Local Void
The Silent Neighbor of the Milky Way
Quick Reader
| Attribute | Details |
|---|---|
| Name | Local Void |
| Type | Cosmic void (nearby underdense region) |
| Location | Adjacent to the Local Group, extending toward constellations Aquila, Ophiuchus, and Sagitta |
| Distance from Earth | Begins just beyond the Local Group (~10–20 million light-years) |
| Size Estimate | ~60–150 million light-years (depending on definition) |
| Density | Very low – significantly fewer galaxies than surrounding zones |
| Discovery | First noted by Tully & Fisher (1987) via velocity field and galaxy mapping |
| Surrounding Structures | Virgo Supercluster, Sculptor Group, Centaurus A Group |
| Dominant Feature | Contributes to the peculiar velocity of the Milky Way |
| Scientific Importance | Key to understanding local gravitational flows and dark energy influence |
| Observation | Detected through redshift surveys and cosmic flow measurements |
| Visibility | Not directly visible; inferred from galaxy distributions and motion |
Introduction – An Invisible Force Behind the Milky Way’s Motion
Though our night sky appears full of stars and galaxies, the universe contains vast regions where almost nothing resides. One such zone — right next to us — is known as the Local Void.
Stretching outward from the edge of the Local Group, the Local Void is a low-density expanse that exerts surprising influence. Despite its emptiness, it is thought to play a major role in:
The movement of the Milky Way
The acceleration of nearby galaxies
The overall gravitational dynamics of the local universe
This nearby underdense region might not be dramatic in appearance, but its presence is dynamically powerful — pushing matter outward and shaping the velocity field we observe in the nearby cosmos.
What Is the Local Void?
The Local Void is a cosmic underdensity that begins just beyond our Local Group (~10–20 million light-years from Earth) and stretches outward for up to 150 million light-years, depending on how its boundaries are defined.
Key Characteristics:
Extremely low galaxy density
Elongated and possibly multi-chambered
Separated from denser regions like Virgo Cluster and Hydra-Centaurus
Dominated by intergalactic gas and sparse dwarf galaxies (if any)
It was first recognized as a true dynamical structure when observations showed that our Local Group is moving away from it — suggesting the void is repelling galaxies gravitationally, while denser zones attract.
Discovery and Mapping
First Identified:
1987 by Brent Tully & Richard Fisher
Based on peculiar velocity flow and galaxy redshift absence
Modern Surveys:
2MASS Redshift Survey: Refined void boundaries
Cosmicflows Project: Measured gravitational influence
ALFALFA Survey: Checked for HI gas and low-luminosity galaxies
These studies confirmed that the Local Void:
Contains fewer galaxies per volume than the cosmic average
Acts as a source of peculiar motion for the Milky Way and nearby groups
Has no central supercluster — it’s defined by the absence of structure
Neighboring Structures and Void Boundaries
The Local Void does not exist in isolation. It is flanked by several important galaxy-rich regions:
| Region | Direction | Distance | Notes |
|---|---|---|---|
| Virgo Supercluster | East/Northeast | ~60 Mly | Major mass attractor |
| Sculptor Group | South | ~10–15 Mly | Loose filament |
| Centaurus A Group | South | ~13 Mly | Bright elliptical core |
| Leo Spur | North | ~10–20 Mly | Local filament connecting to Virgo |
| Great Attractor Region | Farther southeast | ~150–200 Mly | Strong gravity source |
These walls form a cavity — the Local Void lies between them, like a balloon enclosed by rigid filaments.
How It Affects the Milky Way
Surprisingly, the Milky Way’s motion through space is not purely due to attraction — a part of it comes from being pushed away from the Local Void.
Motion Breakdown:
Local Group is moving at ~600 km/s
Toward Virgo Supercluster (~200–300 km/s component)
Toward Great Attractor (additional ~200–300 km/s)
And away from the Local Void (~100–200 km/s)
This suggests the Local Void acts like a gravitational low-pressure zone, where underdensity leads to expansion and pushes galaxies outward.
Inside the Void – What (Little) We Find
While the Local Void is defined by its emptiness, it is not a perfect vacuum. A few scattered galaxies, mostly dwarfs and irregulars, do exist within or near its edges. These rare inhabitants are critical for understanding galaxy formation in isolation.
Types of Galaxies Inside the Local Void
| Galaxy Name | Type | Distance | Notes |
|---|---|---|---|
| KK 246 | Dwarf irregular | ~23 Mly | One of the most isolated galaxies known |
| ESO 461-36 | Dwarf elliptical | ~22 Mly | Possibly embedded within void core |
| UGC 4879 | Dwarf irregular | ~13 Mly | Edge of void, studied in detail |
| Leo T | Dwarf spheroidal | ~1.4 Mly | Very faint, low surface brightness |
| ALFA ZOA Objects | Various | ~10–30 Mly | Detected in HI surveys across Zone of Avoidance |
These galaxies typically:
- Show low star formation rates
- Contain high gas fractions
- Lack clear structure or rotation
- Appear dynamically primitive
Star Formation in the Void
Galaxies inside the Local Void often exhibit:
Isolated star-forming regions
Bursty star formation history (sporadic rather than steady)
High ratio of neutral hydrogen (HI) to stellar mass
Because they are far from external tidal forces or environmental effects like ram-pressure stripping, they offer a clean environment for testing theories of:
Primordial galaxy formation
Internal regulation of star formation
Dark matter halo development in isolation
Observational Challenges
Despite its proximity, the Local Void is difficult to study due to:
1. Low Surface Brightness
Most void galaxies are:
Faint
Diffuse
Easy to miss in optical surveys
2. Zone of Avoidance
Part of the Local Void overlaps the Milky Way’s disk, obscuring it behind:
Dust and gas
Foreground stars
Infrared noise
This region is only accessible through:
Infrared surveys (e.g., 2MASS)
Radio HI surveys (e.g., ALFALFA, HIPASS)
3. Distance Measurement Complexity
Galaxies in or near the void often have:
Low redshift
Motion influenced more by peculiar velocities than cosmic expansion
Requiring distance indicators like:
Tip of the Red Giant Branch (TRGB)
Cepheid variables
Surface brightness fluctuations
Thus, mapping the void demands precision distance measurements, not just redshift catalogs.
