arXiv subjects

James Bullock

Publications and source records attributed to James Bullock.

36 records · Page 2Linked to original sources

arXiv2019

Astro2020 Science White Paper: Construction of an L* Galaxy: the Transformative Power of Wide Fields for Revealing the Past, Present and Future of the Great Andromeda System

The Great Andromeda Galaxy (M31) is the nexus of the near-far galaxy evolution connection and a principal data point for near-field cosmology. Due to its proximity (780 kpc), M31 can be resolved into individual stars like the Milky Way (MW). Unlike the MW, we have the advantage of a global view of M31, enabling M31 to be observed with techniques that also apply to more distant galaxies. Moreover, recent evidence suggests that M31 may have survived a major merger within the last several Gyr, shaping the morphology of its stellar halo and triggering a starburst, while leaving the stellar disk largely intact. The MW and M31 thus provide complementary opportunities for in-depth studies of the disks, halos, and satellites of L* galaxies. Our understanding of the M31 system will be transformed in the 2020s if they include wide field facilities for both photometry (HST-like sensitivity and resolution) and spectroscopy (10-m class telescope, >1 sq. deg. field, highly multiplexed, R~ 3000 to 6000). We focus here on the power of these facilities to constrain the past, present, and future merger history of M31, via chemo-dynamical analyses and star formation histories of phase-mixed stars accreted at early times, as well as stars in surviving tidal debris features, M31's extended disk, and intact satellite galaxies that will eventually be tidally incorporated into the halo. This will yield an unprecedented view of the hierarchical formation of the M31 system and the subhalos that built it into the L* galaxy we observe today.

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arXiv2018

Predicting the binary black hole population of the Milky Way with cosmological simulations

Binary black holes are the primary endpoint of massive stellar evolution. Their properties provide a unique opportunity to constrain binary evolution, which is still poorly understood. In this paper, we predict the inventory of binary black holes and their merger products in/around the Milky Way, and detail their main properties. We present the first combination of a high-resolution cosmological simulation of a Milky Way-mass galaxy with a binary population synthesis model. The hydrodynamic simulation, taken from the FIRE project, provides a cosmologically realistic star formation history for the galaxy and its stellar halo and satellites. We apply a metallicity-dependent evolutionary model to the star particles to produce individual binary black holes. We find that a million binary black holes have merged in the model Milky Way, and 3 million binaries are still present, with an average mass of 28 Msun per binary. Because the black hole progenitors are biased towards low metallicity stars, half reside in the stellar halo and satellites and 40 per cent of the binaries were formed outside the main galaxy. This trend increases with the masses of the black holes. The numbers and mass distribution of the merged systems is compatible with the LIGO/Virgo detections. Observations of these black holes will be challenging, both with electromagnetic methods and LISA. We find that a cosmologically realistic star formation history, with self-consistent metal enrichment and Galactic accretion history, are key ingredients for determining binary black hole rates that can be compared with observations to constrain massive binary evolution.

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arXiv2017

High Angular Momentum Halo Gas: a Feedback and Code-Independent Prediction of LCDM

We investigate angular momentum acquisition in Milky Way-sized galaxies by comparing five high resolution zoom-in simulations, each implementing identical cosmological initial conditions, but utilizing different hydrodynamic codes: Enzo, Art, Ramses, Arepo, and Gizmo-PSPH. Each code implements a distinct set of feedback and star formation prescriptions. We find that while many galaxy and halo properties vary between the different codes (and feedback prescriptions), there is qualitative agreement on the process of angular momentum acquisition in the galaxy's halo. In all simulations, cold filamentary gas accretion to the halo results in ~4 times more specific angular momentum in cold halo gas ($λ_{cold} \gtrsim 0.1$) than in the dark matter halo. At z>1, this inflow takes the form of inspiraling cold streams that are co-directional in the halo of the galaxy and are fueled, aligned, and kinematically connected to filamentary gas infall along the cosmic web. Due to the qualitative agreement among disparate simulations, we conclude that the buildup of high angular momentum halo gas and the presence of these inspiraling cold streams are robust predictions of Lambda Cold Dark Matter galaxy formation, though the detailed morphology of these streams is significantly less certain. A growing body of observational evidence suggests that this process is borne out in the real universe.

