arXiv subjects

Sijing Shen

Publications and source records attributed to Sijing Shen.

At least 55 records · Page 3Linked to original sources

arXiv2016

Diffuse coronae in cosmological simulations of Milky Way-sized galaxies

We investigate the properties of halo gas using three cosmological `zoom-in' simulations of realistic Milky Way-galaxy analogs with varying sub-grid physics. In all three cases, the mass of hot ($T > 10^6$ K) halo gas is $\sim 1\%$ of the host's virial mass. The X-ray luminosity of two of the runs is consistent with observations of the Milky Way, while the third simulation is X-ray bright and resembles more closely a very massive, star-forming spiral. Hot halos extend to 140 kpc from the galactic center and are surrounded by a bubble of warm-hot ($T = 10^5 - 10^6$K) gas that extends to the virial radius. Simulated halos agree well outside 20-30 kpc with the $β$-model of Miller & Bregman (2013) based on OVII absorption and OVIII emission measurements. Warm--hot and hot gas contribute up to $80\%$ of the total gas reservoir, and contain nearly the same amount of baryons as the stellar component. The mass of warm-hot and hot components falls into the range estimated for $L^*$ galaxies. With key observational constraints on the density of the Milky Way corona being satisfied, we show that concealing of the ubiquitous warm-hot baryons, along with the ejection of just $20-30 \%$ of the diffuse gas out of the potential wells by supernova-driven outflows, can solve the "missing baryon problem". The recovered baryon fraction within 3 virial radii is $90\%$ of the universal value. With a characteristic density of $\sim 10^{-4}$ cm$^{-3}$ at $50-80$ kpc, diffuse coronae meet the requirement for fast and complete ram--pressure stripping of the gas reservoirs in dwarf galaxy satellites.

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arXiv2015

A lower fragmentation mass scale in high redshift galaxies and its implications on giant clumps: a systematic numerical study

We study the effect of sub-grid physics, galaxy mass, structural parameters and resolution on the fragmentation of gas-rich galaxy discs into massive star forming clumps. The initial conditions are set up with the aid of the ARGO cosmological hydrodynamical simulation. Blast-wave feedback does not suppress fragmentation, but reduces both the number of clumps and the duration of the unstable phase. Once formed, bound clumps cannot be destroyed by our feedback model. Widespread fragmentation is promoted by high gas fractions and low halo concentrations. Yet giant clumps $M > 10^8 M_{\odot}$ lasting several hundred Myr are rare and mainly produced by clump-clump mergers. They occur in massive discs with maximum rotational velocities $V_{max} > 250$ km/s at $z \sim 2$, at the high mass end of the observed galaxy population at those redshifts. The typical gaseous and stellar masses of clumps in all runs are in the range $\sim 10^7-10^8 M_{\odot}$ for galaxies with disc mass in the range $10^{10}-8\times 10^{10} M_{\odot}$. Clumps sizes are usually in the range $100-400$ pc, in agreement with recent clump observations in lensed high-z galaxies. \\ We argue that many of the giant clumps identified in observations are not due to in-situ fragmetation, or are the result of blending of smaller structures owing to insufficient resolution. Using an analytical model describing local collapse inside spiral arms, we can predict the characteristic gaseous masses of clumps at the onset of fragmentation ($\sim 3-5 \times 10^7 M_{\odot}$) quite accurately, while the conventional Toomre mass overestimates them. Due to their moderate masses, clumps which migrate to the centre have marginal effect on bulge growth.

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arXiv2015

The History of R-Process Enrichment in the Milky Way

We investigate the production sites and the enrichment history of $r$-process elements in the Galaxy, as traced by the [Eu/Fe] ratio, using the high resolution, cosmological zoom-in simulation `Eris'. At $z=0$, Eris represents a close analog to the Milky Way, making it the ideal laboratory to understand the chemical evolution of our Galaxy. Eris formally traces the production of oxygen and iron due to Type-Ia and Type-II supernovae. We include in post-processing the production of $r$-process elements from compact binary mergers. Unlike previous studies, we find that the nucleosynthetic products from compact binary mergers can be incorporated into stars of very low metallicity and at early times, even with a minimum delay time of 100 Myr. This conclusion is relatively insensitive to modest variations in the merger rate, minimum delay time, and the delay time distribution. By implementing a first-order prescription for metal-mixing, we can further improve the agreement between our model and the data for the chemical evolution of both [$α$/Fe] and [Eu/Fe].We argue that compact binary mergers could be the dominant source of $r$-process nucleosynthesis in the Galaxy.

