arXiv subjects

Kathryn V. Johnston

Publications and source records attributed to Kathryn V. Johnston.

At least 127 records · Page 7Linked to original sources

arXiv2006

The Effect of Substructure on Mass Estimates of Galaxies

Large galaxies are thought to form hierarchically, from the accretion and disruption of many smaller galaxies. Such a scenario should naturally lead to galactic phase-space distributions containing some degree of substructure. We examine the errors in mass estimates of galaxies and their dark halos made using the projected phase-space distribution of a tracer population (such as a globular cluster system or planetary nebulae) due to falsely assuming that the tracers are distributed randomly. The level of this uncertainty is assessed by applying a standard mass estimator to samples drawn from 11 random realizations of galaxy halos containing levels of substructure consistent with current models of structure formation. We find that substructure will distort our mass estimates by up to ~20% - a negligible error compared to statistical and measurement errors in current derivations of masses for our own and other galaxies. However, this represents a fundamental limit to the accuracy of any future mass estimates made under the assumption that the tracer population is distributed randomly, regardless of the size of the sample or the accuracy of the measurements.

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arXiv2005

Phase-Space Distributions of Chemical Abundances in Milky Way-type Galaxy Halos

[Abridged] Motivated by upcoming data from astrometric and spectroscopic surveys of the Galaxy, we explore the chemical abundance properties and phase-space distributions in hierarchically-formed stellar halo simulations set in a LambdaCDM Universe. Our sample of Milky-Way type stellar halo simulations result in average metallicities that range from [Fe/H] = -1.3 to -0.9, with the most metal poor halos resulting from accretion histories that lack destructive mergers with massive (metal rich) satellites. Our stellar halo metallicities increase with stellar halo mass. The slope of the [Fe/H]-stellar mass trend mimics that of the satellite galaxies that were destroyed to build the halos, implying that the relation propagates hierarchically. All simulated halos contain a significant fraction of old stellar populations accreted more than 10 Gyr ago and in a few cases, some intermediate age populations exist. In contrast with the Milky Way, many of our simulated stellar halos contain old stellar populations which are metal rich, originating in the early accretion of massive satellites. We suggest that the (metal rich) stellar halo of M31 falls into this category, while the more metal poor halo of the Milky Way is lacking in early massive accretion events. Interestingly, our hierarchically-formed stellar halos often have non-negligible metallicity gradients in both [Fe/H] and [alpha/Fe]. These gradients extend a few tens of kpc, and can be as large as 0.5 dex in [Fe/H] and 0.2 dex in [alpha/Fe]. Finally, we find that chemical abundances can act as a rough substitute for time of accretion of satellite galaxies. We propose a criterion for identifying tidal streams spatially by selecting stars with [alpha/Fe] ratios below solar.

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arXiv2005

Lambda-Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance Pattern

(Abridged) The hierarchical formation scenario for the stellar halo requires the accretion and disruption of dwarf galaxies, yet low-metallicity halo stars are enriched in alpha-elements compared to similar, low-metallicity stars in dwarf spheroidal (dSph) galaxies. We address this primary challenge for the hierarchical formation scenario for the stellar halo by combining chemical evolution modelling with cosmologically-motivated mass accretion histories for the Milky Way dark halo and its satellites. We demonstrate that stellar halo and dwarf galaxy abundance patterns can be explained naturally within the LCDM framework. Our solution relies fundamentally on the LCDM model prediction that the majority of the stars in the stellar halo were formed within a few relatively massive, ~5 x 10^10 Msun, dwarf irregular (dIrr)-size dark matter halos, which were accreted and destroyed ~10 Gyr in the past. These systems necessarily have short-lived, rapid star formation histories, are enriched primarily by Type II supernovae, and host stars with enhanced [a/Fe] abundances. In contrast, dwarf spheroidal galaxies exist within low-mass dark matter hosts of ~10^9 Msun, where supernovae winds are important in setting the intermediate [a/Fe] ratios observed. Our model includes enrichment from Type Ia and Type II supernovae as well as stellar winds, and includes a physically-motivated supernovae feedback prescription calibrated to reproduce the local dwarf galaxy stellar mass - metallicity relation. We use representative examples of the type of dark matter halos we expect to host a destroyed ``stellar halo progenitor'' dwarf, a surviving dIrr, and a surviving dSph galaxy, and show that their derived abundance patterns, stellar masses, and gas masses are consistent with those observed for each type of system.

