arXiv subjects

Gregory S. Stinson

Publications and source records attributed to Gregory S. Stinson.

18 recordsLinked to original sources

arXiv2019

NIHAO XVI: The properties and evolution of kinematically selected discs, bulges and stellar haloes

We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large scale single discs, classical and pseudo bulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of $2.5\times10^{\rm~11}M_{\rm\odot}$, all galaxies have a classical bulge and 70\% have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momentum and stellar mass $J_{\rm *}=3.4M_{\rm *}^{\rm1.26\pm0.06}$, in very good agreement with the prediction based on the empirical stellar-to-halo mass relation in the same mass range, and show a strong correlation between maximum `observed' rotation velocity and dark matter halo circular velocity $v_{\rm c}=6.4v_{\rm max}^{0.64\pm0.04}$. Tracing back in time these structures' progenitors, we find all to lose a fraction $1-f_j$ of their maximum angular momentum. Thin discs are significantly better at retaining their high-redshift spins ($f_j\sim0.70$) than thick ones ($f_j\sim0.40$). Stellar haloes have their progenitor baryons assembled the latest ($z_{\rm~1/2}\sim1.1$) and over the longest timescales ($τ\sim6.2$~Gyr), and have the smallest fraction of stars born in-situ ($f_{\rm in-situ}=0.35\pm0.14$). All other structures have $1.5\lesssim z_{\rm1/2}\lesssim3$, $τ=4\pm2$~Gyr and $f_{\rm in-situ}\gtrsim0.9$.

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arXiv2018

Introducing galactic structure finder: the multiple stellar kinematic structures of a simulated Milky Way mass galaxy

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high resolution galaxies to separate the stars into multiple components. We exemplify this method using a simulated Milky Way analogue, whose stellar component hosts: thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z~3 and the end of halo virialization, z~1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (~95%) and the thin disc the least (~60%). Both bulges formed their stars in-situ at high redshift, while the thin disc formed ~98% in-situ, but with a constant SFR~1.5M$_{\rm\odot}$/yr$^{\rm-1}$ over the last ~11 Gyr. Accreted stars (6% of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex-situ 8% and 45% of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf.

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arXiv2017

How to bend galaxy disc profiles II: stars surfing the bar in Type-III discs

The radial profiles of stars in disc galaxies are observed to be either purely exponential (Type-I), truncated (Type-II) or anti-truncated (Type-III) exponentials. Controlled formation simulations of isolated galaxies can reproduce all of these profile types by varying a single parameter, the initial halo spin. In this paper we examine these simulations in more detail in an effort to identify the physical mechanism that leads to the formation of Type-III profiles. The stars in the anti-truncated outskirts of such discs are now on eccentric orbits, but were born on near-circular orbits at much smaller radii. We show that, and explain how, they were driven to the outskirts via non-linear interactions with a strong and long-lived central bar, which greatly boosted their semi-major axis but also their eccentricity. While bars have been known to cause radial heating and outward migration to stellar orbits, we link this effect to the formation of Type-III profiles. This predicts that the anti-truncated parts of galaxies have unusual kinematics for disc-like stellar configurations: high radial velocity dispersions and slow net rotation. Whether such discs exist in nature, can be tested by future observations.

