dSph Galaxies Research Articles

The mass and velocity anisotropy of the Carina, Fornax, Sculptor and Sextans dwarf spheroidal galaxies

Abstract We model the large kinematic data sets for the four Milky Way dwarf spheroidal (dSph) satellites: Carina, Fornax, Sculptor and Sextans, recently published by Walker et al. The member stars are selected using a reliable dynamical interloper removal scheme tested on simulated data. Our member selection is more restrictive than the one based on metallicity indicators as it removes not only contamination due to Milky Way stars but also the unbound stars from the tidal tails. We model the cleaned data sets by adjusting the solutions of the Jeans equations to the profiles of the projected velocity dispersion and kurtosis. The data are well reproduced by models where mass follows light and the best-fitting stellar orbits are isotropic to weakly tangential, as expected from the tidal stirring scenario. The Fornax dwarf, with more than 2400 member stars, is a dSph galaxy with the most accurately determined mass to date: its 1σ error following from the sampling errors of the velocity moments is below 5 per cent. With mass-to-light ratio of 97 solar units, Sextans seems to be the most dark matter dominated of the four dSph galaxies.

Could the Fermi Large Area Telescope detect γ-rays from dark matter annihilation in the dwarf galaxies of the Local Group?

Context. The detection of γ-rays from dark matter (DM) annihilation is among the scientific goals of the Fermi Large Area Telescope (formerly known as GLAST) and Cherenkov telescopes. Aims. In this paper we investigate the chances of such a discovery, selecting some nearby dwarf spheroidal galaxies (dSph) as a target, and adopting the DM density profiles derived from both astronomical observations and N-body simulations. We also make use of recent studies about the presence of black holes and of a population of sub-subhalos inside the Local Group (LG) dwarfs to carry out boost factor studies. Methods. We study the detectability with the Fermi-LAT of the γ-ray flux from DM annihilation in four of the nearest and highly DM-dominated dSph galaxies of the LG, namely Draco, Ursa Minor, Carina, and Sextans, for which state-of-art DM density profiles were available. We assume the DM is made of weakly interacting massive particles such as the lightest supersymmetric particle and compute the expected γ-ray flux for estimations of the unknown underlying particle physics parameters. We then compute the boost factors due to the presence of DM clumps and of a central supermassive black hole. Finally, we compare our predictions with the Fermi-LAT sensitivity maps. Results. We find that the dSph galaxies shine above the Galactic smooth halo: e.g., the Galactic halo is brighter than the Draco dSph only for angles smaller than 2.3 degrees above the Galactic Center. We also find that the presence of a cusp or a constant density core in the DM mass density profile does not produce any relevant effects in the γ-ray flux due to the fortunate combination of the geometrical acceptance of the Fermi-LAT detector and the distance of the galaxies. Moreover, no significant enhancement is given by the presence of a central black hole or a population of sub-subhalos. Conclusions. We conclude that, even for the most optimistic scenario of particle physics, the γ-ray flux from DM annihilation in the dSph galaxies of the LG would be too low to be detected with the Fermi-LAT.

STELLAR VELOCITIES IN THE CARINA, FORNAX, SCULPTOR, AND SEXTANS dSph GALAXIES: DATA FROM THE MAGELLAN/MMFS SURVEY

We present spectroscopic data for individual stars observed from 2004 March through 2008 August as part of our Michigan/MIKE Fiber System (MMFS) survey of four dwarf spheroidal (dSph) galaxies: Carina, Fornax, Sculptor, and Sextans. Using MMFS at the Magellan/Clay Telescope at Las Campanas Observatory, we have acquired 8855 spectra from 7103 red-giant candidates in these Galactic satellites. We list measurements of each star's line-of-sight velocity (median error ±2.1 km s–1) and spectral line indices for iron and magnesium absorption features. We use globular cluster spectra to calibrate the indices onto standard [Fe/H] metallicity scales, but comparison of the resulting metallicities with published values suggests that the MMFS indices are best used as indicators of relative, not absolute, metallicity. The empirical distributions of velocity and spectral indices also allow us to quantify the amount of contamination by foreground stars. In a companion paper, we develop an algorithm that evaluates the membership probability for each star, showing that the present MMFS sample contains more than 5000 dSph members, including 774 Carina members, 2483 Fornax members, 1365 Sculptor members, and 441 Sextans members.

