Blind zone: What kind of huge object was found behind our Galaxy

(ORDO NEWS) — The least explored is that region of space, which is closed from views from the Earth by our own Galaxy.

Researchers have discovered a previously unknown cluster of galaxies in the most unexplored part of the sky. There are at least 58 “star islands” in it, but it is quite possible that there are many hundreds of such “islands” (that is, galaxies, in strict scientific terms).

Astronomers still have a lot of work to do to get a good look at what lies under the “blind spot”, for the existence of which we must “thank” our own Galaxy.

Twilight Zone

Anyone who has looked into the night sky is familiar with the beautiful whitish streak across the firmament.

Hundreds of billions of stars are concentrated in the disk of the Galaxy. It would seem that the Milky Way should shine brightly in the sky, like the sun, but that was not the case.

It’s all about interstellar gas, which in terms of total mass is not inferior to stars (both in the Universe as a whole and in our galaxy in particular), and in the dust accompanying it. Gas and dust clouds absorb light.

So the beautiful whitish haze of the Milky Way is just a small handful of luminaries closest to the Earth, the rest are simply not visible.

There is no point in thinking to pierce the entire thickness of the Galaxy from edge to edge and see what the big universe looks like in this direction.

This piece of space with all its galaxies just hid behind an impenetrable gas curtain (social phobes, take note). The disk of the Milky Way, together with its central bulge, covers about a fifth of the sky from telescopes.

English-speaking astronomers call this area the zone of avoidance, which is usually – and not very successfully.

It would be more successful to translate avoidance as “exception”, if you really want a literal, rather than contextual translation.

Optical telescopes (more precisely, astronomers working with them) have to exclude this part of the sky from surveys of the Universe.

And, of course, scientists are very curious: what is there, in the “forbidden zone”? Suddenly something especially interesting?

Mouth of universal rivers

By the way, there is something interesting here. In accessible areas of the sky, telescopes see entire streams of galaxies flowing towards a certain point hidden from us in the zone of avoidance. This point, invisible from the Earth, was called the Great Attractor.

Scientists believe that the Great Attractor is the center of mass of a supercluster of galaxies with the beautiful name Laniakea (translated from Hawaiian as “vast skies”).

Embracing Laniakea is really not easy: it is a hundred thousand times more massive than the Milky Way and five thousand times its diameter.

Note, to avoid confusion, that the Great Attractor is not a body with anomalous gravity, like a black hole, and indeed not a body at all. At each point of the huge Laniakea, the attraction of many star systems is summed up.

It is easy to guess that at some point (the center of mass) this amount will be the largest. This point is the Great Attractor.

Galaxies flock there for the same reason that a gravitating ball shrinks towards the center or water rushes to the bottom of the vessel.

Beyond the visible

Of course, not the entire zone of avoidance is so interesting. Probably most of it is as ordinary as any randomly selected section of the sky.

But even so, it is not in the rules of proud explorers to put up with white spots on the maps.

Fortunately, on optical instruments, the light did not converge like a wedge. That is, it is light that may have converged, and radio waves converged on radio telescopes, X-rays on X-ray telescopes, and so on.

All these radiations pass through clouds of gas and dust better than light.

However, different telescopes for an astronomer are like different orchestral instruments for a composer. Each of them has its own role and its own limitations.

For example, not every galaxy that could be seen with an optical telescope is visible to an X-ray or radio instrument.

And in the very center of the Galaxy, almost any telescopes are blinded by the radiation of matter falling on the central black hole of the Milky Way.

In general, it is not surprising that the zone of avoidance remains the most unexplored sector of the sky. Astronomers are interested in any news from this terra incognita.

Blind zone what kind of huge object was found behind our Galaxy 2
Image of the studied area of the sky (circled in red) in false colors. The red squares show galaxies with studied spectra. On the right, 58 open galaxies on an enlarged scale

Twin Gift

The authors of the new study used the southern Gemini telescope. Gemini (from the English Gemini – “Twins”) is a pair of identical instruments with a diameter of 8.1 meters. This size puts them among the largest telescopes in the world.

