Object of the Week April 8, 2018 - The forgotten Trio Holmberg 173 - NGC 3165/3166/3169

NGC 3165, Holm 173C
RA: 10h 13m 31.4s
Dec: +03° 22’ 30”
Magnitude: 13.9v

NGC 3166, Holm 173A, KPG 228A
RA: 10h 13m 45.5s
Dec: +03° 25’ 23”
Magnitude: 10.5v

NGC 3169, Holm 173B, KPG 228B
RA: 10h 14m 14.7s
Dec: +03° 28’ 01”
Magnitude: 10.3v

In the neighbourhood of the famous galaxies (galaxy-groups) in Leo, the interacting galaxy group Holm 173 aka NGC 3165/3166/3169 is often forgotten. In the northern field of the constellation Sextans the brightest two galaxies are almost a target for the smallest apertures. Larger telescopes shows much detail and so theses galaxies stands not back against their northern colleagues.

The brightest two members NGC 3166 and NGC 3169 were discovered in 1783 from Wilhelm Herschel. NGC 3165 followed in 1856 by William Parsons. In 1937 the Swedish astronomer Erik Holmberg summarized all three mentioned galaxies as a trio [1937AnLun...6....1H]. Karachentsev concentrated on the two brightest members NGC 3166/3169 for the entry in his compilation of galaxy pairs [1972AISAO...7....3K]. Arp, Zwicky, Markarian or Vorontsov-Velyaminov misses to catalogue the group or any individual galaxy.

The similar red shifts (distances ~70 million light-years) and the distorted shapes indicates the gravitational interaction. Also an observed bridge of neutral hydrogen (HI) between the galaxies confirms the common bond between the trio.

The challenges for the galaxies are so different as the used apertures. While small apertures shows the galaxies as small fuzzy patches, mid size telescopes shows the different shapes of all three galaxies as dark lanes in NGC 3169 or the faint bar of NGC 3166. Giant telescopes could perhaps show the extremely faint outer envelope of NGC 3169 or the faint companion galaxy NW of NGC 3166.

DSS blue 20'x20'
Attachment 2973

sketch: 4" binocular telescope, 55x, NELM 6m5+
Attachment 2970

sketch: 14.5", 202x, NELM 6m5+, Seeing III
Attachment 2971

sketch: 27", 172x, NELM 6m5+, Seeing III
Attachment 2972

Give it a go and let us know!

I can add a 4.3" drawing from 2002 Fabruary. No real details were visible. Only star-like nuclei surrounded by round bright central parts and very faint elongated outer parts in both galaxies. NGC 3166 is brighter and bigger. It would be time to revisit them with a bigger scope...

4.3", f/7.3 Newtonian, 96x, 25' field. Sorry for the low quality scan.
Attachment 2975

Uwe, the NGC numbers are mistyped in the data section.

Thanks Peter for the typo.

Nice sketch. Cool, that you can pick up the elongation of the galaxies.

Hi Uwe,

I observed them this weekend. This galaxy trio reminded me of the NGC 2964 / 2968 / 2970 trio, which was object of the month 03/2018 at the German astrotreff forum. Two brighter galaxies and one smaller and fainter galaxy.
In my 12" Dobsonian at 275x magnification and wide-angle eyepiece all the 3 of them fit into the field of view together.

NGC 3169
I saw a stellar nucleus, surrounded by an oval glow and a diffuse outer region that was rather diffuse at its northwestern side. At the eastern side it was sharper.

NGC 3166
Its surface brightness declined gradually from center to outer regions. Surrounded by a diffuse glow.

NGC 3165
Diffuse, elongated and slightly brigther at the center.

I also observed NGC 3156, which is located approximately 25 arc minutes southwest of the Holmberg 173 trio. According to its radial velocity it seems to be gravitationally tied to the trio. So is it actually a galaxy quartet?


Clear skies

Robin