NGC 4945

The morphological classification of NGC 4945 is SB(s)cd?edge,^[4] 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.^[4] NGC 4945 is thought to be similar to the Milky Way Galaxy, although the maximum rotation rate of ~ 180 km/s is lower.^[12] It has a combined estimated mass of 1.4+1.4
−0.7×10¹¹ M_☉.^[6] The stellar mass of the galaxy is 3.8×10¹⁰ M_☉, or 38 billion times the mass of the Sun.^[12]

In 1964, this galaxy was classified as a radio source at the Parkes Observatory.^[13] The nucleus was obscured in the optical band but was found to be a quite prominent source of infrared emission.^[14] It is the third brightest galaxy in the IRAS point source catalogue, with most of the emission coming from the core.^[15] Most of the Galaxy shows a linear rotation curve, although the southwest region showed an infall suggestive of a bar.^[16]

In 1979, strong H₂O maser emission was detected from the central region.^[17] The properties of the nuclear region are suggestive of both a Seyfert type galaxy and an intense starburst region with a complex structure.^[18] 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).^[7] 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,^[15] and likely harbors a supermassive black hole.^[19]

The starburst region is thought to be at least 5×10⁶ years old and contributes at least half of the luminosity coming from the core.^[20] It is concentrated in the central 330 ly (100 pc) and includes a conical cavity likely produced by supernovae-driven winds.^[21] This is taking place in a region of dense molecular clouds,^[22] forming a disk of dust and gas, along with many rich star clusters.^[23][24] A hot wind of gas from the nuclear region is carrying away 1.6 M_☉ per year.^[25]

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 ~ 3.5×10⁹ 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 ~ 1.5×10⁹ M_☉, which is roughly the same as the Large Magellanic Cloud.^[12]

In October 2023, researchers using ALMA discovered an unknown object around 200 light years from the center of the galaxy. This object, named Punctum (Latin for "point" or "dot"), is a highly-polarized millimeter continuum source of synchrotron radiation. Punctum has a high non-visible luminosity, with archive data from Chandra reporting a 1×10³⁷ erg s^-1 X-ray luminosity in the 3–6 keV range and ATCA data reporting a radio luminosity of 5×10³⁵ erg s^-1 at 23 GHz. In a paper published in July 2025 about the object, the researchers said it most resembles a magnetar due to its polarization of 50%±14%, however the millimeter luminosity of said objects is usually much lower.^[26]

Two supernovae have been observed in NGC 4945:

• SN 2005af (Type II-P, mag. 12.8) was discovered by CEAMIG/REA Supernovae Search on 8 February 2005, reaching a peak magnitude of 12.5 on 12 February.^[27][28][29]
• SN 2011ja (Type II-P, mag. 14) was discovered by Libert "Berto" Monard on 18 December 2011, and achieved a maximum magnitude of 11.7 on 9 January 2012.^[30][31][32] The progenitor star for 2011ja may have been massive at 25 M_☉ and located within a massive stellar cluster.^[33]

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

• 
  The location of NGC 4945 (labelled in red)