Nuclear Properties of Nearby Spiral Galaxies From Hubble Space Telescope NICMOS Imaging and STIS [Space Telescope Imaging Spectrograph] Spectroscopy

We investigate the central regions of 23 spiral galaxies using archival NICMOS imaging and STIS spectroscopy. The sample is taken from our program to determine the masses of central massive black holes (MBH) in 54 nearby spiral galaxies. Stars are likely to contribute significantly to any dynamical central mass concentration that we find in our MBH program and this paper is part of a series to investigate the nuclear properties of these galaxies. We use the Nuker law to fit surface brightness profiles, derived from the NICMOS images, to look for nuclear star clusters and find possible extended sources in 3 of the 23 galaxies studied (13 per cent). The fact that this fraction is lower than that inferred from optical Hubble Space Telescope studies is probably due to the greater spatial resolution of those studies. Using R-H and J-H colors and equivalent widths of Hα emission (from the STIS spectra) we investigate the nature of the stellar population with evolutionary models. Under the assumption of hot stars ionizing the gas, as opposed to a weak AGN, we find that there are young stellar populations (∼10–20Myr) however these data do not allow us to determine what percentage of the Centre for Astrophysical Research, STRI, University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK. Departamento de Astrofisica Molecular e Infrarroja, CSIC, Madrid, Spain. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy INAF-Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy. Oxford University, Theoretical Physics, Keble Road, Oxford, OX1 3NP, UK Eidgenoessische Technische Hochshule Zuerich, Hoenggerberg HPF G4.3, CH-8092 Zuerich, Switzerland School of Physics and Astronomy, University of Nottingham, NG7 2RD, UK Center for Astrophysical Sciences, Johns Hopkins University, 239 Bloomberg Center for Physics & Astronomy, 3400 North Charles Street, Baltimore, MD 21218 Obserwatorium Astonomiczne Uniwersytetu Jagiellonskiego, Poland Deptartment of Physics, RIT, 84 Lomb Memorial Dr., Rochester, NY 14623-5603 Department of Physics, University of Durham, Rochester Building, Science Laboratories, South Road, Durham, DH1 3LE, UK