S. Dye; A. Lawrence; Mike Read; Xiaohui Fan; Tom Kerr; W. P. Varricatt; K Furnell; A. C. Edge; M. J. Irwin; N. C. Hambly; P. W. Lucas; O. Almaini; K. C. Chambers; R. Green; P. C. Hewett; Michael C. Liu; Ian D. McGreer; William M. J. Best; Zhoujian Zhang; E. Sutorius; D. Froebrich; E. A. Magnier; G. Hasinger; Susan M. Lederer; Matthew Bold; Jonathan Tedds
Monthly Notices of the Royal Astronomical Society · DOI ↗
This paper defines the UK Infra-Red Telescope (UKIRT) Hemisphere Survey (UHS) and release of the remaining ∼12 700 deg2 of J-band survey data products. The UHS will provide continuous J- and K-band coverage in the Northern hemisphere from a declination of 0° to 60° by combining the existing Large Area Survey, Galactic Plane Survey and Galactic Clusters Survey conducted under the UKIRT Infra-red Deep Sky Survey (UKIDSS) programme with this new additional area not covered by UKIDSS. The released data include J-band imaging and source catalogues over the new area, which, together with UKIDSS, completes the J-band UHS coverage over the full ∼17 900 deg2 area. 98 per cent of the data in this release have passed quality control criteria. The remaining 2 per cent have been scheduled for re-observation. The median 5σ point source sensitivity of the released data is 19.6 mag (Vega). The median full width at half-maximum of the point spread function across the data set is 0.75 arcsec. In this paper, we outline the survey management, data acquisition, processing and calibration, quality control and archiving as well as summarizing the characteristics of the released data products. The data are initially available to a limited consortium with a world-wide release scheduled for 2018 August.
C. Gónzalez-Fernández; S. T. Hodgkin; M. J. Irwin; E. González-Solares; S. E. Koposov; J. R. Lewis; J. P. Emerson; P. C. Hewett; A. Yoldaş; M. Riello
Monthly Notices of the Royal Astronomical Society · DOI ↗
In this paper, we describe the routine photometric calibration of data taken with the VISTA infrared camera (VIRCAM) instrument on the ESO Visible and Infrared Survey Telescope for Astronomy (VISTA) telescope. The broad-band ZYJHKs data are directly calibrated from Two Micron all Sky Survey (2MASS) point sources visible in every VISTA image. We present the empirical transformations between the 2MASS and VISTA, and Wide-Field Camera and VISTA, photometric systems for regions of low reddening. We investigate the long-term performance of VISTA+VIRCAM. An investigation of the dependence of the photometric calibration on interstellar reddening leads to these conclusions: (1) For all broad-band filters, a linear colour-dependent correction compensates the gross effects of reddening where E(B − V) < 5.0. (2) For Z and Y, there is a significantly larger scatter above E(B − V) = 5.0, and insufficient measurements to adequately constrain the relation beyond this value. (3) The JHKs filters can be corrected to a few per cent up to E(B − V) = 10.0. We analyse spatial systematics over month-long time-scales, both inter- and intradetector and show that these are present only at very low levels in VISTA. We monitor and remove residual detector-to-detector offsets. We compare the calibration of the main pipeline products: pawprints and tiles. We show how variable seeing and transparency affect the final calibration accuracy of VISTA tiles, and discuss a technique, grouting, for mitigating these effects. Comparison between repeated reference fields is used to demonstrate that the VISTA photometry is precise to better than ≃ 2 per cent for the YJHKs bands and 3 per cent for the Z bands. Finally, we present empirically determined offsets to transform VISTA magnitudes into a true Vega system.
A. Cellino; P. Tanga; Marco Delbò; L. Galluccio; Philippe Bendjoya; F. De Angeli
Proceedings of the International Astronomical Union · DOI ↗
Abstract A new asteroid taxonomy will be an important result of Gaia observations of Solar System objects. Since Gaia observes asteroids in observing conditions and in an interval of wavelength which are slightly different with respect to normal ground-based observations, a dedicated observing campaign has been carried out at the Telescopio Nazionale Galileo in La Palma (Canary Islands, Spain). The obtained spectra have been used to generate a large number of synthetic clones, each one having slight changes with respect to its parent spectrum. These synthetic spectra are then used to feed the algorithm of taxonomic classification developed to reduce Gaia asteroid spectra. Processing of these data is in progress.