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arXiv2016

The Halos and Environments of Nearby Galaxies (HERON) Survey

We have used dedicated 0.7m telescopes in California and Israel to image the halos of ~200 galaxies in the Local Volume to 29 mag/sq arcsec, the sample mainly drawn from the 2MASS Large Galaxy Atlas (LGA). We supplement the LGA sample with dwarf galaxies and more distant giant ellipticals. Low surface brightness halos exceeding 50 kpc in diameter are found only in galaxies more luminous than L* and classic interaction signatures are relatively infrequent. Halo diameter is correlated with total galaxy luminosity. Extended low surface brightness halos are present even in galaxies as faint as M_V=-18. Edge-on galaxies with boxy bulges tend to lack extended spheroidal halos, while those with large classical bulges exhibit extended round halos, supporting the notions that boxy or barlike bulges originate from disks. Most face-on spiral galaxies present features that appear to be irregular extensions of spiral arms, although rare cases show smooth boundaries with no sign of star formation. Although we serendipitously discovered a dwarf galaxy undergoing tidal disruption in the halo of NGC 4449, we found no comparable examples in our general survey. A search for similar examples in the Local Volume identified hcc087, a tidally disrupting dwarf galaxy in the Hercules Cluster, but we do not confirm an anomalously large half-light radius reported for the dwarf VCC 1661.

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arXiv2014

ELVIS: Exploring the Local Volume in Simulations

We introduce a set of high-resolution dissipationless simulations that model the Local Group (LG) in a cosmological context: Exploring the Local Volume in Simulations (ELVIS). The suite contains 48 Galaxy-size halos, each within high-resolution volumes that span 2-5 Mpc in size, and each resolving thousands of systems with masses below the atomic cooling limit. Half of the ELVIS galaxy halos are in paired configurations similar to the Milky Way (MW) and M31; the other half are isolated, mass-matched analogs. We find no difference in the abundance or kinematics of substructure within the virial radii of isolated versus paired hosts. On Mpc scales, however, LG-like pairs average almost twice as many companions and the velocity field is kinematically hotter and more complex. We present a refined abundance matching relation between stellar mass and halo mass that reproduces the observed satellite stellar mass functions of the MW and M31 down to the regime where incompleteness is an issue, $M_\star \sim 5\times 10^5 \, M_\odot$. Within a larger region spanning approximately 3 Mpc, the same relation predicts that there should be $\sim$ 1000 galaxies with $M_\star > 10^{3}\,M_\odot$ awaiting discovery. We show that up to 50% of halos within 1 Mpc of the MW or M31 could be systems that have previously been within the virial radius of either giant. By associating never-accreted halos with gas-rich dwarfs, we show that there are plausibly 50 undiscovered dwarf galaxies with HI masses $> 10^5\,M_\odot$ within the Local Volume. The radial velocity distribution of these predicted gas-rich dwarfs can be used to inform follow-up searches based on ultra-compact high-velocity clouds found in the ALFALFA survey.

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arXiv2013

Can Feedback Solve the Too Big to Fail Problem?

The observed central densities of Milky Way dwarf spheroidal galaxies (dSphs) are significantly lower than the densities of the largest (Vmax about 35 km/s) subhalos found in dissipationless simulations of Galaxy-size dark matter hosts. One possible explanation is that gas removal from feedback can lower core densities enough to match observations. We model the dynamical effects of supernova feedback through the use of a time-varying central potential in high resolution, idealized numerical simulations and explore the resulting impact on the mass distributions of dwarf dark matter halos. We find that in order to match the observed central masses of M_star about 10^6 M_sun dSphs, the energy equivalent of more than 40,000 supernovae must be delivered with 100% efficiency directly to the dark matter. This energy requirement exceeds the number of supernovae that have ever exploded in most dSphs for typical initial mass functions. We also find that, per unit energy delivered and per cumulative mass removed from the galaxy, single blow-out events are more effective than repeated small bursts in reducing central dark matter densities. We conclude that it is unlikely that supernova feedback alone can solve the "Too Big to Fail" problem for Milky Way subhalos.

