Showing papers by "Mike Irwin published in 2007"

The UKIRT Infrared Deep Sky Survey (UKIDSS)

Abstract: Final published version including significant revisions. Twenty four pages, fourteen figures. Original version April 2006; final version published in MNRAS August 2007

Cats and dogs, hair and a hero: A quintet of new milky way companions

Abstract: We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies--one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules--together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness {mu}{sub v} {approx}> 28 mag arcsec{sup -2} have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between {approx} 40 pc and {approx} 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action*

Abstract: We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of the CFHT and INT telescopes. The surveyed area covers the inner 50kpc of the galaxy and the Southern quadrant out to a projected distance of ∼ 150kpc. A survey extension to M33 at > 200kpc probes the interface between the halos of these two galaxies. This survey is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of largescale structures of low surface brightness, including several streams. Significant variations in stellar populations due to intervening stream-like structures are detected in the inner halo along the minor axis. This, together with the fact that the light profile between 0 ◦ .5 < R < 1 ◦ .3 follows the exponential “extended disk”, is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Two new relatively luminous (MV ∼ −9) dwarf galaxies And XV and XVI are found in the study; And XVI is a particularly interesting specimen being located 270kpc in front of M31, towards the Milky Way. Underlying the many substructures that we have uncovered lies a faint, smooth and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. This is consistent with recent claims based on spectroscopy of a small sample of stars. We find that the smooth halo component in M31 has a radially-decreasing profile that can be fit with a Hernquist model of immense scale radius ∼ 55kpc, almost a factor

Discovery of an unusual dwarf galaxy in the outskirts of the Milky Way

Abstract: We announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of ~420 kpc and has an intermediate-age stellar population with a metallicity of [Fe/H] = -1.6, together with a young population of blue stars of age ~200 Myr. There is a compact cloud of neutral hydrogen with mass ~105 M☉ and radial velocity +35 km s-1 coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.

The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action

Abstract: We present a deep photometric survey of M31, conducted with the CFHT and INT, covering the inner 50 kpc of the galaxy, the Southern quadrant out to 150 kpc, and extending to M33. This is the first systematic panoramic study of this very outermost region of galaxies. We detect several streams and other large-scale structures, and two new dwarf galaxies: And XV and XVI. The discovery of substructure on the minor axis, together with the fact that the light profile between 0.5 < R < 1.3 follows the exponential ``extended disk'', is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the substructures lies a faint, metal-poor, smooth and extremely extended halo, reaching out to at least 150 kpc. The smooth halo component in M31 has a profile that can be fit with a Hernquist model of immense scale radius ~55 kpc, almost a factor of 4 larger than theoretical predictions. Alternatively a power-law with exponent -1.91 +/- 0.11 can be fit to the profile. The total luminosity of this structure is similar to that of the halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical monolithic model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fit with a Hernquist model also of scale radius ~55 kpc. These extended slowly-decreasing halos will provide a challenge and strong constraints for further modeling. [Abridged]

A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies

Abstract: We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Bootes, Ursa Major I, Ursa Major II and Willman 1 (Will). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have obtained samples that contain from ∼ 15 to ∼85 probable members of these satellites for which we derive radial velocities precise to a few km s -1 down to i ∼ 21-22. About half of these stars are observed with a high enough signal-to-noise ratio to estimate their metallicity to within ±0.2 dex. The characteristics of all the observed stars are made available, along with those of the Canes Venatici I dwarf galaxy that have been analysed in a companion paper. From this data set, we show that Ursa Major II is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (υ r = 115 ± 5kms -1 ) is in good agreement with simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark matter dominated systems (under the usual assumptions of symmetry and virial equilibrium). In particular, we show that despite its small size and faintness, the Will object is not a globular cluster given its metallicity scatter over -2.0 ≤ [Fe/H] ≤ -1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3 +2.3-1.3 km s -1 around a systemic velocity of -12.3 ± 2.3 km s -1 which implies a mass-to-light ratio of ∼700 and a total mass of ∼5 x 10 5 M O for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 10 7 M O limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modelling and an extended search for potential extratidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction.

