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Register a new userThe Variation of Rotation Curve Shapes as a Signature of the Effects of Baryons on Dark Matter Density Profiles
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Chris Brook
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Rotation curves of galaxies show a wide range of shapes, which can be paramaterized as scatter in Vrot(1kpc)/Vmax i.e.the ratio of the rotation velocity measured at 1kpc and the maximum measured rotation velocity. We examine whether the observed scatter can be accounted for by combining scatters in disc scale-lengths, the concentration-halo mass relation, and the M*-Mhalo relation. We use these scatters to create model galaxy populations; when housed within dark matter halos that have universal, NFW density profiles, the model does not match the lowest observed values of Vrot(1kpc)/Vmax and has too little scatter in Vrot(1kpc)/Vmax compared to observations. By contrast, a model using a mass dependent dark matter profile, where the inner slope is determined by the ratio of M*/Mhalo, produces galaxies with low values of Vrot(1kpc)/Vmax and a much larger scatter, both in agreement with observation. We conclude that the large observed scatter in Vrot(1kpc)/Vmax favours density profiles that are significantly affected by baryonic processes. Alternative dark matter core formation models such as SIDM may also account for the observed variation in rotation curve shapes, but these observations may provide important constraints in terms of core sizes, and whether they vary with halo mass and/or merger history.
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We use the Evolution and assembly of galaxies and their environments (EAGLE) cosmological simulation to investigate the effect of baryons on the density profiles of rich galaxy clusters. We focus on EAGLE clusters with $M_{200}>10^{14}~M_odot$ of which we have six examples. The central brightest cluster galaxies (BCGs) in the simulation have steep stellar density profiles, $rho_*(r) propto r^{-3}$. Stars dominate the mass density for $r<10~rm{kpc}$, and, as a result, the total mass density profiles are steeper than the Navarro-Frenk-White (NFW) profile, in remarkable agreement with observations. The dark matter halo itself closely follows the NFW form at all resolved radii ($rgtrsim3.0~rm{kpc}$). The EAGLE BCGs have similar surface brightness and line-of-sight velocity dispersion profiles as the BCGs in the sample of Newman et al., which have the most detailed measurements currently available. After subtracting the contribution of the stars to the central density, Newman et al. infer significantly shallower slopes than the NFW value, in contradiction with the EAGLE results. We discuss possible reasons for this discrepancy, and conclude that an inconsistency between the kinematical model adopted by Newman et al. for their BCGs, which assumes isotropic stellar orbits, and the kinematical structure of the EAGLE BCGs, in which the orbital stellar anisotropy varies with radius and tends to be radially biased, could explain at least part of the discrepancy.
We use numerical simulations to investigate how the statistical properties of dark matter (DM) haloes are affected by the baryonic processes associated with galaxy formation. We focus on how these processes influence the spin and shape of a large number of DM haloes covering a wide range of mass scales, from galaxies to clusters at redshifts zero and one, extending to dwarf galaxies at redshift two. The haloes are extracted from the OverWhelmingly Large Simulations, a suite of state-of-the-art high-resolution cosmological simulations run with a range of feedback prescriptions. We find that the median spin parameter in DM-only simulations is independent of mass, redshift and cosmology. At z=0 baryons increase the spin of the DM in the central region (<=0.25 r_200) by up to 30 per cent when feedback is weak or absent. This increase can be attributed to the transfer of angular momentum from baryons to the DM, but is no longer present at z=2. We also present fits to the mass dependence of the DM halo shape at both low and high redshift. At z=0 the sphericity (triaxiality) is negatively (positively) correlated with halo mass and both results are independent of cosmology. Interestingly, these mass-dependent trends are markedly weaker at z=2. While the cooling of baryons acts to make the overall DM halo more spherical, stronger feedback prescriptions tend to reduce the impact of baryons by reducing the central halo mass concentration. More generally, we demonstrate a strongly positive (negative) correlation between halo sphericity (triaxiality) and galaxy formation efficiency, with the latter measured using the central halo baryon fraction. In conclusion, our results suggest that the effects of baryons on the DM halo spin and shape are minor when the effects of cooling are mitigated, as required by realistic models of galaxy formation, although they remain significant for the inner halo.
We present the first simulated galaxy clusters (M_200 > 10^14 Msun) with both self-interacting dark matter (SIDM) and baryonic physics. They exhibit a greater diversity in both dark matter and stellar density profiles than their counterparts in simulations with collisionless dark matter (CDM), which is generated by the complex interplay between dark matter self-interactions and baryonic physics. Despite variations in formation history, we demonstrate that analytical Jeans modelling predicts the SIDM density profiles remarkably well, and the diverse properties of the haloes can be understood in terms of their different final baryon distributions.
We review the~current status of the~study of rotation curve (RC) of the~Milky Way, and~present a~unified RC from the~Galactic Center to the galacto-centric distance of about 100 kpc. The~RC is used to directly calculate the~distribution of the~surface mass density (SMD). We then propose a~method to derive the~distribution of dark matter (DM) density in the~in the~Milky Way using the~SMD distribution. The~best-fit dark halo profile yielded a local DM density of $rho_odot = 0.36pm 0.02$ GeV/cc. We also review the~estimations of the~local DM density in the~last decade, and~show that the~value is converging to a~value at $rho_odot=0.39pm 0.09$ GeV/cc.
The impact of the streaming between baryons and dark matter on the first structures has been actively explored by recent studies. We investigate how the key results are affected by two popular approximations. One is to implement the streaming by accounting for only the relative motion while assuming ``baryons trace dark matter spatially at the initialization of simulation. This neglects the smoothing on the gas density taking place before the initialization. In our simulation initialized at $z_i=200$, it overestimates the gas density power spectrum by up to 40% at $kapprox10^2~h~mbox{Mpc}^{-1}$ at $z=20$. Halo mass ($M_h$) and baryonic fraction in halos ($f_{b,h}$) are also overestimated, but the relation between the two remains unchanged. The other approximation tested is to artificially amplify the density/velocity fluctuations in the cosmic mean density to simulate the first minihalos that form in overdense regions. This gives a head start to the halo growth while the subsequent growth is similar to that in the mean density. The growth in a true overdense region, on the other hand, is accelerated gradually in time. For example, raising $sigma_8$ by 50% effectively transforms $zrightarrowsqrt{1.5}z$ in the halo mass growth history while in 2-$sigma$ overdensity, the growth is accelerated by a constant in redshift: $zrightarrow{z+4.8}$. As a result, halos have grown more in the former than in the latter before $zapprox27$ and vice versa after. The $f_{b,h}$-$M_h$ relation is unchanged in those cases as well, suggesting that the Pop III formation rate for a given $M_h$ is insensitive to the tested approximations.
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