Showing papers by "Mike Irwin published in 2009"

The remnants of galaxy formation from a panoramic survey of the region around M31

Abstract: In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than M(v) = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

The UKIRT wide field camera ZYJHK photometric system: calibration from 2MASS

Abstract: 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 broad-band ZYJHK data are directly calibrated from Two-Micron All-Sky Survey (2MASS) point sources which are abundant in every WFCAM pointing. We perform an analysis of spatial systematics in the photometric calibration, both inter- and intradetector 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 the application of the correction procedure, the photometric calibration of WFCAM is found to be accurate to ≃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 calibrations are 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.

Substructure revealed by RR Lyraes in SDSS Stripe 82

Abstract: We present an analysis of the substructure revealed by RR Lyraes in Sloan Digital Sky Survey Stripe 82, which covers 2°5 in declination on the celestial equator over the right ascension range α = 20 h 7 to 3 h 3 We use the new public archive of light-motion curves in Stripe 82, published by Bramich et al in 2008, to identify a set of high-quality RR Lyrae candidates Period estimates are determined to high accuracy using a string-length method A subset of 178 RR Lyraes with spectrally derived metallicities are employed to derive metallicity-period-amplitude relations, which are then used, together with archive magnitude data and light-curve Fourier decomposition, to estimate metallicities and hence distances for the entire sample The RR Lyraes lie 5-115 kpc from the Galactic Centre, with distance estimates accurate to ∼8 per cent The RR Lyraes are further divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity We fit a smooth density law to the distribution as a simple representation of the data For Galactocentric radii 5-25 kpc the number density of RR Lyraes falls as r ―2,4 , but beyond 25 kpc, the number density falls much more steeply, as r ―45 However, we stress that in practice the density distribution is not smooth, but dominated by clumps and substructure Samples of 55 and 237 RR Lyraes associated with the Sagittarius Stream and the Hercules-Aquila Cloud, respectively, are identified Hence, ∼70 per cent of the RR Lyraes in Stripe 82 belong to known substructure, and the sharp break in the density law reflects the fact that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the Sagittarius Stream -lie within 40 kpc In fact, almost 60 per cent of all the RR Lyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which emphasizes the cloud's pre-eminence Additionally, evidence of a new and distant substructure - the Pisces Overdensity ― is found, consisting of 28 faint RRLyraes centred on Galactic coordinates (l ≈ 80°, b ≈ -55°), with distances of ∼80 kpc The total stellar mass in the Pisces Overdensity is ∼10 4 M ⊙ and its metallicity is [Fe/H]∼ ―15

Substructure revealed by RR Lyraes in SDSS Stripe 82

Abstract: We present an analysis of the substructure revealed by 407 RR Lyraes in Sloan Digital Sky Survey (SDSS) Stripe 82. Period estimates are determined to high accuracy using a string-length method. A subset of 178 RR Lyraes with spectrally derived metallicities are employed to derive metallicity-period-amplitude relations, which are then used to find metallicities and distances for the entire sample. The RR Lyraes lie between 5 and 115 kpc from the Galactic center. They are divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity. The density distribution is not smooth, but dominated by clumps and substructure. Samples of 55 and 237 RR Lyraes associated with the Sagittarius Stream and the Hercules-Aquila Cloud respectively are identified. Hence, ~ 70 % of the RR Lyraes in Stripe 82 belong to known substructure. There is a sharp break in the density distribution at Galactocentric radii of 40 kpc, reflecting the fact that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the Sagittarius Stream - lies within 40 kpc. In fact, almost 60 % of all the RR Lyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which emphasises its pre-eminence. Additionally, evidence of a new and distant substructure - the Pisces Overdensity - is found, consisting of 28 faint RR Lyraes centered on Galactic coordinates (80 deg, -55 deg) and with distances of ~ 80 kpc. The total stellar mass in the Pisces Overdensity is ~10000 solar masses and its metallicity is [Fe/H] ~ -1.5.

The discovery of Segue 2: a prototype of the population of satellites of satellites

Abstract: We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of M v =− 2.5, a half-light radius of 34 pc and a systemic velocity of ∼− 40 km s −1 . Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s −1 and the possible stream as ∼ 7k m s −1 . This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites – survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z> 10 and are good candidates for fossils of the reionization epoch.