CLEAN KINEMATIC SAMPLES IN DWARF SPHEROIDALS: AN ALGORITHM FOR EVALUATING MEMBERSHIP AND ESTIMATING DISTRIBUTION PARAMETERS WHEN CONTAMINATION IS PRESENT

We develop an algorithm for estimating parameters of a distribution sampled with contamination. We employ a statistical technique known as expectation maximization (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to update parameter estimates for member and contaminant distributions. The EM approach has wide applicability to the analysis of astronomical data. Here we tailor an EM algorithm to operate on spectroscopic samples obtained with the Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples, to be presented in a companion paper, contain discrete measurements of line-of-sight velocity, projected position, and pseudo-equivalent width of the Mg-triplet feature, for ~1000-2500 stars per dSph, including some fraction of contamination by foreground Milky Way stars. The EM algorithm uses all of the available data to quantify dSph and contaminant distributions. For distributions (e.g., velocity and Mg-index of dSph stars) assumed to be Gaussian, the EM algorithm returns maximum-likelihood estimates of the mean and variance, as well as the probability that each star is a dSph member. These probabilities can serve as weights in subsequent analyses. Applied to our MMFS data, the EM algorithm identifies more than 5000 stars as probable dSph members. We test the performance of the EM algorithm on simulated data sets that represent a range of sample size, level of contamination, and amount of overlap between dSph and contaminant velocity distributions. The simulations establish that for samples ranging from large (N ~ 3000, characteristic of the MMFS samples) to small (N ~ 30), resembling new samples for extremely faint dSphs), the EM algorithm distinguishes members from contaminants and returns accurate parameter estimates much more reliably than conventional methods of contaminant removal (e.g., sigma clipping).

CHEMICAL ABUNDANCES OF THE LEO II DWARF GALAXY

We use previously published moderate-resolution spectra in combination with stellar atmosphere models to derive the first measured chemical abundance ratios in the Leo II dwarf Spheroidal (dSph) galaxy. We find that for spectra with signal-to-noise ratio greater than 24, we are able to measure abundances from weak Ti, Fe, and Mg lines located near the calcium infrared triplet (CaT). We also quantify and discuss discrepancies between the metallicities measured from Fe i lines and those estimated from the CaT features. We find that while the most metal-poor ([Fe/H] <−2.0]) Leo II stars have Ca and Ti abundance ratios similar to those of Galactic globular clusters, the more metal-rich stars show a gradual decline of Ti, Mg, and Ca abundance ratio with increasing metallicity. Finding these trends in this distant and apparently dynamically stable dSph galaxy supports the hypothesis that the slow chemical enrichment histories of the dSph galaxies is universal, independent of any interaction with the Milky Way. Combining our spectroscopic abundances with published broadband photometry and updated isochrones, we are able to approximate stellar ages for our bright red giant branch stars to a relative precision of 2–3 Gyr. While the derived age-metallicity relationship of Leo II hints at some amount of slow enrichment, the data are still statistically consistent with no enrichment over the history of Leo II.

A SPECTROSCOPIC CONFIRMATION OF THE BOOTES II DWARF SPHEROIDAL

We present a new suite of photometric and spectroscopic data for the faint Bootes II dwarf spheroidal galaxy (dSph) candidate. Our deep photometry, obtained with the Isaac Newton Telescope/Wide Field Camera, suggests a distance of 46 kpc and a small half-light radius of 4.0' (56 pc), consistent with previous estimates. Follow-up spectroscopy obtained with the Gemini/GMOS instrument yielded radial velocities and metallicities. While the majority of our targets covers a broad range in velocities and metallicities, we find five stars that share very similar velocities and metallicities and that are all compatible with the colors and magnitudes of the galaxy's likely red giant branch. We interpret these as a spectroscopic detection of the Bootes II system. These stars have a mean velocity of −117 km s−1, a velocity dispersion of (10.5 ± 7.4) km s−1, and a mean [Fe/H] of −1.79 dex, with a dispersion of 0.14 dex. At this metallicity, Boo II is not consistent with the stellar-mass–metallicity relation for the more luminous dwarf galaxies. Coupled with our distance estimate, its high negative systemic velocity rules out any physical connection with its projected neighbor, the Bootes I dwarf spheroidal, which has a velocity of ∼+100 km s−1. The velocity and distance of Bootes II coincide with those of the leading arm of Sagittarius, which passes through this region of the sky, so that it is possible that Bootes II may be a stellar system associated with the Sagittarius stream. Finally, we note that the properties of Bootes II are consistent with it being the surviving remnant of a previously larger and more luminous dSph galaxy.