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arXiv2014

Faint dwarfs as a test of DM models: WDM vs. CDM

We use high resolution Hydro$+$N-Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass ~ 10$^{6-7}$ M$\odot$, total mass 10$^{10}$ M$\odot$ in $Λ$ dominated CDM and 2keV WDM cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1-2 Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the DM model, but proportionally to the SF efficiency, gas outflows lower the central mass density through `dynamical heating', such that all realizations have circular velocities $<$ 20kms at 500$~$pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial color-magnitude diagrams and star formation histories in order to directly compare to available observations. The simulated galaxies formed most of their stars in many $\sim$10 Myr long bursts. The CDM galaxy has a global SFH, HI abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including `baryon physics' in simulations when 1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and 2) differentiating between CDM and non-standard models with different DM or power spectra.

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arXiv2014

Direct formation of supermassive black holes in metal-enriched gas at the heart of high-redshift galaxy mergers

We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergers between the most massive galaxies at $z \sim 8 - 10$, designed to scrutinize the direct collapse formation scenario for massive black hole seeds proposed in \citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically-thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidly accretes higher angular momentum gas from its surroundings reaching $\sim 5$ pc and a mass of $\gtrsim 10^9$ $M_{\odot}$ in only a few $10^4$ yr. Outside $\gtrsim 2$ pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm ($\sim 3000-6000$ K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows ($> 10^4$ $M_{\odot}$ yr$^{-1}$), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass $10^8-10^9$ $M_{\odot}$ via the relativistic radial instability. Such a "cold direct collapse"' explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the planned eLISA interferometer.

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arXiv2014

The Role of Cold Flows and Reservoirs in Galaxy Formation With Strong Feedback

We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations (Stinson et al. 2010). Accreted gas is bimodal with a natural temperature division at $10^5$ K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z=2-4 and declining exponentially below z$\sim$2. Hot-mode accretion peaks near z=1-2 and declines gradually. Hot-mode exceeds cold-mode accretion at z$\sim$1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2-8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z=1-2 from the accretion peak at z=2-4.

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arXiv2014

The Baryon Cycle of Dwarf Galaxies: Dark, Bursty, Gas-Rich Polluters

We present results from a fully cosmological, very high-resolution, LCDM "zoom-in" simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M_vir=4.4e8-3.6e10 Msun. The simulation includes a blastwave scheme for supernova feedback, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background that modifies the ionization and excitation state of the gas. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M_vir < 1e9 Msun are devoid of stars, as they never reach the density threshold for star formation of 100 atoms/cc. The other four, M_vir > 1e9 Msun dwarfs have blue colors, low star formation efficiencies, high cold gas to stellar mass ratios, and low stellar metallicities. Their bursty star formation histories are characterized by peak specific star formation rates in excess of 50-100 1/Gyr, far outside the realm of normal, more massive galaxies, and in agreement with observations of extreme emission-line starbursting dwarfs by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a Mpc-size region of the intergalactic medium, but are not sufficient to completely quench star formation activity and are not ubiquitous in our dwarfs. Within the limited size of the sample, our simulations appear to simultaneously reproduce the observed stellar mass and cold gas content, resolved star formation histories, stellar kinematics, and metallicities of field dwarfs in the Local Volume.

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arXiv2014

Dark Matter Heating and Early Core Formation in Dwarf Galaxies

We present more results from a fully cosmological LCDM simulation of a group of isolated dwarf galaxies that has been shown to reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of dwarfs in the Local Volume. Here we investigate the energetics and timetable of the cusp-core transformation. As suggested by previous work, supernova-driven gas outflows remove dark matter (DM) cusps and create kpc-size cores in all systems having a stellar mass above 1e6 Msun. The "DM core mass removal efficiency" -- dark mass ejected per unit stellar mass -- ranges today from a few to a dozen, and increases with decreasing host mass. Because dwarfs form the bulk of their stars prior to redshift 1 and the amount of work required for DM heating and core formation scales approximately as Mvir^{5/3}, the unbinding of the DM cusp starts early and the formation of cored profiles is not as energetically onerous as previously claimed. DM particles in the cusp typically migrate to 2-3 core radii after absorbing a few percent of the energy released by supernovae. The present-day slopes of the inner dark matter mass profiles, Gamma=d log M/dlog R=2.5-3, of the simulated "Bashful" and "Doc" dwarfs are similar to those measured in the luminous Fornax and Sculptor dwarf spheroidals. None of the simulated galaxies has a circular velocity profile exceeding 20 km/s in the inner 1 kpc, implying that supernova feedback is key to solve the "too-big-to-fail" problem for Milky Way subhalos.

