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Home , EROSITA, Redshift survey

Surveys DOI: 10.18727/0722-6691/5124

4MOST Consortium Survey 5: eROSITA Cluster Survey

Alexis Finoguenov 1, 2 clusters of so as to perform (DESI) and the 4MOST Andrea Merloni 1 dynamical estimates of the total mass ­Redshift Survey, provide geometrical Johan Comparat 1 and to measure the properties of the cosmological constraints, which are Kirpal Nandra 1 member galaxies. The survey aims to complementary; together they can con- Mara Salvato 1 obtain precise­ redshift measurements strain a larger variety of cosmological Elmo Tempel 3, 4 of the photometrically identified bright- models. Anand Raichoor 5 est cluster galaxies at redshift z > 0.7. Johan Richard 6 At lower (z < 0.7) the pro- Spectroscopic identification of galaxy Jean-Paul Kneib 5 gramme aims to sample over 15 member clusters has been a cornerstone of all Annalisa Pillepich 7 galaxies per cluster and enable dynami- surveys. Recently, this field Martin Sahlén 8 cal mass measurements to calibrate the has expanded due to developments on Paola Popesso 9 clusters for cosmological experiments. measurements of dynamical and caustic Peder Norberg 10 At z < 0.2, eROSITA will also detect mass and the availability of precise mem- Richard McMahon 11 X-ray emission from galaxy groups and berships of cluster galaxies. In the last and the 4MOST collaboration filaments. 4MOST spectroscopic data few years, a number of large-area galaxy from the survey will be used for optical cluster surveys have been carried out. identification of galaxy groups down The pioneering work on this has been 1 Max--Institut für extraterrestrische to eROSITA’s mass detection limits of done within the 13 Physik, Garching, Germany 10 M⊙, as well as the detection of the SDSS-IV survey (Clerc et al., 2016), with 2 University of Helsinki, Finland largest filaments for pioneering studies a goal to follow up on 4000 clusters. A 3 Tartu Observatory, University of Tartu, of their X-ray emission. much larger demand on cluster follow-up Estonia is set by the eROSITA survey, and the 4 Leibniz-Institut für Astrophysik Potsdam goal of the 4MOST cluster survey is to (AIP), Germany Scientific context follow up as many as 40 000 groups and 5 Laboratoire d’astrophysique, École clusters of galaxies, the precise details ­Polytechnique Fédérale de Lausanne, The experimental measurement of the depending on the performance of the Switzerland precise values of the cosmological eROSITA survey. In addition to the larger 6 Centre de Recherche Astrophysique de parameters is the accepted way to pro- number of systems, eROSITA will detect Lyon, France gress the field of cosmology. These clusters to higher redshifts than before, 7 Max-Planck-Institut für Astronomie, observational measurements include: reaching beyond z = 1, which requires a Heidelberg, Germany measuring the fluctuations of the cosmic deeper optical survey than SDSS-IV. The 8 Department of Physics and , microwave background, which describe survey will constrain the physics of the Uppsala universitet, Sweden the inhomogeneities of the Universe at warm baryons, which in turn traces the 9 Physics Department, Technische a redshift of 1100; geometric tests of the evolution of the cosmic feedback. In ­Universität München, Germany expansion of the Universe (using Super- addition to the properties of warm inter- 10 Department of Physics, Durham novae and Baryonic Acoustic Oscillations); galactic gas in groups and clusters, X-ray ­University, UK and the growth of large-scale structure emission traces the state of the warm 11 Institute of Astronomy, University of throughout cosmic time. As outlined in the gas in the filaments, which completes Cambridge, UK Task Force report (Albrecht the picture of warm baryons in the Uni- et al., 2006), no single method can con- verse and complements similar studies strain the cosmological parameters pre- using absorption techniques (Nicastro et Groups and clusters of galaxies are a cisely and a combination of methods is al., 2018). current focus of astronomical research required. owing to their role in determining the environmental effects on galaxies and The performance of a particular survey Specific scientific goals the constraints they provide to cosmol- is judged by its ability to constrain the ogy. The eROSITA X-ray telescope on dark energy equation of state. The 4MOST The primary scientific goal of the survey board the Spectrum Roentgen spectroscopic survey will provide the vali- is to provide a spectroscopic study of observatory will be launched in 2019 dation of the cluster catalogue produced 40 000 groups and clusters of galaxies and will have completed eight scans of by the eROSITA survey, which tests cos- within the German eROSITA sky (for the full sky when 4MOST starts operat- mology through the growth of structure details of the eROSITA survey, see Merloni ing. The experiment will detect groups as reflected in the mass function of gal- et al., 2012), reaching a redshift of 1.4 and clusters of galaxies through X-ray axy clusters, and it belongs to the highest to maximise the expected cosmological emission from the hot intergalactic tier of dark energy experiments. The performance of the survey (for details of medium. The purpose of the 4MOST 4MOST eROSITA Galaxy Cluster Redshift the cosmological forecast, see Pillepich eROSITA Galaxy Cluster Redshift Sur- Survey is the only currently planned et al., 2018). The primary uses of the vey is to provide spectroscopic red- large-scale programme of this kind. Other 4MOST spectroscopic observations per- shifts of the optical counterparts to the redshift survey experiments, such as the formed for the eROSITA Galaxy Cluster X-ray emission from 40 000 groups and Dark Energy Spectroscopic Instrument Redshift Survey are:

