arXiv subjects

James S. Bullock

Publications and source records attributed to James S. Bullock.

At least 127 records · Page 7Linked to original sources

arXiv2013

Segue 2: The Least Massive Galaxy

Segue 2, discovered by Belokurov et al. (2009), is a galaxy with a luminosity of only 900 L_sun. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2--a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is sigma_v < 2.2 (2.6) km/s, the most stringent limit for any galaxy. The corresponding limit on the mass within the 3-D half-light radius (46 pc) is M_1/2 < 1.5 (2.1) x 10^5 M_sun. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based the wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. The stars' [alpha/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, <[Fe/H]> = -2.22 +/- 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low-mass dark matter subhalo (v_max < 10 km/s), below the atomic hydrogen cooling limit.

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arXiv2013

On the stark difference in satellite distributions around the Milky Way and Andromeda

We compare spherically-averaged radial number counts of bright (> 10^5 Lsun) dwarf satellite galaxies within 400 kpc of the Milky Way (MW) and M31 and find that the MW satellites are much more centrally concentrated. Remarkably, the two satellite systems are almost identical within the central 100 kpc, while M31 satellites outnumber MW satellites by about a factor of four at deprojected distances spanning 100 - 400 kpc. We compare the observed distributions to those predicted for LCDM suhbalos using a suite of 44 high-resolution ~10^12 halo zoom simulations, 22 of which are in pairs like the MW and M31. We find that the radial distribution of satellites around M31 is fairly typical of those predicted for subhalos, while the Milky Way's distribution is more centrally concentrated that any of our simulated LCDM halos. One possible explanation is that our census is bright (> 10^5 Lsun) MW dwarf galaxies is significantly incomplete beyond ~ 100 kpc of the Sun. If there were ~8 - 20 more bright dwarfs orbiting undetected at 100 - 400 kpc, then the Milky Way's radial distribution would fall within the range expected from subhalo distributions and alos look very much like the known M31 system. We use our simulations to demonstrate that there is enough area left unexplored by the Sloan Digital Sky Survey and its extensions that the discovery of ~10 new bright dwarfs is not implausible given the expected range of angular anisotropy of subhalos in the sky.

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arXiv2013

The Space Motion of Leo I: Hubble Space Telescope Proper Motion and Implied Orbit

We present the first absolute proper motion measurement of Leo I, based on two epochs of HST ACS/WFC images separated by ~5 years. The average shift of Leo I stars with respect to ~100 background galaxies implies a proper motion of (mu_W, mu_N) = (0.1140 +/- 0.0295, -0.1256 +/- 0.0293) mas/yr. The implied Galactocentric velocity vector, corrected for the reflex motion of the Sun, has radial and tangential components V_rad = 167.9 +/- 2.8 km/s and V_tan = 101.0 +/- 34.4 km/s, respectively. We study the detailed orbital history of Leo I by solving its equations of motion backward in time for a range of plausible mass models for the Milky Way and its surrounding galaxies. Leo I entered the Milky Way virial radius 2.33 +/- 0.21 Gyr ago, most likely on its first infall. It had a pericentric approach 1.05 +/- 0.09 Gyr ago at a Galactocentric distance of 91 +/- 36 kpc. We associate these time scales with characteristic time scales in Leo I's star formation history, which shows an enhanced star formation activity ~2 Gyr ago and quenching ~1 Gyr ago. There is no indication from our calculations that other galaxies have significantly influenced Leo I's orbit, although there is a small probability that it may have interacted with either Ursa Minor or Leo II within the last ~1 Gyr. For most plausible Milky Way masses, the observed velocity implies that Leo I is bound to the Milky Way. However, it may not be appropriate to include it in models of the Milky Way satellite population that assume dynamical equilibrium, given its recent infall. Solution of the complete (non-radial) timing equations for the Leo I orbit implies a Milky Way mass M_MW,vir = 3.15 (-1.36, +1.58) x 10^12 Msun, with the large uncertainty dominated by cosmic scatter. In a companion paper, we compare the new observations to the properties of Leo I subhalo analogs extracted from cosmological simulations.