An orphan in the "Field of Streams"

Abstract: We use Sloan Digital Sky Survey Data Release 5 photometry and spectroscopy to study a tidal stream that extends over ~50° in the north Galactic cap. From the analysis of the path of the stream and the colors and magnitudes of its stars, the stream is ~20 kpc away at its nearest detection (the celestial equator). We detect a distance gradient: the stream is farther away from us at higher declination. The contents of the stream are made up from a predominantly old and metal-poor population that is similar to the globular clusters M13 and M92. The integrated absolute magnitude of the stream stars is estimated to be Mr ~ -7.5. There is tentative evidence for a velocity signature, with the stream moving at ~-40 km s-1 at low declinations and ~+100 km s-1 at high declinations. The stream lies on the same great circle as Complex A, a roughly linear association of H I high-velocity clouds stretching over ~30° on the sky, and as Ursa Major II, a recently discovered dwarf spheroidal galaxy. Lying close to the same great circle are a number of anomalous, young, and metal-poor globular clusters, including Palomar 1 and Ruprecht 106.

The United Kingdom Infrared Telescope Infrared Deep Sky Survey First Data Release

Abstract: The First Data Release (DR1) of the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS) took place on 2006 July 21. The UKIDSS is a set of five large near-infrared surveys, covering a complementary range of areas, depths and Galactic latitudes. The DR1 is the first large release of survey-quality data from the UKIDSS and includes 320 deg 2 of multicolour data to (Vega) K = 18, complete (depending on the survey) in three to five bands from the set ZYJHK, together with 4 deg 2 of deep JK data to an average depth K = 21. In addition, the release includes a similar quantity of data with incomplete filter coverage. In JHK, in regions of low extinction, the photometric uniformity of the calibration is better than 0.02 mag in each band. The accuracy of the calibration in ZY remains to be quantified, and the same is true of JHK in regions of high extinction. The median image full width at half-maximum across the data set is 0.82 arcsec. We describe changes since the Early Data Release in the implementation, pipeline and calibration, quality control, and archive procedures. We provide maps of the areas surveyed, and summarize the contents of each of the five surveys in terms of filters, areas and depths. The DR1 marks completion of 7 per cent of the UKIDSS seven-year goals.

The Hercules-Aquila Cloud

Abstract: We present evidence for a substantial overdensity of stars in the direction of the constellations of Hercules and Aquila. The cloud is centered at a Galactic longitude of l ≈ 40° and extends above and below the Galactic plane by at least 50°. Given its off-centeredness and height, it is unlikely that the Hercules-Aquila cloud is related to the bulge or thick disk. More likely, this is a new structural component of the Galaxy that passes through the disk. The cloud stretches ~80° in longitude. Its heliocentric distance lies between 10 and 20 kpc so that the extent of the cloud in projection is ~20 kpc by ~15 kpc. It has an absolute magnitude of Mv = -13, and its stellar population appears to be comparable to, but somewhat more metal-rich than, M92.

Direct Detection of the Asteroidal YORP Effect

Abstract: The Yarkovsky-O9Keefe-Radzievskii-Paddack (YORP) effect is believed to alter the spin states of small bodies in the solar system. However, evidence for the effect has so far been indirect. We report precise optical photometric observations of a small near-Earth asteroid, (54509) 2000 PH5, acquired over 4 years. We found that the asteroid has been continuously increasing its rotation rate ω over this period by dω/dt = 2.0 (±0.2) × 10–4 degrees per day squared. We simulated the asteroid9s close Earth approaches from 2001 to 2005, showing that gravitational torques cannot explain the observed spin rate increase. Dynamical simulations suggest that 2000 PH5 may reach a rotation period of ∼20 seconds toward the end of its expected lifetime.

The Discovery of Two Extremely Low Luminosity Milky Way Globular Clusters

Abstract: We report the discovery of two extremely low luminosity globular clusters in the Milky Way halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at ~40-50 kpc and appear to have old stellar populations and luminosities of only MV ~ -1 mag. Their observed sizes of ~3 pc are well within the expected tidal limit of ~10 pc at that distance. Together with Palomar 1, AM 4, and Whiting 1, these new clusters are the lowest luminosity globular clusters orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only ~70 and ~55 Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as ~0.1tHubble. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.