Constraints on the chemical yields of the first stars

Constraints on the chemical yields of the first stars and supernova can be derived by examining the abundance patterns of different types of metal-poor stars. The concept of stellar archaeology, that stellar abundances reflect the chemical composition of the earliest times, is a vital assumption. The accretion history of a sample of metal-poor stars has been re-examined in a cosmological context and found to have no impact on the observed abundances. Predictions are made for the lowest possible Fe and Mg abundances observable in the Galaxy, [Fe/H]min=- 7.3 and [Mg/H]min=- 6.0. The absence of stars below these values is so far consistent with a top-heavy initial mass function (IMF). These predictions are directly relevant for future surveys and the next generation of telescopes. The most Fe-poor stars, such as HE 1327–2326, are used to constrain the chemical yields of 25 to 60 M⊙ SNe. The nucleosynthetic yields of 8–10 M⊙ SNe can be probed with r-process-enhanced metal-poor stars, such as HE 1523–0901, which display large amounts of neutron-capture elements. Constraints on the chemical evolution of faint dSph galaxies can soon be obtained from detailed analyses of newly discovered extremely metal-poor stars in several systems.

The Abundance Spread in the Boötes I Dwarf Spheroidal Galaxy

We present medium-resolution spectra of 16 radial velocity red-giant members of the low-luminosity Bootes I dwarf spheroidal (dSph) galaxy, that have sufficient S/N for abundance determination, based on the strength of the Ca II K line. Assuming [Ca/Fe] ~ +0.3, the abundance range in the sample is Delta [Fe/H] ~ 1.7 dex, with one star having [Fe/H] = -3.4. The dispersion is sigma([Fe/H]) = 0.45 +/- 0.08 -- similar to those of the Galaxy's more luminous dSph systems and Omega Centauri. This suggests that the large mass (greater than approximately 10 million solar masses) normally assumed to foster self-enrichment and the production of chemical abundance spreads was provided by the non-baryonic material in Bootes I.

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 Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy

We report on the abundance analysis of two red giants in the faint Hercules dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of <-2 dex and ~+0.8 dex, respectively, with essentially no detection of other n-capture elements. In contrast to the only other dSph star with similar abundance patterns, Dra 119, which has a very low metallicity at [Fe/H]=-2.95 dex, our objects, at [Fe/H]~-2.0 dex, are only moderately metal poor. The measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass (~35 M_sun) Type II supernovae progenitors are the main, if not only, contributors to the enrichment of this galaxy. This suggests that star formation and chemical enrichment in the ultrafaint dSphs proceeds stochastically and inhomogeneously on small scales, or that the IMF was strongly skewed to high mass stars. The neutron capture deficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the extremely low metallicity Galactic halo. This suggests that either our stars are composed mainly of the ejecta from the first, massive, population III stars (but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with ~30 times less hydrogen than typical for extreme metal-poor stars.

A Deep Survey of the Fornax dSph. I. Star Formation History

Based on a deep imaging survey, we present the first homogeneous star formation history (SFH) of the Fornax dwarf spheroidal (dSph) galaxy. We have obtained two-filter photometry to a depth of B ~ 23 over the entire surface of Fornax, the brightest dSph associated with the Milky Way, and derived its SFH using a CMD-fitting technique. We show that Fornax has produced the most complex star formation and chemical enrichment histories of all the Milky Way dSphs. This system has supported multiple epochs of star formation. A significant number of stars were formed in the early Universe, however the most dominant population are the intermediate age stars. This includes a strong burst of star formation approximately 3 to 4 Gyr ago. Significant population gradients are also evident. Similar to other dSphs, we have found that recent star formation was concentrated towards the centre of the system. Furthermore, we show that the central region harboured a faster rate of chemical enrichment than the outer parts of Fornax. At the centre, the ancient stars (age > 10 Gyr) display a mean metallicity of [Fe/H] ~ -1.4, with evidence for three peaks in the metallicity distribution. Overall, enrichment in Fornax has been highly efficient: the most recent star formation burst has produced stars with close to solar metallicity. Our results support a scenario in which Fornax experienced an early phase of rapid chemical enrichment producing a wide range of abundances. Star formation gradually decreased until ~4 Gyr ago, when Fornax experienced a sudden burst of strong star formation activity accompanied by substantial chemical enrichment. Weaker star forming events followed, and we have found tentative evidence for stars with ages less than 100 Myr.