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arXiv2013

The Space Motion of Leo I: The Mass of the Milky Way's Dark Matter Halo

We combine our Hubble Space Telescope measurement of the proper motion of the Leo I dwarf spheroidal galaxy (presented in a companion paper) with the highest resolution numerical simulations of Galaxy-size dark matter halos in existence to constrain the mass of the Milky Way's dark matter halo (M_MW). Despite Leo I's large Galacto-centric space velocity (200 km/s) and distance (261 kpc), we show that it is extremely unlikely to be unbound if Galactic satellites are associated with dark matter substructure, as 99.9% of subhalos in the simulations are bound to their host. The observed position and velocity of Leo I strongly disfavor a low mass Milky Way: if we assume that Leo I is the least bound of the Milky Way's classical satellites, then we find that M_MW > 10^{12} M_sun at 95% confidence for a variety of Bayesian priors on M_MW. In lower mass halos, it is vanishingly rare to find subhalos at 261 kpc moving as fast as Leo I. Should an additional classical satellite be found to be less bound than Leo I, this lower limit on M_MW would increase by 30%. Imposing a mass weighted LCDM prior, we find a median Milky Way virial mass of M_MW=1.6 x 10^{12} M_sun, with a 90% confidence interval of [1.0-2.4] x 10^{12} M_sun. We also confirm a strong correlation between subhalo infall time and orbital energy in the simulations and show that proper motions can aid significantly in interpreting the infall times and orbital histories of satellites.

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arXiv2013

The Outer Limits of the M31 System: Kinematics of the Dwarf Galaxy Satellites And XXVIII and And XXIX

We present Keck/DEIMOS spectroscopy of resolved stars in the M31 satellites And XXVIII & And XXIX. We show that these are likely self-bound galaxies based on 18 and 24 members in And XXVIII & And XXIX, respectively. And XXVIII has a systemic velocity of -331.1 +/- 1.8 km/s and velocity dispersion of 4.9+/-1.6 km/s, implying a mass-to-light ratio (within r_1/2) of ~ 44 +/- 41. And XXIX has a systemic velocity of -194.4 +/- 1.5 km/s and velocity dispersion of 5.7+/-1.2 km/s, implying a mass-to-light ratio (within r_1/2) of ~ 124 +/- 72. The internal kinematics and implied masses of And XXVIII & And XXIX are similar to dwarf spheroidals (dSphs) of comparable luminosities, implying that these objects are dark matter-dominated dwarf galaxies. Despite the large projected distances from their host (380 and 188 kpc), the kinematics of these dSph suggest that they are bound M31 satellites.

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arXiv2012

Vertical density waves in the Milky Way disc induced by the Sagittarius Dwarf Galaxy

Recently, Widrow and collaborators announced the discovery of vertical density waves in the Milky Way disk. Here we investigate a scenario where these waves were induced by the Sagittarius dwarf galaxy as it plunged through the Galaxy. Using numerical simulations, we find that the Sagittarius impact produces North-South asymmetries and vertical wave-like behavior that qualitatively agrees with what is observed. The extent to which vertical modes can radially penetrate into the disc, as well as their amplitudes, depend on the mass of the perturbing satellite. We show that the mean height of the disc is expected to vary more rapidly in the radial than in the azimuthal direction. If the observed vertical density asymmetry is indeed caused by vertical oscillations, we predict radial and azimuthal variations of the mean vertical velocity, correlating with the spatial structure. These variations can have amplitudes as large as 8 km/s.

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arXiv2012

On the Hot Gas Content of the Milky Way Halo

The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea supported indirectly by observations of warm gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray observations have established that hot gas does exist in our Galaxy beyond the local hot bubble; however, it may be distributed in a hot disk configuration. Moreover, recent investigations into the X-ray constraints have suggested that any Galactic corona must be insignificant. Here we re-examine the observational data, particularly in the X-ray and radio bands, in order to determine whether it is possible for a substantial fraction of the Galaxy's baryons to exist in ~ 10^6 K gas. In agreement with past studies, we find that a baryonically closed halo is clearly ruled out if one assumes that the hot corona is distributed with a cuspy NFW profile. However, if the hot corona of the galaxy is in an extended, low-density distribution with a large central core, as expected for an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10^11 M_sun of material, possibly accounting for all of the missing Galactic baryons. We briefly discuss some potential avenues for discriminating between a massive, extended hot halo and a local hot disk.

