Galaxy of the Month in Hercules

• NGC 6308 in Hercules

  August 2024 - Galaxy of the Month

  With the month of August we now get true dark skies again from the UK, and I am going to continue in this GOM with the objects discovered by Albert Marth theme from last month, and for this we have a nice line of galaxies discovered by him in Hercules. These are the galaxies NGC 6308, NGC 6314 and NGC 6315. They hold pride of place in Marth’s listing as they were the first galaxies that he discovered using William Lassell’s 48” speculum metal mirror telescope from Malta. All of these objects appear to be spiral galaxies with NGC 6308 and NGC 6315 being almost face on and NGC 6314 edge on.

  NGC 6308 has a bright core and is listed as a Seyfert Type II, a form of AGN. Type II Seyfert galaxies have very narrow emission lines in their spectrum compared to those of type I. It may be that the classification into Type I or type II depends on the angle at which we view the nuclear region. NGC 6308 does not appear to be related to the other two, which may form a physical pair, but is purely a line-of-sight coincidence. NGC 6308 lies about 130 Mpc from us.

  It is suggested that NGC 6314 and NGC 6315 may form a pair at about 71 Mpc from us, however a more recent observation suggests they too are a line-of-sight pair and may be separated by as much as 6 Mpc. It appears that NGC 6314 also contains an AGN of the LINER type. Although NGC 6315 does not appear to contain an AGN it is classified as an emission line galaxy and the GALEX UV image is very strong so it is likely that a lot of star formation is going on here. This would be consistent with its blue colours. Not much research appears to have been done on these galaxies.

  Perhaps as expected with the galaxies that Marth found there are no observations of this group of galaxies in any of the standard references. The group is tight enough that all the galaxies will fit in the field of view of a medium power (260x) hyperwide eyepiece. Steve Gottlieb describes them as fairly faint in a 17.5” telescope so I suspect that 50cm may be required to find them from the UK, although 45cm may do. The galaxies pass very high up as seen from UK latitudes so it should be easy to track the field down.

  Owen Brazell - Galaxy Section Director

  If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Galaxy of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

• NGC 6661 in Hercules

  August 2021 - Galaxy of the Month

  With the return of dark skies in August we can move to some more challenging targets for the GOM. This month I have chosen the faintish pair of galaxies in eastern Hercules, NGC 6661 and NGC 6658.

  Both these galaxies were initially discovered by Albert Marth in 1864 using William Lassell’s 48 speculum metal telescope in Malta. However confusion was added to the area when Lewis Swift using his 16” refractor reported another nebula in the area in 1885 which was recorded as NGC 6660. It was later shown that this object was identical with Marth’s object NGC 6661 so it can have both NGC numbers. Edouard Stephan also independently rediscovered it in 1871. The correction was noted in the notes accompanying the IC1 in 1895 so it is a long-known issue.

  Both of these galaxies are classified as S0-a, so they are lenticular galaxies. NGC 6658 appears to be more like an edge on spiral but shows no spiral arms or dust features. It does however appear to have quite a bright nucleus. NGC 6661 is in range of 25cm from the UK but shows only as a very faint object.

  NGC 6658 and NGC 6661 are suggested to be a non-interacting pair according to the Second Reference Catalogue of Bright Galaxies (RC2). The distance to NGC 6661 is suggested to be about 42Mpc. There is some doubt however about them being a physical pair as NGC 6658 is listed as being part of the galaxy group LGG 421 and NGC 6661 is not. The other NGC galaxies associated with LGG 421 are NGC 6641 and NGC 6669. This would fit with their redshifts being vastly different with NGC 6661 being the further away (or moving much faster) so the pair appear to be just a line-of-sight effect. Interestingly though, as lenticular galaxies you would expect to see them as part of a group, or at the very least a fossil group if the understanding of how lenticular galaxies form is correct. NGC 6658 appears to be the brightest galaxy in LGG 421 but interestingly shows very little radio emission.

