Sculptor Dwarf Research Articles

Asymptotic giant branch stars in the Sculptor dwarf spheroidal galaxy

JHK_S photometry is presented for a 35 arcmin square field centred on the Sculptor dwarf spheroidal galaxy. With the aid of published kinematic data definite galaxy members are identified and the width in J-K of the colour-magnitude diagram is shown to be consistent with an old population of stars with a large range in metal abundance. We identify two Asymptotic Giant Branch variables, both carbon Miras, with periods of 189 and 554 days, respectively, and discuss their ages, metallicities and mass loss as well as their positions in the Mira period-luminosity diagram. There is evidence for a general period-age relation for Local Group Miras. The mass-loss rate for the 554-day variable, MAG29, appears to be consistent with that found for Miras of comparable period in other Local Group galaxies.

Deep wide-field imaging down to the oldest main sequence turn-offs in the Sculptor dwarf spheroidal galaxy

We present wide-field photometry of resolved stars in the nearby Sculptor dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude Diagrams can be used to constrain the ages of different stellar populations, and also their spatial distribution. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old) which can be easily resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. By using spectroscopic metallicities of RGB stars in combination with our deep Main Sequence Turn-Off photometry we can constrain the ages of different stellar populations with particular accuracy. We find that the known metallicity gradient in Sculptor is well matched to an age gradient. This is the first time that this link with age has been directly quantified. This gradient has been previously observed as a variation in Horizontal Branch properties and is now confirmed to exist for Main Sequence Turn-Offs as well. It is likely the Sculptor dSph first formed an extended metal-poor population at the oldest times, and subsequent more metal-rich, younger stars were formed more towards the centre until the gas was depleted or lost roughly 7 Gyr ago. The fact that these clear radial gradients have been preserved up to the present day is consistent with the apparent lack of signs of recent tidal interactions.

An Accurate Timescale for Star Formation and Chemical Enrichment of the Sculptor dSph

The Sculptor dwarf Spheroidal is a Milky Way satellite with a dominant old (>10 Gyr) population that displays at least two distinct stellar populations. We present new sensitive imaging data from CTIO/MOSAIC and make a detailed study of the stellar populations of Sculptor, including the main sequence turnoff (MSTO) properties. We present the Star Formation History (SFH) and Metallicity Distribution Function (MDF) of the Sculptor dSph and find that a well-defined radial gradient is present not only in metallicity but now also in age.

H.E.S.S. constraints on dark matter annihilations towards the sculptor and carina dwarf galaxies

The Sculptor and Carina dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8 h and 14.8 h of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220 GeV and 320 GeV, respectively. Upper limits on the gamma-ray fluxes at 95% CL assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10 −13–10 −12 cm −2 s −1 in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from 〈 σv〉 ∼ 10 −21 cm 3 s −1 down to 〈 σv〉 ∼ 10 −22 cm 3 s −1 depending on the dark matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions.

The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies

We use N‐body simulations to study the effects of tides on the kinematical structure of satellite galaxies orbiting a Milky Way‐like potential. Here we focus on the evolution of a spherical, dark matter dominated satellite, which is modelled with two stellar components set ab initio to be spatially and kinematically segregated, in a way that resembles the configuration of the metal poor and metal rich stellar populations in several dwarf spheroidals of the Local Group. We find that an important attenuation of the initial differences in the distribution of the two stellar components occurs for orbits with small pericentric radii. This is mainly due to (i) the loss of the gravitational support provided by the dark matter component after tidal stripping takes place and (ii) tides preferentially affect the more extended stellar component, leading to a net decrease in its velocity dispersion as a response for the mass loss, which thus shrinks the kinematical gap. We apply these ideas to the Sculptor and Carina dwarf spheroidals. Differences in their orbits might help to explain why in the former a clear kinematical separation between metal poor and metal rich stars is apparent, while in Carina this segregation is significantly more subtle.

Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor

Current cosmological models indicate that the Milky Way's stellar halo was assembled from many smaller systems. On the basis of the apparent absence of the most metal-poor stars in present-day dwarf galaxies, recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt on this conclusion. Verification of the iron-deficiency, however, and measurements of additional elements, such as the alpha-element Mg, are necessary to demonstrate that the same type of stars produced the metals found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars be conclusively linked to early stellar halo assembly. Here we report high-resolution spectroscopic abundances for 11 elements in S1020549, confirming its iron abundance of less than 1/4,000th that of the Sun, and showing that the overall abundance pattern follows that seen in low-metallicity halo stars, including the alpha-elements. Such chemical similarity indicates that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs, and suggests that the early chemical enrichment of all galaxies may be nearly identical.

MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. I. THE SCULPTOR DWARF SPHEROIDAL GALAXY

We present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a satellite of the Milky Way. This is the largest sample of individual alpha element (Mg, Si, Ca, Ti) abundance measurements in any single dSph. The measurements are made from Keck/DEIMOS medium-resolution spectra (6400-9000 A, R ~ 6500). Based on comparisons to published high-resolution (R >~ 20000) spectroscopic measurements, our measurements have uncertainties of sigma([Fe/H]) = 0.14 and sigma([alpha/Fe]) = 0.13. The Sculptor [Fe/H] distribution has a mean <[Fe/H]> = -1.58 and is asymmetric with a long, metal-poor tail, indicative of a history of extended star formation. Sculptor has a larger fraction of stars with [Fe/H] < -2 than the Milky Way halo. We have discovered one star with [Fe/H] = -3.80 +/- 0.28, which is the most metal-poor star known anywhere except the Milky Way halo, but high-resolution spectroscopy is needed to measure this star's detailed abundances. As has been previously reported based on high-resolution spectroscopy, [alpha/Fe] in Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] ~ -1.5) have lower [alpha/Fe] than Galactic halo field stars of comparable metallicity. This indicates that star formation proceeded more gradually in Sculptor than in the Galactic halo. We also observe radial abundance gradients of -0.030 +/- 0.003 dex per arcmin in [Fe/H] and +0.013 +/- 0.003 dex per arcmin in [alpha/Fe] out to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly other surviving dSphs as representative of the dwarf galaxies from which the metal-poor tail of the Galactic halo formed.

Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: The Carina, Fornax, Sculptor, and Sextans Dwarf Spheroidals

The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+48 +/- 15,-25 +/- 14) mas/century, that agrees with published HST astrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+25 +/- 36,+16 +/- 43) mas/century, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})= (-40 +/- 29, -69 +/- 47) mas/century, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia et al. (2008). For Sextans, which at present lacks an astrometric measurement, we measure (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(-26 +/- 41, +10 +/- 44) mas/century.

The Kinematic Status and Mass Content of the Sculptor Dwarf Spheroidal Galaxy

We present VLT/FLAMES spectroscopic observations (R~6500) in the CaII triplet region for 470 probable kinematic members of the Sculptor (Scl) dwarf spheroidal galaxy. The accurate velocities (+- 2 km/s) and large area coverage of Scl allow us to measure a velocity gradient of 7.6_{-2.2}^{+3.0} km/s deg^{-1} along the Scl projected major axis, likely a signature of intrinsic rotation. We also use our kinematic data to measure the mass distribution within this system. By considering independently the kinematics of the distinct stellar components known to be present in Scl, we are able to relieve known degeneracies, and find that the observed velocity dispersion profiles are best fitted by a cored dark matter halo with core radius r_c= 0.5 kpc and mass enclosed within the last measured point M(< 1.8 kpc)=3.4 +- 0.7 x 10^8 M_sun, assuming an increasingly radially anisotropic velocity ellipsoid. This results in a mass-to-light ratio of 158+-33 (M/L)_sun inside 1.8 kpc. An NFW profile with concentration c=20 and mass M(< 1.8 kpc) = 2.2_{-0.7}^{+1.0} x 10^8 M_sun is also statistically consistent with the observations, but it tends to yield poorer fits for the metal rich stars.

