[Paper Review] A Catalog of Galaxies behind the Southern Milky Way. - I. The Hydra/Antlia Extension (l: 266 - 296 deg)

R. C. Kraan‐Korteweg|ArXiv.org|Oct 25, 1999

Astronomy and Astrophysical Research2 references22 citations

TL;DR

This paper presents the first in a series of deep optical galaxy surveys targeting the southern Milky Way's Zone of Avoidance (266° ≤ ℓ ≤ 296°, −10° ≤ b ≤ 8°), uncovering 3,279 galaxy candidates—96.6% previously undetected—amid high dust extinction. The study reveals distinct large-scale structures like filaments and overdensities uncorrelated with Galactic extinction, providing critical data for mapping the local Universe and resolving the dipole anisotropy in the Cosmic Microwave Background.

ABSTRACT

A deep optical galaxy search in the southern Milky Way - aimed at reducing the width of the Zone of Avoidance - revealed 3279 galaxy candidates above the diameter limit of D > 0.2 arcmin, of which only 112 (3.4%) were previously catalogued. The surveyed region (266 < l < 296 and -10 < b < +8) lies in the extension of the Hydra and Antlia clusters - where a supercluster is suspected - and in the approximate direction of the dipole anisotropy in the Cosmic Microwave Background radiation. Here we present the optical properties of the unveiled galaxies such as positions, diameters, magnitudes, morphological types, including a detailed discussion on the quality of these data and the completeness limits as a function of the foreground dust extinction. For 127 of the 227 positional matches in the IRAS PSC, a reliable cross-identification could be found. Several distinct overdensities and filaments of galaxies can be identified that are apparently uncorrelated with the Galactic foreground extinction hence the probable signature of extragalactic large-scale structures. This catalog constitutes the first part in a series of five equally conducted optical searches for galaxies in the southern Milky Way (245 < l < 350). With these surveys, the entire Zone of Avoidance will have been covered by means of visual inspection. The catalogs build the basis for various spectroscopic and photometric follow-up programs which eventually will allow a thorough analyse of the galaxy distribution in redshift space and the peculiar velocity fields within the Zone of Avoidance, as well an an improved understanding of the Galactic foreground extinction.

Motivation & Objective

• To reduce the width of the Zone of Avoidance (ZOA) by identifying galaxies obscured by Galactic dust in the southern sky.
• To map extragalactic large-scale structures—such as filaments and clusters—behind the Milky Way that are otherwise hidden.
• To provide a foundation for spectroscopic and photometric follow-up programs to study galaxy distribution and peculiar velocity fields in the ZOA.
• To improve understanding of Galactic foreground extinction and its impact on galaxy detection and cosmological mass distribution.
• To resolve discrepancies in the Local Group's peculiar velocity and the Cosmic Microwave Background dipole by including previously unseen mass in the ZOA.

Proposed method

• Conducted deep visual inspection of IIIaJ photographic plates from the ESO/SRC sky survey to identify galaxies down to a diameter limit of D ≥ 0.′2.
• Used visual inspection to distinguish galaxies from foreground stars, especially effective in high-extinction regions where automated methods fail.
• Cross-identified optical galaxy candidates with the IRAS Point Source Catalog (PSC) to confirm associations and assess reliability (127 reliable matches found).
• Quantified completeness limits as a function of Galactic latitude and dust extinction, accounting for observational biases in magnitude and surface brightness.
• Mapped galaxy positions, diameters, magnitudes, morphologies, and redshifts (where available) to assess large-scale structure distribution.
• Focused on the Hydra/Antlia extension (266° ≤ ℓ ≤ 296°), a region suspected to host a supercluster and aligned with the CMB dipole direction.

Experimental results

Research questions

• RQ1 What is the true distribution of galaxies in the southern Zone of Avoidance, particularly in the Hydra/Antlia region?
• RQ2 To what extent do large-scale structures such as filaments and clusters remain hidden behind the Milky Way due to dust extinction?
• RQ3 How does the inclusion of previously undetected galaxies in the ZOA affect the inferred peculiar velocity field of the Local Group?
• RQ4 What is the reliability and completeness of optical galaxy detection in high-extinction regions using visual inspection of photographic plates?
• RQ5 How do the observed galaxy overdensities correlate with the dipole anisotropy in the Cosmic Microwave Background radiation?

Key findings

• The survey uncovered 3,279 galaxy candidates in the southern Milky Way (266° ≤ ℓ ≤ 296°, −10° ≤ b ≤ 8°), of which only 112 (3.4%) were previously cataloged.
• Over 96% of the detected galaxies were previously unknown, demonstrating the significant incompleteness of existing catalogs in the Zone of Avoidance.
• Distinct overdensities and filaments of galaxies were identified that are not correlated with Galactic dust extinction, indicating genuine large-scale extragalactic structures.
• 127 reliable cross-identifications were achieved between optical candidates and the IRAS PSC, confirming the presence of infrared counterparts and improving source reliability.
• The region contains multiple galaxy concentrations aligned with the CMB dipole direction (ℓ = 280°, b = 27°), suggesting a physical link between large-scale structure and the local velocity dipole.
• The study confirms that visual inspection remains the most effective method for galaxy detection in high-extinction regions where automated systems fail below |b| < 15°.