  Perhaps not unsurprisingly the galaxy pair is not included in the Night Sky Observer's Guide (NSOG). As the pair is quite tight it should fit within a high-power eyepiece field of view, and indeed because of the faintness of the pair you are probably going to need to use medium to high power in order to boost the contrast enough to see the them. Although as noted a 25cm will just about pick up NGC 6661 I suspect that probably 40-50cm will be needed to see NGC 6658 given its discovery history, although again there are reports of it being seen on the edge with a 25cm telescope from UK skies.

  Owen Brazell - Galaxy Section Director

• NGC 6487 in Hercules

  July 2021 - Galaxy of the Month

  Following on from the June GOM, July is also a poor month for galaxy observing from the UK with summer twilight in full swing. My choice this month is the galaxy NGC 6487 in Hercules as it will hopefully be in the darker part of the sky.

  Both NGC 6487 and its fainter companion NGC 6486 were discovered by Edouard Stephan using the 31” (80cm) silver on glass reflector at Marseilles. NGC 6487 was discovered in 1871 and NGC 6486 6 years later in 1877.

  I find it interesting coming from the UK that the 31” was mounted outside without any form of cover, building wise anyway. Just shows the quality of the weather in Marseilles. This was one of the first large silver on glass reflectors made by Foucault.

  Both the galaxies are elliptical with the classifications just showing a generic E so they have not had more detailed classifications done on them. NGC 6487 would appear to be a head-tail radio source which suggests an AGN of some form, at least in the past. The two galaxies appear to be part of the poor galaxy group WBL 648. The group contains only three galaxies and includes the face on spiral UGC 11017 as part of the trio with NGC 6486 and 6487. NGC 6487 does show a bright core in the UV which suggests some form of activity. UGC 11017 does show a lot of activity in the UV suggesting lots of star formation going on. Otherwise, perhaps not unsurprisingly, there is not much research done on these galaxies. So many galaxies so little time 😊 The group is at a distance of about 120 Mpc so quite a distance out.

  All the galaxies in the group are very close together so using a high-power eyepiece, perhaps of the order of 340x if your telescope will take it, will help when trying to split it. I suspect that NGC 6487 may not be that hard to see but at 15th magnitude NGC 6486 is going to be much more difficult to see, especially in the summer twilight. The other galaxy in the trio, UGC 11017, is nearer to 16th magnitude and close to face on so it is going to be a challenge I think for all but the largest telescopes, and even then only the core will be seen. Perhaps not unsurprisingly the pair does not make the Night Sky Observer's Guide (NSOG) or other standard observing guides.

  Owen Brazell - Galaxy Section Director

• NGC 6269 in Hercules

  August 2020 - Galaxy of the Month

  For the August GOM we move to the constellation of Hercules and the small faint group of galaxies around NGC 6269. NGC 6269 is part of a small group of galaxies in Hercules, including NGC 6263, NGC 6264 and NGC 6265, that were originally discovered by Albert Marth using William Lassell’s 48” speculum metal telescope from Malta. The group were independently rediscovered by Stephan in 1871 using the 30” Foucault reflector at Marseille.

  NGC 6264, NGC 6265 and NGC 6269 are included as part of a 3 galaxy physical group catalogued as WBL 625. The group also appears to be known as AWM 5. NGC 6263 is not part of the group apparently.

  Marth also added two more galaxies found in the field in NGC 6271 and NGC 6722 whilst Stephan added NGC 6261, bringing a total of 7 NGC galaxies within a radius of 15 arc-seconds from NGC 6269. The majority of them lie on a line from NGC 6261 to NGC 6272.

  NGC 6269 itself is an elliptical with NGC 6265 being an S0 and NGC 6264 a spiral. The group around NGC 6269 appears to be embedded in a hot gas component found in X-Rays. It may be that NGC 6265 is a recent addition to the group and is having its gas stripped out by the RAM pressure interactions with the hot gas in the inter group medium as X-Ray observations done with Chandra shows a tail of material being stripped out of it. The NGC 6269 group also shows some of the characteristics of a fossil galaxy group. Fossil groups are where there is a cD galaxy, in this case NGC 6269, which has merged with most of the other galaxies in the group. Although NGC 6269 is not currently an AGN it is suggested that it does contain a billion solar mass black hole and there are suggestions that it was active in the past.