N. A. Walton; M. Rejkuba; J. R. Walsh; Albert A. Zijstra
Proceedings of the International Astronomical Union · DOI ↗
Abstract This paper comments on the use Gaia in studying the internal dynamics and morphologies of Planetary Nebulae (PN). It is noted that the second and subsequent releases of Gaia data, will have significant potential in unravelling PN morphologies.
F. van Leeuwen; D. W. Evans; F. De Angeli; C. Jordi; G. Busso; C. Cacciari; M. Riello; E. Pancino; G. Altavilla; A. G. A. Brown; et al.
Astronomy and Astrophysics · DOI ↗
Context. This paper presents an overview of the photometric data that are part of the first Gaia data release.
J. M. Carrasco; D. W. Evans; P. Montegriffo; C. Jordi; F. van Leeuwen; M. Riello; H. Voss; F. De Angeli; G. Busso; C. Fabricius; C. Cacciari; M. Weiler; E. Pancino; A. G. A. Brown; G. Holland; P. Burgess; P. Osborne; G. Altavilla; M. Gebran; S. Ragaini; S. Galleti; G. Cocozza; S. Marinoni; M. Bellazzini; A. Bragaglia; L. Federici; L. Balaguer-Núñez
Astronomy and Astrophysics · DOI ↗
Context. Gaia is an ESA cornerstone mission launched on 19 December 2013 aiming to obtain the most complete and precise 3D map of our Galaxy by observing more than one billion sources. This paper is part of a series of documents explaining the data processing and its results for Gaia Data Release 1, focussing on the G band photometry.
Gerry Gilmore; F. van Leeuwen
Physics World · DOI ↗
The Gaia spacecraft is conducting the most ambitious and thorough census of our galaxy ever attempted, gathering data on 100,000 stars every hour. With the mission's first major data release due this month, Gerry Gilmore and Floor van Leeuwen explain how the spacecraft works and assess its likely impact on the field of astrophysics
Camila Navarrete; Vasily Belokurov; S. E. Koposov; M. J. Irwin; M. Catelan; S. Duffau; A. J. Drake
Monthly Notices of the Royal Astronomical Society · DOI ↗
We take advantage of the deep and wide coverage of the VLT Survey Telescope (VST) ATLAS survey to study the line-of-sight structure of the Sagittarius (Sgr) stellar stream in the Southern hemisphere, only ∼40° away from the progenitor. We use photometrically selected subgiant branch (SGB) stars to reveal a complex debris morphology of the trailing arm and detect at least two clear peaks in the SGB distance modulus distribution. The separation between the two line-of-sight components is at least 5 kpc at the edge of the VST ATLAS footprint, but appears to change along the stream, which allows us to conclude that these detections correspond to two physically independent stellar structures, rather than a mix of co-distant stellar populations within a single stream. Our discovery of a fork in the Sgr trailing arm is verified using blue horizontal branch stars, and our distance measurements are calibrated using RR Lyrae stars from the Catalina Real-Time Transient Survey. Comparing with numerical simulations of the Sgr dwarf disruption, the more distant of the two components in the fork matches perfectly with the track of the trailing debris. However, no obvious counterpart exists in the simulation for the closer line-of-sight component.
G. Torrealba; S. E. Koposov; Vasily Belokurov; M. J. Irwin; Michelle Collins; M. Spencer; Rodrigo Ibata; Mario Mateo; Ana Bonaca; Prashin Jethwa
Monthly Notices of the Royal Astronomical Society · DOI ↗
We announce the discovery of the Aquarius 2 dwarf galaxy, a new distant satellite of the Milky Way, detected on the fringes of the VLT Survey Telescope (VST) ATLAS and the Sloan Digital Sky Survey (SDSS) surveys. The object was originally identified as an overdensity of red giant branch stars, but chosen for subsequent follow-up based on the presence of a strong blue horizontal branch, which was also used to measure its distance of ∼110 kpc. Using deeper imaging from the Inamori-Magellan Areal Camera and Spectrograph camera on the 6.5m Baade and spectroscopy with DEep Imaging Multi-Object Spectrograph on Keck, we measured the satellite's half-light radius 5.1 ± 0.8 arcmin, or ∼160 pc at this distance, and its stellar velocity dispersion of |$5.4^{+3.4}_{-0.9}$| km s−1. With μ = 30.2 mag arcsec−2 and MV = −4.36, the new satellite lies close to two important detection limits: one in surface brightness; and one in luminosity at a given distance, thereby making Aquarius 2 one of the hardest dwarfs to find.
