We present HST/STIS spectra of the Sbc spiral galaxy NGC 4041 which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r ≃ 0.′′4 ≃ 40 pc), high surface brightness, rotating nuclear disk co-spatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak to peak amplitude of ∼ 40 km s−1 and is systematically blueshifted by ∼ 10−20 km s−1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1+0.6 −0.7) × 107 M⊙ is needed to reproduce the observed rotation curve. However the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ∼ 2 or that the inclination is allowed to vary. This model results in a 3 upper limit of 6×106 M⊙ on the mass of any nuclear black hole. Overall, our analysis only allows us to set an upper limit of 2×107 M⊙ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of −17.7 and with a central stellar velocity dispersion of ≃ 95 km s−1, then these results are not inconsistent with both the MBH-Lsph and the MBH- ⋆ correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce and therefore the present result adds an important new datapoint to our understanding of BH demography. (Refer to PDF file for exact formulas).
Department, Program, or Center
School of Physics and Astronomy (COS)
Astrophysical Journal 586 (2003) 868-890
RIT – Main Campus