The Local Group of Galaxies

Home , Andromeda Galaxy, Local Group, Small Magellanic Cloud

Domingos Soares

The closest star to Earth is the Sun. The Sun has billions of companions, the great majority of them invisible to our eyes. With the “unaided” sight, the human being is able to see about 9,000 stars all over the night sky and, to get that, one must have a cloudless night and no Moon light. The clouds obstruct the light of the stars and the Moon brightness dazzles the fainter stars. All Sun’s companion stars are part of our galaxy, known as “Galaxy”, with capital “g”, or “Milky” Way. Both terms are related to “milk”, the ﬁrst one originating from the Greek language. The explanation for the name is the appearance that all stars, grouped on the sky, exhibit to us in an exceptionally clear and dark night. The Milky Way is, in a certain sense, our “home” in the immense universe! The Milky Way, by its turn, has billions of companions as well! Most of them are invisible to our eyes. Actually, we can see, with the naked eye, only three galaxies: the great “nebula” — galaxy — located in the constellation of Andromeda, the Large Magellanic Cloud and the Small Magellanic Cloud, being the latter two satellite galaxies of our Milky Way, and much smaller than it.

1 Image of the sky, in infrared light, recorded by one of the instruments aboard the American satellite COBE, “Cosmic Background Explorer”. The human eye does not see this light. The dominant source of infrared in this image are the stars in the Milky Way disk. The image is possible because the Sun is located at the end of the galactic disk, at 28,000 light-years from its center, and the disk appears projected in proﬁle against the sky background (Image: NASA).

The most spectacular conﬁrmation that the Milky Way is really a galaxy with a disk comes from its image obtained in the near infrared. But what is near infrared? The visible spectrum, that is, the set of electromagnetic radiations that are detected by the human eye, goes from the red up to the violet, passing through the yellow and the other characteristic colors of the rainbow. The “colors” just below the red are called “infrared radiation” or, simply, “infrared”. The infrared closest to red is called “near infrared”. The great advantage of the near infrared is that it is not signiﬁcantly absorbed or scattered by the interstellar dust. Hence, since the majority of stars emit infrared radiation, an image of the Milky Way in this color will clearly reveal the structure of the galaxy, without obscuration caused by the interstellar dust. As the Sun — and, consequently, the Earth — are located in the periphery of the Milky Way, far away from its center, we can have a privileged view of its disk projected against the sky background. The result is a very clear image of a disk seen edge-on, proving, in an unequivocal way, that the Milky Way is in fact a disk galaxy. The image shown here was obtained by the American satellite COBE, the acronym for “Cosmic Background Explorer”. A little less than 40 galaxies are located within a radius of approximately

2 3 million light-years centered at the Milky Way. These galaxies — including the Milky Way and the three mentioned above — are part of a quite special group called Local Group of galaxies or, just, Local Group.

The great spiral galaxy located in the Andromeda constellation. Its catalog name is Messier 31 or, simply, M31. The galaxy can be viewed with the naked eye. M31 is the largest galaxy of the Local Group, followed by our Milky Way, a spiral galaxy as well (Image: Robert Gendler).

