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The Halos and Environments of Nearby Galaxies (HERON) Survey IV: Complexity in the boxy galaxies NGC 720 and NGC 2768

Andreas J. Koch-HansenAnna PasqualiR. Michael RichOrtwin GerhardOliver M\"uller
Nov 2023
The shapes of galaxies, in particular their outer regions, are important guideposts to their formation and evolution. Here we report on the discovery of strongly box-shaped morphologies of the, otherwise well-studied, elliptical and lenticular galaxies NGC 720 and NGC 2768 from deep imaging. The boxiness is strongly manifested in the shape parameter $A_4/a$ of $-0.04$ in both objects, and also significant center shifts of the isophotes of $\sim$ 2--4 kpc are seen. One reason for such asymmetries commonly stated in the literature is a merger origin, although the number of such cases is still sparse and the exact properties of the individual boxy objects is highly diverse. Indeed, for NGC 2768, we identify a progenitor candidate (dubbed Pelops) in the residual images, which appears to be a dwarf satellite that is currently merging with NGC 2768. At its absolute magnitude of M$_r$ of $-$12.2 mag, the corresponding Sersic radius of 2.4 kpc is more extended than those of typical dwarf galaxies from the literature. However, systematically larger radii are known to occur in systems that are in tidal disruption. This finding is bolstered by the presence of a tentative tidal stream feature on archival GALEX data. Finally, further structures in the fascinating host galaxy comprise rich dust lanes and a vestigial X-shaped bulge component.
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