Cosmic yeast

The densest regions of galaxies similar to that of the Milky Way provide gas and dust for the formation of other stars

Ronaldo de Souza and Dimitri Gadotti, astronomers at the University of São Paulo (USP), were decided, in the last five years, to investigate how and when galaxies form. Today they do not have all of the answers, clearly, but they have managed to better explain the formation and development of close to one third of the one billion galaxies existing in the universe. The observation of almost one hundred of these conglomerates of stars, allied to the shrewdness of turning back to an ancient theorem of classical mechanics, has permitted the two astronomers to set out a computer program that calculates the age and the structural dimensions peculiar to galaxies similar to that of the Milky Way, which houses our very own Solar System. Souza and Gadotti verified that these structures with the approximate shape of rectangles – or bars – could be relatively recent or, in extreme cases, almost as old as the very galaxies called ‘barred’ galaxies. It is the bars, as they verified, that feed the central region of these galaxies with dust and gas that go to form new stars. The mathematical model that they created is helping to even reclassify other types of galaxies.

The barred galaxies are similar to the Milky Way, classified as a spiral galaxy, because it also shows hundreds of millions of stars in the central region in the form of a sphere – the nucleus – and other hundreds of millions dispersed in a fine disc of gas and dust similar to a cosmic whirlpool. A characteristic of bar galaxies is that, within that luminous band in the form of a rectangle, the density of the stars is greater than in the disc, but lower than in the nucleus, also called the bulge.

A series of theoretical studies attributed to the bar galaxies the role of galactic yeast. Formed in the regions of greatest concentration of stars in the disc, these structures grow like a loaf of bread, but much more slowly – up to billions of years. As they become  thicker than the disc, the bars feed the bulge of the galaxies with dust and gas, the raw material for the production of stars, contributing to the accumulation of nucleus material. However, this was a panorama built up starting from computer simulations. Direct observation data that would confirm if the behavior of the Cosmos was truly in this manner was lacking. “Five years ago, almost nothing was known about the age, dimensions and the evolution of bar galaxies”, comments Souza, the coordinator of this line of research that integrated a thematic project on the evolution of galaxies, carried out by Sueli Viegas, from the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) of USP.

The first signs that the model had been correct came about in 2001. In partnership with the astronomer Sandra dos Anjos, also from IAG, Dr. Gadotti analyzed the images of 257 spiral galaxies. He verified that in reality there was a greater concentration of younger stars in the bulge of bar galaxies – such as NGC 4314 (shown on the right) – than in the nucleus of those without being bar shaped. This was an indication that the bar galaxies fed the central region of these galaxies, since the stars in general form themselves in regions distant from it, in the disc.

Galaxy measurer
With the help of a telescope located in the northern hemisphere and another in the southern hemisphere, the USP astronomers observed the characteristics of 14 galaxies that appear in the sky close to the projection of the line of the equator, the so called Celestial Equator. Over a period of ten nights during1999, 2000 and 2002, astronomers Souza and Gadotti registered at points of the disc, bar and bulge of each galaxy a measurement of the velocities with which the stars changed position, either moving away from or getting closer to the observer on earth – a measurement known as velocity dispersion. They discovered that, in the disc, the stars moved with velocities that, on average, varied from 5 to 20 kilometers per second (km/s), whilst these values got close to 100 km/s at the bulge.

It was these measurements that allowed the researchers to estimate the age of the bars. “We identified very young bars, formed some one billions years ago, and others that were more evolved, almost as old as the galaxies themselves, formed around 10 billion years ago”, affirmed Dr. Gadotti, currently working in the laboratory of the Greek astronomer Lia Athanassoula, at the Marselha-Provença Astronomy Observatory, in France. In isolation, however, this data was insufficient to determine the width of the bars and the time of formation of these structures.

In order to define the width of the bars, the astronomers fell back upon an ancient theorem of classical mechanics – the Virial Theorem, proposed in 1870 by the German physicist Rudolf Clausius –, by way of which they associated the dispersion of the velocities of the stars to the mass of the different regions of the galaxies. When the calculations were completed they concluded: the formation of bars lasts between 1 and 2 billion years, when they reach their maximum width, corresponding to two or three times that of the disc. In a barred galaxy with the dimensions of the Milky Way, the width of the disc would be close to 9.5 quadrillion kilometers and that of the bar, from 19 to 27 quadrillion kilometers – the crew of a spaceship capable of traveling at speeds close to that of light would take between 19,000 and 27,000 years to cover the thickness of the bar. As well, they observed that these bars could disappear and later reappear in a cyclic process that continually feeds the bulge of the galaxy.

New forms
Another surprising finding: two galaxies with a well developed bar, but without the disc that originated it – an unusual structure. A more detailed evaluation revealed that, in reality, the internal region of the disc had disappeared, only a trace remaining: a ring that surrounded the bar and the bulge. As yet there is no consensus of opinion for the absence of the disc. In an article published in Astrophysical Journal during 2003, Souza and Gadotti proposed two possibilities: either these galaxies are extreme examples in which the formation of the bar consumed almost all of the disc, or they would be surrounded by a halo literally flattened from a form of material that would not emit light and indeed could not be observed by telescopes – the so called black material. “We tested the model of the black material and verified that the halo in an elliptical form could induce the formation of bars even without the existence of the disc”, explained astronomer Gadotti.

The computer program that he along with Souza and Sandra have created should also facilitate the lives of the astronomers who dedicate themselves to the classification of galaxies in accordance with their shape. This methodology, created by the astronomer Edwin Hubble in 1926, separates the galaxies into ten categories, which include the spheroids, with a bulge similar to a perfect sphere and without a disc, the elliptical and the spiral, with and without bars. Given the acronym BUDDA (Bulge Disk Decomposition Analysis), the program makes use of equations developed by the IAG team to analyze 11 parameters related to the luminosity and geometry of the disc and the bulge of the galaxy – where before they observed only three parameters. In the first test images of 51 galaxies were examined at the National Astrophysics Laboratory in Minas Gerais. And the BUDDA was impressive in identifying unknown structures – such as discs that cannot be observed or the existence of secondary bars – and to detect errors in the classification of 15 galaxies. It is estimated that from 10% to 15% of the galaxies have been classified in the wrong categories.