On-going and future work

There are a quite number of additional applications of the Bayesian aperture photometry method and the source stacking procedure. In the following two of the most relevant are mentioned.

Wavelength bias in galaxy cluster samples. The next step in the research of wavelength bias in large galaxy cluster samples is to extend the study to larger samples. The XXL survey is the largest and deepest XMM-Newton project approved so far, covering two extragalactic areas of 25 deg2_{(Pierre et al.} 2015). The XMM-LSS survey was a pilot survey for XXL. The XXL has detected ∼ 315 galaxy clusters at z < 1 with a flux limit of ∼ 10−14_{erg s}−1_cm−2_{. For all these clusters there exists redshift information} (photometric or spectroscopic). The XXL survey also has a well-defined selection function in terms of flux and source extension. However, the influence of cluster astrophysics on X-ray observability is not well understood.

XXL has ∼ 30 associated follow-up programmes, which allow joint and diverse studies on cluster and AGN physics and cosmology. Within these programmes there are two optical spectroscopic surveys covering the XXL northern field that their own galaxy cluster searching programme. Then, both surveys can be used in the wavelength bias comparison of galaxy cluster samples. Such surveys are:

• VIMOS Public Extragalactic Redshift Survey (VIPERS; Guzzo & The Vipers Team2013) is a

spectroscopic on-going Large Programme to map in detail the large-scale distribution of ∼ 105 galaxies at 0.5 < z < 1.2, and overlaps by 16 deg2 _{with XXL. Its galaxy colour selection was} based on 5-bands of the CFHTLS. So far, VIPERS has detected ∼ 1400 groups, out of which 560 possess 5 or more members, in the redshift range of 0.5 < z < 1.2 within the overlap region with XXL (A. Iovino, priv. comm.)

• Galaxy And Mass Assembly (GAMA; Driver et al. 2011) is a completed spectroscopic survey

of ∼ 2 × 105_{galaxies at z < 0.5, and overlaps with XXL by ∼ 20 deg}2_{. In this region, GAMA} has ∼ 95% redshift completeness (Liske et al.2015). The target galaxies were also selected from the CFHTLS. So far, GAMA has detected ∼ 1380 groups in redshift range of 0 < z < 0.5 in ∼20 deg2of the sky with an approximate mass limit of & 1013M.

These larger galaxy samples will be analysed in the same way as in Section 6.4.3to perform a com- prehensive comparison of cluster catalogues. The goal is to understand how each waveband-dependent detection technique generates cluster samples of different physical properties.

Gas mass fraction in low-mass haloes. Recent studies (Planck Collaboration et al.2013b; Anderson et al.2015) have used a sample of locally brightest galaxies (LBGs) to compare the properties of dark matter haloes in optical, X-ray, SZ and lensing. The LBGs are selected from the Sloan Digitized Sky Survey4 _{and are assumed to be central galaxies within their respective dark matter haloes. The LBGs}

were stacked in the SZ using data from the Planck satellite, and in X-rays using data from the ROSAT All-Sky Survey (RASS). Their results showed that unified SZ and X-ray scaling relations apply for galaxies, galaxy groups and galaxy clusters, i.e. from halo masses of ∼ 1012.5_{M(similar to the Milky} Way) to ∼ 1015_M.

Lead by Dr. Jean Coupon at the Astronomical Observatory of the University of Geneva, a similar approach has been implemented in XXL north field. The main goal is to probe the gas- and stellar-mass content of dark-matter halos down to low masses. Although the XXL volume is much smaller, the XXL data is much deeper and has a better PSF than RASS.

Using optical/IR data from CFHTLS and WIRCAM, a preliminary sample of ∼ 3, 000 LBGs galaxies with M & 1011_{Mat 0.2 < z < 1.0 has been set. The LBGs sample has been divided into four stellar} mass bins and in two redshift bins, and for each mass bin, a luminosity stacked map has been produced using the stacking method described in Section6.3. Preliminary results show that a strong and extended X-ray signal is detected in all four mass bins in the low redshift bin (0.2 < z < 0.5). Null stacked tests performed on random positions confirm the high signal-to-noise ratio of the stacked maps.

