1 Attachment(s)
Object of the Week January 13th - The Eridanus Trio
Eridanus Triplet - NGC 1618, 1622 and 1625
NGC 1622
R.A.: 04h36m36.6s (2000)
Dec.: -03°11'20" (2000)
Magnitude: 13.40 B
Size: 3.0'x 0.5'
Although this trio has popped up in the discussions around the NGC 1637 OOTW at the end of 2018 I had already been researching it for this OOTW
so hopefully people will not mind resubmitting their observations to tie in here.
NGC 1622, along with NGC 1618 and 1625 form a nice triplet just north of the bright star Nu Eri. The discovery history of the group is quite interesting
as they are all relatively bright galaxies. NGC 1618 was found by William Herschel in 1786, NGC 1625 by John Herschel in 1827 whilst re-observing his
fathers’ discoveries. NGC 1622 was discovered by George Johnstone Stoney whilst using Lord Rosse’s 72” at Birr in Janaury 1850. NGC 1622 was also
independently discovered by d’Arrest on the 1st Jan 1862 using the 11” Refractor at Copenhagen, no New Year’s Eve’s parties for him! Given the brightness
of NGC 1625 you would have thought that perhaps William Herschel could also have picked up it as well, although 1622 may have been to faint for him.
Perhaps scattered light from Nu Eri impacted on his observations. John Herschel originally thought that the discoveries by Stoney and d’Arrest were two
separate objects and added them into the GC as such (numbers 881 and 878). Dreyer spotted the error and merged both into NGC 1622. The galaxies
may form a physical system as they all have similar redshifts. All three are spiral galaxies with an inclination close to edge on, with the exception of
NGC 1618 which is rather wider open. None of the galaxies show any obvious signs of interactions, although the outer disk of NGC 1622 does appear
to show quite a strong warp. All the galaxies in the group appear to be barred spirals which is relatively unusual. Of the three it would appear that
NGC 1618 is the one most actively forming stars at this time. NGC 1625 also seems to show a slight warp in its disk and also appears to have a galaxy
superimposed on its disk. Given the radial velocity of the object it may well be a dwarf galaxy associated with NGC 1625. The group would appear to be
about 220 million light years away. The group is tight enough that it will fit in the field of a modern hyperwide eyepiece at medium power. The NSOG
suggests that it is a tough target for 8-10” telescopes but should be easy in 14-18” ones. I suspect the main challenge seeing the galaxies here is
going to be keeping the 4th mag star Nu Eri out of the field whilst looking for the galaxies, a similar challenge to that with NGC 404 and Mirach in And.
If these are not enough of a challenge then the compact galaxy group Hickson 30 lies only 20’ north of NGC 1622. Hickson 30 contains no NGC galaxies,
although the brightest galaxies are around 14th mag so it should be visible in larger scopes. Unfortunately from my latitude at 51.5N the Hickson group
galaxies are always to far into the crud for me to pick up.
Attachment 3302
As always give it a go and lets us know
