Research Topics
Resonant dynamics in dark matter halo and satellite interaction
with Martin D. Weinberg, Neal Katz (UMass)
Interactions between bar-dark matter halo and satellite-primary galaxy play an important role in explaining observed present day galaxy characteristics.
Galaxy evolution by these interactions mostly occurs through resonance coupling of orbits.
Current N-body simulations are possibly inadequate to resolve the appropriate resonant dynamics, resonant dynamics is easily distorted or erased by astronomically unrealistic noise sources, such as Poisson noise and two-body relaxation (Weinberg & Katz 2007ab).
Failure of reproducing resonant dynamics in N-body simulation possibly causes (or enhances) the "small scale" CDM crises.
We are investigating galaxy evolution due to satellite-dark matter halo interaction in detail using both state-of-the-art N-body simulations and linear perturbation theory calculation to determine the validity of the simulation and to understand what processes really drive galaxy evolution.
According to our study, the resonant dynamics plays an important role in galaxy evolution.
First, the resonant interaction significantly changes satellite decay rate.
Second, the resonant heating is important mechanism in satellite disruption.
We find the resonant torque, the angular momentum transfer from time dependent host halo potential to satellite particles, enhances satellite mass loss.
Third, we expect that owing to global excitation by resonant interaction, the satellite halo interaction can cause the halo cusp evolution.
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Gas physics in cosmological simulation
with Kentaro Nagamine (UNLV)
Due to the resolution limitation, current cosmological simulations use simplified phenomenological relationships to implement SF and its feedback.
In Choi & Nagamine 2009ab, we are developing improved models in cosmological N-body/Hydrodynamics simulation, GADGET.
We implemented metal enrichment, metal cooling, and evolution of mean atomic weight.
Owing to the additional metal cooling, global SFR is enhanced by two different mechanisms; 1) increase of SF efficiency in ISM, and 2) increase of IGM accretion onto galaxies.
The former process is effective throughout most of the cosmic history, while the latter is effective only at $z<3$ when the IGM is sufficiently enriched by metals ow ing to feedback.
Due to the former process, simulated galaxies with metal cooling become more gas poor than without metal cooling.
We developed a new SF model that takes the effect of molecular hydrogen formation into account.
Comparing with the previous conventional SF model, the new SF models show better agreement with the observations of the Kennicutt law.
Our new model suppresses the early SF and shifts the peak of the cosmic SF history toward low redshift, more consistently with the recent observations.
Our results suggest that there are still rooms left in the model uncertainties to reconcile the discrepancy that was found between the theory and observations of cosmic SF history and stellar mass development.
We also tested the effect of cosmological parameters on the cosmic SF history, and found that changes in cosmological parameters mostly affect the amplitude of the cosmic SF history.
The SFR is sensitive to the cosmological parameters that control the primordial power spectrum at high redshifts, while the cosmic matter content becomes important at lower redshifts.
Recent observations show that the galactic outflows are observed in three forms: hot X-ray winds, warm-ionized winds, and cold winds detected in absorption lines.
These outflows have different characteristics such as temperatures, wind velocities and mass loading factors.
In order to capture these complicated properties, we have developed a new wind model which reproduces multiphase outflows with variable wind speed depending on the galaxy SFR.
The new wind model does not overheats IGM, while significantly suppresses SFR and spreads metals into IGM, because large fractions of galactic winds in this model are warm and cold gas and contain considerable metals.
Using cosmological hydrodynamic simulations based on this new SF and feedback model, we are currently investigating many observational properties of galaxies such as mass metallicity relation, population of damped Lyα absorbers, and properties of GRB host galaxies.
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Dynamics of Super Massive Binary Black Hole
with Kentaro Nagamine, Daniel Proga, Kurosawa Ryuichi (UNLV)
Recent numerical simulations suggest that the galaxy merger and consequent super massive black hole (SMBH) merger triggers active galactic nuclei, and they are strongly supported by observed quasar population such as an X-ray and optical luminosity function of quasars.
However, our current understanding of binary SMBH formation and the outflow remains poor.
Many recent galaxy formation simulations include SMBH evolution and its outflow effects, but most of them simply assume instantaneous binary SMBH merger and ignore evolution effect during binary phase and outflow from binary SMBH.
We will use both large-scale cosmological simulation (such as Gadget) and the small-scale hydro simulation (such as ZEUS-MP) to make a clear connection of detail black hole physics and galaxy formation.
One of our proposed studies is the kinematic effect on the morphology of outflows owing to the binary motion.
We expect the outflow morphology from this system to be significantly different from that of a single SMBH.
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Tidal tail from satellite galaxy
with Martin D. Weinberg, Neal Katz (UMass)
Satellite galaxy tidal tails are an important observable fossil signature to help understand the formation history of the Milky Way and to test CDM theory as a consequence.
We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations.
We identify two important dynamical co-conspirators: 1) the points where the attractive force of the host halo and satellite are balanced (X-points) do not occur at equal distances from the satellite center or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails; and 2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite's gravity leading to large offsets of the ejecta orbits from the satellite orbit.
Although these findings complicate the interpretation of stellar streams and moving groups, the intrinsic mass dependence provides additional leverage on both halo and progenitor satellite properties.
In future, we plan to include the disk potential to predict realistic tidal tail morphology.
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Dark matter halo response on the disk growth
with Lu Yu, Houjun Mo, Martin D. Weinberg (UMass)
We thoroughly investigated the sensitivity of the circular-orbit adiabatic contraction approximation to the baryon condensation rate and the orbital structure of dark matter halos under the CDM model.
Using one-dimensional hydrodynamic simulations including the dark matter halo mass accretion history and gas cooling, we demonstrated that the adiabatic approximation is approximately valid even though halos and disks may assemble simultaneously, because the disk growth time is still much longer than the dark matter halo dynamical time in the disk vicinity.
Using N-body simulations, we demonstrated the validity of the circular-orbit approximation for isotropic CDM halos, because an isotropic velocity distribution in a cuspy halo requires more circular orbits than radial orbits.
In the extreme case of a radial orbit dominant halo, the approximation is failed.
This results suggest that the adiabatic contraction approximation is useful in modeling the response of dark matter halos to the growth of a disk.
Consequently the adiabatic contraction approximation is unlikely to result in the Tully-Fisher zero point problem.
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Galaxy mass growth history in &LambdaCDM cosmology
with Neal Katz, Chigurupati Murali (UMass), David Weinberg (OSU), Romeel Dave (U. Arizona), Lars Hernquist (Harvard)
We investigated that the importance of smooth accretion and mergers on galaxy mass growth as a function of environment, using cosmological N-body/Hydrodynamics simulations.
We found that smooth accretion dominates the mass growth of galaxies at all redshift and in all three environments, and mergers become progressively important later on.
For each process, the group environment is the preferred environment for merging and the field is preferred for smooth accretion.
Cluster galaxies assemble at higher redshift, while the central cluster galaxy keeps growing through the merging up to the present time.
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