On the Nature of the Faint Compact Narrow Emission-Line Galaxies: The Half-Light Radius–Velocity Width Diagram

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Abstract

We present new measurements of emission-line velocity widths for five faint compact narrow emission-line galaxies (CNELGs) with 20.5 ≤ B ≤ 21.5 and redshifts z ~ 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-line profiles are roughly Gaussian with velocity widths σ ~ 30 to 50 km s-1. The new σ data, in combination with Hubble Space Telescope (HST) measurements of half-light radii Re, indicate that these CNELGs are low-mass stellar systems (i.e., M ~ 109 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 Re and σ are roughly independent of the fading of the stellar population, the Re-σ diagram is particularly useful for comparing the properties of these young galaxies to those of evolved stellar systems. Several physical processes that may modify the position of CNELGs in this diagram during their evolution, including dissipation, mergers, stripping, and winds, are discussed briefly. We conclude that the new data support a simple evolutionary scenario in which these low-mass, young galaxies will fade after the starburst, without major changes in Re or σ, to become today's spheroidal galaxies.

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