pylgframework

A software framework for the Local Group.

The barycentre of the Local Group is routinely adopted as a point estimate. pylgframework replaces the single-line centre-of-mass calculation with a Monte Carlo that propagates the observational uncertainty of the member galaxies, and returns the barycentre together with its uncertainty region.

Frutos Plaza, M. (2026). Membership Systematics of the Local Group Barycentre: Uncertainty Region, Dependence on Member Galaxies, and Consistency with the CMB Dipole. (manuscript; data and code at doi:10.5281/zenodo.23440772)

Status

Installation

pip install -e .                 # core: numpy, pandas, astropy
pip install -e ".[rebound]"      # adapter for REBOUND simulations
pip install -e ".[test]"         # development and tests

Quick start

from pylgframework import barycentre, compare, fiducials

res = barycentre(fiducials.WU, n=1000, seed=2000)
print(res)                       # position, velocity, principal axes, CMB-frame speed, assumptions

res.median_position              # kpc, Galactocentric
res.axes                         # 1-sigma principal axes of the barycentre cloud (kpc)
res.vcmb("4")                    # speed w.r.t. the CMB (16th, 50th, 84th percentile, km/s)
res.cloud                        # DataFrame with every realization

# Paired audit of two literature fiducials (identical random draws)
compare(fiducials.WU, fiducials.VDM2012, n=1000, seed=3000)

Available fiducials: WU (Wu et al. 2026), SAWALA (Sawala et al. 2025), VDM2019, VDM2012 and VDM2012_3B (van der Marel et al. 2012, four-body and faithful three-body variants). Each one carries its reference and notes (fiducials.WU.reference).

Memberships: "2" (MW+M31), "3M" (+M33), "3L" (+LMC), "4" (all four).

Assumptions

• Observables (distance modulus, proper motions, radial velocity, mass) are drawn from Gaussians truncated at ±2σ (the effective standard deviation is about 0.88σ).
• Sky positions are held fixed; the MW is at rest at the origin of the Galactocentric frame (R0 = 8.122 kpc, v⊙ = (12.9, 245.6, 7.78) km/s).
• The output is the uncertainty induced by the declared errors, not a complete physical uncertainty region.
• The uncertainty model is fixed across fiducials unless sigmas= is supplied.

Reproducibility

Realization i of an ensemble with seed s uses numpy.random.default_rng([s, i]). This scheme is part of the public interface. The test suite reproduces Table 1 and Sects. 3 and 4 of the manuscript:

pytest            # fast tests

Verified against the published values (Table 1 of the manuscript) with astropy 7.2.0 (Linux, Python 3.12) and astropy 8.0.2 (Linux, Python 3.12; Windows, Python 3.11).

References

The bibliography of the manuscript, including the sources of every fiducial and of the CMB dipole, is in docs/references.bib.

License

MIT.