Galaxy of the Month in Aquarius

• NGC 7585 in Aquarius

  September 2020 - Galaxy of the Month

  The small group of galaxies around NGC 7585 (NGC 7576 and NGC 7592) in Aquarius were all discovered by William Herschel. He found NGC 7585 and NGC 7592 in September 1784 but NGC 7576 had to wait until the following year in October 1785 to be picked up, although Herschel did not spot that NGC 7592 was a double galaxy.

  NGC 7585 was later added by Halton Arp to his catalogue of Peculiar galaxies as Arp 223 and by Vorontsov-Velyaminov to his interacting galaxy catalogue as VV 1973. Arp did not spot that NGC 7592 was also an interacting pair although Vorontsov-Velyaminov did and it is classified as VV 731.

  NGC 7585 appears to be a shell galaxy although Arp catalogued in his group of galaxies with amorphous spiral arms. The suggestion is that it is the result of the merger of two galaxies. Its current classification is fairly complex as (R’) SA0 +(s) pec. The suggestion is it is probably a lenticular galaxy.

  The three galaxies in the field do not seem to be related, although NGC 7576 is also a disturbed galaxy, possibly a rare ring one. NGC 7592 is a much more distant object. NGC 7585 itself maybe 145 million light-years away or so. NGC 7576 is, by some measurements, at a similar distance. The RC2 suggests they are a non-interacting pair. NGC 7576 does show up quite brightly in the ultraviolet GALEX survey which normally suggests some kind of star formation activity. Only the nucleus of NGC 7585 shows up in the UV. Both galaxies show up well in the infrared WISE images. Hubble has looked at NGC 7585 and in the near infra-red views with the NIC instrument a strong bar appears to show up.

  Perhaps surprisingly given the nature of the group not much individual research appears to have been done on them. NGC 7592 on the other had has had a lot of work done and the interacting pair have been well imaged in various wavebands by Hubble and shown to be Seyfert type AGN’s. Unfortunately I can’t find a colour processed version of the Hubble images of NGC 7592.

  Observational from UK latitudes these galaxies will be a challenge as they do not rise much above the 30 degree altitude line and will be best observed when on the meridian. Although Luginbuhl and Skiff (L&S) does suggest that NGC 7585 will be visible in good skies with a 25cm Night Sky Observers Guide (NSOG) Vol 1 suggests they are more of a challenge for 30cm+. UK observations of NGC 7585 with 25cm suggest it is not very impressive. I suspect that probably 40cm plus would be needed to see NGC 7592 well and to split them perhaps 55cm plus. None of these galaxies were bright enough to make it into the Herschel I or II lists although NGC 7585 did make it into the Herschel 3 list. The NGC 7585/7576 pair are close enough together to make it into the same field of a modern hyperwide eyepiece at perhaps 260x.

  The Arp 223 pair was also covered in the DeepSkyForum (DSF) Object of the Week for October 20th.

  Owen Brazell - Galaxy Section Director

• September 2017 - Galaxy of the Month

  Hickson 88 in Aquarius

  For September's GOM we stay in the constellation of Aquarius and look at the galaxy group Hickson (HCG) 88.

  This group unfortunately does not rise that high from the UK, only getting above one airmass in September so it may be a challenging target. HCG 88 is however one of the brighter Hickson groups, although that is not saying much.

  The main group consists of four spiral galaxies NGC 6975, NGC 6976, NGC 6977 and NGC 6978, although the classification of the edge on galaxy (NGC 6975) is uncertain, it could be a barred spiral.

  NGC 6975 was discovered by Bigourdan in 1886 whilst NGC 6876, NGC 6877 and NGC 6878 were discovered by Marth in 1863 and 1864 using Lassells 48” speculum metal telescope from Malta. Bigourdan also thought he had discovered two other galaxies in the area, NGC 6973 and 6980, but these turned out just to be stars that looked nebulous in poor seeing, something we have all had issues with 😊

  Strangely SkyTools plots NGC 6980 as a cluster in the same field. I am not sure where that information came from.

  The group appears to be physically interacting with the usual signs of distortions and tidal tails visible in deep images.

  In the original Hickson catalogue the group was identified as having four members but deep imaging has added two more to the group. Redshift surveys suggest that there may even be more members.

  The group appears to have a very low velocity dispersion therefore any interactions cannot have been between the main members of the group as the crossing time is equal to the Hubble time (age of the Universe).

  There does appear however to be some controversy about the interaction state of this group. It is probable that HCG 88 is incorporated in a loose group of galaxies (not unusual for Hickson groups). That group may be UGCl 458, a poorly studied galaxy cluster that was also noted by Zwicky in the CGCG. The group is at a distance of about 84 Mpc from the redshift data.

  NGC 6978 itself is also classified as a LINER (a form of AGN).

  The group is likely to be fairly challenging to observe and may need apertures of the order of 40cm or more to see much, except from very dark skies. I have seen observations however that suggest that the cores of the brighter galaxies may be seen with smaller instruments.

  The main group appears as a nice triple but my suspicions are that from typical UK skies a 50cm telescope may be needed to see NGC 6975. It would seem that medium to high power are best to use when observing this object. As always try when the group is within a couple of hours of the meridian. There is also the 6th magnitude star 4 Aqr nearby so you will need to use reasonable magnification to keep that out of the field.

