NGC 5557 and Arp 199 in Boötes

June 2020 - Galaxy of the Month

Writing the galaxy of the month piece for June and July is often tricky as, certainly from the latitude of the UK, we never get any astronomical darkness during these months, and this won’t return until August. As such the choice has to be bright galaxies. For this month’s object I have chosen the elliptical galaxy NGC 5557 in Boötes.

Discovered by William Herschel in May 1785, NGC 5557 is part of a group of galaxies catalogued as LGG 378 which includes the edge on spiral NGC 5529, covered in an earlier GOM piece. William on the same night also found the nearby galaxy NGC 5544 but did not spot it was a double nebula, this was left to his son John to find in 1827 when he saw it split in two and the second galaxy was catalogued as NGC 5545, The pair was also independently split by Bindon Stoney, one of Lord Rosse’s assistants in 1852 using the 72” at Birr. The pair is now known as Arp 199, although it was earlier noted to be an interacting pair and was catalogued as VV 210.

Although looking fairly uninteresting to visual observers’ deep images show that in fact NGC 5557 is a shell galaxy with shells of gas and stars showing that in the last few (3-5) billion years or so NGC 5557 has undergone a number of interactions with other galaxies, and indeed merged with them. This is well illustrated in this image by Mark Hanson. Very deep images with the CFHT also show the presence of tidal tails and a number of dwarf galaxies accompanying NGC 5557. It has been suggested that some of these dwarf galaxies may have formed in the tidal tails. It has also been suggested that NGC 5557 may well have been formed from the merger of two spiral galaxies in the recent, astronomical, past.

NGC 5557 is classified either as an E1 or E2 galaxy, so pretty much round as seen from our perspective. Distance estimates vary widely but it would seem to be at about 33 Mpc from us. Interestingly Lord Rosse thought that NGC 5557 was a spiral galaxy, one of his few misjudgements in that area.

Although NGC 5557 is bright enough to show faintly in a 15cm telescope it will probably require something in the 25-30cm size to show its brighter core and surrounding haze, certainly from UK skies. Arp 199 lies perhaps 16’ away from it. I found that with a 37cm telescope under twilight skies that it was fairly easy, although admittedly the galaxy was overhead and not in the best position to view with a Dobsonian telescope. Both NGC 5557 and Arp 199 will appear in the same medium power field, say around 200x.

NGC 5557 is bright enough to have made the Astronomical League’s Herschel 400 list. There is a bright star involved in the halo of NGC 5557 which could be confused with a SN so beware before reporting it. Arp 199 is a very tight pair and much fainter than NGC 5557 so will require larger telescopes and high power to split. The pair consists of two spirals. Arp 199 was covered in the DSF OOTW in 2014.

Owen Brazell - Galaxy Section Director