Low-ionization emission regions in the nuclei of spiral galaxies

Physics – Nuclear Physics

Scientific paper

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Digital Techniques, Image Processing, Ionization, Nuclei (Nuclear Physics), Spiral Galaxies, Emission Spectra, Luminosity, Spectrophotometry, Star Clusters

Scientific paper

New spectrophotometry and narrow-band digital images have been obtained for bright spiral galaxies with low-ionization emission regions in their nuclei. These regions are characterized by strong forbidden lines from neutral or once-ionized species, with relatively weak Balmer emission. A survey of H alpha and n ll lambda 6584 emission in a complete sample of 93 spirals shows emission in all nuclei, with low-ionization emission seen in 85% of galaxies of types SO/a through Sbc. For a given stellar continuum luminosity, emission regions excited by stellar photoionization (H ll regions) are more luminous than low-ionization regions; H ll regions may mark or replace existing low-ionization regions when bursts of star formation occur. The narrow-band images have been processed to yield pure H alpha + n ll images. These show that the optically emitting gas in low-ionization spiral nuclei is usually concentrated in a nearly spherical core of half-intensity diameter typically 200 pc, often surrounded by a disk, in the galactic plane, traceable over a 600-900 pc diameter.

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