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arXiv2012

Stellar Kinematics of the Andromeda II Dwarf Spheroidal Galaxy

We present kinematical profiles and metallicity for the M31 dwarf spheroidal (dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy of 531 red giant branch stars. Our kinematical sample is among the largest for any M31 satellite and extends out to two effective radii (r_eff = 5.3' = 1.1 kpc). We find a mean systemic velocity of -192.4+-0.5 km/s and an average velocity dispersion of sigma_v = 7.8+-1.1 km/s. While the rotation velocity along the major axis of And II is nearly zero (<1 km/s), the rotation along the minor axis is significant with a maximum rotational velocity of v_max=8.6+-1.8 km/s. We find a kinematical major axis, with a maximum rotational velocity of v_max=10.9+-2.4 km/s, misaligned by 67 degrees to the isophotal major axis. And II is thus the first dwarf galaxy with evidence for nearly prolate rotation with a v_max/sigma_v = 1.1, although given its ellipticity of epsilon = 0.10, this object may be triaxial. We measured metallicities for a subsample of our data, finding a mean metallicity of [Fe/H] = -1.39+- 0.03 dex and an internal metallicity dispersion of 0.72+-0.03 dex. We find a radial metallicity gradient with metal-rich stars more centrally concentrated, but do not observe a significant difference in the dynamics of two metallicity populations. And II is the only known dwarf galaxy to show minor axis rotation making it a unique system whose existence offers important clues on the processes responsible for the formation of dSphs.

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arXiv2012

Cosmological Simulations with Self-Interacting Dark Matter I: Constant Density Cores and Substructure

We use cosmological simulations to study the effects of self-interacting dark matter (SIDM) on the density profiles and substructure counts of dark matter halos from the scales of spiral galaxies to galaxy clusters, focusing explicitly on models with cross sections over dark matter particle mass σ/m = 1 and 0.1 cm^2/g. Our simulations rely on a new SIDM N-body algorithm that is derived self-consistently from the Boltzmann equation and that reproduces analytic expectations in controlled numerical experiments. We find that well-resolved SIDM halos have constant-density cores, with significantly lower central densities than their CDM counterparts. In contrast, the subhalo content of SIDM halos is only modestly reduced compared to CDM, with the suppression greatest for large hosts and small halo-centric distances. Moreover, the large-scale clustering and halo circular velocity functions in SIDM are effectively identical to CDM, meaning that all of the large-scale successes of CDM are equally well matched by SIDM. From our largest cross section runs we are able to extract scaling relations for core sizes and central densities over a range of halo sizes and find a strong correlation between the core radius of an SIDM halo and the NFW scale radius of its CDM counterpart. We construct a simple analytic model, based on CDM scaling relations, that captures all aspects of the scaling relations for SIDM halos. Our results show that halo core densities in σ/m = 1 cm^2/g models are too low to match observations of galaxy clusters, low surface brightness spirals (LSBs), and dwarf spheroidal galaxies. However, SIDM with σ/m ~ 0.1 cm^2/g appears capable of reproducing reported core sizes and central densities of dwarfs, LSBs, and galaxy clusters without the need for velocity dependence. (abridged)

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arXiv2012

Cosmological Simulations with Self-Interacting Dark Matter II: Halo Shapes vs. Observations

If dark matter has a large self-interaction scattering cross section, then interactions among dark-matter particles will drive galaxy and cluster halos to become spherical in their centers. Work in the past has used this effect to rule out velocity-independent, elastic cross sections larger than sigma/m ~ 0.02 cm^2/g based on comparisons to the shapes of galaxy cluster lensing potentials and X-ray isophotes. In this paper, we use cosmological simulations to show that these constraints were off by more than an order of magnitude because (a) they did not properly account for the fact that the observed ellipticity gets contributions from the triaxial mass distribution outside the core set by scatterings, (b) the scatter in axis ratios is large and (c) the core region retains more of its triaxial nature than estimated before. Including these effects properly shows that the same observations now allow dark matter self-interaction cross sections at least as large as sigma/m = 0.1 cm^2/g. We show that constraints on self-interacting dark matter from strong-lensing clusters are likely to improve significantly in the near future, but possibly more via central densities and core sizes than halo shapes.

