Lenticular Galaxy, a Bit Spiral, a Bit Elliptical

Lenticular galaxy, a bit spiral, a bit elliptical Domingos Soares Lenticular galaxies were the last to be included in the morphological classification of galaxies designed by the American astronomer Edwin Hubble (1889-1953). Morphology refers to the shapes of galaxies. The lenticular are galaxies whose stars are distributed in the form of a disk and a central spheroid, like spiral galaxies, but they do not have spiral arms. Besides, they are \clean", like elliptical galaxies, that is, they lack interstellar gas and dust, but unlike those, they do not show a global spheroidal shape. All in all, lenticular galaxies have a bit of each one of the two classes of galaxies. When Hubble put forward his classification of galaxies, for the first time, in 1926, the lenticular class was not recognized. But through the years, Hub- ble began to believe that there should be a class of galaxies that could make a bridge between ellipticals, that have the general form of a spheroid, and spiral galaxies, that are predominantly disk galaxies, with bright and majes- tic arms. In his 1936 book, \The Realm of the Nebulae", he included this class in his galaxy tunning fork. The elliptical galaxies were in the handle of the tunning fork and the spiral galaxies, with and without bars, in the two arms of the fork. In the vertex of the tunning fork, representing a morphological transition between ellipticals and spirals, he postulated the existence of a new class, that of the lenticular galaxies. They were designated by the letter \S" followed by \0", that is, S0. It is always worthwhile seeing again the Hubble tunning fork, the diagram created by Hubble for the schematic representation of his morphological classification. Look to the figure, and notice in it the location of S0 galaxies. 1 The morphological classification of galaxies: Hubble's tunning fork. Lenticular galaxies, designated by \S0", are located in the vertex of the tunning fork. There were not yet concrete observational evidences of their existence. Between the years 1936 and 1950, especially since the beginning of operation of the 200-inch aperture telescope, at Palomar Mountain, empirical evidences showed that, indeed, the class of S0s was real. We witness here, one more \stroke of genius" of the extraordinary astronomer, without a doubt, the greatest exponent of astronomy, in the age of large telescopes. Incidentally, one can highlight three great names, amongst astronomers, whose contributions to the development of modern science were fundamental: the Danish Tycho Brahe (1546-1601), the Italian Galileo Galilei (1569-1642) and Edwin Hubble. Tycho, the greatest genius of observational astronomy before the age of telescopes | his observations were visual |, introduced the scientific precision in the record of astronomical observations. For him, an observation should be always accompanied by the error assumed in its acquisition. Galileo was the first to use the telescope, a simple eyeglass, for undertaking astronomical observations. And he made sensational discoveries. Hubble inaugurated a new age in astronomy: expanded our horizons with the discovery of galaxies and the investigation of the extragalactic universe. What precisely are lenticular galaxies? Why do they have this name? And, if they are disk galaxies, like spiral galaxies, why do not they posses spiral arms? We begin to answer these questions with our first specimen. See the lenticular galaxy NGC 2549 in the figure below. Notice that 2 the stars are distributed in the shape of a biconvex lens, the lens that one finds in a common eyeglass. That is the reason for the name of this class of galaxies. The stars of lenticular galaxies are distributed in a disk, and when seen projected in the plane of the sky they show their peculiar shape. It is because the disk most of the times is seen tilted with respect to our line of sight. Hence, the appearance of the disk, observed in the sky, is that of a biconvex lens. When it is seen head-on, it presents an approximate circular shape and may be very easily mistaken with an elliptical galaxy. NGC 2549 is a lenticular galaxy seen edge-on. Notice the smooth distribution of stellar light, as it occurs in elliptical galaxies. Lenticular galaxies are found in densely populated regions of galaxy clusters. The majority of the small galaxies seen around NGC 2549 are distant galaxies seen in projection in the plane of sky. These are galaxies that inhabit the cluster outskirts. One of the most accepted theory amongst astronomers, about the partic- ular shape of lenticular galaxies, is based in the fact that these galaxies