"The learned say that your lights will one day be no more," said the firefly to the stars.
The stars made no answer.
Sweep on through glittering star fields and long for endless night! More nebulae, more stars. Here a bright and beautiful star overpowering in its brilliancy, and there close to it a tiny point of light seen with the greatest difficulty, a large star and its companion. How plentiful the stars now appear. Each sweep increases their number. The field is sprinkled with them, and now we suddenly sweep into myriads and swarms of glittering, sparkling points of brilliancy - we have entered the Milky Way. We are in the midst of millions and millions of suns - we are in the jewel house of the Maker, and our soul mounts up, up to that wonderful Creator, and we adore the hand that scattered the jewels of heaven so lavishly in this one vast region. No pen can describe the wonderful scene that the swinging tube reveals as it sweeps among that vast array of suns.
Edward Emerson Barnard in The Nashville Artisan (1883).
The quotation is extracted from The Immortal Fire Within by W. Sheehan
(Cambridge University Press).
ir Isaac Newton and Sir William Herschel believed in the transparency of interstellar space. In the period spanning 1840 and 1930, astronomers who believed otherwise (such as Angelo Secchi, Friedrich Struve, Hugo Seeliger, Max Wolf and Heber Curtis) were in the minority, despite the grand photographic efforts of Edward Barnard. Virginia Trimble recalls: "Only with Detectives in Chile Trumpler's 1930 publication of the relationship between angular diameter 19
and apparent brightness of star clusters did the community accept that all of interstellar space was capable of swallowing starlight ... not until 1952, at the Rome International Astronomical Union, did most astronomers catch on."
Light obeys an inverse square law: observe the headlight of a car from a fixed distance and then let the car travel four times that distance: the intensity of the headlight's beam will have dropped by a factor of sixteen (the square of four is sixteen). If one assumes that open clusters of stars may be categorized into certain subclasses with fixed diameters (in light years, for example), Trumpler showed that the distances to the further clusters were systematically too large. The reason is this: foreground dust between a cluster of stars and the observer fools the astronomer in thinking that the cluster is dimmer (and therefore, further) than it actually is. Distances derived from the inverse square law for clusters obscured by foreground clouds of dust are too large. A ship sailing in thick fog may actually be relatively nearby, but it may appear to be much farther away on account of the dimness of the lights on deck as seen through the fog!
Once astronomers had accepted a dusty Milky Way, the question beckoned as to the amounts of dust in spiral galaxies external to the Milky Way. Prior to 1994, the prevailing opinion amongst astronomers was that our Milky Way was particularly rich in cosmic dust - in technical terms, its ratio of "dust-to-gas" mass was believed to be ten times higher than that for other spiral galaxies. We shall see the raison d'etre behind this argument, presently.
How prevalent is cosmic dust in our Universe? It was time for detectives to seek for clues. We shall never forget those nights spent on a mountain in Chile's Atacama Desert in the early 1990s. It is one of the supremely dry places on Earth, and there are said to be patches where the rain falls but once in a century. If ever a place deserves to be called "dry as dust," it is here.
The Atacama desert is an ideal place from which to study dust - of the cosmic, rather than terrestrial, variety (Figures 8-16). At La Silla (the Saddle), the European Southern Observatory (whose headquarters are in Garching, Germany) has stationed some of its most sophisticated instruments, due to the clearness and dryness of the atmosphere. Our team had set our hearts and minds on testing new methods we had devised for unveiling cold (minus 210 degrees Centigrade) and very cold (minus 250 degrees Centigrade) cosmic dust grains in galaxies beyond the Milky Way!
Before we started our work, we stepped out of the observatory control-room into the crisp, clear evening air, climbed onto the catwalk circling the great dome, and gazed upon the Milky Way, stretching majestically across the sky. We were indeed "in the midst of millions and millions of suns" to quote Edward Barnard.
Viewing the blazing Milky Way above our heads, spawning countless myriads of stars, we returned to the telescope control room with a renewed determination and enthusiasm to penetrate Cosmic Shrouds of the Night in spiral galaxies beyond our Galaxy (Figure 17), and to determine their masses.
On the face of it, ours might have seemed like a totally impossible mission. After all, how could one hope to "see" anything within these distant cosmic mists? How could one actually "view" or "penetrate" these cold cosmic dust shrouds, where some grains may have temperatures only a few degrees above absolute zero - and, in galaxies millions of light years away? And why should we even bother?