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arXiv2012

Global Properties of M31's Stellar Halo from the SPLASH Survey. I. Surface Brightness Profile

We present the surface brightness profile of M31's stellar halo out to a projected radius of 175 kpc. The surface brightness estimates are based on confirmed samples of M31 red giant branch stars derived from Keck/DEIMOS spectroscopic observations. A set of empirical spectroscopic and photometric M31 membership diagnostics is used to identify and reject foreground and background contaminants. This enables us to trace the stellar halo of M31 to larger projected distances and fainter surface brightnesses than previous photometric studies. The surface brightness profile of M31's halo follows a power-law with index -2.2 +/- 0.2 and extends to a projected distance of at least ~175 kpc (~ 2/3 of M31's virial radius), with no evidence of a downward break at large radii. The best-fit elliptical isophotes have b/a=0.94 with the major axis of the halo aligned along the minor axis of M31's disk, consistent with a prolate halo, although the data are also consistent with M31's halo having spherical symmetry. The fact that tidal debris features are kinematically cold is used to identify substructure in the spectroscopic fields out to projected radii of 90 kpc, and investigate the effect of this substructure on the surface brightness profile. The scatter in the surface brightness profile is reduced when kinematically identified tidal debris features in M31 are statistically subtracted; the remaining profile indicates a comparatively diffuse stellar component to M31's stellar halo exists to large distances. Beyond 90 kpc, kinematically cold tidal debris features can not be identified due to small number statistics; nevertheless, the significant field-to-field variation in surface brightness beyond 90 kpc suggests that the outermost region of M31's halo is also comprised to a significant degree of stars stripped from accreted objects.

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arXiv2012

Addressing Decadal Survey Science through Community Access to Highly Multiplexed Spectroscopy with BigBOSS on the KPNO Mayall Telescope

This document summarizes the results of a community-based discussion of the potential science impact of the Mayall+BigBOSS highly multiplexed multi-object spectroscopic capability. The KPNO Mayall 4m telescope equipped with the DOE- and internationally-funded BigBOSS spectrograph offers one of the most cost-efficient ways of accomplishing many of the pressing scientific goals identified for this decade by the "New Worlds, New Horizons" report. The BigBOSS Key Project will place unprecedented constraints on cosmological parameters related to the expansion history of the universe. With the addition of an open (publicly funded) community access component, the scientific impact of BigBOSS can be extended to many important astrophysical questions related to the origin and evolution of galaxies, stars, and the IGM. Massive spectroscopy is the critical missing ingredient in numerous ongoing and planned ground- and space-based surveys, and BigBOSS is unique in its ability to provide this to the US community. BigBOSS data from community-led projects will play a vital role in the education and training of students and in maintaining US leadership in these fields of astrophysics. We urge the NSF-AST division to support community science with the BigBOSS multi-object spectrograph through the period of the BigBOSS survey in order to ensure public access to the extraordinary spectroscopic capability.

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arXiv2011

A Complete Spectroscopic Survey of the Milky Way satellite Segue 1: Dark matter content, stellar membership and binary properties from a Bayesian analysis

We introduce a comprehensive analysis of multi-epoch stellar line-of-sight velocities to determine the intrinsic velocity dispersion of the ultrafaint satellites of the Milky Way. Our method includes a simultaneous Bayesian analysis of both membership probabilities and the contribution of binary orbital motion to the observed velocity dispersion within a 14-parameter likelihood. We apply our method to the Segue 1 dwarf galaxy and conclude that Segue 1 is a dark-matter-dominated galaxy at high probability with an intrinsic velocity dispersion of 3.7^{+1.4}_{-1.1} km/sec. The dark matter halo required to produce this dispersion must have an average density of 2.5^{+4.1}_{-1.9} solar mass/pc^3 within a sphere that encloses half the galaxy's stellar luminosity. This is the highest measured density of dark matter in the Local Group. Our results show that a significant fraction of the stars in Segue 1 may be binaries with the most probable mean period close to 10 years, but also consistent with the 180 year mean period seen in the solar vicinity at about 1 sigma. Despite this binary population, the possibility that Segue 1 is a bound star cluster with the observed velocity dispersion arising from the orbital motion of binary stars is disfavored by the multi-epoch stellar velocity data at greater than 99% C.L. Finally, our treatment yields a projected (two-dimensional) half-light radius for the stellar profile of Segue 1 of 28^{+5}_{-4} pc, in excellent agreement with photometric measurements.

