Tilted outer and inner structures in edge-on galaxies?

Abstract

<jats:title>Abstract</jats:title> <jats:p>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, NGC 4469 and NGC 4452, which exhibit apparent tilted outer discs with respect to the inner structure. In NGC 4469, this structure has a boxy shape, inclined by ΔPA ≈ 3○ with respect to the inner disc, whereas NGC 4452 harbours a discy outer structure with ΔPA ≈ 6○. In spite of the different shapes, both structures have surface brightness profiles close to exponential and make a large contribution (∼30%) to the total galaxy luminosity. In the case of NGC 4452, we propose that its tilted disc likely originates from a former fast tidal encounter (probably with IC 3381). For NGC 4469, 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.</jats:p>

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).XII. Ionised gas emission in the inner regions of lenticular galaxies

“…The Hα image shows a thin star-forming disc with filaments of ionised gas on the z-plane. The analysis of deep optical images of this galaxy revealed the presence of a tilted outer disc which has been interpreted by Mosenkov et al (2020) as a possible result of harassment, in agreement with the boxy morphology witnessing a recent gravitational instability (e.g. Gadotti 2012), although gas accretion of gas-rich satellites occuring several Gyr ago cannot be ruled out.…”

“…Another interesting example is the X-shaped, boxy stellar morphology with a tilted outer disc of NGC 4469. This peculiar morphology has been interpreted as a possible result of harassment (Mosenkov et al 2020), or of a recent merging event (Mihos et al 1995), although the presence of an ionised gas tail at the SW of the galaxy due to a ram pressure stripping event is a further evidence that different mechanisms can be at place simultaneously. Three out of eight galaxies (NGC 4262,4469,and 4477) have a prominent bar, often indicating a recent instability of the stellar disc resulting after a strong gravitational perturbation (e.g.…”

“…This exercise can be done only in this galaxy since the g-band image of NGC 4469 is saturated in the centre, and has a pronounced boxy-shape(Mosenkov et al 2020) hardly reproducible with a 2D fitting model, thus affecting the determination of A(g) in the star-forming disc and in the outer, low surface brightness regions.5 This value is derived on the disc region where dust is seen in absorption, which for geometrical effects fairly corresponds to half of the disc, and extrapolated to the rest of the disc.…”

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

“…Another interesting example is the X-shaped, boxy stellar morphology with a tilted outer disc of NGC 4469. This peculiar morphology has been interpreted as a possible result of harassment (Mosenkov et al 2020), or of a recent merging event (Mihos et al 1995), although the ionised gas tail SW of the galaxy due to a ram pressure stripping event is further evidence that different mechanisms can act simultaneously. Three out of eight galaxies (NGC 4262, 4469, and 4477) have a prominent bar, which often indicates a recent instability of the stellar disc resulting from strong gravitational perturbation (e.g.…”

“…where E(B − V) BD is the attenuation of the gaseous component as derived from the Balmer decrement, and E(B − V) star is that of the stellar component derived from the g-band image. This exercise can be made only in this galaxy because the g-band image of NGC 4469 is saturated in the centre and has a pronounced boxy shape (Mosenkov et al 2020) that can hardly be reproduced with a 2D fitting model. It therefore affects the determination of A(g) in the star-forming disc and in the outer low surface brightness regions.…”