NGC 5921 in Serpens Caput

June 2021 - Galaxy of the Month

June at the latitude of the UK has to be one of the worst two months for galaxy observing, at least visually, as we do not get astronomical dark until mid-August again. As such any object for the GOM has to be brighter than the usual case.

My choice this month is the 11th magnitude galaxy NGC 5921 in Serpens Caput. NGC 5921 was discovered by William Herschel in 1786. Mitchell using Lord Rosse’s 72” at Birr suggested that it showed spiral structure, although it does not seem to have been added to their list of spiral galaxies.

NGC 5921 is a barred spiral classified as SBbc, although it has also been classified as SB(r)bc, which suggests that the spiral arms may start from a circumnuclear ring. In this case the spiral arms start from the end of the bar but appear to form a ring afterwards. The galaxy has a small bright nucleus with open spiral arms containing a lot of HII regions. The central bar does appear to have a prominent dust lane across it. Interestingly the form of the galaxy depends on what wavelength it is photographed in and only the blue images show spiral structure. The red image just show dust.

The distance measurements seem uncertain and vary between 65 and 80 million light-years. Hubble has imaged the galaxy. There is also a superb amateur image of it by Adam Block. NGC 5921 does appear to contain a supermassive black hole at its centre. If this is the case the galaxy may also be a mild form of AGN known as a LINER.

Unusually NGC 5921 does not appear to be a part of any galaxy group and is a field galaxy. It was home to the Type II supernova 2001x. If the longer distance computations are right then NGC 5921 is a relatively large galaxy at about 100,000 light-years across, about the size of our own Milky Way.

Interestingly the Night Sky Observer's Guide (NSOG) Vol. 2 suggested that you need a 30cm telescope to see it and then it only shows the bright core and a faint haze. NSOG suggests that a 40-45 cm telescope should show some detail in the haze. Observations from the UK suggest that the core at least can be seen in 25cm from moderately dark skies. NGC 5921 is also featured in The Cambridge Photographic Atlas of Galaxies. For those that like working from lists, NGC 5921 is included in the Herschel 3 list: a list of 300 galaxies after the H400 and H400 part II from the Astronomical League.

Owen Brazell - Galaxy Section Director