Ursa Minor Dwarf Research Articles

Measuring the Dark Matter Scale of Local Group Dwarf Spheroidals

We reanalyze published Draco and Ursa Minor dwarf spheroidal (dSph) stellar velocity data. Previous studies of dSph stellar distributions have concentrated principally on the spatial distribution of stars or on computation of a global velocity dispersion. A detailed understanding of the spatial behavior of the velocity distribution, however, is necessary to distinguish among competing theories of dSph dynamics. Here we apply a maximum-likelihood technique to fit the set of individual velocity measurements to a global distribution. Rather than calculating mean local velocity dispersions in regions where the data are sparse, this technique has the advantage of using the velocity measurements individually. We confirm earlier findings of a radial falloff in the velocity distribution of UMi, which is consistent with a mass-follows-light (MFL) King model. We also confirm an apparent radial rise in the velocity dispersion of Draco. However, these data do not suffice to distinguish among extended halo and MFL models. Finally, we perform simulations to determine the number of additional observations required to clearly differentiate between different dynamical models. Only ~10–20 additional observations at 0.75 times the tidal radius would be required to distinguish clearly between an MFL distribution and an extended halo or disrupted remnant model with a flat or radially rising velocity dispersion. We describe a method for treating the effect of binary stars, which may be a source of contamination.

A [ITAL]V[/ITAL] and [ITAL]I[/ITAL] CCD Mosaic Survey of the Ursa Minor Dwarf Spheroidal Galaxy

We discuss a Johnson-Cousins V- and I-band CCD mosaic survey of the Ursa Minor dwarf spheroidal galaxy (dSph) to V ~ 22. We covered UMi with 27 105 × 105 overlapping CCD frames, each frame consisting of two 300 s exposures in each of V and I. We also observed several regions ~3° from UMi to obtain an estimate of contamination by galaxies and Galactic stars. We report the first H-R diagram of an entire dSph. Separation of dwarf stars from foreground stars by color allows a robust estimation of the structural parameters of UMi. We examine earlier evidence of two lumps in the UMi stellar distribution. We detect a statistically significant asymmetry in the stellar distribution of UMi along the major axis. Structure in the stellar distribution of UMi might indicate a tidal origin for UMi's high observed mass-to-light ratio. We demonstrate a technique for obtaining the absolute magnitude of UMi from horizontal-branch star counts compared with M92; this method of luminosity estimation is independent of the distance to UMi. We obtain MV = -8.87 ± σ , where σ = 0.14 if M92 is a perfect calibrator and σ 0.35 is an upper bound on the error arising from differences in stellar populations. Our value for MV is consistent with earlier measurements and has a smaller uncertainty.

The Mass-to-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies. II. The Binary Population and its Effects on the Measured Velocity Dispersions of Dwarf Spheroidals

The Mass-to-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies. II. The Binary Population and its Effects on the Measured Velocity Dispersions of Dwarf Spheroidals

The Mass-To-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies.I. Radial Velocities from Multifiber Spectroscopy

We have measured 206 radial velocities for 94 probable members in the Ursa Minor dwarf spheroidal galaxy and 167 velocities for 91 probable members in the Draco dwarf spheroidal galaxy. These velocities were obtained using the KPNO 4 m telescope with the Hydra multifiber positioner and bench spectrograph in 1992,1993, and 1994. Both simulations and repeated measurements show that the median uncertainty in these velocities is 4.0 km/s. Our velocities are compared with the other major datasets for Draco and Ursa Minor. Aside from a small zero-point difference, there is excellent agreement between the Hydra velocities and the Olszewski et al. MMT echelle velocities. Comparison with the Hargreaves et al. Ursa Minor velocities reveals inconsistencies which we ascribe to underestimated errors in the Hargreaves et al. data, After correcting for zero-point differences, we combine the three major sets of velocities. Our spectra recovered all of the known Carbon (C) stars in Draco and Ursa Minor and revealed one new C star in Draco and two in Ursa Minor. In addition, one star whose velocity is consistent with membership in Ursa Minor has a deep and wide Hβ absorption line in its spectrum. An initial discussion of the kinematics of the two galaxies is presented. The systemic velocities of Draco and Ursa Minor are -293.3 +/- 1.0 and -247.4 +/- 1.0 km s^-1^, respectively. Ursa Minor shows apparent rotation about a position angle of 75^deg^ with an amplitude of about 3 km s^-1^; the morphological major axis is 53^deg^ +/-5^deg^. No such rotation is found in Draco. We find a velocity dispersion of 10.4 +/- 0.9 km s^-1^ for Ursa Minor including all stars and 8.8 +/- 0.8 km s^-1^ excluding the star with the most extreme velocity. In Draco we find 10.7 +/- 0.9 km s^-1^ including all stars and 8.5 +/- 0.7 km s^-1^ excluding the three stars with the most extreme velocities. These dispersions are consistent with the dispersions measured in the other two studies and confirm that Draco and Ursa Minor have the largest mass-to-light ratios of any of the dwarf spheroidals.