J. Swan; Andrew A. Cole; Eline Tolstoy; M. J. Irwin
Monthly Notices of the Royal Astronomical Society · DOI ↗
We present a detailed analysis of the chemistry and kinematics of red giants in the dwarf irregular galaxy NGC 6822. Spectroscopy at ≈8500 Å was acquired for 72 red giant stars across two fields using FORS2 at the VLT. Line-of-sight extinction was individually estimated for each target star to accommodate the variable reddening across NGC 6822. The mean radial velocity was found to be 〈vrad〉 = −52.8 ± 2.2 km s−1 with dispersion σv = 24.1 km s−1, in agreement with other studies. Ca ii triplet equivalent widths were converted into [Fe/H] metallicities using a V magnitude proxy for surface gravity. The average metallicity was 〈[Fe/H]〉 = −0.84 ± 0.04 with dispersion σ = 0.31 dex and interquartile range 0.48. Our assignment of individual reddening values makes our analysis more sensitive to spatial variations in metallicity than previous studies. We divide our sample into metal-rich and metal-poor stars; the former were found to cluster towards small radii with the metal-poor stars more evenly distributed across the galaxy. The velocity dispersion of the metal-poor stars was found to be higher than that of the metal-rich stars (|$\sigma _{v_{\rm MP}}=27.4$| km s−1; |$\sigma _{v_{\rm MR}}=21.1$| km s−1); combined with the age–metallicity relation this indicates that the older populations have either been dynamically heated during their lifetimes or were born in a less disc-like distribution than the younger stars.. The low ratio vrot/σv suggests that within the inner 10 arcmin, NGC 6822's stars are dynamically decoupled from the H i gas, and possibly distributed in a thick disc or spheroid structure.
Vasily Belokurov; Denis Erkal; Alis J. Deason; S. E. Koposov; F. De Angeli; D. W. Evans; Filippo Fraternali; Dougal Mackey
Monthly Notices of the Royal Astronomical Society · DOI ↗
We present the discovery of stellar tidal tails around the Large and the Small Magellanic Clouds (LMC and SMC, respectively) in the Gaia DR1 data. In between the Clouds, their tidal arms are stretched towards each other to form an almost continuous stellar bridge. Our analysis relies on the exquisite quality of the Gaia's photometric catalogue to build detailed star-count maps of the Clouds. We demonstrate that the Gaia DR1 data can be used to detect variable stars across the whole sky, and, in particular, RR Lyrae stars in and around the LMC and the SMC. Additionally, we use a combination of Gaia and GALEX to follow the distribution of Young Main Sequence stars in the Magellanic System. Viewed by Gaia, the Clouds show unmistakable signs of interaction. Around the LMC, clumps of RR Lyrae are observable as far as ∼20°, in agreement with the most recent map of Mira-like stars reported in Deason et al. The SMC's outer stellar density contours show a characteristic S-shape, symptomatic of the onset of tidal stripping. Beyond several degrees from the centre of the dwarf, the Gaia RR Lyrae stars trace the Cloud's trailing arm, extending towards the LMC. This stellar tidal tail mapped with RR Lyrae is not aligned with the gaseous Magellanic Bridge, and is shifted by some ∼5° from the Young Main Sequence bridge. We use the offset between the bridges to put constraints on the density of the hot gaseous corona of the Milky Way.
P. Cruz; D. Barrado; J. Lillo-Box; Marcos Hernandez Díaz; Mercedes López‐Morales; Jayne Birkby; Jonathan J. Fortney; S. T. Hodgkin
Proceedings of the International Astronomical Union · DOI ↗
Abstract The Calar Alto Secondary Eclipse study was a program dedicated to observe secondary eclipses in the near-IR of two known close-orbiting exoplanets around K-dwarfs: WASP-10b and Qatar-1b. Such observations reveal hints on the orbital configuration of the system and on the thermal emission of the exoplanet, which allows the study of the brightness temperature of its atmosphere. The observations were performed at the Calar Alto Observatory (Spain). We used the OMEGA2000 instrument (Ks band) at the 3.5m telescope. The data was acquired with the telescope strongly defocused. The differential light curve was corrected from systematic effects using the Principal Component Analysis (PCA) technique. The final light curve was fitted using an occultation model to find the eclipse depth and a possible phase shift by performing a MCMC analysis. The observations have revealed a secondary eclipse of WASP-10b with depth of 0.137%, and a depth of 0.196% for Qatar-1b. The observed phase offset from expected mid-eclipse was of −0.0028 for WASP-10b, and of −0.0079 for Qatar-1b. These measured offsets led to a value for | e cosω| of 0.0044 for the WASP-10b system, leading to a derived eccentricity which was too small to be of any significance. For Qatar-1b, we have derived a | e cosω| of 0.0123, however, this last result needs to be confirmed with more data. The estimated Ks-band brightness temperatures are of 1647 K and 1885 K for WASP-10b and Qatar-1b, respectively. We also found an empirical correlation between the ( R ′ HK ) activity index of planet hosts and the Ks-band brightness temperature of exoplanets, considering a small number of systems.
