Galaxy Properties

Galaxies are cool.

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Disk Galaxies

Simlated Disk Galaxy Galaxy simulations can be roughly divided into two major types: cosmological simulations and simulations of individual galaxies. Both of these approaches still suffer from significant weaknesses. Although cosmological simulations produce galaxies which form self-consistently within their environments, the galaxies themselves and especially their internal structure cannot be adequately resolved due to computational limits on the total number of particles that can be taken into account using current hardware. In contrast, simulations of individual galaxies resolve the internal structure of the galaxies very well, but the simulations do not contain any realistic cosmological environment and the galaxies themselves are artificially set up. In order to avoid these disadvantages, we aim to conduct cosmological zoom-simulations of Milky-Way-like disk galaxies. The CAST group has already conducted zoom-simulations of structures on many scales, ranging from superclusters over clusters of galaxies down to groups and large elliptical galaxies, and we will extend this unique set of simulations down to Milky-Way-like disk galaxies with masses around 1012 M and even below. Of particular significance in this regard is the large size (1 Gpc/h) of the parent simulation on which these zoom simulations are based, which allows us to select galaxies from many different cosmological environments.

Due to the small masses of the target galaxies and the intended very high resolution level, improving the simulation setup is indispensable. In particular, AGN and stellar feedback models require several enhancements and will be tested extensively. In addition, we implemented the self-consistent positioning of the SMBHs in the centres of their host galaxies.

The final set of high-resolution disk galaxies will provide a unique tool for investigating the interplay between internal secular evolution processes and the direct cosmological environment of the galaxies. The setup will allow reproducing structural features which are solely triggered by external processes and may enable quantifying which type of process, external or internal, has dominated the evolution of the galaxy. Further considerations could include the formation of bars and spiral structure, which are present in preliminary simulation runs. Moreover, the spatial distribution of metals in the disk and even globular star clusters will also be investigated.

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Spheroidal Galaxies and Scaling Relations

Fundamental Plane in Magneticum

Spheroidal galaxies have been studied in many different kinds of simulations within the last years. With the advent of new observational methods, it was discovered that there are actually multiple different pathways for forming spheroidal galaxies, which in particular lead to a large variety of kinematical features that are now observed and can be reproduced successfully in simulations. Additionally, large samples of spheroidal galaxies can be studied statistically with respect to the galaxies’ scaling properties.

The physical origins of observed scaling relations like the mass–size relation, the Faber–Jackson Relation, or the Fundamental Plane are still nof fully understood, and simulations are a very useful tool for understanding the dependence of the scaling relations on different physical processes such as feedback from AGN or stellar components, the interplay between the dark and the baryonic components of galaxies, and mass accretion. A large part of the work done within our group deals with these questions, as the origin of this kind of fundamental galaxy properties is one of the major backbones in understanding galaxy formation and evolution processes.

For more details, see Remus et al., 2017, Remus et al., 2016, and Remus et al., 2013.

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Galaxies at z = 2

Rotation curves of z=2 Magneticum Galaxies

The observations and measurements of rotation curves are an important tool in deducing the (invisible) mass of a (disk) galaxy. For the rotational velocities of the visible mass, i.e. gas and stars, a Keplerian decrease proportional to r−1/2 is expected in the outer parts of the galaxy. What is seen instead for (local) disk galaxies is that the rotation curves remain flat. This can be explained by the presence of an invisible component and thus, lead to the acceptance of the concept of dark matter.

Recently, observations of disk galaxies at high redshift (z = 2) have found that half of the gas disks exhibit declining rotation curves. This lead to the discussion whether dark matter is important, at all, and even the existence of it was put into question. In our hydrodynamical ΛCDM simulation Magneticum Pathfinder we find that about half of our disk galaxies also show declining rotation curves when calculated directly from the particle data. If we calculate the theoretical rotation curve, which takes all the mass including dark matter into account, the rotation curves remain flat. This shows that this decline is not due to the lack of dark matter; it is caused by the kinetic pressure of the gas in the disk, which is turbulent at high redshift, as the disks are in the process of forming. Interestingly, even disks in (gas-rich) elliptical galaxies have declining rotation curves, which can be important for observations, as mostly gas is used to identify disks at high redshifts. The disk galaxies with declining rotation curves evolve into disks, ellipticals and most of them into transition types.

In summary, galactic disks with declining rotation curves at high redshift emerge naturally within the ΛCDM paradigm. For more details, see Teklu et al., 2018.