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Cosmic Challenge: Seyfert's Sextet


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Cosmic Challenge: Seyfert's Sextet

 

July 2020

Phil Harrington

This month's suggested aperture range:

Monster scopes

 

15-inch (38cm) and up

 

 

 

Target

Type

RA

DEC

Const.

Magnitude

Size

Seyfert's Sextet

Galaxy group

15h 59.2m

+20° 45'

Serpens

see detailed list below

1'

 

Seyfert's Sextet, known to many as Hickson Compact Galaxy Group 79, is a tight gathering of galaxies in the northern corner of Serpens Caput. Serpens Caput is the western segment of this bisected constellation, marking the triangular head of the serpent that Ophiuchus is handling. Observing Seyfert's Sextet has been one of my pet projects for years. It's a fun little galactic rat pack for summer outings before we plunge headlong into the summer Milky Way.

 

Above: Spring star map showing the location of this month's Cosmic Challenge.

 

Credit: Map adapted from Star Watch by Phil Harrington

 

Above: Finder chart for this month's Cosmic Challenge.

 

Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window

 

 

The nickname "Seyfert's Sextet," however, is inaccurate for a couple of reasons. First, technically speaking, Karl Seyfert did not discover Seyfert's Sextet. Instead, the group's primary galaxy, NGC 6027, was found by Édouard Stephan, director of the Marseilles Observatory, while observing through the observatory's 31.5-inch (80-cm) reflector in June 1882. That was the same telescope that Stephan was using when he found another, more famous galactic bunch, his namesake Stephan's Quintet in Pegasus. 

 

Stephan's notes record only a single object, however, although he also noted two very faint stars as "involved." In historical retrospect, those two stars were actually two of the other galaxies that likely went unrecognized because of their small size. Seyfert discovered their true nature, and also spotted several additional members while scrutinizing a photographic plate taken of the region at Harvard College Observatory in 1951. Seyfert later reported his findings in a short paper entitled A Dense Group of Galaxies in Serpens, where he also noted that the group was 27 million light years away.

 

Although five auxiliary NGC numbers -- NGC 6027a through 6027e --- were assigned to the galaxies recorded on the Harvard plate, Seyfert questioned whether or not he was looking at a group of six galaxies, or perhaps only four or five. He acknowledged that his colleague Walter Baade, a staff member of Mount Wilson Observatory, believed that the objects labeled as NGC 6027c and 6027d were actually tidal anomalies creating by the interactions among the others. 

 

Seyfert's Sextet

Galaxy

RA

Dec

Magnitude

Size (')

NGC 6027

15 59.2

+20 45.8

14.7

0.5'x0.3'

NGC 6027A

15 59.2

+20 45.3

15.4

0.9'x0.6'

NGC 6027B

15 59.2

+20 45.8

15.4

0.5'x0.3'

NGC 6027C

15 59.2

+20 44.8

16

0.7'x0.2'

NGC 6027D

15 59.2

+20 45.6

15.6

0.3'x0.3'

NGC 6027E

15 59.2

+20 46.0

16.5

0.9'x0.4'

 

Credit: Hubble Legacy Archive, NASA, ESA; Processing: Judy Schmidt

 

 

It turns out that Baade was correct. Seyfert's Sextet does not contain six galaxies; it contains only four. He was wrong, however, about which were tidally created mirages and which were actual galaxies. NGC 6027c and NGC 6027d are bona fide galaxies. Studies conducted with the Hubble Space Telescope, however, show quite clearly that NGC 6027e is not a separate galaxy, but instead, a gravitationally created plume extending away from NGC 6027. Such distortions are common features of galaxy groups as they swirl around each other and draw closer as time goes on. Ultimately, after hundreds of millions of years doing a galactic pirouette, the galaxies will ultimately merge to form a single, giant elliptical galaxy. That is, all will except NGC 6027d. This spiral galaxy is the background and just happens to lie along the same line of sight.

 

Seyfert's original distance estimate was off by a factor of seven. Current estimates place the four gravitationally related galaxies at about 190 million light-years away. NGC 6027d is well beyond, at an estimated 877 million light years.

 

 

 

Above: Seyfert's Sextet as sketched through the author's 18-inch (45.7cm) reflector.

 

 

Numbers and names aside, Seyfert's Sextet can be found 2° east-southeast of 5th-magnitude Rho (ρ) Serpentis, itself 3° north of the Serpent's triangular head. As a reference, look for a right triangle of 9th- and 11th-magnitude stars 10' southwest of the group, and a closer pair of 11th-magnitude stars 7' to its northwest

 

NGC 6027 should be a fairly easy catch in your scope, but the rest will take some effort to see. Under suburban skies, my 18-inch reflector at 171x shows it as a dim, glow, slightly elongated east-to-west and accented by a very dim central core, but that's about it. There is no sign of the dimmer group members until the magnification is increased to 294x, when the very faint glimmer from NGC 6027A can be seen just 36" to the south-southwest. NGC 6027B can also be suspected with averted vision just 20" west of NGC 6027. Darker skies are needed to confirm it with direct vision, as well as to suspect even the slightest hint of the other members of the bunch.

