6 Attachment(s)
Object of the Week, September 26 2021 -- Fornax Dwarf, NGC 1049, and 5 other extragalactic globulars
[Posting in lieu of Alvin Huey, who will instead post on Oct 3rd]
All objects are in the constellation of Fornax.
| Identifier(s) |
Type |
RA (J2000) |
Dec (J2000) |
B mag |
V mag |
Discoverer |
| NGC 1049 = Fornax 3 |
Globular Cluster |
02:39:48 |
-34:15:29 |
13.6 |
|
John Herschel (1835) |
| Fornax 4 |
Globular Cluster |
02:40:08 |
-34:32:11 |
|
13.9 |
Edwin Hubble and Walter Baade (1939) |
| Fornax 5 |
Globular Cluster |
02:42:21 |
-34:06:06 |
14.5 |
|
Paul Hodge (1961) |
| Fornax 2 |
Globular Cluster |
02:38:40 |
-34:48:05 |
14.5 |
|
Harlow Shapley (1939) |
| Fornax 1 |
Globular Cluster |
02:37:02 |
-34:11:00 |
|
16.6 |
Paul Hodge (1961) |
| Fornax 6 |
Globular Cluster |
02:40:04 |
-34:25:13 |
16.6 |
|
Harlow Shapley (1939) |
| Fornax Dwarf = Fornax dSph |
Galaxy |
02:39:59 |
-34:26:57 |
9.0 |
7.4 |
Harlow Shapley (1938) |
History and Science
NGC 1049 was discovered by John Herschel on Oct 19, 1835, and Dreyer described it as "pB, S, R, stellar" in the NGC [1]. Much later, in 1938, Harlow Shapley reported on "Two Stellar Systems of a New Kind" [2]. Using plates obtained from the Boyden Observatory in South Africa, he had just discovered the first dwarf spheroidal galaxies -- the Sculptor and Fornax dwarfs. He recognized that these were part of the local group and that they had properties that were in common with globular clusters, spheroidal galaxies, and the Magellanic clouds. Whereas Shapley first wrote "There are no irregular nebulosities, no clumping of stellar images, no sharp or bright nuclei...", Hubble and Baade [3] subsequently identified that it has "at least two globular clusters" -- NGC 1049, and what is now known as Fornax 4. Shapley subsequently [4] reported another globular cluster (Fornax 2), and one more "very faint cluster of unidentified character", which is now known as Fornax 6. In 1961, Paul Hodge identified [5] two new globular clusters, now known as Fornax 5 and Fornax 1, by using larger photographs of the Fornax Dwarf.
Now comes an interesting subject. Hodge does reference Fornax 6 from Shapley's paper ([4]), but writes in [5] that it "is probably rather bluer than the globular clusters" ... "appears as a group of five stars of approximately 21st mag", and does not seem to consider it a globular. What is Fornax 6? Throwback to 2016, Steve Gottlieb made a post here on DSF [6] on this subject, in which he quotes a 1998 paper [7]: "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". But in the 5 years between then and now, a couple new papers have emerged on Fornax 6. One of these recent papers from 2019 [8] asserts that it is a "diffuse but bona fide cluster that is likely undergoing tidal disruption" and that "most of the objects discussed in [7] that were thought to be nonstellar, are clearly multiple closely located stars". They provide a picture:
Attachment 4463
(Picture of Fornax 6 from the Dark Energy Survey [8])
Further, they use Gaia data to corroborate that some Fornax 6 cluster stars have similar proper motion to the Fornax dSph. Even more hot-of-the-press material from May 2021 [9] used spectroscopy to identify 15--17 members of Fornax 6. By analyzing the chemical content and the statistics of the motions of the stars, they are able to further assert that Fornax 6 is a globular cluster, perhaps a younger one compared to the other clusters.
It appears that the recent scientific interest in Fornax Dwarf and its globulars stems from its relevance to understanding dark matter. Firstly, it is apparently anomalous for a dwarf galaxy like Fornax dSph to harbor as many as 6 globular clusters [9]. Further, my understanding of a brief literature search [9--11] is that, according to the prevalent model of how dark matter is distributed in such galaxies, the globulars in Fornax dSph should spiral rapidly (on astronomical timescales) into the core of the galaxy, and aggregate there to form a nucleus. Then, given that we are seeing multiple globulars close to the core, and no nucleus, is it just pure chance that we caught glimpses of these globulars towards the end of their life, or do we need to refine our models of dark matter? I believe this constitutes the "Fornax timing problem" [11], on which you can find many sophisticated papers.
