ALBERT

Description: We present the metallicity distribution of a sample of 471 RR Lyrae (RRL) stars in the Sculptor dSph, obtained from the I -band period–luminosity relation. It is the first time that the early chemical evolution of a dwarf galaxy is characterized in such a detailed and quantitative way, using photometric data alone. We find a broad metallicity distribution (full width at half-maximum equals to 0.8 dex) that is peaked at [Fe/H] ~= –1.90 dex, in excellent agreement with literature values obtained from spectroscopic data. Moreover, we are able to directly trace the metallicity gradient out to a radius of ~55 arcmin. We find that in the outer regions ( r 〉 ~32 arcmin) the slope of the metallicity gradient from the RRLs (–0.025 dex arcmin –1 ) is comparable to the literature values based on red giant (RG) stars. However, in the central part of Sculptor, we do not observe the latter gradients. This suggests that there is a more metal-rich and/or younger population in Sculptor that does not produce RRLs. This scenario is strengthened by the observation of a metal-rich peak in the metallicity distribution of RG stars by other authors, which is not present in the metallicity distribution of the RRLs within the same central area.

Description: While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep Hubble Space Telescope /Advanced Camera for Surveys data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour–magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of ‘disc-like’ fields indicate that most of their mass has formed since z  ~ 1, with one quarter of the mass formed in the last 5 Gyr. We find ‘stream-like’ fields to be on average 1.5 Gyr older, with 10 per cent of their stellar mass formed within the last 5 Gyr. These fields are also characterized by an age–metallicity relation showing rapid chemical enrichment to solar metallicity by z  = 1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from the thin disc through disc heating in the recent past.

Description: We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for six GCs, two of which had no previous spectroscopic information. We construct optical-near-IR colour–colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between –1.6  [Fe/H]  –0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is 10 km s –1 , approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within ~11 kpc and we formally find it to be in the range between (3 and 4)  x  10 9 M . This implies an overall mass-to-light ratio in the range of ~30–40 and indicates that NGC 6822 is highly dark-matter-dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties.

Description: We identified and characterized the largest (536) RR Lyrae (RRL) sample in a Milky Way dSph satellite (Sculptor) based on optical photometry data collected over ~24 years. The RRLs display a spread in V -magnitude (~0.35 mag) which appears larger than photometric errors and the horizontal branch (HB) luminosity evolution of a mono-metallic population. Using several calibrations of two different reddening free and metal independent period–Wesenheit relations we provide a new distance estimate μ = 19.62 mag ( μ  = 0.04 mag) that agrees well with literature estimates. We constrained the metallicity distribution of the old population, using the M I period–luminosity relation, and we found that it ranges from –2.3 to –1.5 dex. The current estimate is narrower than suggested by low and intermediate spectroscopy of RGBs ( [Fe/H]≤ 1.5). We also investigated the HB morphology as a function of the galactocentric distance. The HB in the innermost regions is dominated by red HB stars and by RRLs, consistent with a more metal-rich population, while in the outermost regions it is dominated by blue HB stars and RRLs typical of a metal-poor population. Our results suggest that fast chemical evolution occurred in Sculptor, and that the radial gradients were in place at an early epoch.

Description: We exploit data from the Pan-Andromeda Archaeological Survey (PAndAS) to study the extended structures of M31's dwarf elliptical companions, NGC 147 and NGC 185. Our wide-field, homogeneous photometry allows us to construct deep colour–magnitude diagrams which reach down to ~3 mag below the red giant branch (RGB) tip. We trace the stellar components of the galaxies to surface brightness of μ g  ~ 32 mag arcsec –2 and show that they have much larger extents (~5 kpc radii) than previously recognized. While NGC 185 retains a regular shape in its peripheral regions, NGC 147 exhibits pronounced isophotal twisting due to the emergence of symmetric tidal tails. We fit single Sérsic models to composite surface brightness profiles constructed from diffuse light and star counts and find that NGC 147 has an effective radius almost three times that of NGC 185. In both cases, the effective radii that we calculate are larger by a factor of ~2 compared to most literature values. We also calculate revised total magnitudes of M g  = –15.36 ± 0.04 for NGC 185 and M g  = –16.36 ± 0.04 for NGC 147. Using photometric metallicities computed for RGB stars, we find NGC 185 to exhibit a metallicity gradient of [Fe/H] ~ –0.15 dex kpc –1 over the radial range 0.125–0.5 deg. On the other hand, NGC 147 exhibits almost no metallicity gradient, ~–0.02 dex kpc –1 from 0.2 to 0.6 deg. The differences in the structure and stellar populations in the outskirts of these systems suggest that tidal influences have played an important role in governing the evolution of NGC 147.

Description: We present a detailed kinematic analysis of the outer halo globular cluster system of the Andromeda galaxy (M31). Our basis for this is a set of new spectroscopic observations for 78 clusters lying at projected distances between R proj  ~ 20–140 kpc from the M31 centre. These are largely drawn from the recent Pan-Andromeda Archaeological Survey globular cluster catalogue; 63 of our targets have no previous velocity data. Via a Bayesian maximum likelihood analysis, we find that globular clusters with R proj  〉 30 kpc exhibit coherent rotation around the minor optical axis of M31, in the same direction as more centrally located globular clusters, but with a smaller amplitude of 86 ± 17 km s –1 . There is also evidence that the velocity dispersion of the outer halo globular cluster system decreases as a function of projected distance from the M31 centre, and that this relation can be well described by a power law of index –0.5. The velocity dispersion profile of the outer halo globular clusters is quite similar to that of the halo stars, at least out to the radius up to which there is available information on the stellar kinematics. We detect and discuss various velocity correlations amongst subgroups of globular clusters that lie on stellar debris streams in the M31 halo. Many of these subgroups are dynamically cold, exhibiting internal velocity dispersions consistent with zero. Simple Monte Carlo experiments imply that such configurations are unlikely to form by chance, adding weight to the notion that a significant fraction of the outer halo globular clusters in M31 have been accreted alongside their parent dwarf galaxies. We also estimate the M31 mass within 200 kpc via the Tracer Mass Estimator (TME), finding (1.2–1.6) ± 0.2 10 12 M . This quantity is subject to additional systematic effects due to various limitations of the data, and assumptions built in into the TME. Finally, we discuss our results in the context of formation scenarios for the M31 halo.