A very cool brown dwarf in UKIDSS DR1

Abstract: We report the discovery of a very cool brown dwarf, ULAS J003402.77−005206.7 (ULAS J0034−00), identified in the UKIRT Infrared Deep Sky Survey First Data Release. We provide optical, near-infrared, and mid-infrared photometry of the source, and two near-infrared spectra. Comparing the spectral energy distribution of ULAS J0034−00 to that of the T8 brown dwarf 2MASS J04151954−0935066 (2MASS J0415−09), the latest type and coolest well-studied brown dwarf to date, with effective temperature Teff ∼ 750 K, we find evidence that ULAS J0034−00 is significantly cooler. First, the measured values of the near-infrared absorption spectral indices imply a later classification, of T8.5. Secondly, the H − [4.49] colour provides an empirical estimate of the effective temperature of 540 < Teff < 660 K (± 2σ range). Thirdly, the J- and H-band peaks are somewhat narrower in ULAS J0034−00, and detailed comparison against spectral models calibrated to 2MASS J0415−09 yields an estimated temperature lower by 60 <� T eff < 120 K relative to 2MASS J0415−09 i.e. 630 < Teff < 690 K (± 2σ ), and lower gravity or higher metallicity according to the degenerate combination −0.5 <� (log g − 2[m/H]) < −0.25 (± 2σ ). Combining these estimates, and considering systematics, it is likely the temperature lies in the range 600 < Teff < 700 K. Measurement of the parallax will allow an additional check of the inferred low temperature. Despite the low inferred Teff we find no evidence for strong absorption by NH3 over the wavelength range 1.51‐1.56 μm. Evolutionary models imply that the mass and age are in the ranges 15‐36 MJup and 0.5‐8 Gyr, respectively. The measured proper motion, of 0.37 ± 0.07 arcsec yr −1 , combined with the photometrically

The Local Group dwarf Leo T: HI on the brink of star formation

Abstract: We present Giant Meterwave Radio Telescope (GMRT) and Westerbork ynthesis Radio Telescope (WSRT) observations of the recently discovered Local Group dwarf galaxy, Leo T. The peak HI column density is measured to be 7x10^20 cm^-2, and the total HI mass is 2.8Xx10^5 Msun, based on a distance of 420 kpc. Leo T has both cold (~ 500 K) and warm (~ 6000 K) HI at its core, with a global velocity dispersion of 6.9 km/s, from which we derive a dynamical mass within the HI radius of 3.3x10^6 Msun, and a mass-to-light ratio of greater than 50. We calculate the Jeans mass from the radial profiles of the HI column density and velocity dispersion, and predict that the gas should be globally stable against star formation. This finding is inconsistent with the half light radius of Leo T, which extends to 170 pc, and indicates that local conditions must determine where star formation takes place. Leo T is not only the lowest luminosity galaxy with on-going star formation discovered to date, it is also the most dark matter dominated, gas-rich dwarf in the Local Group.

A very cool brown dwarf in UKIDSS DR1

Abstract: (abridged) We report the discovery of a very cool brown dwarf, ULAS J003402.77-005206.7 (ULAS J0034-00), identified in UKIDSS DR1. We provide optical, near-infrared, and mid-infrared photometry of the source, and two near-infrared spectra. Comparing the spectral energy distribution of ULAS J0034-00 to that of the T8 brown dwarf 2MASS J0415-09, the latest-type and coolest well-studied brown dwarf to date, with Teff~750 K, we find evidence that ULAS J0034-00 is significantly cooler. First, the measured values of the near-infrared absorption spectral indices imply a later classification, of T8.5. Second, the H-[4.49] colour provides an empirical estimate of the temperature of 540

The Monitor project: data processing and light curve production

Abstract: We have begun a large-scale photometric survey of nearby open clusters and star-forming regions, the Monitor project, aiming to measure time-series photometry for > 10 000 cluster members over > 10 deg 2 of sky, to find low-mass eclipsing binary and planet systems. We describe the software pipeline we have developed for this project, showing that we can achieve peak rms accuracy over the entire data set of better than ∼2 mmag using aperture photometry, with rms < 1 per cent over ∼4 mag, in data from 2- and 4-m class telescopes with wide-field mosaic cameras. We investigate the noise properties of our data, finding correlated 'red' noise at the ∼ 1-1.5 mmag level in bright stars, over transit-like time-scales of 2.5 h. An important source of correlated noise in aperture photometry is image blending, which produces variations correlated with the seeing. We present a simple blend index based on fitting polynomials to these variations, and find that subtracting the fit from the data provides a method to reduce their amplitude, in lieu of using techniques, such as point spread function fitting photometry, which tackle their cause. Finally, we use the SYSREM algorithm to search for any further systematic effects.

