Ursa Minor Research Articles

The extended ‘stellar halo’ of the Ursa Minor dwarf galaxy

ABSTRACT Stellar candidates in the Ursa Minor (UMi) dwarf galaxy have been found using a new Bayesian algorithm applied to Gaia EDR3 data. Five of these targets are located in the extreme outskirts of UMi, from ∼5 to 12 elliptical half-light radii (rh), where rh(UMi) = 17.32 ± 0.11 arcmin, and have been observed with the high-resolution Gemini Remote Access to CFHT ESPaDOnS Spectrograph at the Gemini North telescope. Precise radial velocities (σRV < 2 km s−1) and metallicities ($\sigma _{\rm {{\rm [Fe/H]}}}\ \lt\ 0.2$ dex) confirm their memberships of UMi. Detailed analysis of the brightest and outermost star (Target 1, at ∼12rh), yields precision chemical abundances for the α (Mg, Ca, and Ti), odd-Z (Na, K, and Sc), Fe-peak (Fe, Ni, and Cr), and neutron-capture (Ba) elements. With data from the literature and APOGEE data release 17, we find the chemical patterns in UMi are consistent with an outside-in star-formation history that includes yields from core-collapse supernovae, asymptotic giant branch stars, and Type Ia supernovae. Evidence for a knee in the [α/Fe] ratios near [Fe/H] ∼ −2.1 indicates a low star-formation efficiency similar to that in other dwarf galaxies. Detailed analysis of the surface number density profile shows evidence that UMi’s outskirts have been populated by tidal effects, likely as a result of completing multiple orbits around the Galaxy.

Multiple chemodynamic stellar populations of the Ursa Minor dwarf spheroidal galaxy

ABSTRACT We present a Bayesian method to identify multiple (chemodynamic) stellar populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity, and positional stellar data without the assumption of spherical symmetry. We apply this method to a new Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS) spectroscopic survey of the Ursa Minor (UMi) dSph. We identify 892 likely members, making this the largest UMi sample with line-of-sight velocity and metallicity measurements. Our Bayesian method detects two distinct chemodynamic populations with high significance (in logarithmic Bayes factor, ln B ∼ 33). The metal-rich ([Fe/H] = −2.05 ± 0.03) population is kinematically colder (radial velocity dispersion of $\sigma _v=4.9_{-1.0}^{+0.8} \, \mathrm{km} \, \mathrm{s}^{-1}$) and more centrally concentrated than the metal-poor ($[{\rm Fe/H}]=-2.29_{-0.06}^{+0.05}$) and kinematically hotter population ($\sigma _v =11.5_{-0.8}^{+0.9}\, \mathrm{km} \, \mathrm{s}^{-1}$). Furthermore, we apply the same analysis to an independent Multiple Mirror Telescope (MMT)/Hectochelle data set and confirm the existence of two chemodynamic populations in UMi. In both data sets, the metal-rich population is significantly flattened (ϵ = 0.75 ± 0.03) and the metal-poor population is closer to spherical ($\epsilon =0.33_{-0.09}^{+0.12}$). Despite the presence of two populations, we are able to robustly estimate the slope of the dynamical mass profile. We found hints for prolate rotation of order ${\sim}2 \, \mathrm{km} \, \mathrm{s}^{-1}$ in the MMT data set, but further observations are required to verify this. The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators, so we computed new astrophysical dark matter annihilation (J) and decay profiles based on the rounder, hotter metal-poor population and inferred $\log _{10}{(J(0{^{\circ}_{.}}5)/{\rm GeV^{2} \, cm^{-5}})}\approx 19.1$ for the Keck data set. Our results paint a more complex picture of the evolution of UMi than previously discussed.

