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NGC 1309: Spiral Galaxy and Friends

Explanation: A gorgeous spiral galaxy some 100 million light-years distant, NGC 1309 lies on the banks of the constellation of the River (Eridanus). NGC 1309 spans about 30,000 light-years, making it about one third the size of our larger Milky Way galaxy. Bluish clusters of young stars and dust lanes are seen to trace out NGC 1309's spiral arms as they wind around an older yellowish star population at its core. Not just another pretty face-on spiral galaxy, observations of NGC 1309's recent supernova and Cepheid variable stars contribute to the calibration of the expansion of the Universe. Still, after you get over this beautiful galaxy's grand design, check out the array of more distant background galaxies also recorded in this sharp, reprocessed, Hubble Space Telescope view.

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I am intrigued by all the red, dust-deficient galaxies in the background of the main subject. Is that concentration unusual? I see other deep-space photos photos that have a lot more active star-formers way in the background.

Great image! Note, however, that NGC 1309 was the APOD of January 16, 2013, and back then its color balance was quite different. Personally I think that today's APOD looks a lot more realistic, since it is hard to believe that a galaxy of NGC 1309's type would be so blue and so non-yellow.
But we can learn a lot from a galaxy's color index. NGC 1309 turns out to be quite blue. Its B-V index is 0.44 and its U-B index is -0.17. Both values are quite blue. Fascinatingly, the B-V index of 0.44 is almost exactly the same as the B-V index of the large Pinwheel Galaxy, M101, whose B-V index is 0.45.

Because I take such a tremendous interest in the color of stars and galaxies, I can say without a doubt that a B-V index of 0.45 is blue for any galaxy, and it is extremely blue for a large galaxy like M101. Those of us who like to look at galaxy pictures see so many photos of M101 that we probably don't realize how extreme it is in its amazing wealth of O, B and A-type stars. NGC 1309, because it is so small, is not nearly as remarkable in its blue color as M101, because small galaxies don't have a huge population of old yellow stars, and a period of vigorous starbirth can easily shift their color to blue.

The point I'm trying to make is that NGC 1309 is just as blue as M101. In view of that, which APOD paints the truer picture, today's APOD of July 14, 2016, or the previous APOD of January 16, 2013?

In my opinion neither APOD makes NGC 1309 appear to be the same color as M101. The APOD of 2013 suppresses the yellow channel too much, and today's APOD enhances it too much. There can be no doubt that the galactic lens of NGC 1309, the dust-free circular area surrounding the brilliant point-shaped nucleus, is predominantly yellow (even though there appears to be star formation close to the nucleus), but it hardly looks yellow at all in the APOD of 2013. On the other hand, I personally would expect the inner spiral arms of NGC 1309 to look bluer than they do in today's APOD.

Still, today's APOD looks very very beautiful.

Ann

wtward wrote:I am intrigued by all the red, dust-deficient galaxies in the background of the main subject. Is that concentration unusual? I see other deep-space photos photos that have a lot more active star-formers way in the background.

As I tried to point out in my post above, I think that the yellow channel - or rather, the red channel - has been enhanced in today's APOD. Or maybe the blue channel has been a bit suppressed? In any case, that in itself will make the background galaxies look "extra red".

But please note that several of the reddest-looking galaxies are seen right through the spiral arms of NGC 1309. These galaxies are undoubtedly reddened by dust in NGC 1309. There are other galaxies some distance away from the main disk and arms of NGC 1309 that do show signs of blue in their arms or disks.

It is not true that most background galaxies in today's APOD look dust-free. On the contrary, many of them display obvious dust lanes. Note, for example, the edge-on spiral galaxy at 11 o'clock. This galaxy does indeed look "all orange" - at least partly because any blue light in it has been filtered away by dust in NGC 1309 - but it does have a very dark, thick dust lane.

Why don't you compare the color of the background galaxies in the two APODs, the one from 2016 and the one from 2013? You'll note that the color of the background galaxies vary a bit more in the APOD of 2013 than in today's APOD.

Finally, the color of the background galaxies also depends on the filters that were used for this APOD. I haven't checked the filters myself, but let's assume, as an example, that this might be a two-filter image, where one filter is sensitive to ultraviolet light and the other to infrared light. Most background galaxies will have lost their ultraviolet light, both because of redshift-reddening and because of dust reddening. If such filters were used, most of the background galaxies will look very orange.

