Galaxy of the Month Archive 2023

In this series of articles we draw your attention to galaxies particularly worthly of an observer's time.

• Hickson 40 in Hydra

  February 2023 - Galaxy of the Month

  After last months really rather hard challenge we have something slightly easier this month in the galaxy group Hickson 40, also known as Arp 321 and VV 116, in Hydra.

  Although this may be challenging because of its altitude from northern climes it is a really nice tight group. Consisting of three spirals, an elliptical and a lenticular galaxy this is one of the tightest groups in the Hickson catalogue. All the galaxies appear to contain compact sources, suggested to be supermassive black holes, at their centres. The size of the group is so small that all five galaxies could fit in a volume less than twice the size of our Milky Way galaxy.

  Although such compact groupings are regularly found in large galaxy clusters Hickson 40 appears to be a rather isolated field group. There actually appear to be 7 galaxies in the group but only 5 are easily seen. The group is located perhaps 300 million light-years away and is in the process of undergoing a merger to form one large elliptical galaxy. This may take place in a billion years or so.

  Hubble imaged these galaxies as part of its 32 birthday study. The group was classified by Arp as part of his class of galaxy groups.

  This group is going to be challenging for observers as none of the galaxies are bright enough to have an NGC or IC designation and the low altitude as seen from the UK, although it does crawl above the 30 degree line, will not help either.

  The brightest galaxy of the group is MCG 1-25-9 at 13.8 B so it should be visible in 30-40cm scopes from a dark sky. The rest of the group is somewhat fainter. The group is so tight that you should try using the highest power your telescope and the conditions will allow as this is also going to be the best when trying to tease out the details of the fainter members of the group.

  Unfortunately, despite being a Hickson group it does not appear in the Night Sky Observer's Guide Vol. 2. There is some information on the group on Reiner Vogel’s Hickson page.

  Owen Brazell - Galaxy Section Director

  If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Galaxy of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

• NGC 1740 in Orion

  January 2023 - Galaxy of the Month

  For this month’s challenge I am going very deep into Orion. I am going to throw this out there and apologise that the challenge may be too extreme. The challenge is the faint galaxy pair NGC 1740 and NGC 1753.

  NGC 1740 was discovered in 1830 by John Herschel using an 18.3 speculum metal telescope. NGC 1753 had to wait another 50 years or so for Lewis Swift to find it using a 16” refractor in 1889, I suspect at this date from the Lowe observatory on Echo Mountain. Unfortunately it seems that confusion arose in the area and it appears that there is some conflict between the NGC numbers 1740 and 1742 due to an error in John Herschel’s positions. NGC 1742 is almost certainly just a star. It was found by Ball using Lord Rosse’s 72” at Birr.

  NGC 1740 is classified as a lenticular galaxy S0 at a distance of perhaps 59 Mpc. NGC 1753 is classified as a spiral SBa (pec) with a redshift derived distance of 57 Mpc so they may be a physical pair. Perhaps unsurpingly for such non-descript galaxies there is not much written on them. Both galaxies do appear surprisingly bright in the GALEX images which suggests that there is some star formation going on, which would be surprising for a lenticular galaxy as most of the star forming gas is normally stripped out of these.

  Holmberg thought NGC 1740 was a double nebula with NGC 1742 and classified it as Holmberg 84 in his catalogue of double and multiple galaxies. Unfortunately, as noted above NGC 1742 does not exist. It is possible that Barrachi using the 40” Great Melbourne telescope found these two galaxies independently.

  I have to say both of these are going to be very challenging objects and probably a bit faint really for any spectacular views. The altitude is also not going to help as seen from the UK but they do rise above 30 degrees. The galaxies are relatively close together and will be in the field of view of a modern widefield eye at a power of 265x. I suspect that using a high power to increase the contrast is probably going to be the only way to pick up NGC 1753. Using a medium power eyepiece may also bring the galaxy NGC 1729 into the field. This is also a faint object though.

  Steve Gottlieb suggests that even with his old 17.5” both these galaxies were no more than faint smudges. Perhaps unsurprisingly they are not in the Night Sky Observer's Guide (NSOG) Vol. 1.

  Owen Brazell - Galaxy Section Director