Ursa Minor Dwarf Research Articles

Kinematics and M/L ratios of dwarf spheroidals

Recent results on the kinematics of the Draco and Ursa Minor dwarf spheroidals are reviewed.

Upper limits on central black hole masses of globular clusters from radio emission and a possible black hole detection in the Ursa Minor dwarf galaxy

Abstract Intermediate-mass black holes (IMBHs) have been alternatively predicted to be quite common in the centres of globular clusters or nearly impossible to form and retain in the centres of globular clusters. As it has been recently shown that radio observations are currently the most sensitive observational technique for detecting such objects, we have obtained new deep radio observations of Omega Cen, and have reanalyzed older observations of M 15 in the hope of constraining the masses of possible black holes in their centres. In both cases, upper limits of about 100 μJy are found at GHz frequencies. We find that if the Bondi–Hoyle accretion rate truly represents the spherical accretion rate onto a black hole, then the masses of the black holes in the centres of these two galaxies are severely constrained – with mass limits of less than about 100 solar masses in both cases. If more realistic assumptions are made based on recent work showing the Bondi rate to be a severe overestimate, then the data for Omega Cen are marginally consistent with a black hole of about 1/1000 of the mass of the cluster (i.e. about 1000 M⊙). The data for M 15 are then only marginally consistent with previous reports of a ~2000 solar mass black hole, and we note that there is considerable hope for either detecting the black hole or improving this upper limit with current instrumentation. Finally, we discuss the possibility that the radio source near the core of the Ursa Minor dwarf spheroidal galaxy is a ~104-M⊙ black hole.

Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Abstract With the HDS (High Dispersion Spectrograph) on the Subaru Telescope, we obtained high-resolution optical region spectra of three red giant stars (COS 4, COS 82, and COS 347) in the Ursa Minor dwarf spheroidal galaxy. The chemical abundances in these stars were analyzed for 26 elements, including $\alpha$-, iron-peak, and neutron-capture elements. All three stars show low abundances of $\alpha$-elements (Mg, Si, and Ca), and two stars (COS 82 and COS 347) show high abundances of Mn compared to Galactic halo stars of similar metallicity. One star (COS 4) has been confirmed to be very metal deficient ($\mathrm{[Fe/H]} =-2.7$) and found to show anomalously low abundances of Mn, Cu, and Ba. In another star, COS 82 ($\mathrm{[Fe/H]} =-1.5$), we have found a large excess of heavy neutron-capture elements with a general abundance pattern similar to the scaled solar system $r$-process abundance curve. These observational results are rather puzzling: low abundances of $\alpha$-elements and high abundance of Mn seem to suggest a significant contribution of SNe Ia at low metallicity, while there is no hint of an $s$-process (i.e., AGB stars) contribution, even at $\mathrm{[Fe/H]} =-1.5$, suggesting a peculiar nucleosynthetic history of the UMi dSph galaxy.

Finding Faint Intermediate-Mass Black Holes in the Radio Band

We discuss the prospects for detecting faint intermediate-mass black holes, such as those predicted to exist in the cores of globular clusters and dwarf spheroidal galaxies. We briefly summarize the difficulties of stellar dynamical searches, then show that recently discovered relations between black hole mass, X-ray luminosity and radio luminosity imply that in most cases, these black holes should be more easily detected in the radio than in the X-rays. Finally, we show upper limits from some radio observations of globular clusters, and discuss the possibility that the radio source in the core of the Ursa Minor dwarf spheroidal galaxy might be a $\sim 10,000-100,000 M_\odot$ black hole.

Flickering Red Giants in the Ursa Minor Dwarf Spheroidal Galaxy: Detection of Low-Amplitude Variability in Faint Red Giant Branch Stars on 10 Minute Timescales

We have analyzed two epochs of Hubble Space Telescope Wide Field Planetary Camera 2 observations of the Ursa Minor dwarf spheroidal galaxy using the HSTphot photometric reduction package. We report the detection of nine faint (MV 0.0 mag) red giant variable stars that exhibit low-amplitude brightness fluctuations on 10 minute timescales with amplitudes ranging from 36 to 130 mmag. We have found variability in 14% of the red giants we have observed. If low-amplitude variability of red giants on 10 minute timescales can be verified and should their numbers prove to be at the 10% level or greater of all red giants in some ancient Population II stellar systems, then the observed color spread of the red giant branch of such systems would be broadened by flickering red giants in color-magnitude diagrams based on short (snapshot) observations of a single pair of 10 minute timescale observations in two different filters.

