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Zone of Avoidance

The Zone of Avoidance (ZoA), also known as the Zone of Galactic Obscuration, is a region of the celestial sphere where the view of distant galaxies and other extragalactic objects is heavily obscured by the interstellar dust and gas concentrated in the disk of the Milky Way galaxy, rendering it appear largely devoid of such sources at optical wavelengths.^[1] This obscuration arises primarily from the absorption and scattering of visible light by dense clouds of gas and dust aligned along the galactic plane, a feature inherent to spiral galaxies like the Milky Way where most of the interstellar medium is confined to a thin disk.^[2] The ZoA encompasses roughly 20% of the sky, forming an irregular band centered on the galactic equator and reaching its widest extent in the direction of the constellation Sagittarius, where extinction levels are particularly high due to elevated concentrations of obscuring material.^[1] Historically, the ZoA was first noted in the early 19th century by astronomers William and John Herschel during their sky surveys, who observed a conspicuous band along the Milky Way with few "nebulae" (now known as galaxies), attributing it to the galaxy's structure rather than a true absence of objects.^[3] This phenomenon played a key role in the 1920 Great Debate between Harlow Shapley and Heber Curtis, where the scarcity of spiral nebulae in the galactic plane supported arguments about the scale of the universe and the position of the Sun within it.^[4] In modern astronomy, the ZoA poses significant challenges for mapping large-scale cosmic structures, as it conceals approximately one-fifth of the extragalactic sky, potentially hiding clusters, superclusters, and anomalies like the Great Attractor—a massive gravitational anomaly influencing the motion of our galaxy. To overcome these obstacles, astronomers employ multi-wavelength observations that penetrate the dust: radio telescopes detect neutral hydrogen emissions from galaxies via the 21 cm line, allowing redshift measurements even in obscured regions; infrared surveys, such as those from the Infrared Astronomical Satellite (IRAS) and the Two Micron All-Sky Survey (2MASS), reduce the effective ZoA to about 10% of the sky by capturing longer wavelengths less affected by extinction; and X-ray observatories like Chandra reveal distant active galactic nuclei and hot gas through high-energy emissions that pass through intervening material.^[1]^[5] These techniques have uncovered hundreds of previously hidden galaxies, enhancing our understanding of the local universe's distribution and dynamics, including evidence of galaxy clusters behind the ZoA that contribute to the Milky Way's peculiar velocity.^[2] Ongoing efforts, such as near-infrared galaxy catalogs from surveys like VVV (VISTA Variables in the Vía Láctea), continue to populate the ZoA, demonstrating that it is not a void but a veiled window into deeper cosmic realms. More recently, as of 2025, the James Webb Space Telescope (JWST) has revealed faint galaxies in the ZoA using near-infrared imaging, further populating this region.^[6]^[7]

Definition and Characteristics

Definition

The Zone of Avoidance (ZOA) is the region of the celestial sphere obscured by the disk of the Milky Way galaxy, which blocks or significantly attenuates light from background extragalactic sources, thereby hindering the detection of distant galaxies and large-scale cosmic structures.^[8] This obscuration primarily arises from interstellar dust and dense gas concentrated along the galactic plane, creating an apparent paucity of objects in optical observations, and to a lesser extent in near-infrared observations.^[9] In sky maps constructed from visible light surveys, the ZOA manifests as a prominent "hole" or gap in the distribution of galaxies, spanning the entire length of the galactic equator and complicating efforts to map the full three-dimensional structure of the local universe.^[10] The region covers approximately 20% of the sky, roughly corresponding to galactic latitudes of |b| ≲ 10°–20°, though the exact boundaries can vary irregularly depending on the wavelength, sensitivity of the observations, and dust distribution.^[9]^[10] While the galactic center refers specifically to the compact, dense core of the Milky Way at galactic coordinates l = 0°, b = 0°, the Zone of Avoidance encompasses the much broader band of obscuration along the entire galactic disk, affecting views toward all longitudes.^[8] Interstellar dust serves as the primary obscuring agent in this region, absorbing and scattering shorter-wavelength light from behind.^[9]

Extent and Boundaries

The Zone of Avoidance is delineated in galactic coordinates as the region where the absolute value of the galactic latitude |b| is roughly less than 10° to 20°, spanning approximately 20% of the celestial sphere.^[11]^[12] This obscured zone manifests as a broad, irregular band aligned with the galactic equator in all-sky maps, its uneven boundaries arising from the patchy distribution of interstellar dust along different lines of sight.^[13]^[14] The zone is widest in the direction of the constellation Sagittarius, where extinction levels are particularly high due to elevated concentrations of obscuring material. Obscuration density within the zone varies significantly with galactic longitude l, reaching higher levels near the galactic center (l ≈ 0°) and anticenter (l ≈ 180°), where sightlines traverse denser concentrations of galactic material.^[15] In terms of quantitative impact, the visual extinction A_V across the galactic plane varies significantly with direction, typically ranging from a few to over 20 magnitudes, and exceeding 30 magnitudes in some directions toward the center due to intensified dust accumulation.^[15]^[16]