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arXiv2017

NIHAO XII: galactic uniformity in a ΛCDM universe

We use a sample of 83 high-resolution cosmological zoom-in simulations and a semi-analytic model (SAM) to study the stochasticity of galaxy formation in haloes ranging from dwarf to Milky Way masses. Our simulated galaxies reproduce the observed inefficiency of galaxy formation as expressed through the stellar, gas and baryonic Tully-Fisher relations. For HI velocities in the range (70 < V < 220 km/s), the scatter is just 0.08 to 0.14 dex, consistent with the observed intrinsic scatter at these scales. At low velocities (20 < V < 70 km/s), the simulated scatter is 0.2-0.25 dex, which could be tested with future observations. The scatter in the stellar mass versus dark halo velocity relation is constant for 30 < V < 180 km/s, and smaller (~0.17 dex) when using the maximum circular velocity of the dark matter only simulation, Vmax, compared to the virial velocity ($V_{200}$ or $V_{200}^{DMO}$). The scatter in stellar mass is correlated with halo concentration, and is minimized when using a circular velocity at a fixed fraction of the virial radius $\simeq 0.4 R_{200}$ or with $V_α=V_{200}^{DMO} (V_{max}^{DMO} / V_{200}^{DMO})^α$ with α~ 0.7, consistent with constraints from halo clustering. Using the SAM we show the correlation between halo formation time and concentration is essential in order to reproduce this result. This uniformity in galaxy formation efficiency we see in our hydrodynamical simulations and a semi-analytic model proves the simplicity and self-regulating nature of galaxy formation in a ΛCold Dark Matter universe.

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arXiv2017

NIHAO VII: Predictions for the galactic baryon budget in dwarf to Milky Way mass haloes

We use the NIHAO galaxy formation simulations to make predictions for the baryonic budget in present day galaxies ranging from dwarf to Milky Way masses. The sample is made of 88 independent high resolution cosmological zoom-in simulations. NIHAO galaxies reproduce key properties of observed galaxies, such as the stellar mass vs halo mass and cold gas vs stellar mass relations. Thus they make plausible predictions for the baryon budget. We present the mass fractions of stars, cold gas ($T<10^4$K), cool gas ($10^4 < T < 10^5$K), warm-hot gas ($10^5 < T < 5\times10^6$K), and hot gas (T$> 5\times10^6$K) inside the virial radius, $R_{200}$. Compared to the predicted baryon mass, using the dark halo mass and the universal baryon fraction, $f_{\rm b}\equiv Ω_{\rm b}/Ω_{\rm m}=0.15$, we find that all of our haloes are missing baryons. The missing mass has been relocated past 2 virial radii, and cool gas dominates the corona at low mass (M$_{200} < 3 \times 10^{11} M_\odot$) while the warm-hot gas dominates at high mass (M$_{200} > 3 \times 10^{11} M_\odot$). Haloes of mass $\sim 10^{10} M_\odot$ are missing $\sim 90\%$ of their baryons. More massive haloes ($\sim 10^{12} M_\odot$) retain a higher fraction of their baryons, with $\sim 30\%$ missing, consistent with recent observational estimates. Moreover, these more massive haloes reproduce the observed fraction of cold, warm-hot and hot gas. The fraction of cool gas we predict ($0.11\pm0.06$) is significantly lower than the observation from COS-HALOs (0.3-0.47), but agrees with the alternative analysis of Stern et al. 2016.

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arXiv2016

The impact of baryons on the direct detection of dark matter

The spatial and velocity distributions of dark matter particles in the Milky Way Halo affect the signals expected to be observed in searches for dark matter. Results from direct detection experiments are often analyzed assuming a simple isothermal distribution of dark matter, the Standard Halo Model (SHM). Yet there has been skepticism regarding the validity of this simple model due to the complicated gravitational collapse and merger history of actual galaxies. In this paper we compare the SHM to the results of cosmological hydrodynamical simulations of galaxy formation to investigate whether or not the SHM is a good representation of the true WIMP distribution in the analysis of direct detection data. We examine two Milky Way-like galaxies from the MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic physics included. The inclusion of baryons drives the shape of the DM halo to become more spherical and makes the velocity distribution of dark matter particles less anisotropic especially at large heliocentric velocities, thereby making the SHM a better fit. We also note that we do not find a significant disk-like rotating dark matter component in either of the two galaxy halos with baryons that we examine, suggesting that dark disks are not a generic prediction of cosmological hydrodynamical simulations. We conclude that in the Solar neighborhood, the SHM is in fact a good approximation to the true dark matter distribution in these cosmological simulations (with baryons) which are reasonable representations of the Milky Way, and hence can also be used for the purpose of dark matter direct detection calculations.

