William E. Kunkel

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 have obtained moderate resolution (~6 km s-1) spectroscopy of several hundred M giant candidates selected from Two Micron All Sky Survey photometry. Radial velocities are presented for stars mainly in the southern Galactic hemisphere, and the primary targets have Galactic positions consistent with association to the tidal tail system of the Sagittarius (Sgr) dwarf galaxy. M giant stars selected from the apparent trailing debris arm of Sgr have velocities showing a clear 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 radial velocity members. The coldness of this stream (σv ~ 10 km s-1) 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. We find that the Sgr trailing arm is consistent with a Galactic halo that contains one dominant, LMC-like lump; however, some lumpier halos are not ruled out. An upper limit to the total mass-to-light ratio of the Sgr core is 21 in solar units. Evidence for other velocity structures is found among the more distant (>13 kpc) M giants. 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 bright, nearby (<13 kpc) M giants toward the south Galactic pole are a number with large velocities that identify them as halo stars; these too may trace halo substructure, perhaps part of the Sgr leading arm near the Sun. The positions and velocities of southern hemisphere M giants are compared with those of southern hemisphere globular clusters potentially stripped from the Sgr system. Support for association of the globular clusters Pal 2 and Pal 12 with Sgr debris is found, based on positional and radial velocity matches. Our discussion includes description of a masked-filtered cross-correlation methodology that achieves better than 1/20 of a resolution element velocities in moderate-resolution spectra. The improved velocity resolution achieved allows tighter constraints to be placed on the coldness of the Sgr stream than previously established.

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 ? in F?K stars, and which is sensed by the flux in the intermediate-band DDO51 filter. Our technique is a simplified variant of the combined Washington/DDO51 four-filter technique described by Geisler, which we modify for the specific purpose of efficiently identifying distant giant stars for follow-up spectroscopic study: We show here that for most stars the Washington T1-T2 color is correlated monotonically with the Washington M-T2 color with relatively low scatter; for the purposes of our survey, this correlation obviates the need to image in the T1 filter, as originally proposed by Geisler. To calibrate our (M-T2, 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 ? 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 ? 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. Finally, we provide conversion formulae from the (M, M-T2) system to the (V, V-I) system, corresponding reddening laws, as well as empirical red giant branch curves from ? Centauri stars for use in deriving photometric parallaxes for giant stars of various metallicities (but equivalent ages) to those of ? Centauri giants.

The Dynamics of the Large Magellanic Cloud Periphery: Mass Limit and Polar Ring

Radial velocities of 759 carbon stars on the periphery of the LMC are used to determine the rotation curve from 3 to 12 kpc. After a peak of 42 km s-1 at 4 kpc, the velocities decline to 35 km s-1 at 6.5 kpc, suggesting that 90% of the LMCs matter lies inside this radius. The rising velocities seen at larger radii are explained by particle-particle N-body simulations as arising from tidal interactions between LMC and both the SMC and the Galaxy. For an inclination of 33° the mass contained within a 5 kpc radius is 6.2 ± 0.9 × 109 M☉. An upper limit on the LMC halo mass is determined.

Constraining the History of the Sagittarius Dwarf Galaxy Using Observations of Its Tidal Debris

We present a comparison of semianalytic models of the phase-space structure of tidal debris with measurements of average distances, velocities, and surface densities of stars associated with the Sagittarius dwarf galaxy, compiled from all observations reported since its discovery in 1994. We find that several interesting features in the data can be explained by these models. The properties of stars about ?10??15? away from the center of Sgr?in particular, the orientation of material perpendicular to Sgrs orbit and the kink in the velocity gradient?are consistent with those expected for unbound material stripped during the most recent pericentric passage ~50 Myr ago. The break in the slope of the surface density seen by Mateo, Olszewski, & Morrison at b ~ -35? can be understood as marking the end of this material. However, the detections beyond this point are unlikely to represent debris in a trailing streamer, torn from Sgr during the immediately preceding passage ~0.7 Gyr ago, as the surface density of this streamer would be too low compared with observations in these regions. The low-b detections are more plausibly explained by a leading streamer of material that was lost more that 1 Gyr ago and has wrapped all the way around the Galaxy to intercept the line of sight. The distance and velocity measurements at b = -40? reported by Majewski et al. in a companion paper also support this hypothesis. We determine debris models with these properties on orbits that are consistent with the currently known positions and velocities of Sgr in Galactic potentials with halo components that have circular velocities vcirc = 140?200 km s-1. In all cases, the orbits oscillate between ~12 and ~40 kpc from the Galactic center with radial time periods of 0.55?0.75 Gyr. The best match to the data is obtained in models where Sgr currently has a mass of ~109 M? and has orbited the Galaxy for at least the last 1 Gyr, during which time it has reduced its mass by a factor of 2?3, or luminosity by an amount equivalent to ~10% of the total luminosity of the Galactic halo. These numbers suggest that Sgr is rapidly disrupting and unlikely to survive beyond a few more pericentric passages. These conclusions are only tentative, because they rely heavily on the less certain measurements of debris properties far from the center of Sgr. However, they demonstrate the immense potential for using debris to determine Sgrs dynamical history in great detail.

