NGC 4939

Background

NGC 4939 was discovered by William Herschel on March 25, 1786. It is a grand design spiral galaxy with two tightly wrapped, thin, smooth arms that can be traced for nearly one and a half revolutions. The galaxy has a large elliptical bulge and possibly a weak bar, and is inclined at 56 degrees, with a rotational speed of about 270 km/s.

The galaxy is classified as a type II Seyfert galaxy, with a bright point-like nucleus. Its X-ray spectrum is consistent with a Compton-thick cold reflection source, meaning the central source is hidden behind dense gas and dust, and the observed X-rays are reflected.

The equivalent width of the FeKα line is large, supporting the Compton-thick interpretation, though a Compton-thin transmission model could not be ruled out. Observations by the Swift Observatory confirmed its Compton-thick nature. The supermassive black hole at the center accretes material at a rate of 0.077 solar masses per year and has been detected in hard X-rays by INTEGRAL.

Five supernovae have been observed in NGC 4939: SN 1968X (mag. 16, discovered by Paul Wild on 27 November 1968), SN 1973J (mag. 16, discovered by Paul Wild on 21 May 1973), SN 2008aw (Type II, mag. 15.9, discovered by LOSS on 2 March 2008), SN 2014B (Type II-P, mag.

17.0, discovered by LOSS on 2 January 2014), and SN 2020nif (Type II, mag. 16.1492, discovered by ZTF on 24 June 2020). NGC 4939 belongs to the NGC 4933 group, a small galaxy group between the Local Supercluster and the Hydra-Centaurus Supercluster.

Did You Know?

• NGC 4939 is about 190,000 light-years across, larger than the Milky Way.