The Box/peanut Phenomenon in Disk Galaxies.

Some edge-on spiral galaxies exhibit distinct box-and peanut-shaped (b/p) isophotes in and/or near the bulge region. Previous surveys relied on the subjective identification of b/p galaxies, resulting in estimates that from 2% to 45% of all disk galaxies possess b/p features. BVRI CCD observations of six b/p galaxies (NGC 1184, NGC 2683, NGC 3079, NGC 4845, NGC 4958, and NGC 5389), chosen from lists in the literature, have been obtained and studied in detail. A new technique for objectively quantifying the amount of boxiness or peanuttiness has been developed and applied to these six objects. The results from this small sample imply that roughly one-third of b/p galaxies do not exhibit b/p brightness contours, indicating that visual inspection alone is insufficient as a classification method. The images of the unambiguous b/p galaxies were searched for evidence of dust as a possible cause of the features, as POSS (National Geographic-Palomar Observatory Sky Survey) prints of some of the literature b/p galaxies suggested. However, the b/p characteristics are as prominent in the I-band images as they are in the B; the phenomenon does not appear to be due to absorption by dust. In addition, there are no obvious color gradients along the minor axis (i.e., in the direction perpendicular to the disk plane), implying that the stars producing the b/p light form a well-mixed population, with colors indicative of disk stars. Finally, a prediction of the collisional models, that the minor axis brightness profiles should be well -fit with a steep powerlaw function, was tested. The profiles from the present sample, however, are not well-fit with such a fitting function, but instead are reasonably well fit with the standard exponential function plus an underlying R^{1/4} function. Along with the lack of environmental correlations noted in the literature, the implication is that collisions may not account for these galaxies.