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James C. Ostheimer

Publications and source records attributed to James C. Ostheimer.

13 recordsLinked to original sources
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.

astro-ph.GA↗
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.

astro-ph.CO↗
arXiv2010

The SPLASH Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, II, III, VII, X, and XIV dSphs

We present new Keck/DEIMOS spectroscopic observations of hundreds of individual stars along the sightline to Andromeda's first three discovered dwarf spheroidal galaxies (dSphs) - And I, II, and III, and leverage recent observations by our team of three additional dSphs, And VII, X, and XIV, as a part of the SPLASH Survey. Member stars of each dSph are isolated from foreground Milky Way dwarf and M31 field contamination using a variety of photometric and spectroscopic diagnostics. Our final spectroscopic sample of member stars in each dSph, for which we measure accurate radial velocities with a median uncertainty (random plus systematic errors) of 4 - 5 km/s, includes 80 red giants in And I, 95 in And II, 43 in And III, 18 in And VII, 22 in And X, and 38 in And XIV. The sample of confirmed members in the six dSphs are used to derive each system's mean radial velocity, intrinsic central velocity dispersion, mean abundance, abundance spread, and dynamical mass. This combined data set presents us with a unique opportunity to perform the first systematic comparison of the global properties (e.g., metallicities, sizes, and dark matter masses) of one-third of Andromeda's total known dSph population with Milky Way counterparts of the same luminosity. We discuss both the luminosity-metallicity relation and the luminosity-size relation of these satellites, and find that the chemical evolution histories of each host's satellites is similar. The dynamical mass estimates of M31's dSphs are similar or smaller than Milky Way dSphs of the same luminosity despite their sizes being similar or larger, suggesting M31 dSphs are less dense than Milky Way counterparts. The implications of these results for general understanding of galaxy formation and evolution is summarized. Abridged.

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arXiv2007

Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream

We present the discovery of a kinematically-cold stellar population along the SE minor axis of the Andromeda galaxy (M31) that is likely the forward continuation of M31's giant southern stream. This discovery was made in the course of an on-going spectroscopic survey of red giant branch (RGB) stars in M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar kinematics are investigated in eight fields located 9-30 kpc from M31's center (in projection). A likelihood method based on photometric and spectroscopic diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky Way dwarf stars: for the first time, this is done without using radial velocity as a selection criterion, allowing an unbiased study of M31's stellar kinematics. The radial velocity distribution of the 1013 M31 RGB stars shows evidence for the presence of two components. The broad (hot) component has a velocity dispersion of 129 km/s and presumably represents M31's virialized spheroid. A significant fraction (19%) of the population is in a narrow (cold) component centered near M31's systemic velocity with a velocity dispersion that decreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to 10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold component matches that of the "Southeast shelf" predicted by the Fardal et al. (2007) orbital model of the progenitor of the giant southern stream. The metallicity distribution of the cold component matches that of the giant southern stream, but is about 0.2 dex more metal rich on average than that of the hot spheroidal component. We discuss the implications of our discovery on the interpretation of the intermediate-age spheroid population found in this region in recent ultra-deep HST imaging studies.

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arXiv2006

A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 Kiloparsecs

We present a method for isolating a clean sample of red giant stars in the outerregions of the Andromeda spiral galaxy (M31) from an ongoing spectroscopic survey using the DEIMOS instrument on the Keck 10-m telescope. The survey aims to study the kinematics, global structure, substructure, and metallicity of M31's halo. Although most of our spectroscopic targets were photometrically screened to reject foreground Milky Way dwarf star contaminants, the latter class of objects still constitutes a substantial fraction of the observed spectra in the sparse outer halo. Our likelihood-based method for isolating M31 red giants uses five criteria: (1) radial velocity, (2) photometry in the intermediate-width DDO51 band to measure the strength of the MgH/Mgb absorption features, (3) strength of the Na I 8190A absorption line doublet, (4) location within an (I, V-I) color-magnitude diagram, and (5) comparison of photometric (CMD-based) versus spectroscopic (Ca II 8500A triplet-based) metallicity estimates. We also discuss K I and TiO diagnostics for giant/dwarf separation that might be useful in future analyses. Training sets consisting of definite M31 red giants and Galactic dwarf stars are used to derive empirical probabilitydistribution functions for each diagnostic. These functions are used to calculate the likelihood that a given star is a red giant in M31 versus a Milky Way dwarf. By applying this diagnostic method to our spectroscopic data set, we isolate 40 M31 red giants beyond a projected distance of R = 60 kpc from the galaxy's center, including three out at R ~ 165 kpc. The ability to identify individual M31 red giants gives us an unprecedented level of sensitivity in studying the properties of the galaxy's outer halo.