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arXiv2014

The effect of models of the interstellar media on the central mass distribution of galaxies

We compare the central mass distribution of galaxies simulated with three different models of the interstellar medium (ISM) with increasing complexity: primordial (H+He) cooling down to 10^4K, additional cooling via metal lines and to lower temperatures, and molecular hydrogen (H_2) with shielding of atomic and molecular hydrogen, in addition to metal line cooling. In order to analyze the effect of these models, we follow the evolution of four field galaxies with V_peak < 120 km/s to a redshift of zero using high-resolution Smoothed Particle Hydrodynamic simulations in a fully cosmological LCDM context. The spiral galaxies produced in simulations with either primordial cooling or H_2 physics have realistic, rising rotation curves. In contrast, the simulations with metal line cooling and otherwise similar feedback and star formation produced galaxies with the peaked rotation curves typical of most previous LCDM simulations of spiral galaxies. The less-massive bulges and non-peaked rotation curves in the galaxies simulated with primordial cooling or H_2 are linked to changes in the angular momentum distribution of the baryons. These galaxies had smaller amounts of low-angular momentum baryons because of increased gas loss from stellar feedback. When there is only primordial cooling, the star forming gas is hotter and the feedback-heated gas cools more slowly than when metal line cooling is included and so requires less energy to be expelled. When H_2 is included, the accompanying shielding produces large amounts of clumpy, cold gas where H_2 forms. Star formation in clumpy gas results in more concentrated supernova feedback and greater efficiency of mass loss. The higher feedback efficiency causes a decrease of low-angular momentum material. (abridged)

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arXiv2013

The AGORA High-Resolution Galaxy Simulations Comparison Project

We introduce the AGORA project, a comprehensive numerical study of well-resolved galaxies within the LCDM cosmology. Cosmological hydrodynamic simulations with force resolutions of ~100 proper pc or better will be run with a variety of code platforms to follow the hierarchical growth, star formation history, morphological transformation, and the cycle of baryons in and out of 8 galaxies with halo masses M_vir ~= 1e10, 1e11, 1e12, and 1e13 Msun at z=0 and two different ("violent" and "quiescent") assembly histories. The numerical techniques and implementations used in this project include the smoothed particle hydrodynamics codes GADGET and GASOLINE, and the adaptive mesh refinement codes ART, ENZO, and RAMSES. The codes will share common initial conditions and common astrophysics packages including UV background, metal-dependent radiative cooling, metal and energy yields of supernovae, and stellar initial mass function. These are described in detail in the present paper. Subgrid star formation and feedback prescriptions will be tuned to provide a realistic interstellar and circumgalactic medium using a non-cosmological disk galaxy simulation. Cosmological runs will be systematically compared with each other using a common analysis toolkit, and validated against observations to verify that the solutions are robust - i.e., that the astrophysical assumptions are responsible for any success, rather than artifacts of particular implementations. The goals of the AGORA project are, broadly speaking, to raise the realism and predictive power of galaxy simulations and the understanding of the feedback processes that regulate galaxy "metabolism." The proof-of-concept dark matter-only test of the formation of a galactic halo with a z=0 mass of M_vir ~= 1.7e11 Msun by 9 different versions of the participating codes is also presented to validate the infrastructure of the project.

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arXiv2013

The Pressure of the Star Forming ISM in Cosmological Simulations

We examine the pressure of the star-forming interstellar medium (ISM) of Milky-Way sized disk galaxies using fully cosmological SPH+N-body, high resolution simulations. These simulations include explicit treatment of metal-line cooling in addition to dust and self-shielding, $\mathrm{H_{2}}$ based star formation. The 4 simulated halos have masses ranging from a few times $10^{10}$ to nearly $10^{12}$ solar masses. Using a kinematic decomposition of these galaxies into present-day bulge and disk components, we find that the typical pressure of the star-forming ISM in the present-day bulge is higher than that in the present-day disk by an order of magnitude. We also find that pressure of the star-forming ISM at high redshift is on average, higher than ISM pressures at low redshift. This explains the why the bulge forms at higher pressures: the disk assembles at lower redshift, when the ISM is lower pressure and the bulge forms at high redshift, when the ISM is at higher pressure. If ISM pressure and IMF variation are tied together as suggested in studies like \cite{Conroy2012}, these results could indicate a time-dependent IMF in Milky-Way like systems, as well as a different IMF in the bulge and the disk.