Historical Background

Early Observations

Early observations of the obscuring effects associated with the Zone of Avoidance emerged from 18th- and 19th-century astronomical surveys, which documented a notable scarcity of visible nebulae aligned with the plane of the Milky Way. William Herschel's systematic sweeps of the sky in the 1780s, using his large reflecting telescopes, produced catalogs of thousands of nebulae and star clusters that revealed a flattened stellar distribution and fewer detectable nebulae in directions toward the galactic plane, hinting at some form of visual obstruction amid the dense star fields. His "star-gauging" method, involving counts of stars across 683 regions, further illustrated this asymmetry, with denser concentrations along the Milky Way but gaps in nebula visibility perpendicular to it.^[17] In the 19th century, John Herschel's comprehensive General Catalogue of Nebulae and Clusters of Stars (1864), based on observations from the Cape of Good Hope and building on his father's work, quantified this pattern by listing over 5,000 objects and highlighting a pronounced depletion of nebulae within about 15 degrees of the Milky Way's plane. This catalog provided the empirical foundation for Richard A. Proctor's 1878 analysis, where he explicitly described the region as the "Zone of Few Nebulae," attributing the scarcity to the distribution of known objects rather than observational bias at the time. The 20th century brought clearer realizations of the galactic disk's role as an observational barrier. Harlow Shapley's seminal 1918 study of 69 globular clusters, using Cepheid variables for distance measurements, mapped their spatial distribution in a halo surrounding a central bulge, with a marked avoidance of the galactic plane that underscored the disk's density as a hindrance to line-of-sight views. This work shifted understanding of the Milky Way's structure, positioning the Sun far from the center and emphasizing the plane's obscuring influence on deeper cosmic features.^[18] Edwin Hubble's photographic surveys in the 1920s at Mount Wilson Observatory captured the paucity of extragalactic "nebulae" (now recognized as galaxies) along the galactic plane, reinforcing the concept of a "zone of few nebulae" through counts that dropped sharply in low-latitude regions.^[19] By the 1930s, Harlow Shapley and Adelaide Ames conducted extensive surveys, including their 1932 catalog of 1,249 external galaxies brighter than 13.2 magnitude, which starkly illustrated the scarcity in the galactic plane—fewer than expected detections within 10 degrees of latitude—confirming the zone's impact on extragalactic astronomy. These efforts highlighted interstellar material as the likely cause, though detailed explanations awaited later investigations.^[19]

Formal Recognition and Naming

The term "Zone of Avoidance" was first used by astronomer Edwin Hubble in 1934 to describe the apparent scarcity of extragalactic nebulae along the plane of the Milky Way, attributing this to obscuration by interstellar dust and gas.^[20] Shapley had earlier described a similar avoidance zone for globular clusters in 1918. This naming reflected the systematic gap in observations along the galactic equator, a concept Shapley further emphasized during the 1920 Great Debate on the scale of the universe, where he applied it to the distribution of spiral nebulae obscured by the Milky Way's structure. In the mid-20th century, particularly during the 1940s and 1950s, the term gained formal recognition in astronomical literature as a designated region of galactic obscuration affecting extragalactic studies. Discussions of this phenomenon, such as those on the role of dust in blocking optical views of distant objects, helped integrate the Zone of Avoidance into standard parlance; for instance, early post-war analyses of galactic structure highlighted its implications for mapping the universe beyond the Milky Way. Key publications by Bart J. Bok and Priscilla F. Bok in the 1950s, including their comprehensive works on interstellar matter and the Milky Way's architecture, formalized the zone as a deliberate avoidance area in optical surveys, emphasizing its boundaries and the need for alternative wavelengths to probe it.^[21] By the 1960s, the term achieved widespread adoption through international astronomical bodies and major catalogs. The International Astronomical Union (IAU) referenced the Zone of Avoidance in proceedings on galactic coordinates and large-scale structure, establishing it as a critical consideration for global surveys. The Uppsala General Catalogue of Galaxies (UGC), initiated in the late 1950s under Bengt Nilson and published in 1973, explicitly delineated the zone by limiting entries to regions with galactic latitudes |b| > 20°, thereby marking it as an excluded band to ensure reliable optical data. A key milestone in the 1970s came through specialized conferences, including IAU Symposium 58 on "The Large Scale Structure of the Universe" in 1973, which standardized the Zone of Avoidance's definition as the low-latitude band (|b| ≲ 10°–20°) where dust extinction renders traditional observations infeasible, prompting advancements in multi-wavelength techniques. This institutional endorsement solidified its role in modern cosmology, distinguishing it from mere observational gaps to a defined astrophysical barrier.

Physical Causes

Interstellar Dust Extinction

The obscuration in the Zone of Avoidance arises primarily from interstellar dust grains, which are composed mainly of amorphous silicates (such as magnesium-iron silicates) and carbonaceous materials, including graphite and polycyclic aromatic hydrocarbons (PAHs). These grains typically range in size from 0.01 to 1 μm, with a power-law size distribution favoring smaller particles. Silicate grains dominate the mass budget, while carbonaceous components contribute significantly to the ultraviolet absorption features; both types efficiently absorb and scatter ultraviolet and optical photons, leading to the dimming and reddening of background starlight.^[22] The wavelength dependence of this extinction follows an approximate inverse relation, A_\lambda \propto 1/\lambda, but is more accurately captured by empirical parametrizations of the interstellar extinction curve. A widely adopted form is that of Cardelli et al. (1989), which expresses the relative extinction as

\frac{A_\lambda}{A_V} = a(\lambda) + \frac{b(\lambda)}{R_V},

where A_V is the visual extinction, R_V \approx 3.1 is the total-to-selective extinction ratio for the diffuse interstellar medium, and a(\lambda), b(\lambda) are functions fitted to observational data across optical, ultraviolet, and near-infrared wavelengths (with \lambda in micrometers). This curve features a prominent bump at 2175 Å due to carbonaceous grains and a steeper rise toward shorter wavelengths, reflecting the combined effects of grain size distribution and composition.^[23] Interstellar dust is not uniformly distributed but exhibits higher densities within the Milky Way's spiral arms, where star formation enhances grain production and accumulation. Additionally, the dust forms in patchy, clumpy structures such as molecular clouds and diffuse filaments, resulting in highly variable extinction maps along different lines of sight through the galactic plane. These inhomogeneities can cause local enhancements in obscuration by factors of several magnitudes. The overall effect is a reduction in the observed flux F_\mathrm{obs} of extragalactic sources by F_\mathrm{obs} = F_0 e^{-\tau}, where the line-of-sight optical depth \tau at visual wavelengths typically ranges from 1 to 10 in the plane, corresponding to visual extinctions A_V of several to tens of magnitudes.^[24]^[16]

