The SRG/eROSITA All-Sky Survey: A comprehensive X-ray analysis of the Hydra I galaxy cluster

TL;DR

This study presents a comprehensive X-ray analysis of the Hydra I cluster (Abell 1060) using eROSITA eRASS:4 and archival Chandra data to map the ICM out to $R_{200}$ and beyond, and to explore X-ray–non-thermal and optical/IR galaxy connections. The authors implement a multi-instrument data reduction and background modelling strategy, employing a modified $eta$-model to fit the surface brightness and a detailed CXB/PIB/nei+ICM spectral framework to extract thermodynamic profiles. They detect two weak ICM shocks near the central galaxy NGC 3311, multiple soft X-ray excesses in the outskirts correlated with the 2D galaxy distribution, and measure an average ICM temperature of $<k_ ext{B}T> \,=\,2.27^{+0.15}_{-0.11}$ keV and metallicity $<Z>\=0.19^{+0.05}_{-0.03} Z_\ obreak_
$, with a temperature profile broadly consistent with simulations between $0.28R_{200}$ and $R_{200}$. The outskirts show active baryon accretion and complex foreground structures, while the central region remains relaxed, supporting Abell 1060 as a transitional WCC system; the work demonstrates eROSITA’s capability to probe cluster outskirts and motivates follow-up with LOFAR, XRISM, and HUBS.

Abstract

The Hydra I galaxy cluster (Abell 1060) is a nearby example of a low-temperature cluster that exhibits intermediate cool core and non-cool core properties. We aim to extend the characterization of the intracluster medium (ICM) properties at least until $R_{200}$ and study the correlation between the X-ray emission and non-thermal emission within $R=0.15$$R_{500}$, and optical/IR galaxy distribution beyond $R_{200}$. We used data from the first four SRG/eROSITA All-Sky Surveys and an archival Chandra observation to image the X-ray emission from Abell 1060. We also used multiwavelength data from TGSS (radio), 2MASS (IR), and NED (optical) to investigate the non-thermal emission, 2D galaxy distribution, and its redshift evolution, respectively. The surface brightness and spectral analyses are also extended until 3$R_{200}$ and $R_{200}$, respectively, following a detailed cosmic X-ray background (CXB) analysis. Our fully corrected eROSITA image showcases a relaxed ICM morphology within $R_{500}$. We detect two weak ICM shocks with Mach number $M\approx 1.5$ near the central galaxy NGC 3311 that coincide with diffuse radio emission along the line of sight. Furthermore, we detect multiple soft X-ray excesses with high spatial correlation with the 2D optical galaxy distribution beyond $R_{200}$. In particular, the excess in the north has a significance of $5.8σ$ above the local CXB level. This suggests that Abell 1060's outskirts are actively accreting baryons. We also estimate the average ICM temperature and metallicity of $\langle k_\mathrm{B}T \rangle=2.27\substack{+0.15\\-0.11}\thinspace$ keV and $\langle Z\rangle=0.19\substack{+0.05\\-0.03}\thinspace Z_\odot$, respectively, from the 0.2-0.5$R_{500}$ annulus. Overall, the temperature profile is broadly consistent with the average temperature profiles from hydrodynamical simulation and Suzaku between 0.28$R_{200}$ and $R_{200}$.

Figures (19)

• Figure 1: Zoom-in on half sky eRASS:1 RGB image prepared using broadband TM8 maps in the energy ranges 0.4-0.6keV (red), 0.6-1.0keV (green), and 1.0-2.3keV (blue) from 2024AA...681A..77Z. The approximate FoV of our eRASS:4 image and other prominent sources in its vicinity are annotated on the image. The circles on the clusters represent their $R_{500}$.
• Figure 2: Combined X-ray (cyan-blue), radio (green), and optical/IR (RGB) overlay of the central region ($\approx$0.8$R_{500}$) of Abell 1060. Some of the prominent member galaxies are labeled on the image. The X-ray image is the 0.2-2.3keV fully corrected TM0 eROSITA image, the radio image is the 150 MHz TGSS image, and the optical image is the DSS2 RGB image (using infrared, red, and blue filters). The $Asinh$ scaling was applied to the X-ray and radio images to enhance them.
• Figure 3: Fully corrected and point source removed Chandra image in the 0.5-2.3keV band overlaid with TGSS radio contours. The image is displayed using a logarithmic scale and is smoothed by a Gaussian kernel of $\sigma=6$ pixels. The black circles mark the excised halos of NGC 3311 and NGC 3309.
• Figure 4: GGM filtered eROSITA images in the 0.2-2.3keV band. The kernel size is denoted in the top left corner of each image.
• Figure 5: GGM filtered (left) and unsharp masked (right) Chandra images in the 0.5-2.3keV band. The kernel sizes and combinations are denoted in the top left corner of each image.