Whirlpools and pinwheels on the sky

Domingos Soares

Galaxies exist in many different forms and count by, at least, the hundreds of billions, according to estimations made from the observations of the . Among all of them, there are no doubts that the most spectacular are the so-called “spiral ”. Our own , the galaxy, is a member of that family. Spiral galaxies are so named because of the distinctive aspect they present to the observer, namely, of a spiral-shaped structure similar to a whirlpool or a pinwheel. They are also called “disk galaxies” because the galactic material — , gas and interstellar dust — is distributed in the shape of a “thick” disk. The stars constitute the main constituents of galaxies, at least with respect to their visual appearance. They emit the major part of the visible light of a galaxy, be it a spiral or not. The typical visual aspect of spiral galaxies is due to the structures that astronomers call “spiral arms”. The most bright stars of a galaxy are those that delineate the arms. These, however, contribute little to the total of the galaxy, but due to their extraordinary brightness they are the most noticeable on a visual inspection. The disks rotate. And this is typical in those galaxies: the galactic material spins around the disk center, which is called “galactic center”. But the rotation is not like the spin of a rigid disk, like, for example, a CD, in which all the points make a full turn in the same time interval. On the galactic disk, there is a different rotational velocity at each distance from the galactic center. This kind of rotation is called “differential rotation”. As we shall see below, the differential rotation of the galactic disk is intimately related to the existence of the spiral arms. Our Sun moves at the fantastic velocity of 900,000 km/h around the Milky Way center. It makes a whole turn each 250 million years! Since the Sun is already 5 billion years-old, it is easy to calculate that it has made this voyage for 20 times! By this token, we could say that the Sun is a 20 galactic-years-old “youngster”.

1 Spiral galaxies present themselves to the observers in a variety of ways, depending of the angle by which they are seen. In other words, their ap- pearance is a question of geometric perspective. We can see them with the galactic disk “edge-on”, “face-on” or in all possible intermediate perspectives. The spiral arms are most spectacularly seen when we watch them face-on. That is what we are going to see next on three “famous” spiral galaxies. They are especial for different reasons, both among professional astronomers and among amateurs. The three are very beautiful examples of face-on spiral galaxies. Their spiral arms can be seen in all of their splendor. The first one is M51, as it appears in the catalog of (1730- 1817), also known as NGC 5194, according to its number of entry in the “New General Catalog”, by John L.E. Dreyer (1852-1926). It is located on the , whose Latin name means “Hunting Dogs”. M51 is popularly known as the “” and was discovered by Messier himself, on October 13, 1773, when he was observing a comet. He described it as a “very faint nebula, without stars”, very difficult to be seen. The distance of M51, according to modern measurements, is 31 million light- years, or about 15 times as far away as our closest companion M31, the Great Andromeda Nebula, that belongs to the Local Group of galaxies, the to which our Milky Way is part of.

2 M51, the so-called “Whirlpool galaxy”. Two spiral arms are clearly seen. Notice the small companion galaxy located on the upper portion of the image, close to the end of one of the arms (Image: Todd Boroson, Kitt Peak National Observatory, United States).

M51 was the first galaxy in which a spiral structure was clearly identified.

3 Its spiral arms are likely to constitute the most fine example amongst the spiral galaxies close to us. The arms exhibit emission in radio waves as well, which indicates that there exists magnetic field along them. Electromagnetic waves in the radio range are emitted by electrons that move around magnetic field lines. The strong emission in radio-frequency, along the spiral arms, indicates that there was out there compression not only of the matter but also of the magnetic field. The second is M83, or NGC 5236, located in the Hydra (Water Snake) constellation, known as the “Southern Pinwheel”. It is easily observed on the southern hemisphere skies, hence its name. M83 was discovered on February 23, 1752, by the French astronomer Nicholas Louis de la Caille, at the Cape of Good Hope. It was, therefore, discovered before M51. Messier included it in his catalog in 1781. It was the first galaxy discovered outside the boundaries of the Local Group. M83 is very difficult to be observed from the northern hemisphere, due to its location on the sky.

4 M83, the so-called “Southern ”. Despite being a bit irregular, the spiral arms can be seen, given the impression of a pinwheel. The image was obtained with an instrument called “Wide Field Imager”, mounted on a 2.2-m aperture telescope, located in Chile, at the European Southern Observatory, ESO (Image: Davide De Martin, ESO).

