Cosmology and Structure Formation

• Testing the standard model, quantifying tensions with other cosmological probes
• Tests of modified gravity using galaxy cluster counts and cluster matter profiles
• Constraints on interacting dark matter models
• Constraints on baryonic feedback and its impact on the matter power spectrum
• Combining weak lensing and galaxy cluster clustering as a probe

• The promise of weak lensing voids as cosmological probes in the era of Euclid and Rubin
• Weak lensing voids in the Dark Energy Survey dataset

• Understanding the linkage between HI and the star formation rate in galaxies over a range of environments
• Creating the largest available samples of X-ray and SZE selected galaxy clusters
• Impact of baryonic feedback on galaxy clusters and the matter power spectrum
• Connections between voids defined using weak lensing, galaxies and galaxy clusters

• Preparation and delivery of the ground based EXTernal datasets
• Optimal photometric calibration across the sky using the Gaia spectrophotometry as a baseline
• Joint cataloging of ground based and Euclid VIS and NIR imaging
• Development of joint Rubin-Euclid Photometry System (REPS) to enable Euclid cosmology with Rubin photometry

• Development and optimization of OTF imaging pipelines for the application to MeerKAT data
• Application of OTF pipelines to the MeerKLASS L-band and UHF-band survey datasets to produce deep radio continuum source catalogs (DR1 end of 2025, DR2 in 2026)
• Scientific exploitation of the MeerKLASS DR1 datasets in UHF and L-band

• Algorithms and code for weak lensing mass calibration of galaxy clusters
• Algorithms and code for studies of the clustering of galaxy clusters
• Development of joint Euclid+Rubin data analysis tools

• Collaborative development of a federated research analysis infrastructure in Germany that serves the Particle, Astro/Cosmology, Nuclear and Particle Astrophysics communities

The MeerKLASS survey is designed primarily as an HI (neutral hydrogen) intensity mapping survey over 10.000 deg² with the purpose of using the large scale structure as traced by the HI to constrain the distance-redshift relation using baryon acoustic oscillations (BAO). The project is led by Prof. Mario Santos (University of the Western Cape, South Africa), and will employ 2000hr of MeerKAT scanning observations to build up sky coverage over a four year period 2025-2028.

Within the Chair for Cosmology and Structure Formation, we have worked since 2020 with Prof. Santos, Prof. Keith Grainge (Uni Manchester, UK) and other collaborators within MeerKLASS to develop a so-called On-the-Fly (OTF) interferometric imaging algorithm (Rozgonyi+25). This OTF algorithm allows the scan data needed for intensity mapping to be simultaneously used to build deep, large solid angle continuum images of the radio sky. In 2025 the OTF pipeline was perfected and the MeerKLASS OTF team, led by Suman Chatterjee (UWC), Sourabh Paul (Uni Manchester) and Sarvesh Mangla (LMU) produced the first L-band and UHF data release over 250 deg² and 800 deg², respectively. This DR1 in December 2025 will be followed by yearly data releases in December hereafter through the end of the MeerKLASS survey.

Currently work continues on preparation for DR2 in 2026 but also focuses on the science analysis of the MeerKLASS OTF data. This includes identifying the optical/NIR hosts for the radio sources and studying the radio luminosity function, the clustering of the continuum sources, the high redshift radio quasars population we have found, and other complementary projects proposed by members of the MeerKLASS collaboration.

The Chair for Cosmology and Structure Formation has been making central contributions to the Euclid mission since its initial approval in 2012. Now that the mission is operating, we are participating in a variety of science projects and planning new ones.

Two early Euclid science projects carried out within the group include a study of Jelly-Fish galaxies in the Euclid Early Release Observations (George+25) and a study of high redshift tSZE selected clusters in the Euclid Q1 dataset (Klein+25).

