arXiv subjects

Steven R. Majewski

Publications and source records attributed to Steven R. Majewski.

At least 271 records · Page 15Linked to original sources

arXiv2004

Star Clusters in the Galactic Anticenter Stellar Structure: New Radial Velocities & Metallicities

The Galactic Anticenter Stellar Structure (GASS) has been identified with excess surface densities of field stars in several large area sky surveys, and with an unusual, string-like grouping of star clusters. Some members of the cluster grouping have radial velocities (RVs) consistent with the observed GASS velocity-longitude trend. We provide new RV measurements of stars in six clusters that have been suggested to be associated with the GASS. We show that the RVs of at least four clusters are consistent with the previously measured RV trend for GASS. We also derive spectroscopic metallicities for four clusters, and provide an improved age-metallicity relation for the clusters apparently associated with GASS.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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).

astro-ph↗

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.

astro-ph↗

arXiv2004

Exploring Halo Substructure with Giant Stars: A diffuse star cloud or tidal debris around the Milky Way in Triangulum-Andromeda

We report here the discovery of an apparent excess of 2MASS M giant candidates with dereddened 0.85 < J-K_S < 1.2 spanning a considerably large area of the celestial sphere between, at least, $100\degr < l < 150\degr$ and $-20\degr > b > -40\degr$, and covering most of the constellations of Triangulum and Andromeda. This structure does not seem to be preferentially distributed around a clear core, but rather lies in a tenuous, clumpy cloud-like structure tens of kiloparsecs away. The reduced proper-motion diagram as well as spectroscopy of a subsample shows these excess stars to be real giants, not contaminating dwarfs. Radial velocity measurements indicate among those M giants the presence of a coherent kinematical structure with a velocity dispersion $σ< 17$ km s$^{-1}$. Our findings support the existence of a quite dispersed stellar structure around the Milky Way that, due to its coreless and sparse distribution, could be part of a tidal stream or a new kind of satellite galaxy.

astro-ph↗

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.

astro-ph↗

arXiv2004

Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters

The Galactic Anticenter Stellar Structure (GASS) has been identified with excess surface densities of field stars in several large area sky surveys, and with an unusual, string-like grouping of five globular clusters. At least two of these are diffuse, young ``transitional'' clusters between open and globular types. Here we call attention to the fact that four {\it younger} open or transitional clusters extend the previously identified, string-like cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17$\arcdeg$ to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and string-like sequence, {\it including almost all fifteen known outer, old open clusters}. Tidal accretion of a dwarf satellite galaxy on a low inclination orbit -- perhaps the GASS system -- appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2003

Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure

To determine the nature of the recently discovered, ring-like stellar structure at the Galactic anticenter, we have collected spectra of a set of presumed constituent M giants selected from the 2MASS point source catalog. Radial velocities have been obtained for stars spanning ~100 degrees, exhibiting a trend in velocity with Galactic longitude and an estimated dispersion of 20 +/- 4 km/sec. A mean metallicity [Fe/H] = -0.4 +/- 0.3 measured for these stars combines with previous evidence from the literature to suggest a population with a significant metallicity spread. In addition, a curious alignment of at least four globular clusters of lower mean metallicity is noted to be spatially and kinematically consistent with this stellar distribution. We interpret the M giant sample position and velocity variation with Galactic longitude as suggestive of a satellite galaxy currently undergoing tidal disruption in a non-circular, prograde orbit about the Milky Way.

astro-ph↗

arXiv2003

The Galactic Anticenter Stellar Stream

The recently discovered, ring-like structure just outside the Galactic disk in Monoceros is detected and traced among 2MASS M giant stars. We have developed a method to recover the signature of this structure from the distance probability density function of stars along a given line of sight. Its detection is possible even when the metallicity is unknown, provided that the structure is not too embedded in the disk. Application of this method reveals the presence of a large group of M giant stars at a Galactocentric distance of 18$\pm 2$ kpc, over $+36\degr<b<+12\degr$ and $100\degr < l <270\degr$. Evidence that the stream extends to high negative latitudes is also found. That the structure contains M giants shows that it contains populations of at least an order of magnitude higher abundance than the [Fe/H] = -1.6 mean metallicity previously reported for this system. The structural characteristics of the stellar stream as traced by M giants do not support the interpretation of this structure as a homogeneously dense ring that surrounds the Galaxy, but as a more localized structure, possibly a merging dwarf galaxy with tidal arms, like the Sagittarius dwarf galaxy.

astro-ph↗

arXiv2003

2MASS Studies of Differential Reddening Across Three Massive Globular Clusters

J, H, and K_S band data from the Two Micron All-Sky Survey (2MASS) are used to study the effects of differential reddening across the three massive Galactic globular clusters Omega Centauri, NGC 6388, and NGC 6441. Evidence is found that variable extinction may produce false detections of tidal tails around Omega Centauri. We also investigate what appears to be relatively strong differential reddening towards NGC 6388 and NGC 6441, and find that differential extinction may be exaggerating the need for a metallicity spread to explain the width of the red giant branches for these two clusters. Finally, we consider the implications of these results for the connection between unusual, multipopulation globular clusters and the cores of dwarf spheroidal galaxies (dSph).

astro-ph↗

arXiv2002

Absolute Proper Motions to B~22.5: V. Detection of Sagittarius Dwarf Spheroidal Debris in the Direction of the Galactic Anticenter

We have detected a population of predominantly blue (B-V <= 1.1) stars in the direction l = 167 deg., b = -35 deg. (Kapteyn Selected Area 71) that cannot be accounted for by standard starcount models. Down to V ~ 20, the colors and magnitudes of these stars are similar to those of the southern overdensity detected by the Sloan Digital Sky Survey, and identified as stripped material from the Sagittarius dwarf spheroidal galaxy. We present absolute proper motions for the stars in SA 71, and we find that the excess blue stars represent a distinct, kinematically cooler component than the Galactic field, and in reasonable agreement with predictions of Sgr disruption models. The density of the excess SA 71 stars at V ~ 18.8 and B-V <=1.1 is within a factor of two of the density of the SDSS-south Sgr stripped material, and of that predicted by the Helmi and White disruption model. Three additional anticenter fields (SA 29, 45 and 118) show very good agreement with standard starcount models.

astro-ph↗

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↗

arXiv2001

On the Distribution of Orbital Poles of Milky Way Satellites

In numerous studies of the outer Galactic halo some evidence for accretion has been found. If the outer halo did form in part or wholly through merger events, we might expect to find coherent streams of stars and globular clusters following similar orbits as their parent objects, which are assumed to be present or former Milky Way dwarf satellite galaxies. We present a study of this phenomenon by assessing the likelihood of potential descendant ``dynamical families'' in the outer halo. We conduct two analyses: one that involves a statistical analysis of the spatial distribution of all known Galactic dwarf satellite galaxies (DSGs) and globular clusters, and a second, more specific analysis of those globular clusters and DSGs for which full phase space dynamical data exist. In both cases our methodology is appropriate only to members of descendant dynamical families that retain nearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) is considered a paradigm for the type of merger/tidal interaction event for which we are searching, we also undertake a case study of the Sgr system and identify several globular clusters that may be members of its extended dynamical family. (ABRIDGED)

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].

astro-ph↗