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arXiv2016

NIHAO IX: the role of gas inflows and outflows in driving the contraction and expansion of cold dark matter haloes

We use ~100 cosmological galaxy formation zoom-in simulations using the smoothed particle hydrodynamics code {\sc gasoline} to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf (M_{200}~10^{10}Msun) to Milky Way (M_{200}~10^{12}Msun) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ~10 changes in the enclosed dark matter mass at one per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation: e_SF=(M_{star}/M_{200})/(Ω_b/Ω_m). In addition we report a new correlation with the compactness of the stellar system: e_R=r_{1/2}/R_{200}. We provide an analytic formula depending on e_SF and e_R for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f, the overall effect is expansion, with more expansion with larger f. For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes.

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arXiv2015

NIHAO V: Too big doesn't fail -- reconciling the conflict between LCDM predictions and the circular velocities of nearby field galaxies

We compare the half-light circular velocities, V_{1/2}, of dwarf galaxies in the Local Group to the predicted circular velocity curves of galaxies in the NIHAO suite of LCDM simulations. We use a subset of 34 simulations in which the central galaxy has a stellar luminosity in the range 0.5 x 10^5 < L_V < 2 x 10^8 L_{sun}. The NIHAO galaxy simulations reproduce the relation between stellar mass and halo mass from abundance matching, as well as the observed half-light size vs luminosity relation. The corresponding dissipationless simulations over-predict the V_{1/2}, recovering the problem known as too big to fail (TBTF). By contrast, the NIHAO simulations have expanded dark matter haloes, and provide an excellent match to the distribution of V_{1/2} for galaxies with L_V > 2 x 10^6 L_{sun}. For lower luminosities our simulations predict very little halo response, and tend to over predict the observed circular velocities. In the context of LCDM, this could signal the increased stochasticity of star formation in haloes below M_{halo} \sim 10^{10} M_{sun}, or the role of environmental effects. Thus, haloes that are "too big to fail", do not fail LCDM, but haloes that are "too small to pass" (the galaxy formation threshold) provide a future test of LCDM.

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arXiv2015

NIHAO project I: Reproducing the inefficiency of galaxy formation across cosmic time with a large sample of cosmological hydrodynamical simulations

We introduce project NIHAO (Numerical Investigation of a Hundred Astrophysical Objects), a set of 100 cosmological zoom-in hydrodynamical simulations performed using the gasoline code, with an improved implementation of the SPH algorithm. The haloes in our study range from dwarf to Milky Way masses, and represent an unbiased sampling of merger histories, concentrations and spin parameters. The particle masses and force softenings are chosen to resolve the mass profile to below 1% of the virial radius at all masses, ensuring that galaxy half-light radii are well resolved. Using the same treatment of star formation and stellar feedback for every object, the simulated galaxies reproduce the observed inefficiency of galaxy formation across cosmic time as expressed through the stellar mass vs halo mass relation, and the star formation rate vs stellar mass relation. We thus conclude that stellar feedback is the chief piece of physics required to limit the efficiency of star formation in galaxies less massive than the Milky Way.

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arXiv2015

The response of dark matter haloes to elliptical galaxy formation: a new test for quenching scenarios

We use cosmological hydrodynamical zoom-in simulations with the SPH code gasoline of four haloes of mass M_{200} \sim 10^{13}\Msun to study the response of the dark matter to elliptical galaxy formation. Our simulations include metallicity dependent gas cooling, star formation, and feedback from massive stars and supernovae, but not active galactic nuclei (AGN). At z=2 the progenitor galaxies have stellar to halo mass ratios consistent with halo abundance matching, assuming a Salpeter initial mass function. However by z=0 the standard runs suffer from the well known overcooling problem, overpredicting the stellar masses by a factor of > 4. To mimic a suppressive halo quenching scenario, in our forced quenching (FQ) simulations, cooling and star formation are switched off at z=2. The resulting z=0 galaxies have stellar masses, sizes and circular velocities close to what is observed. Relative to the control simulations, the dark matter haloes in the FQ simulations have contracted, with central dark matter density slopes d\logρ/d\log r \sim -1.5, showing that dry merging alone is unable to fully reverse the contraction that occurs at z>2. Simulations in the literature with AGN feedback however, have found expansion or no net change in the dark matter halo. Thus the response of the dark matter halo to galaxy formation may provide a new test to distinguish between ejective and suppressive quenching mechanisms.

