arXiv subjects

James S. Bullock

Publications and source records attributed to James S. Bullock.

At least 109 records · Page 6Linked to original sources

arXiv2015

XMM-Newton Survey of Local O VII Absorption Lines in the Spectra of Active Galactic Nuclei

Highly ionized, z=0 metal absorption lines detected in the X-ray spectra of background active galactic nuclei (AGNs) provide an effective method to probe the hot ($T\sim10^6$ K) gas and its metal content in and around the Milky Way. We present an all-sky survey of the $K_α$ transition of the local O VII absorption lines obtained by Voigt-profile fitting archival XMM-Newton observations. A total of 43 AGNs were selected, among which 12 are BL Lac-type AGNs, and the rest are Seyfert 1 galaxies. At above the $3σ$ level the local O VII absorption lines were detected in 21 AGNs, among which 7 were newly discovered in this work. The sky covering fraction, defined as the ratio between the number of detections and the sample size, increases from at about 40% for all targets to 100% for the brightest targets, suggesting a uniform distribution of the O VII absorbers. We correlate the line equivalent width with the Galactic coordinates and do not find any strong correlations between these quantities. Some AGNs have warm absorbers that may complicate the analysis of the local X-ray absorber since the recession velocity can be compensated by the outflow velocity, especially for the nearby targets. We discuss the potential impact of the warm absorbers on our analysis. A comprehensive theoretical modelling of the X-ray absorbers will be presented in a later paper.

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arXiv2014

Core Formation in Dwarf Halos with Self Interacting Dark Matter: No Fine-Tuning Necessary

We investigate the effect of self-interacting dark matter (SIDM) on the density profiles of $V_{\rm max} \simeq 40~km~s^{-1}$ isolated dwarf dark matter halos -- the scale of relevance for the too big to fail problem (TBTF) -- using very high-resolution cosmological zoom simulations. Each halo has millions of particles within its virial radius. We find that SIDM models with cross sections per unit mass spanning the range σ/m = $0.5 - 50$ $cm^2~g^{-1}$ alleviate TBTF and produce constant density cores of size 300-1000 pc, comparable to the half-light radii of $M_\star$ ~ $10^{5-7}$ $M_\odot$ dwarfs. The largest, lowest density cores develop for cross sections in the middle of this range, σ/m ~ $5-10~cm^2~g^{-1}$. Our largest SIDM cross section run (σ/m = $50~cm^2~g^{-1}$) develops a slightly denser core owing to mild core-collapse behavior, but it remains less dense than the CDM case and retains a constant density core profile. Our work suggests that SIDM cross sections as large or larger than $50~cm^2~g^{-1}$ remain viable on velocity scales of dwarf galaxies ($v_{\rm rms}$ ~ $40~km~s^{-1}$). The range of SIDM cross sections that alleviate TBTF and the cusp/core problem spans at least two orders of magnitude and therefore need not be particularly fine-tuned.

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arXiv2014

Running with BICEP2: Implications for Small-Scale Problems in CDM

The BICEP2 results, when interpreted as a gravitational wave signal and combined with other CMB data, suggest a roll-off in power towards small scales in the primordial matter power spectrum. Among the simplest possibilities is a running of the spectral index. Here we show that the preferred level of running alleviates small-scale issues within the $Λ$CDM model, more so even than viable WDM models. We use cosmological zoom-in simulations of a Milky Way-size halo along with full-box simulations to compare predictions among four separate cosmologies: a BICEP2-inspired running index model ($α_s$ = -0.024), two fixed-tilt $Λ$CDM models motivated by Planck, and a 2.6 keV thermal WDM model. We find that the running BICEP2 model reduces the central densities of large dwarf-size halos ($V_\mathrm{max}$ ~ 30 - 80 km s$^{-1}$) and alleviates the too-big-to-fail problem significantly compared to our adopted Planck and WDM cases. Further, the BICEP2 model suppresses the count of small subhalos by ~50% relative to Planck models, and yields a significantly lower "boost" factor for dark matter annihilation signals. Our findings highlight the need to understand the shape of the primordial power spectrum in order to correctly interpret small-scale data.

