Dark Matter in Elliptical Galaxies

Abstract

We review X-ray constraints on dark matter in giant elliptical galaxies (10 12 M ⊙ < ∼ M vir < ∼ 10 13 M ⊙ ) obtained using the current generation of X-ray satellites, beginning with an overview of the physics of the hot interstellar medium and mass modeling methodology. Dark matter is now firmly established in many galaxies, with inferred NFW concentration parameters somewhat larger than the mean theoretical relation. X-ray observations confirm that the total mass profile (baryons+DM) is close to isothermal (M ∼ r), and new evidence suggests a more general power-law relation for the slope of the total mass profile that varies with the stellar half-light radius. We also discuss constraints on the baryon fraction, supermassive black holes, and axial ratio of the dark matter halo. Finally, we review constraints on non-thermal gas motions and discuss the accuracy of the hydrostatic equilibrium approximation in elliptical galaxies.

Constraining the Physical State of the Hot Gas Halos in NGC 4649 and NGC 5846

“…X-ray observations of the hot interstellar medium (ISM) can be used to measure the total mass within a given radius if the hot gas is hydrostatic equilibrium (HE), but departures from HE can seriously affect mass measurements (e.g., Fabricant et al 1984;Buote & Humphrey 2012; see also Trinchieri & Fabbiano 1985;Canizares et al 1987;David et al 1990David et al , 1991Kim et al 1992aKim et al , 1992bBuote & Humphrey 2012). Recently, comparisons with mass profiles derived from optical indicators (GCs/PNs) have uncovered significant discrepancies (e.g., see Figure 8 in Pota et al 2015 for NGC 1407; Figure 7 in Napolitano et al 2014 for NGC 5846).…”

Dark Matter Mass Fraction in Lens Galaxies: New Estimates From Microlensing

“…Existing estimates use X-ray observations, stellar dynamics, or gravitational lensing, and each of these methods has its own advantages and difficulties. X-ray observations of the hot gas in massive galaxies can provide an estimate of the total mass under the assumption of hydrostatic equilibrium (see Buote & Humphrey 2012). This method is very robust and simple, with its main uncertainties coming from the robustness of the hydrostatic equilibrium hypothesis, the possibility of nonthermal contributions to the pressure, and contamination from emission by larger-scale group/cluster halo gas.…”

X-Ray Measurements of the Mass Profiles in Massive Isolated Elliptical Galaxies

“…Although presently one cannot rely on direct kinematic constraints on non-thermal gas motions, there are still reasonable steps that can be taken to mitigate their impact on mass-profile studies (Buote & Humphrey 2012a). First, for the hydrostatic equilibrium approximation to be most accurate, one should focus on those systems that are the most regular in terms of their image morphology.…”

“…We find an entropybased procedure is particularly effective since it allows us to easily enforce the additional constraint of convective stability (e.g., Buote & Humphrey 2012a). We begin by assuming parameterized forms for the entropy and mass profiles which are input into the equation of hydrostatic equilibrium.…”

“…Only a few elliptical galaxies have detailed mass measurements from X-rays because of the lack of suitable targets in terms of total mass ( < ∼ 10 13 M ), ISM X-ray flux, and relaxed dynamical states appropriate for hydrostatic analysis (e.g., for a review see Buote & Humphrey 2012a). We have identified a sample of 11 (hot) gas-rich isolated elliptical galaxies, most of which were discovered from previously awarded XMM-Newton and Chandra snapshot programs to search for optimal targets for X-ray studies of mass profiles on the galaxy scale.…”