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arXiv2013

The circumgalactic medium of massive galaxies at z~3: a test for stellar feedback, galactic outflows, and cold streams

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift 3, using the "Eris" suite of cosmological "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. Synthetic spectra through the multiphase CGM produce interstellar absorption line strengths of Lya, CII, CIV, SiII, and SiIV as a function of galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high-redshift by Steidel et al. (2010). Only about one third of all the gas within R_vir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick material is 27%, in agreement with recent observations by Rudie et al. (2012). Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar, give origin to strong (log N > 13) CII absorption with a covering factor of 22% within R_vir and 10% within 2 R_vir. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large OVI halo, with a typical column density log N>14 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. (2011). Our zoom-in simulations of this single system appear to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings. (Abridged)

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arXiv2012

Baryons Matter: Why Luminous Satellite Galaxies Have Reduced Central Masses

Using high resolution cosmological hydrodynamical simulations of Milky Way-massed disk galaxies, we demonstrate that supernovae feedback and tidal stripping lower the central masses of bright (-15 < M_V < -8) satellite galaxies. These simulations resolve high density regions, comparable to giant molecular clouds, where stars form. This resolution allows us to adopt a prescription for H_2 formation and destruction that ties star formation to the presence of shielded, molecular gas. Before infall, supernova feedback from the clumpy, bursty star formation captured by this physically motivated model leads to reduced dark matter (DM) densities and shallower inner density profiles in the massive satellite progenitors (Mvir > 10^9 Msun, Mstar > 10^7 Msun) compared to DM-only simulations. The progenitors of the lower mass satellites are unable to maintain bursty star formation histories, due to both heating at reionization and gas loss from initial star forming events, preserving the steep inner density profile predicted by DM-only simulations. After infall, tidal stripping acts to further reduce the central densities of the luminous satellites, particularly those that enter with cored dark matter halos, increasing the discrepancy in the central masses predicted by baryon+DM and DM-only simulations. We show that DM-only simulations, which neglect the baryonic effects described in this work, produce denser satellites with larger central velocities. We provide a simple correction to the central DM mass predicted for satellites by DM-only simulations. We conclude that DM-only simulations should be used with great caution when interpreting kinematic observations of the Milky Way's dwarf satellites.

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arXiv2012

MAGICC haloes: confronting simulations with observations of the circumgalactic medium at z=0

We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L ~ 0.1 and 1 L* generated using the smooth particle hydrodynamic code GASOLINE. These simulations are part of the Making Galaxies In a Cosmological Context (MAGICC) program in which the stellar feedback is tuned to match the stellar mass-halo mass relationship. For comparison, each galaxy was also simulated using a 'lower feedback' (LF) model which has strength comparable to other implementations in the literature. The 'MAGICC feedback' (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O VI absorption. The simulations do not yet address the question of what happens to the O VI when the galaxies stop forming stars. Our models also predict a reservoir of cool H I clouds that show strong Lyαabsorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O VI and H I gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H I gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes.

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arXiv2012

The origin of metals in the circum-galactic medium of massive galaxies at z=3

We present a detailed study of the metal-enriched circum-galactic medium of a massive galaxy at z=3 using "ErisMC", a new cosmological hydrodynamic "zoom-in" simulations of a disk galaxy with mass comparable to the Milky Way. The run adopts a blastwave scheme for supernova feedback that generates galactic outflows without explicit wind particles, a star formation recipe based on a high gas density threshold, and high temperature metal cooling. ErisMC's main progenitor at z=3 resembles a "Lyman break" galaxy of mass M_vir=2.4e11 M_sun, virial radius R_vir=48 kpc, and star formation rate 18 M_sun/yr, and its metal-enriched CGM extends to 200 (physical) kpc from its center. Approximately 41, 9, and 50 percent of all gas-phase metals at z=3 are locked in a hot (T> 3e5 K), warm (3e5 >T> 3e4 K), and cold (T< 3e4 K) medium, respectively. We identify three sources of heavy elements: 1) the main host, responsible for 60% of all the metals found within 3R_vir; 2) its satellite progenitors, responsible for 28% of all the metals within 3R_vir, and for only 5% of those beyond 3R_vir; and nearby dwarfs, which give origin to 12% of all the metals within 3R_vir and 95% of those beyond 3R_vir. Late (z<5) galactic "superwinds" account for only 9% of all the metals observed beyond 2R_vir, the bulk having been released at redshifts 5< z < 8 by early star formation and outflows. In the CGM, lower overdensities are typically enriched by `older', colder metals. Heavy elements are accreted onto Eris along filaments via low-metallicity cold inflows, and are ejected hot via galactic outflows at a few hundred km/s. The outflow mass-loading factor is of order unity for the main halo, but can exceed 10 for nearby dwarfs. We stress that our "zoom-in" simulation focuses on the CGM of a single massive system and cannot describe the enrichment history of the IGM as a whole. (abridged)