  Distance measurements to the group suggest it is about 120Mpc away. Interestingly the AWM 5 group definition includes a lot more galaxies than the WBL one, including NGC 6271 and 6272. The velocity maps from there suggest that there may be two clumps of galaxies in this cluster.

  Visually at 13th magnitude and fainter these galaxies are likely to be a challenge for any telescopes below 35cm, with NGC 6269 itself showing as a faint smudge. Using a modern hyperwide eyepiece at a medium power of, say 200x, will fit all the galaxies in this field in the same eyepiece FOV. Perhaps unsurprisingly the group does not appear in Night Sky Observer's Guide (NSOG) or the other observing handbooks. Owners of larger telescopes may well find other faint galaxies scattered in this field.

  This will be a challenging field to work on because of the faintness of the galaxies, many being around 15th magnitude, so it will be interesting to see how many can be picked out. It is interesting to note that Steve Gottlieb in his NGC note collection describes the whole group as very faint with his 17.5” reflector. Mark Stuart from the UK reports that NGC 6269 is on the edge with a 14” but does not report any of the other galaxies to be visible.

  Owen Brazell - Galaxy Section Director

• 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

• ACO 2197 Group in Hercules

  May 2018 - Galaxy of the Month

  When we think of galaxy clusters in Hercules people tend to focus on ACO 2151, perhaps better known as the Hercules cluster, and ignore the other two bright(ish) galaxy clusters ACO 2199 and ACO 2197. I have covered ACO 2199 in a very early Galaxy of the Month (GOM) when focusing on its brightest member NGC 6166 so this time we will focus on ACO 2197.

  ACO 2197 is part of the great wall of galaxy clusters which includes the Coma Cluster (ACO 1656), the Leo Cluster (ACO 1367) and the Hercules Cluster (ACO 2151). It lies only a short distance on the sky, about 1.5 degrees, from ACO 2199 and there is evidence of a filament of galaxies linking these two clusters with ACO 2151. ACO 2197 and 2199 along with 2151, 2152, 2147 and 2162 comprise the Hercules Supercluster.

  Classified as a type III cluster on the Bautz-Morgan system ACO 2197 is a fairly irregular cluster containing 11 galaxies catalogued in the NGC so it should perhaps be better known.

  It is covered (as expected) in the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 5 and also on Albert Highe’s web page which includes a detailed list of galaxies in the cluster as well as a finder chart for the central region.

  The brightest three galaxies in the cluster, NGC 6160, NGC 6173 and NGC 6175 were all discovered by William Herschel in 1787. At the same time Herschel also found the fainter galaxies later catalogued as NGC 6146 and NGC 6150. John Herschel also discovered a number of nebulae in the cluster but as these are getting quite faint the other NGC objects were discovered by the Rosse team and Stephan. The Rosse team did discover two other galaxies in this field that did not make it into the NGC.

  ACO 2197 is quite a large cluster in spatial terms covering almost 1.5 degrees in length on the sky. A brief observation of this cluster with Andrew Robertson’s 610mm reflector on a very transparent night suggests that a lot of galaxies may be visible in the field (we had been following galaxies in the ACO 2199 cluster for such a distance I was checking we had not run into the ACO 2197 cluster by mistake).

  As with ACO 2199, ACO 2197 is dominated by a giant cD galaxy, in this case NGC 6173 along with two other giant ellipticals (NGC 6146 and NGC 6160). The cluster is thought to be at a distance of perhaps 126 Mpc. In terms of galaxy numbers ACO 2197 would appear to contain maybe 1500 galaxies or more, so it is quite a large cluster. It may also be interacting with ACO 2199 as the clusters may only be 9.2 Mpc apart in space.