The Messenger 175 – March 2019 39 Surveys Finoguenov A. et al., 4MOST Consortium Survey 5

1. To spectroscopically confirm the 360° 270° 180° 90°0° 30° photo­metric counterpart of the X-ray emission by removing projection effects in photometric galaxy membership assignment for galaxy clusters (Clerc 0° et al., 2016) and to provide the spectro- scopic counterparts to the X-ray emis- sion coming from the galaxy groups –30° 14 (halos with total mass below 10 M⊙).

2. To obtain a precise distance estimate –60° required to compute X-ray luminosity, which enters the mass function cosmo- 1752 962 targets Equatorial logical study as a mass proxy (Grandis et al., 2018). 110100 Object counts per degree2 3. To perform dynamical mass calibration (Capasso et al., 2019). The survey will a cross-correlation analysis between Figure 1. Target density of the 4MOST Cluster Sur- provide dynamical mass as well as the position of X-ray photons, detected vey, based on the mock catalogue tailored to the expectations of the eROSITA X-ray survey. About caustic mass measurements for 10 000 by eROSITA and the 2D (sky) projec- one third of the targets belong to the low-redshift clusters of galaxies at redshifts z < 0.6 tions of the filaments. (z < 0.2) survey. 14 and total masses > 10 M⊙, which enables us to very accurately link the 7. The eROSITA Galaxy Cluster Redshift cluster observables to their total mass Survey will coordinate with the 4MOST Target selection and survey area — critical for constraining the cosmol- WAVES Survey, where the eROSITA ogy. A comparison of these measure- sample will provide high-signal-to-noise The target selection for the eROSITA ments to the weak lensing mass esti- (S/N) observations of rare systems not cluster identification is carried out using mates can further constrain the models sampled within the WAVES Survey area; the position of the extended X-ray source of modified gravity (Wilcox et al., 2015). 4MOST spectra with S/N > 10 per Å and running the red sequence cluster are suitable for galaxy evolution science. finder on deep provided by 4. To enable the analysis of clustering Examples of these types of rare sys- the Dark Energy Survey (DES), the Dark of groups and clusters, which have a tems are galaxies in rare high-density Energy Camera Legacy Survey (DECaLS), ­different sensitivity in constraining environments in massive clusters and Pan-STARRS1, VST ATLAS and ongoing ­cosmology through the large-scale X-ray selected lowest mass galaxy imaging surveys with the Dark Energy structure growth estimates. Clustering groups that are selected in the Local Camera (DECam) and VST (DeROSITAs/ analysis requires at least 1000 systems Volume. KABS). Altogether, these will cover in order to detect significant signal. A 10 000 square degrees accessible by sample of 40 000 groups and clusters 4MOST (see Figure 1). allows us to sample the mass function Science requirements with five mass bins and the redshift In addition, the project to search for fila- range with eight bins, which is required The science requirements for the 4MOST ments has a strong synergy with the to break the degeneracy of the cluster- eROSITA Galaxy Cluster Redshift Survey 4MOST Cosmology Redshift Survey in ing amplitude between mass, redshift consist of: achieving highly complete terms of targets. Therefore this part of and cosmology (Pillepich et al., 2018). sampling of target galaxies required for the 4MOST Cluster survey will only be the galaxy group and filament searches; carried out over the survey area in com- 5. To improve the link between the various covering a large area to maximise the mon with the Bright Galaxy sub-survey baryonic phases of halos and improve number of spectroscopically confirmed of the 4MOST Cosmology Redshift Sur- the value of eROSITA cosmology by groups and clusters of galaxies in order vey (7500 square degrees). In order to addressing the effects of baryons on to improve the cosmological constraints; estimate the resulting target density, we the growth of structure (Bocquet et al., obtaining uniform coverage of sufficiently have produced a mock target catalogue 2016). large areas to perform the clustering by combining the expected eROSITA analysis to obtain additional cosmological ­performance at cluster detection with the 6. To spectroscopically identify filaments constraints; delivering accurate calibra- mocks based on the MultiDark simulation to a redshift of 0.2, where a detection tion of cluster mass using dynamical and (Comparat et al., 2017 & in preparation). of the X-ray signal with eROSITA is caustic mass measurement; and achiev- The low-redshift (z < 0.2) part of the sur- expected. The study of the Warm Hot ing high S/N for a subsample selected for vey has about one million targets, which Intergalactic Medium (WHIM) in emis- galaxy evolution studies. are bright (Ks < 18 magnitudes) and sion will be based on the combined require short exposures (10 minutes). Multi- 4MOST and eROSITA study, through member cluster identification at high

