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SatGen: a semi-analytic satellite galaxy generator

Halo merger trees

  • We compare different algorithms (based on the extended-Press-Schechter formalism) for generating halo merger trees (Jiang & van den Bosch 2014) and show that the algorithm by Parkinson+ (2008) yields merger statistics in best agreement with those from cosmological simulations.

  • We characterize halo progenitor mass functions for halos in different environments, and re-calibrate the EPS merger tree algorithm for different environments (Jiang+ in prep).

Statistics of dark matter subhalos

  • We characterize the statistics of dark matter halo substructure using the SatGen model and N-body cosmological simulations (Jiang & van den Bosch 2015;  16;  17; van den Bosch & Jiang 2016). We show that ~1% of Milky-Way sized halos are free from the too-big-to-fail problem due to the dramatic halo-to-halo variance of the formation histories. We also show that the halo-to-halo variance of substructure abundance is important in multiple contexts, including subhalo detection by gravitational lensing, and halo models. I have recently overhauled the model to emulate zoom-in hydro-simualtions, incorporating orbit integration in composite potentials with dynamical friction, tidal evolution, ram pressure, as well as halo response to baryon feedback (Jiang + 2020; Green, van den Bosch, Jiang + 2020 submitted to MNRAS). 

Galaxy-halo connection

Is dark matter halo spin a predictor of galaxy spin and size?

  • We find with different hydro-cosmological simulations that galaxy spin and size are barely correlated with the spin of host dark matter halos; instead, galaxy size correlates with the halo concentration parameter, : R_eff ~ c^-0.7 R_vir (Jiang+2019). More generally, we look for secondary halo parameters systematically that affect galaxy morphology using cosmological simulations and machine learning algorithms (Liang, Jiang+2024a,b). 

Dwarf galaxy structure

  • We characterize the formation of ultra-diffuse galaxies (UDGs) in cosmological simulations. We show that field-UDGs dwell in cored, dwarf-mass dark matter halos with normal spin. Additionally, dense environment can transform normal dwarf satellites into red UDGs through impulsive tidal heating and ram pressure stripping at orbital percenters (Jiang, Dekel, Freundlich+2019). This two-channel formation picture has since then been confirmed by other simulation studies. 

  • We use the SatGen semi-analytic model to propagate the effects on galaxy structure of different feedback models adopted in hydro-simulations onto satellite galaxies of clusters and groups (that are not properly resolved in simulations), and quantify their influence on the statistics of dwarf satellites in "bright dwarfs" (M_star ≈ 10^{7-9}M_sun or M_vir ≈ 10^{9.5-11.3}M_sun). (Jiang + , in prep)

  • We parameterize the "burstiness" of star-formation histories (SFH), and quantify in detail the correlation between dwarf-galaxy structure and SFH burstiness using cosmological simulations. (Jiang + in prep) 

Cusp-core transformation

  • We present a simple analytic model that predicts the change of halo density profile upon mass increments or decrements from the center (which mimic gas inflows and outflows). The model makes use of a 3-parameter density profile that has good analytic properties and the flexibility in the inner slope, and assumes energy conservation (Freundlich, Dekel, Jiang+2020) . 

  • We extract the response of dark-matter haloes to baryonic feedback in cosmological simulations, utilizing a halo density profile that is a subclass of the alpha-beta-gamma family and of good analytical properties. We provide analytic expressions of gravitational potential, velocity dispersion, and surface density for this profile, which are useful for dynamical modeling and models of gravitational lenses (Freundlich, Jiang +2020). 

Self-interacting dark matter

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We study the density profile of self-interacting dark-matter (SIDM) halos using analytic models, cosmological simualtions, as well as idealized simulations. Combining models for adiabatic contraction of halos, and self-interaction induced isothermal cores, we analytically predict SIDM profiles given self-interacting cross section and the final baryon distribution. This model is in excellent agreement with predictions from zoom-in cosmological simulations and idealized simulations with DM self-interaction, and allows for faster semi-analytic calculations of SIDM halo profiles. (Jiang + in prep).  We incorporate this model as well as physical recipes for mass loss and deceleration of SIDM substructures into semi-analytic models SatGen and Galacticus, in order to predict satellite statistics in SIDM Universe (work in progress with collaborators at the Carnegie Observatories, Ohio State University, Princeton, and UC Irvine).

Galaxy morphology

Low-surface brightness

  • Deep imaging reveals the low-surface brightness components of massive ellipticals out to several effective radii (e.g., Jiang, Huang, Gu 2011). We are trying to infer the triaxial 3D stellar distribution from deep imaging data, using Bayesian inference and machine learning tools.

Red nugget

  • We examine a nearby galaxy that resembles a high-z "red nugget", and show that there is a dynamically important stellar disk in the center. This kind of disky structures could be prevalent in early-type galaxies at z~2, and thus naturally account for the observed ellipticities of red-nuggets in the early universe, but they remain undetected because they are too compact at z~2 to be resolved by HST.  (Jiang, van Dokkum, Bezanson, Franx 2012).​

Post-compaction disks and rings

  • Using simple analytic arguments and high-resolution cosmological simulations, we show that high-redshift galaxies won't form stable gas discs until they reach a threshold mass of M_vir ~ 10^{11}M_sun or M_star ~ 10^9M_sun, when they tend to undergo dramatic angular-momentum loss and formed a compact star-formation core (a process that we dub as "compaction") (Dekel, Ginsburg, Jiang+2020).  We also show that the bulge formed in the center of a post-compaction system serves as a stabilizing mechanism for a clumpy gas rings (or tightly wound spiral structures) that form with the newly accreted high-angular-momentum gas. (Dekel+ (including F. Jiang) 2020

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