CHRISTOPHER BRYAN BROOK . - Academia.edu (original) (raw)
Papers by CHRISTOPHER BRYAN BROOK .
GCD+ (Galactic Chemodynamics Plus) is a soon-to-be publically available N-body/SPH code being dev... more GCD+ (Galactic Chemodynamics Plus) is a soon-to-be publically available N-body/SPH code being developed at Swinburne University for modeling the formation and evolution of galaxies within a cosmological framework. A sophisticated chemical evolution module as been incorporated within GCD+ making use of the latest stellar yields on the market; a self-consistent treatment of energy feedback from Type Ia and II supernovae (relaxing the instantaneous recycling approximation) cooling and star formation is standard within GCD+. Spatially resolved synthetic maps can be generated ranging from stellar populations to the hot and warm X-ray emitting properties of clusters. We will demonstrate GCD+'s application to simulating the 7-dimensional phase space (position velocity chemistry) distribution of the oldest stars in the Milky Way in addition to its seemless predictive power in regards to spatial and temporal evolution of the age-metallicty relationship metallicity distribution functions and the disruption of the Magellanic System
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Monthly Notices of the Royal Astronomical Society, Aug 24, 2010
We present the McMaster Unbiased Galaxy Simulations (MUGS), the first nine galaxies of an unbiase... more We present the McMaster Unbiased Galaxy Simulations (MUGS), the first nine galaxies of an unbiased selection ranging in total mass from 5 × 1011Msolar to 2 × 1012Msolar simulated using N-body smoothed particle hydrodynamics at high resolution. The simulations include a treatment of low-temperature metal cooling, UV background radiation, star formation and physically motivated stellar feedback. Mock images of the simulations show that the simulations lie within the observed range of relations such as that between colour and magnitude and that between brightness and circular velocity (Tully-Fisher). The greatest discrepancy between the simulated galaxies and observed galaxies is the high concentration of material at the centre of the galaxies as represented by the centrally peaked rotation curves and the high bulge-to-total ratios of the simulations determined both kinematically and photometrically. This central concentration represents the excess of low angular momentum material that long has plagued morphological studies of simulated galaxies and suggests that higher resolutions and a more accurate description of feedback will be required to simulate more realistic galaxies. Even with the excess central mass concentrations, the simulations suggest the important role merger history and halo spin play in the formation of discs.
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Observations suggest that the structural parameters of disk galaxies have not changed greatly sin... more Observations suggest that the structural parameters of disk galaxies have not changed greatly since redshift 1. We examine whether these observations are consistent with a cosmology in which structures form hierarchically. We use SPH/N-body galaxy-scale simulations to simulate the formation and evolution of Milky-Way-like disk galaxies by fragmentation, followed by hierarchical merging. The simulated galaxies have a thick disk, that forms in a period of chaotic merging at high redshift, during which a large amount of alpha-elements are produced, and a thin disk, that forms later and has a higher metallicity. Our simulated disks settle down quickly and do not evolve much since redshift z~1, mostly because no major mergers take place between z=1 and z=0. During this period, the disk radius increases (inside-out growth) while its thickness remains constant. These results are consistent with observations of disk galaxies at low and high redshift.
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The Astrophysical Journal, Mar 20, 2009
We use high-resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas ... more We use high-resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas accretion, particularly along filaments, modifies the standard picture of gas accretion and cooling onto galaxy disks. In the standard picture, all gas is initially heated to the virial temperature of the galaxy as it enters the virial radius. Low-mass galaxies are instead dominated by accretion of gas that stays well below the virial temperature, and even when a hot halo is able to develop in more massive galaxies there exist dense filaments that penetrate inside of the virial radius and deliver cold gas to the central galaxy. For galaxies up to ~L*, this cold accretion gas is responsible for the star formation (SF) in the disk at all times to the present. Even for galaxies at higher masses, cold flows dominate the growth of the disk at early times. Within this modified picture, galaxies are able to accrete a large mass of cold gas, with lower initial gas temperatures leading to shorter cooling times to reach the disk. Although SF in the disk is mitigated by supernovae feedback, the short cooling times allow for the growth of stellar disks at higher redshifts than predicted by the standard model.
