HST/ACS observations of shell galaxies: inner shells, shell colours and dust

Abstract

Context. Shells in Elliptical Galaxies are faint, sharp-edged features, believed to provide evidence for a merger event. Accurate photometry at high spatial resolution is needed to learn on presence of inner shells, population properties of shells, and dust in shell galaxies. Aims. Learn more about the origin of shells and dust in early type galaxies. Methods. V − I colours of shells and underlying galaxies are derived, using HST Advanced Camera for Surveys (ACS) data. A galaxy model is made locally in wedges and subtracted to determine shell profiles and colours. We applied Voronoi binning to our data to get smoothed colour maps of the galaxies. Comparison with N-body simulations from the literature gives more insight to the origin of the shell features. Shell positions and dust characteristics are inferred from model galaxy subtracted images. Results. The ACS images reveal shells well within the effective radius in some galaxies (at 0.24 r e = 1.7 kpc in the case of NGC 5982). In some cases, strong nuclear dust patches prevent detection of inner shells. Most shells have colours which are similar to the underlying galaxy. Some inner shells are redder than the galaxy. All six shell galaxies show out of dynamical equilibrium dust features, like lanes or patches, in their central regions. Our detection rate for dust in the shell ellipticals is greater than that found from HST archive data for a sample of normal early-type galaxies, at the 95% confidence level.Conclusions. The merger model describes better the shell distributions and morphologies than the interaction model. Red shell colours are most likely due to the presence of dust and/or older stellar populations. The high prevalence and out of dynamical equilibrium morphologies of the central dust features point towards external influences being responsible for visible dust features in early type shell galaxies. Inner shells are able to manifest themselves in relatively old shell systems.

Testing MOND gravity in the shell galaxy NGC 3923

“…This can happen because even several of the already known shells are at the limits of detectability (see Fig. D.1 of Sikkema et al 2007). Shells 6 and 7 should be located approximately at the radii of −390 and 450 arcsec.…”

“…2, a significant part of the deviations can be caused by observational uncertainty of the shell radii, for example, as can be seen in Fig. D.1 of Sikkema et al (2007), their shell No. 16 is very faint and diffuse.…”

“…It is not easy to say where the edge of a shell for an observed galaxy lies exactly, because it is always blurry (see, e.g., Figs. 3 and D.4 in Sikkema et al 2007). The observers measured the position of the maximum of brightness, which does not necessarily need to be the same as the position of the edge.…”

“…The disruption of the secondary is actually a gradual rather than an instantaneous Notes. d -the observed distance of the shell from the center of NGC 3923, data taken from Prieur (1988) and Sikkema et al (2007). The plus sign means that the shell is situated on the northern side of the galaxy and minus on the southern; n -identified shell number; Gidentified shell generation; o -the sign of generation; d model -modeled distance of the shell; ∆ -relative difference between observed and modeled distance.…”

“…The shells with the "+" sign lie on the northern side of the galaxy and those with "−" on the southern. The data for shell distances greater than 130 are taken from Prieur (1988), the rest from Sikkema et al (2007). It is not easy to say where the edge of a shell for an observed galaxy lies exactly, because it is always blurry (see, e.g., Figs.…”