Galaxy clusters from the DESI Legacy Imaging Surveys -- III. Star-forming fraction of brightest cluster galaxies

TL;DR

This work addresses how star formation in Brightest Cluster Galaxies evolves within clusters by exploiting DESI Legacy Imaging Surveys photometry and DESI spectroscopic data to measure the star-forming fraction $F_{\,mathrm{sf}}$ for BCGs in the range $0.1<z<0.8$, contrasted with field galaxies. The authors identify star-forming systems using the color boundary in $g-z$ calibrated to BC03 templates and examine $F_{\,mathrm{sf}}$ as a function of redshift $z$, halo mass $M_{200}$, and stellar mass $M_\ ext{\star}$, including a relative fraction to control selection effects. They find that $F_{\,mathrm{sf}}$ in BCGs increases with redshift, while it decreases with halo mass and stellar mass; low-mass BCGs show enhanced star formation relative to field galaxies. A two-component model suggests cooling flows fuel SF at low redshift and gas-rich mergers dominate at higher redshift, with a transition around $z\sim0.5$ that coincides with the rapid rise in $F_{\,mathrm{sf}}$. These results illuminate the evolving growth channels of BCGs and the interplay between cooling, mergers, and environment in shaping star formation in cluster centers.

Abstract

This study investigates the evolution of the star-forming fraction ($F_{\mathrm{sf}}$) of Brightest Cluster Galaxies (BCGs) at $z<0.8$, using the galaxy clusters identified from the Legacy Imaging Surveys from the Dark Energy Spectroscopic Instrument (DESI). Star-forming galaxies are identified using the $g-z$ color, and $F_{\mathrm{sf}}$ is measured as a function of redshift, cluster halo mass, and galaxy stellar mass. Field galaxies are used as a comparison sample to reduce selection effects. For BCGs, $F_{\mathrm{sf}}$ increases with redshift, showing a slow rise below $z \sim 0.4 - 0.5$ and a more rapid increase above this range. In contrast, $F_{\mathrm{sf}}$ decreases with increasing cluster halo mass and BCG stellar mass. At the low stellar mass end, BCGs exhibit higher star-forming fractions than field galaxies, suggesting enhanced star formation likely fueled by cold gas accretion from the intracluster medium. Also, star-forming BCGs tend to show larger projected offsets from the optical cluster density peak than quenching BCGs, indicating ongoing assembly. The analysis of the specific star formation rate (sSFR) further indicates a transition in the dominant mechanism driving star formation in BCGs: cooling flows are likely responsible at low redshift, while gas-rich mergers play a greater role at higher redshift. The shift in dominance occurs around $z \sim 0.5$, aligning with the steep rise in $F_{\mathrm{sf}}$ of BCG.

Figures (14)

• Figure 1: Top panel: the comparison between $z_{\mathrm{spec}}$ and $z_{\mathrm{photo}}$. The gray scale represents the density. The red solid line represents $z_{\mathrm{photo}} = z_{\mathrm{spec}}$. The red dashed line represents $|\Delta z_{\mathrm{norm}}| = 0.1$, and $\eta_{0.1}$ represents the fraction of galaxies outside the red dashed line. The vertical blue solid line represents the cutoff of the photo-z. The values of the three quantities (bias, dispersion and outlier rate) are marked in blue in the lower right corner. Bottom panel: $\Delta z_{\mathrm{norm}}$ as a function of $z_{\mathrm{photo}}$. The red solid line and dashed line represent $\Delta z_{\mathrm{norm}} = 0$ and $|\Delta z_{\mathrm{norm}}| = 0.1$, respectively.
• Figure 2: $\sigma_{\Delta z_{\mathrm{norm}}}$ as a function of $z_{\mathrm{photo}}$. The red solid line represents the average $\sigma_{\Delta z_{\mathrm{norm}}}$ for all galaxies that have both photo-z and spec-z, while the blue solid line represents the average $\sigma_{\Delta z_{\mathrm{norm}}}$ for the galaxies selected after applying the photo-z cut at $z_{\mathrm{photo}}<0.8$.
• Figure 3: Top panel: the stellar mass comparison between the DESI catalog and the MPA-JHU catalog. The red dashed line represents $M_{\star\mathrm{DESI}} = M_{\star\mathrm{MPA-JHU}}$. The red solid line represents the relation of stellar mass between the two catalogs. Bottom panel: residuals as a function of $M_{\star}$. The red dashed line represents the bias of stellar mass between the two catalogs. The gray scale represents the density.
• Figure 4: $g-z$ color as a function of redshift for two subsamples. The black solid line represents the BC03 template with $\tau = 2 \mathrm{Gyr}$BC03. Galaxies below the solid black line are classfied as star-forming galaxies. The color scale represents the density.
• Figure 5: The evolution of the $F_{\mathrm{sf}}$ as a function of redshift for BCGs and field galaxies. The solid line represents the relative $F_{\mathrm{sf}}$ for BCGs. The Poisson errors are too small to be ignored.