The SPT-Chandra BCG Spectroscopic Survey I: Evolution of the Entropy Threshold for Cooling and Feedback in Galaxy Clusters Over the Last 10 Gyr

Abstract: We present a multi-wavelength study of the brightest cluster galaxies (BCGs) in a sample of the 95 most massive galaxy clusters selected from South Pole Telescope (SPT) Sunyaev-Zeldovich (SZ) survey. Our sample spans a redshift range of 0.3 < z < 1.7, and is complete with optical spectroscopy from various ground-based observatories, as well as ground and space-based imaging from optical, X-ray and radio wavebands. At z~0, previous studies have shown a strong correlation between the presence of a low-entropy cool core and the presence of star-formation and a radio-loud AGN in the central BCG. We show for the first time that a central entropy threshold for star formation persists out to z~1. The central entropy (measured in this work at a radius of 10 kpc) below which clusters harbor star-forming BCGs is found to be as low as $K_\mathrm{10 ~ kpc} = 35 \pm 4$ keV cm$^2$ at z < 0.15 and as high as $K_\mathrm{10 ~ kpc} = 52 \pm 11$ keV cm$^2$ at z~1. We find only marginal (~1$\sigma$) evidence for evolution in this threshold. In contrast, we do not find a similar high-z analog for an entropy threshold for feedback, but instead measure a strong evolution in the fraction of radio-loud BCGs in high-entropy cores as a function of redshift. This could imply that the cooling-feedback loop was not as tight in the past, or that some other fuel source like mergers are fueling the radio sources more often with increasing redshift, making the radio luminosity an increasingly unreliable proxy for radio jet power. We also find that our SZ-based sample is missing a small (~4%) population of the most luminous radio sources ($\nu L_{\nu} > 10^{42}$ erg/s), likely due to radio contamination suppressing the SZ signal with which these clusters are detected.

• The paper demonstrates that a consistent low-entropy threshold governs cooling-induced star formation in BCGs over a 10 Gyr period.
• Using multi-wavelength analysis from optical, X-ray, and radio data, it quantifies entropy values evolving marginally from 35±4 to 52±11 keV cm².
• The study highlights distinct feedback mechanisms at high redshifts, questioning the role of AGN activity versus cluster mergers.

Evolution of the Entropy Threshold for Cooling and Feedback in Galaxy Clusters

This paper presents a detailed spectroscopic investigation of the brightest cluster galaxies (BCGs) in a sample of 95 galaxy clusters, which are among the most massive selected from the South Pole Telescope (SPT) Sunyaev-Zel'dovich (SZ) survey. The focus is on examining the evolution of the entropy threshold which governs cooling and feedback processes in these galaxies over a significant cosmic time span of 10 billion years.

The study employs multi-wavelength data, integrating optical spectroscopy, X-ray, and radio imaging, to assess the cooling processes in the intracluster medium (ICM) and resultant star formation in BCGs, which often correlate with radio-loud active galactic nuclei (AGN) presence at $z \sim 0$. Historically, a low-entropy core threshold has correlated with the activity levels in BCGs, implying that below a certain entropy, gas becomes unstable, precipitating star formation and AGN activity due to enhanced cooling. This paper breaks ground in proving the persistence of an entropy threshold for star formation out to $z \sim 1$.

Highlights from the numerical findings indicate that the entropy threshold remains largely consistent across time, with central ICM entropy values at a 10 kpc cluster radius ranging between $35 \pm 4$ keV cm$^2$ at $z < 0.15$ to $52 \pm 11$ keV cm$^2$ at $z \sim 1$. The analysis supports the proposition that these entropy thresholds for cooling are stable over a large portion of the observable universe’s timeline, exhibiting only a marginal ($\sim 1\sigma$) case for evolution.

A contrasting observation to note is the absence of a distinct entropy threshold linked to AGN feedback activity at high redshifts, as opposed to the clear correlations found at lower redshifts. This raises intriguing questions regarding the stability and mechanisms of cooling-feedback cycles in early epochs of cluster formation. The study suggests alternate interpretations, such as the potentially looser regulation of these cycles in the past or the impact of mergers providing an additional mechanism for AGN fueling at higher redshifts. Mergers are more frequent at these redshifts, potentially distorting the strict entropy correlations seen at low $z$.

An insight into the radio dataset reveals a noticeable absence of clusters associated with extremely high radio luminosity (exceeding $10^{42}$ erg s$^{-1}$). According to analyses, this may stem from a selection bias inherent in SZ surveys, potentially leading to these high-luminosity systems being undetected due to radio contamination filling in the SZ signal. However, the detection gap cannot account for the observed lack of an entropy-threshold dichotomy regarding radio AGN feedback.

In summary, this study reinforces the presence of a consistent entropy threshold for star formation across a substantial fraction of the universe's history while also contending with distinctions in feedback processes that challenge current understandings of cosmic cluster development. Future work leveraging such findings should aim to further elucidate the implications for cluster evolution, the role of mergers, and refine selection methodologies to account for potential biases in cosmological surveys.