Description: We present the fiducial sequences of a sample of Galactic star clusters in the five bands of the Pan-STARRS1 (PS1) photometric system ( g P1 , r P1 , i P1 , z P1 , and y P1 ). These empirical sequences – which include the red giant and sub-giant branches, the main sequence, and the horizontal branch – were defined from deep colour–magnitude diagrams reaching below the oldest main-sequence turn-offs of 13 globular and 3 old open clusters covering a wide range of metallicities ( – 2.4 [Fe/H] +0.4). We find excellent agreement for the nine clusters in common with previous studies in similar photometric systems when transformed to the PS1 system. Because the photometric and spectroscopic properties of these stellar populations are accurately known, the fiducials provide a solid basis for the interpretation of observations in the PS1 system, as well as valuable constraints to improve the empirical colour– T eff relations.

Description: We present the first survey of resolved stellar populations in the remote outer halo of our nearest giant elliptical (gE), Centaurus A ( D  = 3.8 Mpc). Using the VIsible Multi Object Spectrograph (VIMOS)/Very Large Telescope (VLT) optical camera, we obtained deep photometry for four fields along the major and minor axes at projected elliptical radii of ~30–85 kpc (corresponding to ~5–14 R eff ). We use resolved star counts to map the spatial and colour distribution of red giant branch (RGB) stars down to ~2 mag below the RGB tip. We detect an extended halo out to the furthermost elliptical radius probed (~85 kpc or ~14 R eff ), demonstrating the vast extent of this system. We detect a localized substructure in these parts, visible in both (old) RGB and (intermediate-age) luminous asymptotic giant branch stars, and there is some evidence that the outer halo becomes more elliptical and has a shallower surface brightness profile. We derive photometric metallicity distribution functions for halo RGB stars and find relatively high median metallicity values (〈[Fe/H]〉 med  ~ –0.9 to –1.0 dex) that change very little with radius over the extent of our survey. Radial metallicity gradients are measured to be –0.002-0.004 dex kpc –1 , and the fraction of metal-poor stars (defined as [Fe/H] 〈 –1.0) is 40–50 per cent at all radii. We discuss these findings in the context of galaxy formation models for the buildup of gE haloes.

Description: We present the identification of 634 variable stars in the Milky Way dwarf spheroidal (dSph) satellite Sculptor based on archival ground-based optical observations spanning ~24 yr and covering ~2.5 deg 2 . We employed the same methodologies as the ‘Homogeneous Photometry’ series published by Stetson. In particular, we have identified and characterized one of the largest (536) RR Lyrae samples so far in a Milky Way dSph satellite. We have also detected four Anomalous Cepheids, 23 SX Phoenicis stars, five eclipsing binaries, three field variable stars, three peculiar variable stars located above the horizontal branch – near to the locus of BL Herculis – that we are unable to classify properly. Additionally, we identify 37 long period variables plus 23 probable variable stars, for which the current data do not allow us to determine the period. We report positions and finding charts for all the variable stars, and basic properties (period, amplitude, mean magnitude) and light curves for 574 of them. We discuss the properties of the RR Lyrae stars in the Bailey diagram, which supports the coexistence of subpopulations with different chemical compositions. We estimate the mean mass of Anomalous Cepheids (~1.5 M ) and SX Phoenicis stars (~1 M ). We discuss in detail the nature of the former. The connections between the properties of the different families of variable stars are discussed in the context of the star formation history of the Sculptor dSph galaxy.

Description: We present a panoramic map of the entire Milky Way halo north of $\delta \sim -30{^\circ }$ (~30 000 deg 2 ), constructed by applying the matched-filter technique to the Pan-STARRS1 3 Survey data set. Using single-epoch photometry reaching to g ~22, we are sensitive to stellar substructures with heliocentric distances between 3.5 and ~35 kpc. We recover almost all previously reported streams in this volume and demonstrate that several of these are significantly more extended than earlier data sets have indicated. In addition, we also report five new candidate stellar streams. One of these features appears significantly broader and more luminous than the others and is likely the remnant of a dwarf galaxy. The other four streams are consistent with a globular cluster origin, and three of these are rather short in projection ( $\lesssim\!\! 10{^\circ }$ ), suggesting that streams like Ophiuchus may not be that rare. Finally, a significant number of more marginal substructures are also revealed by our analysis; many of these features can also be discerned in matched-filter maps produced by other authors from SDSS data, and hence they are very likely to be genuine. However, the extant 3 data is currently too shallow to determine their properties or produce convincing colour–magnitude diagrams. The global view of the Milky Way provided by Pan-STARRS1 provides further evidence for the important role of both globular cluster disruption and dwarf galaxy accretion in building the Milky Way's stellar halo.