June 2015 - Galaxy of the Month

NGC 6745 in Lyra

Surprisingly a galaxy that William Herschel missed, NGC 6745 was discovered in 1875 by Edouard Stephan using a 31" silver on glass reflector.

He described it as vF, IE, ns. This does not suggest a promising target but in fact it can be seen with much smaller telescopes. Made famous by the Hubble image it was described as a bird eating its prey.

There are two galaxies catalogued here in NGC 6745A and B. The larger galaxy NGC 6745A has suffered a direct collision with the smaller galaxy NGC 6745B which is now leaving the scene rather than just gravitationally interacting with it. A galactic hit and run ☺. This has stirred up a large amount of star formation in NGC 6745 but little in the other.

Many sources suggest there are three galaxies at this location but I think there are only two currently with the large area of enhanced star formation from the collision being listed as the third object. It maybe that in a 100 million years or so that this may be classified as a triple system because the large area of star formation does appear to be moving away from the larger galaxy and could then be classified as a compact dwarf galaxy. This large area of young massive star clusters may have been formed from a large knot of gas that was pulled from NGC 6745.

The pair is estimated to be about 206 million light years (63.5 Mpc) away. The blue starburst may be only 10 million years old or so. The total collision time is likely to be of the order of hundreds of millions of years. Although the galaxies collided directly it is unlikely that any stars directly collided Instead what we see is that gas and dust that had been stirred up by the collision is now forming a new generation of stars in a form of starburst. The line of blue white stars traces the path of the smaller galaxy through the larger.

NGC 6745 used to be a spiral galaxy but the collision has severely distorted its form. I find it interesting that this pair did not make Halton Arp’s famous catalogue of peculiar galaxies. There are observations of the galaxy on the Deep Sky Forum.

Interestingly there is a small chain of MCG galaxies about 20 arcminutes north east of NGC 6745. See the accompany chart and image from the SDSS. The brightest two galaxies are at 15.5 magnitude and the third at about 17 magnitude so these will be targets for large telescopes I think.

Owen Brazell - Galaxy Section Director