NGC 3432 in Leo Minor

April 2023 - Galaxy of the Month

As we move into the brighter spring skies, I have stayed in the constellation of Leo Minor for this month’s GOM.

NGC 3432 was first discovered by William Herschel in 1785 and was included by Halton Arp in his atlas of peculiar galaxies as Arp 206. It was also added as VV11 in Vorontsov-Velyaminov’s catalogue.

NGC 3432 appears to be a barred spiral galaxy seen edge-on and is interacting with its neighbour the dwarf galaxy UGC 5983. This interaction appears to be causing intense star formation in NGC 3432 as well as filaments between them. The GALEX satellite image in the UV shows that star formation is occurring throughout the galaxy. Due to the amount of dust the core of the galaxy is not well seen.

Interestingly despite all the star formation going on NGC 3242 is not classified as a starburst galaxy. This maybe because the number of really massive stars being formed is not that high yet. It is suggested that the interaction with UGC 5983 took place very recently, say less than 400 million years ago and that is not enough for a full starburst to get going, in fact the star formation rate is unusually low at the moment. It appears that both ends of NGC 3432 are strongly warped from the gravitational attraction.

The pair is thought to lie about 30 million light-years from us. NGC 3432 was home to an unusual transient event catalogued as SN 2000ch, initially catalogued as a type II supernova it was later shown to be an LBV like Eta Car as it underwent numerous brightening events.

There is a Hubble image on NASA's website, although a more normal coloured image by the HST can be seen on NGC 3432’s Wikipedia page.

Interestingly the interaction does not appear to have sparked that much star formation in UGC 5983. There is an odd condensation though in NGC 3432 off to its northern end. At about 55,000 light-years across NGC 3432 is a relatively small galaxy. UGC 5983 is only 12-15 thousand light-years across and as such is a dwarf galaxy, about the size of the LMC. Observations suggest that NGC 3432 may hide an AGN of the LINER type at its core.

The galaxy makes it into a number of popular observing lists including the Astronomical Leagues H400 list and Stephen O’Meara’s The Secret Deep.

The visibility of the galaxy seems to depend on the observer’s location and the companion galaxy UGC 5983 appears to be very difficult to spot so that is the challenge for larger telescope observers.

The Night Sky Observer's Guide (NSOG) suggests that in 20-25cm telescopes the galaxy is visible as a long spindle, which is backed up by observations from the UK. In 40-45cm telescopes the suggestion is that it will show some of the mottling in the spindle. Unfortunately, there are no large telescope observations in NSOG. There are also observations of the galaxy with a wide range of telescopes on the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 4.

I suspect that using a medium power may be best to see this galaxy, although if the seeing and transparency conditions are good then high power may tease out UGC 5983.

Owen Brazell - Galaxy Section Director

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