Were Bars Common in the Early Universe

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The high redshift (young) Universe, as spectacularly imaged in very "deep" (long) exposures of the Hubble Deep Field and Ultra-Deep Field, shows many galaxies which present a rather chaotic and disorganized morphology. At a recent conference on galaxy morphology in South Africa, one of the delegates was ready to abandon the current Hubble classification entirely, specifically because many galaxies at high redshift "are such a mess." The shapes of galaxies may, however, actually be more orderly at high redshift than they look! The large velocities at which these galaxies are moving away from us implies that the wavelength of the light coming to us is redshifted: very different from the original wavelength at which it left the galaxy.

For example, when astronomers say that galaxies in the early Universe have a redshift of 4, for example, they mean that the wavelength of the light as we see it is (4 + 1) times longer than the wavelength at which it was emitted by the galaxy. Images taken of such galaxies in the infrared are actually revealing to us the masks in the optical and ultraviolet "rest-frames" of the galaxy. The effects of dust and of star formation are strongly enhanced in the optical and ultraviolet, as we have repeatedly seen in earlier chapters of this book. Bars in our nearby Universe are very common. Does this fraction dramatically change with the age of a galaxy, as we probe the structure of galaxies at earlier and earlier epochs? In order for us to truly exit our Dark Ages in this regard, we need to be able to penetrate the masks of these galaxies, which are traveling away from us at huge velocities.

It is fitting here to pay tribute to the amazing insight of Gerard de Vaucouleurs, who strove to understand galaxies long before it became possible to penetrate dusty masks. With his experienced eye, he was able to examine photographs of galaxies secured using a blue filter and was "somehow" able to discern what lay underneath the mask. For example, he classified the almost edge-on spiral NGC 253 from such images. What is truly remarkable is that de Vaucouleurs was able to "see" the optically well-concealed underlying bar in NGC 253 (Figure 140), and his classification corresponds closely to what we now observe in the new view of galaxies provided by infrared detectors!

It should be remembered that Gerard de Vaucouleurs also discerned the true structure of the Large Magellanic Cloud. In de Vaucouleurs' time, most astronomers regarded the Large Magellanic Cloud as an irregular galaxy, without any defining structure like spiral arms or a central bar.

It was de Vaucouleurs who correctly recognized that the Large Magellanic Cloud was one of a class of so-called Magellanic spirals, which have a small bar located not quite in the center of the galaxy. Moreover, its spiral arms are not symmetrical - one arm to the north is much more prominent than two stubby southern counterparts.

As noted earlier, Sir John Herschel, working at the Cape in South Africa, had produced an amazingly accurate drawing of the Large Magellanic Cloud as seen with the naked eye. Herschel's perceptive drawing, published in 1847, shows not only the bar of the Large Magellanic Cloud, but also its dominant spiral arm to the north and the embryonic arms to the south (see Figure 172). An early claim of spiral structure in the Large Magellanic Cloud by means of photography only came 43 years later, when H.C. Russell from Sydney used a 6-inch aperture portrait lens in 1890 (Figure 173). However, the prevailing view, pre-1950, was that the Large Magellanic Cloud was an irregular galaxy. This view was forever changed in the years spanning 1952-1955 when de Vaucouleurs photographed the Large and Small Magellanic Clouds from Mt. Stromlo. de Vaucouleurs made a Christmas card from Herschel's drawing and sent it to one particular astronomer who was very resistant to the idea that the Large Magellanic Cloud was a spiral galaxy!

In hindsight, one of de Vaucouleurs' most important achievements as a morphologist concerned the nature of the central bars in spiral galaxies. He recognized that it is not merely a question of whether galaxies have bars or do not have bars. He saw from his optical photographs what we Eyes to the Future can now see much more clearly in the infrared images: many galaxies have relatively small inner 323


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bars. He introduced a classification branch to include these "weakly barred" galaxies. Ordinary unbarred spirals are designated SA, galaxies with strong bars are classified SB, while intermediate systems with weak bars are denoted as SAB. Our Milky Way has a weak central bar, so is a member of the SAB class. de Vaucouleurs recognized the diversity of the underlying barred structure of galaxies, which we can now quantify by measuring the torques (angular pulls) of bars seen in our infrared images. We know from near-infrared surveys that about two-thirds of spiral galaxies display some level of bar structure in their inner regions.

de Vaucouleurs was probably not as concerned as we are about the dynamical importance of bars. As a morphologist, with a remarkably trained eye, he meticulously studied photographs (many secured with the Reynolds telescope discussed earlier), and he could see which features which varied from one galaxy to another; such as the presence of inner bars as well as the grandeur of inner and outer rings. Some astronomers thought that this level of detail in classifying galaxies was not warranted, but we now know otherwise: our understanding truly depends on knowing the details.

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