Astron. Astrophys. 364, 479-490 (2000)

Decoupled nuclei and nuclear polar rings in regular spiral galaxies

NGC 7217 ^*

O.K. Sil'chenko ^{***  1} and V.L. Afanasiev <sup>****  2

1</sup> Sternberg Astronomical Institute, University av. 13, Moscow 119899, Russia
² Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia

Received 21 July 2000 / Accepted 26 September 2000

Abstract

The regular isolated Sab galaxy NGC 7217 has been studied with the Multi-Pupil Fiber Spectrograph of the 6m telescope of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) in two spectral ranges, the blue one including the strong absorption lines MgI and FeI and the red one including the emission lines H and [NII ]6583. We confirm the existence of a circumnuclear gaseous polar disk with a radius of 3" which we reported earlier. The same area, with a radius of 3" -4", elongated orthogonally to the line of nodes, is distinguished by high values of the Lick index and shows a Mg/Fe ratio lower than solar. This implies that there were at least two discrete star formation bursts in the circumnuclear region with a temporal separation of a few Gyrs. We relate this pair of bursts to the complex structure of the global brightness profile of the galaxy, which may be decomposed into three exponential segments with different scalelengths.

Key words: galaxies: kinematics and dynamics galaxies: structure galaxies: spiral galaxies: evolution galaxies: nuclei galaxies: photometry galaxies: individual: NGC 7217

* Partly based on observations collected with the 6m telescope of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (RAS) which is operated under the financial support of Science Department of Russia (registration number 01-43) and on data from the ING Archive and the HST Archive.
** Isaac Newton Institute of Chile, Moscow Branch
*** UK Astronomy Data Centre, Guest Investigator
**** Isaac Newton Institute of Chile, SAO Branch

Send offprint requests to: O.K. Sil'chenko (olga@sai.msu.su)

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. A magnesium distinct nucleus and a Fe-rich circumnuclear disk
• 4. Kinematics of stars and gas in NGC 7217
  • 4.1. Circumnuclear region
  • 4.2. The whole galaxy
• 5. Surface photometry and central structure of NGC 7217
  • 5.1. The central region of NGC 7217
  • 5.2. The whole galaxy
• 6. Conclusions and discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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