Kinematic properties of the field elliptical NGC 7507

TL;DR

This study investigates the kinematic properties of the isolated elliptical galaxy NGC 7507, finding that its dynamics can be explained without dark matter using simple models, challenging previous claims of significant dark matter presence.

Contribution

The paper provides detailed kinematic analysis of NGC 7507 and demonstrates that its observed properties can be modeled without dark matter, contrasting prior evidence suggesting its presence.

Findings

Jeans models without dark matter fit the data well

Dark matter halo mass is constrained to be relatively low

Radial anisotropy may be necessary to explain the data

Abstract

The dark matter (DM) halos of field elliptical galaxies have not been well studied and their properties appear controversial. While some galaxies appear to be nearly devoid of DM, others show clear evidence of its presence. Furthermore, modified Netonian 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 are measured out to ~90 arcsec. The galaxy, although almost circular, has significant rotation along the minor axis and a rapidly declining velocity dispersion along both axes. Jeans 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+3.1-2.1E11 solar masses, 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+2.0-1.2E11 solar masses. Modeling of the h_4 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 can also reproduce the observed velocity dispersion profile. 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. (Abridged).