*chto@uchicago.edu

Clusters as cosmological and astrophysical probes

Chun-Hao To*

I also work on...

Lensing around dwarf galaxies

Pixel-level Synergies between Roman and Rubin

Galaxy Cluster physics

Dark Matter Halo

5\times 10^{14} M_\odot

Red galaxies

~ 2% of the mass

Hot gas

~ 10% of the mass

Cosmology

Galaxy physics

Baryonic feedback

Yang+03, Lin&Mohr04, More+16,
Kravstov+18, Shin+19, To+20 (APJ 897,15), ... many more

Vikhlinin+06, Eckert+16,
To+24 (JCAP 2024, 037), Dalal+26, Kovac+25, Siegel+25, ...many more

Clusters

Dark Matter Halo:
Rare peaks in the density field.

250 Mpc/h

Cluster Cosmology

Dark Matter Halo:
Rare peaks in the density field.

Abundances are exponentially sensitive to the amount of structure.

More structures

Bahcall 95, Wang&Steinhardt 98, Borgani+ 01, Allen+ 03, Vikhlinin+ 09, DeHann+16, To+ 21, Bocquet+24,... many more

Cluster Cosmology

Text

Cluster mass

Cluster Cosmology

Bocquet+24 (SPT), Ghirardina+24 (eROSITA), Mantz+14 (WtG),... many many more

Cluster Cosmology

~6500 clusters found in DES Y1

Cluster Cosmology

DES+20

Photo-z uncertainty at z=0.2 ~= 100 Mpc/h

Optical clusters have 20-30 % contributions from line-of-sight galaxies.
Modeling these components (and their impact on the weak lensing profile) is essential for unbiased cluster cosmology.

To+21 (MNRAS 502,4093), Lee+22, Wu+ (incl. CT)22, Zhang+ (incl. CT)23, Costanzi+ (incl. CT)26, and many more

How should we proceed?

Clusters are really part of the large-scale structure.

How should we proceed?

Clusters are really part of the large-scale structure.
Analyze clusters in the same way as galaxies.

Why do we want to make it even more complicated?

Adding more data makes modeling simpler:

Focusing on large scales where we can model the data perturbatively (>0.2 Mpc/h vs >8 Mpc/h).
Self-calibrating projection effects.
Easily integrated into joint analyses.

Reanalysis of DES-Y1 data

Large-scale only reanalysis

DES-Y1 clusters

To&Krause+ 21 (PRL, 126, 141301)

DES Y1

DES Y3 (>16k clusters)

Unique redshift and mass range

Merloni+24, Kornoelje+25, DES+25, see also ACTDESHSC Collaboration+25 (ACT DR6)

[SPT deep]

Y3 cluster cosmology are challenging due to high S/N

19 astrophysical parameters describing the connection of measurements and matter fluctuations (linear galaxy bias, intrinsic alignments, mass—observable relations, and projection effects)
12 observational parameters describing observational systematics (photometric redshift and shear measurement)

DES standard!

To+25 (PRD, 112, 063537)

Exhaustively test possible systematics

Several systematics are worrisome:

Nonlinear bias of tracers
Modeling of the projection effect
Mass-observable relation

Some are not bad:

Halo Mass Function/ Halo Bias

New simulation to test the model

[\frac{\text{simulations}}{\text{data}}]

# Cluster ratios

To+24, APJ,961,59

Title Text

Subtitle

Simulations (Cardinal, To+24, APJ,961,59) to test cluster finding algorithm.

To+25 (PRD, 112, 063537)

DES cluster

Cosmology constraints (50% improvements relative to DES-Y1):

S_8=0.864\pm 0.04

\Omega_{\rm{m}}=0.27^{+0.02}_{-0.03}

DES+25

DES internal consistency

Posterior Predictive Distribution (PPD) with a criteria PPD (d1|d2)>0.01.

P(cosmic shear | CL+GC, ΛCDM) = 0.04> 0.01
P(cosmic shear + galaxy-galaxy lensing| CL+GC) = 0.07>0.01

DES+25

All probes combined

DES joint analyses of clusters, galaxies, and weak lensing (CL+3x2pt):

S_8=0.81\pm 0.02

\Omega_{\rm{m}}=0.27^{+0.02}_{-0.03}

24% improvements compared to 3x2pt.

DES+25

All probes combined

DES+25

Bocquet+ (Include CT) 25

SPT clusters + DES 3x2pt

Galaxy Cluster physics

Dark Matter Halo

5\times 10^{14} M_\odot

Red galaxies

~ 2% of the mass

Hot gas

~ 10% of the mass

Cosmology

Galaxy physics

Baryonic feedback

Yang+03, Lin&Mohr04, More+16,
Kravstov+18, Shin+19, To+20 (APJ 897,15), ... many more

Vikhlinin+06, Eckert+16,
To+24 (JCAP 2024, 037), Dalal+25, Kovac+25, Siegel+25, ...many more

Galaxy Cluster physics

Hot gas

~ 10% of the mass

Millimeter wave (Thermal Sunyaev-Zel'dovich effect)

X-ray

\frac{\Delta T}{T_{CMB}} \propto \int n_e T_e d\ell

S_X \propto \int_{\sim 0.5\ KeV}^{\sim 2\ KeV} n_e n_H \Lambda(T,E) dE

Line of sight

Cooling function

Why should we care?

To+24, see also Lucie-Smith+ 25

A significant amount of information in cosmic shear below or near the scale cut comes from galaxy clusters.

What are the observables

Weak lensing

CMB

Y_{SZ} \times 10^{-4}

\langle M|Y_{SZ} \rangle (10^{14} M_\odot)

What are the observables

Y_{SZ} \times 10^{-4}

\langle M|Y_{SZ} \rangle (10^{14} M_\odot)

Change of Matter clustering

Model

Godmax:
A Schneider-Teyssier-type model with parametrized gas distribution implemented in Jax
(Pandey+ (incl. CT) 24)

See also Schneider&Teyssier 15, Giri+ 21, Schneider+ 25, and many more

Simulation validation

Text

Measurement

Prediction

+ Godmax model

To+ 24

Simulations validation

Text

To+ 24

Pathfinder study

Dalal+ (incl. CT) 26

ACT cluster finder

Nemo

Shin+25

Godmax
Model

Pathfinder study

Dalal+ (incl. CT) 26

Pathfinder study

Dalal+ (incl. CT) 26

We find remarkably consistent results compared to other probes.

What is next?

Lots of data exist and have been used to constrain feedback. Here are a very incomplete list:
- WL: Ario+22, Anbajagane+25, DES+26, Xu+25
- tSZxtSZ: Raghunathan+26, Chaubai+26, Efstathiou&McCarthy25
- tSZxothers: Sanchez+22, Dalal+25, Pandey+25
- kSZ x others: Hadzhiyska+24, Bigwood+25, Ropper+25, Hotinli, Smith, Ferraro 25.
- X-ray: Kovac+25, Eckert+25, Siegel+25, Zhang+26.
- FRB: Reischke & Hazstotz25, Wang+25.
- QSO absorption: Chen & Zahedy26, Qu+23, 24, Zahedy+19.

What is next?

Most of the baryon probes involve galaxies.
- Clusters rely on galaxies for redshift and selections.
- kSZ stacks on galaxy samples.
- FRB needs to be cross-correlated with galaxies if DM (host) is unknown.
Weak lensing is sensitive to a broad redshift range.
- Future weak lensing surveys (such as Roman) will be sensitive to z=0-3.