Chemical consequences of low star formation rates: stochastically sampling the initial mass function

When estimating the abundances which result from a given star formation event, it is customary to treat the initial mass function (IMF) as a series of weight factors to be applied to the stellar yields, as a function of mass, implicitly assuming one is dealing with an infinite population. However, when the stellar population is small, the standard procedure would imply the inclusion of fractional numbers of stars at certain masses. We study the effects of small number statistics on the resulting abundances by performing a statistical sampling of the IMF to form a stellar population out of discrete numbers of stars. A chemical evolution code then follows the evolution of the population, and traces the resulting abundances. The process is repeated to obtain a statistical distribution of the resulting abundances and their evolution. We explore the manner in which different elements are affected, and how different abundances converge to the infinite population limit as the total mass increases. We include a discussion of our results in the context of dwarf spheroidal galaxies and show the recently reported internal dispersions in abundance ratios for dSph galaxies might be partly explained through the stochastic effects introduced by a low star formation rate, which can account for dispersions of over 2 dex in [C/O], [N/O], [C/Fe], [N/Fe] and [O/Fe].

Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies

We present new metallicity measurements for 298 individual red giant branch stars in eight of the least luminous dwarf spheroidal galaxies (dSphs) in the Milky Way (MW) system. Our technique is based on medium resolution Keck/DEIMOS spectroscopy coupled with spectral synthesis. We present the first spectroscopic metallicities at [Fe/H] < -3.0 of stars in a dwarf galaxy, with individual stellar metallicities as low as [Fe/H] = -3.3. Because our [Fe/H] measurements are not tied to empirical metallicity calibrators and are sensitive to arbitrarily low metallicities, we are able to probe this extremely metal-poor regime accurately. The metallicity distribution of stars in these dSphs is similar to the MW halo at the metal-poor end. We also demonstrate that the luminosity-metallicity relation previously seen in more luminous dSph galaxies (M_V = -13.4 to -8.8) extends smoothly down to an absolute magnitude of M_V = -3.7. The discovery of extremely metal-poor stars in dSphs lends support to the LCDM galaxy assembly paradigm wherein dwarf galaxies dissolve to form the stellar halo of the MW.

The evolved stars of Leo II dSph galaxy from near-infrared UKIRT/WFCAM observations

Abstract We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Leo II, based on JHKs observations obtained with the near-infrared array WFCAM at the UKIRT telescope. Combining the new data with optical data, we derived photometric estimates of the distribution of global metallicity [M/H] of individual red giant stars from their V−Ks colours. Our results are consistent with the metallicities of red giant branch (RGB) stars obtained from Ca ii triplet spectroscopy, once the age effects are considered. The photometric metallicity distribution function has a peak at [M/H]=−1.74 (uncorrected) or [M/H]=−1.64 ± 0.06 (random) ±0.17 (systematic) after correction for the mean age of Leo II stars (9 Gyr). The distribution is similar to a Gaussian with σ[M/H]= 0.19 dex, corrected for instrumental errors. We used the new data to derive the properties of a nearly complete sample of asymptotic giant branch (AGB) stars in Leo II. Using a near-infrared two-colour diagram, we were able to obtain a clean separation from Milky Way foreground stars and discriminate between carbon- and oxygen-rich AGB stars, which allowed us to study their distribution in Ks-band luminosity and colour. We simulate the JHKs data with the trilegal population synthesis code together with the most updated thermally pulsing AGB models, and using the star formation histories derived from independent work based on deep Hubble Space Telescope photometry. After scaling the mass of Leo II models to the observed number of upper RGB stars, we find that present models predict too many O-rich thermally pulsing AGB (TP-AGB) stars of higher luminosity due to a likely underestimation of either their mass-loss rates at low metallicity, and/or their degree of obscuration by circumstellar dust. On the other hand, the TP-AGB models are able to reproduce the observed number and luminosities of carbon stars satisfactorily well, indicating that in this galaxy the least massive stars that became carbon stars should have masses as low as ∼1 M⊙.

