Deep Chandra X-ray Observations of Abell 2029: the Merger History of a Relaxed, Strong Cool Core Cluster

TL;DR

Deep 485 ks Chandra data of Abell 2029 reveal a striking sloshing spiral extending ~600 kpc, a SE splash wake, and a NW weak shock, all indicative of a late-time minor, off-axis merger. A 1:10 mass-ratio merger simulation with b = 500 kpc reproduces these features at ∼4.3 Gyr after the initial passage, supporting a coherent merger history for A2029. Spectral maps show a multi-temperature ICM with a cool, metal-rich sloshing component and a hot shock region, with a cooling luminosity L_cool ≈ 2.3×10^45 erg s⁻¹ exceeding the AGN power ∼(1–3)×10^44 erg s⁻¹, implying large-scale motions contribute to heating. The optical galaxy distribution and nearby bound groups corroborate a dynamically evolving environment, demonstrating that even the most relaxed clusters can hide a rich dynamical past that shapes their thermodynamics and feedback processes.

Abstract

We present results from very deep (485 ks) Chandra X-ray observations of the relaxed, cool core cluster Abell 2029 (z = 0.0767). A2029 hosts one of the longest, most continuous sloshing spirals ever observed, which we find extends nearly 600 kpc from the cluster core. In addition to providing detailed views of the sloshing spiral, imaging and spectroscopic analysis reveals ICM substructure related to the merger history including a broad ``splash'' of cooler gas and a potential merger shock. The radio lobes of the central WAT source show evidence of alignment with the sloshing motions, consistent with ICM bulk flow, rather than host-galaxy motion, being the primary driver of lobe bending. Comparison to a 1:10 mass-ratio off-axis merger simulation indicates that the observed ICM structures are relics of a second core passage of a subcluster ~4 Gyr after the start of the merger, where the ``splash'' feature is revealed to be a wake of cool gas trailing behind the subcluster. Overall, our results suggest that A2029 is still settling from past interactions -- showing that even the initially most relaxed-looking clusters can be hiding a rich history of dynamical activity.

Figures (23)

• Figure 1: Left: Background subtracted and exposure corrected X-ray image of A2029 in the 0.7-7 keV range, smoothed with a $\sigma = 1\farcs5$ Gaussian, with point sources included, covering a 40$\times$ 40 area, the full FoV of the observations. Right: Diffuse emission image of A2029, with point sources removed, in the same energy band, smoothed with a Gaussian of radius 15, and showing the inner 5$\times$ 5 region. A foreground, edge-on spiral galaxy, seen in absorption, is indicated with a white arrow. Note: $1\arcmin = 87$ kpc.
• Figure 2: False color optical image of A2029 taken from the Legacy Survey Sky Browser, comprised of SDSS g-, r-, and z-band images. The image has a scaling of 05/px. Red ellipses mark X-ray point sources detected by wavdetect. The 1.4 GHz VLA FIRST radio emission is shown with cyan contours which are drawn at levels of 0.0005, 0.0036, 0.0129, 0.0283, and 0.05 Jy beam$^{-1}$ and smoothed with a Gaussian kernel of width 4 pixels. The central red rectangle denotes the foreground spiral galaxy that is seen in absorption in the X-ray imaging (see Fig.\ref{['fig:xray']}). Inset are thumbnails showing zoom-ins of some radio sources in the field. The central WAT source of A2029 is shown in a $5\arcmin\times 5\arcmin$ inset thumbnail (a; upper right). The smaller radio source to the SW, is shown in a 1$\times$ 1 thumbnail (b; middle right). The southernmost source is shown in a 2$\times$2 thumbnail (c; lower right), with the detected X-ray point source indicated by a red circle. The two southern sources are discussed in more detail in §\ref{['sec:optical']}. Image credits: Legacy Surveys / D. Lang (Perimeter Institute).
• Figure 3: Residual emission (top center) in the 0.7-7 band, created by subtracting a 2D $\beta$-model that was fit to the diffuse source image (bottom left). The residual image was smoothed using a Gaussian of $\sigma=$75, while the diffuse image was smoothed with $\sigma=3\arcsec$, and both panels cover a 25$\times$25 area. Arrows indicate notable features, which are illustrated schematically in the cartoon (bottom right) and examined in more detail in §\ref{['sec:azprofs']}. Note: $1\arcmin = 87$ kpc.
• Figure 4: Gaussian gradient magnitude (GGM) filtered, with $\sigma = 4\arcsec$ (top) and $16\arcsec$ (bottom), images of A2029 in the 0.7-7 keV band covering a 25$\times$ 25 area. The brightest regions correspond to the steepest changes in X-ray surface brightness.
• Figure 5: Gaussian gradient magnitude (GGM) filtered, with $\sigma$ = 2, image of A2029 in the 0.7-7 keV range and covering a 3$\times$ 3 area. The brightest regions correspond to the steepest changes in X-ray surface brightness. The inner region of the sloshing spiral can be seen, starting in the cluster core and passing between the radio lobes, shown by the 1.4 GHz radio emission in cyan.