Object of the Week, September 30, 2018 - NGC 247 and Burbridge’s Chain

[Howard B]

NGC 247 and Burbridge’s Chain

Spiral galaxy with background chain if galaxies

Cetus
RA 00 47 08
DEC -20 45 38 (coordinates for NGC 247)
Magnitude 9.1, 14.6, 15.7, 17.0, 14.4 and 15.5
Size: 21.4’ x 6’ to 0.6” x 0.2’

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Screen shot of Megastar chart, north is up.

Forgive me if this is a repeat of an old OOTW, but searching the site I couldn’t find an instance where NGC 247 and Burbridge’s Chain have been the subject. It just seems like they should have been by now.

I have one observation of NGC 247 and two observations of Burbridge’s Chain to share. The first is with my 28-inch from Steens Mountain:

“Great field! NGC 247 is huge, bright and detailed and worth a sketch on nits own, but with Burbridge’s Chain in the same fov this is irresistible. The faintest galaxy in the chain is magnitude 17 and has a surface brightness of 14.2 so it took some extreme averted vision to barely detect…an added bonus is the ESO galaxy 540-19, a nice magnitude 14.1 galaxy on the other side of 247. Sweet! 253x and 408x, 21.95 SQM.” (south is up in my sketch)

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My second observation of Burbridge’s Chain was with Jimi’s 48-inch scope a few years ago:

“Terrific view! The faintest member (MCG -4-3-12) is actually a pair of galaxies that I could see as double with averted vision. The small galaxy below the chain (MAC 0047-2023) is slightly brighter than the faint double galaxy. 488x, 21.37 SQM.” (south is up in my sketch)

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Burbridge’s Chain is a wonderful series of test objects for different size telescopes, but NGC 247 can look great for anyone under a dark sky and a good scope. I'm visiting Jimi next week and hope we'll get an even better view of these great objects.

"GIVE THEM A GO AND LET US KNOW"

[obrazell]

It was covered in 2013 Howard under pretty much the same title :-)

Owen

[Howard B]

Thanks Owen - and my apologies! It still doesn't come up when I search though, I must not be using the search function properly. Anyway, I hope five years is long enough for a fresh comparison of what we've seen since 2013.

[obrazell]

http://www.deepskyforum.com/showthread.php?427-Object-of-the-Week-August-18-2013-NGC-247-amp-Burbidge-s-Chain - and you contributed as well :-)

Owen

[Bertrand Laville]

Hi All

Here is my drawing and report: http://www.deepsky-drawings.com/ngc-0247/dsdlang/fr

Clear skies
Bertrand
http://www.deepskyforum.com



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and cluster and / or HII regions perceived

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[KidOrion]

Still a great object, regardless of whether it's been done before or not. My take:

10/24-10/25/17

EAGLE’S RIDGE
MOON: 5 days (set at 9:46 PM), 22% illumination
SEEING: 6
TRANSPARENCY: 7
SQM: 21.3 (at midnight)
NELM: not checked
WEATHER CONDITIONS: temps in mid-upper 50s, air still, no dew

Others present: JO, RA

All observations: 12.5″ f/5 Discovery truss-tube Dobsonian, 14mm ES 82˚ eyepiece (112x, 0.7˚ TFOV)

12:14
NGC 247, PGC 2791 (Cet): way low in the sky–-247: huge–elongated almost N-S–12.0′ x 3.0′–very diffuse–low surface brightness–irregularly bright across middle–at S end, about 80% of the way to S end on P edge, is a 10th-mag star; N of that star is a 14th-mag star still within galaxy; N slightly P 10th-mag star by 6′ is a 12th-mag star outside P edge of halo–N end of galaxy less distinct, halo fades away–wider at N end than at S end–almost as if galaxy “fans out” from the 10th-mag star–14′ NF the 10th-mag star is an 11.5-mag star which has a 13th-mag companion NP by 1.5′–those two stars are signpost for PGC 2791; NP those stars by 13′ is a 13th-mag star–just SP that 13th-mag star, not quite but almost in contact with star, is faint elongated glow (PGC 2791)–elongated SP-NF–very difficult–0.5′ x 0.25′–brightest in Burbidge Chain–galaxy at other end of Chain (PGC 2796, second-brightest in Chain) was fleetingly visible in averted but lost by the time of recording notes