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The Astrophysical Journal, Apr 22, 2009
We investigate the formation of the stellar halos of four simulated disk galaxies using high-reso... more We investigate the formation of the stellar halos of four simulated disk galaxies using high-resolution, cosmological SPH + N-body simulations. These simulations include a self-consistent treatment of all the major physical processes involved in galaxy formation. The simulated galaxies presented here each have a total mass of ~1012 M sun, but span a range of merger histories. These simulations allow us to study the competing importance of in situ star formation (stars formed in the primary galaxy) and accretion of stars from subhalos in the building of stellar halos in a ΛCDM universe. All four simulated galaxies are surrounded by a stellar halo, whose inner regions (r < 20 kpc) contain both accreted stars, and an in situ stellar population. The outer regions of the galaxies' halos were assembled through pure accretion and disruption of satellites. Most of the in situ halo stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies' potential wells, possibly as part of their primordial disks. These stars were displaced from their central locations into the halos through a succession of major mergers. We find that the two galaxies with recently quiescent merger histories have a higher fraction of in situ stars (~20%-50%) in their inner halos than the two galaxies with many recent mergers (~5%-10% in situ fraction). Observational studies concentrating on stellar populations in the inner halo of the Milky Way will be the most affected by the presence of in situ stars with halo kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.
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A sample of very high resolution cosmological disk galaxy simulations is used to investigate the ... more A sample of very high resolution cosmological disk galaxy simulations is used to investigate the evolution of galaxy disk sizes back to redshift 1 within the Lambda CDM cosmology. Artificial images in the rest frame B band are generated, allowing for a measurement of disk scale lengths using surface brightness profiles as observations would, and avoiding any assumption that light must follow mass as previous models have assumed. We demonstrate that these simulated disks are an excellent match to the observed magnitude - size relation for both local disks, and for disks at z=1 in the magnitude/mass range of overlap. We disentangle the evolution seen in the population as a whole from the evolution of individual disk galaxies. In agreement with observations, our simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness dimming since z=1. We find evidence that evolution in the magnitude - size plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more evolution in size than luminosity, while dwarf galaxies tend to evolve potentially more in luminosity. The disks grow in such a way as to stay on roughly the same stellar mass - size relation with time. Finally, due to an evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or size) at z=1 will reside in a more massive halo and have a higher SFR, and thus a higher luminosity, than a counterpart of the same stellar mass at z=0.
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Bulletin of the American Astronomical Society, Dec 1, 2007
The gas accretion histories for simulated disk galaxies spanning two orders of magnitude in mass ... more The gas accretion histories for simulated disk galaxies spanning two orders of magnitude in mass (3.4x1010 to 3.3x1012 M⊙) are investigated. We confirm previous findings that the standard model, in which gas is shock heated to the virial temperature of the galaxy when accreted, requires significant modification. In our simulations, no gas entering galaxies with masses below 1011 M⊙ is shock heated at the virial radius, while a mixture of shocked and unshocked gas enters the galaxy above this mass. For galaxies more massive than the Milky Way, a strong shock develops, making it difficult for shocked gas to cool onto the disk. We examine the fraction of stellar disk mass that forms from (a) smoothly accreted, shock heated gas, (b) smoothly accreted, unshocked gas (e.g., in filaments), and (c) "clumpy” accretion of gas from satellite galaxies. We find that for the whole range of masses, smoothly accreted gas (whether shocked or unshocked, but never in another galaxy) dominates. Higher mass galaxies merge with other galaxy halos that have a higher baryon-to-dark matter ratio, while low mass galaxies accrete "darker” halos with a lower baryon-to-dark matter ratio. This leads to a trend in which the fraction of gas accreted ("clumpy") from other galaxies increases with mass.
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The Astrophysical Journal, Sep 10, 2004
The disk galaxy simulated using our chemodynamic galaxy formation code, GCD+, is shown to have a ... more The disk galaxy simulated using our chemodynamic galaxy formation code, GCD+, is shown to have a thick-disk component. This is evidenced by the velocity dispersion versus age relation for solar neighborhood stars, which clearly shows an abrupt increase in velocity dispersion at a look-back time of approximately 8 Gyr, and is in excellent agreement with observation. These thick-disk stars are formed from gas that is accreted to the galaxy during a chaotic period of hierarchical clustering at high redshift. This formation scenario is shown to be consistent with observations of both the Milky Way and extragalactic thick disks.
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A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo densi... more 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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For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (C... more For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate. Here we report hydrodynamical simulations (in a framework assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies-bulgeless and with shallow central dark-matter profiles-arise naturally in these simulations.
