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Two challenging galaxies in Leo.

By midnight in February the prominent winter constellations are starting to slip into the west, to be replaced by those of spring. And while Orion dominated the December and January skies it is Leo who takes centre stage as the months move on. Its distinctive shape makes it one of the most conspicuous of the constellations, although whether it looks particularly like a lion is open to debate. Leo is home to numerous galaxies, with something to suit everyone, from the spectacular NGC 2903 that Messier and his colleagues could have discovered (see Observers' Forum 120(2), 2010) to the distant galaxy cluster Abell 1367. Two galaxies that are challenging for both the visual observer and the imager are Leo I and Leo II. These dwarf spheroidal galaxies, which are members of the Local Group, were only discovered in 1950 by US astronomers Robert Harrington and Albert Wilson examining the Palomar Observatory Sky Survey (POSS) plates. Interestingly, 60 years on discoveries are still being made on these plates showing what a valuable resource they remain.

Dwarf spheroidal galaxies are similar in many respects to elliptical galaxies. They show little evidence of nebulosity or recent star formation and contain few heavy elements. In fact deep images of these two Leo galaxies make them look remarkably like low-density resolved globular clusters. However, due to their closeness it has been possible to examine them in detail, and the range of stellar populations is greater than in a globular, showing evidence of star formation over an extended period, although none in recent times. In 2007 the 8.2m Subaru telescope on Mauna Kea, Hawaii observed Leo II and detected over 80,000 stars down to a visible magnitude of 26. The mass to light ratio in these galaxies is also larger than in a globular cluster, suggesting a large dark matter component.

Of the two galaxies, Leo I (UGC 5470) is the easier to locate, in fact it is probably the easiest galaxy in the entire sky to locate as it lies just 20 arcmin north of mag 1.4 Regulus (alpha Leonis) at RA 10h 08.5min and Dec +12[degrees] 18.5' (2000.0). Seeing the galaxy however is entirely another matter. At a distance of just 900,000 light years it presents a size on the sky of 9.8x7.4 arcmin and although it has a visual magnitude of 9.8 the large size means that its surface brightness is extremely low. This, coupled with the proximity to Regulus, makes it a very challenging visual target.

Choice of magnification is important as it is essential to put Regulus out of the field of view while at the same time having a large enough field so that there is dark sky around the galaxy. Do not expect to see any individual stars--the best that can be hoped for is a slight change in the background glow as the galaxy moves through the field. All the tricks of deep sky observing at the limit will be necessary, such as full dark adaptation, averted vision and a pristine dark sky site, along with a telescope in the half-metre class. Even in a 64cm (25-inch) Dobsonian from high altitude in Tenerife the Director and Owen Brazell found Leo I only appeared as a circular patch of haze, very slightly brighter than the background sky.

[ILLUSTRATION OMITTED]

Due to the proximity of Regulus Leo I is also a tricky photographic target and the only image in the Section archives was obtained by Grant Privett using a 10-inch (25cm) f/4.4 Newtonian under poor seeing and conditions of heavy dew in 2007 February. The resulting composite image shown here consists of 80x30s individual frames stacked and processed in batches of 20. The bright flare is from Regulus, accentuated in this case by the poor atmospheric conditions. An excellent David Malin image from the Australian Astronomical Observatory (AAO), along with accompanying text, is available at http:// www.aao.gov.au/images/captions/aat051. html. A Russell Croman image of Leo I also featured as Astronomy Picture of the Day on 2006 June 19. This is a fascinating image as it shows both Regulus and the galaxy in the same field of view.

Leo II (UGC 6253) is slightly closer to us than Leo I at around 760,000 light years. It is located at the rear of Leo and lies 1.6[degrees] almost due north of Zosma (delta Leonis) at RA 11h 13.5m and Dec +22[degrees] 09.2'. With a magnitude of 11.9 and a size of 10.1x9.0 arcmin it is a more challenging target than Leo I and I am not aware of any visual observations from the UK. US astronomer Phillip Harrington describes his observation of it in his book Cosmic Challenges (Cambridge University Press, 2011). Grant Privett has also photographed this galaxy from his home in Fovant, Wiltshire and his image (20x4min sub images) using the same equipment as for Leo I is shown here.

The 2007 image from the Japanese Subaru is available online at http://www.naoj.org/ Pressrelease/2007/11/28/fig01_h.jpg. It is interesting to compare this Subaru image with Grant's--one from an 8.2m telescope on the summit of Mauna Kea and the other from a 0.25m telescope in a back garden in Wiltshire.

Stewart L. Moore, Director, Deep Sky Section
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Title Annotation:Observers' Forum
Author:Moore, Stewart
Publication:Journal of the British Astronomical Association
Date:Feb 1, 2011
Words:902
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