ACT-CL J0215.4+0030 Thermodynamics

Associated publications: Kéruzoré, F. et al. (2020), A&A, 644, A93., Kéruzoré, F. et al. (2020), mm Universe @ NIKA2

I led the comprehensive analysis of the thermodynamic properties of one of the most challenging clusters within the NIKA2 LPSZ. ACT-CL J0215.4+0030 (ACTJ0215 for short) was initially identified in the first ACT tSZ survey. This cluster has a particularly important place within the LPSZ, as one of its lowest mass, highest redshift targets. Such systems are particularly interesting to study: because of their shallower gravitational potential, they are more susceptible to non-gravitational processes, thus offering valuable insights into the thermodynamic properties of the intracluster medium. However, measuring their properties is challenging, as the amplitude of the tSZ signal of a cluster is closely tied to its mass, and its spatial extension to redshift. These systems therefore appear as dim and small in tSZ data. Moreover, in the case of ACTJ0215, an additional difficulty came from a strong contamination of the tSZ signal by point sources. The main bandwidth of NIKA2 for the observation of the tSZ effect is around 150 GHz; at this frequency, the tSZ effect manifests as a decrement in the surface brightness of the CMB. Therefore, the positive flux from astrophysical sources can compensate – partially or totally – for the tSZ decrement in NIKA2 maps, and thus alter the reconstructed cluster properties.

The measurement and analysis of the thermodynamic properties of ACTJ0215 were, therefore, both crucial steps of the LPSZ, and required a large number of observational difficulties to be solved for the exploitation of the rest of the sample. I carried out this analysis by first constructing the tSZ maps of this cluster from NIKA2 raw data, and then quantifying the remaining noise in the maps and the filtering undergone by the signal. The pipeline I developed to perform this analysis is still used by the NIKA2 collaboration to reduce tSZ data. I then used a Markov chain Monte Carlo algorithm and a forward modeling approach to fit the pressure profile of the intracluster medium on the NIKA2 data. In this approach, I accounted for the point source contamination by jointly modeling the NIKA2 map as the sum of tSZ signal and point source emission, with source fluxes treated as parameters of the model. This approach enables propagating the uncertainty on this contamination to the final results, by adding informative priors on the flux values, which are then marginalized over.

Combining the measured pressure profile with X-ray observations from the XMM-Newton satellite allowed me to characterize the cluster’s thermodynamic properties. The integration of NIKA2 and XMM-Newton data revealed a disturbed core, and provided precise constraints on the cluster’s mass and integrated Compton parameter \(Y_{500}\), which is a key tSZ survey observable. These results demonstrated that the combination of NIKA2 and XMM-Newton observations could establish precise constraints on properties in the mass-observable scaling relation, even for complex clusters like ACTJ0215. This is highly promising for the future results of the large NIKA2 LPSZ, indicating a potentially substantial constraining power on the final sample properties.