Kinematic properties of the field elliptical NGC 7507

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 +3.1 −2.1 × 10 11 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 +2.0 −1.2 × 10 11 M . 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 r 0 = 7 kpc and v 0 = 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.

Isolated ellipticals and their globular cluster systems

“…A.4), similar to the one measured on the field elliptical NGC 7507 (Salinas et al 2012). This agrees with the shallow age and metallicity gradients found by Reda et al (2007).…”

Isolated elliptical galaxies and their globular cluster systems

“…Given the structural parameters, the central velocity dispersion is closely related to the stellar mass-to-light ratio (M/L), which in turn can indicate the age. An example is NGC 7507 with an M/L value of 3.1 in the R-band, which is too low for an old population (Salinas et al 2012).…”

“…For example, NGC 7507 had the reputation of a dark matter dominated galaxy (Kronawitter et al 2000), but a closer look revealed that it is possible to construct models without dark matter (Salinas et al 2012).…”

Dark matter deprivation in the field elliptical galaxy NGC 7507

“…Therefore, if the paucity of DM in some IEs can be corroborated, and even evidenced in different galactic environments, this could have serious implications for ΛCDM cosmological models, at least in their current form. A&A 574, A93 (2015) One example of an IE that may not require DM to explain its dynamics is NGC 7507, as shown recently by (Salinas et al 2012; hereafter S12). Models with a cored logarithmic DM halo could also reproduce the observed velocity dispersion profile, assuming some radial anisotropy, however, these models only contained a maximum 7.4% of the total mass as DM within 2 effective radii (R e ).…”

“…Kronawitter et al 2000;Kelson et al 2002;Cappellari & Emsellem 2004;Samurović & Danziger 2005;Salinas et al 2012), and more recently in the context of mask spectroscopy (e.g. Norris et al 2008;Proctor et al 2009;Foster et al 2009Foster et al , 2011Foster et al , 2013.…”