Galaxy of the Month in Libra

• NGC 5903 in Libra

  April 2024 - Galaxy of the Month

  With the month of April comes the real end of the galaxy observing season at higher latitudes as the increase in daylight combined with the move to Daylight Savings Time means we have less hours of useable darkness.

  My choice of galaxy this month is NGC 5903 in Libra. Discovered in 1784 by William Herschel, NGC 5903 is an elliptical galaxy classified as an E2. It is going to be a challenge to observe from the latitude of the UK as it never rises that high above the horizon.

  It forms a non-interacting pair with the elliptical galaxy (E0) NGC 5898 and also a triplet with the lenticular MCG-4-36-7, which is a very much fainter galaxy. It is also part of a small 5 galaxy group catalogued as LGG 398. The group is thought to lie at a distance of perhaps 36 Mpc. More recent observation suggest that there may be as many as 30 members of the group, although most of these are small and faint.

  The faint galaxy MCG-4-36-7 appears to be a strong double lobed radio and X-Ray source.

  NGC 5903 is unusual in that it appears to have a long filament of neutral hydrogen lying across the galaxy. The filament is approximately 100kpc long. Its origin is unknown but it is probable that it was caused by AGN activity in NGC 5903 in the past.

  Both NGC 5903 and NGC 5898 are about half the size of the Milky Way given their distances. Perhaps surprisingly there does not seem to have been much other research done on this group.

  The group is in the Astronomical Leagues Herschel 3 list of mostly faint galaxies. It is also in Alvin Huey’s guide to galaxy triplets and in Miles Paul’s atlas of compact galaxy triplets (downloadable from the Webb Society’s web site).

  Both Luginbuhl and Skiff (L&S) and the The Night Sky Observer's Guide Vol.2 suggest a 20-25 cm scope are needed to see this pair, which may well translate to 25-30cm from the UK given the fact that they never rise that high. The best chances to observe them, as ever, will be when they are on the meridian and with a good light pollution free southern horizon. As both of the main galaxies are ellipticals there will not be much detail to be seen and smaller telescopes may only show the bright cores. Larger ones may show the haze surrounding the core. The triplet is close enough together that they will all fit in the field of view in a high power eyepiece and this may be the best way of finding them as it will increase the contrast between the galaxies and the background sky.

  Owen Brazell - Galaxy Section Director