NGC 7217 explained

NGC 7217
Credit:	NASA/STScI/WikiSky
Epoch:	J2000
Type:	(R)SA(r)ab^[1]
Dist Ly:	50.0 Mly
Z:	952 ± 2 km/s
Appmag V:	11.0
Size V:	3.9 × 3.2
Constellation Name:	Pegasus
Names:	UGC 11914, PGC 68096

NGC 7217 is an unbarred spiral galaxy in the constellation Pegasus.

Features

NGC 7217 is a gas-poor system^[2] whose main features are the presence of several rings of stars concentric to its nucleus: three main ones –the outermost one being of the most prominent and the one that features most of the gas and star formation of this galaxy –^[2] plus several others inside the innermost one discovered with the help of the Hubble Space Telescope; a feature that suggests NGC 7217's central regions have suffered several starbursts.^[3] There is also a very large and massive spheroid that extends beyond its disk.^[4]

Other noteworthy features this galaxy has are the presence of a number of stars rotating in the opposite direction around the galaxy's center to most of them^[5] and two distinct stellar populations: one of intermediate age on its innermost regions and a younger, metal-poor version on its outermost ones.^[6]

It has been suggested these features were caused by a merger with another galaxy^[7] and, in fact, computer simulations show that NGC 7217 could have been a large lenticular galaxy that merged with one or two smaller gas-rich ones of late Hubble type becoming the spiral galaxy we see today;^[6] however right now this galaxy is isolated in space, with no nearby major companions.^[6] More recent research presents a somewhat different scenario in which NGC 7217's massive bulge and halo would have been formed in a merger and the disk formed later (and is still growing) either accreting gas from the intergalactic medium or smaller gas-rich galaxies, or most likely from a previously existing reserve.^[8]

See also

• Spiral Galaxy NGC 1512
• NGC 7742, a very similar galaxy in the same constellation

External links

• NGC 7217
• http://atlas.zevallos.com.br/

Notes and References

1. Web site: NASA/IPAC Extragalactic Database . Results for NGC 7217 . 2006-11-25 .
2. Verdes-Montenegro . L. . Bosma . A. . 1995 . The three rings of the isolated galaxy NGC 7217 . Astronomy and Astrophysics . 300 . 65 . 1995A&A...300...65V.
3. Combes . F. . 4 . García-Burillo . S. . Boone . F. . Hunt . L.K. . Baker . A.J. . Eckart . A. . Englmaier . P. . Leon . S. . Neri . R. . Schinnerer . E. . Tacconi . L.J. . 2004 . Molecular gas in NUclei of GAlaxies (NUGA). II. The ringed LINER NGC 7217 . Astronomy and Astrophysics . 414 . 3 . 857–872 . 2004A&A...414..857C . 10.1051/0004-6361:20031664. astro-ph/0310652 . 12941641 .
4. Buta . R. . 4 . van Driel . W. . Braine . J. . Combes . F. . Wakamatsu . K. . Sofue . Y. . Tomita . A. . 1995 . NGC 7217: A Spheroid-dominated, Early-Type Resonance Ring Spiral Galaxy . The Astrophysical Journal . 450 . 593 . 1995ApJ...450..593B . 10.1086/176169.
5. Merrifield . M.R. . Kuijken . K. . 1994 . Counterrotating stars in the disk of the SAB galaxy NGC 7217 . Astrophysical Journal . 432 . 2 . 575–589 . 1994ApJ...432..575M . 10.1086/174596. 1887/92276. free.
6. Sil'Chenko . O.K. . Chillingarian . I.V. . Sotnikova . N.Y. . Afanasiev . Victor L. . 2011 . Large-scale nested stellar discs in NGC 7217 . . 414 . 4 . 3645–3655 . 2011MNRAS.414.3645S . 10.1111/j.1365-2966.2011.18665.x. 1103.1692 . 119299160 .
7. Sil'Chenko . O.K. . Moiseev . A.V. . 2006 . Nature of Nuclear Rings in Unbarred Galaxies: NGC 7742 and NGC 7217 . The Astronomical Journal . 131 . 6 . 1336–1346 . 2006AJ....131.1336S . 10.1086/499945. astro-ph/0512431 . 14589261 .
8. Fabricius . M. H. . Coccato . Lodovico . Bender . R. . Drory . N. . Gössl . C. . Landriau . M. . Saglia . R. P. . Thomas . J. . Williams . M. J. . 2009 . Regrowth of stellar discs in mature galaxies: the two-component nature of NGC 7217 revisited with VIRUS-W . Monthly Notices of the Royal Astronomical Society . 441 . 3 . 2212–2229 . 2014MNRAS.441.2212F . 10.1093/mnras/stu694. 1404.2272. 119269888 .