  Owen Brazell - Galaxy Section Director

• August 2017 - Galaxy of the Month

  NGC 6962 in Aquarius

  August and astronomical dark finally returns to the UK. For this month’s GOM I have chosen the galaxy group around NGC 6962 in Aquarius.

  NGC 6962 and its companion NGC 6964 were first discovered in 1785 by William Herschel (and not as some Wikipedia articles have it John). John Herschel then remeasured the positions later on. The situation in the area was then confused by observations by Lord Rosse and his team at Birr and Bigourdan. They discovered as many as 5 new nebulae in the field and Bigourdan getting confused managed to add two IC objects, that both turned out to be stars.

  The whole story of the galaxies in this area is discussed by Harold Corwin in his notes on the NGC and IC catalogues. The original Rosse observations are attached to this piece as a PDF to give an idea of what their observations looked like. Remember that Rosse used GC (Herschel’s General Catalogue) numbers so the object that they knew as GC 4601 is equivalent to our NGC 6962.

  NGC 6962 is probably the centre of a group of perhaps 7 galaxies listed as WBL 666. There are suggestions the group may contain up to 28 galaxies, however most of these are likely to be dwarfs discovered on the SDSS.

  The core of the group has a number of peculiar E and S0 galaxies that suggests evidence of past interactions. The group is likely to be quite old and shows evidence of having a core halo structure. NGC 6962 is also a mild form of AGN as it is listed as a LINER.

  The group is likely to be challenging to see visually and, whereas NGC 6962 and NCG 6964 are probably going to be visible in telescopes of perhaps 30cm-40cm aperture, I think to see the other galaxies in the group (those discovered by Rosse, well technically by Mitchell, using the 72”) may require telescopes of 50cm plus and good skies. The Night Sky Observer's Guide (NSOG) has NGC 6962 and NGC 6964 as targets for 40cm telescopes. It does not have any observations of the other galaxies in the area.

  NGC 6962 itself is perhaps 180 million light years away and it had a supernova in 2002ha quickly followed by another in 2003dt. The arms that show up so well in images are very faint and it is likely that only the core of the galaxy will be visible. NGC 6292 is likely to be an intrinsically large galaxy. It has quite a complex morphological classification as SAB (r)ab and appears to be the only obvious spiral in the group, the others being lenticular or elliptical. That is unless UGC 11626 is part of the group.

  Owen Brazell - Galaxy Section Director

• August 2016 - Galaxy of the Month

  NGC 7727 in Aquarius

  This interactive image of the NGC 7727 was provided by the Digitised Sky Survey using Aladin Sky Atlas. You can download a finder chart for this galaxy and another for the wider group.

  NGC 7727, also known as Arp 222, is a face on spiral galaxy in Aquarius.

  First discovered by William Herschel in 1785, the galaxy has numerous star streams and plumes associated with it which are probably the result of a merger with another spiral galaxy about 1 billion years ago. It may take another billion years for this merger to settle down. It was this odd shape that led to its inclusion in Arp’s catalogue of peculiar galaxies. The support for the merger scenario comes from the fact that there are two star like objects near the core of NGC 7727, one of which may be the core of the merging galaxy and the other the main core of NGC 7727.

  NGC 7727 does not seem to have a large reservoir of hydrogen gas to form new stars so it is probably going to become an elliptical galaxy in the future. Images from GALEX, an ultraviolet satellite, show very little star formation going on at the current time in NGC 7727 compared to its neighbour NGC 7724.

  Deep images such as those at Kent Biggs' website, the Sedona Stargazer Observatory gallery and captured by the Chilean Advanced Robotic Telescope 32 inch show the tails and plumes in the galaxy very well. The second link also shows an enhanced version of the core area which shows the dual cores as well.

  NGC 7727’s classification of SAB(s)a pec also gives an idea of its jumbled state. The globular cluster system of NGC 7727 is also strange and it appears to contain 25 young globular star clusters which also support the merger hypothesis. Deep Hubble images show a string of dust clouds, perhaps remnants of spiral arms, in projection across the front of the galaxy. Hubble legacy images can be obtained from their website if you want to do your own processing.

  NGC 7727 is also potentially part of a small group of galaxies listed as LGG 480, which also includes the nearby NGC galaxies 7723 and 7724, along with a couple of MCG galaxies, MCG-2-60-7 and MCG-2-60- 10. The reason I suggest that this group assignment may not be correct is that although NGC 7727 and 7723 are at approximately the same distance of 27 Mpc, NGC 7724 is listed at 37 Mpc in NED. The recession velocities however are fairly close.

  The Night Sky Observer's Guide (NSOG) reports all the galaxies should be visible in a 20-22cm telescope but to see detail you may need 35cm telescope. NGC 7727 is also in the Herschel 400 list from the AL. A medium power field, say 200x centred on NGC 7727 should show NGC 7724 as well using a modern hyperwide eyepiece (100 AFOV). Modern visual observations of the galaxy can also be found on the Deep Sky Observer's Companion – the online database (DOCdb).

  Owen Brazell - Galaxy Section Director