NGC 6500 in Hercules
July 2019 - Galaxy of the Month
Although the July skies are now starting to darken after the summer solstice at my latitude of 52N we don’t really get and truly dark skies until mid-August, as such and deep sky object chosen for observation this month must be by necessity rather bright.
Accordingly I have chosen the galaxy pair NGC6500/6501 in Hercules as this month’s challenge. The pair were first discovered by William Herschel in 1799, although they were independently rediscovered by Stephan in 1880.
The galaxies almost certainly form a physical pair. Both are spirals, although NGC 6501 maybe a lenticular, and NGC 6500 shows some distortion of its spiral arms that suggests a tidal interaction.
NGC 6500 is classified as a LINER i.e. it shows emission lines in its spectrum. Sometimes these are from an AGN or some form of starburst. Observations seem to show that NGC 6500 seems to have a bipolar outflow coming out of it, although this is suggested to be more like to be a starburst wind, like that of M82, rather than a wind from a black hole. Observations in the UV suggest that they may be coming from an obscured population of WR stars which would back up the starburst theory.
NGC 6500 along with NGC 6501 and NGC 6467 are included in LGG 414 along with 3 UGC galaxies making a small group of 6 galaxies. The group is probably at a distance of 140 million light-years which would make NGC 6500 about 90,000 light-years across and NGC 6501 about 85,000 light-years across, so very similar sizes and not far short of the size of the Milky Way. However there appear to be some disagreements as to its distance with some sources placing it around 200 million light-years away.
There are numerous much fainter galaxies in the field, although interestingly one of the brighter ones does not seem to appear in the catalogues, probably because it is in the glare of STF 2245, although Howard Banich does draw it in his Object of the Week post at DeepSkyForum and suggests it is about 16th magnitude. He was using a 28” reflector though.
The pair does not make it into either Luginbuhl and Skiff (L&S) or the Night Sky Observer's Guide (NSOG) which is perhaps rather surprising as there are reports of the pair being seen with a 22cm telescope from UK skies.
Nearby in the same medium power field are two much fainter elliptical galaxies NGC 6490 and 6495, both discovered by Marth in 1864 with Lassell’s 48” reflector in Malta. These are much further away and not associated with the NGC 6500 grouping. These will be challenges for telescopes in the 45-50cm telescope class I would think.
Also in the field is the coloured double star STF 2245 for those amongst our members who enjoy all forms of deep sky objects.
As an aside I must apologise for the quality of the Megastar chart. Unfortunately, Megastar has stopped being able to download DSS images due to changes in the URL and it is unclear if this will be fixed so I may have to move to another charting program going forward.
Owen Brazell - Galaxy Section Director