Inner Ring

tranquility is replaced by expanding rings of fire. Just as in our analogy of throwing a stone into a pond of water discussed above: One small galaxy had plunged head-long almost through the center of the Andromeda Galaxy, creating two outwardly expanding rings of dust imaged using the Spitzer Space Telescope. The two rings truly glow in these images; hence the use of the word "fire."

Galaxy morphology can dramatically change over very short periods of time; in the case of the Andromeda Spiral, the morphology we see today (Figures 144-146) originated from a collision only 200 million years ago. Members of the genus Allosaurus would have been roaming the Earth (Figure 148) ... the drawing serves to highlight the recent timescale of the tumultuous event, gauged in millions (rather than billions) of years.

Many years have passed since 1918, when the astute eyes of Heber Curtis led him to the "bar phenomenon." The Lick Observatory Volume 13, containing Curtis' paper, forever stands out in the history of galaxy morphology. Never could Curtis have dreamt, however, what he had truly opened up to the world - that the new view of galaxies in the dust penetrated Universe would reveal a ubiquity of extragalactic bars outside of the Milky Way.

In fact, over seventy percent of spiral galaxies in our local Universe show bars, even if we see no evidence for the oval or bar in optical images. In some instances, these bars are very striking, covering possibly a third of the diameter of the disk in a typical image. In other cases, the bar is simply a small feature near the center of the galaxy.

How do stars move within these bars? We know that the bars are themselves rotating but a little more slowly than the galaxy as a whole. Opinion changes about whether the bars are made up always of the same stars, or whether stars pass through the rotating bar, somewhat like water particles passing through a water wave. The current viewpoint is that stars become trapped in the bars. One can imagine that the bar is made up of stars following noncircular, elongated orbits, all locked together by gravity to form the rotating bar. The stars themselves move in the elongated bar but, at the same time, are also The Dust Penetrated Universe the building blocks of the elongated bar itself. The presence of mass has an associated 241

gravitational field; it is the gravitational field of the bar which causes the individual orbits of the stars to be elongated, and it also keeps the orbits elongated in the same direction. These elongated orbits actually constitute the bar feature so ubiquitously lurking behind cosmic Shrouds of the Night. If the orbits of the stars were not elongated but almost

circular, then one would not have a bar at all - simply a galaxy possessing symmetry about some axis.

Similarly, if the orbits were elongated but drifted away from being all elongated in the same direction, then again one would not form a bar. The existence of the rotating bar needs the elongated orbits to be locked together and remain elongated in the same direction. There is no room here for individuality.

At this juncture, an analogy may be useful. One of us (Ken) is an avid bird-watcher, traveling across the globe to see new and rare species of birds. Let us imagine arriving at a site on some birding expedition, and watching an elongated flock of birds flying overhead. The ground based birdwatcher of course views the flock from an appreciable distance and the observer takes cognizance of the fact that the overall morphology or shape of the flock is elongated. Within the flock, however, individual birds are flying around; they are by no means stationary and furthermore, each bird is not always in the same locale within the flock. The birds themselves define the shape of the flock. To maintain the shape of the flock, however, the birds must not venture outside its elongated boundary. Some cooperation is needed! It is much the same for the stars that constitute the plethora of bars in our local Universe. The stars move around within the bar, defining the shape of the bar, but they must stay within the bar itself.

The disks of spiral galaxies are truly flat, typically 100 000 light years in the plane of the galaxy (for a galaxy such as the Milky Way) and some 10 000 light years in thickness. Bars may have the same thickness as that of the disk - or sometimes even thicker. Stars within a bar would typically move at speeds of 100-150 kilometers each second in the plane of the galaxy, and possibly 30 kilometers each second in the direction perpendicular to the plane. The shape of the bar itself rotates, but more slowly in angular terms than the stars of the disk just beyond the ends of the bar. Astronomers can actually measure the rotation speed of bars (known technically as their pattern speeds) using spectrometers on large telescopes.

There is no obvious reason why the bar itself needs to be rotating. In principle, it could be fixed in space and still be defined by the orbits of the stars within it. Nature is so rich in structure and in diversity: in reality, astronomers observe that the bars themselves do rotate.