Nine Seasons of Velocity Measurements in the Draco and Ursa Minor Dwarf Spheroidal Galaxies with the MMT Echelle

We have used the Multiple Mirror Telescope echelle spectrograph to measure 112 velocities of 42 stars in the Draco and Ursa Minor dwarf spheroidal galaxies and three velocities of three foreground stars between 1982 April and 1990 September. We used 11 A resolution spectra obtained with the MX multifiber spectrograph at the Steward 90" to find additional giant candidates; 5 UMi and 13 Draco stars were then observed at the MMT and added to the original sample of velocity members. In addition, the MX spectra were used to eliminate 74 stars in the direction of UMI and 59 stars in Draco as likely foreground dwarfs. We detected 7 velocity variables, defined as those stars whose probability of exceeding the measured X^2^ by chance is less than 1.5%. Three of these stars are Carbon (C) stars (UMi K and VA 335 and Draco C1); two have emission lines (Draco CI and UMi M). We show that the C star Draco C4, with a proper motion membership probability of 7%, has a velocity consistent with membership. It is not surprising that these C (most likely CH) stars are binaries because McClure has shown that most Galactic CH stars are in binary systems. Of the remaining 35 stars, only 4 are velocity variables, with measured velocity extrema of 29.1 km s^-1^ (UMi M), 7.2 km s^-1^ (Draco XI-2), 9.0 km s^-1^ (Draco 24), and 8.3 km s^-1^ (Draco 473). The velocity dispersions are 10.1 +/- 1.7 km s^-1^ for UMi, and 9.9 +/- 1.4 km s^-1^ for Draco. These dispersions change to 10.5 +/- 2.0 for UMi, and 8.2 +?- 1.3 for Draco if we eliminate the velocity variables. Our dispersion for UMi differs from that of Hargreaves et al. [MNRAS, 271,693 (1994b)] by 1.3σ of the combined errors. These velocities are combined with the one-component King models of Pryor & Kormendy [AJ, 100,127 (1990)] to give M/L = 73 for UMi, and 77 for Draco.

The Velocity Dispersions of the Draco and Ursa Minor Dwarf Spheroidal Galaxies

We have used the Hydra fiber positioner and the bench spectrograph on the KPNO 4 m telescope to measure radial velocities for giants in the Draco (Dra) and Ursa Minor (UMi) dwarf spheroidal galaxies. The measurement uncertainties are 1–10 km s−1 per observation, with the median uncertainty being 3.6 km s−1. To date, we have reduced the data from two of our three runs. In UMi these have yielded 150 velocities for 85 member giants; 52 stars have two or more measurements. In Dra we have 108 velocities for 84 member giants and 20 stars have two or more measurements. There is good agreement between the repeat observations and with the Olszewski, Aaronson, & Hill (1995) MMT echelle velocities.

A dynamical study of the Ursa Minor dwarf spheroidal galaxy

A dynamical study of the Ursa Minor dwarf spheroidal galaxy

An Update on the Velocity Dispersions in the Draco and Ursa-Minor Dwarf Galaxies

An Update on the Velocity Dispersions in the Draco and Ursa-Minor Dwarf Galaxies

Proper motions and bright-star photometry in the Ursa Minor dwarf galaxy

The authors present astrometry and photographic photometry of the central region of the Ursa Minor dwarf spheroidal galaxy. These studies, based on first-epoch Hale Observatory 5 m plates, second-epoch KPNO 4 m plates, and a photoelectric and CCD sequence, provide a color-magnitude diagram to m<SUB>v</SUB> ≡ 20.5, slightly below the horizontal branch. The well-known features of the Ursa Minor color-magnitude diagram, namely a steep, relatively blue giant branch, and a blue horizontal branch are shown. Evidence for a small population of red horizontal-branch stars is given. The RR Lyrae variables, anomalous Cepheids, and suprahorizontal-branch stars are all likely members, although one suprahorizontal-branch "star" is believed to be a QSO. This QSO may provide a satisfactory zero-proper-motion reference frame for third-epoch plates, and permit the orbit of Ursa Minor to be usefully constrained.

The URSA Minor dwarf galaxy - Still an old stellar system

view Abstract Citations (103) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Ursa Minor dwarf galaxy : still an old stellar system. Olszewski, E. W. ; Aaronson, M. Abstract The authors have constructed a color-magnitude diagram of the Ursa Minor dwarf spheroidal to mV = 24.8 mag from charged-coupled device (CCD) observations with the Kitt Peak 4 m telescope. The main sequence turnoff is easily visible. Fits to evolutionary isochrones and the globular cluster M92 indicate that Ursa Minor has an age and metal abundance very similar to that of the latter cluster. No evidence for stars younger than about 16 billion years is seen, with the possible exception of approximately 20 stars believed to be blue stragglers. Ursa Minor is therefore an extreme-age galaxy, unlike superficially similar objects such as the Carina dwarf. Indeed, Ursa Minor may be the only outer-halo spheroidal whose stellar content lives up to the classical ideals of a Population II system. A surprising result of the study is the discovery of clumpiness in the distribution of stars. This finding may give more weight to the idea that dwarf spheroidal galaxies were previously dwarf irregular galaxies, although clearly, if so, Ursa Minor must have lost its gaseous content very soon after formation. Publication: The Astronomical Journal Pub Date: November 1985 DOI: 10.1086/113925 Bibcode: 1985AJ.....90.2221O Keywords: Abundance; Dwarf Galaxies; Elliptical Galaxies; Galactic Evolution; Star Clusters; Stellar Evolution; Stellar Systems; Binary Stars; Charge Coupled Devices; Color-Magnitude Diagram; Irregular Galaxies; Main Sequence Stars; Milky Way Galaxy; Star Distribution; Astrophysics full text sources ADS | data products SIMBAD (3) NED (1) CDS (1)