The southern “twin” is set in the Andes, and the northern one is in Hawaii. Both instruments work high above sea level, in very clean and dry air. This allows them to “perceive” not only light, but also infrared waves, which are strongly absorbed by water vapor and other atmospheric impurities.

True, these are near infrared waves, which are only slightly longer than light waves. It is still almost light, although already invisible to the eye. But even such waves pass well through the gas and dust clouds of the Milky Way, which means they allow one to look into the zone of avoidance.

Astronomers focused on their work in 2021. Then they processed data from the VISTA telescope, also obtained in the near infrared range.

The researchers found 607 objects similar to previously unknown galaxies in the zone of avoidance. But it was difficult to establish their nature – primarily because the distance to them was not known.

Any gas cloud within the Milky Way has the same apparent size as a huge but distant galaxy. So a flower standing on a windowsill may seem larger than a tree on a nearby street.

Now scientists have re-examined the desired area of the sky. True, not all, but only a small part of it with a radius of six arc minutes (a fifth of the apparent diameter of the Moon). Here they found 58 galaxy candidates, as cautious astronomers put it.

For five of them, the spectrograph of the Gemini telescope obtained spectra, from which the authors calculated the redshift.

And it, in turn, can be converted into distance. Astronomers also determined the photometric redshift, which coincided with the spectral one (Naked Science has already told you what all these buzzwords mean).

It turned out that the redshift of these galaxy candidates is 0.225 ± 0.014. This means that they are almost three billion light years away. For comparison: the diameter of the disk of the Milky Way is about a hundred thousand light years.

That is, these objects not only look like galaxies, but are so far away that they simply cannot be anything else. The candidates, so to speak, won the elections. In addition, their spectra are also typical of galaxies.

Finally, assuming that their infrared “light” is stellar, astronomers calculated the distance to these stars by an independent method. And it coincided with the distance determined by the redshift.

Knowing the distance, the researchers calculated the mass of the cluster (about ten trillion suns) and its diameter (millions of light years). Both are typical of clusters of galaxies at such distances from the Earth.

In general, the authors considered that they had discovered a new cluster of galaxies in the zone of avoidance, and it is difficult to disagree with them. Scientists did not invent a romantic name and assigned this cluster the furious designation VVVGCl-B J181435-381432.

Most likely, this object is only the “tip of the iceberg”. Recall that in the review of the VISTA telescope, the authors counted more than 600 candidates.

And let’s not forget that through the gas and dust curtain of the Milky Way, even with infrared telescopes, only the largest and brightest galaxies can be seen. So there could very well be a larger cluster or multiple clusters.

Heavenly hierarchy

Clusters of galaxies are a fairly common thing. Most of the “star islands” do not shine in the sky in splendid isolation, but gather in company. These are either pairs, or groups of galaxies, or, finally, clusters.

The boundaries between these categories are blurred. Thirty-year-old textbooks say that in a group of galaxies there are from three members to several tens, and in a cluster – from several tens to several thousand.

But today, more than a hundred systems have already been counted in the Local Group of galaxies, which includes the Milky Way.

This is natural: telescopes have become better, and more observations have accumulated. So small and dim galaxies were discovered that were not previously visible. And no one seems to be in a hurry to rename the Local group to the Local Cluster.

Some clusters and groups, in turn, combine into superclusters. Moreover, superclusters are part of larger superclusters.

Thus, the Local Group is included in the Virgo supercluster, and then, in turn, in the Laniakea supercluster. Astronomers seem to have run out of words for ever larger systems. It’s a pity, one could come up with “megaclusters” and “super-duper-clusters”.

But these are not the largest structures in the universe. Clusters and superclusters of galaxies are assembled into threads (they are also fibers and filaments), stretched out over many hundreds of millions of light years.

These filaments border voids (voids), where there are relatively few galaxies. It looks like a grandiose web of space. And this is the largest known level in the structure of the universe.

A piece of the cosmic web a billion or two light years in size is like two drops of water similar to any other piece of the same size. However, astronomers do not rule out that this is not a universal law, we are just lucky to live in a relatively homogeneous region of boundless space.


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