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arXiv2006

Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy

Understanding galaxy formation involves look-back and fossil-record studies of distant and nearby galaxies, respectively. Debris trails in our Galaxy's spheroidal halo of old stars provide evidence of "bottom-up" formation via tidal disruption/merging of dwarf satellite galaxies, but it is difficult to study our Galaxy's large-scale structure from within. Studies of our neighbouring Andromeda galaxy have concluded that its spheroid contains chemically enriched ("metal-rich") stars out to a radius of 30 kiloparsecs with an exponential r^1/4 fall-off in density thereby resembling a galactic "bulge". Were Andromeda's true halo to be found, our detailed yet global view of its stellar dynamics, substructure, chemical abundance, and age distribution would directly constrain hierarchical halo formation models. We report here on the discovery of a hitherto elusive halo of metal-poor Andromeda stars, distinct from its bulge, with a power-law brightness profile extending beyond r = 160 kiloparsecs. This is 3 - 5 times larger than any previously mapped Andromeda spheroidal/disk component. Together, the Galactic and Andromeda halos span >1/3 of the distance between them, suggesting that stars occupy a substantial volume fraction of our Local Group, and possibly most galaxy groups.

astro-ph↗
arXiv2005

Exploring Halo Substructure with Giant Stars VIII: The Extended Structure of the Sculptor Dwarf Spheroidal Galaxy

(Abridged) We explore the spatial distribution of stars in the Sculptor dwarf spheroidal (dSph) galaxy over an area of 7.82 deg^2. We identify red giant branch (RGB) starts via Washington M, T_2+DDO51 photometry and a blue horizontal branch (BHB) population to map the spatial structure of the dSph. A spectroscopically observed subset of Sculptor candidate stars yield a systemic heliocentric velocity for the system of v_{hel}=110.43 km/s, in good agreement with previous studies, and a global velocity dispersion of sigma_v=8.8 km/s, which may rise slightly past 0.4r_{lim}. To a limit of M~19, we find 94% of the photometrically-selected Sculptor giant star candidates with spectrocopic measurements are kinematically associated with Sculptor, and four of ten stars not selected photometrically are selected kinematically; our candidate samples are likely to be very pure. We take considerable care in assessing the contaminating background level in our photometric sample to ensure accurate density profiles. These assessments verify that we detect a considerable stellar density of Sculptor stars to the limits of our survey area in both the RGB and BHB samples. We find the Sculptor density profile is well-fit by a King profile of limiting radius r_{lim} = 79.6 within ~60 arcmin, beyond which a "break" to a power law profile occurs. This break population must be either a bound group of "halo stars" around the Sculptor dSph or unbound tidal debris. The latter is supported by 2D distribution analyses and, if true, implies a fractional mass-loss rate of ~0.042 Gyr^{-1} for Sculptor. Finally, likely more metal-poor RGB stars (as selected by color and magnitude) are significantly less centrally concentrated and, therefore, constitute the primary contributor to the likely tidally-stripped parts of the dSph.

