Record-breaking cosmic tail reveals why galaxies are too small

(ORDO NEWS) — While amateur astronomers are amazed at the size of galaxies, professionals are puzzled over why they are so small.

It is believed that immediately after the Big Bang, matter was distributed almost evenly throughout the newborn Universe. But where there was a little more matter, and gravity was stronger, because it depends on mass. Therefore, more and more portions of the substance were attracted there.

This, in turn, increased gravity, and everything repeated in a circle. This is the principle of a snowball, or, as the boring scientists say, positive feedback. Everyone who has built a snowman knows that the snowball grows the further, the faster.

This is understandable: the larger the surface of the coma, the more snow it wraps around itself, and this will make it even more.

So the galaxies grew until they absorbed almost all the surrounding matter. It would seem that everything is very simple. But if we move from reasoning “on the fingers” to quantitative models, a different picture emerges.

True, it is very difficult to model the evolution of an entire galaxy. It has too many heterogeneous components: stars, gas, magnetic fields, and mysterious dark matter . And all of this bizarrely interacts with each other.

But scientists are not used to giving in to difficulties. Armed with supercomputers, they built a great variety of models of the birth, life and death of the “star islands”. And most of these models agree on one thing: there must be much more gas and stars formed from it in galaxies than is actually observed.

It turns out two options. Or different models, developed by different and very competent people, chronically lie on this point, and so far no one has explained why. Or at some stage of their evolution, galaxies lose a significant part of the gas.

And because of this, the formation of stars, which are molded from this gas, almost stops. For example, in the Milky Way today only a few stars are formed per year, and once the rates were tens or even hundreds of times higher. Similar figures apply to other modern galaxies.

Record breaking cosmic tail reveals why galaxies are too small 2
Artistic depiction of a quasar with jets

Gone With the Wind

Observations show that some galaxies actually emit streams of matter. There are several types of such jets, and it is important not to confuse them with each other.

The general public is most familiar with jets. These are jets of hot particles accelerated to near-light speeds. Jets flow from active galactic nuclei, this is literally seen in the images of galaxies (in the radio range).

The jets come in a variety of lengths and sometimes extend far beyond the parent galaxy. At the same time, they are extremely rarefied, so that they practically do not reduce the mass of the galaxy.

A more serious candidate for a galactic leak is quasar outflows. For brevity, we will simply refer to them as quasar flows. As the name implies, they are associated with quasars, the most active of the galactic nuclei.

Recall that a supermassive black hole is hidden in such a core. This predator is surrounded by a disk of matter falling on it in a spiral.

On the way to the abyss, the streams of matter collide with each other and are heated by friction. As a result, the disk surrounding the black hole emits powerful electromagnetic radiation: light, X-rays, and so on.

This radiation goes far beyond the vicinity of a black hole. It is visible even at intergalactic distances, thanks to which we know about active nuclei. Well, in the thickness of the parent galaxy, the pressure of photons is especially high.

There, the radiation literally scoops up the gas in front of it, like a bulldozer. This happens due to the pressure of light, discovered by our great compatriot Peter Nikolaevich Lebedev. The flow of gas pushed by the radiation of a quasar is the quasar flow.

These streams are very impressive. The power of some of them is measured in trillions of solar luminosities. According to experts, no other phenomenon carries more mechanical energy.

Quasar streams can be traced due to their ultraviolet radiation. But not far, at best, thousands of light-years – about a percent of the diameter of a large galaxy.

Further, the gas cools and ceases to glow in the ultraviolet. Whether he eventually goes beyond the “father’s house” and in what quantity is a debatable question.

Another and most serious candidate for the role of “gas leak” is superwinds. These are powerful streams of cold gas leaving the galaxy at a speed of several thousand kilometers per second.

Where and why they begin is not very clear. Telescopes are unable to trace the flows of cold matter in the galactic thickness. One of the hypotheses is that these are the same quasar streams, only thoroughly cooled down along the way.

Of course, not every galaxy can boast of a quasar or even any active nucleus. The core of the Milky Way, for example, to our happiness, is calm.

On the other hand, we know little about what it was like in its youth. Perhaps the current quiet cores have simply already used up the reserves of matter falling on them. No wonder quasars are observed only in the early Universe.

What does a galaxy have to do with a toilet?

Another model links superwinds to supernova explosions. As you know, the life of the most massive stars (heavier than ten suns) ends with an impressive flash.

The explosion creates a shock wave that rakes matter in front of it. The constant crackling fireworks of such explosions can give rise to a superwind, consisting of many shock waves.