The mass content of the Sculptor dwarf spheroidal galaxy

AbstractWe present a new determination of the mass content of the Sculptor dwarf spheroidal galaxy, based on a novel approach which takes into account the two distinct stellar populations present in this galaxy. This method helps to partially break the well-known mass-anisotropy degeneracy present in the modelling of pressure-supported stellar systems.

Stars at Low Metallicity in Dwarf Galaxies

AbstractDwarf galaxies offer an opportunity to understand the properties of low metallicity star formation both today and at the earliest times at the epoch of the formation of the first stars. Here we concentrate on two galaxies in the Local Group: the dwarf irregular galaxy Leo A, which has been the recent target of deep HST/ACS imaging (Cole et al. 2007) and the Sculptor dwarf spheroidal, which has been the target of significant wide field spectroscopy with VLT/FLAMES (Battaglia 2007).

Masses, Tidal Radii, and Escape Speeds in Dwarf Spheroidal Galaxies under MOND and Dark Halos Compared

We investigate the success and problems of MOdified Newtonian Dynamics (MOND) in explaining the structural parameters and dynamics of remote Galactic globular clusters (GCs) and dwarf spheroidal (dSph) galaxies. Using the MOND value for the mass of the Milky Way as inferred from the Galactic rotation curve, we derive the tidal radii of Galactic GCs, and compare to observed values. Except for Pal 14, the predicted tidal radii of GCs are systematically larger than the observed nominal values. However, the associated uncertainties are so large that tidal radii are consistent on the $1\sigma$ level. We have considered the importance of the Galactic tidal force on the survival of dSphs under MOND. Assuming mass-to-light ratios compatible with a naked stellar population, we found that the present Galactic dSphs preserve their integrity over one Hubble time, except Sextans which may survive the tidal interaction only for several Gyr. Mass-to-light ratios as inferred from the internal kinematics of dSph galaxies can be used, but they appear too large to be accounted for only by the stellar population in Willman 1, Coma Berenice, Ursa Minor, Draco, Ursa Major and possibly Bo\"{o}tes dwarves. Finally, the ability of the Sculptor dwarf to retain the observed population of low-mass X-ray binaries (LMXBs) is examined. Under the MOND paradigm, we find that the retention fraction in Sculptor is likely not larger than a few percent. Compared to the dark matter scenario, it turns out that MOND makes the retention problem worse. We propose that measurements of the radial velocities of the observed LMXBs in Sculptor could provide a way to distinguish between modified gravities or extended and massive dark matter halos.

Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group

Parkes neutral hydrogen 21 cm line (H I) observations of the surroundings of nine early-type Local Group dwarfs are presented. We detected numerous H I clouds in the general direction of those dwarfs, and these clouds are often offset from the optical center of the galaxies. Although all the observed dwarfs, except Antlia, occupy phase-space regions where the high-velocity cloud (HVC) density is well above average, the measured offsets are smaller than one would expect from a fully random cloud distribution. Possible association is detected for 11 of the 16 investigated clouds, while for two galaxies, Sextans and Leo I, no H I was detected. The galaxies in which H I clouds were found not to coincide with the optical yet have a significant probability of being associated are the Sculptor dwarf, Tucana, LGS 3, Cetus, and Fornax. If the clouds are indeed associated, these galaxies have H I masses of M = 2 × 105, 2 × 106, 7 × 105, 7 × 105, and 1 × 105 M⊙, respectively. However, neither ram pressure nor tidal stripping can easily explain the offsets. In some cases, large offsets are found where ram pressure should be the least effective.