We have no reason to think that the background galaxies in today's APOD are abnormal in any way or intrinsically different than the background galaxies in other galaxy images.

Ann

I love seeing galaxies THROUGH galaxies.
Does anyone know of any shot which shows a galaxy-through-a-galaxy-through-a-galaxy (three deep)?
In this shot, depending on how strictly one defines the boundary of 1309, the satellite galaxy at upper left might qualify for my criterion, but I want the real McCoy.

Ann wrote:Finally, the color of the background galaxies also depends on the filters that were used for this APOD. I haven't checked the filters myself, but let's assume, as an example, that this might be a two-filter image, where one filter is sensitive to ultraviolet light and the other to infrared light.

It is a three channel dataset, basically infrared, green, and blue. The blue has some near UV mixed in, red is almost entirely missing.

It seems to be at the tail end of a merger... and has had many... and gee, it is just a solid looking mass of stars...wow...

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I just checked out NGC 1309 in my trusted The Color Atlas of Galaxies by James D Wray. Wray photographed a thousand galaxies or so through U, B and V filters. His equipment was poor compared with today's standards, and the resolution of his galaxy images leaves much to be desired. But the color information is extremely interesting, because Wray went to rather great lengths to make sure that the color information for each galaxy was reliable and standardized, so that the galaxies could be reliably compared with one another. I think Wray did an admirable job at determining the color of his galaxies, and I trust his images as far as their resolution allows me to.

This is my point: In Wray's atlas NGC 1309 looks "all blue". Wray wrote about NGC 1309 (my emphasis):

From an evolutionary perspective we note that the galaxy is dominated by a present burst of star formation activity which, on the basis of the general lack of any significant yellow disk population, may be unprecedented in its evolutionary history.

I can't resist showing you a galaxy, NGC 3642, that looks almost all yellow in James D Wray's atlas. Wray described NGC 3642 in his atlas like this:

Only the yellow nuclear region and several very faint blue knots are bright enough to record in this photograph.

But in the SDSS image at left, the blue arms are almost as bright as the yellow nuclear region. I think the SDSS image has enhanced the blue arms to bring them out.

The fascinating thing is that NGC 3642 does indeed have a yellow nuclear region, and its blue arms are faint, but still this galaxy is almost as blue as NGC 1309. The B-V index of NGC 3642 is 0.47 according to Wray's atlas and 0.49 according to my software. The reason for its blue color index is probably that its yellow nuclear region isn't all that bright after all, and its blue arms are extended.

But it would really seem that NGC 1309 is a very blue and a very non-yellow galaxy. It looked that way in the APOD of January 16, 2013, too, and I'm beginning to suspect that that APOD got the color of NGC 1309 right. Because of that, as handsome as today's APOD truly is, I think it might lead us to think that NGC 1309 has a much larger yellow population than it actually does.

Ann

I'm always surprised at how a small (lower mass) galaxy can look really identical to a larger (higher mass) galaxy. Classical example is M33, which is tiny in fact, but heck, looks like it could rule the Universe!

tomatoherd wrote:I love seeing galaxies THROUGH galaxies.
Does anyone know of any shot which shows a galaxy-through-a-galaxy-through-a-galaxy (three deep)?
In this shot, depending on how strictly one defines the boundary of 1309, the satellite galaxy at upper left might qualify for my criterion, but I want the real McCoy.

I'm afraid I haven't got anything like that for you, but isn't NGC 6050 from the Hercules Cluster amazing? The two main galaxies are entangled with one another and seem to share a spiral arm. But note that there is a third galaxy enmeshed in this very spiral arm, and that galaxy has a bar which is as bright as the bar of NGC 6050B, the (large) galaxy at right.

But that's not all. To the right of the third galaxy, the one enmeshed in the spiral arm which joins NGC 6050A and 6050B, there is an oval "opening" in which we can see a fourth, quite small galaxy. The fourth one might be a background galaxy.

Still, it's amazing, isn't it - four galaxies for the price of one!

Ann

When we look at the APOD photo of NGC 1309 are we able to see individual stars? Or are the bright points of light in the photo of the galaxy actually clusters of stars?

EArHog wrote:When we look at the APOD photo of NGC 1309 are we able to see individual stars? Or are the bright points of light in the photo of the galaxy actually clusters of stars?

Some of both. It's difficult to tell just by looking at a picture like this, but the individually resolved points represent either very bright individual stars or star systems.