ALBERT

Description: We assemble 121 spectroscopically confirmed halo carbon stars, drawn from the literature, exhibiting measurable variability in the Catalina Surveys. We present their periods and amplitudes, which are used to estimate distances from period–luminosity relationships. The location of the carbon stars – and their velocities when available – allow us to trace the streams of the Sagittarius (Sgr) dwarf spheroidal galaxy. These are compared to a canonical numerical simulation of the accretion of Sgr. We find that the data match this model well for heliocentric distances of 15–50 kpc, except for a virtual lack of carbon stars in the trailing arm just north of the Galactic plane, and there is only tentative evidence of the leading arm south of the plane. The majority of the sample can be attributed to the Sgr accretion. We also find groups of carbon stars which are not part of Sgr; most of which are associated with known halo substructures. A few have no obvious attribution and may indicate new substructure. We find evidence that there may be a structure behind the Sgr leading stream apocentre, at ~100 kpc, and a more distant extension to the Pisces Overdensity also at ~100 kpc. We study a further 75 carbon stars for which no good period data could be obtained, and for which NIR magnitudes and colours are used to estimate distances. These data add support for the features found at distances beyond 100 kpc.

Description: We present the first detailed analysis of the East Cloud, a highly disrupted diffuse stellar substructure in the outer halo of M31. The core of the substructure lies at a projected distance of ~100 kpc from the centre of M31 in the outer halo, with possible extensions reaching right into the inner halo. Using Pan-Andromeda Archaeological Survey photometry of red giant branch stars, we measure the distance, metallicity and brightness of the cloud. Using Hubble Space Telescope data, we independently measure the distance and metallicity to the two globular clusters coincident with the East Cloud core, PA-57 and PA-58, and find their distances to be consistent with the cloud. Four further globular clusters coincident with the substructure extensions are identified as potentially associated. Combining the analyses, we determine a distance to the cloud of $814^{+20}_{-9}$  kpc, a metallicity of [Fe/H] = –1.2 ± 0.1, and a brightness of M V = –10.7 ± 0.4 mag. Even allowing for the inclusion of the potential extensions, this accounts for less than 20 per cent of the progenitor luminosity implied by the luminosity–metallicity relation. Using the updated techniques developed for this analysis, we also refine our estimates of the distance and brightness of the South-West Cloud, a separate substructure analysed in the previous work in this series.

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: Using data from the Pan-Andromeda Archaeological Survey (PAndAS), we have discovered four new globular clusters (GCs) associated with the M31 dwarf elliptical (dE) satellites NGC 147 and NGC 185. Three of these are associated with NGC 147 and one with NGC 185. All lie beyond the main optical boundaries of the galaxies and are the most remote clusters yet known in these systems. Radial velocities derived from low-resolution spectra are used to argue that the GCs are bound to the dwarfs and are not part of the M31 halo population. Combining PAndAS with United Kingdom Infrared Telescope (UKIRT)/WFCAM (Wide-Field Camera) data, we present the first homogeneous optical and near-IR photometry for the entire GC systems of these dEs. Colour–colour plots and published colour–metallicity relations are employed to constrain GC ages and metallicities. It is demonstrated that the clusters are in general metal poor ([Fe/H] 〈 –1.25 dex), while the ages are more difficult to constrain. The mean ( V  –  I ) 0 colours of the two GC systems are very similar to those of the GC systems of dEs in the Virgo and Fornax clusters, as well as the extended halo GC population in M31. The new clusters bring the GC-specific frequency ( S N ) to ~9 in NGC 147 and ~5 in NGC 185, consistent with values found for dEs of similar luminosity residing in a range of environments.

Description: We report the discovery of 59 globular clusters (GCs) and two candidate GCs in a search of the halo of M31, primarily via visual inspection of Canada–France–Hawaii Telescope/MegaCam imagery from the Pan-Andromeda Archaeological Survey (PAndAS). The superior quality of these data also allows us to check the classification of remote objects in the Revised Bologna Catalogue (RBC), plus a subset of GC candidates drawn from Sloan Digital Sky Survey (SDSS) imaging. We identify three additional new GCs from the RBC, and confirm the GC nature of 11 SDSS objects (8 of which appear independently in our remote halo catalogue); the remaining 188 candidates across both lists are either foreground stars or background galaxies. Our new catalogue represents the first uniform census of GCs across the M31 halo – we find clusters to the limit of the PAndAS survey area at projected radii of up to R proj  ~ 150 kpc. Tests using artificial clusters reveal that detection incompleteness cuts in at luminosities below M V  = –6.0; our 50 per cent completeness limit is M V   –4.1. We construct a uniform set of PAndAS photometric measurements for all known GCs outside R proj  = 25 kpc, and any new GCs within this radius. With these data, we update results from Huxor et al., investigating the luminosity function (LF), colours and effective radii of M31 GCs with a particular focus on the remote halo. We find that the GCLF is clearly bimodal in the outer halo ( R proj  〉 30 kpc), with the secondary peak at M V  ~ –5.5. We argue that the GCs in this peak have most likely been accreted along with their host dwarf galaxies. Notwithstanding, we also find, as in previous surveys, a substantial number of GCs with above-average luminosity in the outer M31 halo – a population with no clear counterpart in the Milky Way.

