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Spiral Galaxies Seen Edge-On: Needles on the Sky

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Spiral Galaxies Seen Edge-On: Needles on the Sky Spiral galaxies seen edge-on: needles on the sky Domingos Soares Galaxies are groups of stars, which are counted from the hundreds of millions to the hundreds of billions. Many stars, indeed. And each one of these groups | these galaxies | is an independent system, because their stars are bound by their mutual gravitational attraction. One can say that each star moves around all the others! A much more simple example, but analogous, is of our Moon, which is gravitationally bound to the Earth and moves around it. The gravitational force is the force that gives the weight of things. The weight we have originates from the gravitational attraction of the Earth onto us. Galaxies have a variety of shapes depending of how stars, the dust and the interstellar gas distribute themselves to form them. One of the most spectacular shapes is exhibited by spiral galaxies. Stars are distributed in a central bulge, in a disk, where spiral structure develops, the so-called \spiral arms", and in an external halo that envelops the two former components. According to the angle of sight from the Earth, we can see spiral galaxies \face-on", \edge-on" and on all possible intermediate orientations. When we see them face-on, we are able to appreciate all the beauty of the spiral arms, which show up as whirlpools and pinwheels on the sky. But when we see them edge-on, one of their most striking feature is uncovered, namely, the presence of a disk. The stars and all the remaining material, constituted by dust and interstellar gas located amongst the stars, are distributed in this disk. The thickness of the disk is very small as compared to its radius. In the extreme cases of edge-on views, they look like a \needle" projected on the plane of the sky. We shall see, then, some examples of edge-on galaxies and, especially, of some \needles on the sky". We begin by the spiral galaxy M104, or NGC 4594, also known by the name of \Sombrero Galaxy", due to its appearance that resembles a Mexican 1 hat. It sits at 50 million light-years from us, at the sky region of the Virgo constellation. It is the brightest galaxy of a small group of galaxies. On the image, we see a dark stripe that cuts through the central bulge. This dark stripe is made of interstellar dust and delineates the galactic disk. For comparison purposes, we see also the image of the central region of the Milky Way. This region is located at only 30 thousand light-years | compare to the distance to Sombrero |, and it is for this reason that we see a very irregular dark stripe of dust, formed by dust clouds. Incidentally, this im- age was acquired in the Pico dos Dias Observatory, National Laboratory of Astrophysics, Braz´opolis, MG, Brazil. The field of view on the sky is very large, 90 by 40 degrees, and the camera used is located on the exterior of the buildings that house the research telescopes. One can see, on the bottom left corner of the image, a portion of the building that houses the dome of the 1.60-m aperture telescope of the observatory, illuminated by the light of the Milky Way! The exposure time for obtaining this image was 60 seconds. The sky is continuously monitored during the night, in order that the quality of sky at the observatory site be evaluated by the astronomers. Any person can access these images through the electronic page of the National Laboratory of Astrophysics at www.lna.br. The similarity between M104 and the Milky Way is misleading because, in reality, they are quite different. One must recall that we are watching two astronomical objects located at very different distances. Next, we shall see a galaxy which is, indeed now, fairly comparable to our Milky Way. After we present this other galaxy, we shall return to the discussion of this issue. 2 The galaxy M104 (left) exhibits a central stellar bulge and a stellar and dusty disk. Our Milky Way shows a similar structure. We see more details in the Milky Way's image because we are situated within it, and, therefore, its bulge is much closer to us (Images: M104, public domain; Milky Way, National Laboratory of Astrophysics, Pico dos Dias Observatory, Braz´opolis, MG, Brazil). We shall have now two edge-on galaxies to the extreme! The first one is called the \Needle Galaxy" or NGC 4565. It is an isolated galaxy, i.e., it is not part of a group or of a pair of galaxies. It is at 30 million light-years. The other one is catalogued in the \Uppsala General Catalogue of Galax- ies" (UGC) with the number 7321. The UGC catalogue was determined by the Swedish