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arXiv2015

How to bend galaxy disc profiles: the role of halo spin

The radial density profiles of stellar galaxy discs can be well approximated as an exponential. Compared to this canonical form, however, the profiles in the majority of disc galaxies show downward or upward breaks at large radii. Currently, there is no coherent explanation in a galaxy formation context of the radial profile per se, along with the two types of profile breaks. Using a set of controlled hydrodynamic simulations of disc galaxy formation, we find a correlation between the host halo's initial angular momentum and the resulting radial profile of the stellar disc: galaxies that live in haloes with a low spin parameter λ <~ 0.03 show an up-bending break in their disc density profiles, while galaxies in haloes of higher angular momentum show a down-bending break. We find that the case of pure exponential profiles (λ ~ 0.035) coincides with the peak of the spin parameter distribution from cosmological simulations. Our simulations not only imply an explanation of the observed behaviours, but also suggest that the physical origin of this effect is related to the amount of radial redistribution of stellar mass, which is anti-correlated with λ.

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arXiv2013

MaGICC-WDM: the effects of warm dark matter in hydrodynamical simulations of disc galaxy formation

We study the effect of warm dark matter (WDM) on hydrodynamic simulations of galaxy formation as part of the Making Galaxies in a Cosmological Context (MaGICC) project. We simulate three different galaxies using three WDM candidates of 1, 2 and 5 keV and compare results with pure cold dark matter simulations. WDM slightly reduces star formation and produces less centrally concentrated stellar profiles. These effects are most evident for the 1 keV candidate but almost disappear for $m_{\mathrm{WDM}}>2$ keV. All simulations form similar stellar discs independent of WDM particle mass. In particular, the disc scale length does not change when WDM is considered. The reduced amount of star formation in the case of 1 keV particles is due to the effects of WDM on merging satellites which are on average less concentrated and less gas rich. The altered satellites cause a reduced starburst during mergers because they trigger weaker disc instabilities in the main galaxy. Nevertheless we show that disc galaxy evolution is much more sensitive to stellar feedback than it is to WDM candidate mass. Overall we find that WDM, especially when restricted to current observational constraints ($m_{\mathrm{WDM}}>2$ keV), has a minor impact on disc galaxy formation.

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arXiv2013

Halo orbits in cosmological disk galaxies: tracers of formation history

We analyze the orbits of stars and dark matter particles in the halo of a disk galaxy formed in a cosmological hydrodynamical simulation. The halo is oblate within the inner ~20 kpc and triaxial beyond this radius. About 43% of orbits are short axis tubes - the rest belong to orbit families that characterize triaxial potentials (boxes, long-axis tubes and chaotic orbits), but their shapes are close to axisymmetric. We find no evidence that the self-consistent distribution function of the nearly oblate inner halo is comprised primarily of axisymmetric short-axis tube orbits. Orbits of all families, and both types of particles are highly eccentric with mean eccentricity >0.6. We find that randomly selected samples of halo stars show no substructure in "integrals of motion" space. However individual accretion events can be clearly identified in plots of metallicity versus formation time. Dynamically young tidal debris is found primarily on a single type of orbit. However, stars associated with older satellites become chaotically mixed during the formation process (possibly due to scattering by the central bulge and disk, and baryonic processes), and appear on all four types of orbits. We find that the tidal debris in cosmological hydrodynamical simulations experiences significantly more chaotic evolution than in collisionless simulations, making it much harder to identify individual progenitors using phase space coordinates alone. However by combining information on stellar ages and chemical abundances with the orbital properties of halo stars in the underlying self-consistent potential, the identification of progenitors is likely to be possible.

