The Atacama Cosmology Telescope: stellar mass growth in massive galaxy clusters from DR5 over the past 7 billion years

TL;DR

This work analyzes the assembly of stellar mass in 568 ACT DR5 SZ-selected clusters across $0.2<z<0.8$ using DECaLS DR10 photometry to construct redshift- and cluster-mass–binned composite SMFs down to $M_* = 10^{9.5}\,M_\odot$. By stacking SMFs and applying probabilistic cluster membership from full redshift posteriors, the authors fit both single Schechter and Schechter+Gaussian models and examine the cluster stellar mass–halo mass scaling. They find only mild evolution in the SMF characteristic mass $M^*$ and a redshift-dependent low-mass slope $\alpha$, with a more pronounced high-mass excess modeled by a Gaussian component, largely attributable to BCGs. The analysis shows that the SMF shape is largely mass-independent above $M_{200m} \sim 3\times10^{14}\,M_\odot$, while the cluster stellar mass fraction grows by a factor of $\sim3.3$ from $z\sim0.8$ to $z\sim0.2$, consistent with hierarchical growth and late-time accretion in dense environments.

Abstract

We probe the stellar mass growth in a sample of 568 Sunyaev-Zel'dovich (SZ) selected galaxy clusters with masses greater than $2.9 \times 10^{14} \mathrm{M_{\odot}}$ and redshifts in the range $0.2 < z < 0.8$, drawn from the fifth data release of the Atacama Cosmology Telescope (ACT DR5). By utilising deep photometry from the tenth data release of the Dark Energy Camera Legacy Survey (DECaLS DR10), we construct redshift- and cluster mass-binned composite cluster stellar mass functions (SMFs), down to $M_* = 10^{9.5} \mathrm{M_{\odot}}$. This work presents the first analysis of the cluster SMF for this specific cluster sample at this epoch. We find that the characteristic stellar mass ($M^*$) of the cluster SMF evolves marginally from $0.55 \leq z < 0.8$, with most of the measurable growth occurring at $ 0.2 < z < 0.55$. This suggests that most of the massive galaxy population in clusters ($M_* \gtrsim 10^{10.75} \mathrm{M_{\odot}}$) is largely established by $z \sim 0.8$, with subsequent evolution driven primarily by late-time assembly processes. The low-mass slope ($α$) of the composite cluster SMF is flat at high-$z$ ($z \sim 0.8$) but steepens at $z < 0.55$, suggesting an abundance of massive galaxies in high-$z$ clusters compared to low-$z$ clusters. Redshift evolution of cluster stellar mass fractions ($f^{\mathrm{cg}}_{*}$) suggest that cluster stellar mass (from galaxies with $M_* > 10^{9.5} \mathrm{M_{\odot}}$) has grown by a factor of $3.3$ since $z = 0.8$.

Figures (13)

• Figure 1: The distribution of log cluster mass ($\log _{10}[M_{\mathrm{200m}}/\mathrm{M_{\odot}}]$) with redshift for the ACT DR5 cluster sample (shown as grey circles), and the cluster sample selected in this work (show as light blue triangles). Our cluster sample has been selected based on the optical depth in DECaLS DR10 (see Section \ref{['subsec:Completeness']}. The redshift (Top panel) and cluster mass (Right panel) normalised distributions for each sample are also displayed.
• Figure 2: A mass-limited sample obtained for a galaxy field (RA $= 45^{\circ}.0$; Dec $= -51^{\circ}.0$) in DECaLS DR10 that also has DES coverage. This galaxy sample is used to gauge a consistent stellar mass depth in DECaLS DR10. Blue points represent all galaxies within the field region. The orange points in the shaded area show the galaxies above the stellar mass limit (log$_{10}[M_{*}/\mathrm{M_{\odot}}] = 9.5$) and within our selected redshift range for this work ($0.2 < z < 0.8$). We select cluster galaxies above this mass limit for completeness.
• Figure 3: The distribution of cluster mass ($M_{\mathrm{200m}}$) for the total cluster sample (shown in black). Also represented are the top (shown in red) and bottom (shown in blue) terciles of the cluster mass distribution for which we measure the composite SMF (see Section \ref{['subsec:SingleSchechter']}).
• Figure 4: The best-fit composite cluster SMFs for 2 redshift bins, $\langle z \rangle = 0.325$ and $\langle z \rangle= 0.625$. Black circles represent the SMF for all cluster galaxies in the redshift bin. Blue diamonds and red squares represent the SMF of cluster galaxies in the low- and high-mass cluster mass subsets, respectively. Error bars show the $1\sigma$ uncertainties calculated using bootstrap resampling. A solid line shows a best-fit single Schechter model for each SMF. The composite SMFs for all 12 redshift bins are shown in Fig. \ref{['fig:SinglSchechterSMFs']}. Parameter values of the best-fit are presented in Table \ref{['tab:smf_table']}. Deviations in stellar mass above the Schechter function are observed at the high-mass end. The shape of both cluster mass subset SMFs is highly comparable to the composite SMF of all clusters in the redshift bin.
• Figure 5: The single Schechter models to the 12 total redshift binned composite SMFs. Each model has been normalised by a common log stellar mass, $\log_{10}[M_*/\mathrm{M_{\odot}}] = 10.75$, in order to emphasise the evolution of the low-mass slope. The colour bar is used to represents the model evolution with redshift.