3rd GWCats Workshop

23 nov. 2026, 09:00 → 25 nov. 2026, 17:30 Europe/Paris

Amphitheatre 3 (Toulouse)

Image credits @ Maria Lisa Brozzetti

GWCats: gravitational waves science with galaxy catalogs

GWCats is a cross-disciplinary initiative which aims at gathering together members of the gravitational wave community and members of the astronomical community in order to explore and enable the science emerging from combining gravitational wave observations and electromagnetic surveys, in particular galaxy catalogues.

Goals of the workshop 

This is the third workshop of the GWCats initiative, with the previous ones held in Glasgow in 2024 and in Leiden in 2025. The goals of the workshop are multiple: discuss results from ongoing collaborative projects within GWCats, promote both internal and external collaborations, explore new opportunities to expand the science of GWCats, and organise its activities.

Workshop organization

The workshop is organised over three days, with the first two fully dedicated to science and the last one dedicated to GWCats scientific and administrative organisation. All participants are encouraged to stay until lunch on the third day.

There are no registration fees, but there is a maximum number of 60 participants.

Coffee breaks and lunches will be provided to all participants.

Apply to the workshop

To apply please follow this link.

lun. 23 novembre

09:00 → 10:40 Welcome and introductory talks

09:00 Registration / Arrival

09:30 Welcome

Orateur: Nicola Tamanini (L2I Toulouse, CNRS/IN2P3, Université de Toulouse)

09:35 Workshop introduction

Orateurs: Kenneth Duncan (University of Edinburgh), Rachel Gray (University of Glasgow)

09:40 LVK Cosmology Update

Orateur: Alexander Papadopoulos (University of Glasgow)

10:10 GWCats Flagship Simulation Project Update

Orateurs: Felipe Andrade-Oliveira (University of Zurich), Felipe Andrade-Oliveira (UZH), Tom Bertheas (L2IT, LPENS)

10:40 → 11:10 Coffee break

11:10 → 12:30 Talk Session 2 - Galaxy Surveys

Président de session: Maciej Bilicki (Center for Theoretical Physics PAS, Warsaw, Poland)

11:10 Status and Plans for Next-Generation Photometric Surveys

Wide-field photometric surveys are entering a new regime, in which depth, area, cadence, and wavelength coverage are all expanding at once. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is building a deep, multi-band, time-domain map of the southern sky. It operates alongside space-based programs such as Euclid and SPHEREx, with the Nancy Grace Roman Space Telescope and CSST following.

Together these surveys will constrain dark energy and modified gravity through weak lensing, galaxy clustering, and supernova cosmology, though they are also becoming increasingly valuable for standard siren cosmology. In addition to these Stage-IV photometric surveys, a variety of Stage-III photometric surveys are available to apply the standard siren method.

In this talk, I review the status of these projects, including expected data quality, data releases, and survey-strategy decisions. I will highlight the photometric surveys that will maximize constraints from standard siren cosmology, and provide the best opportunity to provide meaningful constraints on cosmological parameters.

Orateur: Sean MacBride (University of Zurich)

11:40 Status and plans for large-area spectroscopic surveys

Wide-area spectroscopic galaxy surveys are becoming increasingly valuable for gravitational-wave (GW) astronomy, providing the three-dimensional maps of the galaxy distribution needed for statistical standard-siren cosmology, host-galaxy identification, and electromagnetic counterpart searches.

In this review, I will provide an overview of the past, present, and future landscape of extragalactic spectroscopic surveys, from legacy programmes such as GAMA, DEEP2, and zCOSMOS to ongoing and upcoming facilities including DESI, 4MOST, and PFS, and next-generation projects such as MUST and WST. I will compare their scientific objectives, sky coverage, depth, redshift ranges, sample completeness, data-release policies, and capabilities for responding to transient alerts.

Particular attention will be given to how these different survey characteristics determine their usefulness for GW science, from constructing sufficiently complete galaxy catalogues for dark-siren analyses to enabling rapid spectroscopic follow-up of potential electromagnetic counterparts. I will conclude by highlighting the opportunities and challenges in coordinating future spectroscopic surveys with the rapidly evolving needs of GW astronomy.

Orateur: Luca Tortorelli (University Observatory, Ludwig-Maximilians Universitaet Muenchen)

12:10 Breakout Session Ideas

12:30 → 14:00 Lunch

14:00 → 15:30 Talk Session 3 - GW Cosmology Methodology

14:00 Dark Sirens Methodology Updates and Breakthroughs

Orateur: Rachel Gray (University of Glasgow)

14:30 Dark siren-galaxy cross-correlations with Peak Sirens: requirements for catalogs,

Peak Sirens employs the cross-correlations of galaxies with dark sirens as a diagnostic for the luminosity distance-redshift relation: the amplitude of their angular cross-spectra peaks for redshift bins and distance bins which fall upon the curve $D_L(z)$. I will present the latest constraints with existing data as well as forecasts. I will also discuss the galaxy catalog requirements for accurate estimates of $H_0$, and how forward-simulations of the relationships between galaxies and dark are key to uncover the connections between Peak Sirens and other methods to infer $H_0$ from dark sirens data.

