Forward-modelling Milky Way Cepheids: selection effects and physical priors in the Gaia-HST calibration

Abstract

The advent of high-precision Gaia parallaxes for Milky Way Cepheids enables per cent-level calibration of the local distance ladder and the Hubble constant $H_0$. We revisit the Milky Way Cepheid calibration from Gaia EDR3 parallaxes using a fully forward-modelled Bayesian framework that simultaneously infers the period--luminosity relation, the Gaia parallax zero-point offset, and individual stellar distances while explicitly incorporating the disc geometry of the Galaxy through the distance prior and the selection functions specified in two HST SH0ES campaigns. We derive an analytic treatment of the detection probability that accounts for magnitude, parallax, period, and extinction cuts and reduces it to a tractable integral over distance and sky position. Posterior predictive checks show that this generative model matches the observed distributions of parallaxes, magnitudes, and periods. Modelling Galactic structure and survey truncation self-consistently in a Bayesian framework yields period--luminosity parameters that agree with the SH0ES maximum-likelihood values at the $<0.5\sigma$ level, a consequence of the small intrinsic scatter of the Cepheid period--luminosity relation. Adopting the uniform-in-volume prior recently advocated by Högås & Mörtsell (2026), without simultaneously accounting for selection, leads to a $\sim 0.05\mathrm{mag}$ bias in the period--luminosity zero-point and posterior predictive distributions incompatible with the observed data; this shift is mostly driven by the omission of the selection model, and produces an apparent and unjustified shift in $H_0$ that reflects this mismodelling. A consistent Bayesian treatment of Galactic structure and selection effects reinforces the local distance-ladder determination of $H_0$, and hence the Hubble tension with early-Universe inferences.

21 pages, 6 figures, 5 appendices. Accepted in MNRAS

References in corpus (53)

• A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team
• A 2.4% Determination of the Local Value of the Hubble Constant
• The No-U-Turn Sampler: Adaptively Setting Path Lengths in Hamiltonian Monte Carlo
• Some Aspects of Measurement Error in Linear Regression of Astronomical Data
• A 3D Dust Map Based on Gaia, Pan-STARRS 1 and 2MASS
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
• Cosmic Distances Calibrated to 1% Precision with Gaia EDR3 Parallaxes and Hubble Space Telescope Photometry of 75 Milky Way Cepheids Confirm Tension with LambdaCDM
• Modelling the Galactic Interstellar Extinction Distribution in Three Dimensions
• A distance to the Large Magellanic Cloud that is precise to one per cent
• Gaia Early Data Release 3 -- Catalogue validation
• The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics
• An Improved Distance to NGC 4258 and its Implications for the Hubble Constant
• Parallax Systematics and Photocenter Motions of Benchmark Eclipsing Binaries in Gaia EDR3
• To H0 or not to H0?
• Correcting Type Ia Supernova Distances for Selection Biases and Contamination in Photometrically Identified Samples
• New Period-Luminosity and Period-Color Relations of Classical Cepheids: I. Cepheids in the Galaxy
• Composable Effects for Flexible and Accelerated Probabilistic Programming in NumPyro
• Fast likelihood-free cosmology with neural density estimators and active learning
• Clarifying the Hubble constant tension with a Bayesian hierarchical model of the local distance ladder
• Cosmicflows-4
• Cluster Cepheids with High Precision Gaia Parallaxes, Low Zeropoint Uncertainties, and Hubble Space Telescope Photometry
• UNITY: Confronting Supernova Cosmology's Statistical and Systematic Uncertainties in a Unified Bayesian Framework
• Improved constraints on cosmological parameters from SNIa data
• Determining $H_0$ with Bayesian hyper-parameters
• Improved Treatment of Host-Galaxy Correlations in Cosmological Analyses With Type Ia Supernovae
• Sensitivity of the Hubble Constant Determination to Cepheid Calibration
• The parallax zero point offset from Gaia EDR3 data
• BAHAMAS: new SNIa analysis reveals inconsistencies with standard cosmology
• The effect of metallicity on Cepheid Period-Luminosity relations from a Baade-Wesselink analysis of Cepheids in the Milky Way and Magellanic Clouds
• An Improved Calibration of the Wavelength Dependence of Metallicity on the Cepheid Leavitt law
• SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field
• The Parallax Zero-Point of Gaia Early Data Release 3 from LAMOST Primary Red Clump Stars
• A 0.9% Calibration of the Galactic Cepheid luminosity scale based on Gaia DR3 data of open clusters and Cepheids
• RR Lyrae variables in Messier 53: Near-infrared Period--Luminosity relations and the calibration using Gaia Early Data Release 3
• The Local Distance Network: a community consensus report on the measurement of the Hubble constant at 1% precision
• A new and homogeneous metallicity scale for Galactic classical Cepheids II. The abundance of iron and alpha elements
• Cepheid metallicity in the Leavitt law (C-MetaLL) survey -- III. Simultaneous derivation of the Gaia parallax offset and Period-Luminosity-Metallicity coefficients
• The Milky Way as Seen by Classical Cepheids II: Spiral Structure
• Marginalised Normal Regression: Unbiased curve fitting in the presence of x-errors
• Two per cent measurement of $H_0$ from Cepheids alone
• Estimation of the Radial Scale Length and Vertical Scale Height of the Galactic Thin Disk from Cepheids
• The Milky Way as Seen by Classical Cepheids I: Distances Based on Mid-infrared Photometry
• A hierarchical Bayesian model to infer PL(Z) relations using Gaia parallaxes
• Orbital parallax of binary systems compared to GAIA DR3 and the parallax zero-point offset at bright magnitudes
• Accounting for Selection Effects in Supernova Cosmology with Simulation-Based Inference and Hierarchical Bayesian Modelling
• Converging on the Cepheid Metallicity Dependence: Implications of Non-Standard Gaia Parallax Recalibration on Distance Measures
• Physically-motivated priors in the local distance ladder significantly reduce the Hubble tension
• The subtle statistics of the distance ladder: On the distance prior and selection effects
• Testing cosmic anisotropy with cluster scaling relations
• Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey: IX. Metallicity dependence of Period-Wesenheit relations based on a homogeneous spectroscopic sample
• The Effect of a Non-universal Extinction Curve on the Wesenheit Function and Cepheid Distances
• No evidence for missing covariance in the Pantheon+ SuperNova sample distance moduli

Cited by in corpus (1)

• Constraints on the gravitational potential from DESI DR2 BAO and its implications for the local void scenario