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arXiv2010

Group Finding in the Stellar Halo using Photometric Surveys: Current Sensitivity and Future Prospects

The ubiquity of substructure in the stellar halo has already been demonstrated by the SDSS and 2MASS and future surveys promise to explore the halo in ever more detail. This paper examines what can be learnt from current and future photometric-databases using group-finding techniques. We compare groups recovered from a sample of M-giants from 2MASS with those found in synthetic surveys of simulated $Λ$CDM stellar halos and demonstrate broad consistency. We also find that these recovered groups are likely to represent the majority of high-luminosity ($\log(L/L_{Sun})>6.67$) satellites accreted within the last 10 Gyr and on orbits with apocenters within 100 kpc. However the sensitivity of the M-giant survey to accretion events that were either ancient, from low-luminosity objects or those on radial orbits is limited because of the low number of stars, bias towards high-metallicity stars and the shallow depth. We examine the extent to which these limitations are addressed by current and future surveys, in particular catalogues of main-sequence turn-off (MSTO) stars from SDSS and LSST, and of RR-Lyrae stars from LSST or PanSTARRS. The MSTO surveys are more sensitive to low-luminosity events ($\log(L/L_{Sun}~5$ or less) than the 2MASS M-giant sample, while the RR-Lyrae surveys, with superior depth, are good at detecting events on highly eccentric orbits. When combined we expect these photometric surveys to provide a comprehensive picture of the last 10 Gyr of Galactic accretion. Events older than this are too phase mixed to be discovered and would require additional information such as velocity and chemical abundance.

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arXiv2010

Group finding in the stellar halo using M-giants in 2MASS: An extended view of the Pisces Overdensity?

A density based hierarchical group-finding algorithm is used to identify stellar halo structures in a catalog of M-giants from the Two Micron All Sky Survey (2MASS). The intrinsic brightness of M-giant stars means that this catalog probes deep into the halo where substructures are expected to be abundant and easy to detect. Our analysis reveals 16 structures at high Galactic latitude (greater than 15 degree), of which 10 have been previously identified. Among the six new structures two could plausibly be due to masks applied to the data, one is associated with a strong extinction region and one is probably a part of the Monoceros ring. Another one originates at low latitudes, suggesting some contamination from disk stars, but also shows protrusions extending to high latitudes, implying that it could be a real feature in the stellar halo. The last remaining structure is free from the defects discussed above and hence is very likely a satellite remnant. Although the extinction in the direction of the structure is very low, the structure does match a low temperature feature in the dust maps. While this casts some doubt on its origin, the low temperature feature could plausibly be due to real dust in the structure itself. The angular position and distance of this structure encompass the Pisces overdensity traced by RR Lyraes in Stripe 82 of the Sloan Digital Sky Survey (SDSS). However, the 2MASS M-giants indicate that the structure is much more extended than what is visible with the SDSS, with the point of peak density lying just outside Stripe 82. The morphology of the structure is more like a cloud than a stream and reminiscent of that seen in simulations of satellites disrupting along highly eccentric orbits.

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arXiv2010

Correcting velocity dispersions of dwarf spheroidal galaxies for binary orbital motion

We show that measured velocity dispersions of dwarf spheroidal galaxies from about 4 to 10 km/s are unlikely to be inflated by more than 20% due to the orbital motion of binary stars, and demonstrate that the intrinsic velocity dispersions can be determined to within a few percent accuracy using two-epoch observations with 1-2 years as the optimal time interval. The crucial observable is the threshold fraction--the fraction of stars that show velocity changes larger than a given threshold between measurements. The threshold fraction is tightly correlated with the dispersion introduced by binaries, independent of the underlying binary fraction and distribution of orbital parameters. We outline a simple procedure to correct the velocity dispersion to within a few percent accuracy by using the threshold fraction and provide fitting functions for this method. We also develop a methodology for constraining properties of binary populations from both single- and two-epoch velocity measurements by including the binary velocity distribution in a Bayesian analysis.