Segue 2: A Prototype of the Population of Satellites of Satellites

Abstract: We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius of 34 pc and a systemic velocity of -40$ km/s. Our MMT data also provides evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighboring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about 7 km/s. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites -- survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch.

Kinematics of SDSS subdwarfs: structure and substructure of the Milky Way halo

Abstract: We construct a new sample of ∼ 1700 solar neighbourhood halo subdwarfs from the Sloan Digital Sky Survey (SDSS), selected using a reduced proper-motion diagram. Radial velocities come from the SDSS spectra and proper motions from the light-motion curve catalogue of Bramich et al. Using a photometric parallax relation to estimate distances gives us the full phase-space coordinates. Typical velocity errors are in the range 30-50 km s ―1 . This halo sample is one of the largest constructed to date and the disc contamination is at a level of ≲1 per cent. This enables us to calculate the halo velocity dispersion to excellent accuracy. We find that the velocity dispersion tensor is aligned in spherical polar coordinates and that (σ r ,σ φ , σ θ ) = (143 ± 2, 82 ± 2, 77 ± 2) km s ―1 . The stellar halo exhibits no net rotation, although the distribution of υ φ shows tentative evidence for asymmetry. The kinematics are consistent with a mildly flattened stellar density falling with distance like r ―3.75 . Using the full phase-space coordinates, we look for signs of kinematic substructure in the stellar halo. We find evidence for four discrete overdensities localized in angular momentum and suggest that they may be possible accretion remnants. The most prominent is the solar neighbourhood stream previously identified by Helmi et al., but the remaining three are new. One of these overdensities is potentially associated with a group of four globular clusters (NGC 5466, NGC 6934, M2 and M13) and raises the possibility that these could have been accreted as part of a much larger progenitor.

On the reported death of the MACHO era

Abstract: We present radial velocity measurements of four wide halo binary candidates from the sample in Chaname & Gould (CG04) which, to date, is the only sample containing a large number of such candidates. The four candidates that we have observed have projected separations >0.1 pc, and include the two widest binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that three of the four CG04 candidates are genuine, including the one with the largest separation. The fourth candidate, however, is spurious at the 5σ level. In the light of these measurements, we re-examine the implications for MAssive Compact Halo Object (MACHO) models of the Galactic halo. Our analysis casts doubt on what MACHO constraints can be drawn from the existing sample of wide halo binaries.

The dynamical and chemical evolution of dwarf spheroidal galaxies

Abstract: We present a large sample of fully self-consistent hydrodynamical Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. The initial total mass (gas + dark matter) of these galaxies is the main driver of their evolution. Star formation (SF) occurs in series of short bursts. In massive systems, the very short intervals between the SF peaks mimic a continuous star formation rate, while less massive systems exhibit well separated SF bursts, as identified observationally. The delay between the SF events is controlled by the gas cooling time dependence on galaxy mass. The observed global scaling relations, luminosity-mass and luminosity-metallicity, are reproduced with low scatter. We take advantage of the unprecedentedly large sample size and data homogeneity of the ESO Large Programme DART, and add to it a few independent studies, to constrain the star formation history of five Milky Way dSphs, Sextans, LeoII, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs. [Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of individual stars are confronted with model predictions. We find that the diversity in dSph properties may well result from intrinsic evolution. We note, however, that the presence of gas in the final state of our simulations, of the order of what is observed in dwarf irregulars, calls for removal by external processes.

Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

Abstract: Context. Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically. Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group. However, our understanding of these stars is very limited because detailed chemical abundance measurements are needed from high resolution spectroscopy. Aims. To constrain the formation and chemical evolution of dwarf galaxies, metallicity and chemical composition of extremely metal-poor stars are investigated. Methods. Chemical abundances of six extremely metal-poor ([Fe/H] < -2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy (R=40 000) with the Subaru Telescope High Dispersion Spectrograph. Results. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H] ≲ -3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H] ≲ -2.5, as found in the Galactic halo.

