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Thread: Fornax globulars

  1. #1
    Member Steve Gottlieb's Avatar
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    Fornax globulars

    The Fornax globulars was also on my observing list for the visit to Jimi's a couple of weeks ago. Through this 48-inch the 4 brighter globulars (Fornax 2, 3, 4, 5) all looked like globulars with bright cores, nice-sized halos and some mottling (no individual stars resolved) -- except for Fornax 1.

    Fornax 1 was by far the faintest of the five and appeared quite dim, fairly small, roundish. It was larger than expected, though, probably 30"-40" diameter (comparable to Fornax 2, 4 and 5) with an extremely low even surface brightness (no central brightening). A mag 13.3 star is 2.5' NW and a 16.5 star is 1' S. Obviously this would be a much easier target from the southern hemisphere (passes overhead in Australia), but even in an 18" scope this is a toughie from the U.S.

    There's also a somewhat mysterious object - Fornax 6 - that Harlow Shapley first reported in his 1939 paper on the Sculptor and Fornax systems. He noted a "very faint cluster of unidentified character, mag 16.6; at 2h 35m 56', -34° 51' (1900), which is probably a part of the Fornax system."

    Visually, Fornax 6 was just an extremely faint, very low surface brightness spot, ~0.3' diameter. Located 7' due north of globular Fornax 4. Pinpointing the location, a mag 15.8 star is 1.6' W and a mag 16.5 star is 2.1' WNW.

    Although its not a globular, what exactly is it? Fornax 6 was discussed in a 1998 paper "Homogeneous Photometry for Star Clusters and Resolved Galaxies. I. A Survey of Bright Stars in the Fornax Dwarf Spheroidal Galaxy". Here's some of the relevant part (see the paper for more):

    "In his early discussion of the globular clusters in Fornax, Shapley (1939) identified a faint object lying some 7' north of globular cluster 4 as a possible additional cluster belonging to the galaxy. Hodge (1961a) described it as a group of five stars of approximately 21st magnitude; the implication was that he regarded its reality as a cluster to be dubious. As recently as 1994, Demers et al. considered the existence of the cluster as still worthy of debate, but stated that in their opinion the region contained “too few stars to be called a cluster."

    "The asterism known as cluster 6 was contained within nine of our frame pairs, and the depth and dynamic range of our stacked image allows us to contribute a new (as far as we can tell) observation to the discussion: nearly half of the “stars” in cluster 6 appear to us to be nonstellar and seem to constitute a very faint compact group of galaxies."

    There are no recent papers on this object in SIMBAD, so if the question "What is Fornax 6?" has been definitively answered, I'm not aware of it.
    Steve
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    I was not aware of the discussion about the status of Fornax 6. Thanks for this info.

    Except Fornax 3 (in the trees) no cluster is in reach from southern Germany or Austria. From Namibia I could pick up all 6 clusters with a 17" telescope. For the both clusters you describe here I noted:

    Fornax 1
    with 114x steadily visible with averted vision; homogeneous small surface

    Fornax 6
    most difficult Fornax cluster to observe; sure and repeated popping with 182x but not steadily visible
    Clear Skies, uwe
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    Member Steve Gottlieb's Avatar
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    Thanks for posting observations of these two objects from Namibia with a 17-inch.

    The only other observation I've seen of Fornax 6 was made by Mike Kerr in Australia (New South Wales) in a 25-inch. He called it "just visible with averted vision as an extremely faint small hazy spot."
    Steve
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    Hi Steve and Uwe,

    I wasn't aware of the status of Fornax globular cluster 6, either. So it seems to be a group of background galaxies shining through the dwarf galaxy? Sounds really interesting! So it's a bit like Maffei 1 shining through the dust of our milky way...

    By the way, there is an interesting article on this galaxy and its globular clusters by Dana de Zoysa at cloudynights:
    http://www.cloudynights.com/page/art...s-part-1-r2696

    This system has already been on my observing list for the next trip to South Africa. I'll only use an 8-inch Dobsonian, so I guess the Fornax 6 object will be out of reach.


    Clear skies

    Robin

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    Big Jim Jim Chandler's Avatar
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    Quote Originally Posted by Steve Gottlieb View Post

    The only other observation I've seen of Fornax 6 was made by Mike Kerr in Australia (New South Wales) in a 25-inch. He called it "just visible with averted vision as an extremely faint small hazy spot."
    We looked at it from my place near Fort Davis on December 30, 2013 using my 30" f/4.5 at 427x:

    Very dim cluster, seen intermittently using averted vision. Round, moderately brighter in center.
    Jim

    Obsession 30" f/4.5

  6. #6
    Member Steve Gottlieb's Avatar
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    Thanks for posting your observation, Jim. I was pretty sure you had looked for this one -- just because it was so obscure.
    Steve
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    Big Jim Jim Chandler's Avatar
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    I observed it thinking it was a sixth Fornax globular. The actually story is much more interesting; thanks for providing that. The question now is how to categorize the log note.
    Jim

    Obsession 30" f/4.5

  8. #8
    Member Steve Gottlieb's Avatar
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    For perhaps the definitive article on Fornax globulars, see Dana De Zoysa's meaty piece (Part I) on Cloudynights. If you're haven't had your fill, there's even more in part 2.
    Steve
    24" f/3.7 Starstructure
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    During the last holidays 2 months ago I had an opportunity to observe Fornax Dwarf Galaxy and its 5 globular clusters from South Africa with my 8-inch travel Dobsonian.
    It was located almost at the zenith.

    At 32x magnification (6.3 mm exit pupil diameter) the galaxy itself appeared as a large-area, low-contrast brightening with respect to the sky background at averted vision.
    In fact, Hodge 3 was quite easy to see and I thought it was fascinating that it appeared non-stellar at 460 000 light years distance.
    I couldn't detect the Hodge 6 group.

    Please find attached my sketches of the galaxy and of individual star fields containing the globular clusters.


    Clear skies

    Robin
    Attached Images Attached Images

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    Member akarsh's Avatar
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    Very interesting thread! Steve, I'm amazed that there was resolution of the stars in Jimi's telescope. While I still haven't read the thread fully, in the interest of sharing, I thought I'll link to my observing reports on these objects which I wrote as an article for the Austin Astronomical Society's newsletter in Feb 2015.

    https://austinastro.wildapricot.org/...e/ST201502.pdf

    The observing report itself is somewhere towards the end, as part of the newsletter.

    I remember someone (probably Steve or Dana?) writing about Fornax 6's mysterious status. Needless to say, I was afraid of its faintness, and used that as an excuse to avoid pursuing it or avoid any mention of it in the report.

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    Akarsh
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  11. #11
    Member akarsh's Avatar
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    Quote Originally Posted by Robin View Post
    During the last holidays 2 months ago I had an opportunity to observe Fornax Dwarf Galaxy and its 5 globular clusters from South Africa with my 8-inch travel Dobsonian.
    It was located almost at the zenith.
    Robin, your observations are incredible! I didn't imagine it would be possible to see so much with an 8" telescope! It's also nice to see someone posting observations of high caliber with small aperture, which we really miss in this forum for some bizzare reason.

    Clear Skies
    Akarsh
    18" f/4.5 Obsession dob "Romela"
    6" SkyQuest Orion dob
    Garrett Optical 25x100
    Homepage
    DSS Tool : Logbook Project : KStars
    The Astronomy Connection : Austin Astronomical Society : Bangalore Astronomical Society

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