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What makes a galaxy a disk or a spheroid?

In the traditional picture, disk galaxies form predominantly in halos with high angular momentum and their recent assembly history is fairly quiet, whereas spheroids are the slowly-rotating remnants of repeated merging events. Scientists at the Max Planck Institute for Astrophysics have now used hydrodynamical numerical simulations of large volumes of the universe to show that, in galaxies like the Milky Way, there is a surprisingly poor correlation between halo properties and morphological features. In the simulations, disks form in halos with both high and low net spin, and mergers play a negligible in the formation of spheroids. More important than the morphology is the angular momentum of the gas that accretes over time to form a galaxy.

Fig. 1: Illustration of the structure of four galaxies in our sample with increasing degree of rotational support. The left and right columns show edge-on and face-on projections of the stellar distribution. The yellow circle marks the radius used to define the galaxy.

Fig. 2: This sequence of images shows how a disk-dominated galaxy assembles over time in our simulations. At early times, the proto-galactic material is distributed over large volumes; gas is shown in red and stars in yellow. Baryons aggregate onto larger and larger units until it all collapses to form a single object at the present day.

Fig. 3: For the galaxies B and D shown in Fig. 1 these plots show the projected particle distribution near turnaround time, about 10 billion years ago. Stars that have already formed are shown in red, particles still in gaseous form in blue. Concentric circles enclose 20%, 50%, and 95% of the mass, and arrows indicate the angular momentum of all material enclosed within each radius. Arrow lengths are normalized to the total value, which defines the z-axis of the projection. Note the misalignment of the angular momentum of various parts of the system for the spheroid- dominated galaxy B. Angular momentum is more coherently acquired in the case of the disk-dominated galaxy D.

Galaxies exhibit a spectacular variety of morphologies, from spheroids to disks to bars, with tidal tails, and a zoo of peculiar objects of irregular shapes. Until recently, astrophysicists thought that the major morphological features of a galaxy are determined by the assembly history and net spin of its surrounding dark matter halo: mergers would create early type elliptical galaxies; whereas disks would form within quiescently built halos whose angular momentum content exceeds the average of the whole population. However, due to the large computational costs of numerical simulations, it was the studies of few individual systems that led to most of the progress in our understanding of morphologies.

Thanks to the combined efforts of several research teams within Europe collaborating as part of the "Virgo Consortium", we have been able to study, for the very first time, the morphology and evolution of a large sample of simulated galaxies that form within the standard cosmological scenario. We focused our attention on objects like our own Galaxy, the Milky Way, and explored the build-up of 100 objects unbiasedly selected from wide, representative volumes of the universe. For that, we use the ``Galaxies-Intergalactic Medium Interaction Calculation'' simulations, which follow the evolution of the dark matter, gas and stars that form within large cosmological volumes throughout cosmic time.

As shown by Fig. 1, our simulated galaxies exhibit a wide variety of morphologies, from dispersion-dominated spheroids (top) to pure disk galaxies (bottom). Numerical simulations are a powerful tool because they allow us to track the temporal evolution of each object, gaining fundamental insight about the physical mechanisms that shape a galaxy. For example, Fig.2 shows different stages of the build-up of one of our simulated objects. Different snapshots show the distribution of gas (red) and stars (yellow) that will collapse to form a realistic looking disk-dominated galaxy at the present time.

We quantify morphologies by using a dynamical indicator, which measures the fraction of the kinetic energy of the stars that is in ordered rotation around a well-defined axis. That fraction is large for disk-dominated galaxies where most stars move in the same plane and close to circular orbits. On the other hand, it is almost null for spheroid-like objects, where the dynamics is dominated by dispersion instead of rotation.

This classification scheme allows us to study systematically the correlations between morphology and properties of the dark matter halos that these galaxies inhabit. Contrary to what is commonly believed, we find that the merger activity and the angular momentum of the dark matter halos relate poorly to the morphology of their central galaxies. However, an interesting hint comes from another correlation: disks preferentially form in objects where the contribution of gas that has cooled from the ``hot corona'' is large, whereas spheroids dominate for stars born from cold flows that deliver the material directly to the central parts of the halo.

The angular momentum properties of the material accreted hot or cold should differ: gas in the hot corona is forced to first homogenize its spin before being aggregated to the central galaxy, whereas cold gas flows unstopped to the centres, bringing along their intrinsic spin without any mixing. Therefore, we look for further clues to galaxy morphologies that might be hidden in the primordial angular momentum distribution of each of our objects. Since the spin of the material destined to form a galaxy is imprinted as soon as the object decouples from the expansion of the Universe (around 10 billion years ago) and remains approximately constant after that, we study the angular momentum distributions of our galaxies at very early times, even before each galaxy has formed.

Surprisingly, in this exercise spheroid- and disk-dominated galaxies showed very different behaviours. Whereas for objects that evolved into spheroids the different parts of the system showed clear misalignments in their angular momentum (left panel Fig. 3), the spins of different collapsing shells in disk-dominated galaxies were remarkably well aligned (right panel Fig. 3). These results are general to our whole sample and not a peculiarity of these two galaxies.

In the scenario we propose, it is not only the net angular momentum content which matters for galaxy morphology, but also the way in which it is distributed across the proto-galaxy. Since gas further out takes longer to accrete, material assembled later will be misaligned with the rest. Stars formed from this gas will tend to destabilize any existing disk and cancel out the net angular momentum of the system, leaving in place a slowly-rotating stellar spheroid. This is a novel mechanism for the formation of spheroids that does not rely on merging. On the other hand, disk-dominated objects show coherence in the angular momentum at turnaround, which allows newly accreted material to settle into a stable disk where star formation can proceed gradually and smoothly.

These considerations suggest that the final morphology of a galaxy is imprinted early on for each object; even before the galaxy is born.

Laura V. Sales, Julio F. Navarro, Tom Theuns, Joop Schaye, Simon D. M. White, Carlos S. Frenk, Robert A. Crain and Claudio Dalla Vecchia


Crain, R.~A., Theuns, T., Dalla Vecchia, C., et al, “Galaxies–intergalactic medium interaction calculation – I. Galaxy formation as a function of large-scale environment”, 2009, MNRAS, 399, 1773

Sales, L.~V., Navarro, J.~F., Theuns, T., J. Schaye, S. White, et al, “The Origin of Disks and Spheroids in Simulated Galaxies”, 2011, linkPfeilExtern.gifarXiv:1112.2220

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