We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance
of $sim$ 170 kpc. Its half-light radius is $r_h=30$ pc and its luminosity is $M_V=-5.5$. The bulk of its stellar population is old and metal-poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of Blue Loop stars and a sparsely populated Red Clump. The existence of the core helium burning population implies that star-formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star-formation by several Gyrs. Therefore, if our hypothesis that the blue bright stars in Crater are Blue Loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only ten degrees to the North of Crater, two ultra-faint galaxies Leo IV and V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.
We present the first deep survey of resolved stellar populations in the remote outer halo of our nearest giant elliptical (gE), Centaurus A (D=3.8 Mpc). Using the VIMOS/VLT optical camera, we obtained deep photometry for four fields along the major a
nd 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 magnitudes 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 localised 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 to -0.004 dex/kpc and the fraction of metal-poor stars (defined as [Fe/H]<-1.0) is ~40-50% at all radii. We discuss these findings in the context of galaxy formation models for the buildup of gE haloes.
In this paper we describe the photometric calibration of data taken with the near-infrared Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope (UKIRT). The broadband ZYJHK data are directly calibrated from 2MASS point sources which are
abundant in every WFCAM pointing. We perform an analysis of spatial systematics in the photometric calibration, both inter- and intra-detector and show that these are present at up to the 5 per cent level in WFCAM. Although the causes of these systematics are not yet fully understood, a method for their removal is developed and tested. Following application of the correction procedure the photometric calibration of WFCAM is found to be accurate to approximately 1.5 per cent for the JHK bands and 2 per cent for the ZY bands, meeting the survey requirements. We investigate the transformations between the 2MASS and WFCAM systems and find that the Z and Y calibration is sensitive to the effects of interstellar reddening for large values of E(B-V), but that the JHK filters remain largely unaffected. We measure a small correction to the WFCAM Y-band photometry required to place WFCAM on a Vega system, and investigate WFCAM measurements of published standard stars from the list of UKIRT faint standards. Finally we present empirically determined throughput measurements for WFCAM.
Stellar photometry derived from the INT/WFC Photometric H$alpha$ Survey of the Northern Galactic Plane (IPHAS) can be used to identify large, reliable samples of A0-A5 dwarfs. For every A star, so identified, it is also possible to derive individual
reddening and distance estimates, under the assumption that most selected objects are on or near the main sequence, at a mean absolute r magnitude of 1.5 -- 1.6. This study presents the method for obtaining such samples and shows that the known reddenings and distances to the open clusters NGC 7510 and NGC 7790 are successfully recovered. A sample of over 1000 A stars is then obtained from IPHAS data in the magnitude range 13.5 < r < 20 from the region of sky including the massive northern OB association Cyg OB2. Analysis of these data reveals a concentration of ~200 A stars over an area about a degree across, offset mainly to the south of the known 1--3 Myr old OB stars in Cyg OB2: their dereddened r magnitudes fall in the range 11.8 to 12.5. These are consistent with a ~7 Myr old stellar population at DM = 10.8, or with an age of ~5 Myr at DM = 11.2. The number of A stars found in this clustering alone is consistent with a lower limit to the cluster mass of ~10000 M-sun.