[Steve Gottlieb]

I thought I'd mention the darker "keyhole" feature, that's shown on Bertrand's sketch. Here's some info from a 2014 study

Wagner-Kaiser et al: "The void in the Sculptor group spiral galaxy NGC 247"

"Closer inspection of NGC 247 shows an apparent void in its disc between the nucleus and outer arm on the north side of the galaxy. This void is visible in bandpasses from the infrared to the ultraviolet and thus far has drawn little attention in the literature beyond noting its existence. At a distance of about 3.5 Mpc, the void region in NGC 247 is approximately 3.4 kpc long. For a dwarf galaxy, this makes up a significant percentage of its stellar disc, which is 11.3 kpc in extent along the major axis."

"The existence of the void in the disc of this dwarf galaxy has been known for some time, but the exact nature and cause of this strange feature has remained unclear. We investigate the properties of the void in the disc of NGC 247 using photometry of archival Hubble Space Telescope data to analyze the stars in and around this region. Based on a grid of isochrones, we assign ages using nearest-neighbor interpolation. Examination of the spatial variation of these ages across the galaxy reveals an age difference between stars located inside the void region and stars located outside this region. We speculate that the void in NGC 247's stellar disc may be due to a recent interaction with a nearly dark subhalo [dark-matter dominated dwarf satellite of ~100 million solar masses] that collided with the disc and could account for the long-lived nature of the void."

[Ivan Maly]

Intriguing hypothesis, but the paper does not convincingly refute the normal spiral arm interpretation of the north side structure. Following the counterarguments in the Discussion, the authors admit: "None the less, spiral density waves provide a plausible scenario for the formation of the void [...]".

[NGC7702]

Hi All,

Only looked at this last Sunday evening, 7 October 2018 with my 63.5cm f/5 at x260 & x346. I am fortunate that these objects pass near Zenith and the SQML reading at the time was 21.86.

Of the four, the northernmost galaxy (ESO 540-23) is a simple catch and is seen as a 40" x 20" oval in PA 30, brightening slightly and broadly to centre where there is a small, slightly brighter core zone. Next brightest is the southernmost -- ESO 540-25 that is only a little fainter and lower in surface brightness with a round 20" diameter halo. Again, it brightens broadly and slightly to centre without apparent zones, core or nucleus.

Next brightest is ESO 540-24 (imm N of -25) that is somewhat dimmer again but can be held steadily with averted vision at x260, about 20" diameter, quite to very LSB, round and brightens weakly to centre. The most difficult is MGC -4-3-12. It takes concentration and x346 for it to be glimpsed from time to time as a 10" diameter spot of weakest gossamer. I detected it more than a few times per minute (about 6-7 times per minute). It takes concentration and patience.

The "fifth" galaxy at the N end of the line -- PGC 842319 is similarly difficult as MGC -4-3-12 and has a similar appearance.

ESO 540-19 (over on the W side of the N tip of NGC 247) is again a more straightforward object, 20-30" diameter brightening slightly and broadly to centre where there is a small weakly brighter core-zone.

My old 46cm f/4.9 could show 3 of the brightest 4 of Burbidge's Chain, but MGC -4-3-12 eluded me -- until now.

Best,

L.

[Uwe Glahn]

Wow Howard, you split the fainter galaxy.

First I add my old sketch with the 16-inch. Three of the five members were visible.
16", 100x-257x, NELM 7m0+, Seeing III
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In addition to that I found an entry with the 27-inch. With 293x I could detect four galaxies of the chain (could not split the faintest galaxy). The northern and the southern galaxies were visible with direct vision, the two between steadily with averted vision.

[obrazell]

I am not sure how many saw this magnificent image of this object at https://apod.nasa.gov/apod/ap180330.html. Perhaps slightly overprocessed but Burbidges chain shows up very well.

Owen