Recent Results

Ghostly Stellar Haloes and their Relationship to Ultra-faint Dwarfs

Ghostly stellar haloes are extended haloes of stars composed solely of debris of pre-reionization fossil galaxies and should exist in dwarf galaxies with total masses < 1010 M . Fossil galaxies are even smaller mass dwarf galaxies that stopped forming stars after the epoch of reionization and have been identified in the Local Group as the ultra-faint dwarf satellites. Using cosmo- logical N-body simulations we present an empirical model for the shape and mass of ghostly stellar haloes. We compare the model to available observations of stellar haloes in six isolated dwarf galaxies in the Local Group (Leo T, Leo A, IC 10, WLM, IC 1613, NGC 6822) to infer the star formation efficiency in dwarf galaxies at the epoch of reionization. We find an efficiency of star formation in dark matter haloes with masses 106 - 108 M at 𝑧 ∼ 7 in rough agreement with independent methods using data on the luminosity function of ultra-faint dwarf galaxies but systematically higher by a factor of 3-5. The systematic uncertainty of our results is still large, mainly because available observations of stellar halo profiles do not extend over a sufficiently large distance from the center of the host dwarf galaxy. Additional observations, easily within reach of current telescopes, can significantly improve the accuracy of this method and can also be used to constrain the present day dark matter masses of dwarf galaxies in the Local Group. Our method is based on a set of observations never used before, hence it is a new independent test of models of hierarchical galaxy formation.

Full Article

Caption: Formation of the stellar halo around a dwarf galaxy (similar to WLM or IC10; beta=-0.5)

PhD Students:

Undergrad Students:

Close Collaborators

Former Postdoctoral Research Associates/Collaborators

Former PhD Students:

  1. Jongwon Park (Current position: Postdoc at Yonsei University, South Korea)

    PhD Thesis (2024): "The Formation of Metal-Free Population III Stars in X-ray and Lyman-Werner Radiation Backgrounds"

  2. Blake Hartley (Current position: Private sector, MD)

    PhD Thesis (2023): "Simulating Bursty and Continuous Reionization Using GPU Computing"

  3. Milena Crnogorcevic (Current position: Postdoc at Stockholm University/Ohio)

    PhD Thesis (2023): "New Messengers and New Physics: A Survey of the High-energy Universe"

  4. ChongChong He (Current position: Postdoc at Australian National University, Camberra, Australia)

    PhD Thesis (2023): "Multiscale Radiation-MHD Simulations of Compact Star Clusters"

  5. Kari Helgason (Current position: Head of Research & Innovation at Carbfix, Iceland)

    PhD Thesis (2014): "The Cosmic Near-Infrared Background: from the Dark Ages to the Present"

  6. Emil Polisensky (Current position: Research Physicist at Naval Research Lab (NRL) Washington, USA)

    PhD Thesis (2014): "Simulations of Small Scale Structures in the Local Universe to Constrain the Nature of the Dark Matter"

  7. Kwang-Ho Park (Current position: Postdoc Gtech Atlanta, USA)

    PhD Thesis (2012): "Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback"

  8. Mia Bovill (Current position: Assistant Professor, TCU, USA)

    PhD Thesis (2011): "The Fossils of the First Galaxies in the Local Universe"

Former Undergrad Students:

Work on Primordial Black Holes in the news)

Old Research Projects:

  1.   The cosmological origin of dwarf galaxies
    Student: Mia Bovill

    Key questions on the origin of dwarf galaxies are still waiting for an answer. The aim of this proposal is to conduct numerical and semianalytical experiments to test against observations competing hypothesis on the cosmological origin of dwarf galaxies. The research that we propose to conduct is founded on the results of cosmological simulations of the formation of the first galaxies at redshifts prior to reionization. Unfortunately existing supercomputers are not powerful enough to evolve primordial dwarf galaxies, including all the relevant physical processes, from high redshifts to the present day. The technical approach that is proposed in this project is a first attempt to circumvent these numerical limitations. The method is an approximation of the full problem but is innovative and complementary to previous studies. The idea is to use the detailed results of high redshift simulations in conjunction with an N-body code to trace the trajectories of each galaxy - identified in the simulation using a halo finding algorithm - from the redshift of formation to the present. The proposed work is an important first step to toward answering important questions such as: are the voids between luminous galaxies populated by faint dwarf Spheroidals? are the voids polluted by heavy elements or they have primordial composition? what is the minimum luminosity and surface brightness of the smallest galaxies that ever formed in the Universe?

