Draco Dwarf Spheroidal Galaxy Research Articles

On the origin of dark matter cores in dwarf galaxies

ABSTRACT In this paper, we study the dynamical stability and time evolution of the central dark mattercores of low-mass (about10 8 − 10 9 M ⊙ ) galactic haloes found in recent cold dark mattersimulations at high redshift. From those simulations we extract three haloes, assembled byhierarchical merging, that at redshift z ∼ > 10 display a core and we evolve them without fur-ther merging to low redshift using direct N-body integration. The central core in the darkmatter profile is found to be dynamically stable: it survives for many crossing times withoutevolution into a cusp. This result supports the claim that the mass dependence of the centraldark matter profiles of simulated haloes is a direct conseque nce of the power spectrum ofprimordial density fluctuations. In addition, we show that t he simulated dark matter profiles,if they evolved in isolation, are consistent with the observed velocity dispersion profile ofstars in the inner parts of the Draco dwarf spheroidal galaxy. Simple scaling arguments arereviewed which explain the evolution of the concentration parameter with redshift. We alsoreview some arguments used to derive the logarithmic slope of the inner and outer densityprofile.Key words: cosmology: theory, dark matter – galaxies: dwarf, clusters, haloes – methods:N-body simulations

First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population

We present a spectroscopic study of D461, a giant star belonging to Draco dwarf spheroidal galaxy. From spectral synthesis in LTE we derive a lithium abundance of log � (Li) = 3.5±0. 4a nd aC /O ratio between 3 and 5. This is the first detection of a lithium rich C-star in a dwarf spheroidal galaxy. Basing on stellar models of appropriate chemical composition, we show that a similar C enrichment is compatible with that expected for a low mass low metallicity thermally pulsing AGB star, undergoing few third dredge up episodes. The position in the log g-log Teff diagram of D461 is also compatible with this theoretical scenario. In particular, the low effective temperature, lower than that expected for a low metallicity giant star, is a consequence of the huge increase of the envelope opacity occurring after the carbon dredge up. The Li enrichment may be explained if a deep circulation would take place during the interpulse period, the so called cool bottom process. In spite of the low resolution of our spectra, we derive a lower limit for the carbon isotopic ratio, namely 12 C/ 13 C > 40, and a constraint for the Ba abundance, namely 0.5 < (Ba/Fe) < 2. The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2 M� , which corresponds to an age ranging between 1 and 3 Gyr. We conclude that this star is more massive and younger than the typical stellar population of Draco.

The RR Lyrae Distance to the Draco Dwarf Spheroidal Galaxy

We present the first CCD variability study of the Draco dwarf spheroidal galaxy. The data were obtained with the Fred Lawrence Whipple Observatory 1.2 m telescope on 22 nights, over a period of 10 months, covering a 22' × 22' field centered at α = 17h19m575, δ = 57°50'05'' (J2000.0). The analysis of the BVI images produced 163 variable stars, 146 of which were RR Lyrae: 123 RRab, 16 RRc, 6 RRd, and one RR12. The other variables include a SX Phe star, four anomalous Cepheids, and a field eclipsing binary. Using the short distance scale statistical parallax calibration of Gould & Popowski and 94 RRab stars from our field, we obtain a distance modulus of (m - M)0 = 19.40 ± 0.02 (stat) ± 0.15 (syst) mag for Draco, corresponding to a distance of 75.8 ± 0.7 (stat) ± 5.4 (syst) kpc. By comparing the spread in magnitudes of RRab stars in B, V, and I, we find no evidence for internal dust in the Draco dwarf spheroidal galaxy. The catalog of all variables, as well as their photometry and finding charts, is available electronically via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

Sloan Digital Sky Survey Catalog of Stars in the Draco Dwarf Spheroidal Galaxy

The Sloan Digital Sky Survey (SDSS) has scanned the entire region containing the Draco dwarf spheroidal galaxy to 23 mag in g*. We present a catalog of stars found in a 453 arcmin2, elliptical region centered on the Draco dwarf spheroidal galaxy. Objects in the catalog are matched with five previously published catalogs. The catalog contains SDSS photometry for 5634 individual objects, and also the photometry from matches to any of the other catalogs. A comparison of the photometry between catalogs allows us to identify 142 candidate variable objects. One hundred and twelve of the suspected variables have colors consistent with RR Lyrae variables.

