NGC 4494
Elliptical galaxy with debated dark matter content and a nuclear dust ring.
NGC 4494 is an elliptical galaxy in the constellation Coma Berenices, about 45 million light-years from Earth. Spanning roughly 60,000 light-years across, it was first observed by William Herschel in 1785.
At its center lies a supermassive black hole with an estimated mass of 26.9 ± 20.4 million solar masses, inferred from velocity dispersion measurements. Observations by the Hubble Space Telescope have revealed a symmetric nuclear dust ring, with a semimajor axis of 0.6 arcseconds (about 60 parsecs at the galaxy’s distance). The ring’s symmetry suggests that dust has settled following a merger with a relatively gas-rich galaxy, which likely also caused the galaxy’s core to be kinematically decoupled—a common feature in elliptical galaxies formed through mergers.
NGC 4494 is unusually faint in X-rays, as seen by the XMM-Newton observatory—nearly 100 times fainter than galaxies of similar optical brightness. This has been attributed to a lack of both dark matter and hot gas. The amount of dark matter in its halo remains debated. Some studies describe the galaxy as “naked,” with a low central dark matter density and an unusually low dark matter fraction of 0.32 ± 0.12 at five effective radii. Other analyses, however, find a higher fraction of about 0.6 ± 0.1 at the same distance, with particularly high dark matter content inside roughly three effective radii. Attempts to tightly constrain the halo mass have had only partial success.
NGC 4494 belongs to the NGC 4565 group, named after the spiral galaxy NGC 4565, which also includes NGC 4525, NGC 4562, NGC 4570, NGC 4725, and NGC 4747. It is also a member of the Coma I Group, part of the larger Virgo Supercluster.
Quick Facts
- Epoch
- J2000
- Ra
- 12 · 31 · 24.1
- Dec
- +25 · 46 · 31
- Dist Ly
- 45 ± 10 Mly (13.7 ± 3.2 Mpc)
- Z
- 1342 ± 5 km/s
- Appmag V
- 9.7
- Constellation Name
- Coma Berenices
- Names
- UGC 7662, MCG +04-30-002, PGC 41441
Facts from the source article.
Lore & Background
NGC 4494 was discovered by William Herschel in 1785. The galaxy hosts a supermassive black hole with an estimated mass of 26.9 ± 20.4 million solar masses, based on velocity dispersion. Observations by the Hubble Space Telescope revealed a nuclear dust ring with a semimajor axis of 0.6 arcseconds, corresponding to 60 parsecs at the galaxy's distance. The ring is symmetric, suggesting that dust has settled after a merger with a relatively gas-rich galaxy that led to its creation. The core of NGC 4494 is kinematically decoupled, a feature common in elliptical galaxies and likely resulting from a galaxy merger.
In X-ray observations by XMM-Newton, the galaxy is quite faint, nearly two orders of magnitude fainter than galaxies with similar optical luminosities. This has been attributed to a lack of dark matter and hot gas. The amount of dark matter in the halo of NGC 4494 has been debated. Some studies characterize the galaxy as 'naked,' while others find an unexpectedly low central density or an unusually low dark matter fraction of 0.32 ± 0.12 at 5Re. Attempts to set strict constraints on the halo mass have had only partial success. In contrast, other analyses find a dark matter fraction of about 0.6 ± 0.1 at 5Re, with particularly high dark matter fraction inside about 3Re.
NGC 4494 belongs to the NGC 4565 group, named after the spiral galaxy NGC 4565, along with NGC 4525, NGC 4562, NGC 4570, NGC 4725, and NGC 4747. It is also a member of the Coma I Group, which is part of the Virgo Supercluster.
Reader's Guide
NGC 4494 holds significance primarily for the ongoing debate about its dark matter content. Its faintness in X-rays, nearly two orders of magnitude below galaxies of similar optical luminosity, suggests a deficiency in both dark matter and hot gas. This has led to conflicting measurements of its dark matter fraction: some studies report a low fraction of 0.32 ± 0.12 at 5Re, while others find a higher fraction of about 0.6 ± 0.1 at the same radius, with a particularly high fraction inside about 3Re. The galaxy's nuclear dust ring, symmetric and settled, indicates a past merger with a gas-rich galaxy, and its kinematically decoupled core further supports a merger origin. As a member of both the NGC 4565 group and the Coma I Group within the Virgo Supercluster, NGC 4494 provides a case study for understanding dark matter distribution in elliptical galaxies and the role of mergers in shaping galaxy structure. The unresolved nature of its dark matter halo makes it a subject of continued interest.
More in Galaxy Clusters and Groups, Part 4 1-24
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