A Spiral Snowflake

Image: The spiral galaxy NGC 6814. Credit:

ESA/Hubble & NASA. Acknowledgement: Judy Schmidt (Geckzilla)

Spiral galaxies together with irregular galaxies make up approximately 60% of the galaxies in the local Universe. However, despite their prevalence, each spiral galaxy is unique - like snowflakes, no two are alike. This is demonstrated by the striking face-on spiral galaxy NGC 6814, whose luminous nucleus and spectacular sweeping arms, rippled with an intricate pattern of dark dust, are captured in this NASA/ESA Hubble Space Telescope image.

NGC 6814 has an extremely bright nucleus, a telltale sign that the galaxy is a Seyfert galaxy.

Seyfert galaxies account for about 10% of all galaxies and are some of the most intensely studied objects in astronomy, as they are thought to be powered by the same phenomena that occur in quasars, although they are closer and less luminous than quasars. These galaxies have supermassive black holes at their centers which are surrounded by accretion discs of in-falling material. The accretion discs are believed to be the source of the observed ultraviolet radiation. Ultraviolet emission and absorption lines provide the best diagnostics for the composition of the surrounding material.

Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, it becomes clear that the luminosity of their cores is of comparable intensity to the luminosity of whole galaxies the size of the Milky Way.

The luminous heart of NGC 6814 is a highly variable source of X-ray radiation, causing scientists to suspect that it hosts a supermassive black hole with a mass about 18 million times that of the Sun.

As NGC 6814 is a very active galaxy, many regions of ionised gas are studded along  its spiral arms. In these large clouds of gas, a burst of star formation has recently taken place, forging the brilliant blue stars that are visible scattered throughout the galaxy.

Links

• A spiral snowflake (SpaceTelescope)
• Seyfert Galaxy (Wikipedia)