Classification of extremely red objects in the Hubble Ultra Deep Field

We present a quantitative study of the classification of Extremely Red Objects (EROs). The analysis is based on the multi-band spatial-and ground-based observa-tions (HST/ACS-BViz, HST/NICMOS-JH, VLT-JHK) in the Hubble Ultra Deep Field (UDF). Over a total sky area of 5.50 arcmin2 in the UDF, we select 24 EROs with the color criterion (i-K)vega 3.9, corresponding to (I-K)vega 4.0, down to KVega = 22. We develop four methods to classify EROs into Old passively evolving Galaxies (OGs) and Dusty star-forming Galaxies (DGs), including (i-K) vs. (J-K) color diagram,spectral energy distribution fitting method, Spitzer MIPS 24μm image matching, and nonparametric measure of galaxy morphology, and found that the classification results from these methods agree well. Using these four classification methods, we classify our EROs sample into 60Gs and 8 DGs to KVega < 20.5, and 80Gs and 16 DGs to KVega < 22, respectively. The fraction of DGs increases from 8/14 at KVega < 20.5to 16/24 at Kvega < 22. To study the morphology of galaxies with its wavelength, we measure the central concentration and the Gini coefficient for the 24 EROs in our sample in HST/ACS-i, z and HST/NICMOS-J, H bands. We find that the morphological param-eters of galaxies in our sample depend on the wavelength of observation, which suggests that caution is necessary when comparing single wavelength band images of galaxies at a variety of redshifts.     

The planet Mercury: Synthesis of resolved images of unknown part in the longitude range 250-290°W

Plans to send orbiter missions to Mercury (e.g., NASA's Messenger and ESA's BepiColombo) have prompted renewed efforts to investigate the surface of Mercury using ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g. HST) cannot be used at small angular distances (<45°) from the Sun (Mercury's elongation never exceeds 28° seen from Earth), advanced ground-based astronomical techniques and modern processing software can be used to construct resolved images of the poorly known part of Mercury. Our observations of the planet presented here were carried out mainly in April and May, 2002, at evening elongation of the planet, at the Skinakas astrophysical observatory of Heraklion University (Crete, Greece). A synthesis of the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974-1975), is presented. A double rim basin with an internal diameter of about 1000 km and an external rim about 2000 km is suggested by the data. We present the observational method, the data analysis approach, and the resulting images.     

Occultation of ε Geminorum by Mars: IV. Oblateness of the martian upper atmosphere

The oblateness of the Martian upper atmosphere was determined from analysis of photoelectric observations of the 8 April 1976 occultation of ε Geminorum by Mars at seven stations. The oblatness is 0.0096 ± 0.0023, consistent with a mean equator-to-pole temperature difference in excess of ～ 50°K, vertically averaged from the surface to the occulation altitude of ～70 km. The astrometric solution provides precise determination of the occultation path relative to the Martian shadow, and absolute vertical alignment of upper atmospheric temperature profiles obtained by inversion of occultation light curves. The observations can be compared directly with models of atmospheric tides computed for the conditions at the suboccultation regions on Mars.     

Observation of Microstructures in Planetary Nebulae

HST images of 5 Planetary nebulae showing small scale structures have been retrieved from the archives. For two PNe, NTT images were also available, and a double channel deconvolution technique applied to combine them to the HST images. A very precise flux calibration was performed on the HST images. Bow shocks and ionization effects are likely detected. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Location of the Nucleus of NGC 1068 from HST Imaging Polarimetry and Absolute Astrometry