Given that millions of stars within the bar of a galaxy are all in motion in elongated orbits described above, it may well be asked how bars are ever able to maintain any shape at all. The answer lies in a concept known as precession. As each star moves in its elongated orbit, the long axis of each stellar orbit also rotates in space, precessing at the same speed as the bar, so that the bar itself maintains its identity. A star whose orbit has precessed so that it no longer lies within the bar does not contribute anything to the presence of the bar. Astronomers call this complex phenomenon figure rotation.

Classification of galaxies has invariably been qualitative, placing galaxies in their respective classes by visual inspection of photographic or digital images. The challenge before us has been to move away from qualitative schemes, to those wherein measurable parameters are made, not by eye, but by computer: in other words, to quantify key parameters of galactic backbones. Not the flesh, but the skeletal structure, using our x-ray analogy. To effectively accomplish this challenge, we must first penetrate our Shrouds of the Night.

David has spent some two decades attempting to quantify the shape of spiral galaxies behind their dust masks, working closely on different aspects of this research with teams of astronomers from France, Germany, Italy, Mexico, Spain, the United Kingdom, the United States of America, Finland and others. Figures 149 and 150 (developed in collaboration with Ivanio Puerari from Mexico and Ronald Buta from the United States) shows our criteria for classifying spiral galaxies in the near-infrared, once we unmask these systems behind their Shrouds of the Night.

One important physical parameter which we use in our near-infrared galaxy classification scheme is the angular pull or torque provided by a rotating bar of stars. We have developed techniques which enable a computer to detect a bar (see Figure 149) and then, to determine its angular gravitational "tug." We recognize seven classes of "bar strength."

The angular pull or tug of a rotating bar influences the behavior of gas, and affects the way in which young stars are distributed in the disk of a spiral galaxy. The stronger the bar, the stronger The Dust Penetrated Universe the angular tug and the greater the effectiveness with which the bar influences the motions of stars 245

and gas. For example, the shapes of galactic rings are tied to the way in which the bar pulls on the stars. The bar can also help to generate some dramatic events near the centers of galaxies.

Many galaxies have massive black holes residing at their centers. The tug which the bar exerts on the gas can funnel some gas into the central parts of the galaxy. This can have very violent consequences. Gas passing close to a central black hole liberates energy in the form of intense radiation extending over the entire range of the spectrum, from gamma rays (whose wavelengths are even shorter than x-rays) to radio waves, with wavelengths of a meter or more. These dramatic events are called active galactic nuclei. Even if this funnelled gas does not reach the central black hole, it can erupt in a giant flash of star formation called a "star burst." Active galactic nuclei are often seen near the centers of barred galaxies.

Secondly, we believe that the degree of openness of the spiral arm pattern of old stars (in technical terms, its pitch angle) is another quantitative parameter, which a computer can readily measure. In Figure 150, we propose using three separate classes (unrelated to the Hubble classification from optical images) for the backbones of spiral galaxies: the alpha, beta and gamma bins.

Our colleagues have empirically demonstrated how the pitch angle of spirals seen in infrared images is related to the shape of the rotation curve. (A rotation curve simply shows how fast stars or gas orbit the center of a galaxy, plotted as a function of radius.) We believe that the alpha, beta and gamma classes yield important insights into the physics of galaxies in the post Hubble era, because the shape of the rotation curve is inextricably tied to the distribution of both stars and of dark matter in a galaxy.

When one studies the appearance of the spiral arms of principally old stars, we find there is little correlation with Hubble's qualitative classification, as alluded to in important earlier studies by Vera Rubin (Carnegie Institute of Washington) and her team of researchers. In Figure 150, it should be remarked that spiral galaxies such as Messier 83, classified by Hubble as "late-type," reside in our tightly wound "alpha" bin. Our classification recognizes the duality of spiral structure found in our local Universe and seeks to classify spiral galaxies Shrouds of the Night behind their Shrouds of the Night using three criteria: the number of dominant spiral arms

246 in the infrared, the bar strength and the openness of the spiral arm pattern of old stars.

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