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We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminositi... more 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 Ly\alpha absorption 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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Medical Journal of Australia, 1996
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The Astrophysical Journal, 2007
We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the exp... more We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (<0.8 Msolar), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.
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Despite having entered the era of “precision cosmology,” the formation of galaxies within the fav... more Despite having entered the era of “precision cosmology,” the formation of galaxies within the favored CDM cosmological paradigm remains problematic. By relating our N-body/smooth particle hydro-dynamical simulation to an extensive range of Galactic and extragalactic observations, we shed light on the formation and evolution of the Milky Way and other late type galaxies. In light of recent observations of the stellar populations of extragalactic thick disks, we examine the proposal that the thick disk formed in a high redshift period characterized by gas rich merging. We show such a scenario to be consistent with color observations. We then follow the evolution of structural parameters of the subsequently formed disk galaxy, from redshift ~1 to the present. Consistent with observation, little evolution in the ratio of scale-height to scale-length is found in our simulated galaxy, despite its somewhat chaotic origins. The simulated galaxy in this report forms part of a larger study of a suite of galaxies, with which these issues are to be studied in detail.
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In Astrophysics and Space Science Kluwer Academic Publ, Apr 1, 2003
The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's... more The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation, supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of `rapid collapse', we suggest that many such stars may have come from recently accreted satellites.
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We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-a... more We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-analogue galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (< 0.8 Msol), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I, and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.
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ABSTRACT Observed kinematical data of 40 Local Group members are used to derive the dark matter h... more ABSTRACT Observed kinematical data of 40 Local Group members are used to derive the dark matter halo mass of such galaxies. Haloes are selected from the theoretically expected Local Group mass function and two different density profiles are assumed, the standard NFW model and a mass dependent profile which accounts for the effects of baryons in modifying the dark matter distribution within galaxies. The resulting relations between stellar and halo mass are compared with expectations from abundance matching. Using the NFW profile, the ensemble of Local Group galaxies is generally fit in relatively low mass haloes, leaving dark many massive haloes of Mhalo&gt;10^10Msun: this reflects the &quot;too big to fail&quot; problem in the Local Group and results in a Mstar-Mhalo relation that differs from abundance matching predictions. Moreover, the star formation efficiency of isolated Local Group galaxies increases with decreasing halo mass when adopting a NFW model. By contrast, using the mass dependent density profile, relatively high stellar mass (Mstar&gt;10^6Msun) dwarf galaxies are assigned to more massive haloes, which have a central cored distribution of dark matter: the &quot;too big to fail&quot; problem is alleviated, the resultant Mstar-Mhalo relation follows abundance matching predictions down to the completeness limit of current surveys, and the star formation efficiency of isolated members decreases with decreasing halo mass, in agreement with theoretical expectations. Several low mass (Mstar&lt;10^6Msun) satellite galaxies are best fit to lower mass haloes than expected from the extrapolation of abundance matching relations, which may result from environmental effects, a scenario favored by the fact that no isolated galaxies fall in this region. Finally, the cusp/core space of Local Group galaxies is presented, providing a framework to understand the non-universality of their density profiles.
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The disk galaxy simulated using our chemo-dynamical galaxy formation code, GCD+, is shown to have... more The disk galaxy simulated using our chemo-dynamical galaxy formation code, GCD+, is shown to have a thick disk component. This is evidenced by the velocity dispersion versus age relation for solar neighbourhood stars, which clearly shows an abrupt increase in velocity dispersion at lookback time of approximately 8 Gyrs, and is in excellent agreement with observation. These thick disk stars are formed from gas which is accreted to the galaxy during a chaotic period of hierarchical clustering at high redshift. This formation scenario is shown to be consistent with observations of both the Milky Way and extragalactic thick disks.
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Acta Endocrinologica, 1991
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In order to explain disk galaxy formation within the hierarchical structure formation, it seems t... more In order to explain disk galaxy formation within the hierarchical structure formation, it seems that gas rich mergers must play an important role. We review here our previous studies which have shown the importance of mergers at high redshift being gas rich, in the formation of both the stellar halo and thick disk components of disk galaxies. Regulation of star formation in the building blocks of our galaxy is required to form a low mass low metallicity stellar halo. This regulation results in high redshift, gas rich mergers during which the thick disk forms. In these proceedings, we categorise stars from our simulated disk galaxy into thin and thick disk components by using the Toomre diagram.
