Non-parametric triaxial deprojection of elliptical galaxies

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>We present a grid-based non-parametric approach to obtain a triaxial 3D luminosity density from its surface brightness distribution. Triaxial deprojection is highly degenerate and our approach illustrates the resulting difficulties. Fortunately, for massive elliptical galaxies, many deprojections for a particular line of sight (LOS) can be discarded, because their projection along other LOSs does not resemble elliptical galaxies. The near-elliptical isophotes of these objects imply near ellipsoidal intrinsic shapes. In fact, deprojection is unique for densities distributed on ellipsoidal shells. The constrained non-parametric deprojection method we present here relaxes this constraint and assumes that the contours of the luminosity density are boxy/discy ellipsoids with radially varying axis ratios. With this approach, we are able to reconstruct the intrinsic triaxial densities of our test models, including one drawn from an N-body simulation. The method also allows to compare the relative likelihood of deprojections at different viewing angles. We show that the viewing orientations of individual galaxies with nearly ellipsoidal isophotal shapes can be constrained from photometric data alone.</jats:p>

Intrinsic shapes of Brightest Cluster Galaxies

“…In this section we describe the deprojection parameters used for the BCGs. An extensive description of the deprojection routine itself is given by de Nicola et al (2020). In short, given the observed surface luminosity I obs = L/pc 2 onto a polar elliptical grid, the code searches for the three-dimensional luminosity density ρ, placed onto an ellipsoidal grid, whose corresponding projected surface luminosity I fit minimizes RMS= (ln(I obs /I fit ))…”

“…Deprojections yield in general a non-unique solution (Gerhard 1996;van den Bosch 1997;de Nicola et al 2020), unless one uses constraints about the galaxy shape. We impose the resulting deprojection to consist of a series of concentric perfect ellipsoids, i.e.…”

“…In a recent paper, de Nicola et al (2020) have presented a triaxial deprojection routine that fits the intrinsic shape of ellipsoidal galaxies and allows to constrain the viewing angles under which an object is seen by photometric data alone. This can be refined further in combination with the dynamical modeling of appropriate stellar kinematics (de Nicola et al, in prep.…”

“…The first goal of this paper is to constrain the intrinsic shapes and viewing angles of a representative subsample (56 objects) of this BCG catalogue with the deprojection method of de Nicola et al (2020). Then, our second goal is to compare the recovered shapes to the ones of simulated massive galaxies and their dark matter halos.…”

Intrinsic Shapes of Brightest Cluster Galaxies

“…In a recent paper, de Nicola et al (2020) have presented a triaxial deprojection routine that fits the intrinsic shape of ellipsoidal galaxies and allows them to constrain the viewing angles under which an object is seen by photometric data alone. This can be refined further in combination with the dynamical modeling of appropriate stellar kinematics (S. de Nicola et al 2022, in preparation), since the number of deprojections that need to be tested is drastically reduced, allowing the study of large samples of galaxies.…”