Galactic Disk Structure

The Milky Way's galactic disk exhibits a flattened, thin structure that significantly contributes to the obscuration in the Zone of Avoidance through its geometry and density distribution. The thin disk has a characteristic scale height of approximately 300 pc, reflecting the vertical exponential decline in stellar and gaseous components away from the midplane. This thin profile results in the disk subtending an angular extent of roughly 20–30° in galactic latitude when viewed from the Solar System's position, encompassing the primary region of the Zone of Avoidance. At the galactic center, a prominent bulge concentrates stellar populations, enhancing the overall density and complicating observations along low-latitude lines of sight.^[25]^[26] Stellar crowding within the disk further exacerbates the challenges posed by the Zone of Avoidance, independent of dust extinction. In the galactic plane near the Solar neighborhood, the stellar number density reaches about 0.14 stars per cubic parsec, leading to source confusion where foreground stars overwhelm background extragalactic signals. This density increases towards the inner disk and bulge, creating a veil of overlapping stellar light that pollutes sightlines and limits the resolution of distant objects. While interstellar dust remains the dominant factor in absorption, the sheer number of stars along these paths amplifies the effective "light pollution" in optical and near-infrared regimes. The gaseous components of the disk, including neutral hydrogen (HI) and molecular clouds, contribute secondarily to the obscuration by scattering and absorbing light, though their impact is less pronounced than dust. The HI disk has a scale height of about 85 pc and extends radially to around 7 kpc, while molecular gas, primarily in dense clouds, is more centrally concentrated with a scale height of roughly 45 pc. These components trace the spiral arms and inner regions, adding patchy absorption that aligns with the disk's overall geometry.^[26] In three-dimensional models using cylindrical coordinates (with radius R from the center, azimuthal angle φ, and height z above the plane), the disk's structure reveals extended path lengths along low-latitude sightlines, reaching up to 10 kpc or more towards the galactic center due to the Sun's position at about 8 kpc from the nucleus. This prolonged traversal through dense material underscores the Zone of Avoidance's role in hiding approximately 20% of the extragalactic sky, necessitating multi-wavelength approaches to peer through the disk.^[27]

Observational Challenges

Optical Limitations

The Zone of Avoidance (ZoA) presents severe challenges for visible-light astronomy primarily due to interstellar dust, which absorbs and scatters over 90% of optical light in its densest regions along the Galactic plane, rendering vast areas appear as "blank" voids devoid of detectable extragalactic sources. This extinction is particularly pronounced at low Galactic latitudes (|b| < 10°), where dust concentrations cause significant dimming, with visual extinctions (A_V) often exceeding 10 magnitudes in the inner plane, effectively blocking most background galaxies from optical detection.^[28] High foreground stellar density exacerbates these issues through source confusion, where overlapping images of Milky Way stars obscure faint background galaxies, limiting the effective magnitude threshold for reliable identification to shallower magnitudes in the plane compared to poleward of the ZoA. This confusion arises from the dense projection of Galactic stars, making it difficult to resolve extended extragalactic structures and leading to systematic undercounting of galaxies.^[28] Historical optical surveys, such as the Palomar Observatory Sky Survey (POSS) from the 1950s, illustrate these limitations vividly, revealing apparent voids in galaxy distributions with significantly reduced detection rates in regions where |b| < 10°, compared to more complete catalogs at higher latitudes. These surveys, reliant on photographic plates sensitive to blue and red light, captured only the brightest and largest galaxies penetrating the dust veil, leaving the underlying large-scale structure largely invisible.^[28] Spectroscopic observations face additional hurdles from dust-induced reddening, which differentially attenuates shorter wavelengths and alters galaxy spectra, shifting emission and absorption features and complicating accurate redshift measurements essential for distance estimation. This reddening, a direct consequence of the primary physical cause—interstellar dust extinction—requires complex corrections that often reduce the quality and reliability of spectra obtained in the ZoA.^[29]^[28]

Effects on Extragalactic Surveys

The Zone of Avoidance (ZoA) introduces significant sampling bias in extragalactic surveys by underrepresenting galaxies due to high interstellar extinction and source confusion along low Galactic latitudes. Early optical redshift surveys were particularly affected, as the ZoA obscures approximately 20% of the optical extragalactic sky, leading to an incomplete mapping of the local universe volume. This bias results in distorted estimates of galaxy densities and large-scale structures, with hidden galaxies potentially altering interpretations of nearby cosmic features.^[30] One notable consequence is the creation of void illusions, where apparent extensions of underdense regions, such as the Local Void, arise from obscured structures behind the ZoA. The Local Void, one of the largest nearby underdense regions, is mostly concealed by the Galactic plane, causing surveys to overestimate its size and connectivity. While the obscuration complicates the reconstruction of the local cosmic web, searches behind the ZoA have not identified major hidden overdensities that fill parts of this apparent void.^[31] To mitigate these biases, statistical corrections for extinction are essential in deriving luminosity functions, often requiring adjustments to models like the Schechter function to account for dimmed fluxes and incomplete samples in the ZoA. These corrections involve estimating extinction based on Galactic dust maps and applying magnitude-dependent factors to recover the underlying galaxy distribution, ensuring more accurate parameterization of the faint-end slope and characteristic luminosity. Without such modeling, luminosity functions derived from ZoA-inclusive data would underestimate galaxy counts at low luminosities.^[32] Large redshift surveys spanning the 1990s and 2000s also suffer from ZoA-induced coverage loss due to avoidance of heavily obscured regions. This gap hinders comprehensive mapping of the nearby large-scale structure, particularly in the Galactic anticenter and plane, where redshift data are sparse. The resulting incompleteness necessitates inpainting techniques or multi-wavelength supplementation to fill these voids in redshift space.^[33]