Selection of Metal‐poor Giant Stars Using the Sloan Digital Sky Survey Photometric System

We present a method for the photometric selection of metal-poor halo giants from the imaging data of the Sloan Digital Sky Survey (SDSS). These stars are offset from the stellar locus in the g-r versus u-g color-color diagram. Based on a sample of 29 candidates for which spectra were taken, we derive a selection efficiency of the order of 50% for stars brighter than r ~ 17 mag. The candidates selected in 400 deg2 of sky from the SDSS Early Data Release trace the known halo structures (tidal streams from the Sagittarius dwarf galaxy and the Draco dwarf spheroidal galaxy), indicating that such a color-selected sample can be used to study the halo structure even without spectroscopic information. This method, and supplemental techniques for selecting halo stars, such as RR Lyrae stars and other blue horizontal-branch stars, can produce an unprecedented three-dimensional map of the Galactic halo based on the SDSS imaging survey.

The Draco and Ursa Minor Dwarf Spheroidal Galaxies: A Comparative Study

We present (V, I) photometry of two wide (≃25 × 25 arcmin2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided [(m-M)0(UMi) = 19.41 ± 0.12 and (m-M)0(Dra) = 19.84 ± 0.14], and a comparative study of their evolved stellar population is presented. We detect for the first time the red giant branch (RGB) bump in the luminosity function of UMi (V = 19.40 ± 0.06), while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies, and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial star experiments. The adopted method is insensitive to stars more metal poor than [Fe/H] ∼ -2.5, and it rests on the assumption that the age spread in the considered populations is small (i.e., the impact of the actual age spread on the colors of the RGB stars is negligible). We find that while the average metallicity of the two galaxies is similar (⟨[Fe/H]⟩UMi = -1.8 and ⟨Fe/H]⟩Dra = -1.7), the metallicity distributions are significantly different, having different peak values ([Fe/H] = -1.9 and [Fe/H] = -1.6) and different maximum metallicities. We suggest that such differences may be partly responsible for the difference in horizontal-branch morphology between the two galaxies. The intrinsic metallicity 1 σ spread is σi = 0.10 in UMi and σi = 0.13 in Draco. We demonstrate that the inner region of UMi is significantly structured, at odds with what is expected for a system in dynamical equilibrium. In particular, we show that the main density peak of UMi is off-center with respect to the center of symmetry of the whole galaxy and shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposed to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass-to-light ratio (M/L) of this galaxy may be as low as M/L ∼ 7, a factor of 5–10 lower than current estimates.

Particle dark matter constraints from the Draco dwarf galaxy

It is widely thought that neutralinos, the lightest supersymmetric particles, could comprise most of the dark matter. If so, then dark halos will emit radio and gamma ray signals initiated by neutralino annihilation. A particularly promising place to look for these indicators is at the center of the local group dwarf spheroidal galaxy Draco, and recent measurements of the motion of its stars have revealed it to be an even better target for dark matter detection than previously thought. We compute limits on WIMP properties for various models of Draco's dark matter halo. We find that if the halo is nearly isothermal, as the new measurements indicate, then current gamma ray flux limits prohibit much of the neutralino parameter space. If Draco has a moderate magnetic field, then current radio limits can rule out more of it. These results are appreciably stronger than other current constraints, and so acquiring more detailed data on Draco's density profile may become one of the most promising avenues for identifying dark matter.

