NGC 1399

Lore & Background

NGC 1399 is a type-cD galaxy with a bright center and a vast, diffuse envelope. As an early-type galaxy, it is much older than spiral galaxies and consists mostly of old, red-colored stars, with very little star formation. The lack of star formation appears to start at the center and slowly propagate outward. The galaxy hosts a supermassive black hole of 510 million solar masses, a factor of over 2 below the correlation of black hole mass and velocity dispersion. Its core shows a dramatic signature of central tangential anisotropy, with velocity profiles on adjacent sides 0.5 arcseconds from the nucleus showing strong bimodality and a large drop in dispersion. The best-fit orbital model suggests a ratio of tangential to radial internal velocity dispersions of 3, the largest seen in any galaxy to date.

The galaxy is exceptionally rich in globular clusters, with an estimated population between 5,700 and 6,500. It has been proposed that NGC 1399 acquired many of these clusters through gravitational interactions with NGC 1404. The bimodal color distribution of the globular cluster system and the different radial distributions of blue and red clusters extend to large distances from the parent galaxy. The azimuthal distribution shows asymmetries that might be understood in terms of tidal stripping of globulars from NGC 1387. The areal density profile of blue clusters agrees well with a projected dark-matter NFW density profile. An ultraluminous X-ray source (ULX) was found in one of NGC 1399's globular clusters in 2010. Evidence initially suggested a white dwarf star torn apart by an intermediate-mass black hole, but a 2019 study disproven this proposal, as the source has remained X-ray bright far longer than theoretically possible in that scenario.

Reader's Guide

NGC 1399 is significant as the central galaxy of the Fornax Cluster, the second richest cluster within 100 megaparsecs. Its role as the central galaxy of the main subgroup places it at the gravitational and dynamical heart of the cluster. The galaxy's globular cluster system is one of the richest known, and its study has provided insights into galaxy interactions, tidal stripping, and dark matter distribution. The supermassive black hole at its core, with its extreme tangential anisotropy, offers a unique laboratory for understanding black hole growth mechanisms. The galaxy's spectrum, studied with Herschel, showed that contrary to previous belief, most giant elliptical galaxies contain plenty of cold gas, but NGC 1399 is an exception—its feedback cycle from the central black hole appears to be at an advanced stage, with jets having cleared almost all trace of cold gas. This makes NGC 1399 a key object for studying how black hole feedback can halt star formation in massive galaxies. The discovery of ultracompact dwarf galaxies surrounding NGC 1399, first found in the Fornax Cluster in 2003, further underscores its central role in the cluster's evolution.

Did You Know?

• The galaxy has an estimated 5,700 to 6,500 globular clusters.
• An ultraluminous X-ray source was found in one of its globular clusters, but its true nature remains a mystery.
• The galaxy is considerably larger than the Milky Way, with a diameter of about 365,000 light-years.