[Paper Review] Cosmicflows-4

R. Brent Tully, Ehsan Kourkchi+15 more|arXiv (Cornell University)|Sep 22, 2022

Gamma-ray bursts and supernovae4 citations

TL;DR

Cosmicflows-4 compiles distances for 55,877 galaxies in 38,065 groups using eight methods, including Tully-Fisher and Fundamental Plane relations, with supernovae and Cepheid/Tip of the Red Giant Branch calibrations for absolute scaling. It reports a Hubble constant of $H_0 = 74.6 \pm 0.8$ km s⁻¹ Mpc⁻¹ (statistical) and a large potential systematic uncertainty of ~3 km s⁻¹ Mpc⁻¹.

ABSTRACT

With Cosmicflows-4, distances are compiled for 55,877 galaxies gathered into 38,065 groups. Eight methodologies are employed, with the largest numbers coming from the correlations between the photometric and kinematic properties of spiral galaxies (TF) and elliptical galaxies (FP). Supernovae that arise from degenerate progenitors (type Ia Sne) are an important overlapping component. Smaller contributions come from distance estimates from the surface brightness fluctuations of elliptical galaxies and the luminosities and expansion rates of core collapse supernovae (SNII). Cepheid period-luminosity relation and tip of the red giant branch observations founded on local stellar parallax measurements along with the geometric maser distance to NGC 4258 provide the absolute scaling of distances. The assembly of galaxies into groups is an important feature of the study in facilitating overlaps between methodologies. Merging between multiple contributions within a methodology and between methodologies is carried out with Bayesian Markov chain Monte Carlo procedures. The final assembly of distances is compatible with a value of the Hubble constant of $H_0=74.6$ km s$^{-1}$ Mpc$^{-1}$ with the small statistical error of $\pm 0.8$ km s$^{-1}$ Mpc$^{-1}$ but a large potential systematic error of ~3 km s$^{-1}$ Mpc$^{-1}$. Peculiar velocities can be inferred from the measured distances. The interpretation of the field of peculiar velocities is complex because of large errors on individual components and invites analyses beyond the scope of this study.

Motivation & Objective

• To compile a comprehensive, multi-method catalog of galaxy distances across the sky to improve constraints on the Hubble constant and peculiar velocity fields.
• To resolve discrepancies in Hubble constant measurements by combining multiple independent distance indicators with a common absolute calibration.
• To enable detailed mapping of large-scale structure and peculiar velocities by integrating galaxy groups into a coherent distance network.
• To reduce statistical errors through Bayesian merging of overlapping distance estimates from multiple methodologies.
• To provide a public, high-precision resource for cosmological and astrophysical studies using a multi-method, multi-calibrator approach.

Proposed method

• Gathers galaxy distances using eight distinct methodologies: Tully-Fisher (TF), Fundamental Plane (FP), Type Ia and II supernovae (SNe Ia, II), surface brightness fluctuations (SBF), Cepheid period-luminosity relation (CPLR), and tip of the red giant branch (TRGB).
• Uses Cepheid and TRGB calibrations, along with geometric maser distance to NGC 4258, to anchor all distance estimates to an absolute scale.
• Applies Bayesian Markov Chain Monte Carlo (MCMC) procedures to merge overlapping distance estimates within and between methodologies, minimizing inconsistencies.
• Groups galaxies into 38,065 systems to enhance cross-methodology consistency and improve distance network density.
• Employs a hierarchical data model where group-level distances are derived from individual galaxy contributions, with error propagation through statistical and systematic uncertainties.
• Relies on extensive observational data from optical, H I, and infrared surveys, with cross-verification across methods to identify and mitigate outliers.

Experimental results

Research questions

• RQ1 What is the most accurate and consistent estimate of the Hubble constant $H_0$ when combining multiple independent distance indicators?
• RQ2 How do different distance indicators (e.g., TF, FP, SNe Ia) compare in consistency and precision when calibrated to a common absolute scale?
• RQ3 To what extent can peculiar velocity fields be reliably reconstructed from a heterogeneous, multi-method distance catalog?
• RQ4 What is the impact of systematic uncertainties on the final Hubble constant estimate, and how can they be quantified?
• RQ5 How does the integration of galaxy groups improve the robustness and spatial coverage of cosmological distance measurements?

Key findings

• The final Hubble constant estimate from Cosmicflows-4 is $H_0 = 74.6 \pm 0.8$ km s⁻¹ Mpc⁻¹ with a small statistical uncertainty.
• A large potential systematic error of approximately 3 km s⁻¹ Mpc⁻¹ remains, indicating unresolved systematics across methodologies.
• The Tully-Fisher and Fundamental Plane relations provide the largest contributions, with over 55,000 galaxies and 38,000 groups included.
• Type Ia supernovae contribute with ~7% accuracy out to $z \sim 0.1$, while SBF and TRGB methods provide high-precision anchors within 20 Mpc.
• The Bayesian MCMC merging process successfully reconciles overlapping distance estimates, reducing inconsistencies across methods.
• The catalog is publicly available as CF4 All Groups and CF4 All Group Velocities via the Extragalactic Distance Database at https://edd.ifa.hawaii.edu.