The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy★

Low-resolution spectroscopy obtained with FORS2 at the Very Large Telescope (VLT) has been used for the measurement of individual metal abundances ([Fe/H]) for 110 variable stars, including 107 RR Lyrae stars and one anomalous Cepheid, and to trace the metal distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy. The RR Lyrae stars are spread over a 15 × 15 arcmin 2 area around the galaxy centre. Their metallicities have an average value of [Fe/H] =− 1.83 ± 0.03 (rms = 0.26 dex) and cover the metallicity range −2.40 −1.3. The star-to-star scatter is larger than typical errors on individual metallicities (±0.15‐0.16 dex), indicating a real spread in metal abundances. The radial velocities measured from the RR Lyrae spectra have a dispersion of 12.9 km s −1 . This value is consistent with the dispersion derived by Tolstoy et al. for metal-poor red giants associated with the blue horizontal branch stars in Sculptor. Along with the metallicity distribution these results suggest that most of the RR Lyrae stars in Sculptor arise from the same burst of stellar formation that produced the metal-poor component, originating the galaxy blue horizontal branch. The metal-rich red horizontal branch population found to be centrally concentrated only produced a few (if any) of the RR Lyrae stars in our sample. The spectroscopic metallicities were used along with the apparent luminosities to study the luminosity‐metallicity relation followed by the RR Lyrae stars in Sculptor, for which we derive a shallow slope of 0.09 mag dex −1 . This result can be due to a high level of evolution off the zero-age horizontal branch of the RR Lyrae stars in this galaxy, again in agreement with their origin from the blue horizontal branch population. Ke yw ords: stars: abundances ‐ stars: evolution ‐ stars: Population II ‐ stars: variables: other ‐g alaxies: individual: Sculptor.

The discovery of X-ray binaries in the Sculptor dwarf spheroidal galaxy

We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with LX of at least 6 × 1033 ergs−1 with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5–8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low-mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.

The Absence of Extratidal Structure in the Sculptor Dwarf Spheroidal Galaxy

The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy are presented. Our aims were to obtain an accurate map of the outer structure of Sculptor and to determine the level of interaction between this system and the Galaxy. Photometry was obtained in two colors down to the magnitude limits of V = 20 and I = 19, covering a 31 ? 31 area centered on Sculptor. The resulting color-magnitude data were used as a mask to select candidate horizontal-branch (HB) and red giant branch stars for this system. Previous work has shown that the red HB stars are more concentrated than the blue HB stars. We have determined the radial distributions of these two populations and show that the overall Sculptor density profile is well described by a two-component model based on a combination of these radial distributions. In addition, spectra of the Ca II triplet region were obtained for over 700 candidate red giant stars over the 10 deg2 region using the 2dF instrument on the Anglo-Australian Telescope. These spectra were used to remove foreground Galactic stars based on radial velocity and Ca II triplet strength. The final list of Sculptor members contains 148 stars, 7 of which are located beyond the nominal tidal radius. Both the photometric and spectroscopic data sets indicate no significant extratidal structure. These results support at most a mild level of interaction between this system and the Galaxy, and we have measured an upper mass limit for extratidal material to be 2.3% ? 0.6% of the Sculptor luminous mass. This lack of tidal interaction indicates that previous velocity dispersion measurements (and hence the amount of dark matter detected) in this system are not strongly influenced by the Galactic tidal field.

Exploring Halo Substructure with Giant Stars. VIII. The Extended Structure of the Sculptor Dwarf Spheroidal Galaxy

