This report documents a study of Milky Way dust filaments that appear in photographs of external galaxies. The technique used in this investigation is the use of software photograph processing tools called effects filters. The software used here is the On One Photo Perfect Effects 9. This allows the adjustment of digital photos to enhance aspects of the photo to permit the investigation of foreground dust in galaxy photographs. The thesis of this study is that the classification and interpretation of galaxy photographs is significantly influenced by foreground dust filaments that appear in projection against the bright background of external galaxies.
The term Milky Way refers to our own galaxy, where the earth is located. It consists of visible stars, and nebula. Nebula refers to what amateur astronomers call deep sky objects. There are various kinds: emission nebula, globular clusters, distant galactic clusters, planetary nebula, and other luminous objects. The term nebula, means cloud, since in the early days of telescopes they appeared as fuzzy faint luminous clouds. The term dark nebula refers to non-luminous cloud-like obscuring formations supposed to consist of gas and dust. Here we will discuss a different type of dark nebula called interstellar dust filaments. They are distinguished from the usual dark nebula in that while they cause an extinction or darkening of objects viewed behind the filaments, they do not absorb strongly enough to totally obscure the background. Thus they are not observed in long exposures using sensitive detectors. Here the main objective will be to reveal the presence of these dark filaments against the luminous backgrounds provided by the external galaxies which are located beyond the Milky Way.
Regarding photographic credits. The writer is an amateur astronomer and has experience taking astronomical photos, but he is no expert at that and has relied upon photos published on the Internet as his sources for research material. The reason for the work discussed here is purely to do research into the nature of the dark filaments that obscure external galaxies. Since it is traditional to present galaxy photographs that are mostly overexposed in order to show the faint outskirts of the distant galaxies, it was necessary to use the Perfect Effects software to show the dark filaments that are projected upon the background galaxies. The writer hopes that this study will encourage other photographers to use the techniques discussed here to present photos that are not over exposed so that the subtle details of the foreground dust can be examined.
Sources Of Material
This study will refer to and use a number of sources of photographs. The primary source for photos being presented is the Internet. However, there will be references to published books. Whenever possible these will refer to versions published electronically. The Hubble Atlas of Galaxies by Alan Sandage is available digitally. Older photographs are much more difficult to obtain from the internet. One of the older sources of photographs that will be relied upon is: A Selection Of Photographs of Stars, Star-Clusters and Nebula, by Isaac Roberts. Catalog of the Universe by Murdin, Allen, and Malin contains a number of older photographs from the 1970s. It is available in digital form. Atlas of Deep-Sky Splendors by Hans Vehrenberg, is a good reference source for studying photographs and does so by showing all of them at the same scale. The Realm Of The Nebula, by E.P. Hubble has a lot of good quality photographic plates that will be referenced. Many of them appear in other places since they were widely used in astronomy textbooks over the years.
The Andromeda Galaxy, Messier 31, or NGC 224
The Andromeda Galaxy is the largest and brightest nearby spiral galaxy and it is a favorite target for amateur astronomers. It is probably the most photographed of all of the galaxies because it is bright and large. There is a very good photograph in the I. Roberts catalog taken in 1895 a 90 minute exposure with a 20-inch reflector. There is a striking difference in brightness of the two different sides of the galaxy in this photograph, that is not very evident in modern photographs, although the effect is clearly evident in the Atlas Of Deep-Sky Splendors photo of M31. If you look closely you will see that the south west end of the galaxy is darker and is not as big. (Lower left in the photo to the right.) That means if you measure the distance to the outermost edge, the north east edge extends farther than the south west edge. That implies a lot of foreground dust that is obscuring the south west side of M31. The Roberts photo also shows evidence of the presence of dark filaments that cover the surface of the galaxy. Unfortunately, they are difficult to see. The objective will be to find and process photos of M31 that can be adjusted using effects software to make these foreground dust filaments more readily visible to the untrained eye.
The method used to process the photo of The Andromeda Galaxy was to make the surface brightness of the image a constant. In other words, the bright nuclear bulge is darkened, and the outer spiral arms are made brighter. The adjustment brush was used, and successive operations of darkening the central regions and brightening the outer regions were performed. Then there was an increase of the contrast over the entire galaxy to bring out the faint dark filaments. The result shows that there is a system of at least four and possibly more parallel dust filaments crossing the lower left of the photograph going diagonally towards the upper right. They do not follow the spiral pattern of the underlying galaxy, and so are certainly foreground Milky Way dark filaments projected in silhouette against the bright background of The Andromeda Galaxy.