C. Crowley; R. Kohley; N. C. Hambly; Michael Davidson; A. Abreu; F. van Leeuwen; C. Fabricius; G. M. Seabroke; J. H. J. de Bruijne; A. Short; L. Lindegren; A. G. A. Brown; G. Sarri; P. Garé; T. Prusti; Thibaut Prod’homme; A. Mora; J Martín-Fleitas; F. Raison; U. Lammers; William O’Mullane; F. Jansen
Dipòsit Digital de la Universitat de Barcelona (Universitat de Barcelona) · DOI ↗
The European Space Agency's Gaia satellite was launched into orbit around L2 in December 2013 with a payload containing 106 large-format scientific CCDs. The primary goal of the mission is to repeatedly obtain high-precision astrometric and photometric measurements of one thousand million stars over the course of five years. The scientific value of the down-linked data, and the operation of the onboard autonomous detection chain, relies on the high performance of the detectors. As Gaia slowly rotates and scans the sky, the CCDs are continuously operated in a mode where the line clock rate and the satellite rotation spin-rate are in synchronisation. Nominal mission operations began in July 2014 and the first data release is being prepared for release at the end of Summer 2016. In this paper we present an overview of the focal plane, the detector system, and strategies for on-orbit performance monitoring of the system. This is followed by a presentation of the performance results based on analysis of data acquired during a two-year window beginning at payload switch-on. Results for parameters such as readout noise and electronic offset behaviour are presented and we pay particular attention to the effects of the L2 radiation environment on the devices. The radiation-induced degradation in the charge transfer efficiency (CTE) in the (parallel) scan direction is clearly diagnosed; however, an extrapolation shows that charge transfer inefficiency (CTI) effects at end of mission will be approximately an order of magnitude less than predicted pre-flight. It is shown that the CTI in the serial register (horizontal direction) is still dominated by the traps inherent to the manufacturing process and that the radiation-induced degradation so far is only a few per cent. We also present results on the tracking of ionising radiation damage and hot pixel evolution. Finally, we summarise some of the detector effects discovered on-orbit which are still being investigated.
J. M. Carrasco; D. W. Evans; P. Montegriffo; C. Jordi; F. van Leeuwen; M. Riello; H. Voss; F. De Angeli; G. Busso; C. Fabricius; C. Cacciari; M. Weiler; E. Pancino; A. G. A. Brown; G. Holland; P. W. Burgess; P. Osborne; G. Altavilla; M. Gebran; S. Ragaini; S. Galleti; G. Cocozza; S. Marinoni; M. Bellazzini; A. Bragaglia; L. Federici; L. Balaguer-Núñez
Data Archiving and Networked Services (DANS) · DOI ↗
Context. Gaia is an ESA cornerstone mission launched on 19 December 2013 aiming to obtain the most complete and precise 3D map of our Galaxy by observing more than one billion sources. This paper is part of a series of documents explaining the data processing and its results for Gaia Data Release 1, focussing on the G band photometry. Aims. This paper describes the calibration model of the Gaia photometric passband for Gaia Data Release 1. Methods. The overall principle of splitting the process into internal and external calibrations is outlined. In the internal calibration, a self-consistent photometric system is generated. Then, the external calibration provides the link to the absolute photometric flux scales. Results. The Gaia photometric calibration pipeline explained here was applied to the first data release with good results. Details are given of the various calibration elements including the mathematical formulation of the models used and of the extraction and preparation of the required input parameters (e.g. colour terms). The external calibration in this first release provides the absolute zero point and photometric transformations from the Gaia G passband to other common photometric systems. Conclusions. This paper describes the photometric calibration implemented for the first Gaia data release and the instrumental effects taken into account. For this first release no aperture losses, radiation damage, and other second-order effects have not yet been implemented in the calibration.