3 The three largest members of the Local Group are, in descending order of mass, the Andromeda galaxy, whose number in the Messier catalog is 31, hence its name M31, the Milky Way, and the galaxy 33 of the Messier catalog, or, M33. The latter is a beautiful galaxy seen head-on, that is, it shows its disk almost perpendicular to the direction of observation. It is located in the constellation Triangulum, a neighbor of the Andromeda constellation, where M31 is located. Both can be seen — M33 with the aid of binoculars or a small telescope — at the end of every year. In the northern hemisphere they are seen high in the sky. In the southern hemisphere skies, M31 shows up not very high above the horizon and M33 can be seen at about 15 degrees above M31. These three galaxies concentrate more than 90% of all visible light emitted by the Local Group. The majority of the other members of the Local Group are dwarf galaxies, of small masses compared to the first three and, consequently, with little luminosity. More than ten of them are satellites of M31 and about the same number are satellites of the Milky Way. Rigorously, the Local Group might be considered a triple group of galaxies. This means that if we were observing a distant group, similar to the Local Group, we would see only the three larger galaxies in the group. The triple groups of galaxies described in catalogs of multiple galaxies do not substantially differ from the Local Group, when one considers the relative brightnesses of their three larger galaxies. Some astronomers treat M33 as being bound to M31, due to their proximity. Both, by their turn, would be gravitationally bound to the Milky Way, making up, therefore, a triple group of galaxies. In some investigations, one may also consider the Local Group as a binary system of galaxies — M31 and the Milky Way. These two have more than 80% of all the group mass, and, thus, depending of the objectives of the study underway, their influence dominates over the influence of the other galaxies, including M33. Even though not being a dwarf galaxy, M33 is at least about six times smaller than M31 and about three times smaller than the Milky Way. The dwarf galaxies in the Local Group are mostly “spheroids”, that is, their stars, gas and dust are distributed in a very diffuse structure of spherical shape. Because they have low mass and are diffuse, they are, in general, very difficult to be observed. Many of them have masses slightly greater than the masses of “star clusters” that exist around the larger galaxies. Then what does distinguish a star cluster from a dwarf galaxy? The main factor is that all the stars in a star cluster are formed in the same epoch, all have the same

4 age. And the stars of a galaxy are formed during the course of time, or, in distinct events of star formation. Some of them have even ongoing star formation. The three major galaxies of the Local Group are spiral galaxies, their masses are distributed in a disk, that presents a very luminous structure with the shape of a pinwheel, exhibiting what is called “spiral arms”. The majority of the dwarfs are irregular, i.e., galaxies without a definite shape. There is only one elliptical galaxy, which is a satellite of M31, and whose catalog name is M32. The elliptical galaxies have this name because they have the characteristic shape of a flattened sphere projected on the sky, and do not show structures like, for example, the spiral arms, typical of spiral galaxies. M31 is approaching the Milky Way with a speed of 120 km/s, or aston- ishing 430,000 km/h! This velocity was measured with high precision by analyzing the spectrum of the light emitted by M31. With that speed, the great Andromeda galaxy would collide with the Milky Way in about 6 billion years. This is not likely to occur because the velocity of 120 km/s is just a part of the relative velocity of the two galaxies. This part — or component — is in the direction of the line that connects the two galaxies, also called “radial direction”. If there is a velocity component perpendicular to that line, then M31 and the Milky Way will be in a mutual orbit of elliptical shape, similar to the orbits of the comets around the Sun. Depending of the value of that velocity component, M31 may pass well far away from the Milky Way in the next billions of years. The major problem is the difficulty of measuring this velocity component. But the astronomers have already made some progress in this direction, and the results indicate that a collision between the two galaxies should not happen.

5 Image of the galaxy M33 obtained with the Isaac Newton Telescope, located at the the Spanish island of La Palma, oﬀ the western African coast. M33 is at 3 million light-years from Earth and can be seen with the aid of binoculars. M33 is close to M31, the largest galaxy of the Local Group (Image: David Malin).

The Local Group may have many other dwarf galaxies that have not been discovered yet, because they are very faint in brightness and because they are hidden by the interstellar dust abundant in the disk of our Milky Way. With the improvement of observational techniques and of telescopes, it is very probable that the number of galaxies in the Local Group increases even

6 more. The Local Group, our “borough” in the universe is, of course, not isolated. It sits in the periphery of a great cluster of galaxies called “Virgo cluster”. This cluster has thousands of galaxies, all of them bound by their mutual gravitational attraction. It has this name because the majority of its galaxies is seen on the region of the sky where the constellation of Virgo is located. In our analogy, it is our “town” in the universe. But the Virgo cluster is not isolated as well. . . And this story, really, has no end. That is good, because we all, human beings, are very curious!