NGC 7331: The Galaxy with the Multicomponent Central Region

Abstract

We present the results of the spectral investigation of the regular Sb galaxy NGC 7331 with the Multi-Pupil Field Spectrograph of the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The absorption-line indices Hβ, Mgb, and Fe are mapped to analyze the properties of the stellar populations in the circumnuclear region of the galaxy. The central part of the disk inside ~3'' (200 pc)—or a separate circumnuclear stellar gaseous disk as it is distinguished by decoupled fast rotation of the ionized gas—is very metal rich, rather young (~2 billion yr old), and its solar magnesium-to-iron ratio evidences for a very long duration of the last episode of star formation there. However, the gas excitation mechanism now in this disk is shocklike. The starlike nucleus probably experienced a secondary star formation burst too: its age is 5 billion yr, much younger than the age of the circumnuclear bulge. But [Mg/Fe] = +0.3, and only solar global metallicity implies that the nuclear star formation burst has been much shorter than that in the circumnuclear disk. The surrounding bulge is rather old, 9–14 billion yr, and moderately metal poor. The rotation of the stars and gas within the circumnuclear disk is axisymmetric, although its rotation plane may be slightly inclined to the global plane of the galaxy. Outside the circumnuclear disk the gas may experience noncircular motions, and we argue that the low-contrast extended bulge of NGC 7331 may be triaxial.