Marcel Popescu; J. Licandro; David Morate; J. de León; Dan Alin Nedelcu; R. Rébolo; R. G. McMahon; E. González-Solares; M. J. Irwin
Astronomy and Astrophysics · DOI ↗
Context. The Sloan Digital Sky Survey (SDSS) and Wide-field Infrared Survey Explorer (WISE) provide information about the surface composition of about 100 000 minor planets. The resulting visible colors and albedos enabled us to group them in several major classes, which are a simplified view of the diversity shown by the few existing spectra. A large set of data in the 0.8−2.5 μm, where wide spectral features are expected, is required to refine and complement the global picture of these small bodies of the solar system.
T. Preibisch; S. T. Hodgkin; Mike Irwin; James R. Lewis; Robert R. King; M. J. McCaughrean; H. Zinnecker; Leisa K. Townsley; Patrick S. Broos
Abbreviated Abstract: The near-infrared study of the Carina Nebula in this paper builds on the results of the Chandra Carina Complex Project (CCCP), that detected 14368 X-ray sources in the 1.4 square-degree survey region, an automatic source classification study that classified 10714 of these as very likely young stars in Carina, and an analysis of their clustering properties. We used HAWK-I at the ESO VLT to conduct a very deep near-IR survey with sub-arcsecond angular resolution, covering about 1280 square-arcminutes. The HAWK-I images reveal more than 600000 individual infrared sources, whereby objects as faint as J ~ 23, H ~ 22, and Ks ~ 21 are detected at S/N >= 3. While less than half of the Chandra X-ray sources have counterparts in the 2MASS catalog, the ~5 mag deeper HAWK-I data reveal infrared counterparts to 6636 (= 88.8%) of the 7472 Chandra X-ray sources in the HAWK-I field. We analyze near-infrared color-color and color-magnitude diagrams to derive information about the extinctions, infrared excesses, ages, and masses of the X-ray selected objects. The near-infrared properties agree well with the results of the automatic X-ray source classification. The shape of the K-band luminosity function of the X-ray selected Carina members agrees well with that derived for the Orion Nebula Cluster, suggesting that, down to the X-ray detection limit around 0.5-1 Msun, the shape of the IMF in Carina is consistent with that in Orion (and thus the field IMF). The fraction of stars with near-infrared excesses is rather small, <=10%, but shows considerable variations between individual parts of the complex. The distribution of extinctions for the diskless stars ranges from ~1.6 mag to ~6.2 mag (central 80th percentile), clearly showing a considerable range of differential extinction between individual stars in the complex.
Clare Higgs; Alan W. McConnachie; M. J. Irwin; N. F. Bate; Geraint F. Lewis; Matthew G. Walker; P. Côté; Kim A. Venn; G. Battaglia
Monthly Notices of the Royal Astronomical Society · DOI ↗
We introduce the Solitary Local dwarfs survey (Solo), a wide-field photometric study targeting every isolated dwarf galaxy within 3 Mpc of the Milky Way. Solo is based on (&lt;it&gt;u&lt;/it&gt;)&lt;it&gt;gi&lt;/it&gt; multiband imaging from Canada–France–Hawaii Telescope/MegaCam for northern targets, and Magellan/Megacam for southern targets. All galaxies fainter than &lt;it&gt;M&lt;/it&gt;&lt;inf&gt;V&lt;/inf&gt; &amp;sime; −18 situated beyond the nominal virial radius of the Milky Way and M31 (&amp;gsim;300 kpc) are included in this volume-limited sample, for a total of 42 targets. In addition to reviewing the survey goals and strategy, we present results for the Sagittarius dwarf irregular galaxy (Sag DIG), one of the most isolated, low-mass galaxies, located at the edge of the Local Group. We analyse its resolved stellar populations and their spatial distributions. We provide updated estimates of its central surface brightness and integrated luminosity, and trace its surface brightness profile to a level fainter than 30 mag arcsec−2. Sag DIG is well described by a highly elliptical (disc-like) system following a single component S&lt;a&gt;&lt;ac&gt;e&lt;/ac&gt;&lt;ac&gt;´&lt;/ac&gt;&lt;/a&gt;rsic model. However, a low-level distortion is present at the outer edges of the galaxy that, were Sag DIG not so isolated, would likely be attributed to some kind of previous tidal interaction. Further, we find evidence of an extremely low level, extended distribution of stars beyond ∼5 arcmin (&gt;1.5 kpc) that suggests Sag DIG may be embedded in a very low-density stellar halo. We compare the stellar and H &lt;scp&gt;i&lt;/scp&gt; structures of Sag DIG, and discuss results for this galaxy in relation to other isolated, dwarf irregular galaxies in the Local Group.