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arXiv2009

Properties of Dark Matter Revealed by Astrometric Measurements of the Milky Way and Local Galaxies

The fact that dark matter (DM), thus far, has revealed itself only on scales of galaxies and larger, again thrusts onto astrophysics the opportunity and the responsibility to confront the age old mystery "What is the nature of matter?" By deriving basic data on the nature of DM - e.g., mass of its particle(s), present mean temperature, distribution in galaxies and other structures in the universe, and capacity for dissipational collapse - we will be uncovering the properties of the dominant species of matter in the universe and significantly extending the standard models of particle physics. Determining the mass of the DM particle to an order of magnitude would help to sort out the particle family to which it (or they) belongs. Beyond mass, there are issues of stability. The DM particle may be unstable with a measurable half-life, or it may become unstable after absorbing a certain amount of energy from collisions. In both cases it would contribute to the present hot dark matter component. Some key parameters of DM can most accurately be measured in the very nearby universe because DM dominates the mass in the outer Milky Way (MW), in other galaxies in the Local Group, and in the Local Group in its entirety. The presence and amount of DM can be quantified by study of dynamical processes observable in fine detail within these entities. Precise measurements of 3-D velocities for stars, coherent star streams, and stars in satellite stellar systems out to the edge of the Galaxy can reveal "what is the shape, orientation, density law, and lumpiness of the dark matter halo" as well as "what is the total mass of the Galaxy?"

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arXiv2009

Galactic Dynamics and Local Dark Matter

The concordance Lambda Cold Dark Matter (Lambda-CDM) model for the formation of structure in the Universe, while remarkably successful at describing observations of structure on large scales, continues to be challenged by observations on galactic scales. Fortunately, CDM models and their various proposed alternatives make a rich variety of testable predictions that make the Local Group and our own Milky Way Galaxy key laboratories for exploring dark matter (DM) in this regime. However, some of the most definitive tests of local DM require microarcsecond astrometry of faint sources, an astrometric regime that is a unique niche of SIM Lite. This chapter explores the important and distinct contributions that can be made by SIM Lite in the exploration of galaxy dynamics and DM on galaxy scales and that have cosmological consequences. Key areas of potential SIM Lite exploration include (1) measuring the shape, orientation, density law, and lumpiness of the dark halo of the Milky Way and other nearby galaxies, (2) determining the orbits of Galactic satellites, which may be representatives of late infall from the hierarchical formation of the Milky Way, (3) ascertaining the distribution of angular momentum and orbital anisotropy of stars and globular clusters to the outer reaches of the Galactic halo, dynamical properties that hold clues to the early hierarchical formation of the Galaxy, (4) measuring the physical nature of DM by placing strong constraints on the phase space density in the cores of nearby dSph galaxies, and (5) reconstructing the dynamical history of the Local Group through the determination of orbits and masses of its constituent galaxies.

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arXiv2007

Discovery of Andromeda XIV: A Dwarf Spheroidal Dynamical Rogue in the Local Group?

In the course of our survey of the outer halo of the Andromeda Galaxy we have discovered a remote, possible satellite of that system at a projected 162 kpc (11.7 degrees) radius. The fairly elongated (0.31 +/- 0.09 ellipticity) dwarf can be fit with a King profile of 1.07 kpc (d/784 kpc) limiting radius, where the satellite distance, d, is estimated at ~630-850 kpc from the tip of the red giant branch. The newfound galaxy, ``Andromeda XIV'' (``AndXIV''), distinguishes itself from other Local Group galaxies by its extreme dynamics: Keck/DEIMOS spectroscopy reveals it to have a large heliocentric radial velocity (-481 km/s), or -206 km/s velocity relative to M31. Even at its projected radius AndXIV already is at the M31 escape velocity based on the latest M31 mass models. If AndXIV is bound to M31, then recent models with reduced M31 virial masses need revision upward. If not bound to M31, then AndXIV is just now falling into the Local Group for the first time and represents a dwarf galaxy that formed and spent almost its entire life in isolation.

