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Getting up close to M66
Posted: 8 April

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This stunningly rich view of the inner portion of the spiral galaxy M66 in the constellation of Leo, captured by the evergreen Hubble Space Telescope, reveals a hooked, distorted spiral arm and a displaced, glowing core, which are both wounds suffered in the gravitational tug of war with its near neighbours, the galaxies M65 and NGC 3628.

The Hubble ACS image of M66, showing unprecedented detail in the galaxy. Click for larger version. Image: NASA/ESA/Hubble Heritage (STScI/AURA)–ESA/Hubble Collaboration.

Collectively this trio of galaxies is known as the Leo Triplet, a favourite target for amateur observers and astrophotographers. Discovered by Charles Messier, it has been long known that M66 displays a slightly odd structure, with a tightly wound eastern arm and a hooked western arm that appears to rise out of the plain of the galaxy’s disc. The chief agitator is NGC 3628, which possesses a faint 400,000 light-year long tidal stream of stars ripped out by M66’s gravity when the two passed close to one another a billion years ago. In return Messier 66’s bulge has been displaced slightly from its centre, making the galaxy lopsided, and it is clear that there is some activity going on around the black hole inside its core, which is being fed gas by the central galactic bar. Some Seyfert type 2 activity (where broad emission lines from a hot accretion disc around the black hole are blocked by obscuring gas and dust) and weak radio galaxy emission has been detected radiating from deep within the core of M66. This is all very typical for a gravitationally perturbed spiral galaxy.

A wide-field view of the Leo Triplet. NGC 3628 is seen at top, M66 at the lower left and M65 to the lower right. Image: NASA/ESA/Digitized Sky Survey 2.

Hubble’s Advanced Camera for Surveys (ACS) is able to pick out long, winding dust lanes in the spiral arms, as well as bright blue and pink knots of young clusters of hot stars, their formation spurred by the gravitational forces of the nearby galaxies.

The Leo Triplet are a varied bunch of spiral galaxies, covering several categories on Edwin Hubble’s tuning fork diagram, which describes the shapes of different types of galaxies. Messier 65 is a type Sa spiral, meaning it has a nicely defined central bulge and neat and smoothly winding spiral arms. Messier 66, as we’ve seen, is a little more ragged around the edges and hence is designated Sb, but with the presence of a bar perhaps SBb (the capital B denoting a bar) would be more appropriate. NGC 3628, seen edge-on, has a weaker core and is designated Sc. Messier 66 is the largest of the three at 100,000 light years across, and the distance between M66 and M65 is just 200,000 light years. Together, the trio lie roughly 33 million light years away.