We explore the spatial distribution of in the Sculptor dwarf spheroidal (dSph) galaxy over an area of 7.82 deg2, including coverage of the central region but extending mostly south and east of the dSph core. Two methods are used to identify that are most likely associated with the dSph, and these filtered samples of are used to map its spatial structure. First, following the method of previous contributions in this series, we utilize Washington M, T2 + DDO51 photometry to identify red giant branch (RGB) star candidates with approximately the same distance and metallicity as the Sculptor dSph. Second, a prominent blue horizontal branch (BHB) population provides a fairly populous and pure sample of Sculptor having broadband colors unlike the bulk of the Galactic field star population. A spectroscopically observed subset of Sculptor candidate (147 total stars: ~5% of all Sculptor candidates and ~10% of Sculptor giant candidates) yields a systemic heliocentric velocity for the system of vhel = 110.43 ± 0.79 km s-1, in good agreement with previous studies. We also find a global velocity dispersion of σv = 8.8 ± 0.6 km s-1, with slight indications of a rise in the velocity dispersion past ~0.4rlim. These spectra also provide a check on the reliability of our candidate Sculptor giant sample to M ~ 19; 94% of the photometrically selected Sculptor giant star candidates with follow-up spectroscopy are found to be kinematically associated with Sculptor, while 4 out of 10 outside of our Sculptor giant star selection criteria that we tested spectroscopically appear to be velocity members of Sculptor. These percentages are in agreement with results for an additional 22 Sculptor field with radial velocities in the literature. All available velocities show that our methodology for picking Sculptor giants is both reliable and conservative. Thus, these giant star samples should provide a reliable means to explore the structure of the Sculptor dSph. Nevertheless, considerable care has been taken to assess the level of background contamination in our photometric sample to ensure an accurately derived density profile of the Sculptor dSph to large radii. Multiple background assessments verify that we detect a considerable stellar density of Sculptor to the limits of our main survey area for both the RGB and BHB candidate samples. While we find that a King profile of limiting radius rlim = 796 fits the density profile of Sculptor well to ~60', beyond this, we identify a in the profile and a clearly detected population of Sculptor following a Σ ∝ r-2 decline to more than 2rlim. This break population must signify either the presence of an extremely broad distribution of bound halo stars around the Sculptor dSph or the presence of unbound tidal debris. If the latter is true, we determine a fractional mass-loss rate of approximately 0.042 Gyr-1 for the Sculptor dSph. Additional support for the notion that there is tidal disruption comes from the two-dimensional distribution of our Sculptor candidate stars; both the RGB and BHB samples show increasingly elongated isodensity contours with radius that point to an apparent stretching reminiscent of what is seen in models of disrupting satellite galaxies. Finally, we find that RGB that are more likely to be metal-poor (based on their color and magnitude) are significantly less centrally concentrated and therefore constitute the primary contributing stellar population to the likely tidally stripped parts of the dSph.

The discovery of X-ray binaries in the Sculptor Dwarf Spheroidal Galaxy

We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with ergs/sec with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.

Chemistry of Stars in the Sculptor Dwarf Galaxy from VLT-FLAMES

The chemical composition of 91 stars in the Sculptor dwarf spheroidal galaxy is presented as determined from spectra taken with the FLAMES multiobject spectrograph in the Medusa mode. The analysis methods are outlined. The [α/Fe] ratios are shown for Mg, Ca, and Ti, and compared with those of Galactic stars. Heavy element abundance ratios (Y, Ba, and Eu) are also presented. Since the Sculptor dwarf galaxy has had a significantly different star formation history and chemical evolution than the Galaxy, then comparison of Sculptor's metal-poor (old) stars to similar metallicity stars in the Galaxy can be used to discuss galaxy formation scenarios, as well as test some of our fundamental assumptions in stellar nucleosynthesis.

"Sculptor-ing" the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy

We have used high-resolution, high signal-to-noise spectra obtained with the VLT and UVES to determine abundances of 17 elements in 4 red giants in the Sculptor dwarf spheroidal galaxy. Our [Fe/H] values range from --2.10 to --0.97, confirming previous findings of a large metallicity spread. We have combined our data with similar data for five Sculptor giants studied recently to form one of the largest samples of high resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range. These properties allow us to establish trends of [X/Fe] with [Fe/H] for many elements, X. The trends are significantly different from the trends seen in galactic halo and globular cluster stars. We compare our Sculptor sample to their most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between [X/Fe] as seen in Sculptor compared with Galactic field stars indicates that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Sculptor. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. A very high percentage of such heavy element stars are now known in dwarf spheroidals compared to the halo, further mitigating against the formation of the halo from such objects.