Orateur: Raul Abramo (São Paulo University)

14:55 Discussion Session - Including breakout planning

15:30 → 16:00 Coffee break

16:00 → 17:30 Breakout session: 1

mar. 24 novembre

09:00 → 10:30 Talk Session 4 - Contributed Talks

Président de session: Kenneth Duncan (University of Edinburgh)

09:00 Multi-Messenger Simulations for 3G Gravitational-Wave Cosmology: Forecasts and Pipeline Development

Third-generation (3G) gravitational-wave (GW) detectors will enable precision cosmology with dark and bright sirens, but realizing this potential requires realistic simulations that jointly model GW sources, electromagnetic (EM) counterparts, and host galaxies. I will present a self-consistent framework that embeds compact-binary mergers in galaxies from the CosmoDC2 LSST synthetic catalog, modeling source populations, host-galaxy association, GW signals for 2G and 3G detector networks, and kilonova counterparts. Key results show that 3G networks dramatically increase detection rates and improve parameter estimation, while retaining 2G detectors alongside 3G facilities significantly enhances sky localization and distance precision, especially for BNS. A simplified Target-of-Opportunity strategy could detect ~5,000 GW-associated kilonovae over 10 years. Beyond forecasts, I will emphasize how such simulated datasets are crucial for developing and validating end-to-end analysis pipelines, and for quantifying systematic uncertainties in the 3G era.

Orateur: RANIER SILVA (UNESP - IFT/Brazil)

09:15 Standard Sirens Enter the Multiprobe Era: A First Assessment of $\Lambda$CDM Tensions Including Gravitational-Wave Distances

Multi-messenger cosmology offers an independent path to stress-test the $\Lambda$CDM model as gravitational-wave (GW) catalogues grow in size. I will present the first multi-probe assessment of $\Lambda$CDM tensions that incorporates GW standard sirens among the probes. Standard sirens from the LIGO-Virgo-KAGRA GWTC-5.0 catalogue are combined with Dark Energy Survey (DES) Year 6 weak lensing and galaxy clustering ($3\times2$pt), baryon acoustic oscillations (BAO) and type Ia supernovae, together with DESI BAO, and are confronted with primary cosmic microwave background (CMB) measurements from Planck, the Atacama Cosmology Telescope and the South Pole Telescope. All probes are analyzed under homogeneous priors and combined via normalizing flows, enabling a consistent quantification of the $S_8$ discrepancy and the $H_0$ tension with respect to the CMB. Adding standard sirens to the $3\times2$pt analysis tightens the constraints in the $\Omega_m$-$S_8$ plane and delivers a competitive low-redshift determination of $H_0$; the resulting joint constraints on $S_8$, $\Omega_m$ and $H_0$ will be shown at the conference. At current GW sensitivity, however, both the $S_8$ discrepancy and the $H_0$ tension remain essentially unchanged. The analysis nonetheless establishes standard sirens as a mature cosmological probe, ready to be combined on equal footing with established datasets in forthcoming surveys such as LSST, Euclid and Roman.

Orateur: David Sánchez Cid (University of Zurich)

09:30 MTC-1.0: The First Mock Gravitational-wave Transient Catalogue

Dark siren analysis is a fast-evolving field, with rich new datasets to be analysed. In order to ensure the robustness of these analyses, it is important to test them on a realistic dataset with a known truth. We present a data challenge, "MTC-1.0", a mock version of a real gravitational-wave transient catalogue (including search results, parameter estimation results, and sensitivity injections) which has been created from a known synthetic universe, which is accompanied by a galaxy catalogue for that universe.
In addition, we introduce bagpuss, an open-source workflow which is able to create the data challenge, and which employs a flexible framework that can be extended to add additional properties to future versions of the dataset. Crucially, bagpuss does not rely on any existing cosmological inference pipelines to produce the data challenge.
This challenge can be used to test analyses which produce estimates of astrophysical and cosmological parameters by analysing BBH signals and assess the ability of these analyses to recover the known parameters used to create MTC-1.0

Orateur: Alexander Papadopoulos (University of Glasgow)

09:45 Testing Cosmological parameter Inference for Dark Sirens with Galaxy Catalog and Low-Latency Skymaps

