Steve Gottlieb
November 12th, 2016, 02:14 AM
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 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1077970/pdf/pnas01610-0017.pdf) 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 (http://www.jstor.org/stable/10.1086/316164) "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.
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 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1077970/pdf/pnas01610-0017.pdf) 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 (http://www.jstor.org/stable/10.1086/316164) "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.