Structure of the Draco Dwarf Spheroidal Galaxy

This article studies the structure of the Draco dwarf spheroidal galaxy, with an emphasis on the question of whether the spatial distribution of its stars has been affected by the tidal interaction with the Milky Way, using R- and V-band CCD photometry for 11 fields. The article reports coordinates for the center, a position angle of the major axis, and the ellipticity. It also reports the results of searches for asymmetries in the structure of Draco. These results and searches for a break in the radial profile and for the presence of principal sequences of Draco in a color-magnitude diagram for regions more than 50' from the center yield no evidence that tidal forces from the Milky Way have affected the structure of Draco.

RR Lyrae Stars and Anomalous Cepheids in the Draco Dwarf Spheroidal Galaxy

AbstractWe present new results on RR Lyrae stars and anomalous Cepheids in the Draco dwarf spheroidal galaxy. We have increased the number of double-mode RR Lyrae stars and found three new anomalous Cepheids. With period-magnitude and period-amplitude diagrams, we discuss the Oosterhoff classification of Draco. Contradictory results were found in that Draco appears to contain both Oosterhoff I and II type RR Lyrae populations.

First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal

We present the first clear evidence for an extended dark matter halo in the Draco dwarf spheroidal galaxy based on a sample of new radial velocities for 159 giant stars out to large projected radii. Using a two-parameter family of halo models spanning a range of density profiles and velocity anisotropies, we are able to rule out (at about the 2.5 ? confidence level) halos in which mass follows light. The data strongly favor models in which the dark matter is significantly more extended than the visible dwarf. However, halos with harmonic cores larger than the light distribution are also excluded. When combined with existing measurements of the proper motion of Draco, our data strongly suggest that Draco has not been tidally truncated within ~1 kpc. We also show that the rising velocity dispersion at large radii represents a serious problem for modified gravity (MOND).

The Star Formation History and Morphological Evolution of the Draco Dwarf Spheroidal Galaxy

The photometric and morphological properties, as well as the star formation history, of the Draco dwarf spheroidal galaxy are analyzed on the basis of wide-field CCD photometry of the resolved stars covering about 1 deg2. Draco is at a distance of d = 80 ? 7 kpc and has a metallicity, [Fe/H], of -1.8 ? 0.2. No metallicity gradient is detected. The star surface density distribution can be fitted by a single exponential law of scale length ? = 50 ? 01. The central surface magnitude is ? = 24.4 ? 0.5, and the core radius is rc = 75 ? 03 (equivalent to rc = 175 ? 7 pc). Within errors, the same scale lengths are found for the density profiles along the semimajor and semiminor axes (rescaled to semimajor-axis units, using the ellipticity of the galaxy) of Draco. There are hence no evidences of a tidal tail associated with Draco. The tidal radius of the galaxy is found to be rt 42' (1 kpc). The possibility that the large mass-to-light relation in Draco could be accounted for by a convenient spatial orientation is tested. An upper limit to Draco's size along the line of sight is ~14 kpc. This is too small to account for the velocity dispersion of Draco if it were due to projection effects only, and it implies that other mechanisms (e.g., dark matter) are required. The stellar population of Draco is mainly old. Although some intermediate-age population is present in Draco, most of the star formation (up to 90%) took place before ~10 Gyr ago. No significant star formation activity is detected in the last ~2 Gyr. Two methods (partial model and subgiant) have been used to investigate the star formation history of Draco, both producing results in good qualitative agreement. No difference is found between the scale lengths of the distributions of old (9 Gyr) and young (~2?3 Gyr) stars, indicating either that both populations were formed under the same kinematic conditions, or that any initial difference was afterward erased.