Subaru Hyper Suprime-Cam Survey for the Local Group Dwarf Galaxies: Ursa Minor

AbstractWe have carried out a wide and deep imaging survey for the Local Group dwarf spheroidal galaxy Ursa Minor (UMi) using Hyper Suprime-Cam (HSC). The data cover out beyond the nominal tidal radius down to ~25 mag in i band, which is ~2 mag below the main sequence turn-off point. The structural parameters of UMi are derived using red giant branch (RGB) stars and sub-giant branch (SGB) stars, and the tidal radius is suggested to be larger than those estimated by the previous studies. It is also found that the distribution of bluer RGB/SGB stars is more extended than that of redder RGB/SGB stars. The fraction of binary systems is estimated to be ~0.4 from the morphology of the main sequences.

HSC Wide and Deep Imaging Survey for the Milky Way Satellite Galaxies

AbstractWe have carried out an imaging survey for Local Group dwarf spheroidal galaxy Ursa Minor (UMi) using Hyper Suprime-Cam (HSC) on the 8.2m Subaru Telescope. Wide and deep data obtained by HSC enable us to investigate the extent of UMi which is revealed to extend out to twice the nominal tidal radius. The fraction of binary systems is also investigated from the morphology of the main sequence and estimated to be ~0.4.

The survival of dynamical fossils in dwarf spheroidal galaxies in conventional and modified dynamics

The survival of unbound density substructure against orbital mixing imposes strong constraints on the slope of the underlying gravitational potential and provides a new test on modified gravities. Here we investigate whether the interpretation that the stellar clump in Ursa Minor (UMi) dwarf spheroidal galaxy is a ‘dynamical fossil’ is consistent with MOdified Newtonian Dynamics (MOND). For UMi mass models inferred by fitting the velocity dispersion profile, the stellar clump around the second peak of UMi is erased very rapidly, within 1.25 Gyr (6.5 orbits), even with the inclusion of self-gravity. We find that the clump can hardly survive for more than 2 Gyr even under more generous conditions. Alternative scenarios which could lead to a kinematically cold clump are discussed but, so far, none of them was found to be fully satisfactory. Our conclusion is that the cold clump in UMi poses a challenge for both Lambda cold dark matter (ΛCDM) and MOND.

AN UPPER LIMIT ON THE MASS OF THE BLACK HOLE IN URSA MINOR DWARF GALAXY

The well-established correlations between the mass of massive black holes (BHs) in the nuclei of most studied galaxies and various global properties of their hosting galaxy lend support to the idea that dwarf galaxies and globular clusters could also host a BH in their centers. Direct kinematic detection of BHs in dwarf spheroidal (dSph) galaxies are seriously hindered by the small number of stars inside the gravitational influence region of the BH. The aim of this Letter is to establish an upper dynamical limit on the mass of the putative BH in the Ursa Minor (UMi) dSph galaxy. We present direct N-body simulations of the tidal disruption of the dynamical fossil observed in UMi, with and without a massive BH. We find that the observed substructure is incompatible with the presence of a massive BH of (2-3)x10^4 Msun within the core of UMi. These limits are consistent with the extrapolation of the M_{BH}-sigma relation to the M_{BH}<10^6 Msun regime. We also show that the BH may be off-center with respect to the center of symmetry of the whole galaxy.

Bounds on the Mass and Abundance of Dark Compact Objects and Black Holes in Dwarf Spheroidal Galaxy Halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal (dSph) galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor (UMi) dSph against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 103 M? at a halo density in UMi's core of 0.35 M? pc-3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses 3 ? 103 M? improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses ~105 M? can comprise no more than a halo mass fraction ~0.01. Better determinations of the velocity dispersion of old overdense regions in dSph galaxies may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km s-1 for the secondary peak of UMi is confirmed, compact objects with masses above ~100 M? could be excluded from comprising all its dark matter halo.