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arXiv2012

Implementing Molecular Hydrogen in Hydrodynamic Simulations of Galaxy Formation

Motivated by the observed connection between molecular hydrogen (H2) and star formation, we present a method for tracking the non-equilibrium abundance and cooling processes of H2 and H2-based star formation in Smoothed Particle Hydrodynamic simulations. The local abundances of H2 are calculated by integrating over the hydrogen chemical network. This calculation includes the gas-phase and dust grain formation of H2, shielding of HI and H2, and photodissociation of H2 by Lyman-Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low-metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H2 abundance. This link limits star formation to "star-forming regions," represented by areas with abundant H2. With this implementation, we determine the effect of H2 on star formation in a cosmological simulation of a dwarf galaxy. This simulation is the first cosmological simulation with non-equilibrium H2 abundances to be integrated to a redshift of zero or to include efficient SN feedback. We find that our simulations are consistent with the observed Tully-Fisher, global Kennicutt-Schmidt, and resolved Kennicutt-Schmidt relations. We find that the inclusion of shielding of both the atomic and molecular hydrogen and, to a lesser extent, the additional cooling from H2 at temperatures between 200 and 5000 K increases the amount of cold gas in the galaxies. The changes to the ISM result in an increased amount of cold, dense gas in the disk of the galaxy and the formation of a clumpier interstellar media (ISM). The explicit link between star formation and H2 and the clumpier ISM results in a bluer galaxy with a greater spatial distribution of star formation at a redshift of zero. (abridged)

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arXiv2011

Halo expansion in cosmological hydro simulations: towards a baryonic solution of the cusp/core problem in massive spirals

A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo density profiles on all mass scales. This is not in agreement with the observed rotation curves of spiral galaxies, challenging on small scales the otherwise successful CDM paradigm. In this work we employ high resolution cosmological hydro-dynamical simulations to study the effects of dissipative processes on the inner distribution of dark matter in Milky-Way like objects (M~1e12 Msun). Our simulations include supernova feedback, and the effects of the radiation pressure of massive stars before they explode as supernovae. The increased stellar feedback results in the expansion of the dark matter halo instead of contraction with respect to N-body simulations. Baryons are able to erase the dark matter cuspy distribution creating a flat, cored, dark matter density profile in the central several kpc of a massive Milky-Way like halo. The profile is well fit by a Burkert profile, with fitting parameters consistent with the observations. In addition, we obtain flat rotation curves as well as extended, exponential stellar disk profiles. While the stellar disk we obtain is still partially too thick to resemble the MW thin disk, this pilot study shows that there is enough energy available in the baryonic component to alter the dark matter distribution even in massive disc galaxies, providing a possible solution to the long standing problem of cusps vs. cores.

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arXiv2011

The First Massive Black Hole Seeds and Their Hosts

We investigate the formation of the first massive black holes in high redshift galaxies, with the goal of providing insights to which galaxies do or do not host massive black holes. We adopt a novel approach to forming seed black holes in galaxy halos in cosmological SPH+N-body simulations. The formation of massive black hole seeds is dictated directly by the local gas density, temperature, and metallicity, and motivated by physical models of massive black hole formation. We explore seed black hole populations as a function of halo mass and redshift, and examine how varying the efficiency of massive black hole seed formation affects the relationship between black holes and their hosts. Seed black holes tend to form in halos with mass between 10^7 and 10^9 Msun, and the formation rate is suppressed around z = 5 due to the diffusion of metals throughout the intergalactic medium. We find that the time of massive black hole formation and the occupation fraction of black holes are a function of the host halo mass. By z = 5, halos with mass M_halo > 3 x 10^9 Msun host massive black holes regardless of the efficiency of seed formation, while the occupation fraction for smaller halos increases with black hole formation efficiency. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems.

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