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تسجيل مستخدم جديدMOND and IMF variations in early-type galaxies from $ATLAS^{3D}$
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MOdified Newtonian dynamics (MOND) represents a phenomenological alternative to dark matter (DM) for the missing mass problem in galaxies and clusters of galaxies. We analyze the central regions of a local sample of $sim 220$ early-type galaxies from the $rm ATLAS^{3D}$ survey, to see if the data can be reproduced without recourse to DM. We estimate dynamical masses in the MOND context through Jeans analysis, and compare to $rm ATLAS^{3D}$ stellar masses from stellar population synthesis. We find that the observed stellar mass--velocity dispersion relation is steeper than expected assuming MOND with a fixed stellar initial mass function (IMF) and a standard value for the acceleration parameter $a_{rm 0}$. Turning from the space of observables to model space, a) fixing the IMF, a universal value for $a_{rm 0}$ cannot be fitted, while, b) fixing $a_{rm 0}$ and leaving the IMF free to vary, we find that it is lighter (Chabrier-like) for low-dispersion galaxies, and heavier (Salpeter-like) for high dispersions. This MOND-based trend matches inferences from Newtonian dynamics with DM, and from detailed analysis of spectral absorption lines, adding to the converging lines of evidence for a systematically-varying IMF.
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G. R. Meurer (International Centre for Radio Astronomy Research
, n University of Western Australia
,
2010
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ulation. The sample consists of seven early-type galaxies (ETGs) at $zsim0.05$, with central velocity dispersion in the range $300<sigma<350$km/s. Using state-of-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiOs and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to $sim4$kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $sim 10^{10} M_odot kpc^{-2}$, or, alternatively, within a characteristic radius of $sim2$~kpc. Radial distance, in physical units, and surface mass density, are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter} that fully explains variations in the stellar IMF, but IMF radial profiles at z$sim$0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.
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