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arXiv2005

Chemical Abundance Distributions of Galactic Halos and their Satellite Systems in a Lambda CDM Universe

We present a cosmologically motivated model for the hierachical formation of the stellar halo which includes a semi-analytic treatment of galactic chemical enrichment coupled to numerical simulations that track the orbital evolution and tidal disruption of satellites. A major motivating factor in this investigation is the observed systematic difference between the chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in neighboring satellite galaxies display significantly lower [alpha/Fe] ratios than stars in the halo. We find that the observed chemical abundance patterns are a natural outcome of the process of hierarchical assembly of the Galaxy. This result follows because the stellar halo in this context is built up from satellite galaxies accreted early on (more than 8-9 Gyr ago) and enriched in alpha-elements produced in Type II supernovae. In contrast, satellites which survive today are typically accreted late (within the last 4-5 Gyr) with nearly solar [alpha/Fe] values as a result of contributions from both Type II and Type Ia supernovae. We use our model to investigate the abundance distribution functions (using both [Fe/H] and [alpha/Fe] ratios) for stars in the halo and in surviving satellites. Our results suggest that the shapes and peaks in the abundance distribution functions provide a direct probe of the accretion histories of galaxies.

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arXiv2005

Tracing Galaxy Formation with Stellar Halos I: Methods

If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted ~100-200 luminous satellite galaxies in the past \~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach which distinguishes explicitly between the evolution of light and dark matter in accreted satellites. This distinction is essential to our ability to produce a realistic stellar halo, with mass and density profile much like that of our own Galaxy, and a surviving satellite population that matches the observed number counts and structural parameter distributions of the satellite galaxies of the Milky Way. Our model stellar halos have density profiles which typically drop off with radius faster than those of the dark matter. They are assembled from the inside out, with the majority of mass (~80%) coming from the \~15 most massive accretion events. The satellites that contribute to the stellar halo have median accretion times of ~9 Gyr in the past, while surviving satellite systems have median accretion times of ~5 Gyr in the past. This implies that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also from stars in the outer halo or those liberated in recent disruption events. We briefly discuss the expected spatial structure and phase space structure for halos formed in this manner. Searches for this type of structure offer a direct test of whether cosmology is indeed hierarchical on small scales.

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arXiv2005

Modeling complete distributions with incomplete observations: The velocity ellipsoid from Hipparcos data

[abridged] A "missing data" algorithm is developed to model (ie, reconstruct) the three-dimensional velocity distribution function of a sample of stars using data (velocity measurements) every one of which has one dimension unmeasured (the radial direction). It also accounts for covariant measurement uncertainties on the tangential velocity components. The algorithm is applied to tangential velocities measured in a kinematically unbiased sample of 11,865 stars taken from the Hipparcos catalog. The local stellar velocity distribution function of each of a set of 20 color-selected subsamples is modeled as a mixture of two three-dimensional Gaussian ellipsoids of arbitrary relative responsibility. In the fitting, one Gaussian (the "halo") is fixed at the known mean velocity and velocity variance tensor of the Galaxy halo, and the other (the "disk") is allowed to take arbitrary mean and arbitrary variance tensor. The mean and variance tensor (commonly the "velocity ellipsoid") of the disk velocity distribution are both found to be strong functions of stellar color, with long-lived populations showing larger velocity dispersion, slower mean rotation velocity, and smaller vertex deviation than short-lived populations. The local standard of rest (LSR) is inferred in the usual way and the Sun's motion relative to the LSR is found to be (U,V,W)_{\odot}=(10.1,4.0,6.7)+/-(0.5,0.8,0.2) km/s. Artificial data sets are made and analyzed, with the same error properties as the Hipparcos data, to demonstrate that the analysis is unbiased. The results are shown to be insensitive to the assumption that the velocity distributions are Gaussian.

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arXiv2005

Dynamics and Stellar Content of the Giant Southern Stream in M31. II. Interpretation

We examine the nature of the progenitor of the giant stellar stream in M31 using as constraints new radial velocity measurements of stream red giant stars (presented in the companion paper by Guhathakurta et al. 2005, astro-ph/0406145) along with other M31 data sets available in the literature. We find that the observations are best fit by orbits that are highly eccentric and close to edge-on, with apo- to peri-center ratios of order 25 - 30, and with apocenters at or only slightly beyond the southern edge of the current data. Among these orbits, we are able to find a few that plausibly connect the stream with the northern spur or with the low-surface-brightness feature of similar high metallicity as the stream (originally reported by Ferguson et al. 2002) to the east of M31's center. In the latter case, if the connection is real, then the eastern debris should lie well in front of M31 near the apocenter of the orbit. Both the width of the debris and velocity dispersion measurements imply a rough lower limit on the mass of the progenitor of 10^8 M_sun. We use this limit and our orbits to discuss which of M31's satellites could be plausibly associated with the stream. In addition, we predict that the width of the stream should increase beyond the southern edge of the current data around the apocenter of the orbit and that the line-of-sight velocity dispersion should exhibit significant variations along the stream.