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arXiv2006

The Absence of Adiabatic Contraction of the Radial Dark Matter Profile in the Galaxy Cluster A2589

We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015-0.25 r_vir using an XMM-Newton observation. Except for a ~16 kpc shift of the X-ray center of the R=45-60 kpc annulus, A2589 possesses a remarkably symmetrical X-ray image and is therefore an exceptional candidate for precision studies of its mass profile by applying hydrostatic equilibrium. The total gravitating matter profile is well described by the NFW model (fractional residuals <~10%) with c_vir=6.1 +/- 0.3 and M_vir = 3.3 +/- 0.3 x 10^{14} M_sun (r_vir = 1.74 +/- 0.05 Mpc) in excellent agreement with LCDM. When the mass of the hot ICM is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, if a component accounting for the stellar mass (M_*) of the cD galaxy is included, then the NFW fit to the DM profile is substantially degraded in the central r ~50 kpc for reasonable M_*/L_V. Modifying the NFW DM halo by adiabatic contraction arising from the early condensation of stellar baryons in the cD galaxy further degrades the fit. The fit is improved substantially with a Sersic-like model recently suggested by high resolution N-body simulations but with an inverse Sersic index, alpha ~0.5, a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient non-thermal pressure support to reconcile the XMM mass profile with adiabatic contraction of a CDM halo assuming reasonable M_*/L_V. Our results support the scenario where, at least for galaxy clusters, processes during halo formation counteract adiabatic contraction so that the total gravitating mass in the core approximately follows the NFW profile.

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arXiv2005

Mass distribution of spiral galaxies from characteristics of spiral structure: Constraints on galaxy formation models

Recently, it has been shown that a correlation exists between the rate of shear and the spiral arm pitch angle in disk galaxies. The rate of shear depends upon the shape of the rotation curve, which is dependent upon the mass distribution in spiral galaxies. Here, we present an imporoved correlation between shear rate and spiral arm pitch angle, by increasing the sample size. We also use an adiabatic infall code to show that the rate of shear is most strongly correlated with the central mass concentration, c_m. The spin parameter, $λ$, and the fraction of baryons that cool, F, cause scatter in this correlation. Limiting this scatter, such that it is equal to that in the correlation between shear rate and pitch angle, and using a value of F=0.1 to 0.2, the spin parameter must be in the range 0.03<lambda<0.09 for spiral galaxies. We also derive an equation which links spiral arm pitch angle directly to c_m.

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arXiv2001

Resolving the Structure of Cold Dark Matter Halos II

We study the effects of mass and force resolution on the density profiles of galaxy-size Cold Dark Matter (CDM) halos in a flat, low-density cosmological model with vacuum energy. We show thatalthough increasing the mass and force resolution allows us to probe deeper into the inner halo regions, it does not lead to steeper inner density profiles. Instead, the halo profiles converge at scales larger than four times the formal resolution or the radius containing more than 200 particles, whichever is larger. In the simulations presented in this paper, we are able to probe density profile of a relaxed isolated galaxy-size halo at scales r=(0.005-1)r_vir. We find that the density distribution can be well approximated by the profile suggested by Moore etal (1998): rho= x^{-1.5}(1+x^{1.5})^-1, where x=r/r_s and r_s is the characteristic radius. The analytical profile proposed by Navarro et al. (1996) rho= x^{-1}(1+x)^-2, also provides a good fit, with the same relative errors of about 10% for radii larger than 1% of the virial radius. Both analytical profiles fit equally well because for high-concentration galaxy-size halos the differences between these profiles become significant only at scales well below 0.01r_vir. We also find that halos of similar mass may have somewhat different parameters (characteristic radius, maximum rotation velocity, etc.) and shapes of their density profiles. We associate this scatter in properties with differences in halo merger histories and the amount of substructure present in the analyzed halos.

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