C. Tortora, F. La Barbera, N. R. Napolitano, R. R. de Carvalho, A. J. Romanowsky
We analyze the central dark-matter (DM) content of $\sim 4,500$ massive
($M_\star \gsim 10^{10} \, M_\odot$), low-redshift ($z<0.1$), early-type
galaxies (ETGs), with high-quality $ugrizYJHK$ photometry and optical
spectroscopy from SDSS and UKIDSS. We estimate the "central" fraction of DM
within the $K$-band effective radius, \Re. The main results of the present work
are the following: (1) DM fractions increase systematically with both
structural parameters (i.e. \Re, and S\'ersic index, $n$) and mass proxies
(central velocity dispersion, stellar and dynamical mass), as in previous
studies, and decrease with central stellar density. 2) All correlations
involving DM fractions are caused by two fundamental ones with galaxy effective
radius and central velocity dispersion. These correlations are independent of
each other, so that ETGs populate a central-DM plane (DMP), i.e. a correlation
among fraction of total-to-stellar mass, effective radius, and velocity
dispersion, whose scatter along the total-to-stellar mass axis amounts to $\sim
0.15$ dex. (3) In general, a Chabrier IMF is favoured with respect to a
bottom-heavier Salpeter IMF, as the latter produces negative (i.e. unphysical)
DM fractions for more than 50% of the galaxies in our sample. For a Chabrier
IMF, the DM estimates agree with $\Lambda$CDM toy-galaxy models based on
contracted DM-halo density profiles. We also find agreement with predictions
from hydrodynamical simulations. (4) The central DM content of ETGs does not
depend significantly on the environment where galaxies reside, with group and
field ETGs having similar DM trends.
View original:
http://arxiv.org/abs/1201.2945
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