Tilted outer and inner structures in edge-on galaxies?

TL;DR

This study presents evidence for tilted outer structures in edge-on galaxies, likely caused by tidal interactions or harassment, supported by deep imaging and numerical simulations, revealing their significant luminosity contribution and potential origins.

Contribution

The paper provides the first observational evidence of tilted outer discs in real edge-on galaxies and explores their possible formation mechanisms through deep imaging and simulations.

Findings

Tilted outer structures contribute about 30% of galaxy luminosity.

NGC4469's tilt may result from galaxy harassment or halo tumbling.

NGC4452's tilt likely caused by past tidal encounters.

Abstract

Tilted and warped discs inside tilted dark matter haloes are predicted from numerical and semi-analytical studies. In this paper, we use deep imaging to demonstrate the likely existence of tilted outer structures in real galaxies. We consider two SB0 edge-on galaxies, NGC4469 and NGC4452, which exhibit apparent tilted outer discs with respect to the inner structure. In NGC4469, this structure has a boxy shape, inclined by $\Delta$PA$\approx$3$^{\circ}$ with respect to the inner disc, whereas NGC4452 harbours a discy outer structure with $\Delta$PA$\approx$6$^{\circ}$. In spite of the different shapes, both structures have surface brightness profiles close to exponential and make a large contribution ($\sim30$%) to the total galaxy luminosity. In the case of NGC4452, we propose that its tilted disc likely originates from a former fast tidal encounter (probably with IC3381). For NGC4469, a plausible explanation may also be galaxy harassment, which resulted in a tilted or even a tumbling dark matter halo. A less likely possibility is accretion of gas-rich satellites several Gyr ago. New deep observations may potentially reveal more such galaxies with tilted outer structures, especially in clusters. We also consider galaxies, mentioned in the literature, where a central component (a bar or a bulge) is tilted with respect to the stellar disc. According to our numerical simulations, one of the plausible explanations of such observed "tilts" of the bulge/bar is a projection effect due to a not exactly edge-on orientation of the galaxy coupled with a skew angle of the triaxial bulge/bar.