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arXiv2014

Galaxies on FIRE (Feedback In Realistic Environments): Stellar Feedback Explains Cosmologically Inefficient Star Formation

We present a series of high-resolution cosmological simulations of galaxy formation to z=0, spanning halo masses ~10^8-10^13 M_sun, and stellar masses ~10^4-10^11. Our simulations include fully explicit treatment of both the multi-phase ISM (molecular through hot) and stellar feedback. The stellar feedback inputs (energy, momentum, mass, and metal fluxes) are taken directly from stellar population models. These sources of stellar feedback, with zero adjusted parameters, reproduce the observed relation between stellar and halo mass up to M_halo~10^12 M_sun (including dwarfs, satellites, MW-mass disks, and small groups). By extension, this leads to reasonable agreement with the stellar mass function for M_star<10^11 M_sun. We predict weak redshift evolution in the M_star-M_halo relation, consistent with current constraints to z>6. We find that the M_star-M_halo relation is insensitive to numerical details, but is sensitive to the feedback physics. Simulations with only supernova feedback fail to reproduce the observed stellar masses, particularly in dwarf and high-redshift galaxies: radiative feedback (photo-heating and radiation pressure) is necessary to disrupt GMCs and enable efficient coupling of later supernovae to the gas. Star formation rates agree well with the observed Kennicutt relation at all redshifts. The galaxy-averaged Kennicutt relation is very different from the numerically imposed law for converting gas into stars in the simulation, and is instead determined by self-regulation via stellar feedback. Feedback reduces star formation rates considerably and produces a reservoir of gas that leads to rising late-time star formation histories significantly different from the halo accretion history. Feedback also produces large short-timescale variability in galactic SFRs, especially in dwarfs. Many of these properties are not captured by common 'sub-grid' galactic wind models.

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arXiv2014

Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile

We present the metallicity distribution of red giant branch (RGB) stars in M31's stellar halo, derived from photometric metallicity estimates for over 1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from 38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to 175 kpc in projected distance from M31's center, and includes 52 confirmed M31 halo stars beyond 100 kpc. While a wide range of metallicities is seen throughout the halo, the metal-rich peak of the metallicity distribution function becomes significantly less prominent with increasing radius. The metallicity profile of M31's stellar halo shows a continuous gradient from 9 to ~100 kpc, with a magnitude of -0.01 dex/kpc. The stellar velocity distributions in each field are used to identify stars that are likely associated with tidal debris features. The removal of tidal debris features does not significantly alter the metallicity gradient in M31's halo: a gradient is maintained in fields spanning 10 to 90 kpc. We analyze the halo metallicity profile, as well as the relative metallicities of stars associated with tidal debris features and the underlying halo population, in the context of current simulations of stellar halo formation. We argue that the large scale gradient in M31's halo implies M31 accreted at least one relatively massive progenitor in the past, while the field to field variation seen in the metallicity profile indicates that multiple smaller progenitors are likely to have contributed substantially to M31's outer halo.

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arXiv2014

The Mass Dependance of Satellite Quenching in Milky Way-like Halos

Using the Sloan Digital Sky Survey, we examine the quenching of satellite galaxies around isolated Milky Way-like hosts in the local Universe. We find that the efficiency of satellite quenching around isolated galaxies is low and roughly constant over two orders of magnitude in satellite stellar mass ($M_{*}$ = $10^{8.5}-10^{10.5} \, M_{\odot}$), with only $\sim~20\%$ of systems quenched as a result of environmental processes. While largely independent of satellite stellar mass, satellite quenching does exhibit clear dependence on the properties of the host. We show that satellites of passive hosts are substantially more likely to be quenched than those of star-forming hosts, and we present evidence that more massive halos quench their satellites more efficiently. These results extend trends seen previously in more massive host halos and for higher satellite masses. Taken together, it appears that galaxies with stellar masses larger than about $10^{8}~M_{\odot}$ are uniformly resistant to environmental quenching, with the relative harshness of the host environment likely serving as the primary driver of satellite quenching. At lower stellar masses ($< 10^{8}~M_{\odot}$), however, observations of the Local Group suggest that the vast majority of satellite galaxies are quenched, potentially pointing towards a characteristic satellite mass scale below which quenching efficiency increases dramatically.