  2.   Intermediate mass black hole accretion in primordial galaxies
    Student: Franciska Koeckert

Ideas for Research Projects and Thesis:

  1.   Formation of Population III stars and black holes in the early Universe
    This project consist on theoretical work on the formation of the first
    galaxies and black holes in the Universe. Feedback processes on
    galactic and cosmological scales regulate the formation history of the
    first galaxies and black holes. As a consequence, galaxies and black
    holes can only form under certain restrictive conditions, poorly
    understood and the focus of the proposed study. The primary technical
    tools of the research are cosmological radiative transfer simulations.
    Numerical simulations with chemo-radiative feedback are needed to
    understand the formation of population III stars and seed black hole
    formation and accretion in the early Universe.
    The main objectives of the proposed research are: (i) to simulate the
    transition from zero metallicity stars (Population III) to normal star
    formation and estimate the importance of Population III stars for the
    reionization of the intergalactic medium (IGM); (ii) to formulate
    physically motivated models for the formation rate of stellar black
    holes from the first stars and their subsequent accretion and merger
    history; (iii) to evaluate the importance of gas accretion onto the
    first black holes for the ionization of the IGM and the build up of
    supermassive black holes.
  2.   Black hole accretion regulated by radiative feedback
     The project will involve writing a hydro-code to simulate the
    effect of UV and X-ray radiation on the rate of gas inflow onto a
    black hole. This code can be easily written by merging two existing
    codes one that calculates the chemistry and radiative transfer in 1D
    and a hydro code with a spherical coordinate grid. We will use this
    code to study how radiation affects the Bondi accretion rate for a
    black hole that moves in a medium or a black hole at rest in the
    center of a galaxy.  
  3.   Properties of the ISM in primordial galaxies and in Lyman Limit systems
     The first small mass galaxies were metal poor because
    their ability to form star was reduced by complex feedback
    processes. In previous works we have studied these feedbacks on
    cosmological scales and we were able to determine the mean and
    statistical properties of the gas and stars in the first galaxies. Due
    to the limited numerical resolution we could not study in detail the
    internal structure of the interstellar medium that appears to be very
    different from the interstellar medium in our Galaxy. This project
    focus on theoretical studies of the multi-phase structure of the
    interstellar medium galaxies with small amounts of metals and dust and
    a much larger gas abundance with respect to present day
    galaxies. These studies are also of crucial importance to understand
    the properties of the ISM in the observed sample of high-redshift
    galaxies. A wealth of observations on the properties of the ISM are
    available from studies of "Lyman-Limit systems" thought to be disk
    galaxies at redshift z = 3-4.  
  4.   Inferring the nature of dark matter from the shape of galaxy's dark halos
    This research is the follow up of a claimed dependence of the
    shape of dark matter halos on their total mass (Ricotti 2003). We will
    test this claim with better simulations and will study how the power
    spectrum of initial perturbation and other cosmological parameters
    affect the shape of the halos. We will also compare observed rotation
    curves of dark matter halos with simulations adopting a uniform
    method. The aim is to better understand the degree of disagreement
    between simulation and observation and from these result infer the
    properties of the power spectrum of perturbations.
  5.   Dark matter "texture" and gamma ray background
     This project aims to understand the small scale clumping of dark
    matter. The dark matter is expected to be concentrated in numerous
    discrete clumps of planet size or smaller, at least at very
    high-redshifts. The evolution of these clumps and their survival from
    high-redshift to the present day is now well understood. We will use
    numerical simulation and analytical calculations to estimate the
    survival of the dark matter clumps. This study is important for
    studies of direct and indirect detection of dark matter particles. It
    is also important to estimate the gamma ray background produced by the
    possible self-annihilation of dark matter particles.  

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