On the Origin of the Angular Momentum Properties of Gas and Dark Matter in Galactic Halos and Its Implications

Abstract

We perform a set of non-radiative hydrodynamical simulations of merging spherical halos in order to understand the angular momentum (AM) properties of the galactic halos seen in cosmological simulations. The universal shape of AM distributions seen in simulations is found to be generically produced as a result of mergers. The universal shape is such that it has an excess of low AM material and hence cannot explain the exponential structure of disk galaxies. A resolution to this is suggested by the spatial distribution of low AM material which is found to be in the center and a conical region close to the axis of rotation. A mechanism that preferentially discards the material in the center and prevents the material along the poles from falling onto the disk is proposed as a solution. We implement a simple geometric criterion for the selective removal of low AM material and show that in order for 90% of halos to host exponential disks one has to reject at least 40% of material. Next, we explore the physical mechanisms responsible for distributing the AM within the halo during a merger. For dark matter there is an inside-out transfer of AM, whereas for gas there is an outside-in transfer, which is due to differences between collisionless and gas dynamics. This is responsible for the spin parameter λ and the shape parameter α of AM distributions being higher for gas compared to dark matter. We also explain the apparent high spin of dark matter halos undergoing mergers and show that a criterion stricter than what is currently used would be required to detect such unrelaxed halos. Finally, we demonstrate that the misalignment of AM between gas and dark matter only occurs when the intrinsic spins of the merging halos are not aligned with the orbital AM of the system. The self-misalignment (orientation of AM when measured in radial shells not being constant), which could be the cause of warps and anomalous rotation in disks galaxies, also occurs under similar conditions. The frequency and amplitude of this misalignment are roughly consistent with the properties of warps seen in disk galaxies.

Connecting Angular Momentum and Galactic Dynamics: The Complex Interplay Between Spin, Mass, and Morphology

“…The inner and outer parts of their DM halos are less aligned, indicating that major merging events are contributing to the buildup of spheroidal galaxies. This is in line with previous studies showing that major mergers with misaligned spins can be responsible for angular momentum misalignments (Sharma et al 2012). …”

“…This is due to the fact that within the halo the stars are more concentrated toward the center, whereas the spin of the DM component is dominated by the outer part of the halo, where most of the angular momentum of the DM component resides, as also shown in the right panel of Figure 22 inAppendix C. In addition, major mergers result in a reduction of the specific angular momentum, as shown in Welker et al (2014). On the contrary, the gas component always has a spin parameter distribution shifted toward larger values, in agreement with more recent studies by Sharma & Steinmetz (2005), Kimm et al (2011), Sharma et al (2012, and Danovich et al (2015), but in contrast to previous studies by van den Bosch et al (2002) and Chen et al (2003), who found nearly the same spin distributions for gas and DM. The larger spin values for the gas reflect the continuous transport of the larger angular momentum from the outer parts into the center due to gas cooling.…”

The role of mergers and halo spin in shaping galaxy morphology

“…This suggests that, in general, galaxy mergers might not decrease the angular momentum of the stellar component, but merely redistribute it to larger radii. This inside-out transport of angular momentum had already been observed for DM (Zavala, Okamoto & Frenk 2008;Sharma, Steinmetz & Bland-Hawthorn 2012). Fig.…”

Kinematic Modeling of the Milky Way Using the Rave and GCS Stellar Surveys

“…Another possibility, proposed by Roškar et al (2010), is misaligned angular momentum of in-falling gas. How the angular momentum of halo gas becomes misaligned is described in Sharma et al (2012). However, Aumer & White (2013) and Sales et al (2012) suggest that misaligned gas can destroy the disks.…”