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arXiv2004

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: III. Constraints on the Flattening of the Galactic Halo

M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 10.4 +- 2.6 degrees between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgr's orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio between the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with q_ρ~ 0.83 - 0.92). Oblate halos are strongly preferred over prolate (q_ρ> 1) halos, and flattenings in the potential of q <= 0.85 (q_ρ<= 0.75) and q >= 1.05 (q_ρ>= 1.1) are ruled out at the 3-sigma level. More extreme values of q <= 0.80 (q_ρ<= 0.6) and q >= 1.25 (q_ρ>= 1.6) are ruled out at the 7-sigma and 5-sigma levels respectively. These constraints will improve as debris with larger separation in orbital phase can be found.

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arXiv2004

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: IV. Modeling the Sagittarius Tidal Tails

M giants recovered from the Two Micron All-Sky Survey (2MASS) have recently been used to map the position and velocity distributions of tidal debris from the Sagittarius (Sgr) dwarf spheroidal galaxy entirely around the Galaxy. We compare this data set to both test particle orbits and N-body simulations of satellite destruction run within a variety of rigid Milky Way potentials and find that the mass of the Milky Way within 50 kpc of its center should be 3.8-5.6 x 10^11 Msun in order for any Sgr orbit to simultaneously fit the velocity gradient in the Sgr trailing debris and the apocenter of the Sgr leading debris. Orbital pole precession of young debris and leading debris velocities in regions corresponding to older debris provide contradictory evidence in favor of oblate/prolate Galactic halo potentials respectively, leading us to conclude that the orbit of Sgr has evolved over the past few Gyr. Based upon the velocity dispersion and width along the trailing tidal stream we estimate the current bound mass of Sgr to be M_Sgr = 2 - 5 x 10^8 Msun independant of the form of the Galactic potential; this corresponds to a range of mass to light ratios (M/L)_Sgr = 14 - 36 (M/L)_Sun for the Sgr core. Models with masses in this range best fit the apocenter of leading Sgr tidal debris when they orbit with a radial period of roughly 0.85 Gyr and have periGalactica and apoGalactica of about 15 kpc and 60 kpc respectively. These distances will scale with the assumed distance to the Sgr dwarf and the assumed depth of the Galactic potential. The density distribution of debris along the orbit in these models is consistent with the M giant observations, and debris at all orbital phases where M giants are obviously present is younger (i.e. was lost more recently from the satellite) than the typical age of a Sgr M giant star.

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arXiv2004

Dynamics and Stellar Content of the Giant Southern Stream in M31. I. Keck Spectroscopy of Red Giant Stars

We present results from a large spectroscopic survey of M31 red giants using the Keck telescope/DEIMOS. Photometric pre-screening, based on the 100A-wide DDO51 band centered on the Mgb/MgH feature, was used to select spectroscopic targets. Red giant candidates were targeted in a small field on M31's giant southern tidal stream at a projected distance of 31kpc from the galaxy center. We isolate a clean sample of 68 giants by removing contaminants (foreground Galactic dwarfs and background galaxies) using spectroscopic, imaging, and photometric methods. About 65% of the M31 stars are found to be members of the stream, while the rest appear to be members of the general halo population. The mean (heliocentric) radial velocity of the stream in our field is -458 km/s, or -158 km/s relative to M31's systemic velocity, in good agreement with recent measurements at other stream locations. The intrinsic velocity dispersion of the stream is constrained to be 15_{-15}^{+8} km/s (90% confidence limits). The companion paper by Font et al. (2004, astro-ph/0406146) discusses possible orbits, implications of the coldness of the stream, and progenitor satellite properties. The kinematics (and perhaps [Fe/H] distribution) of our halo sample indicate that it is different from other M31 halo samples; this may be an indication of substructure in the halo. The stream seems to have a higher mean [Fe/H] than the halo, -0.51 vs -0.74 dex, and a smaller [Fe/H] spread. The stream's high metallicity implies that its progenitor must have been a luminous dwarf galaxy. The CaII triplet strengths of the M31 giants are generally consistent with photometric estimates of their metallicity (derived by fitting RGB fiducials in the color-magnitude diagram). There is indirect evidence of intermediate-age stars in the stream.