are always found in the most densely populated regions of clusters of galaxies. In these regions, besides the existence of many galaxies, there is much interstellar gas as well. According to the theory, a disk galaxy, originally with a lot of 3 gas and interstellar dust, looses the gas and dust as it moves throughout the intergalactic gas. The galaxy stars go unaffected by the intergalactic gaseous medium, but the gas and the dust strongly interact with the exterior gas, being stripped out from the galaxy. Another possibility, similar to the previous one, invokes a head-on colli- sion between two spiral galaxies, which, as we know, are disk galaxies plenty of interstellar gas and dust. The gas and the dust give rise to new stars that shine in the spiral arms. Now, as they collide, the stars of both galaxies do not interact, because they are separated by immense spaces, but the gas and the dust are distributed in an uniform way in the galaxy disks. Thus, galaxies pass one through the other and loose their gas and dust, because they strongly interact giving rise to a cloud of gas and dust at high temper- ature, which, subsequently, spread through the intergalactic medium. The end result will be the formation of two lenticular galaxies. The conclusion we draw from those two scenarios is that lenticular galaxies were spiral galaxies in the past, which lost the constituents of their interstellar media, that are the raw materials for the formation of new stars. A consequence of this is that there is not formation of new stars in lenticular galaxies, contrary to what copiously occurs in spiral galaxies. Lenticular galaxies are, therefore, constituted by old stars. They posses, many times, besides a stellar disc, a spheroidal bulge in the nuclear region, as mentioned above. Such a bulge is formed by stars that move on radial orbits, that is, with trajectories that pass close to the galactic center. The galaxy shown next exhibits these two features: a stellar disk and a large stellar bulge. That is NGC 3115, located at 32 million light-years. 4 NGC 3115, a lenticular galaxy where one can see the stellar disk and a large stellar bulge in the central region. Notice the absence of interstellar dust, which would show up as dark lanes all over the galaxy, blocking stellar light. This is common in spiral galaxies (Image: John Kormendy). When a S0 galaxy is seen edge-on, as is the case of NGC 3115, it can be confused with a spiral also seen edge-on. High-quality observations are necessary in order to rule out the presence of spiral arms. Dynamically, lenticular galaxies are indistinguishable from spirals, because both have a rotating disk. To distinguish between them it is necessary a careful analysis of the distribution of light emitted by the stars, trying to detect the presence of spiral 5 arms or of interstellar dust in appreciable amount, which are the features of spiral galaxies. On the other hand, when a lenticular galaxy is seen head-on another difficulty arises. They become visually indistinguishable of an elliptical galaxy! Again, the distinction must be made from the distribution of galactic light. The light intensity of lenticular galaxies decreases progres- sively from the galactic center in a gentler way than in elliptical galaxies. It is not possible to resort to kinematic or dynamical methods, such as, for ex- ample, an investigation of galactic rotation, since it is impossible to measure the rotation of head-on galaxies. It is worthwhile remember that elliptical galaxies have too little rotation. Unfortunately, this fact does not help in the distinction between them, as mentioned, due to the observational difficulty of getting rotational velocities. Our next case, NGC 4612, is a lenticular galaxy seen with a small incli- nation with respect to the line of sight. It can be confused with an elliptical galaxy. But the analysis of its light shows that it is a genuine lenticular. And with an interesting feature: it has a bar! Lenticular, as spiral galaxies, can have bars also. In these cases, they are called \SB0" galaxies, where the letter \B" announces the presence of a stellar bar. 6 The lenticular galaxy NGC 4612 can be mistaken with an elliptical galaxy. But it is not, as it is evidenced by the analysis of its light. This galaxy has a stellar bar, which is not visible on this image (Image: Space Telescope Science Institute). As we saw, lenticular galaxies present a whole world of particularities. This shows how rich is the Hubble tunning fork: each class of galaxies ap- pearing there open up endless horizons of astronomical research. 7.

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