M83 is at a distance of approximately 15 million light-years. There are red and blue “lumps” distributed along its spiral arms. The red lumps are diffuse gaseous nebulae, in which formation is going on. Recent formed stars shine intensely in the ultraviolet light and “excite” the nebula — constituted mainly of hydrogen —, making it to shine with the red color. To excite a nebula means, in this case, to ionize its gas, i.e., “to pull out” the electrons from the neutral hydrogen atoms. The energy necessary for the excitation comes from the ultraviolet radiation of the newborn stars. The whitish blue lumps are star clusters recently formed — some tens of million years ago. These colored and bright lumps are present in all spiral

5 arms, and are a feature of spiral galaxies. Because of the innumerable bright lumps — especially the red ones —, M83 is often called “the Thousand Ruby Galaxy”. Despite being almost three times smaller than the Milky Way, M83 is very similar to what astronomers imagine our galaxy would look like, if it could be seen face on. Moreover, in the image shown here, one can notice a bar-like stellar structure, crossing the nucleus of the galaxy. The spiral arms emerge from this bar. That is also what seems to happen in our galaxy, that is, the Milky Way, like M83, is, almost certainly, a barred galaxy. The observations of the nucleus of M83 in X-rays show that there is ongo- ing strong there. The nucleus is inside a cloud of superheated gas, at temperatures around 7 million degrees centigrade. Let us now finish with M101, or NGC 5457, on the boreal constellation of , also known as the “Pinwheel galaxy”. It was discovered by another French astronomer, Pierre M´echain, on March 27, 1781, and was one of the last additions to the Messier catalog. It is located at 27 million light-years, according to recent determinations. It is a very big galaxy, about twice as large as the Milky Way. M101 is the brightest member of a group with at least nine galaxies, which includes M51, the Whirlpool galaxy, previously described. As can be seen in the M101 image, its nucleus is slightly displaced from the center of the galactic disk. This was, very likely, caused by the gravitational interaction with one of the closest members of the group, that occurred in recent times — from the galactic point of view, i.e., some hundreds of millions of years ago.

6 M101, or the “Pinwheel Galaxy”. Image obtained by the Hubble Space Telescope in 2006 (Image: Hubble Space Telescope, NASA/ESA).

M101 was observed by the , which is specialized in infrared radiation. Spiral galaxies have much dust, mainly located in the spiral arms. The dust is constituted by microscopic particles of graphite and of giant organic molecules, especially the polycyclic aromatic hydrocarbons. The latter are found in the Milky Way as well and, on Earth, in the prosaic soot. Such a dust sits in regions of star formation and is heated by the stellar radiation. The Spitzer Space Telescope detected the warmed dust clouds of M101, which emit infrared radiation. But the astronomers observed a curious fact. In the external regions of M101 the hydrocarbons are lacking. The most probable explanation for this is that the radiation from the new formed stars, located in these regions, is so energetic that, instead of simply warming up the dust clouds, it destroys the organic macromolecules. As we saw, all three galaxies described above were discovered in the XVIII century, when their true nature was still unknown. They were, then, called “spiral nebulae”. Later, in the XX century, it was discovered that they were independent stellar systems, similar to the Milky Way. How are the spiral arms formed? This is a current issue because there

7 is not a definitive answer yet. As it is apparent on the images shown, with the exception of M51, the spiral structure can be very fragmented. Spiral galaxies like M51 are called “Grand Design” spirals, due to the great regu- larity observed in their arms. These are the preferred ones for the tests of the theories of spiral structure formation. The most popular theory amongst astronomers is the “density wave the- ory”, authored by the American astronomers C.C. Lin (1916-2013) and his doctoral student Frank H. Shu (1943- ), whom put it forward in the mid- 1960s. It has been applied to Grand Design spirals with great success. In this theory, the spiral arms represent the crests of a wave that propagates through the galactic disk. The wave is generated in the disk and due to the disk’s differential rotation it propagates in the form of a spiral. This is a compression wave that, when passing along the disk, forces the gas and the dust present there to approach each other. Such an approaching, or com- pression, leads to the gravitational collapse of the large clouds of gas and dust, resulting into the formation of stars. The brightness of these stars and of these enormous clouds of star formation uncover the spiral shape of the compression wave. The process of the wave propagation is similar to what happens with an ocean wave, with the great difference that, here, the waves are spirals and not parallel to a line, as it is the case with the waves that break on the beach. Like in the sea and in the ocean, the spiral waves are generated by internal energetic processes. In the case of the spiral galaxies, the energy comes from the gravitational field, that is, from the existing grav- itational attraction between the components of the galaxy. But the waves may be also stimulated by an external force, like the gravitational force of a companion galaxy. This seems to be the case in M51 that, as we saw, has a very close companion. This kind of excitation by an external force is very common in nature. A familiar example is the ocean waves, which are generated by the combined gravitational forces of the Moon and the Sun on the oceans.

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