PAndAS’ CUBS: DISCOVERY OF TWO NEW DWARF GALAXIES IN THE SURROUNDINGS OF THE ANDROMEDA AND TRIANGULUM GALAXIES*

Abstract: We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey, a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam Wide-Field Camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H] = -1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (M{sub V} = -9.9 +- 0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II, and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (M{sub V} = -6.5 +- 0.8) and lies a lot closer in projection to M33 than it does to M31 (42 versus 224 kpc), suggestingmore » that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20.« less

A Spectroscopic Confirmation of the Bootes II Dwarf Spheroidal

Abstract: 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.

Life at the periphery of the Local Group: the kinematics of the Tucana dwarf galaxy

Abstract: Aims. Dwarf spheroidal galaxies in the Local Group are usually located close to the Milky Way or M31. Currently, there are two clear exceptions to this rule, and the Tucana dwarf galaxy is the most distant at almost 1 Mpc from the Milky Way. Our aim is to learn more about the nature of Tucana by measuring its radial velocity and internal kinematics. Methods. Using the VLT/FORS2 spectrograph in multi-object mode we were able to measure the velocities of 23 individual red giant branch stars in and around Tucana using the Ca II triplet absorption lines. From this sample 17 reliable members have been identified. Results. We measured the systemic velocity and dispersion of Tucana to be upsilon(hel) = +194.0 +/- 4.3 km s(-1) and sigma(l.o.s) = 15.8(-3.1)(+4.1) km s(-1), respectively. These measures are obtained after removing the signature of rotation using a linear gradient of 6.5 x R/R(core) +/- 2.9 km s(-1) which corresponds to a rotation of approximate to 16 km s(-1) at the reliable limit of our data. Our systemic velocity corresponds to a receding velocity from the barycentre of the Local Group of upsilon(LG) = +73.3 kms(-1). We also determined the mean metallicity of Tucana to be [Fe/H] = -19.5 +/- 0.15 with a dispersion of 0.32 +/- 0.06 dex. Conclusions. Our study firmly excludes any obvious association of Tucana with the HI emission in the vicinity and shows that Tucana is a genuine dwarf spheroidal with low metallicity stars, no gaseous ISM and no recent star formation. The present location and relatively high recession velocity are consistent with Tucana having been an isolated Local Group galaxy for the majority of its existence.

The Monitor project: rotation periods of low-mass stars in M50

Abstract: We report on the results of a time-series photometric survey of M50 (NGC 2323), a ~130Myr open cluster, carried out using the Cerro Tololo Inter-American Observatory (CTIO) 4-m Blanco telescope and Mosaic-II detector as part of the Monitor project. Rotation periods were derived for 812 candidate cluster members over the mass range 0.2 <~ M/Msolar <~ 1.1. The rotation period distributions show a clear mass-dependent morphology, statistically indistinguishable from those in NGC 2516 and M35 taken from the literature. Due to the availability of data from three observing runs separated by ~10 and 1month time-scales, we are able to demonstrate clear evidence for evolution of the photometric amplitudes, and hence spot patterns, over the 10month gap. We are not able to constrain the time-scales for these effects in detail due to limitations imposed by the large gaps in our sampling, which also prevent the use of the phase information.

Density and Kinematic Cusps in M54 at the Heart of the Sagittarius Dwarf Galaxy: Evidence for A 104 M ☉ Black Hole?

Abstract: We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ± 0.7 km s-1, decreasing to 16.4 ± 0.4 km s-1 at 25 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M ☉. However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.

