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Register a new userDynamical Mass Determination for Elliptical Galaxies
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Authors
Ortwin Gerhard
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The mass and anisotropy of an elliptical galaxy can be simultaneously determined from velocity dispersion and line profile shape measurements. We describe the principles, techniques, and limitations of this approach, and the results obtained sofar. We briefly discuss how best to combine these stellar-dynamical results with X-ray measurements and gravitational lensing analyses.
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Elements of kinematical and dynamical modeling of elliptical galaxies (Es) are presented. In projection, NFW models resemble Sersic models, but with a very narrow range of shapes (m=3+/-1). The total density profile of Es cannot be NFW-like because the predicted local M/L and aperture velocity dispersion within an effective radius (Re) are much lower than observed. Stars must then dominate Es out to a few Re. Fitting an NFW model to the total density profile of Sersic+NFW (stars+dark matter [DM]) Es results in very high concentration parameters, as found by X-ray observers. Kinematical modeling of Es assuming an isotropic NFW DM model underestimates M/L at the virial radius by a factor of 1.6 to 2.4, because dissipationless LCDM halos have slightly different density profiles and slightly radial velocity anisotropy. In N-body+gas simulations of Es as merger remnants of spirals embedded in DM halos, the slope of the DM density profile is steeper when the initial spiral galaxies are gas-rich. The Hansen & Moore (2006) relation between anisotropy and the slope of the density profile breaks down for gas and DM, but the stars follow an analogous relation with slightly less radial anisotropies for a given density slope. Using kurtosis (h_4) to infer anisotropy in Es is dangerous, as h_4 is also sensitive to small levels of rotation. The stationary Jeans equation provides accurate masses out to 8 Re. The discrepancy between the modeling of Romanowsky et al. (2003), indicating a dearth of DM in Es, and the simulations analyzed by Dekel et al. (2005), which match the spectroscopic observations of Es, is partly due to radial anisotropy and to observing oblate Es face-on. However, one of the 15 solutions to the orbit modeling of Romanowsky et al. is found to have an amount and concentration of DM consistent with LCDM predictions.
The formation and evolution of galaxies is imprinted on their stellar population radial gradients. Two recent articles present conflicting results concerning the mass dependence of the metallicity gradients for early-type dwarf galaxies. On one side, Spolaor et al. show a tight positive correlation between the total metallicity, Z/H and the mass. On the other side, in a distinct sample, we do not find any trend involving Fe/H (Koleva et al.). In order to investigate the origin of the discrepancy, we examine various factors that may affect the determination of the gradients: namely the sky subtraction and the signal-to-noise ratio. We conclude that our detection of gradients are well above the possible analysis biases. Then, we measured the Mg/Fe relative abundance profile and found moderate gradients. The derived Z/H gradients scatter around -0.4 dex/r_e. The two samples contain the same types of objects and the reason of the disagreement is still not understood. Based on observations made with ESO telescopes at La Silla Paranal observatory under program ID076.B-0196.
Early-type galaxies exhibit thermal and molecular resonance emission from dust that is shed and heated through stellar mass loss as a subset of the population moves through the AGB phase of evolution. Because this emission can give direct insight into stellar evolution in addition to galactic stellar mass loss and ISM injection rates, we conducted a program to search for this signature emission with CAM on ISO. We obtained 6-15 micron imaging observations in six narrow bands for nine elliptical galaxies; every galaxy is detected in every band. For wavelengths shorter than 9 microns, the spectra are well matched by a blackbody, originating from the K and M stars that dominate the integrated light of elliptical galaxies. However, at wavelengths between 9 and 15 microns, the galaxies display excess emission relative to the stellar photospheric radiation. Additional data taken with the fine resolution circular variable filter on one source clearly shows broad emission from 9 to 15 microns, peaking around 10 microns. This result is consistent with the known, broad silicate feature at 9.7 microns, originating in the circumstellar envelopes of AGB stars. This emission is compared with studies of Galactic and LMC AGB stars to derive cumulative mass loss rates. In general, these mass loss rates agree with the expected ~0.8 solar masses per year value predicted by stellar evolutionary models. Both the photospheric and circumstellar envelope emission follow a de Vaucouleurs R^{1/4} law, supporting the conclusion that the mid-infrared excess emission originates in the stellar component of the galaxies and acts as a tracer of AGB mass loss and mass injection into the ISM.
We study the radio emission of the most massive galaxies in a sample of dynamically relaxed and un-relaxed galaxy groups from Galaxy and Mass Assembly (GAMA). The dynamical state of the group is defined by the stellar dominance of the brightest group galaxy, e.g. the luminosity gap between the two most luminous members, and the offset between the position of the brightest group galaxy and the luminosity centroid of the group. We find that the radio luminosity of the most massive galaxy in the group strongly depends on its environment, such that the brightest group galaxies in dynamically young (evolving) groups are an order of magnitude more luminous in the radio than those with a similar stellar mass but residing in dynamically old (relaxed) groups. This observation has been successfully reproduced by a newly developed semi-analytic model which allows us to explore the various causes of these findings. We find that the fraction of radio loud brightest group galaxies in the observed dynamically young groups is ~2 times that in the dynamically old groups. We discuss the implications of this observational constraint on the central galaxy properties in the context of galaxy mergers and the super-massive blackhole accretion rate.
We aim at reproducing the mass- and sigma-[alpha/Fe] relations in the stellar populations of early-type galaxies by means of a cosmologically motivated assembly history for the spheroids. We implement a detailed treatment for the chemical evolution of H, He, O and Fe in GalICS, a semi-analytical model for galaxy formation which successfully reproduces basic low- and high-redshift galaxy properties. The contribution of supernovae (both type Ia and II) as well as low- and intermediate-mass stars to chemical feedback are taken into account. We find that this chemically improved GalICS does not produce the observed mass- and sigma-[alpha/Fe] relations. The slope is too shallow and scatter too large, in particular in the low and intermediate mass range. The model shows significant improvement at the highest masses and velocity dispersions, where the predicted [alpha/Fe] ratios are now marginally consistent with observed values. We show that this result comes from the implementation of AGN (plus halo) quenching of the star formation in massive haloes. A thorough exploration of the parameter space shows that the failure of reproducing the mass- and sigma-[alpha/Fe] relations can partly be attributed to the way in which star formation and feedback are currently modelled. The merger process is responsible for a part of the scatter. We suggest that the next generation of semi-analytical model should feature feedback (either stellar of from AGN) mechanisms linked to single galaxies and not only to the halo, especially in the low and intermediate mass range. The integral star formation history of a single galaxy determines its final stellar [alpha/Fe] as it might be expected from the results of closed box chemical evolution models. (abridged)
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