New Insights on the Draco Dwarf Spheroidal Galaxy from the Sloan Digital Sky Survey: A Larger Radius and No Tidal Tails

We have investigated the spatial extent and structure of the Draco dwarf spheroidal galaxy by using deep wide-field multicolor CCD photometry from the Sloan Digital Sky Survey (SDSS). Our study covers an area of 27 deg2 around the center of the Draco dwarf and reaches 2 mag below the level of Draco's horizontal branch. The SDSS photometry allows very effective filtering in color-magnitude space. With such filtering the density of the foreground of Galactic field stars is decreased by more than an order of magnitude, and the stellar population of the Draco dwarf galaxy thus stands out with much higher contrast than in former investigations. We show that the spatial distribution of Draco's red giants, red horizontal-branch stars, and subgiants down to i* = 21.7 mag does not provide evidence for the existence of tidally induced tails or a halo of unbound stars. The projected surface density of the dwarf galaxy is flattened with a nearly constant ellipticity of 0.29 ± 0.02 at position angle 88° ± 3°. The radial profile can be fitted by King models, as well as by a generalized exponential. Using the empirical King profile, the core radius and the limiting (or tidal) radius along the major axis are rc = 77 and rt = 401, respectively; the latter means that the size of the Draco dwarf galaxy is 40% larger than previously measured. Fitting the profile of King's theoretical models yields a still larger limiting radius of rt = 495. There is no clear indication of a taillike extension of the Draco population beyond this radius. A break in the radial surface density profile, which might indicate a halo of extratidal stars, is also not found in our Draco data. We conclude that down to the above magnitude limit tidal effects can exist only at a level of 10-3 of the central surface density of Draco or below. The regular structure of Draco found from the new data argues against its being a portion of an unbound tidal stream and lends support to the assumption of dynamical equilibrium, which is the basis for mass estimation. The changes in the values for the core radius and limiting radius imply that the total mass of Draco is higher by more than a factor of 2. Using a King spherical model of equivalent size as a reference and adopting a line-of-sight velocity dispersion of either 10.7 or 8.5 km s-1, we derive estimates of the total mass within radius rt of (3.5 ± 0.7) × 107 M⊙ and (2.2 ± 0.5) × 107 M⊙, respectively. From the combined i-band flux of all possible Draco members that lie within major-axis radius rt we determine the total luminosity of the Draco dwarf galaxy as (L/L⊙)i = (2.4 ± 0.5) × 105. This includes corrections for the flux of the foreground stars and the unseen fainter part of the Draco population. We thus obtain overall mass-to-light ratios M/Li of 146 ± 42, or 92 ± 28 in solar units. In summary, our results strengthen the case for a strongly dark matter–dominated, bound stellar system.

Dark matter in dwarf spheroidals II. Observations and modelling of Draco

We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude V ≈ 19.5 with a mean velocity precision of ≈2 km s−1. As this survey is based on a high-precision photometric target list, it contains many more Draco members at large radii. For the first time, this allows a robust determination of the radial behaviour of the velocity dispersion in a dSph. We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood. We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law (υcirc ∝ r0.17). We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3σ significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.

The Nature of the Red Giant Branches in the Ursa Minor and Draco Dwarf Spheroidal Galaxies11This work is based on observations obtained with the Hobby‐Eberly Telescope, which is a joint project of the University of Texas at Austin, Pennsylvania State University, Stanford University, Ludwig‐Maximillians‐Universität München, and Georg‐August‐Universität Göttingen.