40 The Messenger 175 – March 2019 ­redshift depends on the total exposure additional requirement is to determine complete. As with the 4MOST WAVES per field, with the trade-off being the redshifts for between 10 and 100 mem- Survey, this places a strong requirement highest redshift achievable within the ber galaxies within the virial radii for for the survey completeness. total available time for the survey. The clusters with z < 0.7. This component brightest cluster galaxies in each cluster requires 0.4 million fibre-hours in dark The FoM of the survey is a function of the are always considered for highest priority sky conditions to complete. ratio of the covered to total area x: observations. – A sufficiently large area to sample FoM = 0.5 + 0.5 erf ((x – 0.75)/0.165) ­massive clusters, which are the sensitive probes of the cosmology. FoM = 0.5 (0.9); for an area of 7500 Spectral success criteria and figure of The requirement (goal) is to survey (9000) square degrees made of con­ merit 7500 (10 000) square degrees. tiguous 500 square-degree patches. – Contiguous areas to enable the The spectral success criteria are the high-order statistical tests (two-point References: measurements of galaxy redshifts. The and three-point correlation functions). figure of merit (FoM) for the entire survey The requirement for the minimum size Albrecht, A. et al. 2006, arXiv:0609591 encompasses multiple components, of each patch of contiguous area on Bocquet, S. et al. 2016, MNRAS, 456, 2361 including: the sky is 500 square degrees. Capasso, R. et al. 2019, MNRAS, 482, 1043 Clerc, N. et al. 2016, MNRAS, 463, 4490 – Obtaining redshifts for a million targets – A million bright targets to sufficiently Comparat, J. et al. 2017, MNRAS, 469, 4157 to sample galaxy cluster members, sample low-redshift filaments and Grandis, S. et al. 2018, arXiv:1810.10553 the brightest cluster member in each groups of galaxies. It requires 0.1 million Merloni, A. et al. 2012, arXiv:1209.3114 cluster being the highest priority. An fibre-hours in bright sky conditions to Nicastro, F. et al. 2018, Nature, 558, 406 Pillepich, A. et al. 2018, MNRAS, 481, 613 Wilcox, H. et al. 2015, MNRAS, 452, 1171 ESO/J. Emerson/VISTA/CASU ESO/J.

A VISTA image of the Fornax Galaxy Cluster, one of the closest ­clusters beyond the Local Group of galaxies.

The Messenger 175 – March 2019 41