Active galactic nucleus activity and black hole masses in low surface brightness galaxies

Abstract

We present medium resolution optical spectroscopy of a sample of nine low surface brightness (LSB) galaxies. For those that show clear signatures of active galactic nucleus (AGN) emission, we have disentangled the AGN component from stellar light and any Fe i and Fe ii contribution. We have decomposed the Hα line into narrow and broad components and determined the velocities of the broad components; typical values lie between 900 and 2500 km s−1. Of the galaxies in our study, UGC 6614, UGC 1922, UGC 6968 and LSBC F568−6 (Malin 2) show clear signatures of AGN activity. We have calculated the approximate black hole (BH) masses for these galaxies from the Hα line emission using the virial approximation. The BH masses are ∼3 × 105 M⊙ for three galaxies and lie in the intermediate-mass BHs domain rather than the supermassive range. UGC 6614 harbours a BH of mass 3.8 × 106 M⊙; it also shows an interesting feature bluewards of Hα and Hβ implying outflow of gas or a one-sided jet streaming towards us. We have also measured the bulge stellar velocity dispersions using the Ca ii triplet lines and plotted these galaxies on the M–σ plot. We find that all the three galaxies UGC 6614, UGC 6968 and F568−6 lie below the M–σ relation for nearby galaxies. Thus, we find that although the bulges of LSB galaxies may be well evolved, their nuclear BH masses are lower than those found in bright galaxies and lie offset from the M–σ correlation.