@c3d
BLUF: Documenting the galaxy-constants errors I mentioned by email. The
Milky Way and Andromeda share identical mass/dark-matter values and several
mis-attributed references in constants.cc/constants.md. Corrected values below;
fix lands in the reference-cleanup PR (issue #1706),
as Reference 30–35.

Bugs

1. MG and MNGC224 are both 1.5E12_M☉ ± 0.5E12 — identical value and
   uncertainty for two different galaxies (copy-paste artifact). Same for
   fDMG = fDMNGC224 = 0.85 ± 0.05.
2. Mis-attributed references:
   • [11] (Andromeda mass/radius/distance) = Peñarrubia et al. 2014 — a paper
     on the Large Magellanic Cloud, not M31.
   • [12] (Andromeda mass) = Veljanoski et al. 2014 "M31 satellite plane" —
     not a mass measurement.

Corrected values (sourced)

• MNGC224 = 1.4 ± 0.4×10¹² M☉ — Watkins, Evans & An 2010 (recognized value,
  keeps M31 ≳ MW); corroborated by Zhang et al. 2024.
• DNGC224 = 2 482 000 ± 36 000 ly — Li & Riess 2021 (761 ± 11 kpc, most
  precise to date). The previous value's reference was the LMC paper.
• fDMG = 0.95 ± 0.02 and fDMNGC224 = 0.92 ± 0.03 — both derived within
  r_vir as fDM = 1 − M_baryonic/M_vir, on a common aperture so the two galaxies
  are comparable (M31 has more stars for a similar halo → slightly lower fDM).
• New sources: Li & Riess 2021, Ibata 2005, Sick 2015, Licquia & Newman 2015,
  Bland-Hawthorn & Gerhard 2016, Nieten 2006 (Reference 30–35).

Note

Fix belongs in the consolidated cleanup PR (post a/b/c merge) so the new 30–35
numbers don't collide with the astronomy reclassification (27–29).