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GCD+ (Galactic Chemodynamics Plus) is a soon-to-be publically available N-body/SPH code being dev... more GCD+ (Galactic Chemodynamics Plus) is a soon-to-be publically available N-body/SPH code being developed at Swinburne University for modeling the formation and evolution of galaxies within a cosmological framework. A sophisticated chemical evolution module as been incorporated within GCD+ making use of the latest stellar yields on the market; a self-consistent treatment of energy feedback from Type Ia and II supernovae (relaxing the instantaneous recycling approximation) cooling and star formation is standard within GCD+. Spatially resolved synthetic maps can be generated ranging from stellar populations to the hot and warm X-ray emitting properties of clusters. We will demonstrate GCD+'s application to simulating the 7-dimensional phase space (position velocity chemistry) distribution of the oldest stars in the Milky Way in addition to its seemless predictive power in regards to spatial and temporal evolution of the age-metallicty relationship metallicity distribution functions and the disruption of the Magellanic System
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Monthly Notices of the Royal Astronomical Society, Aug 24, 2010
We present the McMaster Unbiased Galaxy Simulations (MUGS), the first nine galaxies of an unbiase... more We present the McMaster Unbiased Galaxy Simulations (MUGS), the first nine galaxies of an unbiased selection ranging in total mass from 5 × 1011Msolar to 2 × 1012Msolar simulated using N-body smoothed particle hydrodynamics at high resolution. The simulations include a treatment of low-temperature metal cooling, UV background radiation, star formation and physically motivated stellar feedback. Mock images of the simulations show that the simulations lie within the observed range of relations such as that between colour and magnitude and that between brightness and circular velocity (Tully-Fisher). The greatest discrepancy between the simulated galaxies and observed galaxies is the high concentration of material at the centre of the galaxies as represented by the centrally peaked rotation curves and the high bulge-to-total ratios of the simulations determined both kinematically and photometrically. This central concentration represents the excess of low angular momentum material that long has plagued morphological studies of simulated galaxies and suggests that higher resolutions and a more accurate description of feedback will be required to simulate more realistic galaxies. Even with the excess central mass concentrations, the simulations suggest the important role merger history and halo spin play in the formation of discs.
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Observations suggest that the structural parameters of disk galaxies have not changed greatly sin... more Observations suggest that the structural parameters of disk galaxies have not changed greatly since redshift 1. We examine whether these observations are consistent with a cosmology in which structures form hierarchically. We use SPH/N-body galaxy-scale simulations to simulate the formation and evolution of Milky-Way-like disk galaxies by fragmentation, followed by hierarchical merging. The simulated galaxies have a thick disk, that forms in a period of chaotic merging at high redshift, during which a large amount of alpha-elements are produced, and a thin disk, that forms later and has a higher metallicity. Our simulated disks settle down quickly and do not evolve much since redshift z~1, mostly because no major mergers take place between z=1 and z=0. During this period, the disk radius increases (inside-out growth) while its thickness remains constant. These results are consistent with observations of disk galaxies at low and high redshift.
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The Astrophysical Journal, Mar 20, 2009
We use high-resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas ... more We use high-resolution cosmological hydrodynamical simulations to demonstrate that cold flow gas accretion, particularly along filaments, modifies the standard picture of gas accretion and cooling onto galaxy disks. In the standard picture, all gas is initially heated to the virial temperature of the galaxy as it enters the virial radius. Low-mass galaxies are instead dominated by accretion of gas that stays well below the virial temperature, and even when a hot halo is able to develop in more massive galaxies there exist dense filaments that penetrate inside of the virial radius and deliver cold gas to the central galaxy. For galaxies up to ~L*, this cold accretion gas is responsible for the star formation (SF) in the disk at all times to the present. Even for galaxies at higher masses, cold flows dominate the growth of the disk at early times. Within this modified picture, galaxies are able to accrete a large mass of cold gas, with lower initial gas temperatures leading to shorter cooling times to reach the disk. Although SF in the disk is mitigated by supernovae feedback, the short cooling times allow for the growth of stellar disks at higher redshifts than predicted by the standard model.
