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Register a new userThe Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy
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V. Belokurov
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We report the discovery of an almost complete Einstein ring of diameter 10 in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6m telescope of the Special Astrophysical Observatory reveals that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km/s and a redshift of 0.444, whilst the source is a star-forming galaxy with a redshift of 2.379. From its color and luminosity, we conclude that the lens is an exceptionally massive Luminous Red Galaxy (LRG) with a mass within the Einstein radius of 5 x 10^12 solar masses. This remarkable system provides a laboratory for probing the dark matter distribution in LRGs at distances out to 3 effective radii, and studying the properties of high redshift star-forming galaxies.
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We report the discovery of a partial Einstein ring of radius 1.48arcsec produced by a massive (and seemingly isolated) elliptical galaxy. The spectroscopic follow-up at the VLT reveals a 2L* galaxy at z=0.986, which is lensing a post-starburst galaxy at z=3.773. This unique configuration yields a very precise measure of the mass of the lens within the Einstein radius, (8.3e11 +- 0.4)/h70 Msolar. The fundamental plane relation indicates an evolution rate of d [log (M/L)B] / dz = -0.57+-0.04, similar to other massive ellipticals at this redshift. The source galaxy shows strong interstellar absorption lines indicative of large gas-phase metallicities, with fading stellar populations after a burst. Higher resolution spectra and imaging will allow the detailed study of an unbiased representative of the galaxy population when the universe was just 12% of its current age.
We report the discovery of a giant, loop-like stellar structure around the edge-on spiral galaxy NGC 4013. This arcing feature extends 6 arcmin (~26 kpc in projected distance) northeast from the center and 3 arcmin (~=12 kpc) from the disk plane; likely related features are also apparent on the southwest side of the disk, extending to 4 arcmin (~17 kpc). The detection of this low surface-brightness muR= 27.0+0.3-0.2 mag/sqarcsec) structure is independently confirmed in three separate datasets from three different telescopes. Although its true three dimensional geometry is unknown, the sky- projected morphology of this structure displays a match with the theoretical predictions for the edge-on, projected view of a stellar tidal streams of a dwarf satellite moving in a low inclined (~25deg), nearly circular orbit. Using the recent model of the Monoceros tidal stream in the Milky Way by Penarrubia et al. as template, we find that the progenitor system may have been a galaxy with an initial mass 6*10^8 Msun, of which current position and final fate is unknown. According to this simulation, the tidal stream may be approximately ~2.8 Gyr of age. Our results demonstrate that NGC 4013, previously considered a prototypical isolated disk galaxy in spite of having one of the most prominent HI warps detected thus far, may have in fact suffered a recent minor merger. This discovery highlights that undisturbed disks at high surface brightness levels in the optical but warped in HI maps may in fact reveal complex signatures of recent accretion events in deep photometric surveys.
An emerging long obstacle placed in a boundary layer developing under a free-surface generates a complex horseshoe vortex (HSV) system, which is composed of a set of vortices exhibiting a rich variety of dynamics. The present experimental study examines such flow structure and characterizes precisely, using PIV measurements, the evolution of the HSV geometrical and dynamical properties over a wide range of dimensionless parameters (Reynolds number $Re_h in [750, 8300]$, boundary layer development ratio $h/delta in [1.25, 4.25]$ and obstacle aspect ratio $W/h in [0.67, 2.33]$). The dynamical study of the HSV is based on the categorization of the HSV vortices motion into an enhanced specific bi-dimensional typology, separating a coherent (due to vortex-vortex interactions) and an irregular evolution (due to appearance of small-scale instabilities). This precise categorization is made possible thanks to the use of vortex tracking methods applied on PIV measurements, A semi-empirical model for the HSV vortices motion is then proposed to highlight some important mechanisms of the HSV dynamics, as (i) the influence of the surrounding vortices on a vortex motion and (ii) the presence of a phase shift between the motion of all vortices. The study of the HSV geometrical properties (vortex position and characteristic lengths and frequencies) evolution with the flow parameters shows that strong dependencies exist between the streamwise extension of the HSV and the obstacle width, and between the HSV vortex number and its elongation. Comparison of these data with prior studies for immersed obstacles reveals that emerging obstacles lead to greater adverse pressure gradients and down-flows in front of the obstacle.
HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ring-like disk of circumstellar debris seen in 1.1 micron light scattered by the disk grains, surrounded by a di use outer region of lower surface brightness. The annular disk appears to be inclined by 31.7 +/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The total 1.1 micron flux density of the light scattered by the disk (at 1.2 < r < 5.0) of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy percent of the light from the scattering grains appears to be confined in a 36 AU wide annulus centered on the peak of the radial surface brightness (SB) profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of thermal emission estimated from IRAS and Spitzer flux densities assuming blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally symmetric, exhibits directionally preferential scattering well represented by a Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy arcsec^-2. No photocentric offset is seen in the ring relative to the position of the central star. A low surface brightness diffuse halo is seen in the NICMOS image to a distance of ~ 4 Deeper 0.6 micron HST/ACS PSF-subtracted coronagraphic observations reveal a faint outer nebulosity, asymmetrically brighter to the North of the star. We discuss models of the disk and properties of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU at the 87.6 AU radius of maximum surface density, and a total maximum dust mass of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~ 4 M(moon).
The SHINE program is a large high-contrast near-infrared survey of 600 young, nearby stars. It is aimed at searching for and characterizing new planetary systems using VLT/SPHEREs unprecedented high-contrast and high-angular resolution imaging capabilities. It also intends at placing statistical constraints on the occurrence and orbital properties of the giant planet population at large orbits as a function of the stellar host mass and age to test planet formation theories. We use the IRDIS dual-band imager and the IFS integral field spectrograph of SPHERE to acquire high-constrast coronagraphic differential near-infrared images and spectra of the young A2 star HIP65426. It is a member of the ~17 Myr old Lower Centaurus-Crux association. At a separation of 830 mas (92 au projected) from the star, we detect a faint red companion. Multi-epoch observations confirm that it shares common proper motion with HIP65426. Spectro-photometric measurements extracted with IFS and IRDIS between 0.95 and 2.2um indicate a warm, dusty atmosphere characteristic of young low surface-gravity L5-L7 dwarfs. Hot-start evolutionary models predict a luminosity consistent with a 6-12 MJup, Teff=1300-1600 K and R=1.5 RJup giant planet. Finally, the comparison with Exo-REM and PHOENIX BT-Settl synthetic atmosphere models gives consistent effective temperatures but with slightly higher surface gravity solutions of log(g)=4.0-5.0 with smaller radii (1.0-1.3 RJup). Given its physical and spectral properties, HIP65426b occupies a rather unique placement in terms of age, mass and spectral-type among the currently known imaged planets. It represents a particularly interesting case to study the presence of clouds as a function of particle size, composition, and location in the atmosphere, to search for signatures of non-equilibrium chemistry, and finally to test the theory of planet formation and evolution.
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