More surviving subhalos in higher resolution than zoom-in simulations

This guest post was written by ChungWen Wang, a MS student at Tsing Hua University studying low mass dark matter halo. Byond doing research, he likes to ride his road bike to mountains or seashores on the weekend.

Title: And In The Darkness Unbind Them: High-Resolution Simulations of Dark Matter Subhalo Disruption in a Milky Way-like Tidal Field

Authors: Jeremy J. Webb & Jo Bovy

First Author’s Institution: Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON, Canada

Status: Submitted to MNRAS

After developing simulations of subhalos in  Milky Way sized galaxies in the ΛCDM model, scientists are building models with both dark matter (DM) and baryonic matter for realistic comparison. As both the mass of dark matter particles and the softening length (a trick used to prevent numerical divergence when particles get too close to another) decrease, there are actually more subhalos that can survive to z=0 than previous zoom-in simulations.

Dissolution of subhalos

When a subhalo fall into a host galaxy in the hierarchical history, it suffers dynamical friction and gradually loses its mass (DM particles of the host galaxy will move towards subhalos. As they fly by, the DM particles of the host galaxy form a denser tail at the trajectory of subhalos, and the tail pulls the DM particles from the subhalos). For a high mass subhalo, the DM particles are tightly bound together, and therefore do not easily lose their mass. However, DM particles in low mass halos are less bound, and have a higher chance of losing mass, especially at the center of the galaxy where there is a deeper potential region.

Our ability to determine if a low mass subhalo is fully dissolved in a host dark matter halo is greatly limited by resolution. Decreasing the DM particles’ mass or the softening length will increase the resolution, and consequently increase the self-gravity of a subhalo, making it stay bound in the external field.

The subhalo mass function of the NFW model and the MW model

The authors use the NFW model to represent dark-matter-only simulation and use the MW model to represent dark matter + baryonic matter simulation. For comparison, the VL2 model is a high resolution dark-matter-only simulation of the Milky Way galaxy. The differences between the NFW model and the VL2 model are some scale shifts. The position and the velocity of subhalos are shifted by 1.27 in VL2 to initiate subhalos distribution.

Figure 1. The subhalo mass function clearly shows that the subhalos in the MW model are lesser than dark-matter-only models. This is caused by the extra potential from the baryonic matter which is present in the MW model but not in the other two models. (bottom panel of Figure 4 in this paper)

Ratio of number of subhalos

Taking the ratio of the number of subhalos in the MW model to the NFW model will give us the ratio of subhalos with/without baryonic influence. The result shows that only 65% subhalos remained if the baryonic matter was included. The effect of the bulge and the disk makes the ratio of the number of subhalos range from 50% ~ 75%.

Figure 2. The ratio of the number of subhalos in the MW model to the number of subhalos in the NFW model. The mean ratio is about 0.65 in the range of subhalo mass ranges from 106 – 107.5 M. Figure 7 in the paper.

If subhalos with different pericentric radii (the closest radius of an object’s trajectory) are separated, the trend can then be seen. For subhalos with rp < 20 kpc, less than 40% of the total number of subhalos have survived. On the other hand, for subhalos with rp > 25 kpc, more than 70% of the total number of subhalos have survived.

Figure 3. The ratio of number of subhalos with different pericentric radii. Subhalos with pericentric radii closer to the center will bring them deeper into the potential well, and thus result in stronger dynamical friction. The ratio of the numbers of surviving subhalos in the MW model to the NFW model increases the more distant they are from the galaxy bulge and disk. (bottom panel in Figure 8 of this paper)

Tracing the subhalos’ history

It is always worth looking back in time at different pericentric radii to find out how subhalos evolved when they are too close to the center. The dissolution of low mass subhalos makes particles less bound, and rmax will increase as vmax decreases.

Figure 4. Radius of maximum circular velocity versus maximum circular velocity as a function of time (Figure 10 in this paper). Bottom left panel shows at lookback time 0 Gyr, the subhalos with pericentric radii smaller than 40 kpc in the MW model have lower density. This is also attributed to the presence of the baryonic disk and bulge.

Subhalos in the NFW model at all times and all pericentric radii correspond roughly to the ΛCDM model (dash line). But subhalos in the MW model have their maximum circular velocity decreased if their pericentric radii are smaller than 40 kpc.

Comparison with previous works and future work

The authors also compared their results with previous studies and they found out that the ΛCDM model overestimated subhalos by a factor of ~65% for all radii. For high resolution comparison, The FIRE and ELVIS projects estimated that no subhalo can survive within 15 kpc of the Galactic center. But if the resolution increases, those previously marked as fully dissolved subhalos can survive.

Figure 5.The ratio of number of subhalos compared with previous works (Figure 12 in this paper). The simulation done by the authors predict more subhalos at all galactocentric radii (radii to the galactic center). However, different studies focused on different mass ranges of subhalos, and so direct comparison is not straightforward.
Table 1.The resolution of this work compared to others (Table 1 in this paper). The authors used simulations with higher resolution than that of FIRE and ELVIS which are high resolution zoom-in simulations. But the mass of the DM particle and softening length in this paper are lower than those from these zoom-in simulations.

Finally, in order to get a correct estimation of subhalos evolving in the Milky Way-like galaxy, both baryonic matter and simulation resolution need to be taken into account. If the mass of DM particles or the softening length are too high, the dissolution of individual subhalos can be artificially accelerated. The authors mentioned that the next step should be including a non-static external field. They hope that this will better match the observations.

Edited by: Luna Zagorac

Featured image credit: Credit: Volker Springel, Aquarius Simulation.

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