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.

Extreme Dark Matter Dominated Dwarfs

The results of a large radial velocity survey of the Draco and Ursa Minor dwarf spheroidal galaxies are presented. the velocity dispersion profiles of both objects are very similar: initially the dispersion increases with radius, while at radii approaching the outer limit of the stellar distribution we observe a sharp fall-off. We present the results of mass modeling based on these new data – the mass to light ratios are found to be about 400M⊙/L⊙. Evidence is also presented that the dark matter in the central regions of UMi has a core-like distribution. Finally, we demonstrate that the inner regions of UMi have not been affected by tides and discuss the implications of our data for MOdified Newtonian Dynamics (MOND).

A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy

The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark matter-dominated galaxies known, with a central mass-to-light ratio M/L ~ 70. Somewhat anomalously, it appears to contain morphological substructure in the form of a second peak in the stellar number density. It is often argued that this substructure must be transient, because it could not survive for the ~12 Gyr age of the system, given the crossing time implied by UMi's 8.8 km s-1 internal velocity dispersion. In this Letter, however, we present evidence that the substructure has a cold kinematical signature and argue that UMi's clumpiness could indeed be a primordial artefact. Using numerical simulations, we demonstrate that substructure is incompatible with the cusped dark matter halos predicted by the prevailing cold dark matter (CDM) paradigm but is consistent with an unbound stellar cluster sloshing back and forth within the nearly harmonic potential of a cored dark matter halo. Thus, CDM appears to disagree with observation at the least massive, most dark matter-dominated end of the galaxy mass spectrum.

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.

A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies

The Wisconsin Hα Mapper has been used to set the first deep upper limits on the intensity of diffuse Hα emission from warm ionized gas in the Draco and Ursa Minor dwarf spheroidal galaxies (dSphs). Assuming a velocity dispersion of 15 km s-1 for the ionized gas, we set limits of IHα ≤ 0.024 R and IHα ≤ 0.021 R for the Draco and Ursa Minor dSphs, respectively, averaged over our 1° circular beam. Adopting a simple model for the ionized interstellar medium, these limits translate to upper bounds on the mass of ionized gas of 10% of the stellar mass, or ~10 times the upper limits for the mass of neutral hydrogen. Note that the Draco and Ursa Minor dSphs could contain substantial amounts of interstellar gas, equivalent to all of the gas injected by dying stars since the end of their main star-forming episodes 8 Gyr in the past, without violating these limits on the mass of ionized gas.

Proper Motions of Dwarf Spheroidal Galaxies from [ITAL]Hubble Space Telescope[/ITAL] Imaging. I. Method and a Preliminary Measurement for Fornax

This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixel (0.25 milliarcseconds) -- an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is mu_alpha = 49 +- 13 milliarcseconds/century and mu_delta = -59 +- 13 milliarcseconds/century. The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 +- 0.8) x 10^{12} solar masses.

The analysis of isochrone fitting methods for red giant branch photometry, and tip red giant branch distance determination

We consider the procedure of isochrone fitting and its application to the study of red giant branch (RGB) photometry in old stellar populations. This is extended to consider the problems introduced by the inclusion of lower-magnitude regions of the colour-magnitude diagram. We refer especially to our previous paper, where the details of our isochrone interpolation and fitting code are explained. We address the systematic errors inherent in the process of isochrone fitting, and we investigate the extent to which simple stellar populations can be recovered from noisy photometric data. We investigate the effects caused by inaccurate distance estimates, isochrone model variation and photometric errors. We present results from two studies of approximately coeval stellar populations, those of the Milky Way globular cluster system, and the Ursa Minor dwarf spheroidal galaxy. In addition, we introduce a new method for estimating distances using photometry of the tip of the RGB, which is significantly more robust than the standard edge-detection filter.

Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population—all of the stars being old and metal-poor—similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to ∼0.3 M ⊙. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.