Observation Techniques

Infrared Astronomy

Infrared astronomy has been instrumental in probing the Zone of Avoidance (ZoA) by leveraging wavelengths in the range of 1 to 1000 μm, where interstellar dust exhibits significantly reduced extinction compared to optical bands. At near- and mid-infrared wavelengths, dust grains are less efficient at absorbing and scattering light, allowing deeper penetration through the Galactic plane; for instance, the extinction in the K-band (A_K) is approximately 0.1 times that in the V-band (A_V), enabling the detection of obscured extragalactic sources that are invisible optically. This wavelength advantage facilitates the identification of galaxies hidden behind dense dust layers, with extinction dropping further at longer mid- and far-infrared wavelengths. The Infrared Astronomical Satellite (IRAS), launched in 1983, conducted the first all-sky infrared survey, detecting approximately 250,000 sources across four bands (12, 25, 60, and 100 μm) and revealing numerous obscured galaxies in the ZoA.^[34] Among its contributions, IRAS data enabled the confirmation of extragalactic counterparts for point sources in the Galactic plane, such as through follow-up observations that identified 97 IRAS-detected galaxies in the ZoA via H I mapping.^[35] These detections highlighted the presence of large-scale structures obscured by the Milky Way, marking a pivotal early application of infrared techniques to the region. Subsequent missions like the Spitzer Space Telescope (operational from 2003 to 2020) advanced ZoA studies with superior sensitivity and resolution in mid- and far-infrared bands using instruments such as the Infrared Array Camera (IRAC) at 3.6–8.0 μm and the Multiband Imaging Photometer for Spitzer (MIPS) at 24–160 μm. Spitzer surveys, including GLIMPSE and MIPSGAL, systematically searched for extended extragalactic sources, leading to the discovery of 25 previously unknown galaxies in the ZoA during the late 2000s and identifications continuing into the 2010s.^[36] These efforts resolved fine-scale structures in obscured galaxies, with typical sensitivity limits around 0.1 mJy, allowing for the detection of star-forming regions behind high extinction. More recent infrared observations, such as those from the James Webb Space Telescope (JWST) using the Near-Infrared Camera (NIRCam), have further enhanced penetration into the ZoA. As of 2025, JWST has identified faint galaxies behind heavily obscured regions, for example detecting 102 galaxies in a field obscured by the NGC 3324 complex using filters at 0.9, 2.0, and 4.4 μm.^[37] These high-resolution, deep near-infrared capabilities allow for the resolution of individual sources in extreme extinction environments (A_V > 20 mag), complementing earlier surveys by revealing lower-mass and more distant objects. A key technique in infrared ZoA observations involves mapping dust emission from foreground interstellar material, often using far-infrared data from IRAS and the Diffuse Infrared Background Experiment (DIRBE) to estimate and subtract extinction along lines of sight. This subtraction corrects for Galactic contamination, isolating extragalactic signals and enabling reliable photometry of background objects in regions of high dust opacity.

Radio and Multi-Wavelength Methods

Radio observations, particularly at the 21 cm wavelength corresponding to neutral hydrogen (HI) emission, provide a powerful means to penetrate the Zone of Avoidance (ZoA), as this spectral line is unaffected by interstellar dust extinction. The 21 cm HI line allows mapping of neutral gas in extragalactic structures, revealing galaxies obscured at optical wavelengths by directly tracing their atomic hydrogen content. Surveys such as the Arecibo L-band Feed Array (ALFA) Zone of Avoidance survey have utilized this technique to detect HI emission from galaxies behind the Galactic plane, with precursor observations identifying 72 HI sources across 138 square degrees at low Galactic latitudes, achieving a sensitivity corresponding to a signal-to-noise ratio of 6.5.^[38] Similarly, the Parkes HI Zone of Avoidance survey covered the southern hemisphere region from longitude 212° to 36° and latitude |b| < 5°, detecting 883 galaxies, of which approximately 90% were previously unknown, with rms sensitivities reaching 1 mJy beam⁻¹ in the deep phase.^[39] These efforts have collectively identified hundreds of HI-selected galaxies in the ZoA by the 2010s, with ongoing surveys adding more and offering insights into the distribution of neutral gas in large-scale structures hidden by the Milky Way. Recent radio advancements include surveys with the MeerKAT telescope, which has conducted blind HI observations in the ZoA, such as the 2024 Vela Supercluster survey detecting 719 galaxies (70% new) with high sensitivity (~0.3 mJy beam⁻¹) and resolution, enabling detailed mapping of nearby structures.^[40] Additionally, the Five-hundred-meter Aperture Spherical Telescope (FAST) has demonstrated potential for detecting over 2000 ZoA galaxies within 300 Mpc (h_{70}^{-1}) at sensitivities below 0.1 mJy, as shown in pilot observations from 2023.^[41] Key radio facilities like the Karl G. Jansky Very Large Array (VLA) enable high-resolution follow-up observations of HI detections in the ZoA, resolving neutral gas distributions on arcsecond scales to study galaxy morphology and dynamics. For instance, VLA 21 cm mapping of the massive spiral galaxy ESO 481-G017 in the ZoA revealed extended HI emission indicative of ongoing star formation, with a rotation curve smoothed over the beam size confirming its properties despite optical obscurity.^[42] The Atacama Large Millimeter/submillimeter Array (ALMA), while primarily operating at higher frequencies, complements radio HI studies by providing high-resolution imaging of molecular gas tracers (e.g., CO lines) in nearby ZoA galaxies, aiding in the characterization of star-forming regions obscured by dust. By the 2010s, these interferometric capabilities had facilitated detailed HI mapping of dozens of ZoA galaxies, enhancing understanding of their neutral and molecular gas content. Machine learning and deep learning methods have emerged as complementary tools for analyzing multi-wavelength data in the ZoA, aiding in galaxy classification and structure reconstruction despite challenges from sparse training data and high confusion. For example, convolutional neural networks applied to near-infrared images have been tested for identifying galaxies, though misclassification rates remain high (~20-30%) due to blending with Galactic sources; deep learning reconstructions of cosmic flows behind the ZoA were demonstrated in 2025 using Bayesian neural networks on existing surveys.^[43]^[44] X-ray and ultraviolet observations supplement radio methods but face significant limitations in the ZoA due to Galactic absorption. Chandra X-ray Observatory glimpses have detected active galactic nuclei (AGN) and extended emission from clusters in the innermost ZoA, such as the identification of diffuse hot gas in the cluster associated with VLSS J2217.5+5943 at z ≈ 0.163, though soft X-ray absorption by the Galactic column density restricts sensitivity to harder energies above 2 keV.^[32] Ultraviolet surveys, like those from GALEX, offer limited penetration, primarily detecting unobscured hot stars or AGN in less extincted ZoA regions, but are generally ineffective for deep extragalactic studies.^[45] Multi-wavelength fusion, particularly leveraging infrared-radio correlations between far-infrared dust emission and 1.4 GHz radio continuum from star formation, helps cross-identify and characterize ZoA sources; for example, combining HI radio data with near-infrared from 2MASS refines distance estimates via the Tully-Fisher relation for obscured galaxies.^[46] These approaches, while constrained, provide critical constraints on AGN activity and total energy output in dust-obscured environments. Resolution remains a primary challenge in radio observations of the ZoA, where single-dish telescopes like Arecibo or Parkes achieve beam sizes of several arcminutes, limiting the ability to resolve individual galaxies amid Galactic confusion. Interferometric arrays such as the VLA address this by providing synthesized beams down to ~1 arcsecond at 21 cm, enabling detailed mapping of HI kinematics and morphology, though baseline coverage and uv-sampling still require careful configuration to mitigate sidelobe contamination from bright Galactic sources. These techniques are essential for disentangling extragalactic signals from the dense foreground emission in the Galactic plane.