At La Silla, the dusty Milky Way, the starry river of the night, appeared to curve, like a majestic archway, from one horizon to the other. Against it were multitudes of cosmic shrouds - dark, mysterious patches -which astronomers have once believed to be devoid of stars. In a paper published in 1946 authored by Jan Oort and Henk van de Hulst, the title was particularly lucid: "Gas and smoke in interstellar space." Lindblad, Oort, van de Hulst, Schalen and other astronomers believed that the apparent vacuities in our Milky Way were clouds of cosmic "smoke" - a smoke whose properties are very effective at blotting out stars lurking behind masks of cosmic dust.
From La Silla, how would our team seek to penetrate these Shrouds of the Night in other galaxies; Shrouds which may span diameters of one hundred thousand light years - sometimes more - and yet appear so small on account of distance?
Detectives in Chile 23
Very fortunately, we had access to special "near-infrared" eyes. If an airport is encased in a fog, it does not imply that the airport is not there: it merely implies that one requires a special set of eyes - infrared eyes - to penetrate the fog! Likewise, the presence of homes in a valley cannot be ascertained if the valley is enshrouded in mist (Figure 18). Herein lies a fundamental question:
What does the unmasked view of our cosmos reveal?
What structures of stars lie behind these dust masks of the night?
"Have you ever, looking up, seen a cloud like to a Centaur, a Part, or a Wolf, or a Bull?" asked the Greek poet and satirist Aristophanes (c. 448-380 BC).
Grains of cosmic dust may be studied in two completely different ways: The one way is by means of the feeble radiation which these tiny grains emit. Special telescopes have been erected which are sensitive to such radiation, called submillimeter telescopes. Submillimeter astronomy is that branch of astronomy based on radiation from space whose wavelengths range from a few hundred microns to a millimeter (one micron is one thousandth of a millimeter). The giant James Clerk Maxwell Telescope on the Big Island of Hawaii, with its impressive 15 meter diameter antenna composed of 276 individually adjustable panels, is a submillimeter telescope (the photons which its receiver detects belong to the submillimeter region of our electromagnetic spectrum). It began observing the skies in 1987.
Dust pervasive as smoke (Figure 19), a fog or an aerosol ... why could such telescopes not have provided definitive answers prior to our investigative work in 1994?
Studies with telescopes such as the James Clerk Maxwell Telescope and the California Institute of Technology (Caltech) Submillimeter Observatory, with its 10.4 meter radio dish, had brought the issue of cold dust in other galaxies to the fore, but with diametrically opposed possibilities. Some research papers published between 1989 and 1993 claimed that very cold dust in galaxies may simply not be present. The reasons for this controversy are manifold, Shrouds of the Night but a principal factor is that it is possible to find markedly different distributions of the tem-
32 peratures of dust grains from an identical set of submillimeter observations. A key unknown
is the efficiency with which dust grains emit radiation at these long wavelengths. Also, dust grains as cold as minus 260 degrees Centigrade simply remain undetected at submillimeter wavelengths.
Large scale, diffuse "cirrus clouds" had been strikingly detected in our Milky Way Galaxy by the space-borne Infrared Astronomical Satellite, launched in January 1983. This satellite was a joint project of the United States, the United Kingdom and the Netherlands. It was equipped with a special 0.57 meter infrared telescope, and during its ten month mission produced a survey spanning the entire sky, at wavelengths of 12, 25, 60 and 100 microns. The satellite was sensitive to the emission from dust grains whose temperatures ranged from about minus 240 degrees Centigrade to 30 degrees Centigrade (the Infrared Astronomical Satellite beautifully observed the zodiacal light between minus 25 degrees and 30 degrees Centigrade; the zodiacal light results from sunlight scattered by dust in the plane of the planets, and is shown in a drawing in Chapter 5 - see Figure 28).
Sky maps of the Milky Way by the Infrared Astronomy Satellite at 100 microns magnificently revealed the presence of dust grains in both "cirrus clouds" and in giant clouds of molecules, in our Galaxy. These cirrus clouds in our Galaxy bear a striking resemblance to terrestrial cirrus clouds; hence their name. However, the Infrared Astronomy Satellite was only sensitive to radiation from hot grains as well as warm grains - not the cold grains of dust which lurk in molecular clouds in which stars are born - grains whose temperatures may be a mere 20 degrees above absolute zero (in other words, minus 253 degrees Centigrade).
Such grains only add only small contributions to the low-frequency end of the emission spectrum (even though these cold grains may constitute by far the bulk of the proportion of the dust mass). When astronomers secured infrared images of galaxies outside of our Milky Way with the Infrared Astronomy Satellite, the emission was masked or dominated by the higher temperature (technically "brighter") dust grains. One may think of a lighthouse analogy; as a ship approaches a coastal city at night, the view from the deck would be dominated not by the distant city lights, but by the emission of blazing rays of light from a lighthouse.
