared wavelengths for millions of galaxies, and enable precise measurements of their star formation histories. Using the first Euclid Quick Data Release (Q1), we developed a probabilistic classification framework that combines the average specific star-formation rate inferred over two timescales ($10^8,10^9$ yr) to categorise galaxies as `ageing' (secularly evolving), `quenched' (recently halted star formation), or `retired' (dominated by old stars). Two classification methods were employed: a probabilistic approach, which integrates posterior distributions, and a model-driven method, which optimises sample purity and completeness using IllustrisTNG. At $z<0.1$ and $M_\ast \gtrsim 3\times10^{8}\,M_\odot$, we obtain Euclid class fractions of 68-72\%, 8-17\%, and 14-19\% for ageing, quenched, and retired populations, respectively. Ageing and retired galaxies dominate at the low- and high-mass end, respectively, while quenched galaxies surpass the retired fraction for $M_\ast \lesssim 10^{10}\,\rm M_\odot$. The evolution with redshift shows increasing and decreasing fractions of ageing and retired galaxies, respectively. More massive galaxies usually undergo quenching episodes at earlier times than to their low-mass counterparts. In terms of the mass-size-metallicity relation, ageing galaxies generally exhibit disc morphologies and low metallicities. Retired galaxies show compact structures and enhanced chemical enrichment, while quenched galaxies form an intermediate population that is more compact and chemically evolved than ageing systems. This work demonstrates Euclid's great potential for elucidating the physical nature of the quenching mechanisms that govern galaxy evolution.

Figures (17)

• Figure 1: Distribution of IllustrisTNG galaxies across the plane defined by the average specific star-formation measured over the last $10^8$ (${\rm sSFR}\xspace_{8}$) and $10^9$ yr (${\rm sSFR}\xspace_{9}$). Quenched galaxies are found below the solid red line, whereas the dotted line delimits the retired domain. Ageing systems are located above both regions. The grey-shaded area denotes the forbidden region of the parameter space.
• Figure 2: IllustrisTNG ${\rm sSFR}\xspace_{\log(\tau)}$ true values versus the median value recovered by BESTA. Coloured maps denote the number of sources in each bin. Bottom to top red lines of every panel correspond to the running 5, 50, and 95 percentiles of ${\rm sSFR}\xspace_{\log(\tau)}$ as a function of the true value. Dashed and dotted black lines illustrate the one-to-one and 0.5 dex offset lines, respectively. Each panel includes at the top-right corner the fraction of sources whose true value lies within the 68 and 90% estimated credible intervals, respectively.
• Figure 3: Statistical distribution of the difference between the input IllustrisTNG values of dust extinction, $A_V$, stellar metallicity, $\log_{10}(Z_\ast / Z_\odot)$, and total stellar mass, $\log_{10}(M_\ast\xspace/ \rm {M_\odot}\xspace)$, versus the median value recovered by BESTA. Each panel includes the fraction of sources whose true value lies within the 68 and 90% estimated credible intervals, respectively.
• Figure 4: Retired (left), quenched (middle), and ageing (right) galaxy classification purity and completeness, as a function of the minimum probability threshold used to perform the classification. Each redshift sample at $z=0$, $z=0.3$, and $z=0.6$ is denoted by the blue, green, and purple lines, respectively. Coloured arrows denote the value of $P_{\rm min}(\rm Quenched)$ that maximises the $F$-score at each redshift (see Sect. \ref{['sec:quenched_selection']}).
• Figure 5: Distribution of IllustrisTNG galaxies across the ${\rm sSFR}\xspace_{9}$ versus ${\rm sSFR}\xspace_{8}$ plane. Left and right columns show the distribution based on the true (null values of ${\rm sSFR}$ are arbitrarily set to $\log_{10}({\rm sSFR}\xspace / \rm yr^{-1})=-15$) and inferred median values, respectively. Top row panels display the full sample RGB colour-coded by the probability of belonging to the retired, quenched, and ageing classes. Solid and dotted lines represent the demarcation lines given by Eqs. \ref{['eq:quenched_line']} and \ref{['eq:retired_line']}, respectively. Middle and bottom rows illustrate the two proposed approaches for selecting quenched galaxies (coloured points) by colouring in black with low transparency those systems classified as retired or ageing.