astronomer Peter Nilson (1937-1998), published in 1973, and contains almost 13,000 galaxies. The majority of them is visible only in sites located in the northern hemisphere. Our second galaxy, UGC 7321, is so thin on the sky that it is known as the \Superthin Galaxy". Its distance is also approximately 30 million light- years, and therefore, like NGC 4565, is close to us. Both are located on the region of the constellation of Berenice's Hair | or of Coma Berenices, in 3 Latin. NGC 4565 (left), the \Needle Galaxy", shows a small central bulge. And UGC 7321, the \Superthin Galaxy", allows us to see only the silhouette of its disk. Its bulge is nonexistent. The central region can be identified by a slight increase of brightness in its disk (Image: Palomar Observatory). Notice, on the image of NGC 4565, a small blueish cloud located well above the galaxy's bulge. It is a dwarf galaxy, satellite of the larger galaxy, and is catalogued in the \Index Catalogue" (IC) with the number 3571. This catalogue complements the \New General Catalog" (NGC) of John L.E. Dreyer (1852-1926). Dreyer himself was responsible for its compliation. The IC catalogue was published, for the first time, in 1895 and nowadays records almost 6,000 objects from which 2,400 are galaxies. Despite being a dwarf galaxy, IC 3571 influences NGC 4565, in a notice- able way, through its gravitational force. This was verified in radio wave observations. The atomic hydrogen in gaseous form is abundant in spiral galaxies, and it can be detected because it emits long waves | in the radio range thus | at the wavelength of 21 cm. The radio observations of NGC 4565 show that there is a gaseous disk much more extensive than the stel- lar disk. At the end, closest to the dwarf galaxy, the hydrogen disk warps itself in the direction of IC 3571, revealing its gravitational influence. It is 4 a known effect, called \tidal interaction", a name that originates from the similar effect observed on the terrestrial oceans due to the gravitational force of the Moon. The effect causes the ocean tides, from which comes the name of this type of interaction. Incidentally, IC 3571 has a lot of atomic hydrogen as well. All this is hardly a surprise, because the chemical element hydro- gen is the most abundant of the universe, followed by the chemical element helium. Hydrogen constitutes about 75% of all existing detectable matter in the universe and helium represents about 24%. The remaining 1%, or even less than that, is responsible for everything more that exists, there in- cluded human beings, all living beings of the planet Earth, other non gaseous planets and all of the non gaseous astronomical objects of the universe. It is worthwhile remembering that the stars and all other gaseous objects are constituted mainly of hydrogen and helium. IC 3571 is located at a distance of 4 arcminutes from the center of NGC 4565, on the plane of the sky. Such an apparent distance corresponds to 35,000 light-years, at the distances that these galaxies are (30 million light- years). There is another satellite galaxy, larger than IC 3571, and located a little more than twice its distance to the center of NGC 4565, and to the opposite direction. It is the galaxy NGC 4562 and does not appear on the sky field shown here. It is larger than IC 3571 but much smaller than NGC 4565. Both NGC 4565 and NGC 4562 exhibit a rotating disk. IC 3571, an irregular dwarf galaxy, does not have rotation. NGC 4565 is a typical giant spiral galaxy, unlike UGC 7321, which is a dwarf galaxy, does not have a central bulge and has little interstellar dust. UGC 7321, the Superthin Galaxy, is part of a galaxy category called \su- perthin galaxies", all of them sharing the same features mentioned before. To illustrate this family, we shall see three more members. They join UGC 7321 on the image I show next. 5 In this set, we see four members of the family of superthin galaxies. From left to right we have IC 2233, in the Lynx constellation, UGC 7170 in Coma, our UGC 7321, also in Coma, and UGC 9242 in the Bo¨otesconstellation (Images: J.W. Goad and M.S. Roberts). The superthin galaxies are \underdeveloped" spiral galaxies, in a certain way. They are dwarf galaxies, where presently there is not star formation, due to the scarcity of interstellar material. These galaxies, like all spirals, have a rotating disk and probably are very common in the universe. Since they have very low brightness, they were not noticed in the past, as well deserved. For example, there are few superthin galaxies in the NGC and IC catalogs, which were determined in the XIX century from direct observations with telescopes, and not from sky photographs.
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