Key Surveys and Discoveries

Pre-2000 Surveys

The Infrared Astronomical Satellite (IRAS), launched in 1983, performed the first all-sky survey at infrared wavelengths (12, 25, 60, and 100 μm), penetrating the dust-obscured Zone of Avoidance (ZoA) to identify candidate galaxies through their far-infrared emission from heated dust.^[35] These candidates, selected from the IRAS Point Source Catalog based on color and flux criteria indicative of extragalactic sources, represented a breakthrough in revealing obscured structures behind the Galactic plane, though many required follow-up observations to confirm their nature. The Cosmic Background Explorer (COBE), operational from 1989 to 1993, utilized its Diffuse Infrared Background Experiment (DIRBE) to map far-infrared emission across the sky at wavelengths from 1.25 to 240 μm, providing high-resolution data on Galactic dust distribution in the ZoA. This mapping confirmed extinction models by quantifying the spatial variation of interstellar dust, revealing how absorption affects optical and near-infrared light and enabling better corrections for obscured extragalactic surveys.^[47] Radio efforts in the 1990s, particularly blind neutral hydrogen (HI) surveys with the Parkes 64-m telescope, detected more than 20 galaxies in the ZoA by targeting 21 cm emission lines unaffected by dust extinction. A key pilot survey conducted in 1997 covered a 24° × 5° strip at low Galactic latitudes, yielding 42 detections up to recession velocities of about 12,000 km/s, highlighting filamentary structures crossing the obscured region.^[12] Optical-infrared hybrid approaches in the 1980s employed UK Schmidt Telescope (UKST) plates, particularly red-sensitive surveys of the southern sky, to visually identify approximately 377 obscured galaxies through faint, extended features amid stellar confusion.^[48] These detections complemented IRAS data by providing positional information for follow-up spectroscopy, though limited by high extinction in denser Galactic fields.^[49]

Post-2000 Developments and Recent Findings

In the 2000s and 2010s, infrared observatories such as NASA's Spitzer Space Telescope and ESA's Herschel Space Observatory significantly advanced the detection of obscured galaxies in the Zone of Avoidance by penetrating interstellar dust. Spitzer's mid-infrared imaging led to the discovery of 25 highly obscured galaxies through systematic searches for extended extragalactic sources, many of which were previously undetected due to optical extinction.^[50] Complementary citizen science analysis of Spitzer data identified an additional 59 galaxies behind the Galactic plane, primarily larger systems with elevated star formation rates. These efforts, combined with Herschel's far-infrared observations, including extensions to structures like the Norma cluster revealed in near-infrared fundamental plane analyses around 2014.^[51] The Wide-field Infrared Survey Explorer (WISE), launched in 2010, conducted an all-sky infrared survey that facilitated the detection of galaxy candidates in the Zone of Avoidance by leveraging mid-infrared data to differentiate extragalactic objects from Galactic foregrounds.^[52] This all-sky coverage complemented ground-based HI surveys, such as the Parkes HI Zone of Avoidance Survey, which integrated WISE imagery to confirm numerous detections and map obscured large-scale structures.^[39] Recent advancements from 2020 to 2025 have further illuminated the region using next-generation facilities. The James Webb Space Telescope (JWST), operational since 2022, employed its Near-Infrared Camera (NIRCam) to resolve faint galaxies in highly obscured fields, identifying 102 extragalactic sources—including compact groups akin to hidden clusters—behind the star-forming complex NGC 3324, many visible through otherwise opaque molecular clouds.^[7] Meanwhile, MeerKAT radio telescope observations in the 2024 Vela-HI survey have produced detailed HI maps that reveal underlying voids and filamentary structures in the Zone of Avoidance, detecting 719 galaxies and enhancing multi-wavelength constraints on gas-rich galaxies.^[53]

