This week on HST

HST Programs: March 31 - April 6, 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 13335: HST and Gaia, Light and Distance

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present SNAP program is part of a suite of HST programs focusing on the Galactic Cepheids that form the foundation for the whole distance ladder. These programs employ a revived version of an old technique to determine both accurate astrometry, hence trigonometric parallaxes and reliable distances, and accurate photometry, hence flux emasurements. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaining high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow not only multiple measurements of position differences for stars in the field but also extremely high signal-to-noise photometry. The latter is crucial in obtaining direct photometry of tghe local calibrations on the same HST system, the same system that is being used for photometry of Cephids in the external galaxies that serve as the basis for the distance scale. The present SNAP program includes 67 longer-period Galactic Cepheids.

GO 13389: The Ultraviolet Frontier: Completing the Census of Star Formation at Its Peak Epoch

Pandora's Cluster, Abell 2744: the Chandra X-ray image, tracking hot gas, is plotted in red; the inferred dark matter distribution in blue

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 Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs, two of which are highlighted here. Program 13389 supplements the visual and near-infrared data in the core Frontier Fields program with deep imaging at near-UV wavelengths using the F275W and F336W filters on WFC3's UVIS camera. At the same time, the ACS-WFC camera is being used to obtain blue (F435W) and red (F606W) data on the associated parallel field from the Frontier Fields program. The UV data will enable investigation of the star formation rates and morphologies of moderate redshift galaxies, 0.5 < z < 3, lying behind the galaxy cluster. Combined with the Frontier Fields photometry, these data will enable more accurate photometric redshift determinations, probe Lyman escape fractions and offer the prospect of mapping the spatial distribution of star formation in lensed systems.

GO 13620: Probing the atmosphere of a transiting ocean world: are there ice fountains on Europa?

An image of Europa taken by Voyage 2 in 1979

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, stongly suggesting that Europa's oceans may be vaporising into space. The present HST program also aims to search for outgassing, but in this by looking for absorption features against the smooth background light of Juptier while Europa is in transit. This DD program follows upon GO 134338, using STIS in time-tag mode to search from transient features in th far-UV, and applying coronagraphy at near-UV wavelengths to look for dust signatures.