We present the results of a program to obtain an accurate (better than 100 mas) astrometry of HST images of NGC 1068 and consequently a direct registration with radio images. The optical peak seen in the HST images is located at α = 02h42m40.711s, δ = -00°00′47.81 (J2000, FK5), with an error of 80 mas. The hidden nucleus, as determined by HST imaging polarimetry, falls at α = 02h42m40.710s, δ = -00°00′48.11. It is offset toward the South, i.e. along the radio axis, with respect to the inverted spectrum radio component, S1, by 170 mas (12 pc). This does not rule out that S1 is indeed associated with the obscuring torus and the central engine, but suggests that the nucleus of NGC 1068 might be radio silent or its emission absorbed also at radio wavelengths. An anti-correlation between radio and optical emission is revealed; the radio jet lies on a region of relatively low optical emission and is surrounded by line-emitting clouds. These results can be understood as due the interaction between the jet and the surrounding medium. The outflowing plasma is sweeping and heating the interstellar gas causing the line-emission to be highly enhanced along the edges of the radio jet. It appears that the morphology of the Narrow Line Region of NGC 1068 is dominated by the presence of a radio outflow, as already revealed by HST observations of several Seyfert galaxies with extended radio emission. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diagnosis and simulation of a rapidly developing cyclone related to a severe dust storm in East Asia

Mongolian cyclones are important to the outbreaks of severe dust storms in Northeast Asia. In this paper, we conduct a diagnostic study and a numerical simulation of a rapidly developing Mongolian cyclone that produced the April 5 to 7, 2000 severe dust storm in East Asia. The surface pressure of the cyclone decreased rapidly on April 5 to 6 causing strong surface winds. The diagnosis shows that the cyclone was a “dry cyclone”, as the convergence of moisture flux was weak and the baroclinic forcing was strong. The analysis of the Q vectors also reveals that ageostrophic wind was significant in the middle and upper part of the cyclone, resulting in strong vertical motion in the lower part. The three-dimensional structure of the cyclone is characterized by an ascending southern warm current and a descending northern cold current. This structure is favorable to the release of available potential energy and the intensification of the cyclone. In the second part of this study, we develop a coupled system of a dust emission scheme and a mesoscale numerical model (MM5V3) and applied the system to the simulation of the Mongolian cyclone. The main features of the cyclone identified from the diagnostic analysis are successfully reproduced. The predicted dust storm-affected area is found to be consistent with the meteorological observations and satellite remote sensing. It is shown that the coupled system is capable of predicting the spatial and temporal variations of the dust storm.     

Martian cratering 12. Utilizing primary crater clusters to study crater populations and meteoroid properties

Images from Mars Global Surveyor and later images from Mars Reconnaissance Orbiter reveal that roughly half of the meteoroids striking Mars (at meter to few decameter crater diameters) fragment in the Martian atmosphere, producing small clusters of primary impact craters. Statistics of these “primary clusters” yield valuable information about important Martian phenomena and properties of interplanetary bodies, including meteoroid behavior in the Martian atmosphere, bulk strengths of bodies striking Mars, and the fraction of Martian “field secondary” craters, a datum that would improve crater count chronometry. Many Martian impactors fragment at altitudes significantly higher than 18 km above the mean surface of Mars, and we find that most bodies striking Mars and Earth have low bulk strengths, consistent with crumbly or highly fractured objects. Applying statistics of primary clusters at various elevations and independent diameter bins, we describe a technique to estimate the percentage of semirandomly scattered “field secondary” craters. Our provisional estimate of this percentage, in the diameter range ~250 m down to ~22 m, is ~40% to ~80% of the total impacts, with the higher percentages at smaller diameters. Our data argue against earlier suggestions of overwhelming dominance by either primaries or secondaries in this diameter range.     

Pixel-based correction for Charge Transfer Inefficiency in the Hubble Space Telescope Advanced Camera for Surveys

Charge Transfer Inefficiency (CTI) due to radiation damage above the Earth's atmosphere creates spurious trailing in Hubble Space Telescope ( HST ) images. Radiation damage also creates unrelated warm pixels – but these happen to be perfect for measuring CTI. We model CTI in the Advanced Camera for Surveys (ACS)/Wide Field Channel and construct a physically motivated correction scheme. This operates on raw data, rather than secondary science products, by returning individual electrons to pixels from which they were unintentionally dragged during readout. We apply our correction to images from the HST Cosmic Evolution Survey (COSMOS), successfully reducing the CTI trails by a factor of ∼30 everywhere in the CCD and at all flux levels. We quantify changes in galaxy photometry, astrometry and shape. The remarkable 97 per cent level of correction is more than sufficient to enable a (forthcoming) reanalysis of downstream science products and the collection of larger surveys.     