Galactic halo stellar structures in the Triangulum-Andromeda region

Abstract: This Letter reports on the Galactic stellar structures that appear in the foreground of our Canada-France-Hawaii-Telecopse/MegaCam survey of the halo of the Andromeda galaxy. We recover the main sequence and main-sequence turnoff of the Triangulum-Andromeda structure recently found by Majewski and collaborators at a heliocentric distance of ~20 kpc. The survey also reveals another less populated main sequence at fainter magnitudes that could correspond to a more distant stellar structure at ~28 kpc. Both structures are smoothly distributed over the ~76 deg2 covered by the survey, although the closer one shows an increase in density by a factor of ~2 toward the northwest. The discovery of a stellar structure behind the Triangulum-Andromeda structure that itself appears behind the low-latitude stream that surrounds the Galactic disk gives further evidence that the inner halo of the Milky Way is of a spatially clumpy nature.

Initial Data Release from the INT Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS)

Abstract: The INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS) is an imaging survey being carried out in H-alpha, r' and i' filters, with the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) to a depth of r'=20 (10 sigma). The survey is aimed at revealing large scale structure in our local galaxy, and also the properties of key early and late populations making up the Milky Way. Mapping emission line objects enables a particular focus on objects in the young and old stages of stellar evolution ranging from early T-Tauri stars to late planetary nebulae. In this paper we present the IPHAS Initial Data Release, primarily a photometric catalogue of about 200 million unique objects, coupled with associated image data covering about 1600 square degrees in three passbands. We note how access to the primary data products has been implemented through use of standard virtual observatory publishing interfaces. Simple traditional web access is provided to the main IPHAS photometric catalogue, in addition to a number of common catalogues (such as 2MASS) which are of immediate relevance. Access through the AstroGrid VO Desktop opens up the full range of analysis options, and allows full integration with the wider range of data and services available through the Virtual Observatory. The IDR represents the largest dataset published primarily through VO interfaces to date, and so stands as an examplar of the future of survey data mining. Examples of data access are given, including a cross-matching of IPHAS photometry with sources in the UKIDSS Galactic Plane Survey that validates the existing calibration of the best data.

Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal

Abstract: This paper presents a study of the Cetus dwarf, an isolated dwarf galaxy within the Local Group. A matched-filter analysis of the INT/WFC imaging of this system reveals no evidence for significant tidal debris that could have been torn off the galaxy, bolstering the hypothesis that Cetus has never significantly interacted with either the Milky Way or M31. Additionally, Keck/Deimos spectroscopic observations identify this galaxy as a distinct kinematic population possessing a systematic velocity of −87 ± 2 km s^(−1) and with a velocity dispersion of 17 ± 2 km s^(−1); while tentative, these data also suggest that Cetus possesses a moderate rotational velocity of ~8 km s^(−1). The population is confirmed to be relatively metal-poor, consistent with [Fe/H]~−1.9, and, assuming virial equilibrium, implies that the Cetus dwarf galaxy possesses a mass-to-light ratio of ~70. It appears, therefore, that Cetus may represent a primordial dwarf galaxy, retaining the kinematic and structural properties lost by other members of the dwarf population of the Local Group in their interactions with the large galaxies. An analysis of Cetus' orbit through the Local Group indicates that it is at apocentre; taken in conjunction with the general dwarf population, this shows the mass of the Local Group to be ≳ 2 × 10^(12) M_⊙.

Is Ursa Major II the Progenitor of the Orphan Stream

Abstract: Prominent in the ''Field of Streams''--the Sloan Digital Sky Survey map of substructure in the Galactic halo--is an ''Orphan Stream'' without obvious progenitor. In this numerical study, we show a possible connection between the newly found dwarf satellite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as -100kms{sup -1}, as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream.

ACS Photometry of Newly Discovered Globular Clusters in the Outer Halo of M31

Abstract: We report the first results from deep ACS imaging of 10 classical globular clusters in the far outer regions (15 kpc Rp 100 kpc) of M31. Eight of the clusters, including two of the most remote M31 globular clusters presently known, are described for the first time. Our F606W, F814W color-magnitude diagrams extend ~3 mag below the horizontal branch and clearly demonstrate that the majority of these objects are old (10 Gyr), metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional four have -1.9 [Fe/H] -1.5. The remaining object is more metal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit the second-parameter effect. Using aperture photometry, we estimate integrated luminosities and structural parameters for all clusters. Many, including all four clusters with projected radii greater than 45 kpc, are compact and very luminous, with -8.9 MV -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, subluminous (-6.0 MV -4.7), and more metal-rich (-1.8 [Fe/H] -1.3).

