This week on HST

HST Programs: January 2 - January 8, 2017

Selected highlights

GO 14164: Exploring the nature and synchronicity of early cluster formation in the Local Group

The cluster NGC 2257 in the Large Magellanic Cloud

Globular clusters are members of the Galactic halo population, which formed during 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. Recent observations have shown that many of these systems have multiple stellar populations, with different chemical abundances and ages, indicating a much more complex star formation history than previously thought. Deep imaging with HST has enabled the construction of colour-magnitude diagrams that offer significant insight into the relative ages of Galactic globulars, revealing strong correlations between age and metallicity that are currently difficult to explain. The present program aims to extend observations to the oldest globular cluster systems in the nearby galaxies, primarily the Large Magellanic Cloud. The Advanced Camera for Surveys and Wide-Field Camera 3 will be used to obtain data in the F336W, F606W and F814W filters, corresponding to the U, R and I photometric passbands. Those data will enable a comparison of the earliest epochs of star formation in the Milky Way and LMC.

GO 14594: QSAGE: QSO Sightline And Galaxy Evolution

An HST GHRS spectrum of the bright quasar, HE 2347-4342

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. In particular, the star formation rate appears to have declined significantly between redshifts z~2 and z~1. The present program aims to compile observations that measure the star formation rate in a large sample of galaxies at z~1. This goal will be achieved by using the G141 grism on Wide Field Camera 3 to target fields centred on quasars where spectroscopy indicates the presence of gaseous absorbers (ie galactic halos) at the appropriate redshift.

GO 14682: A Search for Methane, Ammonia, and Water on Two Habitable Zone Super-Earths

Artist's impression of a planet an M dwarf system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. The succeeding two decades saw first 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. In 1999 the first transiting system, HD 209458b, was discovered.Increasing precision led to discoveries of lower mass planets and systems around lower-mass stars. The Kepler satellite, in particular, has made major contributions in this area, adding close to 3,000 confirmed exoplanets, both in its original incarnation and more recently in the extended Kepler 2 mission. These transiting systems are invaluable not only in providing unambiguous measurements of mass and diameter, but also in providing an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. The current program focuses on two "super-Earths" discovered in the Kepler 2 survey: K2-3d (1.6 Earth radii, 11.1 Earth masses) and K2-18b (2.24 Earth radii, no precise mass). In both cases, the parent star is an M-dwarf: K2-3 is one of three planets circling an M0 dwarf lying at a distance of ~40 parsecs; K2-18b is the as-yet only known companion of a more distant M0 dwarf. Both planets have orbits that lie within the nominal habitable zones of the parent stars. The present program will use the WFC3/IR grisms to map the near-IR spectral energy distribution through several transits, searching for characteristic features due to water, ammonia and (perhaps) methane, probing the relative atmospheric structure of these systems.

GO 14762: A UV census of the sites of core-collapse supernovae

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. Supernovae are intrinsically rare: Tycho's star (1604) was the last Galactic supernova identified by contemporary astronomers, although the radio remnant Cas A (identiied as 3C 461 in the 1959 Third Cambridge Catalogue of Radio Stars) may have been recorded, if not recognised as unusual, by Flamsteed in 1680. Understanding their progenitors therefore demands that we expand observations to external galaxies. The present program builds on several from previous cycles, and uses HST to obtain high-resolution imaging of the sites of several Type II supernovae in nearby galaxies. By now, the supernova themselves have faded from view, but the HST data can reveal the stellar population in the immediate environs. detailed analysis of the colour magnitude diagrams can probe the local environment,the likely age of the star forming regions, and hence set limits of the progenitor mass.