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UV-Optical-IR emission-line ratios are powerful diagnostics tools to discriminate between starbursts and AGNs. The following ratios have been shown to be the most reliable tools for this purpose.

  1. Ultraviolet: N V lambda1240 / Lyalpha, N V lambda1240 / He II lambda1640, C IV lambda1548 / Lyalpha.
  2. Optical: [O III] lambda5007 / Hbeta, [N II] lambda6583 / Halpha, [S II] lambdalambda6724 / Halpha, [O I] lambda6300 / Halpha, [O II] lambdalambda7324 / Halpha, [Fe VII] lambda6087 / Halpha, [Ne V] lambda3426 / Hbeta, He II lambda4686 / Hbeta.
  3. Near-infrared: Obscured broad Paalpha 1.875 µm, [Si VI] 1.962 µm / Paalpha.
  4. Mid-Infrared: [Ne V] 14 µm / [Ne II] 12.8 µm, [O IV] 26 µm / [Ne II] 12.8 µm, EW(PAH 7.7 µm), overall SEDs especially 25 µm / 60 µm colors.

A number of issues complicate the use of line ratios as discriminants between starburst and active galaxies, but additional measures can be used to clarify the situation:

  1. Shock ionization: If shocks are important, one would generally expect correlations between the line ratios and gas kinematics, a UV continuum extended on the same scale as the shock structure, and high gas temperatures.
  2. Aperture effects: One should use a constant linear aperture to avoid variations in the contributions from circumnuclear starbursts.
  3. Morphological bias: The spectral classification is likely to depend on the morphology of the host, especially the merger phase. Selection methods based on morphology will bias the sample.
  4. Metallicity: Massive host galaxies in the local universe have larger metallicity, but high-redshift galaxies should be less dusty and less metal rich. One needs to use emission-line diagnostics which are properly calibrated as a function of metallicity and reddening.

Several new instruments will help refine the diagnostic tools discussed in this paper. The Cosmic Origins Spectrograph (COS), to be installed in 2003 on HST, will provide the high ultraviolet throughput needed to calibrate the UV diagnostic tools as a function of metallicity, evaluate the importance of shock ionization with the use of the C III and NIII temperature-sensitive line ratios, and to help resolve the circumnuclear starbursts and shock-excited winds around AGNs. The advent of SIRTF will help in the calibration of the infrared diagnostic tools as a function of metallicity and dust extinction. This spacecraft will also be a powerful instrument to search for infrared-bright AGNs. Ground-based work with adaptive optics and integral-field units will improve the sensitivity of searches for obscured AGNs by focussing on the inner regions of galaxies and avoiding the circumnuclear material associated with other phenomena. Spectroscopic follow-ups from the ground will help identify and classify AGN candidates in space-based and submm-selected samples.

Acknowledgments. The ground-based study on ultraluminous infrared galaxies discussed in this paper is done in collaboration with Drs. D. B. Sanders and D.-C. Kim. The author gratefully acknowledges the financial support of NASA through LTSA grant number NAG 56547.

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