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Register a new userMass-to-light ratio gradients in early-type galaxy haloes
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Added by
Nicola R. Napolitano
Publication date
2004
fields
Physics
and research's language is
English
Authors
N.R. Napolitano
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Since the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterising the halo masses from both observational and theoretical results: dML, the logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from LCDM simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies $epsilon_{SF}$. As a pilot study, we assemble the available dML data from kinematics in early-type galaxies - representing the first unbiassed study of halo masses in a wide range of early-type galaxy luminosities - and find a correlation between luminosity and dML, such that the brightest galaxies appear the most dark-matter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the LCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency $epsilon_{SF} > 1$. This difficulty is eased if dark haloes are not assumed to have the standard LCDM profiles, but lower central concentrations.
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We present models for the dark and luminous mass structure of 12 strong lensing early-type galaxies (ETGs). We combine pixel-based modelling of multiband HST/ACS imaging with Jeans modelling of kinematics obtained from Keck/ESI spectra to disentangle the dark and luminous contributions to the mass. Assuming a gNFW profile for the dark matter halo and a spatially constant stellar-mass-to-light ratio $Upsilon_{star}$ for the baryonic mass, we infer distributions for $Upsilon_{star}$ consistent with IMFs that are heavier than the Milky Ways (with a global mean mismatch parameter relative to a Chabrier IMF $mu_{alpha c} = 1.80 pm 0.14$) and halo inner density slopes which span a large range but are generally cuspier than the dark-matter-only prediction ($mu_{gamma} = 2.01_{-0.22}^{+0.19}$). We investigate possible reasons for overestimating the halo slope, including the neglect of spatially varying stellar-mas-to-light ratios and/or stellar orbital anisotropy, and find that a quarter of the systems prefer radially declining stellar-mass-to-light ratio gradients, but that the overall effect on our inference on the halo slope is small. We suggest a coherent explanation of these results in the context of inside-out galaxy growth, and that the relative importance of different baryonic processes in shaping the dark halo may depend on halo environment.
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