Anne’s Picture of the Day: The Sculptor Galaxy

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February 25, 2013

The Sculptor Galaxy, a bright intermediate spiral galaxy

Image Credit: ESO/IDA/Danish 1.5 m/ R. Gendler, U. G. Jørgensen, J. Skottfelt, K. Harpsøe

The Sculptor Galaxy (also known as the Silver Dollar Galaxy or NGC 253) is a bright intermediate spiral galaxy of about 70 thousand light-years across that lies some 11,4 million light-years away in the southern constellation Sculptor, while it is moving away from us at approximately 243 kilometers per second.

It is the largest and brightest member of the Sculptor Group of Galaxies, one of the nearest groups of galaxies to the Milky Way. The Sculptor Galaxy together with the companion galaxies NGC 247, PGC 2881, PGC 2933, Sculptor-dE1, and UGCA 15 form a gravitationally bound core near the center of the galaxy group. Most of the other galaxies in the Sculptor Group are only weakly gravitationally bound to this core.

We see the Sculptor Galaxy almost edge-on, and although very bright, this galaxy is also very dusty. In addition to the dust lanes along its spiral arms, tendrils of dust seem to be rising from the galactic disk. This galaxy also shows clumpy gas clouds, and young, luminous star clusters.

The Sculptor Galaxy is known as a starburst galaxy for its current high rate of star formation, one result of which is its superwind, a stream energetic material spewing out from the center of the galaxy out into space. Another result of the starburst is the creation of several super star clusters.

The central 10,000 light years shows evidence of a violent burst of star formation that began some 30 million ago. The purple light comes from that frenzy of star formation, while the yellowish colour is created by dust lit up by young, massive stars. (Contrary to normal galactic cores which contain older population stars, the core of the Sculptor Galaxy consists of young massive stars.)

The super star clusters are lying in this central region: two with masses of 50 thousand solar masses; one with a mass of 1,5 million solar masses; and a heavily obscured one with 14 million solar masses and an age of around of around 5,7 million years. The last mentioned super cluster is rich in Wolf-Rayet stars.

Wolf-Rayet stars are extremely rare and short-lived super-hot stars which start their lives with dozens of times the mass of our Sun, but rapidly loose most of it through a powerful stellar wind, with speeds up to 2000 km/s. They are also highly luminous, from tens of thousands to several million times the luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays. Wolf-Rayet stars are probably progenitors of long-duration Gamma Ray Bursts. Often occurring in binary systems, they are doomed, within a few million years, to explode as supernovae.

Star formation is also high in the northeast of the Sculptor’s disk, where a number of red supergiant stars can be found, and on its halo are young stars present as well as some amounts of neutral hydrogen. This, along with other peculiarities found on the Sculptor Galaxy, suggests that a gas-rich dwarf galaxy collided with it 200 million years ago, disturbing its disk and starting the present starburst.

The Sculptor Galaxy is also known to be a strong source of high-energy x-rays and gamma rays, likely due to a number of intermediate black holes all within a 3000 light-year radius of the galactic center.

Although supernovae are generally associated with starburst galaxies, only one supernova has been detected within the Sculptor galaxy (in November 1940). The supernova, named SN 1940E, is located southwest of the galaxy’s nucleus.

This image combines observations performed through three different filters with the 1.5-meter Danish telescope at the ESO La Silla Observatory in Chile.

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