# NGC 4945

NGC 4945 is a widely-studied barred spiral galaxy in the constellation Centaurus, visible near the optical double star Xi Centauri. It is also known as Caldwell 83. The galaxy was discovered by Sottish astronomer James Dunlop in 1826. It is located at a distance of approximately 11.0 million light-years (3.36 Mpc) from the Milky Way. NGC 4945 hosts one of the closest active galactic nuclei to Earth and is classified as a Seyfert 2 galaxy.
NGC 4945 one of the brightest galaxies of the Centaurus A/M83 Group, a large, nearby group of galaxies. The galaxy is the second brightest galaxy in the subgroup centered on Centaurus A.

## Observations

The morphological classification of NGC 4945 is SB(s)cd?edge, indicating this is a barred spiral galaxy (SB) with no inner ring structure (s) and possibly loosely-wound spiral arms (cd?). It is inclined at an angle of 90° to the plane of the sky, which means the galactic plane is being viewed edge-on. NGC 4945 is thought to be similar to the Milky Way Galaxy, although the maximum rotation rate of ~ 180 km/s is lower. It has a combined estimated mass of 1.4+1.4 −0.7×10$^{11}$ M$_☉$. The stellar mass of the galaxy is 3.8×10$^{10}$ M$_☉$, or 38 billion times the mass of the Sun.
In 1964, this galaxy was classified as a radio source at the Parkes Observatory. The nucleus was obscured in the optical band but was found to be a quite prominent source of infrared emission. It is the third brightest galaxy in the IRAS point source catalogue, with most of the emission coming from the core. Most of the Galaxy shows a linear rotation curve, although the southwest region showed an infall suggestive of a bar.
In 1979, strong H$_2$O maser emission was detected from the central region. The properties of the nuclear region are suggestive of both a Seyfert type galaxy and an intence starburst region with a complex structure. The active nucleus is heavily obscured by dust. These dust lanes approach the nucleus, forming a tightly-wound structure in the inner 650 ly (200 pc). X-ray emission from the nuclear region indicates a Type 2 Seyfert galaxy. It is a strong emitter of hard X-rays, second only to NGC 4151, and likely harbors a supermassive black hole.
The starburst region is thought to be at least 5×10$^6$ years old and contributes at least half of the luminosity coming from the core. It is concentrated in the central 330 ly (100 pc) and includes a conical cavity likely produced by supernovae-driven winds. This is taking place in a region of dense molecular clouds, forming a disk of dust and gas, along with many rich star clusters. A hot wind of gas from the nuclear region is carrying away 1.6 M$_☉$ per year.
NGC 4945 was the first galaxy outside the Local Group to have stars resolved within its galactic halo. The halo mass is relatively large at ~ 3.5×10$^9$ M$_☉$ and it is metal-rich, both of which are typical for a Milky Way-like galaxy. It appears to be counter-rotating compared to the main disk, suggesting the halo has been accreted. The mass of the dominant satellite accreted into the halo is ~ 1.5×10$^9$ M$_☉$, which is roughly the same as the Large Magellanic Cloud.
Two supernovae have been observed in NGC 4945. Type II-P supernova SN 2005af was discovered by CEAMIG/REA Supernovae Search on 8 February 2005, reaching a peak magnitude of 12.8. Type II-P supernova SN 2011ja was discovered by Libert "Berto" Monard on 18 December 2011, and achieved a maximum magnitude of 14. The progenitor star for 2011ja may have been massive at 25 M$_☉$ and located within a massive stellar cluster.

## In popular culture

"NGC 4945" is the title of a song by Brett Domino on the album Funk.