The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128

Abstract

The halos of elliptical galaxies are faint and difficult to explore, but they contain vital clues to both structure and formation. We present the results of an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby elliptical NGC 5128. We extend the work of Hui and coworkers well into the halo of the galaxy-out to distances of 100 and 50 kpc along the major and minor axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these 780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe exist at distances up to 80 kpc ($15r e ), showing that the stellar halo extends to the limit of our data. This study represents by far the largest kinematic study of an elliptical galaxy to date, both in the number of velocity tracers and in radial extent. We confirm the large rotation of the PNe along the major axis and show that it extends in a disklike feature into the halo. The rotation curve of the stars flattens at $100 km s À1 with V/ between 1 and 1.5 and with the velocity dispersion of the PNe falling gradually at larger radii. The two-dimensional velocity field exhibits a zero-velocity contour with a pronounced twist, showing that the galaxy potential is likely triaxial in shape, tending toward prolate. The total dynamical mass of the galaxy within 80 kpc is $5 Â 10 11 M , with M =L B $ 13. This mass-to-light ratio is much lower than what is typically expected for elliptical galaxies.

Planetary nebulae as kinematic tracers of galaxy stellar halos

“…Here we focus on the "Gaussian Kernel smoothing" technique developed in Peng et al (2004), with "Adaptive Kernel" as introudced in Coccato et al (2009). Basically, the kinematics at each point in the field of view is obtained computing weighted moments of the velocity distribution of the entire PNe population.…”

“…Initial works that included a significant amount of PNe in the analysis were carried on massive nearby ellipticals, such as NGC 1316 (Arnaboldi et al 1998), NGC 5128 (Hui et al 1995;Peng et al 2004), and more recently on NGC 4649 (Teodorescu et al 2011) -just to cite few examples. The signature of the presence of dark matter was clearly present in the kinematic profile at large radii.…”

Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter

“…Globular clusters (Puzia et al 2004;Bergond et al 2006;Romanowsky et al 2009;Shen & Gebhardt 2010;Schuberth 2010;Woodley et al 2010) or planetary nebulae (PNe, see, e.g. Napolitano et al 2001Napolitano et al , 2002Mendez et al 2001;Peng et al 2004;Douglas et al 2007;Coccato et al 2009;Teodorescu et al 2010;Napolitano et al 2011) have made it possible to extend kinematics up to 5−7 R eff , from the ≈R eff achievable with only stellar observations. Furthermore, the analysis of satellites orbiting the galaxies could extend up to 50−500 kpc ( 10 R eff ) (Klypin & Prada 2009).…”

Dynamical constant mass-to-light ratio models of NGC 5128

“…The results of dynamical studies of NGC 5128 obtained so far suggest the existence of dark matter: Mathieu et al (1996) used a quadratic programming method to infer that the mass-to-light ratio increases from the center and that about ∼50 per cent of the galaxy is dark; Peng et al (2004) also concluded that about 50 per cent of the galaxy is dark and that M/L B ∼ 13, but they note that the mass-to-light ratio they obtained is "much lower than what is typically expected for elliptical galaxy". Recently Łokas (2008) used the PNe data from Peng et al (2004) and concluded that in the V-band, M/L V = 26.3 ± 16.9, which in the B-band becomes M/L B = 36.3 ± 23.3 (note the large error bars). We use PNe as tracers of the gravitational potential and try to describe the kinematics of NGC 5128 without inclusion of dark matter: we apply both the Newtonian and the MOND approach in the spherical approximation allowing for anisotropies in the motion of the tracers.…”