A Prominent Apparition of Neptune's South Polar Feature

During the last week of June 2001, a bright apparition of Neptune's South Polar Feature (SPF) at 70°S was observed to develop and decay in less than 30 hours, displaying contrast of ∼2.5 at 619 nm. Assuming that the same SPF was observed on two consecutive rotations of Neptune, the feature moved eastward at 3.2±1.8° hr⁻¹ (130±80 m s⁻¹). The SPF made no obvious appearances during eight other Hubble Space Telescope (HST) observations of Neptune between July 2000 and June 2001, although there was a faint feature at 70°S in one image in October 2000. A prominent SPF was present in near-IR Keck Telescope images in August 2000. Bright SPFs are seen on ∼10% of the HST images of Neptune obtained since 1994, and a fainter SPF is visible on another ∼10%. An SPF bright enough to be visible at HST resolution was present around half the time during the last week of Voyager's approach to Neptune in August 1989, with one prominent brightening, suggesting that the SPF is less visible now than in 1989.     

Keck Adaptive Optics Images of Uranus and Its Rings

We present adaptive optic images of Uranus obtained with the 10-m W. M. Keck II telescope in June 2000, at wavelengths between 1 and 2.4 μm. The angular resolution of the images is ∼0.06-0.09″. We identified eight small cloud features on Uranus's disk, four of which were in the northern hemisphere. The latter features are ∼1000-2000 km in extent and located in the upper troposphere, above the methane cloud, at pressures between 0.5 and 1 bar. Our data have been combined with HST data by Hammel et al. (2001, Icarus153, 229-235); the combination of Keck and HST data allowed derivation of an accurate wind velocity profile. Our images further show Uranus's entire ring system: the asymmetric ? ring, as well as the three groups of inner rings (outward from Uranus): the rings 6+5+4, α+β, and the η+γ+δ rings. We derived the equivalent I/F width and ring particle reflectivity for each group of rings. Typical particle albedos are ∼0.04-0.05, in good agreement with HST data at 0.9 μm.     

The new gravitational lens system B1030 + 074

We report the discovery of a new double-image gravitational lens system, B1030 + 074, which was found during the Jodrell Bank–VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA, and optical observations using WFPC2 on the HST . The lensed images are separated by 1.56 arcsec and their flux density ratio at centimetric wavelengths is approximately 14:1, although the ratio is slightly frequency-dependent and the images appear to be time-variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be a spiral arm or an interacting galaxy. We have modelled B1030 + 074 using a singular isothermal ellipsoid which yields a time delay of 156/ h ₅₀ d. This lens is likely to be suitable for the measurement of the Hubble constant.     

Bi-Dimensional Spectrography at HST-Like Spatial Resolution: [OIII] Kinematics and Ionization in the NLR of NGC 1068

We present a study at HST-like spatial resolution of the gas kinematics in the innermost part of NGC 1068. This work is based on observations with the integral field spectrograph TIGER mounted on the CFH Telescope. We first derived the final spatial PSF of our data and checked its photometric integrity by comparison with two independent [OIII] HST images (FOC and WFPC2). We then performed a spatial deconvolution of the data-cube constraining the sum of the deconvolved velocity-slices to be as close as possible to the HST image. The resulting deconvolved data-cube has then a resolution similar to the HST image. It shows that the velocity distribution of the inner clouds detected with the HST differs strongly from a simple NE-SW ejection flow. We thus show that the previous interpretations of the double-peaked lines NE of the nucleus must be revisited. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Olympus volcano on Mars: Geometry and characteristics of lava flows

The HRSC (image 0037) and MOC imagery and MOLA altimetry were used to determine the following parameters of the lava flows typical of the southern slope of the Martian volcano Olympus: the length (13–35 km), the width (0.2–4.8 km), and the angles of ground slopes along which these flows advanced (3.4°–6.9°). To measure the thickness of the flows, we applied a method which had never been used before for Mars. In this method, the apparent thickness obtained from the MOC images and the slope steepness obtained from the MOLA data are used to determine the true thickness. The average estimates of the thickness of lava flows vary from 4 to 11 m and from 4 to 26 m for the volcano flanks and caldera scarps, respectively. These values are close to those of terrestrial basalt flows and to the lower limit found for the Martian flows by other researchers. Based on the performed measurements, we estimated the lava yield strength (0.9 × 10³?3.6 × 10⁴ Pa), the supply rate (24–137 m³/s), and the viscosity (1.4 × 10³?2.8 × 10⁷ kg/m s). These values are close to the estimates found for the Martian lavas by other researchers and to the characteristic values of these parameters for terrestrial lava flows with basalt and basalt-andesite composition.     