An examination of the lower outer edge of the galaxy shows the presence of dark patches that do not follow the spiral pattern, since they seem to cut across the outer spiral arms rather than follow the spiral pattern. These are certainly foreground dark filaments. They can also be seen in the outer spiral arm towards the top of the photo, where they appear as bands that cross the outer arms. The main point is that when we look at the dust that we see, there is a lot of it that does not follow the spiral pattern and so it must be foreground dust silhouetted against the bright galaxy background.
The third photo is an infrared photo designed to show the hot dust in the Andromeda Galaxy. The dust is supposed to be shown by the red or orange areas. There is no apparent spiral pattern, but as discussed before, the left hand side is darker than the right and if you look closely you can see the dark filaments crossing from the left to right in the left side of M31. In particular notice the dark patches that obscure the outer ring in the lower part of the photo. The conclusion that results from examining this photograph is that there can still be seen the presence of foreground dust that obscures the background, but because the photo is in infrared light the effect is much less evident than in the previous photos. Finally, notice that there is no clearly apparent spiral pattern. The hot dust forms rings, and not spiral arms. The only spiral form is near the nucleus.
The Triangulum Galaxy, Messier 33 or NGC 598
The Triangulum Galaxy is also a spiral galaxy that is a popular photographic subject, although it is much further away, and so appears much smaller than M31. In the I. Roberts photograph, there is also a difference in brightness between the two sides since the northern side is obviously obscured by a large cloud of dark matter, while the southern is not. However, on the southern side, lower right in the Robertson photo, there are a bunch of dark patches sprinkled about, that are clearly created by the overlapping of dust filaments running in different directions. The dark cloud on the northern side is present in the Hubble Atlas photo, but the patches are not evident in this photo. In the photographs usually seen of M33, it appears as a Type Sc galaxy, with “S” shaped spiral arms. However, this writer thinks that this classification is mistaken because of the obscuring clouds of foreground Milky Way dust. The thesis is that this is actually a disk shaped galaxy of Type late Sb similar to M31.
The photo above right of Messier 33, has been processed using photo effects software in a manner similar to M31. The idea was to make the surface brightness uniform and to increase the contrast. In this photo the northern side is on the left and the photo clearly shows there are dark filaments crossing this side of M33 that cut across the spiral arms, and so they must be foreground dust silhouetted in projection. These filaments rise up from the bottom of the photo and continue up towards the top. Others cross diagonally from the lower left towards the upper right. They overlap and crisscross and so create the dark patches that obscures the northern side of M33.
The next photo is an ultraviolet photo of M33, and in it there are obvious dark filaments that cut across the nuclear region from top to bottom. There is a lot of dust silhouetted in this photo and, by looking closely, there is a lot of dark matter that does not follow the underlying spiral pattern. So it must be caused by foreground dust. Other dark filaments crossing the face of M33 from left to right are not as obvious, but can be seen with careful study. When the crisscrossing dark filaments intersect, they create dark patches. These seem to be isolated dust clouds in longer exposure photos but become apparent only because the absorption effect is multiplied when the filaments overlap. The result in the photo is to create dark diagonal bands that cross the vicinity of the nuclear region.
Bode’s Galaxy, Messier 81, NGC 3031
Bode’s Galaxy is another very popular photographic subject. Like M31, M81 is brighter on one side than the other, but the effect is less dramatic. There is one important difference, which is that the cause of this difference in brightness has been noticed by a professional astronomer. In the Hubble Atlas we are told that: “There is an even more intricate pattern of straight dust lanes , which has no connection with the spiral pattern, on the north-preceding end of the major axis. The parallel streaks can be traced across the central lens and across two branches of the brighter spiral arm on the north-preceding side.” This quotation is unique in that it is almost the only record we have of a published report of foreground dust filaments seen in projection against a galaxy. While there are other places where there are comments that suggest foreground filaments, this is the only one that clearly states that the filaments seen do not participate in the spiral pattern. There is however, no conclusion that this is an example of foreground Milky Way dust seen in projection, and it seems to have not been followed up in any other study of galaxies.
Close examination shows that there is a more complex situation than noticed by Alan Sandage in The Hubble Atlas. There are in fact at least two systems of filaments that cross the north-preceding side of M81. The more prominent one is that noted by Sandage, and it crosses in a north-east to south-west direction diagonally in the Hubble Atlas. There is a less prominent system overlapping the first one and it crosses diagonally from south-east to north-west. It is more readily visible on the northern side to the east of the central bulge of M81. In some photographs, particularly those by amateur astronomers using small telescopes, the two systems of dark filaments discussed above, cause a significant dimming of the galaxy.
