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Description
We have obtained U,B, and R CCD surface photometry for a sample of 39 elliptical galaxies. For each galaxy we have determined the surface brightness profile, U-R and B-R color profiles, and the ellipticity and position angle profiles, all as function of major axis radius, using a two-dimensional ellipse fitting program. In addition, we have derived the sin and cos(3.theta) and 4.theta terms that describe the high-order deviations of the B and R isophotes from ellipses. While it is very common for ellipticals to display measurable 3.theta and 4.theta terms, the amplitudes of these terms rarely exceed 0.5%. The isophotes of elliptical galaxies are very well characterized by ellipses. The surface brightness and color profiles are given to radii at which the error in the profile reaches 0.1 mag from the uncertainty in the brightness of the night sky. We have carried out a series of simulations of the effects of seeing on luminosity and ellipticity profiles, to determine the radius beyond which the errors in our data from seeing are less than 0.05 mag and 0.02 in ellipticity. Measurable effects of seeing extend to surprisingly large radii, as much as 5-10 seeing radii, depending upon the ellipticity of the galaxy and the form of the surface brightness profile. Ellipticity and position angle profiles are usually the same in all passbands with no indication that the contours of constant color are more or less flattened than the isophotes, i.e., the isochromes and isophotes have the same shapes, but the insensitivity of the ellipticity to differences between these properties makes this a weak argument. The high-order terms, particularly the 3.theta terms, appear to be sensitive diagnostics for the existence of dust in ellipticals. We find that all the galaxies in this sample either become bluer in B-R and U-R with increasing radius or are of constant color. Mean values for the logarithmic gradients in color are -0.09 mag/arcsec2 per dex in radius in B-R, and -0.20 mag/arcsec2 per dex in radius in U-R. These color changes are consistent with a decrease in the [Fe/H] of approximately 0.20 per decade in radius. Surprisingly, there is no correlation of color gradient with luminosity. It is striking, however, that the lowest luminosity galaxies in the sample (i.e., those with M(B) > -20) do not show any color gradients. They have boxy isophotes, and are also rotationally flattened. While these properties may be related to the fact that they are companions of larger ellipsoidal systems, it could provide an important clue to the formation of ellipticals. Low luminosity ellipticals that are not close companions to giant ellipticals need to be studied.
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