Nebula size compared to galaxy
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Nebula Size Compared to Galaxy: Key Observations
Typical Sizes of Nebulae and Galaxies
Large emission-line nebulae, such as Lyα nebulae (also called Lyα blobs), can reach sizes from about 40 kpc up to 400 kpc, with some of the most gigantic examples spanning over 400 kpc in diameter 47. In contrast, typical galaxies, especially at high redshift, have much smaller effective radii, often just a few kpc to a few tens of kpc . For example, the [O III] nebula around the radio galaxy 4C +19.71 is 74 × 9 kpc, while the host galaxy itself is much more compact .
Nebulae Exceeding Host Galaxy Size
Many nebulae are much larger than the galaxies they surround. For instance, the enormous Lyα nebula (ELAN) discovered in a massive overdensity at z = 2.3 has an end-to-end spatial extent of 442 kpc, far exceeding the size of any individual galaxy within the region . Similarly, the Ivory Nebula, the largest in a recent survey, spans about 365 kpc, dwarfing the galaxies associated with it . Another example is a nebula detected around a galaxy at z = 5.89, which extends 25 kpc in diameter—much larger than the central galaxy’s effective radius .
Environmental Context and Overdensities
These large nebulae are often found in regions with a high density of galaxies, such as galaxy overdensities or proto-clusters 2457. The presence of such nebulae is a signpost of ongoing massive galaxy formation and dynamic interactions in these environments. Galaxies within these nebulae tend to be brighter, more massive, and have higher star formation rates than typical field galaxies, but the nebulae themselves still extend far beyond the galaxies’ visible boundaries .
Morphological Features and Special Cases
Some nebulae display unique features, such as the giant "jellyfish" nebula around the quasar 3C 275.1, which has ionized gas tails extending up to 170 kpc—again, much larger than the host galaxy . These features are not only larger than the galaxies but also show complex interactions with their environments, such as outflows, ram pressure stripping, or AGN-driven processes 38.
Comparison with Local Universe Nebulae
At lower redshifts, nebulae like the so-called "Green Bean" galaxies are larger and less concentrated than compact star-forming galaxies but are similar in size to typical Type 2 AGN host galaxies . However, even in these cases, the nebulae can extend well beyond the main stellar body of the galaxy.
Conclusion
In summary, nebulae—especially Lyα and [O III] emission-line nebulae—are often much larger than the galaxies they surround, sometimes by an order of magnitude or more. While typical galaxies have sizes of a few to a few tens of kpc, nebulae can span hundreds of kpc, making them some of the largest coherent structures associated with galaxies. These vast nebulae are key tracers of galaxy evolution, environmental effects, and the dynamic processes shaping the universe’s largest structures 1234+4 MORE.
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