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arXiv2004

A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy: II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream

We present moderate resolution (~6 km/s) spectroscopy of 284 M giant candidates selected from the Two Micron All Sky Survey photometry. Radial velocities (RVs) are presented for stars mainly in the south, with a number having positions consistent with association to the trailing tidal tail of the Sagittarius (Sgr) dwarf galaxy. The latter show a clear RV trend with orbital longitude, as expected from models of the orbit and destruction of Sgr. A minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is implied by verified RV members. The coldness of this stream (dispersion ~10 km/s) provides upper limits on the combined contributions of stream heating by a lumpy Galactic halo and the intrinsic dispersion of released stars, which is a function of the Sgr core mass. The Sgr trailing arm is consistent with a Galactic halo containing one dominant, LMC-like lump, however some lumpier halos are not ruled out. An upper limit to the total M/L of the Sgr core is 21 in solar units. A second structure that roughly mimics expectations for wrapped, leading Sgr arm debris crosses the trailing arm in the Southern Hemisphere; however, this may also be an unrelated tidal feature. Among the <13 kpc M giants toward the South Galactic Pole are some with large RVs that identify them as halo stars, perhaps part of the Sgr leading arm near the Sun. The positions and RVs of Southern Hemisphere M giants are compared with those of southern globular clusters potentially stripped from the Sgr system and support for association of Pal 2 and Pal 12 with Sgr debris is found. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element RVs in moderate resolution spectra.

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arXiv2004

Cosmology and the Stellar Halo

If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted and subsequently tidally destroyed ~100 low-mass galaxies in the past ~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach and show that the disrupted systems lead naturally to stellar halos with masses and density profiles much like the stellar halo of our own Galaxy. We present predictions for the properties of stellar halos and show that ours is likely dominated by substructure beyond \~50kpc and more spatially smooth within that radius. The average stellar halo density profile is expected to drop off with radius more quickly than that of the dark matter because the stellar halo is formed from the most tightly bound material in infalling systems, while the majority of the accreted dark matter is stripped and deposited at larger radii. We argue that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also different from stars in the outer halo or those liberated in recent disruption events. We discuss how deep halo surveys and chemical probes may be useful tools for uncovering evidence of accretion. Searches of this kind offer a direct test of whether cosmology is indeed hierarchical on small scales.

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arXiv2003

Dark Matter Constraints from the Sagittarius Dwarf and Tail System

2MASS has provided a three-dimensional map of the >360 degree, wrapped tidal tails of the Sagittarius (Sgr) dwarf spheroidal galaxy, as traced by M giant stars. With the inclusion of radial velocity data for stars along these tails, strong constraints exist for dynamical models of the Milky Way-Sgr interaction. N-body simulations of Sgr disruption with model parameters spanning a range of initial conditions (e.g., Sgr mass and orbit, Galactic rotation curve, halo flattening) are used to find parameterizations that match almost every extant observational constraint of the Sgr system. We discuss the implications of the Sgr data and models for the orbit, mass and M/L of the Sgr bound core as well as the strength, flattening, and lumpiness of the Milky Way potential.

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arXiv2003

Modeling the Tidal Tails of the Sagittarius Dwarf Galaxy

N-body simulations are used to model the tidal disruption of the Sagittarius (Sgr) dwarf galaxy with constraints set by the positions and velocities of M giants in the Sgr tidal arms recently revealed by the Two Micron All-Sky Survey (2MASS). The simulated Sgr dwarf is placed on a variety of orbits within a Milky Way potential parameterized by variable circular velocities, halo flattenings and radial profiles. Two hundred separate test particle orbits have been used to explore a wide range of model Milky Way potentials and dwarf galaxy characteristics. The family of models is delimited by the data to a relatively narrow allowed range of parameters, and then input into N-body simulations. We present our best-fitting model, and discuss the orbital period, apoGalacticon distance, current space velocity, mass-to-light ratio, and other characteristics of the Sgr dwarf. In addition, we discuss the implications of this model for the flattening of the Galactic halo.