 

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my web site or post to this month's discussion forum.

 

Until next month, remember that half of the fun is the thrill of the chase.  Game on!



About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2018 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.

 


  • John O'Hara, Knasal and jeremiah2229 like this


4 Comments

I've only seen three brightest members in my 18" (not too difficult), but Uwe Glahn managed to glimpse five in a 16".  It is a fun challenge to try and bust the knot apart.

 

The faintest two members (based on observations in a 48") of the Sextet are not physical members --  namely, the tidal plume and the small background galaxy

    • PhilH likes this

Thanks for the guide, Phil.  I had this one on my list for this month after doing other Serpens Hicksons last month.

 

Last night I gave this a shot with the 20" in 21.6 MPSAS conditions.  The three brightest members popped out immediately.  In many ways each of these three looks larger than its published size, probably because each has relatively high surface brightness.  Stellar/semi-stellar cores were seen in all three at 278x (roughly the seeing limit at the time.)   

 

To my surprise, I found the 6027E plume to be readily seen in averted vision.  While it was subtle, it was also quite recognizable.  The brighter central portion of it reminded me of a faint extended arm.  I couldn't see anything directly, but there was enough separation from 6027 that this faint arm and its orientation stood out in averted vision. 

 

6027D proved to be a challenge and I am not sure why.  It might have had something to do with the seeing, or the brighter galaxies nearby, but I could not get a definite fix on it.  I was seeing something separate from the glow of 6027 at moments, but not coherent and persistent enough to satisfy.  I tried 357, 417, and 500x, and tried using my other eye, but none of it helped.  I would like to try it in better seeing.

 

6027C was visible in averted vision, much like a ghostly low surface brightness near edge on.  While it was AV only, it could be held well enough after a time, and the orientation remained consistent when seen.     

 

In checking Wikisky I noticed another tiny PGC to the west, PGC 1635684.  I didn't have a chart made for it, but knew its relative orientation to a 14 and 16.6 mag star, so I gave it a go. The galaxy has a green magnitude of about 17.2.  I was getting a hit of something in the general vicinity, and roughly the right size, but I could not pin it down well enough to say I definitely had it.  Had I made a chart showing the other 17.1 mag star beyond it, I probably could have nailed it down.  

    • PhilH and sgottlieb like this

I'm not surprised NGC 6027D (=HCG 79E) was challenging. It was really small in the 48" (estimated as 6" diameter) and barely off the south side of HCG 79B (just 13" between center). So although it wasn't difficult, it wasn't immediately obvious either.

 

By the way, Seyfert's original letter designations in his paper "A Dense Group of Galaxies in Serpens" and shown in Phil's article was not followed by Hickson, who skipped the tidal plume (NGC 6027E).  So, sometimes I've seen the identifications confused.

    • PhilH and Redbetter like this

Steve,

 

Yes, the Hickson/NGC designations are a jumble.  I have used Phil's above to try to be consistent.  It is a challenge when some are using Hickson designations, some NGC letters and the NGC numbers differ depending on source.  In addition to Phil's guide, Wikisky has one (same as LEDA I believe), Reiner Vogel has Hickson's(I think)...I can't figure out what Uranometria is trying to show, it is tiny and seems at odds with the others--I haven't tried to sort based on DSFG's coordinates.  I haven't checked NED.  I can't keep them straight...I would have to spend a few hours printing each chart out for each source and each designation (6027's and HCG 79's separately in each source) and make a cross reference table for all of them by source. But I think I am just going to attach Phil's chart to my log.

 

I had a clean sweep on Hickson 85 in Draco last night.  Those tiny C & D components were immediately picked up in averted vision at 278x, simultaneously, and could be held that way.  They seemed to have good surface brightness considering how dim their stated magnitudes are (mid 17 B mags.)  I would put them at somewhere around 16.5+ visual.  It doesn't hurt that this is the darkest section of sky for me and seeing was peaking at the time.  The contrast was better than it had been on Hickson 79.  Preceding a bit to the WSW is a field star with a galaxy just south of it, easily seen.  And further to the west of HCG 79 is CGCG 341-9 with a dim field star on the east body of the galaxy, following the core. I was able to pick up a tough PGC 2754094 well south of the HCG.  But the reason for mentioning those is the interesting and difficult UGC 11377 that is north of the bright (9th mag) field star.  This galaxy was very difficult to see, somewhat more so than the elongated NGC 6027C had been.   Mostly I saw the long N/S line in averted vision, but as glimpses sweeping back and forth.  Eventually I found that I could hold the slightly thicker central region in AV, but when I did I could not see the longer strand.  Anyway, this last galaxy made for a good comparison visually as it shared some aspects but differed in others.

    • PhilH likes this


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