Exploring Halo Substructure with Giant Stars: The Velocity Dispersion Profiles of the Ursa Minor and Draco Dwarf Spheroidal Galaxies at Large Angular Separations

We analyze velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal (dSph) galaxies based on published and new Keck HIRES spectra for stars in the outer UMi field. Washington + DDO51 filter photometric catalogs provide additional leverage on membership of individual stars and, beyond 0.5 King limiting radii (rlim), identify bona fide dSph members up to 4.5 times more efficiently than simple color-magnitude diagram selections. Previously reported cold populations at rlim are not obvious in the data and appear only with particular binning; more or less constant and platykurtic dispersion profiles are characteristic of these dSphs to large radii. We report the discovery of UMi stars to at least 2.7rlim (i.e., 210' or 4 kpc). Even with conservative assumptions, a UMi mass of M > 4.9 × 108 M☉ is required to bind these stars, implying an unlikely global mass-to-light ratio of M/L > 900 (M/L)☉. We conclude that we have found stars tidally stripped from UMi.

Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidal Galaxies

We present projected velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar velocities, respectively. Both profiles show a sharp decline in the velocity dispersion outside ~30' (Draco) and ~40' (UMi). New deep photometry of Draco reveals a break in the light profile at ~25'. These data imply the existence of a kinematically cold population in the outer parts of both galaxies. Possible explanations of both the photometric and the kinematic data in terms of both equilibrium and nonequilibrium models are discussed in detail. We conclude that these data challenge the picture of dwarf spheroidal galaxies as simple, isolated stellar systems.

Evidence for Light-weight Local Group Dwarf Spheroidal Galaxies

A simple and natural explanation for the dynamics and morphology of the Local Group Dwarf Spheroidal galaxies, Draco (Dra) and Ursa Minor (UMi), is that they are weakly unbound stellar systems with no significant dark matter component. A gentle, but persistent, Milky Way (MW) tide has left them in their current kinematic and morphological state (the “parametric tidal excitation”). A new test of a dark matter dominated dS potential follows from a careful observation of the “clumpiness” of the dS stellar surface density.

A New Method of Estimating the Mass-to-Light Ratio of the Ursa Minor Dwarf Spheroidal Galaxy

Dwarf satellite galaxies undergo strong tidal forces produced by the main galaxy potential. These forces disturb the satellite and produce asymmetries in its stellar distribution, tidal tail formation, and modifications of the velocity dispersion profiles. Most of these features are observed in the Ursa Minor (UMi) dwarf spheroidal galaxy, which is one of the closest satellites of the Milky Way. These features show that UMi is being tidally disrupted and is probably not in virial equilibrium. The high-velocity dispersion of UMi would also be a reflection of this tidal disruption and is not the signature of the large dark matter content that would be deduced if virial equilibrium is assumed. In order to avoid the uncertainty produced when virial equilibrium is assumed in systems in strong tidal fields, we present a new method of calculating the mass-to-luminosity ratio of disrupted dwarf galaxies. This method is based on numerical simulations and only takes into account the shape of the dwarf density profile and the tidal tail brightness, but it does not depend on the kinematics of the dwarf. Applying this method to UMi, we obtain a mass-to-luminosity relation of 12, which is lower than the value obtained assuming virial equilibrium (M/L = 60). In addition, if UMi has a large dark matter content, it will be impossible to reproduce a tidal tail as luminous as the one observed.

Parallel programming and performance evaluation with the URSA tool family

This paper contributes to the solution of several open problems with parallel programming tools and their integration with performance evaluation environments. First, we propose interactive compilation scenarios instead of the usual black-box-oriented use of compiler tools. In such scenarios, information gathered by the compiler and the compiler's reasoning are presented to the user in meaningful ways and on-demand. Second, a tight integration of compilation and performance analysis tools is advocated. M any of the existing, advanced instruments for gathering performance results are being used in the presented environment and their results are combined in integrated views with compiler information and data from other tools. Initial instruments that assist users in “data mining” this information are presented and the need for much stronger facilities is explained. The URSA Family provides two tools addressing these issues. URSA MINOR supports a group of users at a specific site, such as a research or development project. URSA MAJOR complements this tool by making available the gathered results to the user community at large via the World-wide Web. This paper presents objectives, functionality, experience, and next development steps of the URSA tool family. Two case studies are presented that illustrate the use of the tools for developing and studying parallel applications and for evaluating parallelizing compilers.

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, &amp; Hill (1995) MMT echelle velocities.