Figure 2.9:Joint 68.3% and 95.4% credibility regions for the cosmological parameters (Ωm, σ8) and (Ωm, w0) applying different cosmological probes. Left: Overlap of the credibility regions for the mean matter densityΩm
and the amplitude of matter perturbationsσ8obtained from cluster counts and observations of the CMB power-spectrum with the two instrumentsWMAP, applying the 9-year survey results, andPlanck. Right: Constraints on the mean matter densityΩmand the dark energy equation of statew0from various probes, including cluster abundances and the joint credibility region for all probes.Credit:Mantz et al. (2015)
The methodology of re-formulating the halo mass function based on X-ray observables is described in detail as well as in a practical context in Sect. 6.3.1.
2.9 The eROSITA -Telescope
Whereas the previous sections emphasised on the cosmological model and the concepts of studying the evolution of our Universe with the help of galaxy clusters, we now introduce a promising instrument for the required cluster observations - theextended ROentgen Survey with an Imaging Telescope Array (eROSITA;Merloni et al. 2012). The main science driver for this telescope is the analysis of the dark energy equation of state, by tracing the evolution of structures with the help of galaxy clusters. It is likely to be the first “Stage IV” dark energy probe, according to the Dark Energy Task Force(DETF) report of 2006 (Albrecht et al. 2006). Such a probe is expected to improve the constraints on dark energy by a factor of∼ 10 compared to the knowledge at the publication date of the report. For this aim, the telescope will need to detect at least∼30,000 clusters of galaxies up to redshifts of∼2.0.
The instrumental set-up as well as the different science goals foreROSITA, including the observation strategy to constrain the characteristics of dark energy, are summarised in the following sections.
2 Introduction
2.9.1 Instrumental Information8
Figure 2.10: Schematic image of the eROSITA-instrument, looking onto the seven X-ray tele-scopes.9
The eROSITA-telescope is a joint X-ray initiative be-tween several institutes and universities in Germany and Russia, led by theMax-Planck Society(MPG) and theGerman Space Agency(DLR) on the one side, and by the Russian space agency ROSCOSMOS and the Space Research Institute for the Russian Academy of Sciences(IKI) on the other side. Currently, the instru-ment is assembled under the leadership of the Max-Planck Institute for Extraterrestrial Physics (MPE) and will then be mounted onto the Russian satel-lite platformSpectrum Roentgen Gamma (SRG). The launch is scheduled for early 2017 from the cosmod-rome in Baikonur to an L2 orbit. The telescope will then perform eight all-sky surveys in total, each last-ing half a year, with a subsequent pointed observation phase of three years.
The instrument consists of seven X-ray mirror tele-scopes, each with its own CCD (Charged Coupled De-vice) in the focal plane (Fig. 2.10). X-ray telescopes as well as CCDs need to follow certain characteristics to collect and focus the energetic photons as well as to measure their energy. X-ray photons are only re-flected by a smooth metal surface and only for suffi -ciently small impact angles, referred to asgrazing in-cidence. For example, for a photon with an energy of
E = 10 keV, which is equivalent to a wavelength ofλ ≈ 1 Å, the incidence angle needs to be < 1◦. To account for the grazing incidence and to focus all incoming light rays into one point, Wolter optics are applied (Wolter 1952a,b), where X-ray instruments, including eROSITA, are based on the Wolter optics Type I, which combine an outer parabolic mirror with an inner hyperbolic mirror. At the same time, these optics allow large numbers of mirror shells to be stacked to increase the effective area of the instrument. Each of the seveneROSITA-telescopes consists of 54 of these mirror shells with a focal length of 1.60 m. The applied X-ray CCDs show a thicker depletion layer than optical CCDs to provide for the sensitivity of high energy photons. Additionally, each X-ray photon is detected individually with its direction as well as with its energy, whereeROSITA’s effective area covers the energy range between (0.1−10.0) keV with an energy resolution of∼5 eV (comp. Fig. 2.11). The effective area is especially large in the range between (0.5−2.0) keV with a sharp drop offfor energies above∼2 keV. Following this shape, the energy range of highest sensitivity overlaps with the position of the main line emission complexes at∼ 1 keV of galaxy clusters (comp. Sec. 2.7.2) and accordingly allows for precise and accurate estimates of various cluster characteristics, including especially the ICM temperature.
Other important information on the instrument include its field-of-view (FoV) of 0.83 deg2 and the angular resolution of∼ 15 arcsec for a pointed on-axis observation. However, the angular resolution highly depends on the observation angle and degrades with increasing off-axis angle. For the scanning observation mode of the all-sky surveys, the resolution is averaged over the entire FoV to show a value
8If not stated otherwise, the information on the instrumental design are published by Merloni et al. (2012).
