ON THE NATURE OF THE FAINT COMPACT NARROW EMISSION-LINE GALAXIES - THE HALF-LIGHT RADIUS-VELOCITY WIDTH DIAGRAM

We present new measurements of emission-line velocity widths for five faint compact narrow emission-line galaxies (CNELGs) with 20.5 less th an or equal to B less than or equal to 21.5 and redshifts z similar to 0.19 to 0.35, The spectra were taken at the Keck telescope using the HIRES spectrograph with a resolution of 8 km s(-1) (FWHM). Emission-li ne profiles are roughly Gaussian with velocity widths sigma similar to 30 to 50 km s(-1). The new a data, in combination with Hubble Space T elescope (HST) measurements of half-light radii R(e), indicate that th ese CNELGs are low-mass stellar systems (i.e., M similar to 10(9) M.), while their unusually low mass-to-light ratios (typically < 0.2 M./L. ) are consistent with being undergoing a major burst of star formation . Since R(e) and sigma are roughly independent of the fading of the st ellar population, the R(e) - sigma diagram is particularly useful for comparing the properties of these young galaxies to those of evolved s tellar systems, Several physical processes that may modify the positio n of CNELGs in this diagram during their evolution, including dissipat ion, mergers, stripping, and winds, are discussed briefly. We conclude that the new data support a simple evolutionary scenario in which the se low-mass, young galaxies will fade after the starburst, without maj or changes in R(e) or sigma, to become today's spheroidal galaxies.