A kinematic study of the Fornax dwarf spheroidal galaxy

Precise radial velocities of 44 stars and four globular clusters located in two fields of the Fornax dwarf spheroidal galaxy are obtained on the basis of photon-counting echelle spectroscopy with a resolution of approximately 14 km/s. BV CCD photometry of the giant branch of Fornax in both fields are presented as well. A variety of kinematic and photometric criteria are used to identify 10-12 probable nonmembers in the present sample of spectroscopically observed stars. Based on the most probable members, the mean heliocentric systemic velocity of Fornax is 53.0 + or - 1.8 km/s, with no evidence of any significant rotation about the minor axis. The intrinsic velocity dispersion of the stars in Fornaxs central field is 9.9 + or - 1.7 km/s, while for the outer field the velocity dispersion is 1.20 + or - 2.8 km/s. The true central velocity dispersion is not more than 1.6 km/s larger than the observed central dispersions for a number of reasonable models.

Starcounts Redivivus. III. A Possible Detection of the Sagittarius Dwarf Spheroidal Galaxy at b = ?40?

As part of the Selected Areas Starcounts Survey, a CCD survey to V > 21, we have obtained VI photometry of two fields at b = ?40? aligned roughly with an extrapolation of the major axis of the Sagittarius dwarf spheroidal galaxy. Comparison of the color-magnitude diagram (CMD) for some of these fields with the CMDs of fields reflected about the Galactic l = 0? meridian reveals an excess of stars at V0 = 17.85 and 0.9 < (V-I)0 < 1.1 in the (l, b) = (11?, -40?) field. The excess stars have colors consistent with the Sgr red clump, and deeper CMD imaging in these locations shows evidence of a main-sequence turnoff (MSTO) at V = 21, with the main sequence extending to the limit of our data (V = 24). The surface brightnesses we derive from either the potential excess of red clump stars or the apparent excess of MSTO stars are consistent with each other and with the results of other surveys at this latitude. No similar excess appears in our northern Galactic hemisphere fields near the (l, b) = (353?, +41?) field. We have obtained spectroscopy of all 30 candidate red clump stars in the range 0.9 < (V-I)0 < 1.1and 17.75 < V0 < 17.95. The radial velocity distribution of the stars, while dissimilar from expectations of Galactic structure models, does not show a contribution by stars near the Galactocentric radial velocity seen in other studies near the Sgr core. It is difficult to reconcile a photometric result that is consistent with other explorations of the Sagittarius stream with a radial velocity distribution that is apparently inconsistent. In a companion paper, we discuss how some of the discrepancies are resolved if our potential Sgr detection corresponds to a different Sgr tidal streamer than that detected by most other surveys.

Phoenix - An intermediate dwarf galaxy in the Local Group

CCD BVI photometry of the Local Group dwarf galaxy Phoenix is presented. Phoenix looks like a dwarf spheroidal with a sprinkle of young blue stars. The apparent V and I magnitudes of its red giants are used to estimate its distance. Phoenix is definitively a member of the Local Group. Its metal abundance, determined from the I vs (V-I) diagram, is found to be low at [Fe/H] = −2.0. The radius of Phoenix is comparable to the dwarf spheroidal Leo II while its absolute magnitude is similar to that of the Carina dwarf. With M V ∼ −9.9, Phoenix is as faint as the dwarf irregular galaxies LGS3 or SagDIG. Its stellar mass is less than one million solar masses

Young stars between the Magellanic Clouds. III - Overall properties of associations: A shallow mass function

We present CCD B, V photometry of three more groups of blue stars located in the bridge of the Magellanic Clouds (MC). Two associations near the SMC Wing tip are members of large complexes of blue stars. The other one, located at equal angular distance between the SMC and the LMC, is isolated. ZAMS fittings yield distances to the Sun that are intermediate between the ones of the LMC and the SMC. We find that the stellar bridge has a large line-of-sight depth near the eastern tip of the SMC Wing.

Magellanic Cloud Periphery Carbon Stars. IV. The SMC

The kinematics of 150 carbon stars observed at moderate dispersion on the periphery of the Small Magellanic Cloud are compared with the motions of neutral hydrogen and early-type stars in the intercloud region. The distribution of radial velocities implies a configuration of these stars as a sheet inclined at 73° ± 4° to the plane of the sky. The near side, to the south, is dominated by a stellar component; to the north, the far side contains fewer carbon stars and is dominated by the neutral gas. The upper velocity envelope of the stars is closely the same as that of the gas. This configuration is shown to be consistent with the known extension of the SMC along the line of sight and is attributed to a tidally induced disruption of the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago. The dearth of gas on the near side of the sheet is attributed to ablation processes akin to those inferred in 1996 by Weiner & Williams to collisional excitation of the leading edges of Magellanic Stream clouds. Comparison with the 1989 kinematic data of Hardy, Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the 1986 and 1988 data of Mathewson et al. leaves little doubt that forces other than gravity play a role in the dynamics of the H I.

DISCOVERY OF VERY RED GIANTS IN THE FORNAX GALAXY.

Sixty-six very red stars were discovered in the Formax galaxy by blinking a pair of B,V plates. Their mean apparent magnitude is V = 18.4 m and their (B-V) color indices range from 1.6 m to 2.9 m. These stars form on the color-magnitude diagram a natural extension of the giant branch of Fornax. Even though no spectroscopic observations are available to establish the nature of the stars, it is reasonable to suspect that a certain number of carbon stars must be present in Fornax.