CHAPTER

7

Constraining the intra-cluster pressure profile

from the thermal SZ power spectrum

The angular power spectrum of the thermal Sunyaev-Zel’dovich (tSZ) effect is highly sensit- ive to cosmological parameters such asσ8andΩm, but its use as a precision cosmological probe is hindered by the astrophysical uncertainties in modeling the gas pressure profile in galaxy groups and clusters. In this paper we assume that the relevant cosmological parameters are accurately known and explore the ability of current and future tSZ power spectrum measurements to constrain the intracluster gas pressure or the evolution of the gas mass fraction, f_gas. We use the CMB bandpower measurements from the South Pole Tele- scope and a Bayesian Markov Chain Monte Carlo (MCMC) method to quantify deviations from the standard, universal gas pressure model. We explore analytical model extensions that bring the predictions for the tSZ power into agreement with experimental data. We find that a steeper pressure profile in the cluster outskirts or an evolving fgashave mild-to-severe conflicts with experimental data or simulations. Varying more than one parameter in the pressure model leads to strong degeneracies that cannot be broken with current observa- tional constraints. We use simulated bandpowers from future tSZ survey experiments, in particular a possible 2000 deg2CCAT survey, to show that future observations can provide almost an order of magnitude better precision on the same model parameters. This will allow us to break the current parameter degeneracies and place simultaneous constraints on the gas pressure profile and its redshift evolution, for example.

Note: This chapter is a reproduction of a paper of the same title, published in the Astronomy & Astrophysics Journal. The reference is Ramos-Ceja, M. E., Basu, K., Pacaud, F., and Bertoldi, F.2015, A&A, 583, A111. The manuscript is reproduced here under the non-exclusive right of re-publication granted by ESO to the author(s) of the paper.

7.1 Introduction

The scattering imprint of the cosmic microwave background (CMB) radiation from the hot, thermalized electrons in the intracluster medium (ICM) is known as the Sunyaev-Zel’dovich (SZ) effect (Sunyaev & Zel’dovich1972,1980), which is playing an increasingly important role in the cosmological and astro- physical research using galaxy clusters. The SZ effect is generally divided into two distinct processes: the kinetic SZ (kSZ) effect describes the anisotropic scattering due to the cluster bulk motion, while the thermal SZ (tSZ) effect describes the inverse Compton scattering of the CMB photons by the thermal distribution of hot electrons in the ICM, which is proportional to the line-of-sight integral of the electron pressure. For the tSZ effect, the energy gain by the CMB photons gives rise to a specific spectral de- pendence of the tSZ signal, such that below roughly 217 GHz, clusters appear as a decrement and above as an increment in the CMB surface brightness. Because the tSZ surface brightness is independent of the redshift of the scattering source, it provides a powerful means to study the structure and dynamics of the hot intracluster gas throughout cosmic history.

Apart from observing individual clusters, the tSZ effect can also be detected in a statistical sense through the excess power over the primordial CMB anisotropies, coming from all the resolved and unresolved galaxy groups and clusters in a CMB map. Unlike the optical and X-ray observables, the redshift inde- pendence of the tSZ signal makes this “confusion noise” a significant source of temperature anisotropies at millimeter/submillimeter wavelengths in the arcmin scale regime, where the primordial CMB aniso- tropies are damped exponentially. Similar to the cluster number counts, the tSZ anisotropy signal is sensitive to the same set of cosmological parameters because its contribution comes primarily from the hot (& 1 keV), ionized ICM bound to groups and clusters (e.g., Hernández-Monteagudo et al.2006). The amplitude of the tSZ power spectrum depends roughly on the eighth power of σ8, the rms amplitude of the matter density fluctuations and on the third power ofΩm(Komatsu & Seljak2002; Trac et al.2011). However, the tSZ power receives significant contribution from the low mass, high redshifts objects. This seriously hinders its use as a precision cosmological probe, since the thermodynamic properties of these systems are not well constrained from direct observations. Thus, the astrophysical uncertainties in mod- eling the tSZ power spectrum are too large to place significant constraints on cosmological parameters. Consequently, attention has moved to measuring the higher order statistics of the correlation function, such as the tSZ bispectrum, which arises mostly from massive systems at intermediate redshifts and is therefore less prone to astrophysical systematics (e.g., Rubiño-Martín & Sunyaev2003; Bhattacharya et al.2012; Wilson et al.2012).

As the measurement precision of the cosmological parameters improves from other methods, the use of the tSZ power spectrum in cosmology can be reversed. The tSZ power can then be used as a probe for measuring the distribution and evolution of the intracluster gas, down to low cluster masses and up to high redshifts, where direct observations are difficult. Similar arguments have been presented by several authors (e.g., Shaw et al.2010; Battaglia et al.2012a), although a quantitative comparison between the results of analytical cluster pressure models and the observations of the tSZ power spectrum has been lacking. It is also of great interest to know how the future ground-based SZ surveys may constrain the intracluster gas models because their resolutions are better suited to constraining the shape of the tSZ power spectrum.

The amplitude of the tSZ power spectrum was predicted analytically by Komatsu & Seljak (2002), with an expected value of 8 − 10 µK2 _{around ` = 3000. Later semi-analytic modeling predicted similar} values (Sehgal et al. 2010), but experimental results have confirmed these early predictions to be too high. The first conclusive measurement of the combined tSZ+kSZ power spectrum came from the South