A reanalysis of the megamaser Hubble constant: from spot catalogues to peculiar velocities
Abstract
The Hubble tension motivates careful scrutiny of the redshift-independent distances underpinning the local distance ladder. Water megamasers afford a purely geometric distance from orbital dynamics in an accretion disc. For five of the six Megamaser Cosmology Project (MCP) galaxies, we present a Bayesian forward model that infers the angular-diameter distance and warped Keplerian disc parameters jointly from the very long baseline interferometry quasi-observables: maser spot positions, velocities, and accelerations. We then constrain H0 from these distances and the host redshifts, including a physical distance prior, modelling the galaxy sample selection and testing a linear and a non-linear peculiar-velocity reconstruction. Accounting for peculiar velocities with the non-linear Manticore reconstruction, we infer H0 = 71.6 ± 3.7 km/s/Mpc, where the uncertainty is statistical. Relative to ignoring peculiar velocities, the linear and non-linear reconstructions lower H0 by $0.8−−1.4$ and $1.6−−2.1$ km/s/Mpc, respectively, whereas changing the selection variable alters it by less than $1$ km/s/Mpc. We analyse the sixth galaxy, NGC 4258, separately. Modelling its disc as eccentric and quadratically warped, we infer an angular-diameter distance of 7.558 ± 0.073 (stat.) Mpc, within 0.13σ of the published maser distance.
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BibTeX
@misc{stiskalek2026reanalysis,
title = {{A reanalysis of the megamaser Hubble constant: from spot catalogues to peculiar velocities}},
author = {Richard Stiskalek and Harry Desmond},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17684},
eprint = {2609.17684},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17684},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A reanalysis of the megamaser Hubble constant: from spot catalogues to peculiar velocities
AU - Richard Stiskalek
AU - Harry Desmond
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2609.17684
UR - https://arxiv.org/abs/2609.17684
AB - The Hubble tension motivates careful scrutiny of the redshift-independent distances underpinning the local distance ladder. Water megamasers afford a purely geometric distance from orbital dynamics in an accretion disc. For five of the six Megamaser Cosmology Project (MCP) galaxies, we present a Bayesian forward model that infers the angular-diameter distance and warped Keplerian disc parameters jointly from the very long baseline interferometry quasi-observables: maser spot positions, velocities, and accelerations. We then constrain H_0 from these distances and the host redshifts, including a physical distance prior, modelling the galaxy sample selection and testing a linear and a non-linear peculiar-velocity reconstruction. Accounting for peculiar velocities with the non-linear Manticore reconstruction, we infer H_0 = 71.6 ± 3.7 km/s/Mpc, where the uncertainty is statistical. Relative to ignoring peculiar velocities, the linear and non-linear reconstructions lower H_0 by $0.8−−1.4$ and $1.6−−2.1$ km/s/Mpc, respectively, whereas changing the selection variable alters it by less than $1$ km/s/Mpc. We analyse the sixth galaxy, NGC 4258, separately. Modelling its disc as eccentric and quadratically warped, we infer an angular-diameter distance of 7.558 ± 0.073 (stat.) Mpc, within 0.13σ of the published maser distance.
ER -