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arXiv2011

Disc Heating: Comparing the Milky Way with Cosmological Simulations

We present the analysis of a suite of simulations run with different particle-and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion - age relation for disc stars at z=0 and find that four of the simulations are more consistent with observations by Holmberg et al. (2008) in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett (2001) observations that suggest a "saturation" in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies' evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger - i.e. once the disc has formed. All of the simulations are significantly "hotter" than the Milky Way disc; on top of the effects of mergers, we find a "floor" in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical codes.

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arXiv2008

Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks

Stars in disks of spiral galaxies are usually assumed to remain roughly at their birth radii. This assumption is built into decades of modelling of the evolution of stellar populations in our own Galaxy and in external systems. We present results from self-consistent high-resolution $N$-body + Smooth Particle Hydrodynamics simulations of disk formation, in which stars migrate across significant galactocentric distances due to resonant scattering with transient spiral arms, while preserving their circular orbits. We investigate the implications of such migrations for observed stellar populations. Radial migration provides an explanation for the observed flatness and spread in the age-metallicity relation and the relative lack of metal poor stars in the solar neighborhood. The presence of radial migration also prompts rethinking of interpretations of extra-galactic stellar population data, especially for determinations of star formation histories.

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arXiv2008

Clues to Radial Migration from the Properties of Outer Disks

The outer disks of galaxies present a unique laboratory for studying the process of disk formation. A considerable fraction of observed disks exhibit a break in their surface brightness profiles. The ubiquity of these features points to a crucial aspect of disk formation which must be explained. Recent theoretical work suggests that such breaks are related to significant amounts of radial migration. We discuss the current observational evidence which supports this picture.

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arXiv2008

Beyond Inside-Out Growth: Formation and Evolution of Disk Outskirts

We have performed a high mass and force resolution simulation of an idealized galaxy forming from dissipational collapse of gas embedded in a spherical dark matter halo. The simulation includes star formation and effects of stellar feedback. In our simulation a stellar disk forms with a surface density profile consisting of an inner exponential breaking to a steeper outer exponential. The break forms early on and persists throughout the evolution, moving outward as more gas is able to cool and add mass to the disk. The parameters of the break are in excellent agreement with observations. The break corresponds with a rapid drop in the star formation rate associated with a drop in the cooled gas surface density, but the outer exponential is populated by stars that were scattered outward on nearly circular orbits from the inner disk by spiral arms. The resulting profile and its associated break are therefore a consequence of the interplay between a radial star formation cutoff and redistribution of stellar mass by secular processes. A consequence of such evolution is a sharp change in the radial mean stellar age profile at the break radius.

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arXiv2005

Magnetic White Dwarfs from the SDSS II. The Second and Third Data Releases

Fifty-two magnetic white dwarfs have been identified in spectroscopic observations from the Sloan Digital Sky Survey (SDSS) obtained between mid-2002 and the end of 2004, including Data Releases 2 and 3. Though not as numerous nor as diverse as the discoveries from the first Data Release, the collection exhibits polar field strengths ranging from 1.5MG to ~1000MG, and includes two new unusual atomic DQA examples, a molecular DQ, and five stars that show hydrogen in fields above 500MG. The highest-field example, SDSSJ2346+3853, may be the most strongly magnetic white dwarf yet discovered. Analysis of the photometric data indicates that the magnetic sample spans the same temperature range as for nonmagnetic white dwarfs from the SDSS, and support is found for previous claims that magnetic white dwarfs tend to have larger masses than their nonmagnetic counterparts. A glaring exception to this trend is the apparently low-gravity object SDSSJ0933+1022, which may have a history involving a close binary companion.

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