Deep Sky Object of the Month - the Leo Trio

Original article appears in GuideStar April, 2017.

by Stephen Jones

Objects: M65, M66, and NGC3628 (aka the Leo Trio)
Constellation: Leo
Type: Spiral Galaxies
Magnitudes: M65 – 9.3, M66 – 8.9, NGC3628 – 9.5
Discoverers: M65 and M66 – Charles Messier, 1780; NGC 3628 – William Herschel, 1784
Equipment necessary:  A small telescope should show them from a dark site; larger scopes needed for finer detail

One of my favorite groups of galaxies visible in the spring sky is this fine trio of bright galaxies near the hind-quarters of the constellation Leo (that is, the eastern part of the constellation).  They are quite easy to find, about halfway in between Theta (θ) and Iota (ι) Leonis (see chart). 

With a small telescope and low power, you may be able to fit all three galaxies in the same field.  With larger scopes and higher powers, this may not be possible, but instead the fun is in the detail visible in the individual galaxies.

M65 is a lovely spiral galaxy, fairly evenly bright.  You can see a bright central nucleus fading gradually outward.  It is seen as oval in shape, because we are seeing the relatively flat spiral galaxy at an inclination of around 30 degrees from edge-on.  Of the three galaxies, M65 has the least obvious detail for visual observers, though some have reported seeing dust lanes in scopes over 16”. 

M66 is the brightest galaxy of the three, right next to a quite obvious y-shaped pattern of fairly bright stars.  It appears oval in shape, with a fairly bright nucleus in the center and fainter outer areas.  In larger scopes, you may be able to tell that the brightness distribution is a little uneven;  Photographs reveal that the reason for this is that one of the main spiral arms is brighter and extends a bit further than the other.  Astrophysicists theorize this is due to a past close encounter with NGC 3628. 

NGC 3628, though the faintest galaxy in the group, is perhaps the most interesting to look at.  It is almost perfectly edge-on to us, so we do not see a bright round nucleus or spiral arms here; instead, we see a big dust lane right down the middle.  Examining the galaxy with high power in a larger scope brings out the dust lane very clearly.  I recently spent a good bit of time on this galaxy with the C14 in the observatory.  I noted that compared to other edge-on galaxies, this one appears very noticeably asymmetrical, with the dust lane looking perhaps slightly warped.  This also shows evidence of the past encounter between this galaxy and M66. 

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