by Ann » Tue Aug 02, 2011 6:56 am
After mulling it over for a moment, I suddenly realized what it was: each of these galaxies must have an extended halo of stars surrounding the central bulge and disk. If the galaxy were just a disk these features would be very difficult to explain, but if there are a few billion stars that exist above and below the plane of the disk, they would "fill in" the dark lanes, making them less dark. And that explains why we see the dust better on the galaxy’s near side: we’re not looking through as many stars. It’s like we’re looking through a fog; stuff nearby is clearer than stuff farther away because we’re looking through less mist. Except in this case, the fog isn’t obscuring anything. It’s stars, adding their light to the overall picture.
That is an astute observation. It is obvious from the Hubble that NGC 634 has a large central bulge, which may indeed function as a sort of "light fog", making the dust lanes on the far side look less dark and less distinct than the dust lanes on the near side.
However, it should be noted that dust lanes on the far side of the bulge may reflect the light from the bulge, thus acting as reflection nebulae. That will make them look quite bright. By contrast, the light from the bulge will be filtered through the dust lanes on the near side, making those dust lanes look quite dark.
This is an interesting example, a Hubble picture of NGC 2841. Not only do the dust lanes look much brighter on the far side than on the near side, but young blue clusters stand out much more clearly on the far side than on the near side. My guess is that this is more of a "reflection nebulae versus dark nebula" effect than the effect of the bright bulge and halo of NGC 2841.
Ann
EDIT: I see that the ESA/Hubble picture of NGC 634 was taken through relatively red filters. That would dampen the "reflection nebulae" effect, since reflection nebulae primarily reflect blue light. The Hubble image of NGC 634 does not detect blue light, and the blue color in the image is just "mapped blue", which explains why the far dust lanes don't look particularly blue. On the other hand, the faint stars of the central bulge will consist mostly of old red stars, which show up very well through red and infrared images. This bulge will therefore look brighter through red and infrared filters than through standard RGB filters, which explains why there may indeed appear to be a "light fog" in front of the far dust lanes, making them look not exactly bluer, but less distinct.
[quote]After mulling it over for a moment, I suddenly realized what it was: each of these galaxies must have an extended halo of stars surrounding the central bulge and disk. If the galaxy were just a disk these features would be very difficult to explain, but if there are a few billion stars that exist above and below the plane of the disk, they would "fill in" the dark lanes, making them less dark. And that explains why we see the dust better on the galaxy’s near side: we’re not looking through as many stars. It’s like we’re looking through a fog; stuff nearby is clearer than stuff farther away because we’re looking through less mist. Except in this case, the fog isn’t obscuring anything. It’s stars, adding their light to the overall picture.[/quote]
That is an astute observation. It is obvious from the Hubble that NGC 634 has a large central bulge, which may indeed function as a sort of "light fog", making the dust lanes on the far side look less dark and less distinct than the dust lanes on the near side.
However, it should be noted that dust lanes on the far side of the bulge may reflect the light from the bulge, thus acting as reflection nebulae. That will make them look quite bright. By contrast, the light from the bulge will be filtered through the dust lanes on the near side, making those dust lanes look quite dark.
[float=right][img2]http://www.todaysphoto.org/potd/ngc-2841.jpg[/img2][/float]
This is an interesting example, a Hubble picture of NGC 2841. Not only do the dust lanes look much brighter on the far side than on the near side, but young blue clusters stand out much more clearly on the far side than on the near side. My guess is that this is more of a "reflection nebulae versus dark nebula" effect than the effect of the bright bulge and halo of NGC 2841.
Ann
EDIT: I see that the ESA/Hubble picture of NGC 634 was taken through relatively red filters. That would dampen the "reflection nebulae" effect, since reflection nebulae primarily reflect blue light. The Hubble image of NGC 634 does not detect blue light, and the blue color in the image is just "mapped blue", which explains why the far dust lanes don't look particularly blue. On the other hand, the faint stars of the central bulge will consist mostly of old red stars, which show up very well through red and infrared images. This bulge will therefore look brighter through red and infrared filters than through standard RGB filters, which explains why there may indeed appear to be a "light fog" in front of the far dust lanes, making them look not exactly bluer, but less distinct.