Strangers in the night : Discovery of a dwarf spheroidal galaxy on its first local group infall

Abstract: We present spectroscopic observations of the And XII dwarf spheroidal galaxy using DEIMOS/Keck II, showing it to be moving rapidly through the Local Group (-556 km s-1 heliocentric velocity, -281 km s-1 relative to Andromeda), falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. And XII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H I gas mass of <3 × 103 M☉ suggests that And XII's gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest that the dwarf is close to the escape velocity of M31 for published mass models. And XII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.

The Monitor project: searching for occultations in young open clusters

Abstract: The Monitor project is a photometric monitoring survey of nine young (1-200Myr) clusters in the solar neighbourhood to search for eclipses by very low mass stars and brown dwarfs and for planetary transits in the light curves of cluster members. It began in the autumn of 2004 and uses several 2- to 4-m telescopes worldwide. We aim to calibrate the relation between age, mass, radius and where possible luminosity, from the K dwarf to the planet regime, in an age range where constraints on evolutionary models are currently very scarce. Any detection of an exoplanet in one of our youngest targets (<~10Myr) would also provide important constraints on planet formation and migration time-scales and their relation to protoplanetary disc lifetimes. Finally, we will use the light curves of cluster members to study rotation and flaring in low-mass pre-main-sequence stars. The present paper details the motivation, science goals and observing strategy of the survey. We present a method to estimate the sensitivity and number of detections expected in each cluster, using a simple semi-analytic approach which takes into account the characteristics of the cluster and photometric observations, using (tunable) best-guess assumptions for the incidence and parameter distribution of putative companions, and we incorporate the limits imposed by radial velocity follow-up from medium and large telescopes. We use these calculations to show that the survey as a whole can be expected to detect over 100 young low and very low mass eclipsing binaries, and ~3 transiting planets with radial velocity signatures detectable with currently available facilities.

Blue straggler stars in dwarf spheroidal galaxies

Abstract: Blue straggler star (BSS) candidates have been observed in all old dwarf spheroidal galaxies (dSphs), however whether or not they are authentic BSSs or young stars has been a point of debate. To both address this issue and obtain a better understanding of the formation of BSSs in different environments, we have analysed a sample of BSS candidates in two nearby Galactic dSphs, Draco and Ursa Minor. We have determined their radial and luminosity distributions from wide field multicolour imaging data extending beyond the tidal radii of both galaxies. BSS candidates are uniformly distributed through the host galaxy, whereas a young population is expected to show a more clumpy distribution. Furthermore, the observed radial distribution of BSSs, normalized to both red giant branch (RGB) and horizontal branch (HB) stars, is almost flat, with a slight decrease towards the centre. Such a distribution is at odds with the predictions for a young stellar population, which should be more concentrated. Instead, it is consistent with model predictions for BSS formation by mass transfer in binaries (MT-BSSs). Such results, although not decisive, suggest that these candidates are indeed BSSs and that MT-BSSs form in the same way in Draco and Ursa Minor as in globular clusters. This favours the conclusion that Draco and Ursa Minor are truly 'fossil' galaxies, where star formation ceased completely more than 8 billion years ago.

Galactic halo stellar structures in the Triangulum-Andromeda region

Abstract: This letter reports on the Galactic stellar structures that appear in the foreground of our Canada-France-Hawaii-Telecopse/MegaCam survey of the halo of the Andromeda galaxy. We recover the main sequence and main sequence turn-off of the Triangulum-Andromeda structure recently found by Majewski and collaborators at a heliocentric distance of ~20 kpc. The survey also reveals another less populated main sequence at fainter magnitudes that could correspond to a more distant stellar structure at ~28 kpc. Both structures are smoothly distributed over the ~76 sq. deg. covered by the survey although the closer one shows an increase in density by a factor of ~2 towards the North-West. The discovery of a stellar structure behind the Triangulum-Andromeda structure that itself appears behind the low-latitude stream that surrounds the Galactic disk gives further evidence that the inner halo of the Milky Way is of a spatially clumpy nature.

Ram Pressure Stripping of an Isolated Local Group Dwarf Galaxy: Evidence for an Intragroup Medium

Abstract: We compare the stellar structure of the isolated, Local Group dwarf galaxy Pegasus (DDO 216) with low-resolution H I maps from L. M. Young et al. Our comparison reveals that Pegasus displays the characteristic morphology of ram pressure stripping; in particular, the H I has a "cometary" appearance that is not reflected in the regular, elliptical distribution of the stars. This is the first time this phenomenon has been observed in an isolated Local Group galaxy. The density of the medium required to ram pressure strip Pegasus is at least 10-5 to 10-6 cm-3. We conclude that this is strong evidence for an intergalactic medium associated with the Local Group.