Attachment 4464
(Another image from [8] showing a few of the globs and the core of Fornax dSph)
Attachment 4465
(Image from the KStars planetarium software showing overlay of globular clusters on the DSS2 Colored Image)
Visual Observation
I believe my interest in the globular clusters objects arose out of perusing Paul Hodge's "An Atlas of Local Group Galaxies" [12] for extragalactic targets. Using my 18" f/4.5, I checked off five of these globulars, excluding Fornax 6, on 20th Jan 2015 from an observing site in Central Texas (~31° N latitude). Subsequently, I submitted a detailed report to the Austin Astro. Soc. newsletter [13], but I will summarize my notes here. I put both the above list and the below based on my estimated order of difficulty.
1. NGC 1049 (Fornax 3): Surely, if there's one object you should attempt from this list, it's this one. I first saw this with my former 17.5" in 2009 from south India, where it rises much higher. With that aperture, despite my inexperience and the humidity, it was not tough. In 2015, I logged this as the easiest of the five globulars. I revisited it with my 18" in 2015 for completeness.
2. Fornax 4: This was the second easiest of the five. It was just a tad more condensed than NGC 1049, and only a small jump in difficulty from NGC 1049.
3. Fornax 5: The third easiest of the five, I logged it as quite reminiscent of Fornax 4, condensed and of nearly equal brightness.
4. Fornax 2: More diffuse than Fornax 4 and NGC 1049, I logged this as "a uniform glow at 205x. Was not at all as obvious as F4 or NGC 1049."
5. Fornax 1: Looking at my logs, this one must have been very challenging. I observed it 1.5 hours after transit, so it was low and I logged "at the @#$@#$ edge of visibility". I sensed it a multiple number of times, with the object popping in and out of the sky background. The size of the very, very, faint glow was comparable, if not larger than F2. I had the best view using a Pentax 14mm XS (147x)
I have been unsuccessful in observing the Fornax dSph itself -- as is well-known, its very low surface brightness makes it a challenging object even for the darkest of skies and the most seasoned observers, even more so in the northern hemisphere. Fornax 6 will be on my list if I get any observing in this season, with the west coast of the US drenched in wildfire smoke.
I already mentioned Steve's observing report of these objects with Jimi's 48" [6] which includes Fornax 6, but here are some more pointers I was able to unearth:
1. A November 2018 Sky & Telescope article by Steve on Extragalactic Globulars includes the 5 globulars, detailing observations with a 13", and noting that they should be visible in a 10".
2. I haven't read this one, but here's an article by Dana De Zoysa on CN that seems to go to greater depths: https://www.cloudynights.com/article...s-part-1-r2696
3. Some of you may remember Fornax Dwarf and its globulars as being the subject of a recent discussion on amastro [14]. Paper [8] also features in this discussion.
This collection of objects should appeal to a wide range of audiences. I imagine that folks with access to big binoculars, southern or low northern latitudes, and extremely dark skies can take a crack at the Fornax Dwarf. Yet again, those who are not fortunate to have access to great skies, but have larger aperture scopes (13") can shoot for NGC 1049, F4 and F5. Those with larger aperture, darker skies or more observing experience can take a crack at F1. Hopefully, all of this is convincing that the Fornax Dwarf system is a target worthy of a try. So, as always:
Give it a go, and let us know!
[1] http://cseligman.com/text/atlas/ngc10.htm#1049
[2] https://www.nature.com/articles/142715b0 (Sadly, this one is paywalled despite being almost a century old)
[3] http://adsabs.harvard.edu/pdf/1939PASP...51...40B
[4] https://www.ncbi.nlm.nih.gov/pmc/art...01610-0017.pdf
[5] http://adsabs.harvard.edu/pdf/1961AJ.....66...83H
[6] https://www.deepskyforum.com/showthr...rnax-globulars
[7] https://www.jstor.org/stable/10.1086...o_tab_contents
[8] https://iopscience.iop.org/article/1...213/ab14f5/pdf
[9] https://arxiv.org/pdf/2105.00064.pdf
[10] https://arxiv.org/pdf/2102.11522.pdf
[11] https://arxiv.org/pdf/1910.11887.pdf
[12] https://www.google.com/books/edition...s/-RoBCAAAQBAJ
[13] http://austinastro.wildapricot.org/r...e/ST201502.pdf (Page 15)
[14] https://groups.io/g/amastro/topic/77...0,0,0,77656115
