The Sculptor Group of Galaxies

NGC 300, NGC 247, NGC 55, NGC 253. NGC 7793

Distance: 9 Million Light Years to the center of the group

copyright Robert Gendler

The Sculptor Galaxy group includes five bright galaxies, NGC 7793, NGC 253, NGC 247, NGC 300, NGC 55 and about 20 dwarf galaxies. The Sculptor group is considered the nearest galaxy group to the Local group. Although a defined group, the Sculptor group is really a loose cloud of galaxies forming a cigar shaped filament some 3 by 20 million light years along our line of sight. The Sculptor group, along with the Canes Venatici complex and the local group appear to form a loose filament of the Virgo supercluster that extends over a distance of 33 million light years.

NGC 253 is the dominant galaxy of the Sculptor group and a prototypical starburst galaxy. The central 10,000 light years of this galaxy shows evidence of a violent burst of star formation that began some 30 million ago. Infrared studies have helped to understand inner nature of NGC 253. Contrary to normal galactic cores which contain older population stars, the conspicuous yellow-orange color of the center of NGC 253 originates not from old stars but rather from the dust attenuated light of young massive stars. The starburst region in NGC 253 is very compact with a diameter of 200 light years and is located just southwest of its nucleus. Like other starburst galaxies, NGC 253 shows evidence of a galactic "superwind" phenomenon. NGC 253's superwind is caused by a massive output of energy and matter, much of it in the form of 5 million degree plasma streaming from the center of the galaxy into the intergalactic medium. The ultrahot gases generate intense radiation at x-ray wavelengths which astronomers have detected. The driving force of the superwind is believed to be a collective energy outflow from episodic starbirth, mass loss from unstable stars, and energy released from supernovas. Galactic winds are always coupled to star formation and supernova rates and are a typical phenomenon of starburst galaxies.


NGC 55 is in a relatively isolated environment and probably has not been disturbed by major merger activity in its recent history. Confusion still remains regarding its overall morphology. Conclusions about its morphological type have ranged from barred spiral to Magellenic irregular type. Recent consensus suggests it is a magellenic irregular type with its bar seen end on. There is considerable active star formation occurring in NGC 55 particularly in its central region. Large shell structures and plumes have been identified which are signatures of recent supernovae and stellar winds from massive stars. Recent star formation has almost certainly occurred near the center of NGC 55 as recently as 8 million years ago although a fairly steady production of stars has occurred during the last 100 to 200 million years. Compared to other galaxies, NGC 55 has experienced a remarkably uniform star forming history. NGC 55 is nearby and inclined some 81 degrees so its edge-on disk is favorably oriented for studies of its extraplanar regions and specifically its halo. Several HII regions with embedded clusters of massive stars have been found within the galaxy's halo at distances up to 5000 light years from the galactic plane.


NGC 300 is a bright nearly face-on spiral similar to M33. Because of its proximity and face-on orientation much is known about NGC 300. The outer regions of NGC 300 show a very large mass to luminosity ratio suggesting a massive halo of dark matter sustains the rotational curve of the outer disk. NGC 300 has been the subject of exhaustive searches for extragalactic Wolf-Rayet stars. Wolf-Rayet stars (WR) are of interest to astronomers as they represent a late evolutionary phase of very massive O type giants with masses greater than 25 suns. They are extremely hot and luminous with temperatures exceeding 50,000 degrees K. and some with luminosities approaching one million suns. WR stars are a rare and exotic breed of stars, dominated by helium rather than hydrogen. Only one in ten million stars is a WR star. Astronomers search for WR stars in other galaxies by detecting specific helium emission lines. Perhaps the most defining feature of Wolf-Rayet stars is their immense stellar winds. The fierce stellar winds can drive extraordinary mass loss rates of one millionth to one hundred thousandth of a solar mass per year (equivalent to an earth sized planet), a rate 3 or 4 time greater than typical O type giants and 10 billion times stronger than our suns stellar wind.


NGC 247 is a well developed dwarf galaxy and a member of the Sculptor group which contains several other prominent members such as NGC 253, NGC 300, NGC 7793 and NGC 55. Dwarf galaxies are defined by their small size and mass and are the most numerous type of galaxies within groups and cluster, exceeding the number of spirals by a factor of ten. In the standard scenario of cosmic evolution, galaxies are built up via hierarchical merging of smaller galaxies. Dwarf galaxies being the most numerous type in the universe are thought to represent simple primordial galaxies unchanged over billions of years. As simple systems they are thought to play a major role in galaxy formation as essential building blocks. Recent observations show that contrary to previous notions about dwarf galaxies, they appear to possess complex structures that on a smaller scale rival larger disk galaxies like the Milky Way. Their stellar and gas content and extended halo structures suggest that dwarf galaxies have had their own complex evolution and formation histories and in turn have formed from lower mass precursors.

NGC 7793 is a spiral galaxy about 12.7 million light-years away in the constellation Sculptor. It was discovered in 1826 by James Dunlop. NGC 7793 is one of the brightest galaxies within the Sculptor Group, a group of galaxies in the constellation of the same name (from Wikipedia). The group itself is an elongated, loosely bound group of galaxies with the Sculptor Galaxy (NGC 253) and its companion galaxies forming a tightly-bound core of galaxies near the center. The Sculptor group, along with the Canes Venatici complex and the local group appear to form a loose filament of the Virgo supercluster that extends over a distance of 33 million light years.