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arXiv2014

Near-Field Limits on the Role of Faint Galaxies in Cosmic Reionization

Reionizing the Universe with galaxies appears to require significant star formation in low-mass halos at early times, while local dwarf galaxy counts tell us that star formation has been minimal in small halos around us today. Using simple models and the ELVIS simulation suite, we show that reionization scenarios requiring appreciable star formation in halos with $M_{\rm vir} \approx 10^{8}\,M_{\odot}$ at $z=8$ are in serious tension with galaxy counts in the Local Group. This tension originates from the seemingly inescapable conclusion that 30 - 60 halos with $M_{\rm vir} > 10^{8}\,M_{\odot}$ at $z=8$ will survive to be distinct bound satellites of the Milky Way at $z = 0$. Reionization models requiring star formation in such halos will produce dozens of bound galaxies in the Milky Way's virial volume today (and 100 - 200 throughout the Local Group), each with $\gtrsim 10^{5}\,M_{\odot}$ of old stars ($\gtrsim 13$ Gyr). This exceeds the stellar mass function of classical Milky Way satellites today, even without allowing for the (significant) post-reionization star formation observed in these galaxies. One possible implication of these findings is that star formation became sharply inefficient in halos smaller than $\sim 10^9 \,M_{\odot}$ at early times, implying that the high-$z$ luminosity function must break at magnitudes brighter than is often assumed (at ${\rm M_{UV}} \approx -14$). Our results suggest that JWST (and possibly even HST with the Frontier Fields) may realistically detect the faintest galaxies that drive reionization. It remains to be seen how these results can be reconciled with the most sophisticated simulations of early galaxy formation at present, which predict substantial star formation in $M_{\rm vir} \sim 10^8 \, M_{\odot}$ halos during the epoch of reionization.

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arXiv2014

The High-$z$ Universe Confronts Warm Dark Matter: Galaxy Counts, Reionization and the Nature of Dark Matter

We use $N$-body simulations to show that high-redshift galaxy counts provide an interesting constraint on the nature of dark matter, specifically Warm Dark Matter (WDM), owing to the lack of early structure formation these models. Our simulations include three WDM models with thermal-production masses of 0.8 keV, 1.3 keV, and 2.6 keV, as well as CDM. Assuming a relationship between dark halo mass and galaxy luminosity that is set by the observed luminosity function at bright magnitudes, we find that 0.8 keV WDM is disfavored by direct galaxy counts in the Hubble Ultra Deep Field at $>\!\!10σ$. Similarly, 1.3 keV WDM is statistically inconsistent at $2.2σ$. Future observations with JWST (and possibly HST via the Frontier Fields) could rule out $1.3$ keV WDM at high significance, and may be sensitive to WDM masses greater than 2.6 keV. We also examine the ability of galaxies in these WDM models to reionize the universe, and find that 0.8 keV and 1.3 keV WDM produce optical depths to the Cosmic Microwave Background (CMB) that are inconsistent at 68% C.L. with current Planck results, even with extremely high ionizing radiation escape fractions, and 2.6 keV WDM requires an optimistic escape fraction to yield an optical depth consistent with Planck data. Although CMB optical depth calculations are model dependent, we find a strong challenge for stellar processes alone to reionize the universe in a 0.8 keV and 1.3 keV WDM cosmology.

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arXiv2014

The surprising inefficiency of dwarf satellite quenching

We study dwarf satellite galaxy quenching using observations from the Geha et al. (2012) NSA/SDSS catalog together with LCDM cosmological simulations to facilitate selection and interpretation. We show that fewer than 30% of dwarfs (M* ~ 10^8.5-10^9.5 Msun) identified as satellites within massive host halos (Mhost ~ 10^12.5-10^14 Msun) are quenched, in spite of the expectation from simulations that half of them should have been accreted more than 6 Gyr ago. We conclude that whatever the action triggering environmental quenching of dwarf satellites, the process must be highly inefficient. We investigate a series of simple, one-parameter quenching models in order to understand what is required to explain the low quenched fraction and conclude that either the quenching timescale is very long (> 9.5 Gyr, a "slow starvation" scenario) or that the environmental trigger is not well matched to accretion within the virial volume. We discuss these results in light of the fact that most of the low mass dwarf satellites in the Local Group are quenched, a seeming contradiction that could point to a characteristic mass scale for satellite quenching.

