Study: all disk galaxies in the universe rotate with the same period

Regardless of the size and mass, all disk galaxies in the Universe make one complete rotation around its axis in approximately 1 billion years. This is the conclusion made by a group of astrophysicists from the International Center for Radio Astronomy Research (ICRAR) from the University of Western Australia. As it is written in the press release on the results of the discovery, in this sense, disk galaxies can be compared with space clocks.

The number of disk galaxies includes lenticular and spiral, like our Milky Way or the Andromeda Galaxy nearby.

In accordance with the modern standard cosmological model CDM (Cold Dark Matter), the main structural and dynamic properties of galactic halos (halo is a spherical invisible component of the galaxy, including dark matter and containing the main mass of the galaxy) and disks (visible part) obey simple virial scale ratio. These properties are usually defined as radius. $$$R$, rotational speed $$$V$and mass $$$M$. Or, alternatively, the brightness $$$L$as a proxy for the masses. Virial equilibrium for halo is observed as $$$V$to $$$R$and to $$$M ^ {1 \ over 3}$. Of course, dark matter is not directly observed, but the scale relations are observed through the properties of baryons, although the exponents in the power law of these relations do not exactly correspond to the model for the halo.

In practice, the scale ratio of speed and luminosity, more commonly known as the Tally-Fisher relationship, is used in practice. This is an empirically derived relationship connecting the mass (luminosity) of a spiral galaxy and its speed of rotation. Baryon physics is very complex, it takes into account many factors that can affect and distort all large-scale relationships. For example, the active core of a galaxy can redistribute baryons, and in the process tighten dark matter into the distribution of baryons, due to which all of the above ratios are distorted.
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Although the relationship between the speed of rotation and luminosity is often mentioned, the relation between the speed of rotation and the radius has so far received little attention. Perhaps the situation here is complicated by the difficulty in measuring the radius on a radial scale.

In the new work, the scientists used to measure the scale ratio for $$$R$not the radial scale, as in previous works, but the outer radius of the galaxy - and showed that in this case there is an almost linear relationship $$$RV$.

The circular velocity of rotation correlates with the radius of all observed galaxies, which differ in size and circular velocity of rotation by 30 times: from dwarf irregular galaxies to giant spiral ones.


The ratio of the radius and circular velocity on a logarithmic scale

In other words, all disk galaxies really work like a clock, making a revolution in about 1 billion years, measured from the very edge of their disks.

“Detecting such a pattern in galaxies helps to better understand the mechanics of their rotation — you will not find a rapidly rotating dense galaxy, while another galaxy of the same size, but lower density, rotates more slowly,” says Professor Gerhardt Meurer of the University of Western Australia



True, the researchers make a reservation that in order to confirm this universal law, measurements should be made on a wider set of disk galaxies in order to completely eliminate bias in the selection.

In addition, scientists note that on the outer edge of the galactic disk there are not only dense clusters of young stars and interstellar gas, but also a large number of much older stars mixed with young and interstellar gas. The galactic disk has a fairly clear boundary. Knowing the speed of rotation, you can calculate the radius and quickly detect this boundary.

The authors of the scientific work say that after the long-awaited launch of the Square Kilometer Array (SKA) radio telescope, they will receive a vast array of data on galaxies. Then accurate guidance, where to find the boundaries of the galaxy, will help in processing this large amount of information.

The scientific work was published on March 9, 2018 in The Monthly Notices of the Royal Astronomical Society (doi: 10.1093 / mnras / sty275, pdf ).