Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.6232t&link_type=abstract
HST Proposal ID #6232
Astronomy and Astrophysics
Astronomy
Hst Proposal Id #6232 Stellar Populations
Scientific paper
Recently, ground-based CCD imaging and spectroscopy of a sample of extreme late-type {Sd-Sm}, low surface brightness spiral galaxies by Matthews and Gallagher {in preparation} have revealed that a significant number of these galaxies contain bright, stellar-like nuclei. In some, but not all cases, the bright nucleus is coincident with observed H alpha emission. Although the compact appearance of the nuclei of these extreme late-type spirals is reminiscent of that of the AGN in more luminous galaxies, the true nature of the nuclei remains unclear. In some very late-type spirals such as M33 {SAcd}, observations have revealed that a supermassive star clusters resides at the center of the galaxy {e.g., Kormendy and McClure 1993: AJ 105:1793}. However, recently Filippenko et al. {1994: ApJ 410,L75} used the WFPC-1 and the Faint Object Spectrograph to establish that the point-like nucleus of the extreme late-type spiral NGC 4395 {Sd IV} is a type 1 Seyfert--the first ever detected in such a late-type, low luminosity galaxy. This observation provides additional incentive to establish just how common such activity is among the latest-type spirals. For example, if such activity is commonplace, it may make a substantial but previously unaccounted for contribution to the cosmic x-ray background {e.g., Koratkar et al.; 1995: ApJ 440,132}. In addition, the study of such objects may help to constrain AGN evolution and to determine whether different physical processes may power nuclear activity in galaxies of different luminosities or Hubble types, such as recent observations of QSOs where the host galaxies, if any, must have low luminosities {Bahcall, Kirhakos, & Schneider 1995: preprint}. Such observations are surprising in light of theoretical arguements suggesting that the potential wells of late-type galaxies are not deep enough to form the massive black holes beleived to power most AGN {see Artyukh and Ogannisyan 1991: Sov Astron Lett 17,377}. To establish the true nature of the nuclei of our galaxy sample requires a multi-faceted process. Matthews and Gallagher have already obtained ground-based spectroscopy of a number of extreme late-type low surface brightness spirals. These spectra cover the wavelength range from 6300-7000 A and include the H alpha, [NII], and [SII] lines. In some cases, the spectral signatures are suggestive of LINER or low-level AGN activity, but higher signal-to-noise spectra with greater wavelength coverage are needed. Moreover, a critical compliment to this investigation is the need for high spatial resolution imaging of the nuclear regions in order to determine the angular sizes/spatial scales of the bright nuclei, to probe the structure of the surrounding medium, and to measure the brightness gradients and colors across the inner regions of these galaxies. In particular the ability of the HST to determine sizes and the possible presence of young, luminous stars in or near the nuclei will be central in eliminating starburst models for active systems, such as NGC 4395 {Filippenko et al. 1994: ApJ 410,L75}.
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