The AAT/WFI survey of the Monoceros Ring and Canis Major dwarf galaxy – I. From l= (193–276)°

Abstract: We present the results of an Anglo-Australian Telescope (AAT) wide field camera survey of the stars in the Monoceros Ring (MRi) and purported Canis Major (CMa) overdensity in the Galactic longitudes of l = (193‐276) ◦ . Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the MRi, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report eight new detections and two tentative detections of the MRi in this survey. The exact nature of the CMa overdensity is still unresolved, although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. We also report that the blue plume stars previously invoked to support the dwarf galaxy hypothesis are unfounded, and associating them with an outer spiral arm is equally problematic. Standard Galactic models are unable to accommodate all the observations of these new structures, leading away from a warped/flared disc explanation for their origins and more toward a non-Galactic source. Additionally, evidence is presented in favour of a detection of the CMa dwarf stream away from the CMa region. As the outer reaches of the Galactic disc continue to be probed, the fascinating structures that are the MRi and CMa overdensity will no doubt continue to inform us of the unique structure and formation of the Milky Way.

Ram pressure stripping of an isolated Local Group dwarf galaxy: evidence for an intra-group medium

Abstract: We compare the stellar structure of the isolated, Local Group dwarf galaxy Pegasus (DDO216) with low resolution HI maps from Young et al. (2003). Our comparison reveals that Pegasus displays the characteristic morphology of ram pressure stripping; in particular, the HI has a ``cometary'' appearance which is not reflected in the regular, elliptical distribution of the stars. This is the first time this phenomenon has been observed in an isolated Local Group galaxy. The density of the medium required to ram pressure strip Pegasus is at least $10^{-5} - 10^{-6}$, cm$^{-3}$. We conclude that this is strong evidence for an inter-galactic medium associated with the Local Group.

Draco, a flawless dwarf galaxy★

Abstract: The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass-to-light ratio and compact size, holds many clues to the nature of the enigmatic dark matter. Here we present deep photometric studies of this dwarf galaxy, undertaken with the MegaCam Camera at the Canada-France-Hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide Field and Planetary Camera onboard the Hubble Space Telescope (HST). The new photometric data cover the entirety of the galaxy, and reach i' = 24.5 at 50 per cent completeness, significantly deeper than previous panoramic studies, allowing searches for tidal disturbances of much lower surface brightness than has been possible before. With these improved statistics, we find no evidence for asymmetric disturbances or tidal tails that possess more than 3 per cent of the stars found within the centre of the galaxy. We find that the central stellar density, as probed by the HST data, rises into the central 0.5 arcmin. Uncertainties in the position of the centroid of the galaxy do not allow us to determine whether the apparent flattening of the profile interior to 0.5 arcmin is reliable or not. Draco is therefore a flawless dwarf galaxy, featureless and apparently unaffected by Galactic tides.

Deconstructing dwarf galaxies: a Suprime-Cam survey of Andromeda II

Abstract: We present deep, subhorizontal-branch, multicolour photometry of the Andromeda II dwarf spheroidal (And II dSph) taken with the Subaru Suprime-Cam wide-field camera. We identify a red clump population in this galaxy, the first time this feature has been detected in an M31 dSph, which are normally characterized as having no significant intermediate-age populations. We construct radial profiles for the various stellar populations and show that the horizontal branch (HB) has a nearly constant density spatial distribution out to large radius, whereas the reddest red giant branch stars are centrally concentrated in an exponential profile. We argue that these populations trace two distinct structural components in And II, and show that this assumption provides a good match to the overall radial profile of this galaxy. The extended component dominates the stellar populations at large radius, whereas the exponential component dominates the inner few arcminutes. By examining colour-magnitude diagrams in these regions, we show that the two components have very different stellar populations; the exponential component has an average age of ∼7-10 Gyr, is relatively metal-rich ([Fe/H] ∼1) but with a significant tail to low metallicities, and possesses a red clump. The extended component, on the other hand, is ancient (∼13 Gyr), metal-poor ([Fe/H] ∼-1.5) with a narrower dispersion σ[Fe/H] ≃ 0.28, and has a well-developed blue HB. The extended component contains approximately three-quarters of the light of And II and its unusual density profile is unique in Local Group dwarf galaxies. This suggests that its formation and/or evolution may have been quite different from other dwarf galaxies. The obvious chemodynamical complexity of And II lends further support to the accumulating body of evidence which shows that the evolutionary histories of faint dSph galaxies can be every bit as complicated as their brighter and more massive counterparts.