Leo V: Spectroscopy of a Distant and Disturbed Satellite

Abstract: We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3 arcmin) and have velocity dispersion 2.4_{-1.4}^{+2.4} km/s. The other two likely members lie near each other but far from the center (R~13 arcmin ~ 700 pc) and inflate the global velocity dispersion to 3.7_{-1.4}^{+2.3} km/s. Assuming the five central members are bound, we obtain a dynamical mass of M=3.3_{-2.5}^{+9.1} x 10^5M_{sun} (M/L_V=75_{-58}^{+230}[M/L_V]_{sun}). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=-2.0\pm 0.2 dex. Thus with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal-rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ~ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

Leo v: spectroscopy of a distant and disturbed satellite*

Abstract: We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3') and have a velocity dispersion of 2.4+2.4 –1.4 km s–1. The other two likely members lie near each other but far from the center (R ~ 13' ~ 700 pc) and inflate the global velocity dispersion to 3.7+2.3 –1.4 km s–1. Assuming the five central members are bound, we obtain a dynamical mass of M = 3.3+9.1 –2.5 × 105 M ☉ (M/LV = 75+230 –58[M/LV ]☉). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=–2.0 ± 0.2 dex. Thus, with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ~ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

Resolving the stellar outskirts of m81: evidence for a faint, extended structural component*

Abstract: We present a wide field census of resolved stellar populations in the northern half of M81, conducted with Suprime-Cam on the 8 m Subaru telescope and covering an area similar to 0.3 deg(2). The resulting color-magnitude diagram reaches over one magnitude below the red giant branch (RGB) tip, allowing a detailed comparison between the young and old stellar spatial distributions. The surface density of stars with ages less than or similar to 100 Myr is correlated with that of neutral hydrogen in a manner similar to the disk-averaged Kennicutt-Schmidt relation. We trace this correlation down to gas densities of similar to 2 x 1020 cm(-2), lower than typically probed with H alpha flux. Both diffuse light and resolved RGB star counts show compelling evidence for a faint, extended structural component beyond the bright optical disk, with a much flatter surface brightness profile. The star counts allow us to probe this component to significantly fainter levels than is possible with the diffuse light alone. From the colors of its RGB stars, we estimate that this component has a peak global metallicity [M/H] similar to -1.1 +/- 0.3 at deprojected radii 32-44 kpc assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of its RGB stars follows a power-law surface density profile, I (r) proportional to r(-gamma), with gamma similar to 2. If this component were separate from the bulge and from the bright optical disk, then it would contain similar to 10%-15% of M81's total V-band luminosity. We discuss the possibility that this is M81's halo or thick disk, and in particular highlight its similarities and differences with these components in the Milky Way. Other possibilities for its nature, such as a perturbed disk or the faint extension of the bulge, cannot be completely ruled out, though our data disfavor the latter. These observations add to the growing body of evidence for faint, complex extended structures beyond the bright disks of spiral galaxies.

Blue straggler stars in dwarf spheroidal galaxies - II. Sculptor and Fornax

Abstract: The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax, there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor, the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax, the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars, and this is consistent with previous studies.

An HST/ACS view of the inhomogeneous outer halo of M31

Abstract: We present a high precision photometric view of the stellar populations in the outer halo ofM31, using data taken with the Hubble Space Telescope/Advanced Camera for Surveys. We analyse the field populations adjacent to 11 luminous globular clusters which sample the galactocentric radial range 18 ≤ R ≤ 100 kpc and reach a photometric depth of ∼2.5 mag below the horizontal branch (m F814W ∼ 27 mag). The colour-magnitude diagrams are well populated out to ∼60 kpc and exhibit relatively metal-rich red giant branches, with the densest fields also showing evidence for prominent red clumps. We use the Dartmouth isochrones to construct metallicity distribution functions which confirm the presence of dominant populations with ([Fe/H]) ≈ ―0.6 to -1.0 dex and considerable metallicity dispersions of 0.2 to 0.3 dex (assuming a 10 Gyr population and scaled-solar abundances). The average metallicity over the range 30-60 kpc is [Fe/H] = ―0.80 ± 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H] ≤ ―1.3) typically account for ≤ 10―20 per cent of the population in each field, irrespective of radius. Assuming our fields are unbiased probes of the dominant stellar populations in these parts, we find that the M31 outer halo remains considerably more metal rich than that of the Milky Way out to at least 60 kpc.

The UV‐excess Survey of the Northern Galactic Plane

Abstract: The UV-Excess survey of the northern Galactic plane images a 10 degrees x 185 degrees wide band, centred on the Galactic equator using the 2.5-m Isaac Newton Telescope in four bands (U, g, r, He 15875) down to similar to 21-22 mag (similar to 20 in He 15875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM Canum Venaticorum stars are made, including the effects of reddening. A first look at the data of the survey (currently 30 per cent complete) is given.

