The Local Void & Milky Way Outflow
The Local Void (identified by Tully & Fisher 1987) is a vast cosmic underdensity beginning immediately at the edge of the Local Group and extending roughly 60 megaparsecs (200 million light-years) across the northern supergalactic hemisphere toward Aquila and Hercules. Because its center lies behind the dust-obscured Galactic Center (the Zone of Avoidance), it remained poorly mapped until 21-cm H I radio and TRGB space-telescope surveys revealed that its gravitational deficit pushes the Local Sheet away at v_push ≈ 260 km/s.
Physical Mechanism & Dynamics
The Milky Way and Andromeda sit directly on the bounding wall (the Local Sheet) of the Local Void. Because matter inside the void has been evacuated to less than 15% of the cosmic mean density, the gravitational attraction from the northern supergalactic hemisphere is much weaker than the pull from the Virgo and Hydra-Centaurus hemisphere to the south.
Using Hubble Space Telescope ACS Tip of the Red Giant Branch (TRGB) distances to dwarf galaxies perched on the rim of the Local Void (such as ESO 461-36 and KK 246), Tully et al. (2008, 2019) measured an orthogonal velocity component of 259 ± 15 km/s directed perpendicular to the Local Sheet away from the void's center.
Quantitatively, this mechanism is governed by Three-Vector Local Group Peculiar Velocity Decomposition (v_LG,CMB (627 km/s) = v_LocalVoid (259 km/s) + v_Virgo (185 km/s) + v_LargeScale (455 km/s)). Three nearly orthogonal dynamical vectors combining in quadrature to produce the Local Group's total 627 km/s velocity relative to the CMB.
Three-Vector Local Group Peculiar Velocity Decomposition
v_LG,CMB (627 km/s) = v_LocalVoid (259 km/s) + v_Virgo (185 km/s) + v_LargeScale (455 km/s)Three nearly orthogonal dynamical vectors combining in quadrature to produce the Local Group's total 627 km/s velocity relative to the CMB.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Local Void diameter | ~60 Mpc (200 million ly) | Cosmicflows-3 3D density reconstruction (Tully et al. 2019) |
| Outflow push on Local Sheet | 259 ± 15 km/s toward SGZ = -1 | TRGB distances to Local Sheet & rim dwarfs |
| Distance to void center | ~23 Mpc (75 million ly) | Located behind Galactic bulge in Sagittarius/Aquila |
| Interior dwarf prototypes | KK 246 (ESO 461-36), Pisces A/B | Isolated gas-rich dwarf irregulars inside/on rim |
Observational Evidence & How Scientists Know
Parkes and Arecibo 21-cm blind H I surveys (HIPASS and ALFALFA) penetrated the dusty Galactic plane where optical surveys are blind, confirming a dramatic dearth of H I-bearing galaxies at cz < 4,000 km/s toward the Galactic Center and verifying that the Local Void is a true physical underdensity rather than an artifact of Galactic dust extinction.
Across reference datasets (Planck 2018 Cosmological Parameters (PR3), SIMBAD Astronomical Database (CDS)), quantitative benchmarks for the local void & milky way outflow are anchored by: Local Void diameter (~60 Mpc (200 million ly), via Cosmicflows-3 3D density reconstruction (Tully et al. 2019)); Outflow push on Local Sheet (259 ± 15 km/s toward SGZ = -1, via TRGB distances to Local Sheet & rim dwarfs); Distance to void center (~23 Mpc (75 million ly), via Located behind Galactic bulge in Sagittarius/Aquila); Interior dwarf prototypes (KK 246 (ESO 461-36), Pisces A/B, via Isolated gas-rich dwarf irregulars inside/on rim).
Uncertainties, Systematics & Open Questions
Mapping the exact internal substructure dividing the Near Local Void from the Far Local Void behind the innermost ±5° of the Galactic bulge continues to improve with MeerKAT and SKA-pathfinder 21-cm surveys.
Real Astronomical Benchmarks & Cross-Domain Links
KK 246 (ESO 461-36)
Isolated dwarf irregular galaxy residing inside the Local Void at d ≈ 7.8 Mpc with an extended H I envelope
The Local Sheet
Flattened galaxy wall forming the southern bounding membrane of the Local Void
Boötes Void
Larger distant supervoid analogue at z ≈ 0.052
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- Planck 2018 Cosmological Parameters (PR3) (PR3 (2020 A&A 641, A6)) — Planck Collaboration, Aghanim, N., et al. 2020, A&A, 641, A6. Archive / Reference ↗
- SIMBAD Astronomical Database (CDS) (2026-10 release) — Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database). Archive / Reference ↗
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