Observations of four nearby galaxies with the Einstein Observatory

Four nearby galaxies, NGC 205, NGC 6822, the dwarf elliptical galaxy in Uras Major, and Maffei 1, were studied with the imaging instruments on the Einstein Observatory, and the results are reported. Fifteen X-ray sources were discovered, eight of which may not be associated with the target galaxies. No X-ray source was found in NGC 205, and one was found in NGC 6822, probably the optical supernova remnant Hodge 12. There are at most three sources in the Ursa Minor galaxy. There is probably an extended source associated with the Maffei 1 galaxy.

ABUNDANCES OF GIANT STARS IN THE DRACO AND URSA MINOR DWARF GALAXIES.

ABSTRACT A number of synthetic spectra have been compared with Stetson's (1984) low-dispersion spectra of stars in the Draco and Ursa Minor dwarf galaxies. Spectral indices have been used to deduce the overall metal abundances of the stars, as indicated by the strengths of the H and K lines, as well as the carbon abundances, from the G bands. Similar analyses have been carried out for stars in the globular clusters M3 and M92. Stars in both dwarf spheroidals have the same mean overall abundances as the M92 stars. Carbon is depleted relative to the other metals in both the Draco and Ursa Minor stars ([C/M] = -0.5), but the depletion is not as great as in M92 where [C/M] = -0.8. The abundances deduced from the Draco color-magnitude diagram are higher than those deduced from the M92 diagram, a factor which may be due to higher CNO abundances.

The metal abundance range in the Ursa Minor dwarf galaxy

A small sample of red giants in the Ursa Minor dwarf spheroidal galaxy has been observed with the multichannel scanner and the digital spectrograph of the Hale 5 m telescope. These observations confirm the large width of the giant branch in the color-magnitude diagram and indicate that it is caused by a range in metal abundance of approx.1 dex (i.e., -2.55< or approx. =(Fe/H)< or approx. =-1.36, according to Cohen's scale for globular clusters). Ursa Minor and the Draco dwarf spheroidal appear to have very similar ranges in metal abundance. These observations and those of the other dwarf spheroidal galaxies are compared with the models of the chemical evolution of elliptical galaxies that include mass loss. The models are consistent with the dispersions in metal abundance that are found in the dwarf spheroidals, but now with the lack of detectable dispersions in globular clusters that are comparable in mass to Ursa Minor and Draco (e.g., M92). Some explanations for this difference between the galaxies and the clusters are discussed.

Abundance variations on the giant branches of the sculptor and Ursa Minor dwarf spheroidal galaxies and the Omega Centauri anomaly

Abundance variations on the giant branches of the sculptor and Ursa Minor dwarf spheroidal galaxies and the Omega Centauri anomaly

A Preliminary Color-Magnitude Diagram for the Ursa Minor Dwarf Galaxy

The Ursa Minor dwarf galaxy (RA = 15h08m.2, Dec = +67°18′ (1950)) was one of four dwarf ellipticals discovered by Wilson (1955) in a search of the Palomar Sky survey plates. The general properties of these galaxies resemble those of galactic globular clusters, linear size and density excepted (see Hodge, 1971, for a review). The characteristics of Ursa Minor are very similar to those of the Draco dwarf, for which a color magnitude diagram was published by Baade and Swope (1961). Van Agt (1967) published a study of the variable stars in Ursa Minor based on Baade's 200-inch plates.

The Chemical Abundances of the Halo Clusters of the Galaxy

Photometric metal abundances of individual red giants in eight extremely distant halo globular clusters and the Draco and Ursa Minor dwarf spheroidal galaxies have been obtained using the Washington broad-band system, C, M, T1, T2(Canterna 1976). Observations were made at the KPNO 2.1-m and CTIO 1.5-m telescopes. In Table I we list for each system the mean value of [Fe/H], the number of stars observed in each system, n, the Galactocentric distance, RGC, the intrinsic color of the giant branch at the level of the horizontal branch (HB), (B-V)o,g, and the fraction of HB stars bluer than the RR Lyrae gap, fB. Sources for unpublished color-magnitude diagram (CMD) data are: Pal 11 (Canterna and Schommer), Pal 12 (Canterna and Harris), and Ursa Minor (Schommer, Olszewski and Kunkel).