Gravitational-wave (GW) standard sirens provide an independent probe of the Hubble constant (H0) by directly measuring the luminosity distance without relying on a cosmic distance ladder calibrations. A primary method to obtain the source redshift is cross-matching the GW localization volumes with galaxy surveys. This procedure, at the baseline of the so-called "Dark siren method" is typically achieved with full parameter estimation samples that constrains all the binary parameters, including sky position and distance. However, low-latency tools such as BAYESTAR provide sky localization that can be already used for the dark siren methods without having to wait for the full parameters estimation. In this presentation, we investigate the possibility of achieving accurate GW cosmology with dark sirens using only low-latency GW skymaps. Using this framework, we demonstrate that sky-dependent selection effects can impact the accuracy on H0 if not taken into account. Further, we show that the approximations assumed for the GW likelihood from BAYESTAR are likely to introduce systematic biases that are stronger in the case that the effective number of galaxies in the localization volume is large. The results obtained in this work indicate that, while low-latency skymaps can be crucial for rapid electromagnetic searches, accurate dark sirens cosmology would require the full GW likelihood encoded in paramter estimation samples.

Orateur: Alessandro Buchicchio (Sapienza)

10:00 Target selection effects in spectroscopic galaxy catalogs on dark sirens

The precision of dark-siren measurements of $H_0$ with the galaxy-catalog method depends on the redshift uncertainties of the potential host galaxies. Spectroscopic surveys greatly reduce these uncertainties. However, they select targets by colour, so their completeness and galaxy populations differ from those of magnitude-limited photometric catalogs. We study how spectroscopic target-selection strategies affect the $H_0$ inference using 5000 mock GW events and excerpts of the Euclid Flagship simulation catalog that reproduce the colour cuts DESI applies to its Bright Galaxy Survey (BGS) and Luminous Red Galaxy (LRG) samples. We also examine the role of catalog incompleteness and how to correct for it without adding new biases, using hybrid catalogs that combine spectroscopic and photometric redshifts. When only spectroscopic redshifts are used, the colour cuts may bias $H_0$. The selection properties of spectroscopic catalogs must therefore be included in the line-of-sight redshift prior if dark sirens are to give an unbiased, sub-percent measurement of $H_0$.

Orateur: Léonard Lebrun (UZH)

10:15 Dark-Siren Cosmology with DESI DR1 BGS Galaxies and LVK detections

We investigate measurements of the Hubble constant, $H_0$, by combining LVK gravitational-wave observations with spectroscopic galaxies from the DESI Data Release 1 Bright Galaxy Sample. The spectroscopic redshifts provide a three-dimensional map of candidate host galaxies within each event's localisation volume. We assess how key catalogue and analysis choices affect the inferred $H_0$, such as the treatement of the redshift uncertainties, and corrections for incompleteness in the galaxy sample. By comparing the resulting constraints under these assumptions, we evaluate their impact on dark-siren measurements and identify sources of systematic sensitivity when using spectroscopic galaxy catalogues for standard-siren cosmology.

Orateur: Dounia Nanadoumgar-Lacroze (IFAE)

10:30 → 11:00 Coffee break

11:00 → 12:20 Talk Session 5 - Contributed Talks + Targeted Follow-up

Président de session: Maria Lisa Brozzetti (Università degli Studi di Perugia)

11:00 Gravitational-wave dark sirens as astrophysical probes: inferring galaxy-merger relations and identifying individual hosts

Gravitational-wave (GW) "dark siren'' techniques, namely the statistical association of GW events with a galaxy catalog, have so far been used as a cosmological tool. Yet this association carries distinctive astrophysical information that remains unused--specifically, how compact binaries populate galaxies and the identities of individual hosts. In this work, we turn the dark-siren program into an astrophysical probe by inferring both simultaneously within a single hierarchical framework. The method reduces the many potential hosts to a compact, probabilistically ranked target list, providing a direct follow-up decision tool. Applied blindly to GW170817, it recovers NGC~4993 as the highest-ranked host and constrains the galaxy luminosity weighting, mildly favoring non-uniform host weights. Fixing the galaxy weighting can instead degrade host identification and even favor the wrong host. This framework recasts dark sirens as a potentially unique probe of the astrophysics of electromagnetically dark compact-binary mergers.

Orateur: Alessandro Agapito (Centre de Physique Théorique (CPT), Aix-Marseille Université (AMU))

11:15 Unifying spatial correlation with hierarchical population inference to constrain the AGN channel for binary black hole mergers

The origin of the stellar-mass binary black holes (BBHs) observed in gravitational wave (GW) events by the LIGO-Virgo-KAGRA detectors remains uncertain. One promising formation pathway is the active galactic nucleus (AGN) channel, in which BBHs embedded in AGN accretion discs experience enhanced merger rates and may produce rapidly spinning black holes in the upper mass gap.

