MeerKAT observations of Abell 1775 and Abell 1795: the discovery of a hadronic radio halo?

TL;DR

This study uses deep MeerKAT L-band and LOFAR data to image Abell 1775 and Abell 1795, unveiling two-component radio haloes in both clusters. Abell 1775 shows a compact, inner halo plus an outer component with ultra-steep, filamentary LOFAR emission, while Abell 1795 hosts a large, flat-spectrum halo extending to ~1 Mpc and exhibiting a strong radio–X-ray correspondence. The work explores hadronic versus turbulent re-acceleration origins for Abell 1795, finding that a hadronic model can reproduce the radial halo profile under specific magnetic-field scaling and CRp-energy-density assumptions, though the linear radio–X-ray relation poses a challenge. The authors also discuss observational biases in cool-core systems that can hide giant haloes and highlight the need for high dynamic range and multi-frequency data to robustly test halo formation scenarios, foreshadowing the role of future SKA surveys in resolving these questions.

Abstract

Giant radio haloes are diffuse synchrotron sources typically found in merging galaxy clusters, while smaller mini-haloes occur in cool-core clusters. Both trace cosmic-ray electrons in the intracluster medium, though recent observations suggest their distinction is not always clear. We present new 903-1655 MHz MeerKAT observations of Abell 1775 and Abell 1795, both hosting cool cores and cold fronts. Combined with reprocessed 120-168 MHz LOFAR Two-metre Sky Survey data, we perform imaging and spectral analyses of their radio emission. In both clusters, we detect radio haloes with distinct inner and outer components. In Abell 1775, the halo appears diffuse at 1.3 GHz, while LOFAR images reveal steep-spectrum filaments. In Abell 1795, the inner component corresponds to a previously reported mini-halo candidate, but the full structure extends to $\sim$1 Mpc with a spectral index of $α=-1.08\pm0.06$. The presence of such a large, flat-spectrum halo in a dynamically relaxed cluster makes Abell 1795 an outlier relative to typical merging systems. This suggests that some relaxed clusters may still retain sufficient turbulence to sustain particle re-acceleration, or that hadronic interactions producing secondary electrons play a significant role. Together with other recent discoveries in cool-core systems, our results indicate that some large radio haloes may have been overlooked in past studies due to limited dynamic range near bright central AGN. Finally, we detect steep-spectrum emission south of Abell 1795's central AGN, tracing a 45 kpc X-ray and optical filament that terminates in an X-ray cavity, likely linked to a past AGN outburst.

Figures (15)

• Figure 1: MeerKAT L-band (left; at a resolution of $9.6\arcsec\times4.7\arcsec$) and LOFAR HBA (right; at a resolution of $9.1\arcsec\times5.0\arcsec$) images of Abell 1775 at central frequencies of 1279 MHz and 144 MHz, respectively. Images were made using Briggs weighting with a robust value of $-0.5$. Various features are labeled in the LOFAR image. The beam sizes are shown at the bottom left corners. The noise levels are reported in Table \ref{['tab:imageproperties']}.
• Figure 2: Left panel: MeerKAT L-band image of Abell 1775 tapered to a resolution of 25 kpc at the cluster's redshift. The emission from compact sources was removed. The dashed circle indicates $0.5\times R_{500}$2016AA...594A..27P. Right panel: Chandra 0.5--2.0 keV X-ray image of Abell 1775. MeerKAT L-band contours, with emission from compact sources subtracted, are overlaid. The two lowest contour levels come from images tapered at 100 kpc resolution (blue) and 50 kpc resolution (green), see also Fig. \ref{['fig:lowres']}. These are drawn at a level of $5\times$ the r.m.s. map noise ($\sigma_{\rm{rms}}$). The white contours come from the 25 kpc resolution image shown in the left panel and are drawn at levels of $[1,2,4,\ldots] \times 5\sigma_{\rm{rms}}$. The noise levels and beam sizes of the radio images are reported in Table \ref{['tab:imageproperties']}.
• Figure 3: Left panel: Radial radio surface brightness profile for Abell 1775 extracted from the MeerKAT 25 kpc resolution image. The solid blue line shows the best fitting double exponential model (Eq. \ref{['eq:profile']}). The dashed lines show the two individual exponential components. Right panel: Spectral index map at 10 resolution between 144 and 1279 MHz for Abell 1775. Contours are from the 144 MHz LOFAR image and are drawn at levels of $[1,2,4,\ldots] \times 3\sigma_{\rm{rms}}$, with $\sigma_{\rm{rms}}=119$$\mu$Jy beam$^{-1}$. The corresponding spectral index uncertainty map is shown in Fig. \ref{['fig:spixerror']}.
• Figure 4: MeerKAT L-band (left; at a resolution of $10.4\arcsec\times4.3\arcsec$) and LOFAR HBA (right; at a resolution of $9.4\arcsec\times5.2\arcsec$) images of Abell 1795 at central frequencies of 1279 MHz and 144 MHz, respectively. Images were made using Briggs weighting with a robust value of $-0.5$. The arrows on the MeerKAT image indicate the location of the cold front 2001ApJ...562L.153M. Various radio features are labeled on the LOFAR image. The beam sizes are shown at the bottom left corners. The noise levels are reported in Table \ref{['tab:imageproperties']}.
• Figure 5: Chandra 0.5–-2.0 keV fractional residual X-ray image. The image was created by subtracting a radially averaged profile from the original X-ray image and dividing the result by the original. LOFAR radio contours at 144 MHz are overlaid in red. These contours are from a high-resolution image with a Briggs robust parameter of $-1.25$, yielding a beam size of $5.3\arcsec \times 3.5\arcsec$ with a position angle of 97 (shown in the bottom left corner). Solid contours are at levels of $[1, 2, 4, 8] \times 25\sigma_{\rm rms}$; dashed contours are at $[0.2, 1, 5] \times 10^{3}\sigma_{\rm rms}$, with $\sigma_{\rm rms} = 139$$\mu$Jy beam$^{-1}$.