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NGC 6946 revisited

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#1 IVM

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Posted 18 September 2013 - 10:45 AM

I an earlier thread I initiated discussion of the astrophysical correlates of the visual features in the nearby galaxy NGC 6946. Since then I have reobserved the galaxy in detail, producing a sketch, and found some additional relevant publications. Below is the annotated sketch. The observation was made on August 14 from Cherry Springs with my 16" Newtonian at 140-225x.

H1-2 are exceptionally bright HII regions whose combined light seems to comprise the compact knot to which the estern arm extends. They were cataloged by Hodge in 1969: http://adsabs.harvar...ApJS...18...73H

A32-35 are OB associations that seem to form the bulk of the main segment of the brightest eastern arm. They were cataloged by Ivanov and Krastev in 2000: http://publications....pdf/081-084.pdf

A36-41 from the same catalog seem to form the bulk of the star cloud in the middle of the inner eastern arm, which appears almost completely detached.

Star cloud "b" (my personal designation) is anchored by two bright foci of the H-alpha emission, HII regions HB171-172. These were cataloged by Bonnarel et al. in 1986: http://adsabs.harvar...A&AS...66..149B

A4 from the Ivanov-Krastev catalog corresponds to the peculiar star complex that is visible as a compact enhancement at the root of the outer western arm.

Star cloud "a" (my personal designation) is an exceptionally bright inner edge of the inner western arm. It is marked by the string of Bonnarel HII regions HB481-486.

A1-3 from the Ivanov-Krastev catalog form a detached group that defines the visual extent of the outer western arm.

A5,6,8 similarly define the visual extent of the inner western arm.

(A similar detached group of OB associations exists inside the triangle of stars that continues the arc of the inner western arm; I saw but did not sketch a nebulous patch in this location, deeming the perceived light illusory despite the match with the photo.)

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#2 Sasa

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Posted 18 September 2013 - 11:52 AM

Wonderful observations! Any chance that some of this features is visible through small (4in class) telescope? For example A4 is pretty bright on your sketch.

#3 blb

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Posted 18 September 2013 - 12:19 PM

I am sure that you had much darker skies at Cherry Springs than I did at our clubs dark sky site. Even so, I could still see the beginnings of the arms and a lumpy appearance with my 10-inch from the light polluted sky. Here is the sketch that I made two years ago.

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#4 uniondrone

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Posted 18 September 2013 - 12:28 PM


This has been a recent target for me using my 10" Dob from our club's orange zone site. There are days when I can't see it at all, and there are days when it is obvious but with only subtle detail visible. It's a pretty elusive object from light polluted skies.

#5 IVM

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Posted 18 September 2013 - 01:08 PM

Thank you for posting the comparison sketch, Buddy. I see most of the familiar details in it.

Sasa, with a small scope under good skies I would rather try for the best separated and brightest compact easternmost knot (H1-2 on my sketch, known to us Sky and Telescope readers as "Gottlieb 1"). I did not record any detail when observing this galaxy first with my 4" refractor, but it means nothing because I was then just surveying the Herschel 400. The OB association A4, in contrast (if that's the right word here), is reported to have Vmag 15.86. I believe this association comprises most of the "A4" knot ("Gottlieb 2") that is seen visually.

#6 uniondrone

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Posted 20 September 2013 - 10:48 AM


Yes, both sketches are beautiful! They're a real testament to your observing and artistry skills.

#7 Bill Weir

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Posted 20 September 2013 - 08:36 PM

Wonderful observations! Any chance that some of this features is visible through small (4in class) telescope? For example A4 is pretty bright on your sketch.


Well.....there is a simple way to find out. ; )

I look forward to your report.

Bill






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