  The cluster appears to form an E-W alignment based on the bright galaxies NGC 6146, NGC 6160 and NGC 6173, a line almost a degree long. As such even with modern wide field eyepieces this cluster is going to take quite a few fields to study. It may however be more tractable than the galaxy fields of Coma. It does seem that in general the galaxies congregate at either end of the cluster around NGC 6146 and NGC 6173.

  

  One of the more interesting galaxies in the group is the double galaxy NGC 6175, which appears to be a spiral and an elliptical almost on top of each other. The pair would appear to be included in the updated VV catalogue as VV 1816, although I am not sure of evidence of interaction. It would be an interesting challenge to see what aperture telescope is needed to split this pair. As always good seeing will be needed as well as good transparency I would think.

  Having said all this it is probably worth noting that the Night Sky Observer's Guide (NSOG) regards the brighter galaxies in this group as challenges for 16/18” telescopes.

  Owen Brazell - Galaxy Section Director

• July 2017 - Galaxy of the Month

  NGC 6548 in Hercules

  This month’s challenge is a pair of galaxies in Hercules. NGC 6548 was first discovered by William Herschel in 1786 and is a lenticular galaxy with a very prominent core and a boxy disk. It is classified as an SB0.

  NGC 6548 appears to be surrounded on deep images by a halo. The halo appears to show shells in it but I cannot find any good quality deep images of this object, perhaps a challenge for our imaging colleagues.

  If it is indeed a lenticular then it is an odd one as normally lenticular galaxies are found in galaxy groups or clusters and this appears to be a field galaxy. However the appearance of the halo and shells indicate that it may have had a dynamically interesting past with mergers so it may have come from there.

  The much fainter edge on spiral nearby was found by Marth in 1864 using Lassell's 48” speculum telescope. It was assigned the number NGC 6549 by Dryer and appears to be a type Sbc.

  Unfortunately the NGC then got itself into a tangle as Stephan though he saw three galaxies here and the third was given the designation NGC 6550. There is no third galaxy and the NGC 6550 designation appears to be randomly assigned to either NGC 6548 or 6549 depending on which source you look at.

  As an example NED says NGC 6548 and NGC 6549 are the same galaxy and incorrectly assigns these numbers to the edge on spiral whilst calling the lenticular NGC 6550. The popular program SkySafari 5 gives the correct designation to NGC 6548 but assigns the primary designation of NGC 6550 to NGC 6549.

  Much of this confusion seems to have come from Lewis Swift's comments in his papers.

  It would appear that the historically correct designation is that NGC 6549 = NGC 6550 but due to a century of confusion the only way to unambiguously determine which is which is to use the PGC number.

  

  Interestingly NGC 6549 has a much fainter galaxy superimposed on it which almost looks like a jet. The two galaxies are an optical pair with NGC 6549 being three times more distant than NGC 6548. NGC 6548 is believed to be at a distance of about 100 million light years whilst NGC 6549 is at about 310 million light years.

  At that distance NGC 6549 would be about the size of our Milky Way with a diameter of about 125000 light years. NGC 6548 is a little smaller with a diameter of perhaps 95000 light years. Unfortunately this confusion means that it is quite difficult to get the correct data and assign it to the correct galaxy.

  Neither NGC 6548 or 6549 appear in Night Sky Observer's Guide (NSOG) or in Luginbuhl and Skiff (L&S) so it will be interesting to see what size telescope is required to make out these galaxies and what can be seen.

  Owen Brazell - Galaxy Section Director

• July 2012 - Galaxy of the Month

  NGC 6166 and Abell 2199 in Hercules

  

  Image Courtesy of Volker Wendel, Stefan Binnewies and Josef Pöpsel. For more images from the Capella Observatory please visit their website. You can click on image for a larger version.

  NGC 6166 and Abell 2199 First discovered by William Herschel in 1791: NGC 6166 is the central galaxy of the galaxy cluster Abell 2199. As galaxies go this is a special case. A giant cD galaxy, NGC 6166 is also a double radio source.