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arXiv2004

Detection of the Main Sequence Turn-off of a Newly Discovered Milky Way Halo Structure in the Triangulum-Andromeda Region

An upper main sequence (MS) and main-sequence turn-off (MSTO) feature appears in the color-magnitude diagram (CMD) of a large area photometric survey of the southern half of M31 stretching to M33. Imaging in the Washington M,T_2,DDO51 photometric system allows us to remove the background M31/M33 giants from our CMD and more clearly define the dwarf star feature, which has an MSTO near M ~ 20.5. The corresponding stellar population shows little density variation over the 12 X 6 square degree area of the sky sampled and is of very low surface brightness, >32 mag/arcsec^2. We show that this feature is not the same as a previously identified, MS+MSTO in the foreground of the Andromeda Galaxy that has been associated with the tidal stream ringing the Milky Way disk at less than half the distance. Thus, the new stellar system is a separate, more distant entity, perhaps a segment of tidal debris from a disrupted satellite galaxy. It is most likely related to the structure with similar distance, location and density uniformity seen as an excess of K and M giants in the Two Micron All-Sky Survey reported in the companion paper by Rocha-Pinto et al. (2004).

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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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arXiv2003

A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: I. Morphology of the Sagittarius Core and Tidal Arms

We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M giant star tracers detected in the complete Two Micron All-Sky Survey (2MASS). The main body is fit with a King profile of 30 deg limiting radius, but with a break in the density profile from stars in tidal tails. We argue that much of the observed structure beyond the 224' core radius may be unbound as the satellite undergoes catastrophic disruption. A striking, >150 deg trailing tidal tail extends from the Sgr center and arcs across the South Galactic Hemisphere. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an ~40 kpc apoGalacticon, loops towards the North Galactic Cap (NGC) and descends back towards the Galactic plane, foreshortened and covering the NGC. The Sgr tails lie along a well-defined orbital plane that shows little precession, which supports the notion of a nearly spherical Galactic potential. The Sun lies near the path of leading Sgr debris; thus, former Sgr stars may be near or in the solar neighborhood. The number of M giants in the Sgr tails is >15% that within the King limiting radius of the Sgr center. That several gigayear old M giants are so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but poses a timing problem that bears on the recent binding energy of the Sgr core and that is naturally explained by recent and catastrophic mass loss. Sgr appears to contribute >75% of the high latitude, halo M giants; no evidence for M giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris with the exception of Sgr carbon stars -- which must be subluminous to resolve the discrepancy.

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arXiv2001

The Metallicity Distribution Function of Omega Centauri

We explore the metallicity distribution function (MDF) of red giant stars in Omega Centauri from a catalogue of Washington M, T_2 and DDO51 photometry covering over 1.1 deg^2 outside the cluster core. Using updated calibrations of giant branch isometallicity loci in this filter system, photometric metallicities, guided by previously published spectroscopic abundances, are derived. Several methods are employed to correct the MDF for contamination by Galactic stars, including: (1) use of the surface gravity sensitivity of the (M-DDO51) color index to eliminate foreground dwarf stars, (2) radial velocities, and (3) membership probabilities from proper motions. The contamination-corrected MDF for Omega Cen shows a range of enrichment levels spanning nearly 2 dex in [Fe/H], and with peaks at [Fe/H]=-1.6, -1.2, and -0.9.

astro-ph↗
arXiv2000

Exploring Halo Substructure with Giant Stars: I. Survey Description and Calibration of the Photometric Search Technique

We have begun a survey of the structure of the Milky Way halo, as well as the halos of other Local Group galaxies, as traced by their constituent giant stars. These giant stars are identified via large area, CCD photometric campaigns. Here we present the basis for our photometric search method, which relies on the gravity sensitivity of the Mg I triplet + MgH features near 5150 Angstroms in F-K stars, and which is sensed by the flux in the intermediate band DDO51 filter. To calibrate our (M-T_2, M-DDO51) diagram as a means to discriminate field giant stars from nearby dwarfs, we utilize new photometry of the main sequences of the open clusters NGC 3680 and NGC 2477 and the red giant branches of the clusters NGC 3680, Melotte 66 and omega Centauri, supplemented with data on field stars, globular clusters and open clusters by Doug Geisler and collaborators. By combining the data on stars from different clusters, and by taking advantage of the wide abundance spread within omega Centauri, we verify the primary dependence of the M-DDO51 color on luminosity, and demonstrate the secondary sensitivity to metallicity among giant stars. Our empirical results are found to be generally consistent with those from analysis of synthetic spectra by Paltoglou & Bell [1994, MNRAS, 268, 793].

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