Orbit histories of satellite galaxies in FIRE

I recently submitted my 3rd first-author paper, which is focused on the orbital dynamics and histories of satellite galaxies in the FIRE simulations. Here are a few takeaways from my paper, but there are MANY more takeaways in the paper here!

I also recently gave a talk on some of my current research, and you can watch it here.

• The orbital velocity, specific angular momentum, and specific total energy strongly depend on the time of infall into the Milky Way-mass halo.

• In ~2/3 of satellites that completed more than one orbit around the MW-mass host, the minimum pericenter was not the most recent pericenter, suggesting that the orbits have grown at some point, and this is primarily due to either a rapid increase in their specific angular momenta near apocenter or more gradual increase over time, suggesting either perturbations at large distances or the slowly growing host potential are driving the orbit growth.

• The difference between the minimum and most recent pericenter distance is ~37% and the minimum pericenter occurred ~6 Gyr before the most recent.

Metal-poor stars on prograde orbits in the FIRE Simulations

This is about my 2nd first-author paper which is focused on studying the origins of old, metal-poor stars with prograde orbits located in the disks of Milky Way-like galaxies using the cosmological zoom-in baryonic simulations from the FIRE project. Read a low-level explanation of the results on the UC Davis College of Letters and Science research blog here! Here is the link to ADS.

Here are a few key takeaways from my paper:

• Metal-poor ([Fe/H] < -2.5) stars located within the disk (R = [4, 12] kpc, |Z| < 3 kpc) of MW-like galaxies preferentially orbit in a prograde manner, and this is a common feature in 11 out of the 12 simulated galaxies in our sample. Here is an example in one galaxy, where the boxes show the prograde and retrograde selection windows.

• This prograde preference of metal-poor stars does not depend on stellar metallicity. We see this prograde bias across -4 < [Fe/H] < -0.

• The largest contribution of these stars is due to a single massive, gas-rich satellite galaxy merger that occurred roughly 7 Gyr ago. The rest of these stars were brought in from lessor contributing galaxy mergers as well as early in-situ formation. Below is an example in one galaxy where the arrows show when the three most massive mergers that contributed these metal-poor, prograde stars (decreasing mass with decreasing opacity in the arrows). The angle shows the relative angular momentum of the merging satellite orbit and the host disk. This shows that the merger helped set the orientation of the disk!

Formation times of MW-mass galaxies

Here are some of the highlights from my 1st first-author paper which was focused on studying the formation times, and building blocks, of progenitors to Milky Way-like galaxies in the cosmological zoom-in baryonic simulations from the FIRE project:

• By looking into when a galaxy's stellar mass growth transitions from being primarily merger-supported, to self-supported (in-situ growth), and by looking into when the most massive galaxy in a system becomes the most dominant component, we can define that the progenitor to the main galaxy "formed".

• With these definitions, galaxies like the Milky Way formed by z ~ 3.4 (11.9 Gyr ago); see top right figure. When a galaxy is able to form more than half of its own stars than it gains from mergers, or when the curves cross the horizontal dotted line at 0.5, the progenitor "forms".

• Galaxies in paired systems (similar to the Milky Way and Andromeda pair) form earlier than isolated galaxies, implying environment may play a role in galaxy formation. In the top right figure, paired galaxies are represented as dotted curves. They cross a fraction of 0.5 before the isolated galaxies (in the solid curves) and hence form earlier!

• Nearly 100 dwarf galaxies helped "build" the main galaxy, about 4 - 5 times as many dwarf galaxies as there are at present-day. The bottom right figure shows how many galaxies, above a given mass, merge into the virial halo of the galaxy, as a function of redshift (time).