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Register a new userA Dearth of Dark Matter in Ordinary Elliptical Galaxies
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Added by
Aaron J. Romanowsky
Publication date
2003
fields
Physics
and research's language is
English
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The kinematics of the outer parts of three intermediate-luminosity elliptical galaxies have been studied using the Planetary Nebula Spectrograph. The galaxies velocity dispersion profiles are found to decline with radius; dynamical modeling of the data indicates the presence of little if any dark matter in these galaxies halos. This surprising result conflicts with findings in other galaxy types, and poses a challenge to current galaxy formation theories.
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We review X-ray constraints on dark matter in giant elliptical galaxies (10^{12} M_sun <~ M_vir <~ 10^{13} M_sun) 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, super-massive 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.
There is strong evidence that the mass in the Universe is dominated by dark matter, which exerts gravitational attraction but whose exact nature is unknown. In particular, all galaxies are believed to be embedded in massive haloes of dark matter. This view has recently been challenged by surprisingly low random stellar velocities in the outskirts of ordinary elliptical galaxies, which were interpreted as indicating a lack of dark matter (Mendez et al. 2001; Romanowsky et al. 2003). Here we show that the low velocities are in fact compatible with galaxy formation in dark-matter haloes. Using numerical simulations of disc-galaxy mergers, we find that the stellar orbits in the outer regions of the resulting ellipticals are very elongated. These stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories. The elongated orbits, combined with the steeply falling density profile of the observed tracers, explain the observed low velocities even in the presence of large amounts of dark matter. Projection effects when viewing a triaxial elliptical can lead to even lower observed velocities along certain lines of sight.
I show that the lensing masses of the SLACS sample of strong gravitational lenses are consistent with the stellar masses determined from population synthesis models using the Salpeter IMF. This is true in the context of both General Relativity and modified Newtonian dynamics, and is in agreement with the expectation of MOND that there should be little classical discrepancy within the high surface brightness regions probed by strong gravitational lensing. There is also dynamical evidence from this sample supporting the claim that the mass-to-light ratio of the stellar component increases with the velocity dispersion.
Given the recently deduced relationship between X-ray temperatures and stellar velocity dispersions (the T-sigma relation) in an optically complete sample of elliptical galaxies (Davis & White 1996), we demonstrate that L>L_* ellipticals contain substantial amounts of dark matter in general. We present constraints on the dark matter scale length and on the dark-to-luminous mass ratio within the optical half-light radius and within the entire galaxy. For example, we find that minimum values of dark matter core radii scale as r_dm > 4(L_V/3L_*)^{3/4}h^{-1}_80 kpc and that the minimum dark matter mass fraction is >~20% within one optical effective radius r_e and is >~39-85% within 6r_e, depending on the stellar density profile and observed value of beta_spec. We also confirm the prediction of Davis & White (1996) that the dark matter is characterized by velocity dispersions that are greater than those of the luminous stars: sigma_dm^2 ~ 1.4-2 sigma_*^2. The T-sigma relation implies a nearly constant mass-to-light ratio within six half-light radii: M/L_V ~ 25h_80 M_sun/L_V_sun. This conflicts with the simplest extension of CDM theories of large scale structure formation to galactic scales; we consider a couple of modifications which can better account for the observed T-sigma relation.
We use observations of gas-rich dwarf galaxies to derive constraints on dark matter scattering with ordinary matter. We require that heating/cooling due to DM interacting with gas in the Leo T dwarf galaxy not exceed the ultra-low radiative cooling rate of the gas. This enables us to set $(i)$ stronger bounds than all the previous literature on ultra-light hidden photon DM for nearly all of the mass range $10^{-23}lesssim m_mathrm{DM} lesssim 10^{-10}$ eV, $(ii)$ limits on sub-GeV millicharged DM which add to the constraints on the recent EDGES 21cm absorption anomaly, and $(iii)$ constraints on DM-baryon interactions directly at low relative velocities $v_mathrm{rel}sim 17$ km/s. Our study opens a new direction at using observations of gas-rich dwarf galaxies from previous, current and upcoming optical and 21cm surveys to probe physics beyond the standard model.
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