In addition to these analyses, we have contributed to a variety of Euclid papers that include and overview of the mission design (Laureijs+11, Laureijs+24, Mellier+25) aspects of the survey planning (Scaramella+22), aspects of the Science Ground Segment (George+20), reports on the data quality for releases where our contributions were central (Aussel+25), and algorithm development for the Euclid science (Fumagalli+25).

Prof. Mohr served on the ESA Euclid Science Team as one of two German representatives between 2013 and 2025. In that position he participated in implementing the mission, resolving the instrumentation problems that arose, finalizing the science requirements document and participating in many project reviews. In the Chair, we have supported a team of between five and six postdoctoral fellows, who have contributed centrally to the Euclid Science Ground Segment during the development and validation phase and now in the operations phase. Prof. Mohr co-leads the SGS unit EXT, which is focused on the external ground based data, and Dr. Kümmel co-leads the SGS unit MER, which is focused on the cataloging of the Euclid VISual, NIR (YJH band) and EXT (ugriz) photometry to support the science analyses within the collaboration. Dr. Bocquet is co-leading a cluster work package focused on developing the cluster cosmology likelihood that will be used as a part of the core Euclid cosmology analysis.

The Chair for Cosmology and Structure Formation has two International Participant PI groups. This gives a limited number of scientists data rights. To obtain these PI groups we made in-kind contributions to the Dark Energy Science Collaboration (DESC) over a two year period.

We are very excited about the opportunity to participate in the Rubin project and to pursue a variety of science analyses both inside DESC and outside.

Because of our dual involvement in Euclid and Rubin, we have taken on the task of developing the Rubin-Euclid Photometry System, which is a pipeline and data management environment meant to extract the Rubin photometry needed for the Euclid cosmological analyses in the later years of the Euclid survey.

The Dark Energy Survey (DES) data have been central to the science of the Chair for Cosmology and Structure Formation over many years. But we not only use DES for science analyses – we have contributed to the project and collaboration over more than two decades.

Within DES our group has led the study of galaxy clusters, using primarily samples coming from the South Pole Telescope (SPT) mm-wave survey but later samples coming from Planck, ACT, ROSAT and eROSITA. We have studied the evolution of cluster galaxies to high redshift using the SPT samples (Hennig+17) as well as carrying out optical followup for redshifts and statistical confirmation using the Multi-Component Matched Filter (MCMF) tool that enables one to suppress noise fluctuations from the thermal Sunyaev-Zel'dovich effect and X-ray selected candidate lists (Klein+18). Finally, we have led the development of weak gravitational lensing methods for calibrating cluster halo masses (Stern+19, Grandis+21, Bocquet+24), and then have led the key galaxy cluster cosmology analyses in the SPTxDES collaboration since 2016. The latest analyses appear in a series of papers led by Dr. Bocquet and two PhD students Dr. Mazoun and Dr. Vogt in our group in 2024 and 2025 (Bocquet+24,Mazoun+25,Vogt+25a,Vogt+25b)

We are proud that many students and postdocs have received their training in modern survey cosmology using DES data in analyses within our group!

In addition to carrying out leading science within DES, our group has developed tools and coordinated the science activities for the DES. Prof. Mohr co-founded the DES in 2004, while a Professor at the University of Illinois, Urbana-Champaign (UIUC). Partnering with Fermilab and the University of Chicago, Prof. Mohr co-wrote and co-presented the initial DES proposal to build a new Giga-pixel camera for the Blanco 4m at CTIO in Chile. The proposal for NOAO to dedicate 500 observing nights in return was accepted, and this gave birth to the DES. Mohr served as PI of the NSF proposal that funded the DES data management, and then he led the data management development at UIUC and at the National Center for Supercomputing Applications until his move to LMU in 2009.

Prof. Mohr served thereafter as project scientist of DES data management from 2009–2012 and continued as galaxy cluster science working group co-chair through 2015. He has maintained participation in the DES management board over the full course of the survey. Other members of the Chair have served as cluster science working group co-chair (Dr. Bocquet), cluster weak lensing science co-chair (Dr. Dietrich), and high-redshift cluster science co-chair (Dr. Klein).