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arXiv2014

Too Big to Fail in the Local Group

We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to the predicted masses of halos in the ELVIS suite of $Λ$CDM simulations, a sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar to the LG. We enumerate unaccounted-for dense halos ($V_\mathrm{max} \gtrsim 25$ km s$^{-1}$) in these volumes that at some point in their histories were massive enough to have formed stars in the presence of an ionizing background ($V_\mathrm{peak} > 30$ km s$^{-1}$). Within 300 kpc of the Milky Way, the number of unaccounted-for massive halos ranges from 2 - 25 over our full sample. Moreover, this "too big to fail" count grows as we extend our comparison to the outer regions of the Local Group: within 1.2 Mpc of either giant we find that there are 12-40 unaccounted-for massive halos. This count excludes volumes within 300 kpc of both the MW and M31, and thus should be largely unaffected by any baryonically-induced environmental processes. According to abundance matching -- specifically abundance matching that reproduces the Local Group stellar mass function -- all of these missing massive systems should have been quite bright, with $M_\star > 10^6M_\odot$. Finally, we use the predicted density structure of outer LG dark matter halos together with observed dwarf galaxy masses to derive an $M_\star-V_\mathrm{max}$ relation for LG galaxies that are outside the virial regions of either giant. We find that there is no obvious trend in the relation over three orders of magnitude in stellar mass (a "common mass" relation), from $M_\star \sim 10^8 - 10^5 M_\odot$, in drastic conflict with the tight relation expected for halos that are unaffected by reionization. Solutions to the too big to fail problem that rely on ram pressure stripping, tidal effects, or statistical flukes appear less likely in the face of these results.

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arXiv2014

M31 Satellite Masses Compared to LCDM Subhaloes

We compare the kinematics of M31's satellite galaxies to the mass profiles of the subhaloes they are expected to inhabit in LCDM. We consider the most massive subhaloes of an approximately M31-sized halo, following the assumption of a monotonic galaxy luminosity-to-subhalo mass mapping. While this abundance matching relation is consistent with the kinematic data for galaxies down to the luminosity of the bright satellites of the Milky Way and M31, it is not consistent with kinematic data for fainter dwarf galaxies (those with L <~ 10^8 Lsun). Comparing the kinematics of M31's dwarf Spheroidal (dSph) satellites to the subhaloes reveals that M31's dSph satellites are too low density to be consistent with the subhaloes' mass profiles. A similar discrepancy has been reported between Milky Way dSphs and their predicted subhaloes, the "too big to fail" problem. By contrast, total mass profiles of the dwarf Elliptical (and similarly bright) satellites are consistent with the subhaloes. However, they suffer from large systematic uncertainties in their dark matter content because of substantial (and potentially dominant) contributions from baryons within their half-light radii.

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arXiv2014

The dynamics of isolated Local Group galaxies

We measured velocities of 862 individual red giant stars in seven isolated dwarf galaxies in the Local Group: NGC 6822, IC 1613, VV 124 (UGC 4879), the Pegasus dwarf irregular galaxy (DDO 216), Leo A, Cetus, and Aquarius (DDO 210). We also computed velocity dispersions, taking into account the measurement uncertainties on individual stars. None of the isolated galaxies is denser than the densest Local Group satellite galaxy. Furthermore, the isolated dwarf galaxies have no obvious distinction in the velocity dispersion--half-light radius plane from the satellite galaxies of the Milky Way and M31. The similarity of the isolated and satellite galaxies' dynamics and structural parameters imposes limitations on environmental solutions to the too-big-to-fail problem, wherein there are fewer dense dwarf satellite galaxies than would be expected from cold dark matter simulations. This data set also has many other applications for dwarf galaxy evolution, including the transformation of dwarf irregular into dwarf spheroidal galaxies. We intend to explore these issues in future work.

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arXiv2013

A Dichotomy in Satellite Quenching Around L* Galaxies

We examine the star formation properties of bright (~0.1 L*) satellites around isolated ~L* hosts in the local Universe using spectroscopically confirmed systems in the Sloan Digital Sky Survey DR7. Our selection method is carefully designed with the aid of N-body simulations to avoid groups and clusters. We find that satellites are significantly more likely to be quenched than a stellar mass-matched sample of isolated galaxies. Remarkably, this quenching occurs only for satellites of hosts that are themselves quenched: while star formation is unaffected in the satellites of star-forming hosts, satellites around quiescent hosts are more than twice as likely to be quenched than stellar-mass matched field samples. One implication of this is that whatever shuts down star formation in isolated, passive L* galaxies also plays at least an indirect role in quenching star formation in their bright satellites. The previously-reported tendency for "galactic conformity" in color/morphology may be a by-product of this host-specific quenching dichotomy. The Sérsic indices of quenched satellites are statistically identical to those of field galaxies with the same specific star formation rates, suggesting that environmental and secular quenching give rise to the same morphological structure. By studying the distribution of pairwise velocities between the hosts and satellites, we find dynamical evidence that passive host galaxies reside in dark matter halos that are ~45% more massive than those of star-forming host galaxies of the same stellar mass. We emphasize that even around passive hosts, the mere fact that galaxies become satellites does not typically result in star formation quenching: we find that only ~30% of ~0.1 L* galaxies that fall in from the field are quenched around passive hosts, compared with ~0% around star forming hosts.