Description: The relation between the size and luminosity for both bright and faint early-type galaxies has been repeatedly discussed as a crucial proxy for understanding evolutionary scenarios, as galaxies grow and lose their stellar mass in different physical processes. The class of compact early-type galaxies (cEs) are, however, distinct outliers from this relation and mainly found around massive galaxies in the centres of groups and clusters. The recent discovery of a cE in isolation provided a new opportunity to understand their formation scenario in a different environment. Here, we report the discovery of an isolated cE, CGCG 036–042, using imagery from the Sloan Digital Sky Survey. The CGCG 036–042 has an r -band absolute magnitude ( M r ) of –18.21 mag and a half-light radius ( $R_{\text{e}}$ ) of 559 pc. Interestingly, it possesses a faint outer stellar component, which extends up to 10 kpc from its centre and has a nearly constant r -band surface brightness of ~26 mag arcsec –2 . European Southern Observatory-Very Large Telescope long-slit spectroscopic data reveal that the simple stellar population (SSP) properties are fairly similar to those of previously identified cEs, with an intermediate age of 7.15 ± 1.17 Gyr, a metallicity of –0.18 ± 0.07 dex and a supersolar alpha-element abundance of 0.2 ± 0.05 dex. The SSP metallicity also shows a significant decline outwards from the centre, by 0.5 dex over one half-light radius. We discuss the possible origin of this galaxy and suggest that it might have originated from a merger of even smaller objects – a well-established scenario for the formation of massive early-type galaxies.

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 discovery of an isolated compact elliptical (cE) galaxy, found during a search of SDSS DR7 for cE galaxies, and for which we obtained William Herschel Telescope/ACAM imaging. It is ~900 kpc distant from its nearest neighbour, has an effective r -band radius of ~500 pc and a B -band mean surface brightness within its effective radius of 19.75 mag arcsec –1 . Serendipitous deep SuprimeCam imaging shows that there is no underlying disc. Its isolated position suggests that there is an alternative channel to the stripping scenario for the formation of compact ellipticals. We also report analysis of recent deeper imaging of the previous candidate free-flying cE, which shows that it is, in fact, a normal dwarf elliptical. Hence, the new cE reported here is the first confirmed isolated cE to be found in the field.

Description: We report a new compact galaxy, SDSS J122958.84+138.0 (SDSS J1229+0001), which has unique morphological and stellar population properties that are rare in observations of the nearby Universe. SDSS J1229+0001 has an r -band absolute magnitude ( M r ) and half-light radius ( R h ) of –17.75 mag and 520 pc, respectively. Located in a fairly low-density environment, morphologically it is akin to a typical early-type galaxy as it has a smooth appearance and red colour. However, interestingly, it possesses centrally concentrated star-forming activity with a significant amount of dust. We present an analysis of its structural and stellar population properties using archival images and VLT/FORS2 spectroscopy. An analysis of the UKIDSS H -band image shows that the observed light distribution is better fitted with a two-component Sérsic function with inner and outer component effective radii of 190 and 330 pc, respectively. The overall half-light radius measured in the H band is much smaller compared to optical, i.e. 290 pc. We prepared a spectral energy distribution from optical to far-infrared and interpret it to derive the star-formation rate, dust mass and stellar mass. We find that SDSS J1229+0001 has dust mass M dust = 5.1 x 10 5  M with a dust-to-stellar mass ratio log ( M dust / M * ) = –3.5. While the observed stellar population properties are – to some extent – like those of a typical S0 galaxy, a unified view from stellar population and structural properties may suggest that SDSS J1229+0001 is a smoking gun example of a compact early-type galaxy in formation.

Description: M31 has a giant stream of stars extending far to the south and a great deal of other tidal debris in its halo, much of which is thought to be directly associated with the southern stream. We model this structure by means of Bayesian sampling of parameter space, where each sample uses an N -body simulation of a satellite disrupting in M31's potential. We combine constraints on stellar surface densities from the Isaac Newton Telescope survey of M31 with kinematic data and photometric distances. This combination of data tightly constrains the model, indicating a stellar mass at last pericentric passage of log 10 ( M s /M ) = 9.5 ± 0.1, comparable to the Large Magellanic Cloud. Any existing remnant of the satellite is expected to lie in the Northeast (NE) Shelf region beside M31's disc, at velocities more negative than M31's disc in this region. This rules out the prominent satellites M32 or NGC 205 as the progenitor, but an overdensity recently discovered in M31's NE disc sits at the edge of the progenitor locations found in the model. M31's virial mass is constrained in this model to be log 10 M 200  = 12.3 ± 0.1, alleviating the previous tension between observational virial mass estimates and expectations from the general galactic population and the timing argument. The techniques used in this paper, which should be more generally applicable, are a powerful method of extracting physical inferences from observational data on tidal debris structures.