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arXiv2012

The SPLASH Survey: Spectroscopy of 15 M31 Dwarf Spheroidal Satellite Galaxies

We present a resolved-star spectroscopic survey of 15 dwarf spheroidal (dSph) satellites of the Andromeda Galaxy (M31) as part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) project. We filter foreground contamination from Milky Way (MW) stars, noting that MW substructure is evident in this contaminant sample. We also filter M31 halo field giant stars, and identify the remainder as probable dSph members. We then use these members to determine the kinematical properties of the dSphs. For the first time, we confirm that And XVIII, XXI, and XXII show kinematics consistent with bound, dark matter-dominated galaxies. From the velocity dispersions for the full sample of dSphs we determine masses, which we combine with the size and luminosity of the galaxies to produce mass-size-luminosity scaling relations. With these scalings we determine that the M31 dSphs are fully consistent with the MW dSphs, suggesting that the well-studied MW satellite population provides a fair sample for broader conclusions. We also estimate dark matter halo masses of the satellites, and find that there is no sign that the luminosity of these galaxies depends on their dark halo mass, a result consistent with what is seen for MW dwarfs. Two of the M31 dSphs (And XV, XVI) have estimated maximum circular velocities smaller than 12 km/s (to 1sigma), which likely places them within the lowest mass dark matter halos known to host stars (along with Bootes I of the MW). Finally, we use the systemic velocities of the M31 satellites to estimate the total mass of the M31 halo: within 139 kpc, M31's mass is 8 +4.1 - 3.7 x 10^11 corresponding to a virial mass for M31's dark matter halo of 1.2 +0.9 -0.7 x 10^12, consistent with previous results.

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arXiv2012

Signatures of minor mergers in the Milky Way disc I: The SEGUE stellar sample

It is now known that minor mergers are capable of creating structure in the phase-space distribution of their host galaxy's disc. In order to search for such imprints in the Milky Way, we analyse the SEGUE F/G-dwarf and the Schuster et al. (2006) stellar samples. We find similar features in these two completely independent stellar samples, consistent with the predictions of a Milky Way minor-merger event. We next apply the same analyses to high-resolution, idealised N-body simulations of the interaction between the Sagittarius dwarf galaxy and the Milky Way. The energy distributions of stellar particle samples in small spatial regions in the host disc reveal strong variations of structure with position. We find good matches to the observations for models with a mass of Sagittarius' dark matter halo progenitor $\lessapprox 10^{11}$ M$_{\odot}$. Thus, we show that this kind of analysis could be used to provide unprecedentedly tight constraints on Sagittarius' orbital parameters, as well as place a lower limit on its mass.

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arXiv2012

The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid

The combination of large size, high stellar density, high metallicity, and Sersic surface brightness profile of the spheroidal component of the Andromeda galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent of the Milky Way. In this work we capitalize on our proximity to and external view of M31 to probe the kinematical properties of this "inner spheroid." We employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to disentangle M31's inner spheroid from its stellar disk. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins after accounting for a locally cold stellar disk as well as the Giant Southern Stream and associated tidal debris. For the first time, we detect significant spheroid rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc, consistent to 2 sigma with existing measurements and models. We calculate the probability that a given star is a member of the spheroid and find that the spheroid has a significant presence throughout the spatial extent of our sample. Lastly, we show that the flattening of the spheroid is due to velocity anisotropy in addition to rotation. Though this suggests that the inner spheroid of M31 more closely resembles an elliptical galaxy than a typical spiral galaxy bulge, it should be cautioned that our measurements are much farther out (2 - 14 r_eff) than for the comparison samples.

