NGC 833 in Cetus

November 2020 - Galaxy of the Month

NGC 833 is part of a quartet of galaxies including NGC 833, 835, 838 and 839 that together make up the galaxy group Hickson 16.

All four were discovered by William Herschel on the same night in November 1785. NGC 833 is also included in the Vorontsov-Velyaminov extended catalogue of interacting galaxies as VV 1007. The original VV catalogue had only 852 members but was extended by his students by adding an extra 1162 systems. The group is also known as Arp 318 and that may consist of the 4 galaxies in the Hickson 16 group as well as NGC 848, which appears to be at the same distance as the others.

NGC 848 is somewhat fainter than the others and was discovered by Ormond Stone in 1885 and then independently by Swift in 1886. NGC 848 appears to have undergone strong gravitational interactions as well. Arp described it as a part of his group of galaxies with faint, diffuse streamers, peculiar galaxies.

The group is at a distance of about 160 million light-years. All of the galaxies in the group show signs of interactions and most of the galaxies in the group are mild AGN’s either LINER’s or Seyfert 2 types. There are suggestions that a number of galaxies in the group may have undergone major merger events in the relatively recent past.

The group is also part of a larger conglomeration of seven galaxies catalogued as LGG 49 which also includes NGC 873 as well, which I am slightly surprised about as it is at least 2 degrees away from the others on the sky.

NGC 838 and 839 are probably star burst dominated galaxies as they show no signs of the expected continuum of an AGN in X-Ray observations. Observations with the XMM—Newton satellite however suggested that NGC 839 is probably an obscured AGN. Hickson 16 is thought to have one of the highest concentrations of starburst/AGN activity in the local universe. Interestingly all the galaxies in the group appear to be spirals.

As one of the brighter Hickson groups the core group of HCG 16 should be visible with difficulty in 20-cm but relatively easily in say 37-cm. The galaxy group, including NGC 848, is pretty tight so should be visible in a medium power hyperwide eyepiece, say 250x. The Night Sky Observer's Guide (NSOG) suggests that perhaps 30-cm is required to see them well, although there is no mention of NGC 848 so this may be more difficult to find, perhaps not a surprise as it was missed by Herschel. NGC 835 is the brightest member of the group.

For observers from the southern UK the group only just makes the 30 degree line so it is best to catch when within two hours of the meridian.

The group (as Hickson 16) made the Deep Sky Forum (DSF) forum object of the week in 2014. There are also observations of the group at Adventures in Deep Space. For owners of large telescopes there are a number of other faint and probably unrelated galaxies in the field.

Owen Brazell - Galaxy Section Director