Unusual comets (?) as observed from the Hubble Space Telescope

In separate projects, the Hubble Space Telescope has been used to assess the nature of 3 unusual objects: Chiron, Pholus and P/Shoemaker-Levy 9. This paper will compare these objects and discuss how the unique capabilities of the HST may be used to address the issue of cometary activity in each. Chiron, which has exhibited obvious cometary characteristics for several years, might have a bound dust coma that is unresolvable from the ground. In an attempt to directly observe this bound coma, we have obtained a series of images of Chiron with the HST Planetary Camera. Inner coma structure out to 0.2 has been detected. From these observations we infer a low bulk nucleus density for Chiron. Both HST and ground-based images of 5145 Pholus have been obtained to search for evidence of activity. The ground-based data give the most sensitive limits; however, it is shown that the WFPC-2 on HST can give limits 2–3 orders of magnitude more sensitive than conventional ground-based limits. Finally, as part of a collaborative effort, we have been obtaining HST observations of SL9 in order to determine the fragment sizes and to assess their nature (i.e., cometary vs. asteroid). Both ground-based observations from the UH 2.2m telescope on Mauna Kea and HST observations show that the near-nucleus dust is redder than the sun. While FOS spectra did not detect OH emission, the WFPC-2 HST data show that the inner coma remained very circular from July 1993 up until 2 weeks prior to impact, implying continued production of dust.     

Unusual comets (?) as observed from the Hubble Space Telescope

In separate projects, the Hubble Space Telescope has been used to assess the nature of 3 unusual objects: Chiron, Pholus and P/Shoemaker-Levy 9. This paper will compare these objects and discuss how the unique capabilities of the HST may be used to address the issue of cometary activity in each. Chiron, which has exhibited obvious cometary characteristics for several years, might have a bound dust coma that is unresolvable from the ground. In an attempt to directly observe this bound coma, we have obtained a series of images of Chiron with the HST Planetary Camera. Inner coma structure out to 0.″2 has been detected. From these observations we infer a low bulk nucleus density for Chiron. Both HST and ground-based images of 5145 Pholus have been obtained to search for evidence of activity. The ground-based data give the most sensitive limits; however, it is shown that the WFPC-2 on HST can give limits 2–3 orders of magnitude more sensitive than conventional ground-based limits. Finally, as part of a collaborative effort, we have been obtaining HST observations of SL9 in order to determine the fragment sizes and to assess their nature (i.e., cometary vs. asteroid). Both ground-based observations from the UH 2.2m telescope on Mauna Kea and HST observations show that the near-nucleus dust is redder than the sun. While FOS spectra did not detect OH emission, the WFPC-2 HST data show that the inner coma remained very circular from July 1993 up until 2 weeks prior to impact, implying continued production of dust.     

A globular cluster in the dwarf galaxy Sextans B

We present spectroscopic observations of a massive globular cluster in the dwarf irregular galaxy Sextans B, discovered by us on the Hubble Space Telescope Wide Field and Planetary Camera 2 (HST WFPC2) images. Long-slit spectra were obtained with the SCORPIO spectrograph on the 6-m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. We determine the age, metallicity and alpha-element abundance ratio for the globular cluster to be 2 ± 1 Gyr, ?1.35 ±0.25 dex, and 0.1 ± 0.1 dex, respectively. Main photometric and structural parameters of it were determined using our surface photometry on the HST images. The mass (～10⁵ M⊙), luminosity and structural parameters appear to be typical of the globular clusters in our own Galaxy. Our findings shed new light on the evolutionary history of Sextans B.     

