Velocity field and physical conditions in the active lenticular galaxy NGC 3998

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Abstract
The velocity field of the emission line elliptical/lenticular galaxy NGC 3998 has been measured using a total of seven long-slit spectrograms in five position angles. A rotating and expanding, flattened distribution of gas is suggested. The kinetic energy of the expanding material is 1053–1054 erg, indicating pronounced nuclear activity, a conclusion also indicated by the flat spectrum radio source and X-ray emission in the nucleus. Spectro-photometry of the nucleus of NGC 3998 has revealed emission line strengths characteristic of shocks rather than photoionization. The shocks are probably driven by the outflow and the nuclear activity. A post-shock temperature of 6 × 104 K and pre-shock density of 25 particles cm−3 are indicated by the observed line ratios. The mass of ionized gas within 0.2 kpc radius is 2 × 105M⊙. Both the stars and the gas have central velocity dispersions of 260 km s−1. The high rotation velocity of the stars suggests that NGC 3998 is a lenticular rather than an elliptical galaxy. The mass within the 20 arcsec (1.2 kpc) maximum radius of the rotation curve is ∼ 1010M⊙ and the mass-to-B-light ratio is ∼ 3.

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