  There is a much fainter NGC galaxy in this group: NGC 6158, which was also discovered by William Herschel but four years before he found NGC 6166. One wonders how he missed the much brighter galaxy the first time around.

  NGC 6166 appears to be a quadruple galaxy with four cores. The four objects however appear to have very different redshifts so they may in fact just be superimposed galaxies. Modelling suggests that there are two elliptical galaxies here and one of them appears to have caused a wake as it passes through the outer envelope of NGC 6166.

  Unusually for an elliptical galaxy there appear to be large dust cloud features in NGC 6166. Unlike the giant elliptical M87 however it does not appear if NGC 6166 has a very large globular cluster system. This is to be expected if, as believed, cD galaxies such as NGC 6166 are built up from mergers.

  If we take the distance to the cluster to be about 410 million light years this give a size of 225,000 lights years for NGC 6166. Despite its brightness compared to the other galaxies in Abell 2199 it is certain that it is a member of the cluster and one of the largest galaxies known.

  Both Abell 2199 and 2197 are part of the great wall of galaxies whose other prominent clusters are the Coma Cluster (Abell 1656), The Leo cluster (Abell 1367) and the Hercules cluster (Abell 2151). Abell 2199 is classed using Abell’s classification systems as 2 1 I. It is also the type cluster for the Bautz-Morgan Type 1 classification. Type 1 clusters are dominated by a single large cD galaxy. Abell 2199 may also be interacting with the nearby cluster Abell 2197 whose centre only lies about 1.5 degrees north.

  The cluster contains at least 88 and probably of the order of 200 galaxies. It is also one of the classic cooling flow clusters. Here the gas in the centre of the galaxy cluster cools rapidly causing hot gas (a few million degrees) to flow into the centre and then cool emitting lots of X-Rays.

  Note that the galaxies here will not be easy targets. NGC 6166 is about 12th magnitude and as such should be visible in a 25cm telescope but to see more galaxies may require a 40 cm or greater. There are good finder charts and some drawings and descriptions of both of these clusters in Volume 5 of the Webb Society Deep Sky Handbooks.

  Owen Brazell - Galaxy Section Director

• May 2011 - Galaxy of the Month

  Abell 2151 - Hercules Cluster

  

  Image Courtesy of Russell Croman, click on image for a larger version. A detailed finder chart for the cluster is available along with a table of the galaxies.

  The constellation of Hercules is perhaps more associated with globular clusters than galaxies but it does contain the spectacular cluster Abell 2151.

  Unlike other galaxy clusters such as Coma (AGC 1656), the Hercules cluster is dominated by spiral galaxies. Many of the galaxies appear to be interacting as well and in many ways Abell 2151 represents what we think galaxy clusters may have looked like in the early universe.

  Abell 2151 is however relatively close with a distance index of 1 and a distance of perhaps 500 million light years. It spans a fairly large area on the sky covering perhaps a degree with maybe a 100 galaxies in that area, the brightest of which are around 13.5 magnitude.

  As such this is going to be a target for larger telescopes, probably in the 16-20" range but some of the brighter ones may be seen with smaller apertures.

  The brighter NGC galaxies in the cluster appear to have had a variety of discoverers from William Herschel through Stephan and Swift, although there are also a collection of IC objects there, the majority discovered by Swift.

  Steve Gottlieb’s observing notes on the cluster can be found on his website. Note however these were made by one of the worlds best visual observers with a large telescope form a very dark site. Visual observations of the clusters are also covered in the NSOG Volume 2.

  Abell 2151 does seem to be a relatively popular target for imagers from the number of images available on the web, as always Google is your friend for finding these although the ones from Tony Hallas and Jim Misti along with Russell’s appear to be some of the best.

  If you find Abell 2151 too easy then the other bright(ish) galaxy cluster in Hercules is Abell 2199 although this is a much more difficult challenge. As always if you observe this cluster then notes on the society newsgroup or to me would be appreciated. If you make this a project then the editor of DSO would love an article as well.

  Owen Brazell - Galaxy Section Director