The metallicity extremes of the Sagittarius dSph: SALT spectroscopy of PNe★

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) contains a youngest stellar population with [O/H] ≈ -0.2 and age t > 1 Gyr, and an oldest population with [O/H] = -2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12 + log(O/H) = 8.57 ± 0.02 dex. The other PN studied, BoBn 1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn 1 yield the original stellar metallicity, corresponding to 12 + log(O/H) = 6.72 ± 0.16 dex which is 1/110 of the solar value. The actual [O/H] is much higher: third dredge-up enriched the material by a factor of ∼12 in oxygen, ∼240 in nitrogen and ∼70 in neon. Neon as well as nitrogen and oxygen content may have been produced in the intershell of low-mass asymptotic giant branch (AGB) stars. Well defined broad WR lines are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn 1. This puts the fraction of [WR]-type central PNe stars to 67 per cent for dSph galaxies.

Proper Motion of Milky Way Dwarf Spheroidals from Line-of-Sight Velocities

Proper motions for several Milky Way dwarf spheroidal (dSph) galaxies have been determined using both ground- and space-based imaging. These measurements require long baselines and repeat observations, and typical errors are of order 10 mas century−1. In this Letter, we utilize the effect of perspective to show that the systematic proper motion of some dSphs can be determined to a similar precision using only stellar line-of-sight velocities. We show that including the effects of small intrinsic rotation in dSphs increases the proper motion errors by about a factor of 2. We provide error projections for future data sets and show that proposed 30 m class telescopes will measure the proper motion of a few dSphs with mas century−1 precision.

The anatomy of Leo I: how tidal tails affect the kinematics

We model the recently published kinematic data set for Leo I dwarf spheroidal (dSph) galaxy by fitting the solutions of the Jeans equations to the velocity dispersion and kurtosis profiles measured from the data. We demonstrate that when the sample is cleaned of interlopers the data are consistent with the assumption that mass follows light and isotropic stellar orbits with no need for an extended dark matter halo. Our interloper removal scheme does not clean the data of contamination completely, as demonstrated by the rotation curve of Leo I. When moving away from the centre of the dwarf, the rotation appears to be reversed. We interpret this behaviour using the results of an N-body simulation of a dwarf galaxy possessing some intrinsic rotation, orbiting in the Milky Way potential and show that it can be reproduced if the galaxy is viewed almost along the tidal tails so that the leading (background) tail contaminates the western part of Leo I while the trailing (foreground) tail the eastern one. We show that this configuration leads to a symmetric and Gaussian distribution of line-of-sight velocities. The simulation is also applied to test our modelling method on mock data sets. We demonstrate that when the data are cleaned of interlopers and the fourth velocity moment is used the true parameters of the dwarf are typically within 1σ errors of the best-fitting parameters. Restricting the fitting to the inner part of Leo I our best estimate for the anisotropy is β= -0.2 +0.3 -0.4 and the total mass M = (4.5 ± 0.7) x 10 7 M ⊙ . The mass-to-light ratio (M/L) including the errors in mass, brightness and distance is M/L V = 8.2 ± 4.5 solar units.