B. Chehade; T. Shanks; Joseph R. Findlay; N. Metcalfe; Utane Sawangwit; M. J. Irwin; E. González-Solares; S. Fine; M. J. Drinkwater; S. M. Croom; Russell J. Jurek; David Parkinson; R. M. Bielby
Monthly Notices of the Royal Astronomical Society · DOI ↗
We present a new redshift survey, the 2dF Quasar Dark Energy Survey pilot (2QDESp), which consists of ≈10 000 quasars from ≈150 deg2 of the southern sky, based on VST-ATLAS imaging and 2dF/AAOmega spectroscopy. Combining our optical photometry with the WISE (W1,W2) bands we can select essentially contamination free quasar samples with 0.8 &lt; z &lt; 2.5 and g &lt; 20.5. At fainter magnitudes, optical UVX selection is still required to reach our g ≈ 22.5 limit. Using both these techniques we observed quasar redshifts at sky densities up to 90 deg−2. By comparing 2QDESp with other surveys (SDSS, 2QZ and 2SLAQ) we find that quasar clustering is approximately luminosity independent, with results for all four surveys consistent with a correlation scale of r0 = 6.1 ± 0.1 h−1 Mpc, despite their decade range in luminosity. We find a significant redshift dependence of clustering, particularly when BOSS data with r0 = 7.3 ± 0.1 h−1 Mpc are included at z ≈ 2.4. All quasars remain consistent with having a single host halo mass of ≈2 ± 1 × 1012 h−1 M⊙. This result implies that either quasars do not radiate at a fixed fraction of the Eddington luminosity or AGN black hole and dark matter halo masses are weakly correlated. No significant evidence is found to support fainter, X-ray selected quasars at low redshift having larger halo masses as predicted by the ‘hot halo’ mode AGN model of Fanidakis et al. (2013). Finally, although the combined quasar sample reaches an effective volume as large as that of the original SDSS LRG sample, we do not detect the BAO feature in these data.
T. Wevers; S. T. Hodgkin; P. G. Jonker; C. Bassa; G. Nelemans; T. van Grunsven; E. A. González-Solares; M. A. P. Torres; C. O. Heinke; D. Steeghs; Thomas J. Maccarone; C. T. Britt; R. I. Hynes; Christopher B. Johnson; Jianfeng Wu
Monthly Notices of the Royal Astronomical Society · DOI ↗
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centred at Galactic latitude b = 1| $_{.}^{\circ}$|5 above and below the Galactic Centre, spanning (l × b) = (6° × 1°). The catalogue consists of two or more epochs of observations for each line of sight in r′, i′ and H α filters. The catalogue is complete down to r′ = 20.2 and i′ = 19.2 mag; the mean 5σ depth is r′ = 22.5 and i′ = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the 1640 unique X-ray sources detected in Chandra observations of the GBS area, and find candidate optical counterparts to 1480 X-ray sources. We use a false alarm probability analysis to estimate the contamination by interlopers, and expect ∼10 per cent of optical counterparts to be chance alignments. To determine the most likely counterpart for each X-ray source, we compute the likelihood ratio for all optical sources within the 4σ X-ray error circle. This analysis yields 1480 potential counterparts (∼90 per cent of the sample). 584 counterparts have saturated photometry (r′ ≤ 17, i′ ≤ 16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i′ band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.
G. Torrealba; S. E. Koposov; Vasily Belokurov; M. J. Irwin
Monthly Notices of the Royal Astronomical Society · DOI ↗
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.We announce the discovery of the Crater 2 dwarf galaxy, identified in imaging data of the VLT Survey Telescope ATLAS survey. Given its half-light radius of ~1100 pc, Crater 2 is the fourth largest satellite of the Milky Way, surpassed only by the Large Magellanic Cloud, Small Magellanic Cloud and the Sgr dwarf. With a total luminosity of MV ≈ -8, this galaxy is also one of the lowest surface brightness dwarfs. Falling under the nominal detection boundary of 30 mag arcsec-2, it compares in nebulosity to the recently discovered Tuc 2 and Tuc IV and UMa II. Crater 2 is located ~120 kpc from the Sun and appears to be aligned in 3D with the enigmatic globular cluster Crater, the pair of ultrafaint dwarfs Leo IV and Leo V and the classical dwarf Leo II. We argue that such arrangement is probably not accidental and, in fact, can be viewed as the evidence for the accretion of the Crater-Leo group.