The Infrared Astronomy Satellite therefore did not detect cold cosmic dust grains in spiral galaxies external to our Milky Way - with one notable exception: the Milky Way itself.
We live in this Galaxy, and therefore we can secure high spatial resolution images of it! With the four bands of Infrared Astronomy Satellite, we can successfully separate high and low temperature regions in such maps of the Milky Way. The Milky Way is so close that we can demarcate areas where the feeble emission from low temperature dust grains is not swamped or masked by regions containing higher temperature cosmic dust grains. On the other hand, for more distant spiral galaxies, astronomers can measure only the total brightness in the four Infrared Astronomy Satellite bands, and therefore the radiation from cold dust is completely swamped by the warm dust.
The prevalent viewpoint in the 1980s was that although cosmic dust certainly was seen in the photographs of Keeler, Curtis and others, the ratio in mass of dust to gas in spiral galaxies (as determined from observations with the Infrared Astronomy Satellite) was about ten times smaller than that determined for our Milky Way Galaxy. Was our Milky Way simply very unusual in having copious amounts of cosmic dust? Or could it be that the Infrared Astronomy Satellite had actually missed ninety percent of the dust content in spiral galaxies external to our Milky Way, as a result of masking by the hotter dust grains?
Let us for a moment think of a soldering iron. The iron will not only warmly glow in the infrared, but it will also block out optical light. Herein lies the second method of detecting cosmic dust - a dust grain will block out background starlight, independent of its temperature. This is the method we invoked at La Silla.
One of the galaxies we observed is known as NGC 2997. An optical image (Figure 20) shows a plethora of bright young stars and a magnificent set of dust lanes. In the infrared (Figure 21), the obscuring effects of dust are enormously reduced. If one subtracts a "dust-free" near-infrared image of a spiral galaxy from an optical image, the result is a map of the distribution of dust in the galaxy without any starlight (see Figure 22). Such maps reveal the presence of dust grains of all temperatures, including the very cold ones which we could not detect directly from their emission. Moreover, it is possible to actually quantify the masses of our Shrouds of the Night and herein lay the crucial clues ... clues Shrouds of the Night which would definitively unravel whether it was only our Milky Way which contained large
36 amounts of cosmic dust.
Detectives in Chile
Figure 20  S7
To our astonishment, we found that ninety percent of the dust mass in spiral galaxies external to the Milky Way had been missed by the Infrared Astronomy Satellite: astronomers had vastly underestimated how thick the galactic fog was through which they had been observing the Universe. At this juncture, we owe an enormous debt to our collaborators from Germany, Italy, Spain, the Netherlands, the United Kingdom and the United States of America, who worked so closely with us prior to our infrared and cold dust publications in 1994.
What we had discovered in Chile was that dust grains do not simply lie along spiral arms of galaxies themselves; rather, vast curtains of dust also lie between spiral arms (technically known as "inter-arm" dust). Shrouds of the Night betray a breathtaking structure of their own (Figure 22). True, there certainly were clearly defined dust lanes along the spiral arms, but these dust lanes alone do not fully describe the complexity of a cosmic dust shroud. We are reminded of the words of the English novelist Ellen Thorneycroft Fowler (1860-1929):
Though outwardly a gloomy shroud, The inner half ofevery cloud Is bright and shining: I therefore turn my clouds about And always wear them inside out To show the lining.
Our discoveries on the prevalence of dusty Shrouds of the Night in spiral galaxies beyond our Milky Way were published during the historic year 1994 - the year that saw the emergence of the New South Africa under Nelson Mandela. Mayo Greenberg, one of the world's foremost laboratory astrophysicists who we will meet in the next chapter, stood up at a meeting at Cardiff (Wales) and exclaimed his excitement at our team's observational confirmation of his 23-year-old prediction that cold cosmic dust was a reality, and that it constituted most Detectives in Chile of the dust mass in spiral galaxies. 39
I've waited 23 years to "see" cold dust. You did it. Thanks! he told our team at the Cardiff meeting.
Nobel-laureate Indian poet Rabindranath Tagore writes in his book entitled Stray Birds:
The dust receives insult and in return offers her flowers.
Without dust, no flowers: do not planets themselves come from the material produced in stars? Planets, icy comets, and life itself: at the kernel, stardust. "Remember, man, that thou art dust. And into dust thou shalt return" writes the author of Genesis.
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