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arXiv2012

The Milky Way's bright satellites as an apparent failure of LCDM

We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have dark matter halos that are a factor of ~5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhalos, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and LCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard LCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2% to 20% of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of LCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.

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arXiv2012

Dark matter concentrations and a search for cores in Milky Way dwarf satellites

We investigate the mass distributions within eight classical Milky Way dwarf spheroidal galaxies (MW dSphs) using an equilibrium Jeans analysis and we compare our results to the mass distributions predicted for subhalos in dissipationless ΛCDM simulations. In order to match the dark matter density concentrations predicted, the stars in these galaxies must have a fairly significant tangential velocity dispersion anisotropy (β~-1.5). For the limiting case of an isotropic velocity dispersion (β=0), the classical MW dSphs predominantly prefer to live in halos that are less concentrated than ΛCDM predictions. We also investigate whether the dSphs prefer to live in halos with constant density cores in the limit of isotropic velocity dispersion. Interestingly, even in this limit, not all of the dSphs prefer large constant-density cores: the Sculptor dSph prefers a cusp while Carina, Draco and Leo I prefer cores. The other four dSphs do not show a statistically significant preference for either cuspy or cored profiles. Finally, we re-examine the hypothesis that the density profiles of these eight MW dSphs can be quantified by a common dark matter halo.

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arXiv2011

Exploring the links between star formation and minor companions around isolated galaxies

Previous studies have shown that galaxies with minor companions exhibit an elevated star formation rate. We reverse this inquiry, constructing a volume-limited sample of \simL\star (Mr \leq -19.5 + 5 log h) galaxies from the Sloan Digital Sky Survey that are isolated with respect to other luminous galaxies. Cosmological simulations suggest that 99.8% of these galaxies are alone in their dark matter haloes with respect to other luminous galaxies. We search the area around these galaxies for photometric companions. Matching strongly star forming (EW(Hα)\geq 35 Å) and quiescent (EW(Hα)< 35 Å) samples for stellar mass and redshift using a Monte Carlo resampling technique, we demonstrate that rapidly star-forming galaxies are more likely to have photometric companions than other galaxies. The effect is relatively small; about 11% of quiescent, isolated galaxies have minor photometric companions at radii \leq 60 kpc h$^{-1}$ kpc while about 16% of strongly star-forming ones do. Though small, the cumulative difference in satellite counts between strongly star-forming and quiescent galaxies is highly statistically significant (PKS = 1.350 \times10$^{-3}$) out to to radii of \sim 100 h$^{-1}$ kpc. We discuss explanations for this excess, including the possibility that \sim 5% of strongly star-forming galaxies have star formation that is causally related to the presence of a minor companion.

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arXiv2011

Infall Times for Milky Way Satellites From Their Present-Day Kinematics

We analyze subhalos in the Via Lactea II (VL2) cosmological simulation to look for correlations among their infall times and z = 0 dynamical properties. We find that the present day orbital energy is tightly correlated with the time at which subhalos last crossed into the virial radius. This energy-infall correlation provides a means to infer infall times for Milky Way satellite galaxies. Assuming that the Milky Way's assembly can be modeled by VL2, we show that the infall times of some satellites are well constrained given only their Galactocentric positions and line-of-sight velocities. The constraints sharpen for satellites with proper motion measurements. We find that Carina, Ursa Minor, and Sculptor were all accreted early, more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are also probable early accreters, though with larger uncertainties. On the other extreme, Leo T is just falling into the Milky Way for the first time while Leo I fell in \sim 2 Gyr ago and is now climbing out of the Milky Way's potential after its first perigalacticon. The energies of several other dwarfs, including Fornax and Hercules, point to intermediate infall times, 2 - 8 Gyr ago. We compare our infall time estimates to published star formation histories and find hints of a dichotomy between ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched star formation after infall but the ultrafaint dwarfs tend to be quenched long before infall, at least for the cases in which our uncertainties allow us to discern differences. Our analysis suggests that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within the last \sim 4 billion years.

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arXiv2011

The Sagittarius impact as an architect of spirality and outer rings in the Milky Way

Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that are reminiscent of the low- latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

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