Galaxies comic

To understand the interplay between the baryonic and the dark matter component of galaxies, especially quiescent (elliptical and S0) galaxies, is one of the main topics of my research. Understanding the mechanisms that are important for the growth of such galaxies, the quenching mechanism that lead to the formation of quiescent massive systems as well as the build-up of global scaling relations of all kind of galaxies, is one of the most enthralling questions in extragalactic astronomy. My work on this subject so far consist of three different contributions, for which short descriptions are given below: The Dark Halo–Spheroid Conspiracy, the Co-Evolution of Total Density Slopes and Central Dark Matter Fractions and The Evolution of the Total Density Slope with Redshift in Simulations and Observations.

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Dark Halo–Spheroid Conspiracy

Radial density profile example Dynamical modelling and strong lensing data indicate that the total density profiles of early-type galaxies are close to isothermal, i.e., ρtot ∝ rγ with γ ≈ −2. To understand the origin of this universal slope we study a set of simulated spheroids formed in isolated binary mergers as well as the formation within the cosmological framework. The total stellar plus dark matter density profiles can always be described by a power law with an index of γ ≈ −2.1 with a tendency towards steeper slopes for more compact, lower-mass ellipticals. In the binary mergers the amount of gas involved in the merger determines the precise steepness of the slope. This agrees with results from the cosmological simulations where ellipticals with steeper slopes have a higher fraction of stars formed in-situ. Each gas-poor merger event evolves the slope towards γ ∼ −2, once this slope is reached further merger events do not change it anymore. All our ellipticals have flat intrinsic combined stellar and dark matter velocity dispersion profiles. We conclude that flat velocity dispersion profiles and total density distributions with a slope of γ ∼ −2 for the combined system of stars and dark matter act as a natural attractor. The variety of complex formation histories as present in cosmological simulations, including major as well as minor merger events, is essential to generate the full range of observed density slopes seen for present-day elliptical galaxies. For more details see Remus et al., 2013.

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Co-Evolution of Total Density Slopes and Central Dark Matter Fractions

Correlations between the central dark matter fractions and the total density slope We present evidence from cosmological hydrodynamical simulations for a co-evolution of the slope of the total (dark and stellar) mass density profiles, γtot, and the dark matter fractions within the half-mass radius, fDM, in early-type galaxies. The relation can be described as γtot = AfDM + B and holds for all systems at all redshifts. We test different feedback models and find that the general trend is independent of the assumed feedback processes and is set by the decreasing importance of dissipative processes towards lower redshifts and for more massive systems. Early-type galaxies are smaller, more concentrated, have lower dark matter fractions and steeper total density slopes at high redshifts and at lower masses for a given redshift. The values for A and B change distinctively with the assumed feedback model, and thus this relation can be used as a test for feedback models. A similar correlation exists between γtot and the stellar mass surface density Σ. The model with weak stellar feedback and, in particular, feedback from black holes is in better agreement with observations. All simulations, independent of the assumed feedback model, predict steeper total density slopes and lower dark matter fractions at higher redshifts. While the latter is in agreement with the observed trends, the former is in conflict with currently available lensing observations, which indicate constant or decreasing density slopes. This discrepancy cannot be overcome by any of the feedback models included in this study. For more details see Remus et al., 2016.

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The Evolution of the Total Density Slope with Redshift in Simulations and Observations

Coming soon ...

Last update 24.03.2016 by Rhea-Silvia