This week on HST

HST Programs: July 8 - July 14, 2013

Selected highlights

GO 12861: Morphologies of the Most UV luminous Lyman Break Galaxies at z~3

The Lyman-break technique for detecting high redshift galaxies (from Burgarella et al, 2006)

Understanding galaxy formation and evolution remains one of the prime goals of 21st century astronomy and the focus of numerous HST observing programs. The present program aims to probe galaxy evolution in the z~3 regime by using HST to obtain high-resolution near-infrared imaging of 18 active star-forming galaxies from that epoch. The galaxies were detected using the Lyman break technique, which localises the redshift from photometric measurements, taking advantage of the sharp decrease in flux below the Lyman limit. Such systems apear as "drop outs" - objects that disappear in images taken shortward of ther redshfted Lyman limit. Conversely, the ability to detect the break demands that these Lyman Break Galaxies (LBGs) have substantial (rest-frame) UV flux, which translates to extensive star formation. The LBGs targeted by this program were identified from wide-field ground-based surveys as fallingin the redshift range 2.2 < z < 3.2. They are generally characterised with luminosities exceeding 6L*, implying that they are among the most luminous galaxies at this epoch, and therefore are likely to represent the formative stages of the most massive galaxies identified at the present epoch.

GO 12970: Completing the Census of Ultracool Brown Dwarfs in the Solar Neighborhood using HST/WFC3

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying a number of extremely interesting sources, including at least 4 objects that have been confirmed as dwarfs with temperatures lower than 350K. These are among the first examples of Y dwarfs, and all are too faint to be characterised with any degree of certainty using ground-based observations. The current program will use WFC3 G102 grism spectroscopy to verify the nature of a further 20 candidates.

GO 13055: Orbital Evolution and Stability of the Inner Uranian Moons

HST ACS images of the planet, part of the ring system and a number of the smaller moons

All of the gas giants in the outer Solar System possess an extensive entourage of small satellite moons. They all also possess ring systems. Uranus has (as of mid-2013) a total of 27 known satellite companions. The two largest moons, Titania and Oberon (almost all of the Uranian moons have Shakespearean names) have diameters of ~1500 km and were discovered by William Herschel shortly after he discovered the planet. Two other moons (Umriel and Ariel) have diameters of more than 1,100 km, while Miranda is around 472 km in diameter, but almost all of the remaining moons are smaller than 100 km in diametr, with the smallest (Mab, Margaret and Trinculo) closer to 20 km. As such, these smaller objects are likely to be captured KBOs. Nine of the moons are characterised as "irregular" moons, with orbits that lie well beyond that of Oberon. These moons form a dynamically chaotic system, and significant changes in orbital properties have been detected over a matter of 10 years or less. There is also evidence for a faint ring within this system. the major rings in the Uranian system were discovered in 1977, when the planet occulted the relatively anonymous 10th magnitude K1 subgiant, HD 128598. Photometry from the Kuiper Airborne Observatory, which was able to continuously monitor the event, showed a symmetric sequence of ten events (five before occultation, five after); combined with ground-based observations from Perth, Australia, a total of nine rings were identified. Subsequent observations by Voyager 2, both occultations and during its flyby, identified a few additional rings and revealed belts of dust. Particles in both the prominent inner rings, which have been imaged with HST, and the more tenuous outer rings have relatively short lifetimes, indicating that the ring system must be replenished on a regular basis. The present program aims to monitor the Uranian moons over the next three cycles to better establish the dynamics of this active system.

GO 13180: Search for a Transit of Alpha Centauri Bb, the First Earth-mass Exoplanet Orbiting a Sun-like Star

An artist's impression of the hypothetical terrestial companions of Alpha Cen B

Alpha Centauri is the closest stellar system to the Sun, lying at a distance of ~1.3 parsecs. Alpha cen is a triple system, comprising two solar-type stars, Alpha Cen A and B, spectral types G2 and K1, in relatively close proximity together with the M5 dwarf, Proxima Centauri, at a separation of 15,000 AU. As our nearest neighbours (Proxima is actually the closest star to the Sun) and among the brightest stars in the sky, the solar-type stars in Alpha Cen have been the target of numerous searches for planetary companions. Specifically, both stars have been subjected to extensive radial velocity monitoring by the Geneva planet-search team using the HARPS echelle spectrograph mounted on the 3.6-metre telescope at ESO's La Silla Observatory. In October 2012, the team announced the discovery of systematic radial velocity variations of amplitude of +/- 51 cm/sec and a period of 3.2 days; assuming moderate inclinations, this correponds to a terrestrial-mass planet (M~0.93 MEarth) in a orbit with semi-major axis 0.04 AU. These observations are on the edge of current detection techniques, and some controversy remains over whether the detection is confirmed or not. Clearly, this detection would not correspond to a habitable planet - the separation from the parent star is approximately one-tenth the radius of Mercury's orbit, leading to exceedingly high temperatures on the planetary surface. Nonetheless, the discovery of a terrestrial planetary companion around our nearest neighbour has strong implications (psychological as much as statistical) for the frequency of such systems. Assuming its existence, there is a 10% chance that the planet transits the host star. HST is the only facility that is currently capable of detecting such a planet in transit. The present observations are timed to coincide with the appropriate orbital phase.