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arXiv2002

Interpreting the Morphology of Diffuse Light Around Satellite Galaxies

Recent observations of surface brightness distributions of both Milky Way and M31 satellite galaxies have revealed many instances of sudden changes or breaks in the slope of the surface brightness profiles (at some break radius, r_break). These breaks are often accompanied by increasingly elliptical isophotes and sometimes by isophote twisting. We investigate the hypothesis of a tidal origin for these features by applying the same ellipse fitting techniques that are used on observed galaxies to numerical simulations of the destruction of satellites, represented by spherical, single-component systems. We examine how observed quantities such as r_break, ellipticity and position angle of the fitted ellipses and amplitude of extra-break population vary with the satellite's orbital eccentricity and phase, and our viewpoint relative to the orbit. We also look at orbit and viewpoint dependence of the rate of change of the latter three quantities with radius. We find that there are trends with orbital phase and eccentricity in all observed quantities, many of which are preserved through a wide variety of viewing angles. In particular, a generic feature of all simulations is a depletion zone just interior to an excess zone, regions in which the surface brightness is lower and higher, respectively, than the initial profile. A clear interpretation of any individual image, however, is likely to be hampered by the dependence of the observable features on these multiple parameters. For example, breaks can be excited by several physical processes and can occur well within the bound satellite population. Nevertheless, we do find we can place loose constraints on the tidal radius, mass loss rate, orbital type and phase of the satellite, and nature of breaks using photometric data alone.

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arXiv2002

Tidal Interaction of M32 and NGC 205 with M31: Surface Photometry and Numerical Simulations

We investigate the interaction history of the M31 sub-group of galaxies by comparing surface photometry of its two nearest satellites, M32 and NGC 205, with N-body simulations of satellite destruction. The recent discovery of a giant stream in M31's outer halo, apparently pointed towards M32 and NGC 205, makes such an investigation particularly relevant. The observational component of this study is based on 1.7 x 5.0 degree B- and I- band CCD mosaic images centered on M31 and covering both satellites. The simulation component of this project is based on the analysis of single-component, spherical model satellites that are tidally disrupted through interactions with their parent galaxy. Generic features of the simulations include an excess in the surface brightness profile at large radii, a depletion zone at intermediate radii, and isophotal elongation and twists that are coincident with breaks in the brightness profile. The satellites M32 and NGC 205 display most of these features consistently across the B and I bands, strongly suggestive of tidal interaction and probable stripping by M31. We discuss what these observed features can tell us about the satellites' orbital parameters and histories. Specifically, M32 is found to be on a highly eccentric orbit and away from pericenter. Investigating M32's unusual combination of high surface brightness and low luminosity (the hallmark of compact ellipticals), we conclude on empirical grounds that M32 is not likely to be the residual core of a tidally-stripped normal elliptical galaxy as has been suggested, but rather that its precursor was intrinsically compact.

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arXiv2001

The Impact of Tidal Interactions on Satellite Galaxies: A Study of the M31 Satellites, M32 & NGC 205

Surface photometry of the M31 satellites M32 and NGC 205 is compared to numerical simulations of satellite destruction to constrain orbital parameters and the interaction history of the M31 subgroup. Our analysis reveals the following preliminary results: (1) Generic features of tidal disruption in the simulations include an extended ``extra-tidal'' excess region and an inner depletion zone, both of which are observed in M32 and NGC 205; (2) M32 is likely to be on a highly eccentric orbit well away from pericenter; (3) Surface brightness and luminosity evolution estimates for M32, the prototypical compact elliptical galaxy, imply that it is not simply the residual core of a tidally-stripped normal elliptical galaxy, but was instead formed in a truncated state.

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arXiv2001

How Lumpy is the Milky Way's Dark Matter Halo?

CDM simulations predict that there are hundreds of lumps of with masses greater than 10e7 solar masses in the Milky Way halo. However, we know of only a dozen dwarf satellites close to this mass. Are these lumps simply lacking in stars or is there a fundamental flaw in our most popular cosmology? By studying the tidal debris of known satellites we can potentially address this question. In this paper, we quantify the the effects of the dark matter lumps on tidal tails. The lumps scatter stars in the tidal tails from their original orbits producing a distinctive signature. We simulate debris evolution in smooth and lumpy halos potentials and use our simulations to motivate and test a statistical measure of the degree of scattering apparent in the angular position and radial velocity measurement of debris stars - the ``scattering index''. We find that the scattering index can in general distinguish between the levels of substructure predicted by CDM cosmologies and smooth Milky Way models, but that the sensitivity of the debris depends on the orientation of the parent satellite's orbit relative to the largest lumps orbits. We apply our results to the carbon star stream associated with the Sagittarius dwarf galaxy (Sgr) and find that these stars appear to be more scattered than we expect for debris orbiting in a smooth halo. However, the degree of scattering is entirely consistent with that expected due to the influence of the Large Magellanic Cloud, which is on an orbit that intersects Sgr's own. We conclude that the current data is unable to constrain CDM models. Nevertheless, our study suggests that future data sets of debris stars associated with other Milky Way satellites could provide strong constraints on CDM models.

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