Hickson 90 in the Southern Fish

hcg90_hst_ — The image shows NGC 7173 (left), NCG 7174 (middle right), and NGC 7176 (lower right). NGC 7174 is a dusty spiral galaxy stretched and distorted by the close encounter, the other two galaxies are large and gorgeous elliptical galaxies. Image credit ESO/Hubble

25 Oct 2016

The Hickson Compact Groups are fascinating to observe – and they offer a really nice range of observations – from easy bright targets (such as Hickson 44 in Leo and Hickson 68 in Canes Venatici) to the targets that are challenging deep sky targets – the realm of the very large aperture telescopes. Hickson 90 in Piscis Austrinus is one of the brighter Hickson groups – a beautiful, albeit small and faint compact grouping of four galaxies that lie in a lovely field sprinkled with stars. My 10″ Dobs was helped along by prime galaxy observing conditions – an incredibly dark, clear, steady Kalahari night, the only disturbance to the air the sound of a gazillion barking geckos going about their nocturnal business in the red sand dunes.

Canadian astronomer Paul Hickson published a catalogue of 100 Compact Groups of Galaxies in 1982, based on his examination of the Palomar Observatory Sky Survey (POSS) red plates. In order to qualify for inclusion in the catalogue a compact group had to meet three rigid criteria: 1) to be compact; 2) to have at least four members in a certain magnitude range; and 3) to be isolated, thus excluding groups belonging obviously to the larger dense galaxy clusters.

Observations

10″ f/5 Dobs; 144x, 230x and 208x

NGC 7172 (Hickson 90A)

RA 22 02 01.5 Dec -31 52 14 Mag 11.9 Size 2.5′ x 1.4′ Surf Br 13.1 PA 100°

This galaxy shows as a small, fairly bright grey oval of silky diffuse light, elongated WNW-ESE. Averted vision reveals it has a very slightly brighter nucleus; a drop of brighter glow against the soft oval glow. A small ash-white mag 11 star lies 2.5’ to the southeast.

NGC 7173 (Hickson 90C)

RA 22 02 03.0 Dec -31 58 25 Mag 12 Size 1.2′ x 0.9′ Surf Br 12.1 PA 143°

Located 6′ due south of NGC 7172 this galaxy shows as a small, round, faint glow of soft light.

NGC 7174 (Hickson 90D)

RA 22 02 05.8 Dec -31 59 33 Mag 13.3 Size 2.3′ x 1.2′ Surf Br 14.3 PA 88°

NGC 7176 (Hickson 90B)

RA 22 02 08.7 Dec -31 59 24 Mag 11.4 Size 0.8′ x 0.8′ Surf Br 11.0

Only 1.5’ southeast of NGC 7173, this contact pair shows as a small, faint silvery glow. Knowing that they are interacting galaxies allows one to discern the two, albeit only just. NGC 7176 on the western end appears as the brighter of the two faint galaxies, a small, round glow of smooth light. NGC 7174, merged on the SW side shows as a very small, very faint and smooth slick of soft light. It is astounding to stare at the faint smooth glow with the beautiful image of it dusty, stretched and distorted in one’s imagination.

I enjoyed sketching this group of galaxies. There is nothing like sketching a group of galaxies 100 million light years away to challenge one’s sense of spacial awareness! Plus there is the bonus: Looking back at a sketch – nothing but small grey pencil smudges in a pencilled circle – evokes, like Proust’s memory-laden madeleines, the same emotion I experienced when I held the object in the eyepiece.