Astronomical Significance

Mapping Large-Scale Structure

Studies of the Zone of Avoidance (ZoA) have been instrumental in unveiling hidden components of the cosmic web, particularly large superclusters that span across the obscured Galactic plane. The 2014 delineation of the Laniakea Supercluster, which encompasses the Local Group and extends over approximately 160 Mpc, relied on peculiar velocity mappings that pierced through the ZoA to reveal basin-like flows and structural extensions previously masked by interstellar dust and stars. Similarly, infrared and radio surveys have traced extensions of the Great Attractor region behind the ZoA, identifying dense concentrations of galaxies that connect to known filaments and contribute to the gravitational pull influencing local peculiar motions. ZoA observations have significantly filled gaps in the mapping of cosmic walls and voids, providing a more complete picture of filamentary structures. Data from near-infrared and H I surveys have complemented the CfA2 Great Wall map by populating the obscured sections, refining the boundaries of adjacent voids. Integrations of the 2MASS and SDSS datasets have enhanced the overall galaxy distribution models by incorporating ZoA sources, leading to refinements in the observed dipole anisotropy of the local universe. These combined surveys reveal a more isotropic large-scale distribution, with the added ZoA galaxies adjusting the dipole direction and amplitude to better align with peculiar velocity predictions. ZoA contributions have increased the known galaxy count in the local volume, thereby smoothing cosmic variance in structure maps and reducing uncertainties in the filamentary web's connectivity.^[54]

Implications for Cosmology

The Zone of Avoidance (ZoA) complicates the assessment of local cosmic density, leading to potential biases in cosmological parameter estimates within the ΛCDM model. Observations indicate that the Milky Way resides in a significant local underdensity, such as the KBC void extending to approximately 300 Mpc, where the relative density contrast is δ ≈ -0.46. This underdensity causes the locally measured Hubble constant (H₀) to appear inflated by about 5.5% compared to the global value inferred from the cosmic microwave background (CMB), partially alleviating the H₀ tension between local (≈73 km s⁻¹ Mpc⁻¹) and CMB-based (≈67 km s⁻¹ Mpc⁻¹) measurements. Post-2020 analyses, incorporating peculiar velocity data and supernova distances, suggest that bias corrections for these local inhomogeneities can shift H₀ estimates by up to 5%, reducing the tension from >4σ to around 2.5σ in ΛLTB models that account for the void profile. The ZoA's obscuration exacerbates these biases by limiting direct mapping of the local structure, necessitating indirect corrections via multi-wavelength surveys to align local observations with global ΛCDM predictions.^[55]^[56]^[57] The presence of obscured structures in the ZoA also serves as a probe for dark matter distributions, particularly in testing cold dark matter (CDM) simulations. CDM models predict a rich population of dwarf satellite galaxies as subhalos within the Milky Way's halo, yet observations reveal fewer than expected, known as the missing satellites problem. The ZoA potentially conceals up to around 33% of these faint dwarfs due to galactic extinction, with estimates suggesting incompleteness in satellite counts from obscuration alone. Recent JWST/NIRCam observations, including 2025 discoveries of faint, low-surface-brightness galaxies in the ZoA, have uncovered candidates for ultra-faint dwarfs that could harbor significant dark matter content, thereby refining CDM simulations by providing hidden tracers of subhalo masses and distributions. These discoveries impact CDM by demonstrating that local underdensities and obscured regions may host unresolved dark matter-dominated systems, helping to calibrate the faint-end luminosity function and alleviate discrepancies in satellite abundance predictions.^[58]^[7] Data from the ZoA contribute to isotropy tests of the universe, particularly in refining the CMB dipole interpretation. The CMB dipole, primarily kinematic from our motion relative to the CMB rest frame at ≈370 km s⁻¹ toward l=264°, b=48°, is expected to align with matter and galaxy distributions under the cosmological principle. However, the ZoA's limited sampling introduces potential hemispheric asymmetries in galaxy catalogs, prompting tests using radio and infrared surveys to probe obscured regions. Analyses of peculiar velocities and supernova samples, including ZoA-piercing data, show no significant deviations from isotropy, with dipole alignments consistent with ΛCDM expectations and no major violations of homogeneity on scales beyond 100 Mpc. These tests confirm that ZoA effects do not induce large-scale anisotropies, supporting the standard model's assumption of statistical isotropy while highlighting the need for complete sky coverage to minimize selection biases.^[59]^[60] Looking ahead, missions like Euclid and the James Webb Space Telescope (JWST) are poised to enhance precision cosmology by resolving the ZoA's obscuration through 2030. Euclid's wide-field infrared imaging will map millions of galaxies in the ZoA, enabling detailed reconstruction of local large-scale structure and void profiles to better constrain H₀ biases at the 1% level. Similarly, JWST's high-resolution mid-infrared capabilities have already revealed faint ZoA galaxies, with ongoing surveys expected to identify dark matter probes like isolated dwarfs, testing CDM on sub-kpc scales. By integrating these data with multi-wavelength observations, these missions will facilitate bias-free adjustments to ΛCDM parameters, potentially resolving remaining tensions in H₀ and isotropy while advancing our understanding of local cosmology's role in global models.^[7]^[61]

