Multiwavelength Characterization of a Dynamically Relaxed Cool Core Galaxy Cluster at $z=1.5$

Abstract

We present imaging and spectroscopic analyses of Chandra and XMM-Newton observations of ACT-CL J0123.5$-$0428, one of the most massive, highest redshift galaxy clusters detected within the survey fields of the Atacama Cosmology Telescope. The Chandra data are sufficient to characterize the morphology of this cluster and constrain the geometrically deprojected temperature in 2 spatial bins out to $r_{2500}$, revealing a dynamically relaxed system whose temperature drops to $kT = 1.8\pm0.6$ keV in the inner $\sim40$kpc. Within this same inner radius, the surface brightness and density of the ICM is sharply peaked, and the cooling time falls to $t_\mathrm{cool}=280^{+150}_{-120}$ Myr. A novel forward-modeling analysis of the XMM data extends imaging and spectroscopic measurements of this system out to $r_{500}$, constraining the redshift to $z=1.50\pm0.03$, with a mean temperature of $kT = 7.3\pm1.1$ keV and an emission-weighted mean metallicity of $Z/Z_\odot = 0.43^{+0.46}_{-0.25}$. We also utilize the limited optical/IR photometric coverage of the cluster to characterize the properties of the brightest cluster galaxy (BCG), which is coincident with the X-ray peak. Despite the high redshift and strong cool core, the BCG exhibits no signs of recent or ongoing star formation, suggesting AGN feedback has been acting persistently to stem star formation since $z\sim 2.5$. These measurements identify ACT-CL J0123.5$-$0428 as the highest redshift, dynamically relaxed, cool core galaxy cluster discovered to date, making it a premier target for future astrophysical and cosmological studies.

Figures (9)

• Figure 1: SZ cluster detections from the Atacama Cosmology Telescope (ACT; Hilton2009.11043) and the South Pole Telescope (SPT; Bleem1409.0850Bleem1910.04121Huang1907.09621), limited to $z>0.7$ and S/N $>5$. Filled markers represent systems with targeted X-ray observations as of Chandra Cycle 27/XMM Cycle 24.
• Figure 2: X-ray and optical images of ACT-CL J0123.5$-$0428. Left: Soft band (0.6--2.0 keV) 51.2ks Chandra image. Center: 0.4--4.0 keV stacked XMM Newton image of 15ks (MOS1/2) and 11.6ks (pn). Right: Legacy Survey DR10 $g, r, z$ image cutout of ACT-CL J0123. In each image, the $1\sigma$ uncertainty in the position of the cluster center (from the Chandra data) is marked with a green ellipse. Point sources identified with the CIAO tool wavdetect are marked with magenta circles of radius $2^{\prime\prime}$. The measured vaue of r$_{2500}$ is denoted by the dashed cyan circle (See Section \ref{['sec:deprojection']}). A $10^{\prime\prime}$ cutout of the center of the optical image is presented to highlight the alignment of the BCG with the X-ray cluster center (white cross).
• Figure 3: Symmetry, Peakiness and Alignment metrics describing the X-ray morphology of ACT-CL J0123, compared with a large sample of clusters Mantz1502.06020. ACT-CL J0123 (orange star) satisfies the criteria for relaxation, which is defined in terms of exceeding thresholds (dashed lines) in all 3 parameters. For the larger sample, blue circles (red crosses) show clusters classified as relaxed (unrelaxed).
• Figure 4: Top: pn spectrum of ACT-CL J0123, extracted from a circular region of radius 80$^{\prime\prime}$ centered on the cluster. The spectrum is binned in energy for clarity, with a finer binning around 2.7 keV (6.7 keV rest frame) to highlight the Fe K-$\alpha$ detection. The vertical dashed grey line indicates Fe K-$\alpha$ emission redshifted by $z_\mathrm{spec}=1.5$. Also plotted are the total model (blue) and individual model components, including the cluster (orange), X-ray foreground (green), cosmic X-ray background (red), quiescent particle background (QPB; purple) and pn out-of-time (OOT) events (brown). Gaps in the model curves and between data points correspond to the excluded energies in our fits due to fluorescence lines. Bottom: Best-fit model residuals to the pn data, normalized by their uncertainty.
• Figure 5: Density (left) and temperature (right) profiles for ACT-CL J0123 as measured by Chandra (red) and XMM (blue). Chandra is able to resolve more of the inner density profile, demonstrating its peakiness, and identifies the cool core. A $\beta$-model fit to the Chandra density (solid red curve) shows good agreement with XMM beyond the central bin, even at large radiii where Chandra cannot probe.