Candidate planetary nebulae in the IPHAS photometric catalogue

Abstract: Context. We have carried out a semi-automated search for planetary nebulae (PNe) in the INT photometric H-alpha survey (IPHAS) catalogue. We present the PN search and the list of selected candidates. We cross correlate the selected candidates with a number of existing infrared galactic surveys in order to gain further insight into the nature of the candidates. Spectroscopy of a subset of objects is used to estimate the number of PNe present in the entire candidate list. Aims. The overall aim of the IPHAS PN project is to carry out a deep census of PNe in the northern Galactic plane, an area where PN detections are clearly lacking. Methods. The PN search is carried out on the IPHAS photometric catalogue. The candidate selection is based on the IPHAS and 2MASS/UKIDSS colours of the objects and the final candidate selection is made visually. Results. From the original list of ∼600 million IPHAS detections we have selected a total of 1005 objects. Of these, 224 are known objects, leaving us with 781 PN candidates. Based on the initial follow-up spectroscopy, we expect the list to include very young and proto-PNe in addition to genuine, normal PNe (∼16%) and emission line objects other than PNe. We present additional criteria to select the most probable PN candidates from our candidate list.

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And XV and And XVI

Abstract: We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc respectively. These satellites lie to the South of M31, in regions of the stellar halo which wide field imaging has revealed as relative voids (compared to the degree-scale coherent stream-like structures). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ~6 km/s down to i~21.5. While the distance to And XVI remains the same as previously reported (525pm50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch (TRGB) in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770pm70 kpc (compared to the previous 630 kpc), increasing the luminosity from MV -9.4 to MV~-9.8. The And XV velocity dispersion is resolved with vr =-339+7-6 km/s and sigma-v = 11+7-5 km/s. The And XVI dispersion is not quite resolved at 1sigma with vr =-385+5-6 km/s and sigma-v = 0+10-indef km/s. Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H]=-1.58, interquar- tile range +-0.08), and And XVI (median [Fe/H]=-2.23, interquartile range +-0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H]=-1.8 (-2.1) for And XV (And XVI), with a uncertainty of ~0.2 dex in both cases. Our measure- ments of And XV reasonably resolve its mass (~10^8 Msun) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31-centric distance.

Candidate planetary nebulae in the IPHAS photometric catalogue

Abstract: Context. We have carried out a semi-automated search for planetary nebulae (PNe) in the INT Photometric H-Alpha Survey (IPHAS) catalogue. We present the PN search and the list of selected candidates. We cross correlate the selected candidates with a number of existing infrared galactic surveys in order to gain further insight into the nature of the candidates. Spectroscopy of a subset of objects is used to estimate the number of PNe present in the entire candidate list. Aims. The overall aim of the IPHAS PN project is to carry out a deep census of PNe in the northern Galactic plane, an area where PN detections are clearly lacking. Methods. The PN search is carried out on the IPHAS photometric catalogues. The candidate selection is based on the IPHAS and 2MASS/UKIDSS colours of the objects and the final candidate selection is made visually. Results. From the original list of ~600 million IPHAS detections we have selected a total of 1005 objects. Of these, 224 are known objects, leaving us with 781 PN candidates. Based on the initial follow-up spectroscopy, we expect the list to include very young and proto-PNe in addition to genuine, normal PNe (~16 %) and emission line objects other than PNe. We present additional criteria to select the most probable PN candidates from our candidate list.

