A Chandra View of Dark Matter in Early-Type Galaxies

Abstract

We present a Chandra study of mass profiles in 7 elliptical galaxies, of which 3 have galaxy-scale and 4 group-scale halos demarcated at 1E13Msun. These represent the best available data for nearby objects with comparable X-ray luminosities. We measure ~flat mass-to-light (M/L) profiles within an optical half-light radius (Reff), rising by an order of magnitude at ~10Reff, which confirms the presence of dark matter (DM). The data indicate hydrostatic equilibrium, which is also supported by agreement with studies of stellar kinematics in elliptical galaxies. The data are well-fitted by a model comprising an NFW DM profile and a baryonic component following the optical light. The distribution of DM halo concentration parameters (c) versus Mvir agrees with LCDM predictions and our observations of bright groups. Concentrations are slightly higher than expected, which is most likely a selection effect. Omitting the stellar mass drastically increases c, possibly explaining large concentrations found by some past observers. The stellar M/LK agree with population synthesis models, assuming a Kroupa IMF. Allowing adiabatic compression (AC) of the DM halo by baryons made M/L more discrepant, casting some doubt on AC. Our best-fitting models imply total baryon fractions \~0.04--0.09, consistent with models of galaxy formation incorporating strong feedback. The groups exhibit positive temperature gradients, consistent with the "Universal" profiles found in other groups and clusters, whereas the galaxies have negative gradients, suggesting a change in the evolutionary history of the systems around Mvir=1E13 Msun.

Figures (6)

• Figure 1: X-ray images of each of the galaxies in the sample. None of the systems show evidence of large-amplitude disturbances which would indicate a violation of hydrostatic equilibrium. Some lower-amplitude asymmetries do persist in some of the images, which we discuss in detail in § \ref{['sect_asymmetry']}.
• Figure 2: Deprojected temperature profiles of each galaxy, ordered by ${\rm M_{vir}}$ (Table \ref{['table_syserr']}). The data-points are shown, along with the best-fit parameterized model determined from simultaneously fitting the ${\rm \rho_g}$ and temperature profiles (see text). Where data-points were excluded from the fitting, they are denoted by dashed lines. Errors shown are 1-$\sigma$.
• Figure 3: Deprojected density-profiles of each galaxy, shown with the best-fit AC NFW+stars model from the "assumed potential" modelling (§ \ref{['sect_potential']}). Data-points excluded from the fit are indicated by dashed lines. Errors shown are 1-$\sigma$.
• Figure 4: 1-$\sigma$ (${\rm M_{vir}}$,c) confidence regions for each object. Panel (a) shows the confidence contours found when fitting the NFW model. In panel (b) we show the contours for the AC NFW+stars model, restricting ${\rm f_{b}}$ by Eq \ref{['eqn_baryons']} (see text), and panel (c) shows contours for the same model, but now restricting 0.032$\leq$${\rm f_{b}}$$\leq$0.16. The results for the NFW+stars model were very similar to those for the AC NFW+stars model. On each plot, we also show the prediction from the "toy model" of bullock01a (solid line) and the approximate 1-$\sigma$ scatter in the simulated DM halos (the region bounded by the dotted lines).
• Figure 5: The K-band mass-to-light profile of each galaxy. Results are shown both for the NFW+stars and the AC NFW+stars models. For the latter the shaded region indicates the approximate 1-$\sigma$ confidence region. We also show data-points derived from the "parameterized profile" mass-modelling described in § \ref{['sect_xmass']}. We stress that these data-points are not fitted by the models shown here, but they are derived independently.