In this model, star formation stops itself. Initially, a lot of stars are formed in the galaxy, including massive ones. The latter quickly (in hundreds or even tens of millions of years) burn out and explode.

The shock waves from the explosions merge into a superwind that sweeps gas out of the galaxy. And when there is too little gas, there is literally nothing to sculpt new portions of luminaries from.

This is an example of not positive, but negative feedback. Everyone who has ever removed the lid from the toilet tank is familiar with it.

The valve is opened by a lever to which a float is attached. While he lies at the very bottom, the tap is open all the way. As the tank fills, the float rises and gradually closes the tap.

This is called negative feedback: the more something (in this case water) is, the less will be added from above. Not only technology is full of negative feedbacks, but also nature, especially living.

This is one of the main stabilization mechanisms that prevent the system from going haywire: the tank overflows, and the wolves eat all the hares. It seems that the number of stars in galaxies is also regulated by such a relationship: the stars themselves take care that their “reproduction” almost stops.

There are strong arguments in favor of the “supernova” origin of superwinds. For example, in a recent study of dozens of galaxies, no statistical relationship was found between the brightness of a quasar and the presence of superwinds around it.

But “superwindiness” correlated well with the rate of star formation. However, supporters of the quasar hypothesis also have references to the literature. It is possible that both mechanisms work in nature.

Record breaking cosmic tail reveals why galaxies are too small 3
The tidal tail of the Arp 188 galaxy

When a galaxy grows a tail

Are superwinds, whatever they are, the only mechanism for gas loss? More likely no than yes. Astronomers know of at least one other way in which gas can leave the home system. For the galaxy, as for a negligent student, tails can become a problem.

What it is? Sometimes galaxies approach each other, and the gravity of one of them literally pulls out a jet of gas from the other – the so-called tidal tail. More precisely, it is tidal forces that do this.

They arise due to the fact that at different distances from the guest galaxy, its gravity has a different value.

Similar forces from the Moon and the Sun cause tides on Earth. Unless they are so powerful, so the ocean water hump does not escape into space, unlike a jet of gas from the galaxy.

Tidal tails are rare because most galaxies don’t have close neighbors. Therefore, they are usually not considered as a brake that stops the growth of galaxies.

But in the past, when the universe had not yet expanded as much, the “star islands” were closer to each other. Then tidal tails could appear more often than today.

Recently, scientists have found important evidence in favor of this view. This is an impressive tail of gas from a galaxy with the furious designation SDSS J144845.91+101010.5 (below, we will simply refer to it as J144845 for brevity).

The gaseous tail of this galaxy stretches over 200,000 light-years and has a mass of 10 billion solar masses. The authors of the study call such scales unprecedented. Indeed, this is almost as much gas as is left in the galaxy itself. Such a loss could easily stop star formation without any superwinds.

What could give rise to such a long tail? Tidal forces. It seems that there are simply no other workable mechanisms: the gas carried by superwinds looks different.

At the same time, J144845 has no close neighbors that claim to be a troublemaker. The researchers explain this simply: the guest galaxy managed to collide and merge with J144845.

Recall that billions of years ago, the galaxies were closer to each other: the expansion of the Universe did not have time to do its job.

Therefore, they encountered much more often than now. For example, in the history of the Milky Way, astronomers have counted five major accidents over the past 11 billion years. There is no reason to believe that this happened less frequently in other star systems.

Dangerous proximity

It is curious that the approach of the visitor system even stimulates the formation of stars at first. Its gravity causes a violent movement of interstellar gas in the host galaxy. And any imbalance creates random inhomogeneities. Gravity is just what is needed to mold stars out of these inhomogeneities.

So the galaxy is experiencing a burst of star formation (starburst). But this flash fades as suddenly as it begins.

And now, it seems, it is clear why it is fading: because an overly approaching guest pulls gas out of the galaxy. Rapprochement is good in moderation, astronomers know this as well as psychologists.

If each collision of galaxies is preceded by the formation of grandiose tidal tails, then it is understandable how star systems lose gas. It is also clear why we do not observe tails in the Milky Way and other neighboring galaxies.

In the billions of years since the last collision, these jets have had time to dissipate. But distant galaxies, unlike close ones, we see young. For example, the light from J144845 took six billion years to reach Earth nearly half the age of the universe.

The authors emphasize that they stumbled upon J144845 to some extent by accident. The researchers did not set themselves the goal of looking for “tailed” galaxies. Indeed, only recently have telescopes become so advanced as to be able to detect such tails at such considerable distances.

A targeted search for such objects is needed, scientists emphasize. Only in this way can we understand what J144845 is: a typical case or a rare exception.


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