This week on HST

HST Programs: May 25 - May 31, 2015

Selected highlights

GO 13667: Observations of the Pluto System during the New Horizons Encounter

The first New Horizon image Pluto and Charon, from a distance of ~200 million km

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, is the primary target of NASA's New Horizons Mission and has been the subject of a range of supporting HST programs over the past few years. Those observing programs have added four moons to the Pluto system. James Christy originally identified the largest moon, Charon, in 1978 from photographic plates Charon has a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble added two small moons, christened Nix and Hydra, approximately 5,000 times fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar. More recent observations, in support of New Horizons mission, aimed to use WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. While no rings were detected unequivocally, two small satellite, christened "P4" and "P5", have been discovered. Both are significantly fainter than Nix and Hydra, and may well be as small as 10-13 km in size. There is also some evidence that might point to the presence of a debris ring within Charon's orbit. Most recently, Hubble carried out an extensive imaging survey to identify KBOs lying beyond pluto that might seve as targets for an extended new Horizons mission. Two such candidates were identified, and hubble will be conducting follow up observations to further constrain their orbits. The present observations are in direct support of the primary New Horizons mission. WFC3 will be used to provide a comprehensive set of direct imaging and grism observations that will provide reference data to the broad context for interpreting New Horizons data compiled during the July 14th system fly-by.

GO 13673: he Metallicity Dependence of the Initial Mass Function

The Small Magellanic Cloud and the globular cluster, 47 Tucanae

The initial mass function is the number of stars forming as a function of mass in star-formation region, and therefore describes how gas re-organises itself into self-gravitating entities. Since the rate of stellar evolution, and the exact path followed during that evolution, depends crucially on stellar mass, this distributiond describes what fraction of the gas content in a galactic system is processed to form other elements and returned to enrich the interstellar medium, and what fraction is locked up in long-lived, low-mass stars. Originally defined over 60 years ago by Edwin Salpeter, numerous investigations of different locations within the Milky Way galaxy and in nearby galaxies have generally concluded that the overall form is broadly consistent with the Ann E. Elk (ms) theory of the dnosaur - small at one end, big in the middle and small at the other; more quantitatively, the high mass stars apear to conform with a power-law distribution, M-2, that flattens near one solar mass, turns over and decreases throught the sub-stellar (<0.075 MSun) brown dwarf regime. Recent observatons, however, have suggested that the low-mass decline may not be as pronounced in the dense central environs of elliptical galaxies, while low-mass stars may be much less abundance in low-density ulrta-faint dwarf galaxies. The present program aims to examine a different facet of this question through deep imaging of the nearby Small Magellanic Cloud. Stars in the SMC are significantly less metal abundance than stars in the Milky Way, and the present program targets a field where the metallicity is [Fe/H]=-1.6, corresponding to stars with metallicities more than 40 times lower than the Sun. ACS and WFC3 will be used to obtain deep V- and I-band image, probing stars with masses less than 0.3 MSun<.sub>.

GO 13790: Frontier Field Supernova Search

Finding chart for the multiply imaged supernova, SN Refsdal, discovered in November 2014 in cluster MACJ1149

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. The present program is using the Frontier Field observations to search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. Recent observations of the fourth cluster, MACSJ1149.5+2223, resulted in the detection of a particularly unusual object - multiple lensed images of a supernova in a redshift z=1.49 galaxy that is itself multiply lensed. Each of those images results from light following a different path due to the gravitational potential of the foreground cluster and galaxies. Dubbed Supernova Refsdahl, after the gravitational lensing pioneer, the present program is continuing to obtain follow-up observations to monitor the light-curves of each component, hence determining the time-delay for each light path. Those measured delays can be matched against the predictions of gravitational lensing models. Moreover, since the galaxy itself has multiple images it is possible that future observations may detect images of the supernova in other components. Thus, this supernova can (perhaps even will) be discovered more than once - and might even be predicted.

GO 13872: The GOODS UV Legacy Fields: A Full Census of Faint Star-Forming Galaxies at z~0.5-2

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, GOODS, originated as a Spitzer Legacy program coupled with a Cycle 12 HST Treasury program. The program was designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field in Ursa Major and the Chandra Deep Field-South in Fornax. Initially, the program combined deep optical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. These two fields have become among the most studied celestial regions. In addition to deep HST data at optical and near-infrared wavelengths (both fields have been covered by NICMOS), the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton, and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Gemini, Surbaru and the ESO VLT. Part of the GOODS South field was covered by the WFC3 Early Release Science observations (see WFC3 ERS ), and both fields are also covered partially by one of the three Multi-Cycle Treasury programs allocated time in Cycle 18-20. Further observations were obtained in Cycle 17, using the G141 grism on the WFC3 IR camera to identify H-alpha+[N II] emission from galaxies at redshifts 0.7 < z < 1.5, and thereby set constraints on star formation at those redshifts. The present program builds on these multiple datasets by adding WFC3-UVIS imaging with the F275W and F336W. These data will cover the CANDELS sections of GOODs and sample far-UV radiation from galaxies at edshifts z > 0.5, tracing the evolution of the FUV luminosity through the peak epoch of star formation.