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Register a new userThe mass discrepancy acceleration relation in early-type galaxies: extended mass profiles and the phantom menace to MOND
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The dark matter (DM) haloes around spiral galaxies appear to conspire with their baryonic content: empirically, significant amounts of DM are inferred only below a universal characteristic acceleration scale. Moreover, the discrepancy between the baryonic and dynamical mass, which is usually interpreted as the presence of DM, follows a very tight mass discrepancy acceleration (MDA) relation. Its universality, and its tightness in spiral galaxies, poses a challenge for the DM interpretation and was used to argue in favour of MOdified Newtonian Dynamics (MOND). Here, we test whether or not this applies to early-type galaxies. We use the dynamical models of fast-rotator early-type galaxies by Cappellari et al. based on ATLAS$^{3D}$ and SLUGGS data, which was the first homogenous study of this kind, reaching ~4 $R_e$, where DM begins to dominate the total mass budget. We find the early-type galaxies to follow an MDA relation similar to spiral galaxies, but systematically offset. Also, while the slopes of the mass density profiles inferred from galaxy dynamics show consistency with those expected from their stellar content assuming MOND, some profiles of individual galaxies show discrepancies.
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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.
We have analyzed the parallelism between the properties of galaxy clusters and early-type galaxies (ETGs) by looking at the similarity between their light profiles. We find that the equivalent luminosity profiles of all these systems in the vfilt band, once normalized to the effective radius re and shifted in surface brightness, can be fitted by the Sersics law Sers and superposed with a small scatter ($le0.3$ mag). By grouping objects in different classes of luminosity, the average profile of each class slightly deviates from the other only in the inner and outer regions (outside $0.1leq r/R_eleq 3$), but the range of values of $n$ remains ample for the members of each class, indicating that objects with similar luminosity have quite different shapes. The Illustris simulation reproduces quite well the luminosity profiles of ETGs, with the exception of in the inner and outer regions where feedback from supernovae and active galactic nuclei, wet and dry mergers, are at work. The total mass and luminosity of galaxy clusters as well as their light profiles are not well reproduced. By exploiting simulations we have followed the variation of the effective half-light and half-mass radius of ETGs up to $z=0.8$, noting that progenitors are not necessarily smaller in size than current objects. We have also analyzed the projected dark+baryonic and dark-only mass profiles discovering that after a normalization to the half-mass radius, they can be well superposed and fitted by the Sersics law.
We discuss the problem of using stellar kinematics of early-type galaxies to constrain the galaxies orbital anisotropies and radial mass profiles. We demonstrate that compressing a galaxys light distribution along the line of sight produces approximately the same signature in the line-of-sight velocity profiles as radial anisotropy. In particular, fitting spherically symmetric dynamical models to apparently round, isotropic face-on flattened galaxies leads to a spurious bias towards radial orbits in the models, especially if the galaxy has a weak face-on stellar disk. Such face-on stellar disks could plausibly be the cause of the radial anisotropy found in spherical models of intermediate luminosity ellipticals such as NGC 2434, NGC 3379 and NGC 6703. In the light of this result, we use simple dynamical models to constrain the outer mass profiles of a sample of 18 round, early-type galaxies. The galaxies follow a Tully-Fisher relation parallel to that for spiral galaxies, but fainter by at least 0.8 mag (I-band) for a given mass. The most luminous galaxies show clear evidence for the presence of a massive dark halo, but the case for dark haloes in fainter galaxies is more ambiguous. We discuss the observations that would be required to resolve this ambiguity.
Early-type galaxies obey a narrow relation traced by their stellar content between the mass and size (Mass- Radius relation). The wealth of recently acquired observational data essentially confirms the classical relations found by Burstein, Bender, Faber, and Nolthenius, i.e. log(R_1/2) propto log(Ms)simeq 0.54 for high mass galaxies and log(R_1/2) propto log(Ms) simeq 0.3 for dwarf systems (shallower slope), where R_1/2 and Ms are the half-light radius and total mass in stars, respectively. Why do galaxies follow these characteristic trends? What can they tell us about the process of galaxy formation? We investigate the mechanisms which concur to shape the Mass-Radius relation, in order to cast light on the physical origin of its slope, its tightness, and its zero point. We perform a theoretical analysis, and couple it with the results of numerical hydrodynamical (NB-TSPH) simulations of galaxy formation, and with a simulation of the Mass-Radius plane itself. We propose a novel interpretation of the Mass-Radius relation, which we claim to be the result of two complementary mechanisms: on one hand, the result of local physical processes, which fixes the ratio between masses and radii of individual objects; on the other hand, the action of cosmological global, statistical principles, which shape the distribution of objects in the plane. We reproduce the Mass-Radius relation with a simple numerical technique based on this view.
We analyze the total and baryonic acceleration profiles of a set of well-resolved galaxies identified in the EAGLE suite of hydrodynamic simulations. Our runs start from the same initial conditions but adopt different prescriptions for unresolved stellar and AGN feedback, resulting in diverse populations of galaxies by the present day. Some of them reproduce observed galaxy scaling relations, while others do not. However, regardless of the feedback implementation, all of our galaxies follow closely a simple relationship between the total and baryonic acceleration profiles, consistent with recent observations of rotationally supported galaxies. The relation has small scatter: different feedback implementations -- which produce different galaxy populations -- mainly shift galaxies along the relation, rather than perpendicular to it. Furthermore, galaxies exhibit a characteristic acceleration, $g_{dagger}$, above which baryons dominate the mass budget, as observed. These observations, consistent with simple modified Newtonian dynamics, can be accommodated within the standard cold dark matter paradigm.
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