9Credit:www.mpg.de/4710144/eROSITA_Dunkle_Energie
2.9 TheeROSITA-Telescope
Figure 2.11: Effective area for the seven eROSITA-telescopes compared to the efficiency of the three XMM-Newton EPIC-PNfilters. In the energy range between (0.5−2.0) keV, where most of the emission lines of galaxy clusters are located,eROSITAshows a higher efficiency than the current instrument.Credit: Merloni et al. (2012).
Figure 2.12:Exposure map for the four years ofeROSITAall-sky survey given in galactic coordinates (FK5) with the colour indicating the exposure time per FoV in seconds. Credit: J. Robrade 2014, eROSITA Collaboration private communication.
2 Introduction
of∼ 28 arcsec. The spatial distribution of exposure times per FoV is presented in Fig. 2.12, with an average effective exposure time of 1.6 ks (Pillepich et al. 2012) after accounting for the subtraction of solar flares and slight instrumental difficulties. Due to the observation strategy, two fields with deep ex-posure times of∼20 ks develop at the ecliptic poles. These fields cover only a very small sky fraction of fsky=0.0034, but allow for more detailed studies of X-ray objects and for predictions on the efficiency of the subsequent pointed observation phase.
2.9.2 Science Goals
The interest in theeROSITAobservations is especially promoted as it will perform the first all-sky survey after theRöntgensatellit(ROSAT) in the 1990s, while allowing for a resolution of the order of current X-ray instruments and for an improved sensitivity by one order of magnitude. The currently mainly applied instruments for galaxy cluster studies are the EuropeanXMM-Newton, the US-american Chan-dra and the JapaneseSuzaku with angular resolutions of ∼ 15 arcsec, ∼ 0.5 arcsec and∼ 2 arcmin, respectively. Accordingly, the resolution ofeROSITA is comparable to that of XMM-Newton for the pointed observation phase, while it still remains four times as good asSuzaku’sresolution during the survey mode. During its four years of all-sky surveyseROSITAwill detect large samples of all types of X-ray emitting objects, including e.g. X-ray binaries, single stars, AGN and galaxy clusters. As AGN are the brightest extragalactic objects in the sky,eROSITA is expected to detect (3−10)·106 of these sources up to redshifts ofz ≈ 7−8. At the same time, the instrument will allow for a detailed study of the accretion processes onto the super massive black holes in the centre of the AGN. However, the main science driver of this telescope is the detection of galaxy clusters as tracers of the LSS and thus of the dark energy characteristics. To achieve this aim, the average flux limit for the observation of galaxy clusters is reduced to 3·10−14erg/s/cm2 in the energy range of (0.5−2.0) keV, which is roughly one order of magnitude below theROSAT limit (Trümper 1985). Along with this sensitivity, forecasts pre-dicteROSITAto detect∼100,000 clusters of galaxies with a minimum ofηmin= 50 observed photons and masses aboveM = 5·1013 M/h. This sample will cover redshifts ofz 2, while including all massive clusters withM3·1014M/hin the observable Universe (Pillepich et al. 2012). With these characteristics, theeROSITAcluster catalogue will extend the presentROSAT-cluster sample by a factor of∼50.
Complementary optical observations are planned to determine the cluster redshifts, such that X-ray fluxes, luminosities as well as redshifts will be available for the entireeROSITAcluster sample. These optical observations include e.g. the multi-band surveysPanSTARRS(Panoramic Survey Telescope&
Rapid Response System, e.g. Ebeling et al. (2013)),DES(Dark Energy Survey, e.g. Crocce et al. (2015)) andVST ATLAS(VLT Survey Telescope ATLAS, Shanks et al. (2015)) for photometric redshifts, while spectroscopic observations with e.g. 4MOST (4m Multi-Object Spectroscopic Telescope for ESO, e.g.
de Jong et al. (2014)) andSPIDERS (SPectroscopic IDentification of eROSITA Sources, e.g. Salvato (2015)) are designed aseROSITAfollow-up. This redshift information is especially important for cos-mological studies and will improve the constraints on the coscos-mological parameters.
Assuming luminosities and redshifts to be accessible for all eROSITA galaxy clusters, first forecasts predict constraints of Δσ8 = 0.014 andΔΩm = 0.012 for a ΛCDM cosmology, Δw0 = 0.053 for a constant dark energy equation of state, and Δwa = 0.48 for a variable dark energy equation of state (Fig. 2.13, Pillepich et al., in prep). These simulations are based on the halo mass function as well as on the angular clustering of galaxy clusters and emphasise thateROSITAwill allow for a significant improvement of the cosmological constraints from galaxy clusters (comp. Sec. 2.8). At the same time, it will decrease the uncertainty on the dark energy equation of state even below the current uncertainty from thePlanck data (Planck Collaboration et al. 2015c). However, the above stated results are still