NGC 4945 is a prominent near edge-on southern galaxy whose physical appearance is dominated by thick obscuring dust which hides its nucleus. The galaxy is part of the Centaurus group which contains another famous galaxy NGC 5128 (Centaurus A). NGC 4945 is a remarkable galaxy in that it is both a starburst and a Seyfert type II galaxy. In fact it has one of the most heavily obscured Seyfert nuclei known.
NGC 4945 is one of three nearby starburst galaxies which includes NGC 253 and M82. Starburst galaxies are characterized by high infrared luminosities, a sign of the immense light and radiation released from its young massive stars. Although it cannot be observed directly because of obscuring dust and an edge-on orientation, astronomers have good evidence for a central bar within NGC 4945. A central bar provides a very efficient means of transporting gas inward towards its nucleus thus setting the stage for the formation of nuclear starbursts. The stars are sufficiently old enough to have enriched the central medium with heavier elements and exotic organic compounds. The starburst ring lies within a compact 300 light year torus (donut shaped structure) of molecular gas that surrounds the nucleus. The visible light of the starburst is heavily blocked by the dust and gas however its presence is revealed at infrared wavelengths. Superwinds of energetic matter from the powerful starburst have carved out an enormous conical cavity within the central gas that stretches some 1630 light years across along the galactic minor axis.
The central region of NGC 4945 shows a high level of energy output at both infrared and x-ray wavelengths which cannot be explained solely by the nuclear starburst. Studies at X-ray wavelengths have revealed a hard x-ray source supporting the existence of an active galactic nucleus, specifically a Seyfert type II nucleus (see M77). The nucleus is obscured at optical wavelengths by thick dust and an opaque molecular gas cloud driven inward from the winds of the starburst ring. The various contributions of both the AGN and the nuclear starburst to the overall energy output of the nucleus remains controversial but each generates substantial energy outflow making NGC 4945 a remarkably energetic galaxy. Velocity measurements of rotating matter in the nucleus points to a supermassive black hole of 1.6 million solar masses as the central engine of the energetic Seyfert nucleus.
NGC 4945 is also known for the first water maser discovered in a galactic nucleus. The water maser has been detected at radio wavelengths to within 3 light years of the center of the galactic nucleus of NGC 4945. A maser is an acronym for "Microwave Amplification by Stimulated Emission of Radiation". Similar to lasers they represent amplified energy emission phenomena and can be created artificially on earth. However when occurring naturally in space a maser is usually a sign of shocked gas either in star forming regions or within a circumnuclear disk surrounding a supermassive black hole. Within NGC 4945 the water maser traces a small circumnuclear disk surrounding its one million solar mass black hole. The shocked gases produce both the temperatures and density required for water production within ambient cloud material.