SPT-CL J0417-4748: A Deep Chandra Study of a Relaxed Galaxy Cluster Without Central Star Formation

TL;DR

This study presents a deep Chandra X-ray analysis of the z = 0.58 galaxy cluster SPT J0417-4748, a massive, dynamically relaxed cool-core system with a BCG showing negligible recent star formation. By combining a total X-ray exposure of $103$ ks with multiwavelength data, the authors derive robust thermodynamic profiles, pseudo-entropy, cooling time, and a hydrostatic NFW mass model, demonstrating that the cluster hosts a pronounced cool core (central $n_e\approx0.08\ \mathrm{cm^{-3}}$, $K_c\approx26\ \mathrm{keV\,cm^{2}}$, $t_{cool}\approx515\ \mathrm{Myr}$) and yet exhibits a suppressed star formation rate ($\mathrm{SFR} < 3.8\ M_\odot\ \mathrm{yr^{-1}}$). The analysis shows $t_{cool}/t_{ff}>10$ at all radii and a classical cooling rate of $\dot{M}_{cool} \approx 560\ M_\odot\ \mathrm{yr^{-1}}$ within $r_{cool}\approx85$ kpc, implying a highly effective self-regulated AGN feedback cycle that quenches cooling-induced star formation. The gas fraction and NFW concentration ($f_{gas}\approx0.13$, $c\approx4.9$) and the placement on Mantz2016 scaling relations indicate SPT J0417 is representative of massive, relaxed cool-core clusters, providing key constraints on how AGN feedback governs the long-term thermal balance of the ICM in such systems.

Abstract

We present an in-depth Chandra X-ray analysis of the galaxy cluster SPT-CL J0417$-$4748 (hereafter SPT J0417), at z = 0.58, with a focus on its thermodynamic properties and the apparent absence of central star formation. Utilizing a total Chandra exposure of 103 ks, we find that the large-scale X-ray morphology is consistent with a dynamically relaxed, cool-core system. The intracluster medium (ICM) shows a central density of 0.08+/-0.01 cm^{-3}, a central pseudo-entropy of 26^{+6}_{-5} keVcm^{2} and a central cooling time of 515^{+96}_{-75} Myr, values typical of massive cool-core clusters. Despite these conditions, no evidence of recent or ongoing star formation is detected in the brightest cluster galaxy (BCG). Spectral energy distribution (SED) fitting of DES photometry indicates that the bulk of the stellar population formed at z~1.25, with no significant star formation over the past ~3 Gyr, while optical spectra from Magellan show no [O II] emission. Complementary ASKAP radio and Spitzer infrared data indicate a lack of strong current AGN activity in the BCG. SPT J0417 exemplifies massive, relaxed, cool-core clusters in which cooling and star formation appear almost completely quenched, providing valuable insights into how AGN feedback regulates the long-term thermal balance of the intracluster medium.

Figures (8)

• Figure 1: The left panel presents a 0.6–7.0 keV Chandra X-ray image of the cluster's central region, constructed from raw pixels of $0.492\times0.492$$\rm{arcsec^{2}}$ and smoothed with a Gaussian filter of 2-pixel (0.984 arcsec) radius. The red crosses indicate the location of the BCG. The SPA center from the morphological analysis is within $0.5^{\prime\prime}$ of the BCG center. The right panel displays the Euclid image Euclid2405.13491, revealing a central red giant elliptical galaxy with other similarly red galaxies nearby.
• Figure 2: Distribution of X-ray peakiness ($p$), symmetry ($s$), and alignment ($a$) for clusters from Mantz2015. The dashed lines indicate the threshold used to define the relaxed sample. Clusters meeting all three criteria are shown in green, while non-relaxed clusters are plotted in blue. The red point marks SPT J0417, which lies well within the relaxed region.
• Figure 3: The left panel displays the optical image of the cluster from Euclid Euclid2405.13491 for reference, with red crosses indicating the position of the BCG and green crosses marking the locations of detected radio sources. The middle panel shows the 4.5 $\mu$m infrared image obtained with Spitzer, providing a view of the stellar and dust content. The right panel presents the 1.4 GHz radio image from ASKAP with the beam size shown in the bottom left. Several radio sources are detected near the cluster center, but none is coincident with the BCG; instead, each is spatially associated with an optical counterpart confirmed to be a cluster member, making an association with the BCG unlikely. From these data, we derive a $3\sigma$ upper limit on the BCG radio luminosity of $\sim\!1\times10^{40}~\rm{erg~s^{-1}}$.
• Figure 4: Top: Observed-frame optical photometry of the BCG in SPT J0417 (DECam $g,r,i,z,Y$; red points with error bars) compared with the best-fit simple stellar population (SSP) model derived using prospector. The model spectrum is shown in blue, with the corresponding synthetic photometry indicated by blue squares. The relative transmission curves of each filter are shown in gray for reference. Bottom: Residuals normalized to the photometric uncertainties. The fit is consistent with an old stellar population in the BCG, with little or no evidence of star formation within the last $\sim\!3$ Gyr.
• Figure 5: Deprojected ICM thermodynamic profiles of SPT J0417. Left: Electron density profile, with the gray dashed line marking $R_{500}$. Middle: Temperature profile showing a clear increase in temperature from the core to the outskirts. Right: Metallicity profile, displaying an enhancement near the cluster core, while the outer regions are consistent with an average metallicity of $\sim\!0.3$ solar Mernier2017.