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Home , Barred spiral galaxy

Breaking the symmetry: barred spiral

Domingos Soares

The American astronomer (1889-1953) proved, in the 1920s, the existence of the galaxies. Because of that, he is considered as the “discov- erer of the galaxies”. Soon after his great discovery, Hubble realized several researches concerning the galaxies. One of those investigations was related to a classification of the galaxies regarding their appearance, in other words, to the shapes that they presented when observed on the plane of the sky. His morphological classification of the galaxies can be represented by a tuning fork, the so-called “Hubble’s tuning fork”.

The morphological classification of galaxies: Hubble’s tuning fork. The barred spiral galaxies appear in one of the tuning fork bifurcations.

Hubble’s classification was first published in his 1936 book entitled “The Realm of the Nebulae”. At the handle of the fork, Hubble put the galaxies he called “elliptical”, because they presented a contour of elliptical shape, that is, with the shape of a flattened circle. Elliptical galaxies have, in general,

1 little gas and little interstellar dust. At the vertex of the bifurcation of Hubble’s tuning fork sit the “lenticular” galaxies, which look like spirals (see below), but do not have spiral arms. They are represented by the letter “S” followed by a “0”, or, “S0”. Their name is related to their shape, which is approximately that of a lens. Lenticular galaxies, like ellipticals, have too little gas and interstellar dust. The bifurcations of Hubble’s tuning fork have the “spiral” galaxies, whose name comes from the fact that they have the so-called “spiral arms”. There are the spiral galaxies said to be “normal” or “ordinary”, in which the arms extend rightly from the nucleus. And the “barred” spirals, in which the arms emanate from a structure similar to a bar, a central linear structure superimposed on the nucleus itself. The latter type was represented by the letters “SB”, where the letter “S” refers to “spiral” and the letter “B” means “barred”. Certainly, barred spirals are very important, because Hubble did not hes- itate in dedicate an entire branch of his tuning fork to them! In fact, it is verified, from astronomical observations, that half of all disk galaxies — spirals and lenticular — have a central bar. And if we take into account all existing types of galaxies, more than one third of them have bars. How are bars formed? What is the importance of the bar in the overall setting of the ? What is its effect on galaxy evolution? Many and many questions can be raised, and many of them are still the object of contemporary astronomical research. The bars are also called “oval distor- tions”, due to its general appearance. Let us, then, dive into the world of barred spiral galaxies! And nothing could be better than examining a fine example of this family: NGC 1300. It is located in the , a typical constellation of the southern hemisphere skies. The galaxy sits in the vertex of a great triangle, whose basis has the brightest of the sky, Sirius and Canopus, on its ends. It can be seen with the aid of a telescope northwest of Canopus. Its distance is 70 million light-years.

2 NGC 1300 is a barred spiral galaxy that clearly exhibits the characteristics of this morphological type: a central nucleus, a bar, formed by stars, gas and dust that cross it over, and spiral arms that extend from the ends of the bar. Notice the interstellar dust lanes that cross over the bar up to the galaxy nucleus (Image: , NASA/ESA).

The first thing that bars do to the disks of spirals is a notable symmetry breaking. Spiral galaxies are disk galaxies, i.e., the stars and all the galactic material are distributed in the shape of a disk. A homogeneous, that is, uniform disk has an axial symmetry, i.e., relative to an axis. In the case of spiral galaxies the axis is perpendicular to the plane of the disk and passes through its center. Axial symmetry means that all the points of the disk located at the same distance from the axis are indistinguishable. Like spiral arms, the bar breaks this symmetry, and does it in a more incisive way than the arms. It suffices to watch NGC 1300 image and we will not doubt of such an important aspect of bars. The bar rotates like a rigid body, in the same way as the hands of a clock. The rest of the disk rotates with a different speed at each distance to the center. This kind of spin is called a “differential rotation”. Which is another characteristic of the bar: the stars within it orbit around the center, but the whole rotates as a rigid body. The stellar orbits inside the bar have a variety of shapes, but none of them is circular. In general, they are “radial” orbits,

3 that is, the trajectories pass close to the . The gas and dust clouds found in the region of the bar have similar trajectories as well, but unlike stars, clouds lose much energy during the orbits. This a feature of a fluid — gas and dust — in motion. In contrast to the stars, these clouds collide and strongly interact, and radiate a lot, therefore, losing energy and falling unto the galactic center. Consequently, bars are regions that facilitate the “supply” of gas and dust to the central regions of the galaxies that have them. Hence, we can have the formation of many stars in these regions. The formation of a bar in disk galaxies is still a puzzle. Some galaxies develop a bar, while others, apparently similar in other aspects, do not. Theoretical astrophysicists, thus, are not yet completely satisfied with the available scenarios. But some details are well established. The formation of a bar is a natural process, that occurs in a rotating disk of stars. The force of gravity tends to make the disk smaller. But the disk is rotating and when it gets smaller its rotation increases greatly, which can be destructive! The way the disk has to get rid of part of its rotation is by redistributing its mass in the form of a bar. This can be achieved also with spiral arms, without a bar. Both, spiral arms and bars, have this important role. The initial studies of bar formation were made by means of computerized numerical simulations. A stellar disk, initially rotating, will develop a bar when the mass of the disk is large compared to the total mass of the galaxy. Furthermore, the disk stars must have random motions superimposed upon their global rotational motion. In reality, at the end, as we saw, what creates a bar is gravity. The random motion of the stars has the effect of disturbing their original circular motion. This gives origin to small concentrations of stars — of mass — in some places, which cause the gravitational attraction of more stars to that region. This process, once started, has no return: the result is the formation of a stellar bar, spinning like a rigid body. The stars have no circular orbits anymore, and start to have orbits more or less radial, i.e., trajectories that pass through the central region of the galaxy. Low mass disks do not develop the so-called “bar instability”, but just a spiral structure that forms from the nucleus. The numerical simulations are made with computational programs. They are called “N-body simulations”. The galaxy is represented by a very large number of masses whose dimensions are much smaller than the size of the whole, that is, of the “galaxy”. They are called “point masses”. The masses interact through the gravitational force given by Newton’s law of universal

4 gravitation.

The time evolution of a rotating disk of stars, initially with axial symmetry. We see the development of a faint spiral structure, which is soon dominated by a bar. This is a N-body computational simulation, where N = 20,000. That is, the stellar disk is represented by 20,000 point masses, which are gravitationally attracted to each other (Figure: J.A. Sellwood).

To exemplify this kind of procedure, I show here one of the simulations undertaken by the astronomer J.A. Sellwood, who specialized in the study of barred galaxies. In this model, the initial galaxy has axial symmetry and spins with differential rotation. The galaxy is represented by 20,000 bodies, or 20,000 “stars”. As time passes by, and the “stars” interact, an instability is develops, which breaks the disk symmetry. First, there appears a spiral structure — two faint arms —, and then a central bar, that soon dominates, clearly breaking the initial symmetry. In order to unveil the properties of barred galaxies it is necessary to perform many simulations, corresponding to different initial models of galaxies. The final results of the simulations are then compared with the observations of real barred galaxies, to determine which model is the most appropriate. In this way, one gets a

5 better understanding of the bar phenomenon in disk galaxies. One conclusion is indisputable: bars are very important substructures in disk galaxies. Maybe they are present — big or small — in all of them. And all of this has much to do with us: it is increasingly evident, from astronomical observations, that our has a small central bar.

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