This week on HST

HST Programs: October 27 - November 2, 2014

Selected highlights

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 13665: Exploring the Diversity of Exoplanet Atmospheres in the Super-Earth Regime

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ confirmed systems and ~2000 additional candidates. With the added numbers, observations have pushed detections to lower and lower masses, and it is now clear that the most common type of planet is the "super-Earth" - planets with masses that are several (3-6) times that of Earth and radii 2-4 times larger than Earth. One of the earliest examples is the planet circling the M dwarf, GJ 1214. Such planets have no obvious analogue in the Solar System, and the measured masses and diameters might reflect a range of interior structurees: large rocky bodies with relatively thin atmospheres; dense cores surrounded by a steam atmosphere; or "mini-Neptunes", with rock or ice cores surrounded by extended hydrogen or helium atmospheres. The present program aims to probe the diversity of these systems by using the G141 grism on Wide-Field Camera 3 to obtain time-resolved scanning observations of five transiting systems. The goal is to obtain data that will clearky distinguish between large scaleheight, hydrogen-dominated atmospheres and a more compact, steam-dominated systems.

GO 13716: Constraining the history of the outer Solar System: Definitive proof with HST

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known TNOs are binary (including Pluto, one of the largest known TNOs, regardless of whether one considers it a planet or not). TNOs are grouped within three broad classes: resonant objects, whose orbits are in mean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter class are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristics, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. Recent observations show a distinct break in the size distribution of these objects at diameters of ~40 km. The present proposal focuses on studying the cold classical objects, using the UVIS channel of Wide-Field Camera 3 to image the five smallest known objects to search for binaries and measure colours. If these objects have colours that are comparable with the larger objects, then the break in the size distribution probably reflects a primordial formation process; if they are bluer, then this suggests that the smaller objects may have formed collisionally.

GO 13745: Resolving the Tip of the Red Giant Branch of Two New Candidate Local Group Dwarf Galaxies

Subaru image of the Leo A dwarf galaxy

The Milky Way Galaxy is a member of a relatively sparse set of galaxies known as the Local Group. Fifty-four members are currently catalogued within ~1.5 Mpc., with the overwhelming majority being dwarf systems. The Milky Way and M31 are the two dominant members, with M33 the only other spiral system. Most of the dwarf galaxies are satellites of the three major systems, typically dwarf spheroidal systems that have exhausted most of their gaseous content and exhibit little or no current star formation. However, there are also a number of irregular dwarfs that are gas rich and exhibit ongoing star formation. HST is well suited to determining the distances of these systems: the high sensitivity of the Advanced Camera for Surveys and Wide-Field Camera 3 combined with the unparalleled angular resolution enables resolution of the most luminous stars; constructing the colour-magnitude provides access to a number of distance indicators, including the tip of the first red giant branch (RGB). Red giants have completed the core hydrogen-burning main-sequence stage of evolution and have moved to burning hydrogen in an inner shell. The maximum luminosity in this phase, and hence the location of the tip of the RGB, is set when the core reaches a sufficiently high temperature to ignite helium burning, the so-called helium flash. At that point, hydrogen shell-burning is extinguished, the star contracts and moves onto the horizontal branch. The present program focuses on two new candidates, identified as stellar concentration coincident with HI clumps detected in the GALFA-HI survey. Both have associated H-alpha emission consistent with dwarf galaxies, and both will be observed using ACS