The Whirlpool Galaxy, Messier 51, NGC 5194-5195
This is the prototype spiral nebula. It is a double galaxy, and the dominant galaxy is a well defined whirlpool spiral. The major spiral galaxy, NGC 5194 demonstrates in a very compelling way evidence for the existence of foreground dark filaments. In the photograph to the right, two dark foreground filaments can be seen silhouetted crisscrossing and forming an X shape directly on top of the nucleus of M51. By looking closely, other less distinct filaments can be seen in the region around the bright over exposed nuclear region. Professional astronomers claim the following: “Pictures of M51 taken with the [Hubble] Planetary Camera show a striking, dark “X” silhouetted across the nucleus. The “X” is due to absorption by dust and marks the exact position of the nuclear black hole…The second bar of the “X” is both interesting and puzzling. The dust in this linear feature could be a second disk seen edge on, or possibly rotating gas and dust in M51 interacting with the jets and ionization cones. The safest interpretation is that the slash is a caution sign, showing that we do not yet fully understand what is happening in the center of M51,” This seems like an absurd thing to claim. There is no evidence to suppose that they are observing a black hole, and that is just wishful thinking. On the other hand, since astronomers can not imagine that foreground dust filaments could exist, they simply neglected to investigate that possibility.
In the Hubble Atlas, Alan Sandage writes: “The dust arms are very highly branched. Thin filaments break away from the main dust path and cross the luminous arms almost at right angles. Multiple secondary dust lanes exist throughout the central lens. Individual dust lanes of the secondary pattern cannot be traced, but rather separate segments exist, which when viewed with other segments, form a rough spiral pattern.” This means that dark foreground filaments are projected or silhouetted onto the galaxy, and they do not follow the spiral pattern, but cut across it and so they are clearly not part of the underlying galaxy. To understand what is being described, the reader should understand that there are dark filaments in the foreground that cut across the face of M51 and this combined with the underlying spiral dust pattern creates the confusing appearance described by Sandage. He does not conceive that there is any foreground dust and so thinks that all that he sees is part of the underlying galaxy structure. We can make sense of the X pattern discovered by the Hubble Telescope and the confusing pattern of dust in M51 simply by understanding that there are foreground dark filaments silhouetted onto the galaxy.
The Sculptor And Fornax Dwarf Galaxies
Up to this point we considered bright and large galaxies. The two galaxies being considered now are both termed dwarf galaxies. They have a low surface brightness, and are nearby. They do not have appreciable internal gas and dust. The website Glittering Lights has an excellent picture of both the Sculptor and Fornax dwarf galaxies. The nice aspect of these photos is that they do not require any additional processing to bring out the dark filaments, because the background is not over exposed, as in other photos. A close look at the Sculptor Dwarf photo shows a straight filament crossing the middle of the galaxy horizontally. It is particularly evident on the right side of the galaxy. There are a number of other filaments that can be seen in this photo. Look for the horizontal filament first and once you have learned to recognize it, look for the other ones. A lot of them are visible in this photo. Some are curved, others cross diagonally, and others cut across the outskirts. Some produce tiny dark patches. The Fornax Dwarf galaxy also shows the presence of numerous dark filaments. Unfortunately they are not as easy to see as in the Sculptor photo. The reader is urged to study the Sculptor photo first and after he has learned to see the filaments in that galaxy, to shift to the Fornax photo and study the filaments there.
Here are a couple of suggestions to help learn to see the filaments. First, dim the monitor. The best way is to tilt the screen. Then step back from the monitor about six feet or so. Don’t try to focus on a particular part of the photo, but let your gaze relax. Once you see a filament, try to follow it as far as possible in the photo.
Once you have trained your eye to see the dark filaments in the Sculptor and Fornax Galaxies, you may want to try detecting the foreground dark filaments in the photograph of the Small Magellanic Cloud at the same website.