Simulation Insights – What the Models Show
Modern cosmological simulations like the Illustris and Millennium Simulation reproduce voids similar to the Local Void, revealing:
Structural Properties:
Irregular in shape (not spherical)
Composed of nested subvoids or chambers
Bordered by walls and filaments
Dynamical Properties:
Voids expand faster than average cosmic regions
Material flows outward from void centers toward denser zones
Galaxy velocity vectors show radial divergence from void cores
In such models, the Local Void behaves like a local repeller, influencing the Milky Way’s peculiar motion by gravitational underdensity — complementing the attractive pull of structures like Virgo and the Great Attractor.
Galaxy Density Profile
| Region | Density (relative to cosmic average) |
|---|---|
| Local Void Core | ~10–15% |
| Intermediate Zone | ~25–30% |
| Void Boundary | ~60–70% |
| Virgo Cluster | ~300–500% |
This illustrates the steep gradient between the Local Void and its surrounding walls, helping shape both local flow fields and observational perspectives.
Scientific Relevance – Why the Local Void Matters
Despite its quiet nature, the Local Void is a key player in our understanding of the local cosmic environment and the forces shaping galaxy motion and structure.
1. The Local Void and the Milky Way’s Motion
The Local Void contributes to the peculiar velocity of the Milky Way — the velocity in excess of what would be expected from the expansion of the universe alone.
Galaxies flee voids: In cosmology, underdense regions like the Local Void are repellers, not attractors.
Milky Way velocity vector: A measurable component of our galaxy’s ~600 km/s motion is directed away from the Local Void.
This motion, mapped in Cosmicflows and redshift-independent distance surveys, confirms that voids shape cosmic flow patterns just as strongly as clusters do — albeit in the opposite way.
2. Implications for Cosmic Expansion and Dark Energy
The Local Void also allows us to:
Study how dark energy accelerates the expansion of low-density regions.
Model the growth of structure from initial quantum fluctuations to large-scale geometry.
Test predictions of ΛCDM (Lambda Cold Dark Matter) on void dynamics.
Since voids expand faster than denser regions, they can be used to:
Measure the local Hubble constant
Compare H₀ locally vs. cosmologically (tension with CMB results)
Place bounds on modified gravity or alternative dark energy models
3. Why the Local Void Is Ideal for Void Studies
Compared to distant voids like Boötes or Delphinus, the Local Void is:
Closer (starts just 10–20 Mly away)
Better resolved in redshift-independent data
Accessible to HI and optical follow-up
Surrounded by well-studied filaments and clusters (Virgo, Sculptor, Centaurus A)
This makes it a perfect test case for understanding:
Void formation
Void–galaxy interactions
Void-based cosmological constraints
Frequently Asked Questions (FAQ)
Q: What is the Local Void?
A: A nearby cosmic void — a region of space with significantly fewer galaxies than average — located just beyond the Local Group. It begins around 10–20 million light-years away and extends up to ~150 million light-years.
Q: Can we see the Local Void?
A: No, not directly. It is detected by:
The absence of galaxies in sky surveys
Velocity flow patterns of galaxies moving away from it
Redshift surveys and HI observations in underdense areas
Q: Are there any galaxies inside the Local Void?
A: Yes, a few isolated dwarf galaxies exist, like KK 246 and ESO 461-36. These are studied to understand star formation in low-density environments.
Q: How does the Local Void affect the Milky Way?
A: It contributes to our galaxy’s peculiar velocity, pushing it away due to gravitational underdensity. This complements the pull from denser regions like the Virgo Cluster and the Great Attractor.
Q: How big is the Local Void?
A: Estimates vary, but it may extend 60–150 million light-years depending on how its boundaries are defined.
Q: Why is the Local Void important for cosmology?
A: It helps:
Understand gravitational flows
Constrain the local value of the Hubble constant
Test models of dark energy and void expansion
Explore galaxy formation in isolation
Comparison with Other Voids
| Void Name | Distance from Earth | Size Estimate | Density | Notes |
|---|---|---|---|---|
| Local Void | 10–20 Mly (edge) | 60–150 Mly | Very low | Directly affects Milky Way motion |
| Boötes Void | ~700 Mly | ~330 Mly | Extremely low | Largest known void |
| Delphinus Void | ~250–300 Mly | ~100 Mly | Low | Bordered by Pegasus and Hercules |
| Eridanus Void | ~400 Mly | ~150 Mly | Moderate | Less mapped; partially obscured |
Final Thoughts – The Void Next Door
The Local Void is an unassuming but powerful force in our cosmic neighborhood. Though it appears invisible, it helps define:
The paths galaxies take
The structure of the local universe
How matter and energy interact on the largest scales
As redshift-independent measurements improve, and radio and infrared surveys deepen, we will uncover more about this quiet void and its subtle impact — proving that even in the universe, what’s missing matters just as much as what’s present.
