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The TNG contributes to the characterization of 200 new SZ clusters of galaxies first detected by the Planck satellite

The PSZ1 and PSZ2 catalogues contain about 2000 sources selected by means of their Sunyaev-Zeldovich signature in the all-sky maps obtained by the Planck mission. The ITP13-08 128-MULTIPLE-16/15B and the programs, entitled “The Planck SZ galaxy clusters: building a reference sample for cosmology", aimed at characterising the optical counterparts of more than 400 cluster candidates of the PSZ1 and PSZ2 catalogues in the northern hemisphere (Dec. > -15°) with unknown counterparts at any wavelength at the time of the publication of the catalogue.

The observations, which have lasted for over more than 4 years, have been made with the telescopes of the Roque de los Muchachos Observatory within two international projects. More than 400 sources associated with Sunyaev-Zel’dovich emission have been studied, and 221 new clusters of galaxies have been characterized using photometry and spectroscopy. More than 100 clusters have been studied by determining their velocity dispersions and dynamical masses applying multi-object spectroscopic techniques using the DOLORES/TNG spectrograph.

The Sunyaev-Zel’dovich Effect

The Sunyaev – Zel'dovich (SZ) effect is caused by distortion of the cosmic microwave background radiation (CMB) in the direction of a galaxy cluster, due to a mechanism called the inverse Compton effect. This mechanism takes place when the electrons of the hot gas in the clusters interact with the photons of the CMB as they pass through the cluster, so producing a distortion in the spectrum of the CMB.

By measuring the SZ emission in the Planck maps at different frequencies it is possible to detect the galaxy clusters. The SZ effect does not depend on the distance of the cluster producing it, so it is necessary to characterize these SZ sources, identifying the clusters which cause them, finding their distances and masses so that they can be used for later cosmological studies.

In the past decade, this technique has been used to make catalogues of clusters of galaxies using data from radio-telescopes such as the South Pole Telescope (SPT), the Atacama Comology Telescope (ACT), and the data obtained from space by the Planck satellite. In 2013 and 2015 the Planck consortium published two catalogues, PSZ1 and PSZ2, which jointly include almost 2,000 sources detected over the whole sky.

The SZ sources is especially useful in cosmology because the abundance of clusters of galaxies as function of their masses and distances is very sensitive to the cosmological parameters of the model assumed. So, the cluster counts provide an independent way to determine cosmological parameters, and compare them with other techniques (supernovae, CMB, ...)". Many of the SZ sources in these catalogues concide with clusters already known. However, one third of them are unknown their validation and characterization is necessary. In this context, this validation program is an important contribution to the Planck consortium (ESA) in recent years has been the characterization by the telescopes at the Roque de los Muchachos.

Methodology

The method of observing consisted, in the first place, of obtaining deep optical images in the direction of the SZ sources, with which it was possible to identify the regions with concentrations of galaxies, first obtaining the photometric redshift (z) estimates, and the richness of the detected clusters. The optical validation of the clusters as true counterparts to the SZ sources was based on the angular distance from the cluster optical centers to the SZ peak and on their richness.

In a second step, the clusters were observed using multi-object spectrosgraphs, in particular the OSIRIS (GTC) and DOLORES (TNG). The more distant clusters (at z>0.4) were observed with the GTC and the nearer clusters (z<0.4) with the TNG. Multi-object spectroscopy allows us to obtain the radial velocities of many galaxies within each cluster, with which it is possible to calculate their velocity dispersions, and finally to obtain their dynamical masses. In other words, the velocities with which the galaxies move in the cluster gravitational well tell us how much mass the cluster contain. We expect that, given the detection sensitivity of the instruments of the Planck satellite, only the most massive galaxies with masses greater than 1 or 2 times 1014 solar masses can produce a detectable SZ effect, and so, be actual sources of the SZ emission.

The follow-up programs have implied a huge observational effort, in more than 50 square degrees of sky have been explored, and have obtained over 10,000 spectra of galaxies. All the data have been included in archives for public access.

Applying this technique, over 400 SZ sources with optical counterparts unknown have been studied, allowing the confirmation and characterization of 221 new clusters of galaxies. The remaining objects (about 40% of these sources) do not show detectable counterparts, and could be associated with false detections due to emission in the SZ maps of Planck with low signal to noise ratio.

This process of detailed characterization of the catalogues is essential to determine the real selection function of the Planck satellite in these large maps of the sky in the microwave wavelength range, and so for cosmological studies, which can then be now performed with high precision.

This work has also found that about 40% of the low SZ signal-to-noise ratio (S/N<6) sources studied showed an almost perfect relation with the thermal emission of dust within our Galaxy in the 857 GHz, which confirms that the main source of low signal-noise contamination of the SZ effect on the Planck maps is caused by non-Gaussian background sources, mainly linked with the diffuse thermal emission from dust clouds in the Milky Way.

Now, the next step is to determine the relation between the masses of the clusters obtained using the SZ effect, and that determined dynamically, in order to study any biases in the estimates of masses using these methods. Our preliminary studies suggest a value of Msz/Mdyn of around 0.85. The main goal of the project is to determine this relation within an accuracy of 2-3% in order to determine cosmological parameters with high precision and constraining the σ8 (amplitude of fluctuations), Ωm (total matter density) and Σmν (sum of neutrino masses) with respect to the results already published: σ8 (Ωm/0.27)0.3 = 0.764±0.025, and Σmν<0.2eV at 95% confidence levels, provided that the mass calibration bias is 1-b=0.8 (+/- 0.1).

This research in observational cosmology with galaxy clusters will be complemented in the near future by projects starting up, such as the WEAVE Cosmological Cluster Survey (WCC), in coordination with the Leiden Observatory (The Netherlands) and the University of Bologna (Italy), among others, and the NIKA2 LPSZ Optical Follow-up, in collaboration with the University of Grenoble-Alpes (France).

The results of this project have been published in six scientific articles in the journal Astronomy & Astrophysics.The research group is composed by Jose Alberto Rubiño Martín (as PI), Rafael Barrena and Alina Streblyanska (as postdocs), Antonio Ferragamo and Alejandro Aguado-Barahona (as PhD students), within the IAC Cosmology Research Group at the IAC, and others as Ricardo Génova, Inés Flores, Angela Hempel, Heidi Lietzen, Denis Tramonte and Rafael Rebolo.

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Contact: Rafael Barrena: rbarrena@iac.es

RGB image for PSZ1 G158.34-47.49

Figure : RGB image for PSZ1 G158.34-47.49, one of the cluster counterparts discovered with ITP13-8, located at redshift z=0.311. A blue gravitational arc is clearly seen in the center of the image rounding the brightest galaxy of the cluster. The photometry was obtained with ACAM/WHT, and spectroscopy with DOLORES/TNG.