Ultraviolet observations of comet Hale-Bopp

Ultraviolet spectroscopy and imaging of comet Hale-Bopp (C/1995 O1) were obtained from a variety of space platforms from shortly after the discovery of the comet through perihelion passage. Observations with the International Ultraviolet Explorer (IUE) and the Hubble Space Telescope (HST) spanned the range of 6.8 to 2.7 AU pre-perihelion, but IUE was decommissioned in September 1996 and HST was precluded from near-perihelion observations because of its solar avoidance constraint. In September 1996, observations were made by the Extreme Ultraviolet Explorer (EUVE) that showed the presence of soft X-rays offset from the optical center of the coma and provided a sensitive spectroscopic upper limit to the Ne/O abundance ratio. During the perihelion period NASA mounted a successful campaign of four sounding rockets that were launched at the White Sands Missile Range, New Mexico, between March 25 and April 8, 1997. The payloads included long-slit spectroscopy and ultraviolet imaging polarimetry. In addition, Hale-Bopp was observed near perihelion by ultraviolet instruments on orbiting spacecraft that were designed for solar or terrestrial observations. Observations with HST, using the Space Telescope Imaging Spectrograph (STIS), installed during the February 1997 servicing mission, resumed in August 1997. Intercomparison of the ultraviolet observations and comparison with the results on gas composition and activity from ground-based visible, infrared and radio observations may permit the resolution of many discrepant results present in the literature. This revised version was published online in July 2006 with corrections to the Cover Date.     

The final phases of the Viking mission to Mars

Of the four spacecraft that the Viking Project put into operation at Mars in the summer of 1976, one continues to acquire data periodically. The missions of the two orbiters were terminated by the depletion of their attitude-control gas: Orbiter 2 in July 1978 and Orbiter 1 in August 1980. Lander 2 was shut down in April 1980 because of degradation of its batteries. Lander 1 is programmed to continue acquiring a modest number of imaging, meteorology, and ranging data periodically until December 1994. During its final year Orbiter 1 continued to produce excellent data from its full complement of instruments—two cameras, two infrared instruments (thermal mapper and water vapor detector), and the radio subsystem. The major emphasis was on photography, with 10,000 images being acquired. These included two very large swaths of high-resolution contiguous coverage of the Martian surface and the completion of the moderate-resolution mapping of nearly the entire surface, as well as miscellaneous other observations. The majority of these images has not been processed and examined, but the others have revealed many previously unobserved features and have greatly enhanced the base for a geological understanding of the planet. The history of Viking mission operations is brought up to date.     

Accurate Mars Express orbits to improve the determination of the mass and ephemeris of the Martian moons

The determination of the ephemeris of the Martian moons has benefited from observations of their plane-of-sky positions derived from images taken by cameras onboard spacecraft orbiting Mars. Images obtained by the Super Resolution Camera (SRC) onboard Mars Express (MEX) have been used to derive moon positions relative to Mars on the basis of a fit of a complete dynamical model of their motion around Mars. Since, these positions are computed from the relative position of the spacecraft when the images are taken, those positions need to be known as accurately as possible. An accurate MEX orbit is obtained by fitting two years of tracking data of the Mars Express Radio Science (MaRS) experiment onboard MEX. The average accuracy of the orbits has been estimated to be around 20–25 m. From these orbits, we have re-derived the positions of Phobos and Deimos at the epoch of the SRC observations and compared them with the positions derived by using the MEX orbits provided by the ESOC navigation team. After fit of the orbital model of Phobos and Deimos, the gain in precision in the Phobos position is roughly 30 m, corresponding to the estimated gain of accuracy of the MEX orbits. A new solution of the GM of the Martian moons has also been obtained from the accurate MEX orbits, which is consistent with previous solutions and, for Phobos, is more precise than the solution from the Mars Global Surveyor (MGS) and Mars Odyssey (ODY) tracking data. It will be further improved with data from MEX-Phobos closer encounters (at a distance less than 300 km). This study also demonstrates the advantage of combining observations of the moon positions from a spacecraft and from the Earth to assess the real accuracy of the spacecraft orbit. In turn, the natural satellite ephemerides can be improved and participate to a better knowledge of the origin and evolution of the Martian moons.