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Register a new userThe two-component virial theorem and the acceleration-discrepancy relation
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Added by
Christine Cordula Dantas
Publication date
2017
fields
Physics
and research's language is
English
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We revisit the two-component virial theorem (2VT) in the light of recent theoretical and observational results related to the dark matter(DM) problem. This modification of the virial theorem offers a physically meaningful framework to investigate possible dynamical couplings between the baryonic and DM components of extragalactic systems. In particular, we examine the predictions of the 2VT with respect to the acceleration-discrepancy relation (ADR). Considering the combined data (composed of systems supported by rotation and by velocity dispersion), we find that: (i) the overall behavior of the 2VT is consistent with the ADR; and (ii) the 2VT predicts a nearly constant behavior in the lower acceleration regime, as suggested in recent data on dwarf spheroidals. We also briefly comment on possible differentiations between the 2VT and some modified gravity theories.
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We examine the origin of the mass discrepancy--radial acceleration relation (MDAR) of disk galaxies. This is a tight empirical correlation between the disk centripetal acceleration and that expected from the baryonic component. The MDAR holds for most radii probed by disk kinematic tracers, regardless of galaxy mass or surface brightness. The relation has two characteristic accelerations; $a_0$, above which all galaxies are baryon-dominated; and $a_{rm min}$, an effective minimum aceleration probed by kinematic tracers in isolated galaxies. We use a simple model to show that these trends arise naturally in $Lambda$CDM. This is because: (i) disk galaxies in $Lambda$CDM form at the centre of dark matter haloes spanning a relatively narrow range of virial mass; (ii) cold dark matter halo acceleration profiles are self-similar and have a broad maximum at the centre, reaching values bracketed precisely by $a_{rm min}$ and $a_0$ in that mass range; and (iii) halo mass and galaxy size scale relatively tightly with the baryonic mass of a galaxy in any successful $Lambda$CDM galaxy formation model. Explaining the MDAR in $Lambda$CDM does not require modifications to the cuspy inner mass profiles of dark haloes, although these may help to understand the detailed rotation curves of some dwarf galaxies and the origin of extreme outliers from the main relation. The MDAR is just a reflection of the self-similar nature of cold dark matter haloes and of the physical scales introduced by the galaxy formation process.
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