NGC 7424

NGC 7424
Observation data (J2000 epoch)
Constellation	Grus
Right ascension	22h 57m 18s[1]
Declination	-41° 04′ 14″ [1]
Redshift	0.003132 (939 +/-2 km/s) [1]
Distance	37.5 Mly (11.5 Mpc)[2]
Type	SAB(rs)cd [1]
Apparent dimensions (V)	9.5 x 8.1 arcmin [1]
Apparent magnitude (V)	11.0 [1]
Notable features	similar to the Milky Way
Other designations
PGC 070096
See also: Galaxy, List of galaxies

NGC 7424 is a barred spiral galaxy located 37.5 million light-years away in the southern constellation Grus (the Crane). Its structure and diameter (about 100,000 light-years) make it similar to our own galaxy, the Milky Way.^[3] It is called a "grand design" galaxy because of its well defined spiral arms. One supernova and two ultraluminous X-ray sources have been discovered in NGC 7424.

Characteristics

NGC 7424 is intermediate between normal spirals (SA) and strongly barred galaxies (SB). Other features include the presence of a central ring-like structure and a relatively low core brightness relative to the arms. The redder color of the prominent bar indicates an older population of stars while the bright blue color of the loose arms indicates the presence of ionised hydrogen and clusters of massive young stars. NGC 7424 is listed as a member of the IC 1459 Grus Group of galaxies, but is suspected of being a "field galaxy"; that is, not gravitationally bound to any group.^[4]

Supernova 2001ig

SN 2001ig was a rare Type IIb supernova discovered by Australian amateur Robert Evans on the outer edge of NGC 7424 on 10 December 2001.^[5] Type IIb supernovae (SNe) initially exhibit spectral lines of hydrogen (like typical Type II's), but these disappear after a short time to be replaced by lines of oxygen, calcium and magnesium (like typical Type Ib's and Ic's).

Spiral Galaxy NGC 7424 and SN2001ig. Credit: ESO

On 28 May 2002 Cambridge University astrophysicist Stuart Ryder et al. found what they believe is the binary companion to SN 2001ig. It is a massive O or B class star that had an eccentric orbit around the progenitor, a Wolf-Rayet star. They believe that the companion periodically stripped the outer hydrogen-rich envelope of the progenitor, accounting for the observed spectral changes.^[6][7] Princeton University fellow Alicia Soderberg et al. also believe that the progenitor was a Wolf-Rayet star, but suggest that the periodic mass loss was a result of the intense stellar wind these stars produce.^[8]

Ultraluminous X-ray sources

In May and June 2002 Roberto Soria and his colleagues at the Harvard-Smithsonian Center for Astrophysics discovered two Ultraluminous X-ray sources (ULXs) with the Chandra X-ray Observatory. ULXs are objects that emit tremendous amounts of X-rays (> 10³² watts or 10³⁹ erg/s), assuming they radiate isotropically (the same in all directions). This amount is larger than currently understood stellar processes (including supernovae) but smaller than the amount of X-rays emitted by active galactic nuclei, which accounts for their alternate name, Intermediate-luminosity X-ray Objects (IXOs). The source designated ULX1 was found in a relatively empty interarm region, far from any bright clusters or star-forming complexes, and showed a 75% increase in X-ray luminosity over the course of 20 days. ULX2 was found in an exceptionally bright young stellar complex, and showed an order of magnitude increase over the same time period.^[2]

References

1. ^ ^{a b c d e f} "NASA/IPAC Extragalactic Database". Results for NGC 7424. http://nedwww.ipac.caltech.edu/. Retrieved 26 November 2008. 
2. ^ ^{a b} Soria, R., et al. (August 2006), "Multi-band study of NGC 7424 and its two newly-discovered ULXs", Monthly Notices of the Royal Astronomical Society 370 (4): 1666–1676, arXiv:astro-ph/0606080, Bibcode 2006MNRAS.370.1666S, doi:10.1111/j.1365-2966.2006.10629.x 
3. ^ "Explosions in majestic spiral galaxy beauties". Spaceflight Now. 5 December 2004. http://www.spaceflightnow.com/news/n0412/05spiralgalaxies/. Retrieved 27 November 2008. 
4. ^ Yahil, A., Vidal, N. V. (June 1977), "The Velocity Distribution of Galaxies in Clusters", Astrophysical Journal 214: 348 (last note), Bibcode 1977ApJ...214..347Y, doi:10.1086/155257 
5. ^ Daniel W. E. Green (13 December 2001). "IAU Circular 7772". http://www.cfa.harvard.edu/iauc/07700/07772.html. Retrieved 14 January 2009. 
6. ^ Ryder, S., et al. (April 2004), "Modulations in the radio light curve of the Type IIb supernova 2001ig: evidence for a Wolf-Rayet binary progenitor?", Monthly Notices of the Royal Astronomical Society 349 (3): 1093–1100, arXiv:astro-ph/0401135, Bibcode 2004MNRAS.349.1093R, doi:10.1111/j.1365-2966.2004.07589.x 
7. ^ Ryder, S., et al. (June 2006), "A post-mortem investigation of the Type IIb supernova 2001ig", Monthly Notices of the Royal Astronomical Society 369 (1): L32–L36, arXiv:astro-ph/0603336, Bibcode 2006MNRAS.369L..32R, doi:10.1111/j.1745-3933.2006.00168.x 
8. ^ Soderberg, A., et al. (October 2006), "The Radio and X-Ray Luminous SN 2003bg and the Circumstellar Density Variations around Radio Supernovae", Astrophysical Journal 651 (2): 1005–1018, arXiv:astro-ph/0512413, Bibcode 2006ApJ...651.1005S, doi:10.1086/507571 

Coordinates: 22^h 57^m 18^s, −41° 04′ 14″