Origin and Evolution of the $Ω$ Structure in the Head-Tail Radio Galaxy of Abell 3322

Abstract

A head-tail galaxy is thought to be a radio galaxy with bent active galactic nuclei (AGN) jets interacting with the intracluster medium (ICM). Study of head-tail galaxies provides us with fruitful insights into the mechanisms of shock waves and turbulence, as well as magnetic-field amplification and cosmic-ray acceleration. A recent MeerKAT observation revealed that a head-tail galaxy in the galaxy cluster, Abell 3322, exhibits a peculiar ``Omega" structure in its shape. In this paper, we investigated this Omega-tail galaxy using the upgraded Giant Meterwave Radio Telescope (GMRT) and the Australia Telescope Compact Array (ATCA). We found that the southern jet tends to be brighter than the northern jet, with a brightness ratio of about 2. This can be attributed to Doppler boost and the inclination of the jets. Our broadband data suggest that the radio spectrum becomes steeper along the jet propagation direction, and the cosmic-ray aging model with a weak reacceleration of cosmic rays is preferable to explain the index profile. We further found a gradient of the spectral index perpendicular to the jet propagation. We discussed the origin of the gradient and suggested that a shock wave along one side of the jets is present. The resultant ram pressure as well as the backflow made at the early stage of the jet may produce the tail component of this Omega-tail galaxy, while the observed Omega-shape structure is more likely due to a twin vortex seen in the low Reynolds number flow.

Figures (11)

• Figure 1: A composite image combining X-ray (cyan-contour), optical (white), and radio (red) data. The X-ray image is obtained by Chandra data in the 0.5-7.0 keV energy band, smoothed with a Gaussian kernel of $\sigma =$ 23 arcseconds, and the contours are overlaid in cyan and drawn at 6 logarithmically spaced levels between the 90th percentile and the maximum of the smoothed X-ray intensity, after linear normalization. The optical data were obtained from the Blue band of the DSS2 survey via SkyView. The radio image corresponds to the frequency-integrated map from the MGCLS survey. The primary target of this study, referred to as the "Omega-tail galaxy", is labeled along with its various structural components. The $\Omega$ symbol is also shown for reference. Alt text: Composite image of the Omega-tail galaxy showing X-ray (cyan contours), optical (white), and radio (red) data with labeled structures.
• Figure 2: Stokes I images of the Omega-Tail galaxy associated with Abell 3322, observed at multiple frequencies. Panels (a) to (f) correspond to the datasets listed in the same order as in Table \ref{['t01']}. Each panel shows the total intensity map at a different frequency or resolution, revealing the detailed morphology of the radio jets and tails. The contours are drawn at $[3, 6, 12, 24, 48]\,\sigma_{\rm rms}$ levels, where $\sigma_{\rm rms}$ is the local root-mean-square noise measured in each map (see Table \ref{['tab:2']} for the $\sigma_{\rm rms}$ values used in each dataset). The HT structure is clearly seen bending toward the south, likely due to ram pressure from the intracluster medium. The highest-frequency image (bottom right) shows compact core-like emission, while the lower-frequency images reveal extended, diffuse tails. These maps provide the basis for subsequent spectral index and spectral aging analyses. Alt text: Six panels of Stokes I images of the Omega-Tail galaxy showing radio jets and tails at different frequencies. The head-tail bends south, with a compact core visible at high frequency.
• Figure 3: The spectral index ($\alpha$) distribution of the Omega-tail galaxy, measured at an effective angular resolution of 15 arcseconds. The numbers from 1 to 5 and the gray dashed lines indicate the labels and positions of the slices used in the later discussion. Alt text: Spectral index ($\alpha$) map of the Omega-tail galaxy at 15 arcseconds resolution. Labels 1 to 5 mark slice positions shown by gray dashed lines.
• Figure 4: The black points represent the observed data. The error bars were calculated as ${ {\sqrtsign{\left( \sigma_{\rm rms} { {\sqrtsign{N_{\rm b}}} } \right)^2 + \left( \sigma_{\rm abs} F_\nu \right)^2}} }$, where $\sigma_{\rm rms}$ is the image noise, $N_{\rm b}$ is the number of beams within the total flux measurement region, and $\sigma_{\rm abs} = 0.1$ represents a 10% uncertainty in the absolute flux scale. The solid line shows the best-fit curve obtained using 'synchrofit' with the CI (continuous injection) model. Alt text: This figure shows the observed spectral data with uncertainties and the best-fit continuous injection (CI) model curve.
• Figure 5: The model fitting results for the northern component. The central panel shows the smoothed radio intensity map (MeerKAT), with circular markers indicating the regions from which the spectral fitting data were extracted. Surrounding the central image, the individual radio spectra corresponding to each extraction region are shown. The inset at the lower left corner illustrates the relative position numbers of circular markers. The black points represent the data with error bars. The green triangles indicate the upper limits for non-detected data, corresponding to 3$\sigma_{\rm rms}$. Note that these upper-limit data were not used in the fitting. Alt text: Model fitting results for the northern component. Central MeerKAT map shows extraction regions with circles. Surrounding panels display radio spectra for each region. Position numbers inset included.