In this talk, I will present a Bayesian framework to constrain the contribution of the AGN channel to the BBH population, based on dark-siren population inference. By combining the electromagnetic AGN catalogue Quaia with the latest GW observations, we set upper limits on the contribution of the AGN channel to the observed merger rate. I will discuss the impact of completeness and redshift measurement precision on our results. In the future, our formalism can be used to infer the properties of GW merger channels, AGN discs and nuclear star clusters.

Orateur: Lucas Pouw (Leiden Observatory)

11:30 Characterizing redshift calibration systematics in the euclid photometric survey

Galaxy surveys provide an unprecedented opportunity to map the large-scale structure of the Universe and constrain the nature of dark matter, dark energy, and gravity. Euclid will observe billions of galaxies through photometric and spectroscopic measurements, probing the distribution and evolution of matter through galaxy clustering and weak gravitational lensing.

A key challenge for photometric analyses is the accurate determination of the redshift distribution of the galaxies entering the sample. Unlike spectroscopic surveys, which can measure individual galaxy redshifts with high precision, photometric surveys rely on information from a limited number of broad photometric bands. The goal is therefore not to precisely determine the redshift of every galaxy, but to accurately characterize the redshift distribution of the galaxy population. This is essential for extracting competitive and robust cosmological constraints from photometric data.

The inferred redshift distribution can be affected by observational effects such as photometric noise, galaxy selection, and variations in imaging depth and quality. Understanding and controlling these effects is therefore crucial for photometric galaxy surveys. In this talk, we present a framework to study these effects in the context of Euclid, combining synthetic source injection with realistic galaxy simulations. We investigate how galaxy selection and photometric measurements impact the inferred redshift distributions, quantify the associated calibration uncertainties, and explore their propagation into the cosmological analysis to ensure robust and unbiased cosmological constraints.

Orateur: Tyann Dumerchat (Aix-Marseille Université CPPM)

11:45 Dark Siren Analysis and Follow-Up in OzGrav

I will give an update on the systematic studies being done with the dark siren technique using the Euclid Flagship mock. I will also give an overview of our follow-up plans with the Anglo-Australian Telescope.

Orateur: Dr Liana Rauf (The Australian National University)

12:00 Sirenas Update and Follow-up proposals Overview

Orateur: Isaac MacMahon (UZH)

12:20 → 14:00 Lunch

14:00 → 15:30 Talk Session 6 - Working with EM Surveys

14:00 Simulation based inference with pop-cosmos

Orateur: Stephen Thorp

14:30 Simulation based inference of galaxy properties with spectroscopic surveys

Accurate and scalable estimates of galaxy physical properties are essential for exploiting the scientific potential of current and upcoming galaxy surveys. I will present the approach of inferring galaxy physical properties using simulation-based inference (SBI), a framework that exploits simulated observations to constrain physical parameters without requiring an explicit analytical likelihood.

At the core of this approach is GalSBI-SPS, a physically motivated galaxy population model that, once constrained against observations, can provide informative population-level priors for galaxy properties. I will discuss its application to KiDS-VIKING photometry, incorporating spectroscopic redshifts to improve the inference of individual galaxy properties, and highlight the advantages and challenges of this approach compared to traditional spectral energy distribution fitting.

Looking ahead, I will discuss how compressed information from full galaxy spectra can be incorporated into SBI and how amortised inference can enable the rapid generation of wide-area catalogues of galaxy physical properties. Such catalogues will support studies of galaxy evolution and provide valuable information for gravitational-wave host-galaxy characterisation and cosmological analyses.

Orateur: Luca Tortorelli (University Observatory, Ludwig-Maximilians Universitaet Muenchen)

14:55 Discussion Session: Incorporating EM survey information in GW cosmology

Orateur: Will Hartley

15:30 → 16:00 Coffee break

16:00 → 17:30 Breakout session: 2

mer. 25 novembre

09:00 → 10:45 Updates from breakout sessions and planning the next year

09:00 Discussion Session Part 1: Reports from breakout groups

09:30 Discussion Session Part 2: What/where next for dark sirens focused EM proposal

Orateurs: Isaac MacMahon (UZH), Liana Rauf (The Australian National University)

10:00 Discussion Session Part 3: Key takeaways New project ideas

10:45 → 11:15 Coffee break

11:15 → 12:30 GWCats wrap up

11:15 Discussion Session: GWCats Project Planning and Overall wrap-up

12:30 → 14:00 Lunch

14:00 → 15:30 MSCA SE: 1

Restricted session

15:30 → 16:00 Coffee break

16:00 → 17:00 MSCA SE: 2

Restricted session