The Star Formation History and Spatial Distribution of Stellar Populations in the Ursa Minor Dwarf Spheroidal Galaxy

As a part of a project devoted to the study of the Ursa Minor dwarf spheroidal (dSph) galaxy, the star formation history of the galaxy is presented. The analysis uses wide-field photometry, encompassing about 1° × 1° (the total covered area being 0.75 deg2), which samples the galaxy out to its tidal radius. Derivation of the star formation history (SFH) has been performed using the synthetic partial-model technique. The resulting SFH shows that Ursa Minor hosts a predominantly old stellar population, with virtually all the stars having formed earlier than 10 Gyr ago and 90% having formed more than 13 Gyr ago. Nevertheless, the Ursa Minor color-magnitude diagram shows several stars above the main, old turnoff forming a blue plume (BP). If these stars were genuine main-sequence stars, Ursa Minor would have maintained a low star formation rate extending up to ~2 Gyr ago. However, several indications (relative amount and spatial distribution of BP stars and difficulty of retaining processed gas) argue against this possibility. In such a context, the most reliable hypothesis is that BP stars are blue stragglers originating in the old population, with Ursa Minor hence remaining the only Milky Way dSph satellite to host a pure old stellar population. A marginally significant age gradient is detected, in the sense that stars in outer regions are slightly younger on average. The distance of Ursa Minor has been calculated using the magnitude of the horizontal branch and a calibration based on Hipparcos data for main-sequence subdwarfs. We estimate a distance of 76 ± 4 kpc, which is slightly higher than previous estimates. From the red giant branch color, we estimate a metallicity [Fe/H] = -1.9 ± 0.2, in agreement with a previous spectroscopic determination. No metallicity gradients have been detected across the galaxy.

The enigma in the heart of Ursa Minor

To investigate the physical nature of the observed deciency of stars in the very center of the Ursa Minor Dwarf Spheroidal galaxy, we recently obtained JCMT-SCUBA 850 m photometry of this region. The observed void could be explained by0.5 mag of extinction. While the amount of dust needed to produce such absorption (under certain assumptions for the dust temperature and the exponent in the dust emissivity term) would yield a 5 mJy signal { easily detectable by SCUBA { we found no signal at 850 m, after 256 min of integration. This nding rules out the presence of a few solar masses of cold dust cloud as the explanation for the central lack of stars observed in Ursa Minor. We discuss possible explanations.

Analysis of a Proper-Motion–Selected Sample of Stars in the Ursa Minor Dwarf Spheroidal Galaxy

We have studied the stellar population and internal structure of the core of the Ursa Minor dwarf spheroidal galaxy, using a sample of stars selected to be members based on their proper motions. In agreement with previous studies, we find Ursa Minor to be dominated by an old, metal-poor stellar population. A small number of stars with high membership probabilities lie redward of the red giant branch. The brightest (V 18) such stars are known to be carbon stars, rather than metal-rich first-ascent giants. A number of stars with high membership probabilities lie blueward of the red giant branch and are more luminous than the horizontal branch. We speculate that these are post-horizontal branch stars. There may also be one or two stars in the post-AGB phase. Spectroscopy of the candidate post-HB and post-AGB stars is required to determine their nature. We recover the internal substructure in Ursa Minor that has been noted by several authors in the last 15 years. Using a variety of two- and three-dimensional statistical tests, we conclude that this substructure is statistically significant at the 0.005 level. There is no evidence that the regions of density excess have stellar populations that differ from the main body of Ursa Minor. The crossing time for a typical density excess is only ~5 × 106 yr. They are therefore clearly not due to intermediate age star-forming bursts. We conclude that they are instead due to tidal interactions between the Galaxy and Ursa Minor.

A Tidal Extension in the Ursa Minor Dwarf Spheroidal Galaxy

We report the detection of main-sequence and blue horizontal-branch stars of the Ursa Minor dwarf spheroidal galaxy beyond its tidal radius, indicating the existence of a possible tidal extension in this satellite of the Milky Way. This tidal extension could spread out well beyond the area covered in our survey (R > 80'), as suggested by the presence of a break to a shallower slope observed in its density profile. The V-band surface brightness for this possible tidal extension ranges from 29.8 to 31.5 mag arcsec-2. The area covered in our survey (~1.65 deg2) is not enough to discriminate if this extratidal population is part of a tidal tail or an extended halo around the galaxy. The existence of this tidal extension in Ursa Minor indicates that this satellite is currently undergoing a tidal disruption process by the Milky Way. We discuss the possibility of a tidal origin for the high mass-to-light ratio observed in this galaxy on the basis of our result and recent theoretical simulations of the tidal disruption of dwarf satellites in the Galactic halo.

Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

The Keck I telescope has been used to obtain HIRES spectra for red giants belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 stars is the most extensive yet assembled for stars in dSph environments. Our main findings are summarized as follows: (1) There is unambiguous evidence for a large internal spread in metallicity in all three galaxies: our program stars span a range of [Fe/H] = 1.53, 1.40 and 0.73 dex in Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that all three galaxies have similar nucleosynthetic histories. (3) A comparison of the measured abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 < [alpha/Fe] < 0.13 dex, whereas the halo field star sample has [alpha/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances which are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity dSphs studied here. (ABRIDGED).

WFPC2 Observations of the Ursa Minor Dwarf Spheroidal Galaxy

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F555W (approximately V) and F814W (approximately I) of the central region of the Ursa Minor dwarf spheroidal galaxy. The V versus V - I color-magnitude diagram features a sparsely populated blue horizontal branch, a steep thin red giant branch, and a narrow subgiant branch. The main sequence reaches approximately 2 magnitudes below the main-sequence turnoff (V(sup UMi, sub TO) approximately equals 23.27 +/- 0.11 mag) of the median stellar population. We compare the fiducial sequence of the Galactic globular cluster M92 (NGC 6341). The excellent match between Ursa Minor and M92 confirms that the median stellar population of the UMi dSph galaxy is metal poor ([Fe/H](sub UMi) approximately equals [Fe/H](sub M92) approximately equals -2.2 dex) and ancient (age(sub UMi)approximately equalsage(sub M92) approximately equals 14 Gyr). The B - V reddening and the absorption in V are estimated to be E(B - V) = 0.03 +/- 0.01 mag and A(sup UMi, sub V) = 0.09 +/- 0.03 mag. A new estimate of the distance modulus of Ursa Minor, (m - M)(sup UMi, sub 0) = 19.18 +/- 0.12 mag, has been derived based on fiducial-sequence fitting M92 [DELTA.V(sub UMi - M92) = 4.60 +/- 0.03 mag and DELTA(V - I)(sub UMi - M92) = 0.010 +/- 0.005 mag] and the adoption of the apparent V distance modulus for M92 of (m - M)(sup M92, sub V) = 14.67 +/- 0.08 mag (Pont et al. 1998, A&A, 329, 87). The Ursa Minor dwarf spheroidal galaxy is then at a distance of 69 +/- 4 kpc from the Sun. These HST observations indicate that Ursa Minor has had a very simple star formation history consisting mainly of a single major burst of star formation about 14 Gyr ago which lasted approximately < 2 Gyr. While we may have missed minor younger stellar populations due to the small field-of-view of the WFPC2 instrument, these observations clearly show that most of the stars in the central region Ursa Minor dwarf spheroidal galaxy are ancient. If the ancient Galactic globular clusters, like M92, formed concurrently with the early formation of the Milky Way galaxy itself, then the Ursa Minor dwarf spheroidal is probably as old as the Milky Way.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL] View of the Heart of Ursa Minor

Hubble Space Telescope F606W observations to 26th magnitude are used to investigate the stellar distribution in the center of the Ursa Minor dwarf spheroidal galaxy. The central surface density is found to be low, less than 700 stars arcmin-2, with no sign of a pronounced cusp. The maximum of the surface density is not found at our adopted center but in a ringlike structure some 13'' from it. We identify this feature with the subclustering identified by Olszewski & Aaronson, who believed that it was off-center. The currently accepted King's structural parameters, rc and rt, of Ursa Minor are found to be unsuitable to represent the stellar surface density near the center. Our star counts are used to obtain a lower limit of the central stellar density without the use of the velocity dispersion. We find a lower limit of 1.7 ± 1.1 M⊙ pc-3 from stars that were counted. By extrapolating the mass function to lower masses, we estimate that the true density could increase be a factor of 7.