Kinematic properties of the field elliptical NGC 7507^⋆,⋆⋆

¹ Departamento de Astronomía, Universidad de Concepción, Concepción, Chile
e-mail: rcsave@utu.fi
² European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
³ Facultad de Ciencias Astronómicas y Geofísicas de la Universidad Nacional de La Plata and Instituto de Astrofísica de la Plata (CCT La Plata-CONICET-UNLP), Paseo del Bosque S/N, 1900 La Plata, Argentina
⁴ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁵ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 14 January 2011
Accepted: 7 November 2011

Abstract

The dark matter (DM) halos of field elliptical galaxies have not been well-studied and their properties appear controversial in the literature. While some galaxies appear to be nearly devoid of DM, others show clear evidence of its presence. Furthermore, modified Newtonian dynamics (MOND), which has been found to have predictive power in the domain of disk galaxies, has not yet been investigated for isolated elliptical galaxies. We study the kinematics of the isolated elliptical NGC 7507, which has been claimed as a clear example of DM presence in early-type galaxies. We obtained major and minor axis long-slit spectroscopy of NGC 7507 using the Gemini South telescope and deep imaging in Kron-Cousins R and Washington C using the CTIO/MOSAIC camera. Mean velocities, velocity dispersion and higher order moments of the velocity distribution are measured out to  ~90′′. The galaxy, although almost circular, has significant rotation along the minor axis and a rapidly declining velocity dispersion along both axes. The velocity dispersion profile is modeled in the context of a spherical Jeans analysis. Models without DM provide an excellent representation of the data with a mass-to-light ratio (M/L) of 3.1 (R-band). The most massive Navarro-Frenk-White (NFW) halo the data allow has a virial mass of only 3.9_-2.1^+3.1×10¹¹ M_⊙, although the data are more consistent with models that have a slight radial anisotropy, which implies the galaxy has an even lower DM halo mass of 2.2_-1.2^+2.0×10¹¹ M_⊙. Modeling of the h₄ Gauss-Hermite coefficient is inconclusive but seems to be consistent with mild radial anisotropy. A cored logarithmic DM halo with parameters r₀ = 7 kpc and v₀ = 100 km s^-1 can also reproduce the observed velocity dispersion profile. The MOND predictions overestimate the velocity dispersion. In conclusion, we cannot easily reproduce the previous findings of a predominance of DM in NGC 7507 within a simple spherical model. DM may be present, but only in conjunction with a strong radial anisotropy, for which there are some indications.