APOD: Facing NGC 6946 (2012 Jan 09)

[APOD Robot]

Facing NGC 6946

Explanation: From our vantage point in the Milky Way Galaxy, we see NGC 6946 face-on. The big, beautiful spiral galaxy is located just 10 million light-years away, behind a veil of foreground dust and stars in the high and far-off constellation of Cepheus. From the core outward, the galaxy's colors change from the yellowish light of old stars in the center to young blue star clusters and reddish star forming regions along the loose, fragmented spiral arms. NGC 6946 is also bright in infrared light and rich in gas and dust, exhibiting a high star birth and death rate. In fact, since the early 20th century at least nine supernovae, the death explosions of massive stars, were discovered in NGC 6946. Nearly 40,000 light-years across, NGC 6946 is also known as the Fireworks Galaxy. This remarkable portrait of NGC 6946 is a composite that includes image data from the 8.2 meter Subaru Telescope on Mauna Kea.

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[Terry P-B]

I have been enjoying APOD intensively for some months now. I have enjoyed the posts without exception; thank you for this wonderful work. Today's image, like so many other of the images of galazies, is seen through a screen of stars in our own galaxy. I was wondering if it would be possible to devise a procedure for subtracting light sources with redshifts below a certain level, to reveal only the galaxy. The same procedure could be used to subtract nearby galaxies to show only the most distant ones.

[Ann]

This is a most remarkable galaxy and a most remarkable portrait of it. Robert Gendler is outstanding at processing images taken by large telescopes.

One thing I find very interesting about this galaxy is that it is apparently quite dusty. I don't have access to my software here, but I can remember that the colors of NGC 6946 are relatively red. This is partly caused by dust in our galaxy, but NGC 6946 is self-reddened by its own dust, too. Star formation produces dust, and the death throes of massive and even mid-sized stars cause copious amounts of dust. And there has been no shortage of such death throes in this galaxy. NGC 6946 has produced nine supernovae in less than a hundred years - compare that with the Milky Way whose latest supernova to be definitely seen and recorded by contemporary witnesses happened in 1604!

So this galaxy is choking with its own dust. However, a few places in the galaxy appear to be relatively dust free. Chief among them is an incredible super star cluster in NGC 6946. In today's APOD, this cluster can be seen to the lower left of the yellow bulge of NHC 6946, looking like a light blue vaguely circular object. Do follow the link in today's APOD called remarkable portrait of NGC 6946 to come to a page where you can read Robert Gendler's own description of this fantastic cluster. One thing is clear - this brilliant cluster of young stars is going to age as a beautiful globular cluster.

In James D Wray's The Color Atlas of Galaxies, where galaxies are photographed in UBV, NGC 6946 looks slightly dingy and dust-dirtened. But the super star cluster shines intensely blue, in spite of the foreground dust in our galaxy. Perhaps the ferocious stellar winds have blown away the dust in this particular part of NGC 6946.

And Robert Gendler's processing has lifted much of the rest of the dust from this galaxy, revealing its colorful splendour.

Ann

[orin stepanek]

9 super novas in 112 years! Is that a pretty high number for a galaxy? Is there a normal number? Sounds like this a a pretty active (young?) galaxy!

[Byork]

An algorithm may be developed to remove dust and vapor component in ground based optical images of objects such as NGC 6946. Algorithm may draw from results of both ground based images and Hubble Space Telescope images of the same object. Testing algorithm on well developed ground based images should yield corrected image comparable to a space telescope portrait. Enhancement may further uncover structural and chemical details in the enhanced portrait. NGC 6946 must be a very bright object considering the manner in which its spiral arms glow despite the dust and particle obstruction. Any corrective algorithm must take into consideration the water vapor component of the Earth's atmosphere; M74 appears blue-green on gound based telescopes whereas Hubble Space Telescope image of galaxy is distinctly blue-white owing to the absence of atmospheric water vapor in the space telescope image. Objects such as IC 342 which are obstructed by the Milky Way galactic disk require significant corrective processing to bring out the inherent detail.

This particular image of NGC 6496 is truly remarkable..astonished by the detail.. unable to speak

[Ann]

9 super novas in 112 years! Is that a pretty high number for a galaxy? Is there a normal number? Sounds like this a a pretty active (young?) galaxy!

Orin, if you click on the link NGC 6946 face-on, you'll come to a page which tells you this:

Nine supernovae have been detected in NGC 6946 as of this writing (March 2009):
...
With this number of supernovae which have been found in this galaxy, NGC 6946 is leading the statistics, now a significant three supernovae more than follow-ups M61 and M83 with six each.

So nine supernovae in about a hundred years is really, really a lot! It is the largest number of reasonably well-recorded recent supernovae that we know of in any nearby galaxy. In fact, it is the greatest number of recent supernovae that we know of in any galaxy, although we can be quite sure that many galaxies in the distant past experienced even greater numbers of them.

As for what is "normal"... well, about one supernova per century, or so they say. (But don't ask me who "they" are!)

Ann

P.S. Tomorrow's APOD will show us a bright star and a dwarf galaxy. Could that by any chance mean that we will get to see something like this?

[saturn2]

The Spiral Galaxy NGC 6946 is interesting.
I think that NGC 6946 is a galaxy very "accelerated".
The star born very fast.
The explosion of 9 Supernovas in " few" time, it´s a record.
It´s a model galaxy, important for the study.

[Anthony Barreiro]

This is a breathtakingly beautiful picture, with an informative caption and erudite discussion. An apod trifecta! Thanks Ann for pointing out the supercluster.