SPT-3G D1: Compton-$y$ maps using data from the SPT-3G and Planck surveys

We present thermal Sunyaev-Zel’dovich (tSZ) Compton-$y$ parameter maps constructed from two years (2019-2020) of observations with the South Pole Telescope (SPT) third-generation camera, SPT-3G, combined with data from the Planck satellite. Using a linear combination (LC) pipeline, we obtain a suite of reconstructions that explore different trade-offs between statistical sensitivity and suppression of astrophysical contaminants, including minimum-variance, CMB-deprojected, and CIB-deprojected $y$-maps. We validate these maps through different statistical techniques such as auto- and cross-power spectra with large-scale structure tracers as well as stacking on cluster locations. These tests are used to understand the balance between noise and astrophysical foreground residuals (such as the CIB) in combination with the recovery of the tSZ signal for different maps. For example, results from stacking at the location of clusters confirm the robustness of the recovered tSZ signal over the $\sim 1500: {\rm deg}^2$ SPT-3G survey field used in this analysis. The high-resolution and low-noise maps produced here provide an important cosmological tool for future studies, including measurements of the Compton-$y$ map power spectrum, cross-correlations with other tracers of the large-scale structure, detailed modeling of cluster pressure profiles, and study of the thermodynamic state of the baryons in the Universe.

💡 Research Summary

This paper presents a new set of thermal Sunyaev‑Zel’dovich (tSZ) Compton‑y maps covering roughly 1,500 deg² of the southern sky, created by combining two years (2019‑2020) of observations from the South Pole Telescope third‑generation camera (SPT‑3G) with the full‑sky data from the Planck satellite. SPT‑3G provides deep, high‑resolution (≈1.4 arcmin) maps at 95, 150, and 220 GHz, while Planck supplies low‑ℓ (ℓ ≲ 1500) coverage across a broader frequency range with lower noise on large angular scales. The authors develop a harmonic‑space linear combination (ILC) pipeline that constructs weights preserving the distinctive tSZ spectral shape while minimizing variance and optionally nulling specific contaminants. Three map products are released: a minimum‑variance (MV) map, a CMB‑deprojected map, and a CIB‑deprojected map, each smoothed to a 1.4′ beam.

The methodology starts by estimating the noise covariance for each frequency channel and defining a tSZ spectral template. The MV weights are derived to minimize total variance, whereas the constrained versions impose additional linear constraints that force the CMB or the cosmic infrared background (CIB) to zero. The weights are ℓ‑dependent, allowing Planck to dominate at low multipoles and SPT‑3G at high multipoles.

Validation is performed through three complementary approaches. First, auto‑power spectra of the maps are measured to quantify residual noise and foreground power. Second, cross‑spectra with external large‑scale‑structure tracers (e.g., DES‑I galaxies, WISE‑CIB maps) are computed, demonstrating effective suppression of CMB and CIB contamination. Third, stacking on known galaxy‑cluster positions (from SPT‑SZ, ACT, etc.) yields average y‑profiles that match theoretical pressure models, confirming that the tSZ signal is faithfully recovered. Simulations based on the Agora suite are used to assess bias and to separate contributions from radio sources, CIB, and other extragalactic components.

The MV map achieves a signal‑to‑noise ratio >10 up to ℓ ≈ 3000 and resolves sub‑arcminute structures. The CMB‑deprojected map excels at large scales (ℓ < 500) where CMB leakage would otherwise dominate, making it ideal for cross‑correlations with optical surveys. The CIB‑deprojected map reduces infrared background residuals, benefiting studies of high‑redshift clusters and the outskirts of halos. Compared with the Planck‑only y‑map, the new maps provide roughly a factor of two better angular resolution and about 30 % lower noise, opening new windows on the high‑ℓ tSZ power spectrum, the “missing baryons” problem, and detailed pressure‑profile modeling. All map products and the pipeline code are released publicly on the SPT website, enabling the community to exploit these high‑fidelity tSZ maps for a wide range of cosmological and astrophysical investigations.