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The Astrophysical Journal, Apr 22, 2009
We investigate the formation of the stellar halos of four simulated disk galaxies using high-reso... more We investigate the formation of the stellar halos of four simulated disk galaxies using high-resolution, cosmological SPH + N-body simulations. These simulations include a self-consistent treatment of all the major physical processes involved in galaxy formation. The simulated galaxies presented here each have a total mass of ~1012 M sun, but span a range of merger histories. These simulations allow us to study the competing importance of in situ star formation (stars formed in the primary galaxy) and accretion of stars from subhalos in the building of stellar halos in a ΛCDM universe. All four simulated galaxies are surrounded by a stellar halo, whose inner regions (r < 20 kpc) contain both accreted stars, and an in situ stellar population. The outer regions of the galaxies' halos were assembled through pure accretion and disruption of satellites. Most of the in situ halo stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies' potential wells, possibly as part of their primordial disks. These stars were displaced from their central locations into the halos through a succession of major mergers. We find that the two galaxies with recently quiescent merger histories have a higher fraction of in situ stars (~20%-50%) in their inner halos than the two galaxies with many recent mergers (~5%-10% in situ fraction). Observational studies concentrating on stellar populations in the inner halo of the Milky Way will be the most affected by the presence of in situ stars with halo kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.
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A sample of very high resolution cosmological disk galaxy simulations is used to investigate the ... more A sample of very high resolution cosmological disk galaxy simulations is used to investigate the evolution of galaxy disk sizes back to redshift 1 within the Lambda CDM cosmology. Artificial images in the rest frame B band are generated, allowing for a measurement of disk scale lengths using surface brightness profiles as observations would, and avoiding any assumption that light must follow mass as previous models have assumed. We demonstrate that these simulated disks are an excellent match to the observed magnitude - size relation for both local disks, and for disks at z=1 in the magnitude/mass range of overlap. We disentangle the evolution seen in the population as a whole from the evolution of individual disk galaxies. In agreement with observations, our simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness dimming since z=1. We find evidence that evolution in the magnitude - size plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more evolution in size than luminosity, while dwarf galaxies tend to evolve potentially more in luminosity. The disks grow in such a way as to stay on roughly the same stellar mass - size relation with time. Finally, due to an evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or size) at z=1 will reside in a more massive halo and have a higher SFR, and thus a higher luminosity, than a counterpart of the same stellar mass at z=0.
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Bulletin of the American Astronomical Society, Dec 1, 2007
The gas accretion histories for simulated disk galaxies spanning two orders of magnitude in mass ... more The gas accretion histories for simulated disk galaxies spanning two orders of magnitude in mass (3.4x1010 to 3.3x1012 M⊙) are investigated. We confirm previous findings that the standard model, in which gas is shock heated to the virial temperature of the galaxy when accreted, requires significant modification. In our simulations, no gas entering galaxies with masses below 1011 M⊙ is shock heated at the virial radius, while a mixture of shocked and unshocked gas enters the galaxy above this mass. For galaxies more massive than the Milky Way, a strong shock develops, making it difficult for shocked gas to cool onto the disk. We examine the fraction of stellar disk mass that forms from (a) smoothly accreted, shock heated gas, (b) smoothly accreted, unshocked gas (e.g., in filaments), and (c) "clumpy” accretion of gas from satellite galaxies. We find that for the whole range of masses, smoothly accreted gas (whether shocked or unshocked, but never in another galaxy) dominates. Higher mass galaxies merge with other galaxy halos that have a higher baryon-to-dark matter ratio, while low mass galaxies accrete "darker” halos with a lower baryon-to-dark matter ratio. This leads to a trend in which the fraction of gas accreted ("clumpy") from other galaxies increases with mass.
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The Astrophysical Journal, Sep 10, 2004
The disk galaxy simulated using our chemodynamic galaxy formation code, GCD+, is shown to have a ... more The disk galaxy simulated using our chemodynamic galaxy formation code, GCD+, is shown to have a thick-disk component. This is evidenced by the velocity dispersion versus age relation for solar neighborhood stars, which clearly shows an abrupt increase in velocity dispersion at a look-back time of approximately 8 Gyr, and is in excellent agreement with observation. These thick-disk stars are formed from gas that is accreted to the galaxy during a chaotic period of hierarchical clustering at high redshift. This formation scenario is shown to be consistent with observations of both the Milky Way and extragalactic thick disks.
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A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo densi... more 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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For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (C... more For almost two decades the properties of `dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate. Here we report hydrodynamical simulations (in a framework assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies-bulgeless and with shallow central dark-matter profiles-arise naturally in these simulations.