SPECTROSCOPY OF CARBON STARS IN THE DRACO AND URSA MINOR DWARF SPHEROIDAL GALAXIES

With the ISIS spectrograph on the William Herschel Telescope, we obtained intermediate-resolution optical spectra in three and five carbon stars belonging to the dwarf spheroidal (dSph) galaxies Draco and Ursa Minor, respectively. The metallicity, carbon isotopic ratios, and high-mass s-element abundances were determined by spectral synthesis in LTE using appropriate spherically symmetric, carbon-rich atmosphere models. The infrared colors and derived luminosities suggest that these stars are equivalent to the classical CH-type stars found in the halo of the Milky Way, although the evidence of luminosity variations in the stars Draco 461 and Draco 20733 may be compatible with these being carbon-rich low-mass asymptotic giant branch stars. The derived overall metallicity in the stellar sample ([M/H] ~–2.0) agrees with the average metallicity of the main stellar component in these dSphs obtained by previous studies. The C/O and 12C/13C ratios, and the average large heavy-element (Ba, La, Nd, Sm) enhancements derived ([hs/M] ≥1) are also similar to the values found in galactic CH-type stars at the same stellar metallicity. Although this average excess in heavy elements can be explained by standard s-process nucleosynthesis models, in two stars of Ursa Minor there is a suggestion that their heavy-element abundance pattern bears a closer resemblance to the scaled solar system r-process than the s-process abundance curve. If this is confirmed, these stars would represent an extragalactic example of the s + r carbon-rich (binary) stars found in the galactic halo. This r-process like abundance pattern has been found previously in other red giant stars belonging to Ursa Minor, suggesting a peculiar chemical evolution history in this dSph galaxy.

Comparing CN and CH line strengths in a homogeneous spectroscopic sample of 8 Galactic globular clusters

Our work focuses on the understanding of the origin of CNO-anomalies, which have been detected in several Galactic globular clusters (GCs). This study is based on a homogeneous data set of hundreds of medium resolution spectra of stars in eight GCs. Two of the clusters are believed to be former members of the Sagittarius dSph galaxy. Our sample comprises stars in different evolutionary states, namely the main sequence turn-off (MSTO) region, the subgiant branch (SGB), and the base of the red giant branch (RGB). We compare the relative CN and CH line strengths of stars in the same evolutionary states. The majority of the examined clusters show significant variations in their CN and CH abundances at the base of the RGB. The two former Sgr dSph clusters do not exhibit any CN-strong stars. Our results suggest that the environment in which the clusters influences existence of CN-strong stars. We confirm the known anticorrelation between CN and CH for most of the observed GCs. Although the signal of CN absorption is weaker for the hotter stars on the MSTO and SGB we observed the same anticorrelation in these less evolved stars for the CN-bimodal clusters. The inclusion of structural parameters taken from literature reveals that the existence of the CN-bifurcation seems to be independent of most other cluster characteristics. In particular, we do not confirm the correlation between cluster ellipticity and number of CN-strong stars. However, there may be a trend of an increased percentage of CN-strong stars with increasing cluster tidal radius and total luminosity. We argue that our findings are consistent with pollution by intermediate AGB stars and/or fast rotating massive stars and two generations of star formation in luminous clusters with larger tidal radii at larger Galactocentric distances.

TRIMMING DOWN THE WILLMAN 1 dSph

Willman 1 is a small low-surface-brightness object identified in the Sloan Digital Sky Survey and tentatively classified as a very low luminosity dSph galaxy. Further study has supported this classification while hinting that it may be undergoing disruption by the Milky Way potential. In an effort to better constrain the nature of Willman 1, we present a comprehensive analysis of the brightest stars in a 0.6 deg2 field centered on the overdensity. High-resolution Hobby-Eberly Terlescope (HET) spectra of two previously identified Willman 1 red giant branch (RGB) stars show that one is a metal-rich foreground dwarf while the other is a metal-poor giant. The one RGB star that we confirm as a member of Willman 1 has a low metallicity ([Fe/H] = −2.2) and a surprisingly low α-element abundance ([α/Fe] = −0.11). Washington+DDO51 photometry indicates that 2–5 of the seven brightest Willman 1 stars identified in previous studies are actually dwarf stars, including some of the more metal-rich stars that have been used to argue both for an abundance spread and a more metal-rich stellar population than galaxies of similar luminosity. The remaining stars are too blue or too faint for photometric classification. The Washington+DDO51 photometry identifies three potential RGB stars in the field but HET spectra show that they are background halo stars. Time series photometry identifies one apparent variable star in the field, but it is unlikely to be associated with Willman 1. Our wide-field survey indicates that over 0.6 deg2, Willman 1 does not have a single RR Lyrae star, a single blue horizontal branch (BHB) star, or a single RGB star beyond its tidal radius. While our results confirm that Willman 1 is most likely a low-luminosity metal-poor dSph galaxy, the possibility remains that it is a tidally disrupted metal-poor globular cluster.