There Are Hundreds Of Galaxies Lurking Behind The Milky Way

An artist’s impression of the galaxies found hidden behind the Milky Way. This scene has been created using the actual positional data of the new galaxies and randomly populating the region with galaxies of different sizes and types. ICARR

A search of the most difficult part of the sky to study has found hundreds of previously unknown galaxies. These objects are close enough that we would see them easily if they were in any other direction, but their visible light is blocked by the dust within our own galaxy. Studying them more deeply will hopefully help us understand the giant structure known as the Great Attractor

The Milky Way galaxy, in which we live, is moving at 2 million kilometers per hour (1.2 million mph) in approximately the direction of the little-known constellation Norma. Observations of other galaxies in the local universe indicate that they are also traveling towards the same location, leading to the area being dubbed the Great Attractor. Unfortunately, by sheer chance, the Great Attractor is almost directly on the opposite side of the Milky Way from Earth, so much of our view is obscured by the stars and dust in between. Naturally, this makes it hard for us to study it effectively.

Fortunately, the 21 centimeter radio signal from galactic hydrogen passes through the galactic obstruction relatively unscathed. However, powerful radio telescopes, which usually have small fields of view, have struggled to locate all the objects of interest in the large area of obscured sky.

Schematic of the newly discovered galaxies hidden behind the Milky Way, and their relationship to Earth. Note that this exaggerates how much they are clumped at the same distance. ICRAR.

The multibeam receiver on the giant Parkes radio telescope has changed this. Professor Lister Staveley-Smith of the University of Western Australia told IFLScience the receiver “Acts like a wide field camera, allowing us to map a much larger area more quickly.” Staveley-Smith is the first author of a paper in The Astronomical Journal reporting on 883 galaxies found this way, almost half of them previously unrecorded through searches at other wavelengths.

The galaxies range in distance from 6 million light-years away, which Staveley-Smith says places them just outside the Local Group of galaxies, to a distance of approximately 500 million light-years. Among the closer galaxies we have been able to pick up some fairly small specimens, but Staveley-Smith told IFLScience that most of those observed are close to the Milky Way's mass or even larger, reflecting the fact that only large galaxies can be seen at greater distances.

The extra mass contained in these galaxies helps explain why the Great Attractor is doing so much attracting. "We don't actually understand what's causing this gravitational acceleration on the Milky Way or where it's coming from," Staveley-Smith said in a statement.

As the densest part of the local universe, the Attractor's gravity is affecting everything around it, but we don't yet know the form that density takes. The favored model of the universe soon after the Big Bang has fluctuations in the density of dark matter, which caused accumulations of ordinary matter, leading to the formation of galaxies. Since some concentrations are larger than others, it is inevitable that particularly dense regions will exist.

Staveley-Smith told IFLScience that the extent to which the Great Attractor is denser than anything else we can see “stretches that theory, but does not break it.”  

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