The Sunflower Galaxy, Messier 63, NGC 5055
This is a bright well developed spiral galaxy with a lot of dark nebula that appear to be internal to the galaxy. In long exposure photographs taken with the large telescopes, the dust appears to be internal to the galaxy and follow the spiral structure. This is exactly the impression that is obtained by examination of this Hubble Telescope image of M63. Contrary to the impression gained from the Hubble photograph, images obtained by amateur astronomers using much smaller telescopes provide evidence for a completely different conclusion. They show that much of the dust that produces the dark nebula is external to the galaxy and is therefore foreground dark nebula in the Milky Way Galaxy. As noticed previously, the dark nebula are straight, long, and thin filaments. A good example of a photograph that gives the impression of foreground dark filaments is this photo by Andrea Tamanti. The photo to the right shows this photo after moderate adjustment using On One Software.
The photo to the right shows some features already noticed that indicate that indicate the presence of foreground Milky Way dark nebula. The first point is that the brightness is asymmetrical. The right side in the photo is darker than the left. Looking at the right side of the galaxy, toward the outer edge there is a distinct vertical dark band that is straight and does not follow the spiral pattern. Closer in, there is a dark filament that cuts diagonally across the upper right hand side of the galaxy, passing from the upper left to the lower right. Where this dark filament cuts across the face of M63, you can see that it enhances a number of dark short filaments that are clearly cutting across the face of M63 horizontally from left to right. In the lower part, below the nucleus, there are a number of straight dark segments that do not follow the spiral pattern. On the right hand side of the center as the bright central region is approach, there is clearly see a straight vertical dark filament that does not seem to follow the spiral pattern as well. Finally, below the galaxy on the left hand side there are also a number of straight dark filaments that do not follow the spiral pattern.
Local Group Galaxies
This section examines the effects of Milky Way foreground dark filaments on the appearance of small local group galaxies. A nice aspect of considering these galaxies is that they show the presence of foreground dark filaments without additional processing. The reader is advised to learn how to identify foreground obscuration in these photographs. They provide very convincing evidence that foreground dust is significant in effecting the appearance of galaxies. In a previous section, the effect of foreground dark filaments was examined with respect to the Fornax and Sculptor Dwrarf galaxies. The reader is referred to the photographs at this page. What they show is the presence of foreground dark filaments that completely cover the galaxies. One galaxy in particular shows this very well. That is the galaxy referred to as WLM. This is an unusual designation as it is not included in the traditional NGC or IC designations. WLM refers to Wolt-Lundmark-Melotte. The photo of this galaxy is here. The reader should click on the image to see a magnified image. The main conclusion to be drawn from examining this is that the obscuration is clearly present and it is in the form of filaments and not a cloud or clouds or a haze. The filaments can be clearly seen as long dark streaks crossing vertically from top to bottom, and crossing diagonally from lower left to upper right. When they intersect they create dark patches. There is also the characteristic texturing pattern that is created by the filaments. That is the dark filaments create the same impression as if a texture pattern were applied to the photograph as is done in photo effects software. Notice that the filaments are more easily seen near the outskirts of the galaxy image. There is however, a very clear and distinct filament that crosses from the top to the bottom. Look for it in the upper part of the galaxy image. It is dark, long and thin.
Once the reader has become familiar with the appearance of obscuring dark filaments by studying the WLM Galaxy, he should take a look at the Sextans A and B Galaxies. These are much fainter and the filaments are very difficult to discern. However, they are present as shown by the very obvious obscuration that is present. You have to be pretty careful in your examination if you what to convince yourself that they are present. The Phoenix Galaxy is an even more difficult example, due to its small size and dimness. In all of these cases discussed so far, there is no evidence of bright blue stars which provide an indication of star formation activity and there is no reason to suppose that the obscuration that is present is caused by internally generated dust clouds.
Consider now the photo of NGC 6822 known as Barnard’s Galaxy. This is also a nearby dwarf, but it is different from the others, in that there is obvious star formation activity in the outer regions. This can be seen in the photograph below the main body of the galaxy. If we study the main body of NGC6822, we see that it is very much like the appearance of the WLM Galaxy. There are a lot of dark filaments crossing the center of the main body from left to right, almost horizontally. There is also a vertical system of filaments that can be discerned. The result is a textured appearance that is typical of obscuring systems of dark filaments. The image to the right is a different photo of Barnard’s Galaxy that has been edited by the author to bring out the dark filaments that overlay the galaxy. Notice that this photo is reversed or upside down compared to the previous one, and that this one shows the presence of the star forming regions more effectively. In both of these photos of NGC 6822, the dark regions do not correspond to the obvious star forming regions. Hence it is clear that the dark ares in the main body of NGC 6822 are caused by foreground obscuring material and not by internally generated dark clouds. In addition, the filaments are obvious as they are long and narrow and not at all like nebulous or formless clouds.