First Sardinia Radio Telescope detection of the Sunyaev-Zel'dovich effect at 18.6 GHz

1. Problem Statement and Scientific Context

Galaxy clusters are the largest gravitationally bound structures in the universe. They imprint a signature on the Cosmic Microwave Background (CMB) via the Thermal Sunyaev-Zel'dovich (tSZ) effect, caused by the inverse Compton scattering of CMB photons by hot electrons in the intracluster medium (ICM). This effect manifests as a decrement in CMB intensity at low frequencies (below ~220 GHz).

While large-scale surveys (e.g., Planck, SPT, ACT) have cataloged thousands of clusters via the tSZ effect, they suffer from low angular resolution (typically 1' to 5'). This limits the ability to resolve fine structures within the ICM, such as shocks or pressure discontinuities. Conversely, high-resolution instruments (e.g., MUSTANG-2, NIKA2) operate at higher frequencies (90–273 GHz) where the tSZ signal is stronger but often lack sensitivity to the low-frequency tail or have smaller fields of view.

The specific challenge addressed in this work:

• Detecting the tSZ effect at low frequencies (18.6 GHz) using a single-dish radio telescope.
• Overcoming the contamination from diffuse radio sources (radio relics) which are often present in merging clusters and can obscure the tSZ decrement.
• Achieving higher angular resolution than all-sky surveys to better reconstruct the plasma distribution.

2. Methodology

Observations

• Target: The galaxy cluster MACS J1752+4440 (redshift $z=0.366$), known for its double radio relic system.
• Instrument: Sardinia Radio Telescope (SRT) equipped with a seven-feed K-band receiver.
• Frequency: Centered at 18.6 GHz with a 1200 MHz bandwidth.
• Strategy: On-the-fly (OTF) mapping in the equatorial frame over 15 days (total 82.5 hours). Orthogonal scanning (RA and DEC) was used to mitigate striping noise.
• Resolution: 0.9 arcminutes (beam size).

Data Reduction Pipeline

The data were processed using the SCUBE software:

1. Flagging: Removal of spectral channels with backend issues and Radio Frequency Interference (RFI).
2. Calibration:
   • Gain and delay corrections using cold sky observations.
   • Atmospheric opacity and system temperature removal via skydip scans.
   • Bandpass calibration using the calibrator 3C286.
   • Flux density scale calibration (using 3C286 and 3C295) to ensure errors remained below 10%.
3. Image Processing:
   • Baseline subtraction using second-order polynomials on individual subscans.
   • Masking of bright radio sources (based on 1.4 GHz NVSS contours) to prevent negative bias at the map center.
   • Creation of Stokes I images by weighted stacking of Left- and Right-Hand Circular Polarizations (LCP/RCP).
   • Merging of RA and Dec maps using a wavelet-based method to enhance Signal-to-Noise Ratio (S/N) and remove residual scanning noise.

Modeling and Analysis

• Signal Extraction: The team identified a surface brightness decrement at the cluster center. The S/N was calculated by comparing the decrement against the outer plateau of the map.
• Physical Modeling: The decrement was modeled using a spherically symmetric $\beta$-model for the electron density distribution:
  $$n_e(r) = n_0 \left(1 + \frac{r^2}{r_c^2}\right)^{-3\beta/2}$$
• Parameter Estimation: A Markov Chain Monte Carlo (MCMC) approach (using the emcee sampler) was employed to retrieve the core radius ($r_c$), central electron density ($n_0$), and the $\beta$ parameter. The model assumed an isothermal cluster with $k_B T_e = 5.9$ keV.

3. Key Results

Detection of the SZ Decrement

• The SRT successfully detected a surface brightness decrement at the center of MACS J1752+4440 with a significance of S/N = 3.
• The decrement is attributed to the thermal SZ effect.
• The detection was achieved despite the presence of radio relics; the northern relic was detected at 3$\sigma$, while the southern relic and a nearby radio galaxy were masked to isolate the SZ signal.

Derived Physical Parameters

The MCMC analysis yielded the following parameters, consistent with expectations for a cluster of this mass:

• Core Radius ($r_c$): $160 \pm 30$ kpc.
• Central Electron Density ($n_0$): $(2.5^{+0.7}_{-0.5}) \times 10^{-3} \text{ cm}^{-3}$.
• Beta Parameter ($\beta$): $0.6 \pm 0.1$.
• Compton-y Parameter: The mean Compton-y parameter within a radius of 3.5' is $(2.6 \pm 0.3) \times 10^{-5}$.

Comparison with Planck

• The derived Compton-y value is higher than that reported by Planck ($y \approx 4 \times 10^{-6}$).
• Explanation: This discrepancy is primarily due to angular resolution. Planck's 7' beam smooths the signal over large scales, leading to an underestimation of the peak Compton-y. When the SRT map is convolved to the Planck beam, the value drops to $(1.8 \pm 0.3) \times 10^{-5}$, which is still higher but closer, confirming that SRT resolves the central concentration better.
• Additionally, SRT's ability to mask contaminating radio sources (which contribute noise in low-resolution surveys) improves the accuracy of the SZ measurement.

4. Significance and Contributions

1. First SRT Detection: This is the first detection of the SZ effect by the Sardinia Radio Telescope, validating its capability for low-frequency SZ studies.
2. Low-Frequency Regime: It demonstrates that the SRT can detect the onset of the SZ decrement at 18.6 GHz. While the signal is weaker here than at 150 GHz, this frequency regime is crucial for:
   • Characterizing the spectral shape of the SZ effect.
   • Studying the ICM in conjunction with radio continuum emission (relics/halos) without the confusion of high-frequency dust emission.
3. Resolution Advantage: The 0.9' resolution provides a significant improvement over Planck (5'), allowing for a more precise reconstruction of the SZ profile and the separation of thermal (SZ) and non-thermal (radio relic) components.
4. Methodological Validation: The study proves that single-dish telescopes, when combined with rigorous data reduction (de-striping, masking, and wavelet merging), can effectively measure diffuse SZ signals, complementing interferometric observations which often miss short baselines (large-scale diffuse emission).
5. Future Prospects: The results highlight the potential for the SRT to serve as a precursor for high-resolution follow-up observations (e.g., with the MISTRAL receiver at 90 GHz) by identifying promising targets and providing preliminary Compton-y estimates.

Conclusion

This paper establishes the Sardinia Radio Telescope as a powerful tool for studying the intracluster medium through the Sunyaev-Zel'dovich effect. By detecting the SZ decrement at 18.6 GHz with high angular resolution, the authors have provided a new perspective on the plasma distribution in MACS J1752+4440, demonstrating that low-frequency, high-resolution observations are essential for a complete understanding of galaxy cluster physics and cosmology.