SatGen: a semi-analytic satellite galaxy generator

Halo merger trees

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  • 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 (i.e., different large-scale density), and re-calibrate the EPS merger tree algorithm for different environments (Jiang+ in prep).

Statistics of dark matter subhalos

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  • 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. We have recently overhauled the model to emulate zoom-on hydro-simualtions, incorporating orbit integration in composite potentials with dynamical friction, tidal evolution, ram pressure, as well as halo response to baryonic effects (Jiang + 2020a). 

 

Galaxy-halo connection

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

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  • 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, Dekel+2018)

Dwarf galaxy structure

  • We characterize the formation of ultra-diffuse galaxies (UDGs) in cosmological simulations. We show that field-UDGs dwell in cored dark matter halos with normal spin, and that normal dwarf satellites can be transformed into red UDGs at orbital pericenters due to tidal heating and ram pressure stripping (Jiang, Dekel, Freundlich+2019).

  • 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 the mass range of "bright dwarfs" (M_star ≈ 10^{7-9}M_sun or M_vir ≈ 10^{9.5-11.3}M_sun). (Jiang + 2020b, 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 + 2020c 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 3-parameter halo density profile that is a subclass of the alpha-beta-gamma family. We also provide analytic expressions of gravitational potential, velocity dispersion, and surface density for this density profile, which could be useful for dynamical modeling and models of gravitational lenses (Freundlich, Jiang+2020). 

 

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); and with S. Huang and A. Leauthaud et al., we are trying to infer the triaxial 3D stellar distribution from deep imaging data.

Red nugget

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  • 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 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 underwent 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 ring (or tightly wound spiral structures) that could form after compaction from the newly accreted high-angular-momentum gas. (Dekel+ (including F. Jiang) 2020

 

Dr. Fangzhou Jiang