References

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Zone Of Avoidance | COSMOS
### Summary of Zone of Avoidance (ZoA)
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X-rays from galaxies in the "Zone of Avoidance" - HEASARC
We realize that this zone of avoidance is produced by the large clouds of gas and dust in the Milky Way. Since the Milky Way is a spiral galaxy, most of the gas ...
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Astronomy 10: Lecture 17
Jun 26, 2002 · Spiral nebulae were not seen at all in the galactic plane on the sky. This is sometimes called the Zone of Avoidance. Shapley thought that the ...Missing: definition | Show results with:definition
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2001 August 14 - X-Rays from the Galactic Plane - APOD
Aug 14, 2001 · Its target was a spot in the small constellation Scutum, within the Milky Way's zone of avoidance where galactic gas and dust clouds block ...
[5]
[PDF] The VVV near-IR galaxy catalogue of the southern Galactic disc
is called the zone of avoidance (ZoA), as defined by Shapley. (1961), and detecting galaxies is a major challenge. The ZoA covers a significant volume of the ...
[6]
[PDF] Predicting Structures in the Zone of Avoidance - arXiv
Jul 13, 2017 · The Zone of Avoidance (ZOA), whose emptiness is an artifact of our Galaxy dust, has been challenging observers as well as theorists for many ...
[7]
Zone Of Avoidance | COSMOS
The Zone of Avoidance (ZoA) is a region of sky that appears devoid of extragalactic objects at optical wavelengths. Covering roughly 20% of the sky and ...
[8]
None
### Definition and Angular Extent of the Zone of Avoidance
[9]
Nearby Groups of Galaxies
The shaded 20° fan along the Z-axis marks the approximate limits of the galactic zone of avoidance. Note the wide gap (not due to obscuration) between the ...
[10]
A BLIND HI SURVEY FOR GALAXIES IN THE ZONE OF ...
A BLIND H I SURVEY FOR GALAXIES IN THE ZONE OF AVOIDANCE, 308¡ ¹ l ¹ 332 ... 1994) suggests that about 20% of the sky in the optical and about 10% in ...
[11]
UKIDSS Galactic Plane Survey - Oxford Academic
This reflects the patchy nature of interstellar extinction. ... For this reason the Galactic plane is referred to as the 'zone of avoidance' or 'ZoA' in ...
[12]
Optical observations of a newly identified compact galaxy group ...
low Galactic latitudes is obscured by a patchy layer of dust, whose inner part (|b| <∼ 10◦. ) is called Zone of Avoidance (ZoA). However, numerous multi ...
[13]
2MASS All-Sky Data Release Explanatory Supplement: Introduction
The so-called "Zone of Avoidance" (ZoA) refers to the area contained within 5° or 10° of the disk plane of the Milky Way. Gas and dust obscures starlight within ...Missing: extent boundaries
[14]
[PDF] Galaxies: an overview - Princeton University
Figure 1.2 Map of the Milky Way derived from William Herschel's \star gauging. ... He further pointed out that the zone of avoidance { a region close to the plane ...
[15]
BACK UP 437PM Science Page - Harlow Shapley Project
Between 1914 and 1918 Shapley discovered that our Milky Way galaxy is way bigger than believed. ... Galaxy Shapley called the dark region the “zone of avoidance.
[16]
[PDF] Beginnings of cosmology in Hubble's time - A.Sandage
... zone of avoidance" is mapped in greater detail than was possible by Seares (1925), (d) the frequency distribution of the numbers of galaxies per square ...
[17]
[PDF] studies based on the colors and magnitudes - NASA ADS
An error of =*= i° in the cluster latitudes might slightly displace, widen, or narrow the zone of avoidance, but nothing short of selectively operative ...
[18]
[PDF] 1954AJ 59. . 318B 318 THE ASTRONOMICAL JOURNAL 59, No ...
Bok, Bart J. and Harold I. Ewen. Research of the 21 cm radiation at ... Thus, the zone of avoidance found by Shapley in his studies of the globular.
[19]
[PDF] Interstellar Dust Grains
A dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons is compared with observed emission and scattering.
[20]
https://adsabs.harvard.edu/pdf/1934ApJ....79....8H
[21]
Distance and extinction to the Milky Way spiral arms along the ...
We also built extinction maps for each spiral arm and find them to be homogeneous and that they might correspond to independent extinction layers. Finally, ...
[22]
Dust extinction map of the Galactic plane based on the VVV survey ...
Oct 18, 2022 · The map has 30 arcseconds spatial resolution and it traces extinctions typically up to about 10-20 mag of visual extinction and maximally up to ...
[23]
Milky Way Disk - an overview | ScienceDirect Topics
The scaleheight of the thin disk perpendicular to the Galactic plane is approximately 300 pc, and the total mass of the thin disk is approximately 5×1010 M⊙.<|control11|><|separator|>
[24]
https://www.aanda.org/articles/aa/full_html/2021/09/aa40073-20/aa40073-20.html
[25]
[1502.03151] The Milky Way disk - arXiv
Feb 10, 2015 · This review summarises the invited presentation I gave on the Milky Way disc. The idea underneath was to touch those topics that can be ...
[26]
https://arxiv.org/pdf/2412.12252.pdf
[27]
The 2MASS Redshift Survey in the Zone of Avoidance - IOPscience
Nov 1, 2019 · This was due to the inherent difficulties of getting good-quality spectra for galaxies affected by increased dust extinction and higher ...Missing: reddening | Show results with:reddening
[28]
A catalogue of galaxies behind the southern Milky Way-II. The Crux ...
Dust and stars in the plane of the Milky Way obscure $\sim$ 20% of the optical extragalactic sky and 10% of the IRAS extragalactic sky. ... Zone of Avoidance'' ( ...
[29]
Observational evidence for a local underdensity in the Universe and ...
Accord- ing to Tully et al. (2019) the “Local Void”, one of the largest underdense structures nearby, is mostly hidden by the zone of avoidance.
[30]
A galaxy cluster in the innermost Zone of Avoidance, close to the ...
