The Three Masks of our Milky Way Galaxy

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In this book, we have, at length discussed dusty Shrouds of the Night - apart from their dust content, these masks also contain fiery young blue stars, which obscure our view of what spiral galaxies actually appear like, beneath their Shrouds. However, we encounter an even more dramatic mask, which we term the mass mask. When galaxies are imaged behind their dust masks, they reveal the impressive backbones of spiral galaxies, as we have repeatedly seen - but it should always be remembered that the actual mass of stars in the disk of a spiral galaxy such as our Milky Way or the Andromeda Spiral is only a small fraction of its total mass. In the inner parts of a spiral galaxy, the stars do indeed contribute most of the mass, but as astronomers probe their disks further and further out in radius, another component of the mass dominates. This is the "missing mass" or enigmatic dark matter (discussed in greater detail later on in this chapter). Eyes to the Future

The first recognition of dark matter in spiral galaxies (as detected from the manner in which the galaxies rotate) was made in 1970. There is a fascinating historical interlude here yet again, this time not from the pen of Edwin Hubble or John Reynolds but from the pen of Princeton astrophysicist, the late John Bahcall.

David shares his thoughts on these letters:

During my visit at the Mt. Stromlo Observatory to complete this book, I came upon a fascinating letter written by John Bahcall, addressed to the astronomer Bart Bok, then in Arizona. The letter is dated December 23, 1982. In it, Bahcall cites a paper published in 1970 in which we find these words:

... there must be undetected matter beyond the optical extent of NGC 300 ... For NGC 300 and M33, the 21-cm data give turnover points near the photometric outer edges of these systems ... there must be in these galaxies additional matter which is undetected ... Its mass must be as least as large as the mass of the detected galaxy .

In another letter, Bahcall comments that this 1970 paper "is the earliest explicit recognition of the problem that I know about from rotation curves." Who was the author of this 1970 paper? I must confess that at first, I was not sure who Bahcall was referring to in his letter to Bok, but upon reading carefully, the penny dropped: John Bahcall was referring to research conducted in 1970 by an astronomer who has an office only two doors away from my office at Mt. Stromlo - coauthor of this book, Ken! Detective trails sometimes lead to the living and not only to the dead ...

While the constituents of dark matter still remain a great mystery (as discussed in the next section below), it constitutes about ninety to ninety-five percent of the total mass of a typical spiral galaxy, so that drawings (Figure 157) and photographs (Figure 158) of the stars themselves give astronomers a very restricted view of the way in which matter in a spiral galaxy is Shrouds of the Night actually distributed. The stars merely form a tiny fraction of the total mass; the disks of spiral

276 galaxies are immersed in extensive envelopes of dark, unseen matter.

Lucus Augusti

At Mt. Stromlo, we still have access to a "light table," a device very rarely used today. It has an illuminated piece of glass which lights up when switching on a diffuse light source below. On the light table itself is placed a negative, much like a radiologist today might examine an x-ray, by placing it on a light table with diffuse light illumination. Before the digital era, astronomers would sit for hours at such light tables, carefully scrutinizing their glass negatives exposed at a telescope.

It is with awe and wonder that we gazed upon a negative of one of the most massive galaxies known - the galaxy NGC 5084 (a positive version appears in Figure 158). The galaxy has been carefully studied by one of us (Ken) and associates several years ago, who determined a mass of possibly 10 times the mass of our Milky Way Galaxy. NGC 5084 is seen "edge-on" -as if viewing a thin plate from one side. How intriguing to almost walk back decades in time, by actually examining these negatives of NGC 5084 visually on a light table, and to realize that while there must be a gigantic halo of dark matter in which the galaxy NGC 5084 is embedded, there is not the slightest visual hint of its existence. All looks so perfectly normal around the environs of NGC 5084; there is no obvious dimming of background galaxies in any way at all. Peeking through the microscope at the negative of NGC 5084, the environs of NGC 5084 look so quiescent. The constituents of what dark matter is actually made of, forms one of the most major discoveries awaiting astronomers. Dark matter - by all accounts a vast constituent of our cosmos - matter which has mass, but which emits absolutely no light whatever. The eye sees nothing amiss: the visual presence on the surroundings of galaxies such as NGC 5084 (Figure 158) and scores of other spiral galaxies with their dark matter halos, is zero.

A thought for the future. Imagine if astronomers could construct a special telescope (which obviously no-one has yet invented) which could directly image how matter in a spiral galaxy is distributed. Such a telescope would image dark matter as well as luminous matter. Galaxies such as NGC 5084 would look supremely different through such mass-detecting telescopes, for their dark matter halos would be imaged too!

Dark matter itself is a mask: although it does not actually obscure any stars in galaxies, it is a mask in the sense that its presence means that astronomers are afforded a misleading picture as to how the total mass in spiral galaxies is distributed.

Johann Wolfgang von Goethe elucidated: "Wo viel Licht is, ist starker Schatten" or translated, "Where there is much light, the shadows are deepest."

The mysteries in our starry skies of dark matter ... the "shadows" of dark matter ... run very deep.

There are thus several levels of masking in our majestic cosmos. If our interest is to study the formation of young bright stars - those fiery energetic furnaces delineating the arms of so many spiral galaxies - then one should examine optical images, such as those secured photographically or digitally. However such young stars themselves mask the bulk of the older stars, which we would therefore miss!