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arXiv2013

How to Zoom: Bias, Contamination, and Lagrange Volumes in Multimass Cosmological Simulations

We perform a suite of multimass cosmological zoom simulations of individual dark matter halos and explore how to best select Lagrangian regions for resimulation without contaminating the halo of interest with low-resolution particles. Such contamination can lead to significant errors in the gas distribution of hydrodynamical simulations, as we show. For a fixed Lagrange volume, we find that the chance of contamination increases systematically with the level of zoom. In order to avoid contamination, the Lagrangian volume selected for resimulation must increase monotonically with the resolution difference between parent box and the zoom region. We provide a simple formula for selecting Lagrangian regions (in units of the halo virial volume) as a function of the level of zoom required. We also explore the degree to which a halo's Lagrangian volume correlates with other halo properties (concentration, spin, formation time, shape, etc.) and find no significant correlation. There is a mild correlation between Lagrange volume and environment, such that halos living in the most clustered regions have larger Lagrangian volumes. Nevertheless, selecting halos to be isolated is not the best way to ensure inexpensive zoom simulations. We explain how one can safely choose halos with the smallest Lagrangian volumes, which are the least expensive to resimulate, without biasing one's sample.

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arXiv2013

The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies

We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z_* ~ M_*^(0.30 +/- 0.02). The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M_* = 10^12 M_sun. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping.

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arXiv2013

Groups of two galaxies in SDSS: implications of colours on star formation quenching time-scales

We have devised a method to select galaxies that are isolated in their dark matter halo (N=1 systems) and galaxies that reside in a group of exactly two (N=2 systems). Our N=2 systems are widely-separated (up to $\sim$\,200\,$h^{-1}$\,kpc), where close galaxy-galaxy interactions are not dominant. We apply our selection criteria to two volume-limited samples of galaxies from SDSS DR6 with $M_{r}-5 \log_{10} h \leq$ -19 and -20 to study the effects of the environment of very sparse groups on galaxy colour. For satellite galaxies in a group of two, we find a red excess attributed to star formation quenching of 0.15\,$\pm$\,0.01 and 0.14\,$\pm$\,0.01 for the -19 and -20 samples, respectively, relative to isolated galaxies of the same stellar mass. Assuming N=1 systems are the progenitors of N=2 systems, an immediate-rapid star formation quenching scenario is inconsistent with these observations. A delayed-then-rapid star formation quenching scenario with a delay time of 3.3 and 3.7\,Gyr for the -19 and -20 samples, respectively, yields a red excess prediction in agreement with the observations. The observations also reveal that central galaxies in a group of two have a slight blue excess of 0.06\,$\pm$\,0.02 and 0.02\,$\pm$\,0.01 for the -19 and -20 samples, respectively, relative to N=1 populations of the same stellar mass. Our results demonstrate that even the environment of very sparse groups of luminous galaxies influence galaxy evolution and in-depth studies of these simple systems are an essential step towards understanding galaxy evolution in general.

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arXiv2013

Angular Momentum Acquisition in Galaxy Halos

We use high-resolution cosmological hydrodynamic simulations to study the angular momentum acquisition of gaseous halos around Milky Way sized galaxies. We find that cold mode accreted gas enters a galaxy halo with ~70% more specific angular momentum than dark matter averaged over cosmic time (though with a very large dispersion). In fact, we find that all matter has a higher spin parameter when measured at accretion than when averaged over the entire halo lifetime, and is well characterized by λ~0.1, at accretion. Combined with the fact that cold flow gas spends a relatively short time (1-2 dynamical times) in the halo before sinking to the center, this naturally explains why cold flow halo gas has a specific angular momentum much higher than that of the halo and often forms "cold flow disks". We demonstrate that the higher angular momentum of cold flow gas is related to the fact that it tends to be accreted along filaments.

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arXiv2013

Cold dark matter: controversies on small scales

The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way's dwarf galaxy satellites. We review the current observational and theoretical status of these "small scale controversies." Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: recent numerical simulations and analytic models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star-formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.

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