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  Current Research Highlight :: November 2013 all highlights

Why do the most massive galaxies in the local Universe stand still?

Over the last two years an international team of astronomers participating in the linkPfeilExtern.gifATLAS3D project has presented the rotation properties of all early-type (elliptical and lenticular) galaxies in a well defined volume (42Mpc) of the nearby Universe. To the surprise of the team the stellar components of the most massive observed galaxies (~ 1011 Msun) in our neighbourhood show no global rotation signatures, in contrast to the regular rotation patterns observed for the majority of lower mass early-type galaxies. With the help of cosmological galaxy formation simulations performed by MPA scientists the team was now able to demonstrate that the giant non rotating galaxies might have special formation histories. Simulated galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. More than half of their stars were born in other galaxies which then have been eaten by the giants. The simulations indicate that over the last 10 Gyrs repeated minor mergers have continuously slowed these initially rotating giants down until they come to a halt.

Fig. 1: Postage stamp of a slowly rotating galaxy (top, NGC4636) and a fast rotating galaxy (bottom, NGC2974) with the corresponding observed two-dimensional ATLAS3D velocity fields (right). NGC4636 has no rotation patterns and the measured velocities do not exceed 40 km/s. NGC2974 shows regular fast (~ 200 km/s) rotation (Krajnovic et al. 2011).

Fig. 2: Rotation properties of all early-type galaxies in the ATLAS3D sample measured by the spin parameter λR. This parameter measures the angular momentum of stellar components of the galaxies and is derived from the two-dimensional velocity fields (see Fig. 1). Most early-type galaxies rotate fast (high λR values) but the rare massive systems (largest symbols in the plot) are slow rotators (Emsellem et al. 2011).

Fig. 3: Two dimensional velocity field of a simulated non-rotating rotating-DELETE galaxy from a cosmological simulation (top panel). These galaxies have special formation histories (class F in Naab et al. 2013). Since redshift z ~ 2 they have experienced repeated minor mergers (~ 100 in this case) with mass ratios larger than 4:1 (counted by the orange histogram, middle panel) and no late major mergers. During their evolution (from high redshift to z=0 today), the galaxies continuously grow in mass (black line, bottom panel) and loose angular momentum (green line) until they stand still.

Within the linkPfeilExtern.gifATLAS3D project, 260 nearby early-type galaxies within a local volume of 42 Mpc have been observed at optical, radio, and millimeter wavelengths. The multi-wavelength coverage enabled the team to determine the dynamics, the star-formation histories, ages and metallicities of the stellar populations as well as a full census of the gas phase (molecular, neutral and ionised) properties. The integral-field observations of the stellar kinematics (see Fig. 1) have revealed a surprising result. Whereas most early-type galaxies (~ 80 per cent) rotate quite regularly - similar to thick stellar disks - the most massive ones rotate very slowly (see Fig. 2) and some of them (7 out of 260) are very round and show no sign of ordered rotation at all. They stand still.

The absence of rotation is difficult to reconcile with current standard formation scenarios and has caused theorists quite a headache. Traditionally, it is assumed that early-type galaxies are burned out spiral galaxies or they formed and evolved by mergers of disk-like or even early-type galaxies of comparable mass. Many studies, however, have demonstrated that these formation paths mostly result in rotating or very elongated galaxies, inconsistent with properties of the observed non-rotating early-type galaxies.

As part of the theoretical efforts within ATLAS3D, a group of MPA scientists have carried out a number of high resolution computer simulations of the formation and evolution of massive galaxies. Analysing the stellar kinematics of the simulated galaxies in the same way as the observers made it possible to identify direct links between the formation history of the galaxies - as recorded by the simulations - and the resulting kinematic properties. The study reveals a surprising wealth of formation histories which are consistent with observations and the scientists were able to demonstrate that every formation history leaves its characteristic imprint on the observable two-dimensional kinematic properties. A most valuable result to interpret the observations.

Similar to the real Universe, most simulated galaxies of lower mass are fast rotating. They either form a thick stellar disk from accreted gas or are still rotating after collisions with companion galaxies of similar size. At higher galaxy masses (~ 1011 Msun), however, the majority of the stars in a typical simulated galaxy do not form in the galaxy itself but formed in other galaxies that have merged with the galaxy progenitor. Some of the major collision wrecks rotate slowly but their very elongated shapes do not agree with observed non-rotators. Only galaxies with a special formation history resemble the observed round and non-rotating galaxies. They acquire about half of their stars from many mergers with much smaller galaxies and experience no major merger. The many repeated merger events over the last ~ 10 Gyrs continuously slow the giant galaxies down so that today they stand sill (Fig. 3).


Thorsten Naab (MPA), Ludwig Oser (MPA, Columbia University) and the ATLAS3D team


References

Cappellari et al., "The ATLAS3D project - I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria", 2011, MNRAS, 413, 813 linkPfeilExtern.gifhttp://adsabs.harvard.edu/abs/2011MNRAS.413..813C

Krajnovic et al., "The ATLAS3D project - II. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies", 2011, MNRAS, 414, 2923 linkPfeilExtern.gifhttp://adsabs.harvard.edu/abs/2011MNRAS.414.2923K

Emsellem et al., "The ATLAS3D project - III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of fast and slow rotators", 2011, MNRAS, 414, 888 linkPfeilExtern.gifhttp://adsabs.harvard.edu/abs/2011MNRAS.414..888E

Naab et al., "The ATLAS3D project - XXV: Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators", 2013, astro-ph linkPfeilExtern.gifhttp://arxiv.org/abs/1311.0284



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