Spectrophotometric Properties of Merging Galaxies

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Abstract

We present quantitative analysis of long-slit spectrophotometry of 40 merging and strongly interacting galaxy systems, in the wavelength range 3650-7100 Å. Along with optically selected objects, the sample includes 10 ultraluminous IRAS galaxies with ongoing merger activity. The mergers exhibit a very large range of spectral properties, ranging from completely evolved stellar populations to emission-line dominated starbursts and absorption-line dominated poststarburst systems. The spectral types are correlated with the morphological types of the merging galaxies, as best as they can be inferred. The sample has much higher mean star-formation rates than isolated galaxies, as measured by Hα + [N II] line emission. The distribution of [0 II] equivalent widths in the sample is very different from that of local field galaxies, but does resemble the EW([O II]) distribution for faint blue field galaxies studied in deep redshift surveys. The far-infrared (FIR) luminosities and L(FIR)/L(B) ratios of the optically selected subsample are substantially higher than that of field galaxies as a whole. The ultraluminous IRAS mergers have distinctly higher L(FIR) and L(FIR)/L(B), but are similar to the optically selected mergers at optical wavelengths; on the average, their optical continuum colors are actually somewhat bluer than the optical subsample, indicating the presence of a global, young stellar population along with heavily dust-enshrouded nuclei. The frequency of the appearance of Seyfert nuclei and LINERs in the sample is also discussed. A considerable fraction of the mergers exhibit spectra with anomalously strong Balmer absorption, when compared to nearby normal galaxies. Many of these systems have spectra resembling those of distant "E + A" galaxies. A quantitative comparison of the Balmer absorption-dominated mergers with several E + A galaxies at high redshift shows that they are indeed similar and suggests that the two kinds of galaxies may be the same class of objects.

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