This week on HST

HST Programs: August 18 - August 24, 2014

Selected highlights

GO 13297: The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. Up until about 5 years ago, the only known counter-example to convention was the cluster Omega Centauri, which is significantly more massive than most clusters and has both a complex main sequence structure and a range of metallicities among the evolved stars. High precision photometric observations with HST has demonstrated that Omega Cen is far from unique, with multiple populations evident in numerous other clusters, including NGC 2808, NGC 1851, 47 Tuc and NGC 6752. Multiple populations have also been discerned in a number of clusters in the Magellanic clouds. Sustaining multiple bursts of star formation within these systems demands that they retain gas beyond the first star forming event, which appears to set a requirement that these clusters were significantly more massive during their epoch of formation; put another way, the current globulars may represent the remnant cores of dwarf galaxy-like systems. That, in turn, implies that the stars ejected from those systems make a significant contribution to the current galactic halo. One of the most effective means of identifying and studying multi-population clusters is combining high-precision photometry over a wide wavelength range, particularly extending to UV wavelengths. Sixty-five globular clusters already have R/I (F606W, F814W) Hubble imaging and photometry thanks to the Cycle 14 program, An ACS Survey of Galactic Globular Clusters (GO 10775). The present program aims to build on those data by adding UV/blue observations using the F275W, F336W and F438W filters on the WFC3-UVIS camera. The colorus derived from these filters enable characterisation of the C, N and O abundances of the component stellar populations in these systems.

GO 13459: The Grism Lens-Amplified Survey from Space {GLASS}

HST imaging of the galaxy cluster MACS0717.5-3745

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Discretionary Time program (GO 13495). The present program supplements those visual and near-infrared data by adding imaging spectrophotometry at near-infrared wavelengths, using the low-resolution G102 and G141 grisms on the WFC3-IR camera. In all, the program targets 10 clusters drawn from both the Frontier Fields sample and from the larger-scale (but less deep) multicolour CLASH program. The goal is to identify and characterise galaxies at relatively high redshifts, close to the epoch of reionisation. The grism data should provide low-resolution spectra for 150-200 galaxies at redshifts z>6, with the potential to detect Lyman alpha emission at redshifts up to z~8.5. The present observations target the Frontier Fields cluster, MACS0717.5+3745.

GO 13633: A Kuiper Belt Object for the New Horizons Mission

Hubble Space Telescope images of the Pluto system, including the recently discovered moons, P4 and P5

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggest an icy composition, similar to Pluto and its main satellite, Charon. In recent years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; indeed, one object, Eris (2003 UB13), is slightly larger than Pluto (2320 km) and 25% more massive. We know the mass for Eris because it has a much lower mass companion, Dysnomia, which orbits Eris with a period of 16 days (see this recent press release ). Pluto itself has at least 5 companions: Charon, which is about 1/7th the mass of Pluto, and the much smaller bodies, Hydra, Nix, P4 and P5 discovered through HST observations within the last few years. The New Horizons Mission was launched on January 19th 2006 with the prime purpose of providing the first detailed examination of Pluto. The Pluto encounter represents the first phase of the originally-proposed mission. Following the fly-by, set for Bastille day in 2015, the aim is to re-direct New Horizons towards one or more smaller members of the Kuiper Belt, with the goal of providing a closer look at these icy bodies. However, New Horizons needs to identify an appropriate target - a KBO with orbital parameters such that New Horizons can use its modest complement of remaining fuel to reach the target. Adding a further complication, Pluto happens to lie within 5 degrees of the Galactic Plane and the consequent high star density has proven a barrier to deep ground-based searches. As a consequence, the New Horizons team was awarded Hubble time to search an area roughly the size of the full moon for a suitable target. These observations are significantly deeper than previous surveys, and as a result will lead to a better understanding of the size distribution of smaller objects (< 20 km diameter) in the Kuiper Belt.

GO 13863:Imaging Polarimetry of the 67P/Churyumov-Gerasimenko with ACS: Supporting the Rosetta Mission

Rosetta image of Comet 67P/Churymunov-Gerasimenko

The Rosetta mission is an ESA-led space mission that was launched in 2 March 2004 with the aim of making a rendezvous with a comet and dropping a small lander onto its surface. The original target was Comet 46P/Wirtanen, but problems with the Ariane launch vehicle led to launch delays, and Rosetta was re-targeted towards a different comet, 67P/Churyumov-Gerasimov. Achieving that rendezvous demanded a complex path through the solar system, with Rosetta undertaking a series of close passages of Mars and Earth that were designed to boost its velocity to sufficiently high values to match the comet. Those measures were successful, and the rendezvous occuring in early August, with Rosetta sending back spectacular pictures of the boot-shaped 67P. The present HST program is one of several using various facilities that aim to provide complementary data. The Advanced Camera for Surveys (ACS) is obtaining polarised images that can be used to probe the dust content in the coma in the immediate vicinity of the nucleau. HST's unparalleled angular resolution at optical wavelengths give it a significant advantage over larger ground-based telescopes.