ABSTRACTSpectra for stars located redward of the fiducial red giant branches (RGBs) of the Ursa Minor and Draco dwarf spheroidal galaxies have been obtained with the Hobby‐Eberly telescope and the Marcario Low Resolution Spectrometer. From a comparison of our radial velocities with those reported in previous medium‐resolution studies, we find an average difference of 10 km s−1 with a standard deviation of 11 km s−1. On the basis of these radial velocities, we confirm the membership of five stars in Ursa Minor and find two others to be nonmembers. One of the confirmed members is a known carbon star that lies redward of the RGB; three others are previously unidentified carbon stars. The fifth star is a red giant that was found previously by Shetrone and coworkers to have [Fe/H] = -1.68 ± 0.11 dex. In Draco, we find eight nonmembers, confirm the membership of one known carbon star, and find two new members. One of these stars is a carbon star, while the other shows no evidence for C2 bands or strong atomic bands, although the signal‐to‐noise ratio of the spectrum is low. Thus, we find no evidence for a population of stars more metal‐rich than [Fe/H] ≃ -1.45 dex in either of these galaxies. Indeed, our spectroscopic survey suggests that every candidate suspected of having a metallicity in excess of this value based on its position in the color‐magnitude diagram is, in actuality, a carbon star. Based on the census of 13 known carbon stars in these two galaxies, we estimate the carbon star specific frequency to be ϵdSph≃2.4 × 10-5 L-1V,⊙, 25–100 times higher than that of Galactic globular clusters.

Stars of the Draco Dwarf Spheroidal Galaxy Beyond Its Measured Tidal Boundary

We report R- and V-band photometry derived from CCD imaging for objects in nine fields in and around the Draco dwarf spheroidal galaxy. The most distant fields are about 13 from the center. We use these data to search for Draco stars outside of its measured tidal boundary. The search involves three methods: (1) plotting color-magnitude diagrams for individual fields, for sections of fields, and for combined fields and sections—a colour-magnitude diagram can reveal a population of Draco stars by the presence of the expected principal sequences; (2) measuring field-to-field fluctuations in the surface density of objects located near the Draco principal sequences in the color-magnitude diagram; (3) measuring intrafield fluctuations in the surface density of those objects. We find evidence for the presence of Draco stars immediately beyond the measured tidal boundary of Draco and place an upper limit on the number of such stars in more distant fields that lie close to the extension of its major axis. The best evidence is the presence of the Draco principal sequences in the color-magnitude diagram for some combined fields and sections of fields. The measurements of the field-to-field fluctuations in the stellar surface density confirm this result.

Keck HIRES Abundances in the Dwarf Spheroidal Galaxy Draco

An analysis of high-resolution Keck HIRES spectra of four giants in the dwarf spheroidal galaxy Draco is presented. We find an unmistakable difference in [Fe/H] among the four stars analyzed, with the range -3.0 < [Fe/H] < -1.5. The abundance ratios are found to be consistent with a Population II sample, although the [Ca/Fe] ratio may be more nearly solar and the [Ba/Fe] ratio exhibits a steeper slope with [Fe/H] than has been found in the Galactic globular clusters or in the Galactic halo field stars. A hint of a deep-mixing pattern is found in one star (D473), suggesting that Draco giants may be more similar to globular cluster giants than they are to the field Population II giants.

ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] Observations of the Draco Dwarf Spheroidal Galaxy

We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The luminosity function is well sampled to ~3 mag below the turnoff. We see no evidence for multiple turnoffs and conclude that, at least over the field of view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 ± 2.5 Gyr, lending support to the argument that the second parameter that governs horizontal-branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.

A dynamical study of the Draco dwarf spheroidal galaxy

A dynamical study of the Draco dwarf spheroidal galaxy

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.

The dark matter halos of Draco and Ursa Minor

Published density profiles and central velocity dispersions place important constraints on the stellar velocity ellipsoid and on the distribution of dark matter (DM) in the dwarf spheroidal galaxies Draco and Ursa Minor. Central velocity dispersions of 9 km/s are adopted for Draco and 11 km/s for Ursa Minor. Then, for an isotropic stellar velocity distribution, the central DM densities are 0.8 and 1.0 solar mass/cu pc, respectively, if visible and dark matter have the same core radius. If DM has a much larger core radius than visible matter but nevertheless dominates the potential, these densities are reduced by a factor of 2. Central DM densities can be lower than this only if the stellar velocity distribution is anisotropic. Simple two-component King models are used to investigate this and to look for the smallest DM densities that are consistent with the observations. 36 refs.