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X-ray and Radio Analysis of Abell 1644: Constraints on Cluster Dynamics

Humaira Bashir, R. Kale, Asif Iqbal, Manzoor A. Malik

TL;DR

Abell 1644 is studied with new uGMRT band-2 observations (120–250 MHz) complemented by archival Chandra X-ray data to constrain its dynamical state and nonthermal content. The results show two compact radio AGN associated with the subclusters, but no diffuse radio halos or relics, implying limited ICM turbulence or advanced relaxation. X-ray maps reveal a cold front east of the southern subcluster and a pronounced temperature asymmetry consistent with long-lived gas sloshing, likely triggered by a past off-axis encounter with a gas-poor subcluster. Taken together, the findings support a post-merger scenario with preserved cool cores and suggest Abell 1644 is transitioning toward dynamical relaxation, illustrating how low-frequency radio and X-ray analyses jointly constrain cluster dynamics.

Abstract

We present the first band-2 (120--250\,MHz) uGMRT (upgraded Giant Metrewave Radio Telescope) observations of the bimodal galaxy cluster Abell\,1644 (\(z = 0.0471\)), complemented by Chandra X-ray data. While weak lensing measurements reveal a third substructure in Abell 1644, our radio analysis reveals only two compact sources coinciding with the respective brightest cluster galaxies (BCGs) of the northern (A1644N1) and southern (A1644S) substructures, seen in the X-ray observations. Radio analysis yields compact active galactic nuclei (AGN) powered sources with radio power $P_{A1644S} = 1.1\times 10^{23} W/Hz$ and $P_{A1644N} = 7.3\times 10^{23} W/Hz$ at 200MHz. We find no evidence of non-thermal diffuse radio emission, such as halos or relics, within the sensitivity of our band-2 image. We measured the flux density of each radio source and performed spectral analysis. A1644N1 exhibits a synchrotron power law spectrum while A1644S shows spectral turnover suggestive of synchrotron self-absorption. Our X-ray analysis confirms the presence of a cold front east of the A1644S subcluster core. The temperature map further reveals a previously unreported asymmetry, with a hot intracluster medium (ICM) region to the east of A1644S and cooler gas to the west, likely representing residual signatures of earlier merger activity. Together, these features indicate that Abell 1644 preserves clear imprints of its merger history through long-lived sloshing motions, while the absence of diffuse radio emission suggests that the past merger was relatively minor not injecting enough turbulence for large scale reacceleration or the cluster is approaching a late stage of ICM relaxation.

X-ray and Radio Analysis of Abell 1644: Constraints on Cluster Dynamics

TL;DR

Abell 1644 is studied with new uGMRT band-2 observations (120–250 MHz) complemented by archival Chandra X-ray data to constrain its dynamical state and nonthermal content. The results show two compact radio AGN associated with the subclusters, but no diffuse radio halos or relics, implying limited ICM turbulence or advanced relaxation. X-ray maps reveal a cold front east of the southern subcluster and a pronounced temperature asymmetry consistent with long-lived gas sloshing, likely triggered by a past off-axis encounter with a gas-poor subcluster. Taken together, the findings support a post-merger scenario with preserved cool cores and suggest Abell 1644 is transitioning toward dynamical relaxation, illustrating how low-frequency radio and X-ray analyses jointly constrain cluster dynamics.

Abstract

We present the first band-2 (120--250\,MHz) uGMRT (upgraded Giant Metrewave Radio Telescope) observations of the bimodal galaxy cluster Abell\,1644 (), complemented by Chandra X-ray data. While weak lensing measurements reveal a third substructure in Abell 1644, our radio analysis reveals only two compact sources coinciding with the respective brightest cluster galaxies (BCGs) of the northern (A1644N1) and southern (A1644S) substructures, seen in the X-ray observations. Radio analysis yields compact active galactic nuclei (AGN) powered sources with radio power and at 200MHz. We find no evidence of non-thermal diffuse radio emission, such as halos or relics, within the sensitivity of our band-2 image. We measured the flux density of each radio source and performed spectral analysis. A1644N1 exhibits a synchrotron power law spectrum while A1644S shows spectral turnover suggestive of synchrotron self-absorption. Our X-ray analysis confirms the presence of a cold front east of the A1644S subcluster core. The temperature map further reveals a previously unreported asymmetry, with a hot intracluster medium (ICM) region to the east of A1644S and cooler gas to the west, likely representing residual signatures of earlier merger activity. Together, these features indicate that Abell 1644 preserves clear imprints of its merger history through long-lived sloshing motions, while the absence of diffuse radio emission suggests that the past merger was relatively minor not injecting enough turbulence for large scale reacceleration or the cluster is approaching a late stage of ICM relaxation.

Paper Structure

This paper contains 12 sections, 3 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Abell 1644
  3. uGMRT band-2 observations and analysis
  4. X-ray Analysis
  5. X-ray surface brightness map
  6. Temperature map
  7. Spectral Extraction and Fitting
  8. Results
  9. Discussion
  10. Radio morphology and absence of diffuse emission
  11. Thermodynamic properties of ICM
  12. Summary and conclusions

Figures (9)

  • Figure 1: A representative plot of the band-2 spectrum on a baseline at a given time stamp. Baseline W03--E03 ('ll' correlation) was used to produce this plot. The amplitude is uncalibrated. The band has a notch filter around 0.17--0.19 GHz. The bright radio frequency interference in the frequencies below the notch filter, 0.15 - 0.17 GHz did not allow its usage.
  • Figure 2: First uGMRT band 2 full resolution image of galaxy cluster A1644 obtained with robust=0.5. The rms is $\sigma_{rms} = 379$$\ \mu Jybeam^{-1}$ and the restoring beam is $16.307" \times 11.384", P.A -9.282\degree$.
  • Figure 3: Digitized Sky Survey 2 (DSS2) colour optical image of the Abell 1644 with overlaid radio continuum contours (Black). Left: A1644N1; Right: A1644S. Black contours show the radio emission at 200 MHz. In both subclusters, the radio emission is compact and centered on the brightest cluster galaxy (BCG). Contours are drawn at $[1,2,4,8...] \times 3\sigma_{rms}$, where $\sigma_{rms} = 379 \ \mu Jybeam^{-1}$.
  • Figure 4: Integrated radio spectrum of A1644N1 and A1644S. Error bars are the uncertainties in flux density. The solid lines represents the best fit.
  • Figure 5: Exposure corrected, background subtracted, point source removed, surface brightness map of galaxy cluster A1644 in 0.7-8.0 keV energy range. Left: Radio contour levels (blue) drawn at are overlaid. Right: Green annular regions on the right image are drawn to get the temperature profile of the two substructures of the galaxy cluster.
  • ...and 4 more figures