The Chair for Cosmology and Structure Formation members have been playing a leading role in the cosmological and astrophysical exploitation of the galaxy cluster and AGN samples identified with the South Pole Telescope (SPT) mm-wave mapping. We continue to focus on SPT data, including the exciting new cosmological analysis of the SPT-3G sample, which is being led by Dr. Bocquet.

Over the years most of the PhD students trained in the group have focused on science with SPT – often in combination with the multi-band optical imaging dataset of the DES. Initially as a student, Dr. Bocquet introduced a new cluster cosmology likelihood that focused more explicitly on the mass calibration (Bocquet+15), and from that time on we have led most of the cluster cosmological analyses for SPT (Bocquet+19, Bocquet+24a, 24b). In addition, we have played central roles in defining the SPT cluster samples (Song+12,Klein+24a24b) and have carried out the initial studies of SPT selected AGN (Gupta+17).

Other PhD students have focused on cluster physics studies (Chiu+16, Chiu+18, Bulbul+19) and mass calibration of cluster halos using dynamical (Capasso+19) and gravitational weak lensing constraints (Dietrich+19, Stern+19).

Prof. Mohr co-founded the SPT in 2002 with Prof. Carlstrom as a co-PI on the NSF proposal to the Office of Polar Programs (Carlstrom+11). He took on the responsibility of arranging for the optical followup of the future SPT cluster sample. That effort led to the founding of the DES, which is a rich multi-band survey that offers much more than just galaxy cluster photometric redshifts and constraints from weak gravitational lensing. Prof. Mohr led the Blanco Cosmology Survey (Desai+12), which is a DES precursor survey using the existing Mosaic camera. Those data were instrumental in confirming the first thermal Sunyaev-Zel'dovich effect selected galaxy clusters, reported by the SPT collaboration in 2009. He served as cluster working group chairs for many years until his move to LMU.

The Chair for Cosmology and Structure Formation has led and contributed to many of the first eROSITA cosmology analyses. In addition, group members contributed cosmological algorithms and optical imaging and weak gravitational lensing data over a decade long period in preparation for the eROSITA launch in 2019.

As Senior Scientist representing LMU in the eROSITA collaboration, Prof. Mohr participated in the eROSITA collaboration between his arrival in Munich in 2009 through the end of 2020. During that time he led the development of the optical followup program, which included, in collaboration with Dr. Klein, designing with the Multi-Component Matched Filter (MCMF- Klein+18) algorithm for optimally identifying the hosts of X-ray selected clusters and suppressing the contamination of that cluster sample through X-ray AGN. Moreover, he, Dr. Bocquet and Dr. Grandis developed the algorithms for using weak gravitational lensing from the Dark Energy Survey to calibrate the eROSITA cluster halo masses and enable their use for robust cosmology (Grandis+19). As cluster cosmology co-convenor over a decade, he worked closely with his partners, first Dr. Boehringer, then later Prof. Finoguenov and finally Prof. Reiprich and Dr. Bulbul to coordinate the planning and development for the cosmology with the eROSITA dataset.

These efforts enabled the first eROSITA galaxy cluster catalog (Klein+22) and – in collaboration with Prof. I-Non Chiu, a former PhD student in the group, the first cosmological analysis of the eROSITA cluster sample (Chiu+23). Finally, the DES weak gravitational lensing mass calibration data needed for the mass calibration of the eRASS1 sample was prepared and delivered by Drs. Grandis and Bocquet in collaboration with Prof. Mohr.

The eROSITA dataset is exquisite and contains a treasure trove of information about cosmology and structure formation that will continue to emerge over the coming years. Scientists continue to reap scientific rewards from the ROSAT All Sky Survey (RASS) that was acquired 35 years ago. Indeed, the largest all-sky X-ray selection galaxy cluster catalog still comes from RASS rather than eROSITA (Klein+19).