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We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminositi... more 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 Ly\alpha absorption 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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Medical Journal of Australia, 1996
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The Astrophysical Journal, 2007
We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the exp... more We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (<0.8 Msolar), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.
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Despite having entered the era of “precision cosmology,” the formation of galaxies within the fav... more Despite having entered the era of “precision cosmology,” the formation of galaxies within the favored CDM cosmological paradigm remains problematic. By relating our N-body/smooth particle hydro-dynamical simulation to an extensive range of Galactic and extragalactic observations, we shed light on the formation and evolution of the Milky Way and other late type galaxies. In light of recent observations of the stellar populations of extragalactic thick disks, we examine the proposal that the thick disk formed in a high redshift period characterized by gas rich merging. We show such a scenario to be consistent with color observations. We then follow the evolution of structural parameters of the subsequently formed disk galaxy, from redshift ~1 to the present. Consistent with observation, little evolution in the ratio of scale-height to scale-length is found in our simulated galaxy, despite its somewhat chaotic origins. The simulated galaxy in this report forms part of a larger study of a suite of galaxies, with which these issues are to be studied in detail.
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In Astrophysics and Space Science Kluwer Academic Publ, Apr 1, 2003
The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's... more The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation, supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of `rapid collapse', we suggest that many such stars may have come from recently accreted satellites.
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We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-a... more We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-analogue galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (< 0.8 Msol), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I, and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.
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ABSTRACT Observed kinematical data of 40 Local Group members are used to derive the dark matter h... more ABSTRACT Observed kinematical data of 40 Local Group members are used to derive the dark matter halo mass of such galaxies. Haloes are selected from the theoretically expected Local Group mass function and two different density profiles are assumed, the standard NFW model and a mass dependent profile which accounts for the effects of baryons in modifying the dark matter distribution within galaxies. The resulting relations between stellar and halo mass are compared with expectations from abundance matching. Using the NFW profile, the ensemble of Local Group galaxies is generally fit in relatively low mass haloes, leaving dark many massive haloes of Mhalo&gt;10^10Msun: this reflects the &quot;too big to fail&quot; problem in the Local Group and results in a Mstar-Mhalo relation that differs from abundance matching predictions. Moreover, the star formation efficiency of isolated Local Group galaxies increases with decreasing halo mass when adopting a NFW model. By contrast, using the mass dependent density profile, relatively high stellar mass (Mstar&gt;10^6Msun) dwarf galaxies are assigned to more massive haloes, which have a central cored distribution of dark matter: the &quot;too big to fail&quot; problem is alleviated, the resultant Mstar-Mhalo relation follows abundance matching predictions down to the completeness limit of current surveys, and the star formation efficiency of isolated members decreases with decreasing halo mass, in agreement with theoretical expectations. Several low mass (Mstar&lt;10^6Msun) satellite galaxies are best fit to lower mass haloes than expected from the extrapolation of abundance matching relations, which may result from environmental effects, a scenario favored by the fact that no isolated galaxies fall in this region. Finally, the cusp/core space of Local Group galaxies is presented, providing a framework to understand the non-universality of their density profiles.
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The disk galaxy simulated using our chemo-dynamical galaxy formation code, GCD+, is shown to have... more The disk galaxy simulated using our chemo-dynamical galaxy formation code, GCD+, is shown to have a thick disk component. This is evidenced by the velocity dispersion versus age relation for solar neighbourhood stars, which clearly shows an abrupt increase in velocity dispersion at lookback time of approximately 8 Gyrs, and is in excellent agreement with observation. These thick disk stars are formed from gas which is accreted to the galaxy during a chaotic period of hierarchical clustering at high redshift. This formation scenario is shown to be consistent with observations of both the Milky Way and extragalactic thick disks.
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Acta Endocrinologica, 1991
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In order to explain disk galaxy formation within the hierarchical structure formation, it seems t... more In order to explain disk galaxy formation within the hierarchical structure formation, it seems that gas rich mergers must play an important role. We review here our previous studies which have shown the importance of mergers at high redshift being gas rich, in the formation of both the stellar halo and thick disk components of disk galaxies. Regulation of star formation in the building blocks of our galaxy is required to form a low mass low metallicity stellar halo. This regulation results in high redshift, gas rich mergers during which the thick disk forms. In these proceedings, we categorise stars from our simulated disk galaxy into thin and thick disk components by using the Toomre diagram.
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