The observed SED had to be corrected for reddening by Galactic dust, which is crucial for fields deep in the ZoA. The IRSA Galactic Dust Reddening and ...
[31]
[1911.02944] The 2MASS Redshift Survey in the Zone of Avoidance
Nov 7, 2019 · However, we were missing redshifts for ~18% of the catalog entries that were located within the "Zone of Avoidance" (|b|<10 deg) -- an important ...Missing: impact SDSS 2dF
[32]
Infrared Astronomy After IRAS | Science
The 250,000 sources in the recently issued Infrared Astronomy Satellite (IRAS) all-sky infrared catalog are a challenge to astronomy. Many of these sources ...
[33]
Identifying Galaxies in the Zone of Avoidance - NASA ADS
371 of the selected sources within a Galactic latitude range |b| = 2^deg^-16^deg^ were searched for H I with the Arecibo 305 m radio telescope.
[34]
DISCOVERY OF HIGHLY OBSCURED GALAXIES IN THE ZONE OF ...
ABSTRACT. We report the discovery of 25 previously unknown galaxies in the zone of avoidance (ZoA). Our systematic search for extended extragalactic sources in ...
[35]
The Arecibo L-band Feed Array Zone of Avoidance Survey I - arXiv
Feb 15, 2010 · The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic latitude survey, to map the distribution of galaxies and large- ...Missing: radio VLA ALMA
[36]
THE PARKES H I ZONE OF AVOIDANCE SURVEY - IOPscience
Feb 9, 2016 · The historical Zone of Avoidance (ZOA) has prevented study of the distribution of galaxies behind the Milky Way and is most pronounced in the ...6.1. Completeness · 7. Large-Scale Structure · 7.1. 2d And Redshift...
[37]
massive spiral galaxy in the Zone of Avoidance - Oxford Academic
By excluding the longest baselines (i.e. all baselines to antenna 6) we obtain an angular resolution of 47.5 arcsec × 36.7 arcsec. We measure an rms per ...
[38]
H I line observations of 2MASS galaxies in the zone of avoidance
The galaxy positions (grey dots) are plotted in Galactic coordinates, centred on the Galactic anticentre. The black dots indicate galaxies observed in this ...
[39]
[astro-ph/0005447] Multi-wavelength Observations of Galaxies in the ...
May 23, 2000 · We discuss the possibilities of extragalactic large-scale studies behind the Zone of Avoidance (ZOA) using complementary multi-wavelength data ...Missing: radio | Show results with:radio
[40]
COBE diffuse infrared background experiment observations of ...
The near-IR reddening observed by DIRBE is consistent with the extinction law tabulated by Rieke & Lebofsky (1985). The distribution of dust and stars in most ...Missing: Zone Avoidance
[41]
Penetrating the “zone of avoidance”. II. Optically detected galaxies in ...
We began an extensive optical survey in late. 1989 along the entire northern Galactic Plane on prints of the Palomar Observatory Sky Survey I (POSS I) and will.
[42]
Introduction: Why and How to Probe the Zone of Avoidance?
The dipole in the distribution of IRAS and optical galaxies lies within 100 ... candidates selected from the IRAS Point Source Catalog or from optical plates.
[43]
[PDF] Discovery of Highly Obscured Galaxies in the Zone of Avoidance
We report the discovery of twenty-five previously unknown galaxies in the. Zone of Avoidance. Our systematic search for extended extra-galactic sources.
[44]
Norma cluster (ACO 3627) – III. The distance and peculiar velocity ...
The Norma cluster (Abell 3627) was identified by Kraan-Korteweg et al. (1996) as the richest cluster in the GA region lying close to the galactic plane at (ℓ, b ...
[45]
A Search for Galaxies in the Zone of Avoidance using WISE - Caltech
Apr 16, 2011 · The WISE all-sky survey, with sensitivies of up to 500-1000x those achieved by IRAS, offers a unique opportunity to extend our knowledge of ...
[46]
Faint galaxies in the Zone of Avoidance revealed by JWST/NIRCam
### Summary of https://arxiv.org/abs/2510.12488
[47]
Exploring the Zone of Avoidance: hidden structures - Astrobites
Dec 5, 2024 · The Zone of Avoidance (ZOA, which lies in the galactic plane) refers to the region of space in the sky that is obscured by the Milky Way's dust ...Missing: standard | Show results with:standard
[48]
Scientists discover massive 'extragalactic structure' behind the Milky ...
Nov 11, 2022 · Infrared surveys of the zone of avoidance have found evidence of thousands of individual galaxies shining through the cosmic fog, though little ...
[49]
zone of avoidance catalogue of 2MASS bright galaxies
Nov 10, 2018 · A zone of avoidance catalogue of 2MASS bright galaxies – I. ... 5 (top panel) the histogram of source counts as a function of Galactic latitude ...
[50]
Observational evidence for a local underdensity in the Universe and ...
Determining the Hubble constant in the redshift range z = 0.018 − 0.85 using a distance calibration from the analysis of Baryonic acoustic oscillations (BAOs) ...Missing: shift post-
[51]
None
### Summary of Key Points from arXiv:1801.01890
[52]
Testing the local supervoid solution to the Hubble tension with direct ...
We evaluate models for the KBC void capable of resolving the Hubble tension with a background Planck cosmology. We fit these models to direct distances from ...
[53]
The spatial distribution of galactic satellites in the Λ cold dark matter ...
We investigate the spatial distribution of galactic satellites in high-resolution simulations of structure formation in the Λ cold dark matter (ΛCDM) model: the ...
[54]
(An)isotropy of the Hubble diagram: comparing hemispheres
The galaxy defines the zone of avoidance and is approximately North-South symmetric. Thus the galactic hemispheres cover approximately the same observable ...
[55]
Euclid opens data treasure trove, offers glimpse of deep fields - ESA
Mar 19, 2025 · On 19 March 2025, the European Space Agency's Euclid mission released its first batch of survey data, including a preview of its deep fields.Missing: Zone Avoidance