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Register a new userMass distribution and orbital anisotropy of early-type galaxies: constraints from the Mass Plane
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C. Nipoti Bologna University
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Massive early-type galaxies are observed to lie on the Mass Plane (MP), a two-dimensional manifold in the space of effective radius R_e, projected mass M_p (measured via strong gravitational lensing) and projected velocity dispersion sigma within R_e/2. The MP is less `tilted than the Fundamental Plane, and the two have comparable associated scatter. This means that c_e2=2*G*M_p/(R_e*sigma^2) is a nearly universal constant in the range sigma=175-400 km/s. This finding can be used to constrain the mass distribution and internal dynamics of early-type galaxies. We find that a relatively wide class of spherical galaxy models has values of c_e2 in the observed range, because c_e2 is not very strongly sensitive to the mass distribution and orbital anisotropy. If the total mass distribution is isothermal, a broad range of stellar luminosity profile and anisotropy is consistent with the observations, while NFW dark-matter halos require more fine tuning of the stellar mass fraction, luminosity profile and anisotropy. If future data can cover a broader range of masses, the MP could be seen to be tilted and the value of any such tilt would provide a discriminant between models for the total mass-density profile of the galaxies. [Abridged]
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We are studying the mass distribution in a sample of 50 early type spiral galaxies, with morphological type betweens S0 and Sab and absolute magnitudes M_B between -18 and -22; they form the massive and high-surface brightness extreme of the disk galaxy population. Our study is designed to investigate the relation between dark and luminous matter in these systems, of which very little yet is known. From a combination of WSRT HI observations and long-slit optical spectra, we have obtained high-quality rotation curves. The rotation velocities always rise very fast in the center; in the outer regions, they are often declining, with the outermost measured velocity 10-25% lower than the maximum. We decompose the rotation curves into contributions from the luminous (stellar and gaseous) and dark matter. The stellar disks and bulges always dominate the rotation curves within the inner few disk scale lengths, and are responsible for the decline in the outer parts. As an example, we present here the decompositions for UGC 9133. We are able to put tight upper and lower limits on the stellar mass-to-light ratios.
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Galaxy mergers are instrumental in dictating the final mass, structure, stellar populations, and kinematics of galaxies. Cosmological galaxy simulations indicate that the most massive galaxies at z=0 are dominated by high fractions of `ex-situ stars, which formed first in distinct independent galaxies, and then subsequently merged into the host galaxy. Using spatially resolved MUSE spectroscopy we quantify and map the ex-situ stars in thirteen massive Early Type galaxies. We use full spectral fitting together with semi-analytic galaxy evolution models to isolate the signatures in the galaxies light which are indicative of ex-situ populations. Using the large MUSE field of view we find that all galaxies display an increase in ex-situ fraction with radius, with massive and more extended galaxies showing a more rapid increase in radial ex-situ fraction, (reaching values between 30% to 100% at 2 effective radii) compared to less massive and more compact sources (reaching between 5% to 40% ex-situ fraction within the same radius). These results are in line with predictions from theory and simulations which suggest ex-situ fractions should increase significantly with radius at fixed mass for the most massive galaxies.
We discuss the problem of using stellar kinematics of early-type galaxies to constrain the galaxies orbital anisotropies and radial mass profiles. We demonstrate that compressing a galaxys light distribution along the line of sight produces approximately the same signature in the line-of-sight velocity profiles as radial anisotropy. In particular, fitting spherically symmetric dynamical models to apparently round, isotropic face-on flattened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face-on stellar disk. Such face-on stellar disks could plausibly be the cause of the radial anisotropy found in spherical models of intermediate luminosity ellipticals such as NGC 2434, NGC 3379 and NGC 6703. In the light of this result, we use simple dynamical models to constrain the outer mass profiles of a sample of 18 round, early-type galaxies. The galaxies follow a Tully-Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag (I-band) for a given mass. The most luminous galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.
We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an IMF mismatch parameter alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing and dynamical models (M*(L+D)) to the current stellar mass inferred from stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (<log alpha>=0.00+-0.03+-0.02), while a Chabrier IMF underestimates them (<log alpha>=0.25+-0.03+-0.02). A tentative trend is found, in the sense that alpha appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a non universal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.
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