Correlations between UBV colors and fine structure in E and S0 galaxies - A first attempt at dating ancient merger events

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Abstract

A study of 69 E and S0 galaxies located mostly in the field and in groups reveals that the UBV colors become systematically bluer at any given luminosity as the amount of merger-induced fine structure increases. To quantify such fine structure, we define an index {SIGMA} that measures ripples, jets of luminous matter, X structures, and boxy isophotes; it ranges between 0 and 7.6 for the above galaxies. The correlations between UBV colors and this index {SIGMA} closely resemble the correlations found earlier between CN, Mg2, and Hβ line strengths and the same {SIGMA} in 36 ellipticals [Schweizer et al. ApJ, 364, L33 (1990)]. Because {SIGMA} is a rough measure of dynamical youth or rejuvenation, both sets of correlations are most likely due to systematic variations in the mean age of the stellar populations rather than to variations in their mean metallicity. The new color correlations emphasize that these suggested age variations are not limited to the nuclei, but occur globally in the stellar populations of E and S0 galaxies. These correlations also yield a rough ranking of E and S0 galaxies by the date of their last major merger event. To calibrate this chronology we develop a simple two-burst model of evolving stellar populations in mergers and apply it to derive heuristic merger ages (HMA) from UBV colors for each galaxy. These HMAs vary mainly as a function of two Parameters: the Hubble type of the premerger components and the gas-to-star conversion efficiency. For representative ranges of these parameters, the HMAs of our 69 E and S0 galaxies spread over at least 5 Gyr and up to 10 Gyr. Hence the scatter in color-magnitude-relations -though relatively small- is fully compatible with the hypothesis that in the field such galaxies formed, or were seriously modified, by major mergers during at least 1/3 to 2/3 the age of the Universe. A mean HMA of ~8 Gyr is Suggested for E's with no fine structure and 4.6 Gyr for those with the most fine structure. Good candidates for dynamically young ellipticals having formed through mergers of disk galaxies during the last 7 Gyr are NGC 3610, 1700, 4125, 4915, and 5322; significant rejuvenation seems to have occurred also in NGC 596, 3640, and 5018.

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