If astronomers wish to study the underlying population of old stars in a spiral galaxy, they need to image the galaxy in the infrared, penetrating the dusty Shrouds of the Night. In this chapter, we have alluded to yet a third mask - a mask containing nonluminous, dark matter. To see most of the mass of a spiral galaxy necessitates a new mass telescope.

Enter the fourth mask, which astronomers may term the dynamical mask of a galaxy.

One of our most important diagnostics in trying to understand spiral galaxies is to measure how the stars actually move. For example, most of the stars in the disk of a spiral galaxy move in almost circular orbits around the galaxy center. The implication is that the gas from which those stars have formed, has already settled into the disk before most of the formation of stars takes place. Yet implicitly intertwined is a story forever masked ... forever lost: it is the story of what happens during the settling process itself. What does the birthing process of spiral galaxies actually reveal? After all, a galaxy becomes quite different from what it was before. Has it not changed from a rather chaotic system into a well-ordered disk? We are reminded of the vibrant youth of the boy in William Wordsworth's "Ode: Intimations of Immortality from Recollections of Early Childhood (V)"

Our birth is but a sleep and a forgetting; The Soul that rises with us, our life's Star, Hath had elsewhere its setting,

And cometh from afar; Not in entire forgetfulness, And not in utter nakedness, But trailing clouds of glory, do we come From God, who is our home. Heaven lies about us in our infancy. Shades of the prison-house begin to close Upon the growing Boy,

But He beholds the light, and whence it flows, He sees it in his joy;

The Youth, who daily farther from the east Must travel, still is Nature's Priest, And by the vision splendid Is on his way attended; At length the man perceives it die away, And fade into the light of common day.

We may see a young boy or girl now, but each one has emerged through a birthing process, the exact details of which are veiled in the mists of time.

A crucial point therefore is that a lot of information is lost during the settling (or birthing) process of a galaxy. By examining stars in the disk of a galaxy now, it is almost impossible to learn much about the properties of the galaxy before the disk formed. One can think of this loss of information as the dynamical mask.

A spiral galaxy may present a rather well-ordered dynamical face to us now, with the stars moving mostly in near-circular orbits. Yet behind this mask lies a history untold ... the chaos which invariably initially reigned supreme before the disk settled. It is as if a cosmic iron has smoothed away the primordial history ofevery spiral galaxy; their birthing processes rapidly are washed away, as with a young child running on a beach at the sea-shore; the child so enjoys leaving footprints in the sand - but these footprints are quickly washed away by the crashing waves of our tumultuous oceans. Those early footprints, almost forgotten, as in a sleep, in Eyes to the Future the words of Wordsworth. 281

Most of the fossil history of spiral galaxies has forever been lost in the processes which led to the formation of galactic disks. If one wishes to recover information about the galaxy in its early history, one may have to look at chemical fossils or signatures rather than dynamical signatures. The way in which stars move today obscure most of the dynamical fossil information. In this sense, this Dynamical Shroud of the Night is different from the dust and mass masks which we have been discussing. The dust and mass masks obscure the underlying information but the information about stars and mass is inherently still there. With a dynamical mask, the information is gone.

The words of Henry Longfellow ring in our ears:

I heard the trailing garments of the Night Sweep through her marble halls.

("Hymn to the Night")

After the sweeping, all is apparently clean. It is akin to an archaeologist who may be studying some ancient manuscripts from a long-gone civilization by candlelight; accidentally the candle sets the manuscripts alight and all information is lost. The archaeologist is forced to seek for clues elsewhere pertaining to the very early days of such a civilization!

One of the great challenges of the new millennium is to learn how to penetrate these various masks to unravel the real structure of spiral galaxies, as well as to try and discover how such spiral pinwheels were actually birthed.

Astronomers use a novel archaeological approach to probe the primordial history of our Milky Way. The goal is to discover what happened when the Milky Way itself (Figures 159 and 160) was being assembled ... the birthing process of our Galaxy. We seek fossil remnants - footprints in the sand, as it were - of star forming events which occurred very long ago. These fossils speak with a more feeble voice, when contrasted to our geological fossils of long-lost plants and animals.

The astronomical fossils are often groups of stars which share common motions in space and with common chemical properties - properties which have been preserved since these stars formed. Unfortunately for us, gas clouds and spiraling waves of stars seek to change or

to perturb the motions of stars in their orbits. For some stars, their stellar orbits have carried them around the Milky Way more that fifty times; their orbits will most certainly have changed during such aeons of time. One therefore cannot always rely on common stellar motions as evidence of astronomical fossils.

For the future, we believe in chemical tagging, much like human beings may be genetically tagged by their DNA. The goal: a chemical identification of stellar fossils in our Galaxy. How do astronomy detectives set upon finding these primordial fossils, with their feeble voices? The technique relies on the patterns of chemical abundances of heavy elements, such as calcium, silicon, iron and barium - the amounts of which do not change as the stars age. Although this is technically more challenging, astronomers can rely on stars maintaining their distinguishing chemical properties as they march forward in their galactic orbits. Chemical archaeology will allow astronomers to penetrate the dynamical mask, and allow us to study a primordial cosmic landscape of our River of the Night - the Milky Way. Such a landscape will not have been created by paint, but by the distinguishing footprints or signatures of the chemical elements of the stars.

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