APOD: Galaxy, Stars, and Dust (2015 Oct 12)

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APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by APOD Robot » Mon Oct 12, 2015 4:11 am

Image Galaxy, Stars, and Dust

Explanation: Is this galaxy trapped in a web of dust? No -- it is far in the background. However, spiky stars and spooky shapes are abound in this deep cosmic skyscape. Its well-composed field of view covers about a Full Moon on the sky toward the constellation Pegasus. Of course the brighter stars show diffraction spikes, the commonly seen effect of internal supports in reflecting telescopes, and lie well within our own Milky Way galaxy. The faint but pervasive clouds of interstellar dust ride above the galactic plane and dimly reflect the Milky Way's combined starlight. Known as high latitude cirrus or integrated flux nebulae they are associated with molecular clouds. In this case, the diffuse cloud cataloged as MBM 54, less than a thousand light-years distant, fills the scene. The galaxy seemingly tangled in the dust is the striking spiral galaxy NGC 7497 some 60 million light-years away. Seen almost edge-on near the center of the field, NGC 7497's own spiral arms and dust lanes echo the colors of the Milky Way's stars and dust.

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Ann
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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by Ann » Mon Oct 12, 2015 5:30 am

NGC 7497 looks like a tiny little fish caught in a gigantic cosmic fishnet. Fascinatingly, the tip of the "upright" galaxy coincides with a "peak" in the dust structure, as if the galactic "fish" was struggling to get free. It is of course an illusion.

There can be no doubt that NGC 7497 is reddened by dust in our galaxy, so it is interesting to check out the colors of NGC 7497. Its B-V index is 0.720 and its U-B index is 0.150. That is relatively red for a galaxy of Hubble class SBc - a barred galaxy with a small bulge and extensive arms. Of course, we see it edge-on too, which will usually make galaxies look redder (because we look straight into their central dust lanes). NGC 7497 isn't perfectly edge-on, and the central dust lane isn't very dark and obvious. But we can see, on the other hand, that the galaxy appears to have relatively large yellow patches with no star formation. The relatively red colors of this galaxy are most likely caused by the nature of the galaxy itself, not by foreground reddening.

And there are many galaxies that are so much more reddened than NGC 7497, for example nearby galaxies IC 342, IC 10 and Maffei 1 and 2.

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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by Chris Peterson » Mon Oct 12, 2015 5:46 am

Ann wrote:There can be no doubt that NGC 7497 is reddened by dust in our galaxy, so it is interesting to check out the colors of NGC 7497. Its B-V index is 0.720 and its U-B index is 0.150.
Color indices are pretty useless for galaxies reddened by intervening dust. That's because you can only use the ratio or difference of a pair of colors to yield temperature information when you are working with a blackbody (as true for galaxies as for stars). That requires no filtering or selective scattering interfering with the light we receive. Otherwise, you need to look at the full spectrum and do a more sophisticated analysis.

Of course, these days color indices aren't much used, anyway, except as screening tools to suggest targets that might be interesting for further study.
Chris

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Ann
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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by Ann » Mon Oct 12, 2015 8:13 am

Chris Peterson wrote:
Ann wrote:There can be no doubt that NGC 7497 is reddened by dust in our galaxy, so it is interesting to check out the colors of NGC 7497. Its B-V index is 0.720 and its U-B index is 0.150.
Color indices are pretty useless for galaxies reddened by intervening dust. That's because you can only use the ratio or difference of a pair of colors to yield temperature information when you are working with a blackbody (as true for galaxies as for stars). That requires no filtering or selective scattering interfering with the light we receive. Otherwise, you need to look at the full spectrum and do a more sophisticated analysis.

Of course, these days color indices aren't much used, anyway, except as screening tools to suggest targets that might be interesting for further study.
Well, the (effective) color indices for IC 342 are 0.280 (U-B) and 1.100 (B-V). That is clearly redder than the colors for NGC 7479, although the intrinsic colors of IC 342 might well be bluer than for NGC 7497. My impression is that the yellow population of IC 342 is not as widespread as the yellow population of NGC 7497.

IC 10 is even more reddened. Its total B-V index is 1.430, while its effective B-V index is 1.280. Its total U-B index is 0.300, while its effective U-B index is 0.360. Fascinatingly, its apparent (unreddened by external factors) B-V index is given as 0.906, which is pretty red, while its apparent U-B index is -0.244, which is pretty blue (or rather, it is pretty ultraviolet). We know that IC 10 is a starburst galaxy that is going to be bright in ultraviolet light, but it will also be dusty. Clearly, however, our own galaxy contributes a lot of reddening to the light that reaches us from IC 10.

As for Maffei 2, a nearby spiral galaxy, its light is so reddened by dust in our own galaxy that Simbad Astronomical database only gives its B magnitude as 16 (with no decimals), while its J, H and K infrared magnitudes are given as seventh, sixth and fifth magnitude (with two or three decimals). Maffei 1, a nearby elliptical (the nearest elliptical to the Local Group) is fainter than Maffei 2 in B magnitude (17 according to Simbad), while its infrared magnitudes are a little brighter than those of Maffei 2.

So I find color indices very interesting and quite revealing.

Ann
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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by starsurfer » Mon Oct 12, 2015 2:01 pm

Wow that looks amazing with lots of depth! There is lots of integrated flux nebulosity everywhere in the sky but a lot of it is faint and only visible in long exposures. I particularly like regions around planetary nebulae with integrated flux such as NGC 7094 and Abell 39.

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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by Boomer12k » Mon Oct 12, 2015 6:59 pm

What an interesting image...a galaxy behind a veil of dust.

A Dragon flying through space, (his head is the main part on the left), guarding the Treasures and Jewels of the Universe...he has a rider on his head directing the way.....

OK...on the right end....is BUGS BUNNY'S HEAD??????

Boy, my Pareidolia really kicked in... :lol2:

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Re: APOD: Galaxy, Stars, and Dust (2015 Oct 12)

Post by DavidLeodis » Tue Oct 13, 2015 1:39 pm

I'm curious why the dust stream of what I assume is MBM 54 seems much more concentrated to its left? It's as if the dust source is like a comet that is dispersing its material in a trail behind.