Testing Verlinde's emergent gravity in early-type galaxies [CEA]


Emergent Gravity (EG) is a new theory which proposes an alternative way to solve the missing mass problem in galactic structures. In this theory the standard gravitational laws are modified on galactic and cluster scales due to the entropy displacement of dark energy by baryonic matter. In EG the excess gravity can be explained by an “apparent” dark matter density, which only depends on the baryonic mass distribution and the Hubble parameter. We test the EG theory using the central dynamics in a sample of local early-type galaxies (ETGs). We use the SPIDER data sample, which consists of massive ETGs ($M_{\rm \star} > 10^{10} \, \rm M_{\odot}$) at redshifts $0.05 < z < 0.095$. We demonstrate that, consistently with a classical Newtonian framework with a dark matter halo component, or alternative theories of gravity as MOND, the central dynamics can be fitted if the IMF is assumed non-universal. However, we find unrealistically low stellar M/L in EG theory. The IMF is Chabrier-like for the highest-$\sigma_{\star}$ ETGs (in contrast with the literature in the field, including stellar population studies). And still more interestingly, extremely low stellar M/L ($\sim 0.25$ times the ones adopting a Chabrier IMF) are found in the lowest-$\sigma_{\star}$ ETGs. These low stellar M/L would imply “ultra-light” IMF, which contrasts with both theoretical predictions and results from spectral gravity-sensitive features. However, if the strain caused by the entropy displacement would be not maximal, then the dynamics of ETGs could be reproduced with more realistic M/L.

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C. Tortora, L. Koopmans and N. Napolitano
Wed, 1 Mar 17

Comments: 5 pages, 2 figures, submitted to MNRAS letters