A Keck/DEIMOS spectroscopic survey of the faint M 31 satellites And XV and And XVI

Abstract: We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc, respectively. These satellites lie to the south of M31, in regions of the stellar halo which wide-field imaging has revealed as relative voids (compared to the ~degree-scale coherent stream-like structures). Using the Deep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ~6 km s^(−1) down to i ~ 21.5. While the distance to And XVI remains the same as previously reported (525 ± 50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770 ± 70 kpc (compared to the previous 630 kpc), increasing the luminosity from M_V ≈ −9.4 to −9.8. The And XV velocity dispersion is resolved with v_r = −339^(+7)_(−6) km s^(−1) and σ_v = 11^(+7)_(−5) km s^(−1). The And XVI dispersion is not quite resolved at 1σ with v_r = −385^(+5)_(−6) km s^(−1) and σ = 0^(+10)_(−indef) km s^(−1) . Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H] = −1.58 , interquartile range ±0.08) and And XVI (median [Fe/H] = −2.23, interquartile range ±0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H] = −1.8 (−2.1) for And XV (And XVI), with an uncertainty of ~0.2 dex in both cases. Our measurements of And XV reasonably resolve its mass (~10^8 M_⊙) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31 centric distance.

The Discovery of Remote Globular Clusters in M33

Abstract: We present the discovery of four remote star clusters in M33, one of which is of an extended nature. Three of the clusters were discovered using survey data from the Isaac Newton Telescope Wide-Field Camera while one was discovered serendipitously in a deep image taken with the Hubble Space Telescope's Advanced Camera for Surveys. With projected radii of 38-113 arcmin (9.6-28.5 kpc for an assumed M33 distance of 870 kpc), these clusters lie significantly beyond all but one of the currently confirmed clusters in M33. The clusters have magnitudes and colors consistent with their being old to intermediate-age globular clusters (GCs). Indeed, they bear a strong resemblance to the outer halo GC population of the Milky Way and M31 in terms (V – I)0 color. The three outermost clusters are projected on the far side of M33 with respect to M31, an asymmetry that could suggest tidal interactions have affected M33's GC distribution at large radii.

Stellar metallicities and kinematics in a gas-rich dwarf galaxy: first calcium triplet spectroscopy of red giant branch stars in wlm

Abstract: We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton ([Fe/H]CG97) metallicity scale. The mean metallicity is [Fe/H] = –1.27 ± 0.04 dex, with a standard deviation of σ = 0.37. We find that the stars in the inner field are more metal-rich by Δ[Fe/H]=0.30 ± 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star-forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (less than 6 Gyr) are more metal-rich by Δ[Fe/H]=0.32 ± 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the H I rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal-poor stars.

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII

Abstract: We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data for two further faint companions, And IX and AndXII. By combining data obtained using the DEIMOS spectrograph mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have calculated global properties for the dwarfs, such as systemic velocities, metallicites and half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve a dispersion for And XI due to small sample size and low S/N, but we set a one sigma upper limit of sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4 (+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies, and the Universal halo profiles established for Milky Way dwarfs (Walker et al. 2009). For both galaxies, this sees them fall below the Universal halo profiles of Walker et al. (2009). When combined with the findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit halos that are significantly different from those of the Milky Way in terms of their central densities (abridged).

Stellar Metallicities and Kinematics in a Gas-Rich Dwarf Galaxy: First Calcium Triplet Spectroscopy of RGB Stars in WLM

Abstract: We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton (1997) metallicity scale. The mean metallicity is = -1.27 +/- 0.04 dex, with a standard deviation of 0.37. We find that the stars in the inner field are more metal rich by [Fe/H] =0.30 +/- 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (< 6 Gyr) are more metal rich by [Fe/H] = 0.32 +/- 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the HI rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal poor stars.

A spectroscopic survey of EC4, an extended cluster in Andromeda's halo★

Abstract: We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our Canada—France—Hawaii Tele- scope/MegaCam survey, overlapping the tidal streams, Streams 'Cp' and 'Cr'. These obser- vations used the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope to obtain spectra around the CaII triplet region with ∼1.3 A resolution. Six stars lying on the red giant branch within two core radii of the centre of EC4 are found to have an average vr =− 287.9 +1.9 −2.4 km s −1 and σ v,corr = 2.7 +4.2 −2.7 km s −1 , taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L = 6.7 +15 −6.7 M � /L � , a value that is consistent with a globular cluster within the 1σ errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with (Fe/H) =− 1.6 ± 0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Streams 'Cp' and 'Cr', and find that EC4 bears a striking resemblance to Stream 'Cp' in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this, we conclude that EC4 is likely related to Stream 'Cp'.