This week on HST

HST Programs: December 9 - December 15, 2013

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 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). The present program supplements those visual and near-infrared data by adding 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 pf star formation in lensed systems.

GO 13414: Reading the Record of Cometary Impacts into Jupiter's Rings

Jupiter's rings as imaged by the New Horizons mission in February 2007

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The Saturnian ring system is the most prominent in the Solar System, visible in Galileo's refractor and first clearly identified by Huyghens in 1679. However, all of the Jovian planets have ring systems. Jupiter's rings are much less substantial, and were first detected by Voyager 1 during its 1979 fly-by. They were surveyed in greater detail by the Galileo probe in the 1990s, by the new Horizons probe during its 2007 fly-by and have also been observed with Hubble. Recent re-analysis of the Galileo data has revealed the presence of significant structure, specifically a pattern of vertical ripples. These appear to have been triggered by dynamical interactions with Comet Shoemaker-Levy 9 during its passage through the rings en route to impact in 1994. Analysis of the New Horizons data appears to show the presence of at least four other spiral-like patterns within the rings, each of which may well be traced back to a past impact event or close comet/asteroid encounter. Jupiter is currently oriented such that the rings are slightly inclined with respect to the view from Earth, and it may therefore be possible to trace out further details of these features using Hubble data. This program therefore aims to use the UVIS channel on Wide-Field Camera 3 to image Jupiter to search for such featurs.

GO 13471: STIS Spectra of the Young SN Ia Remnant SN 1885 in M31

WFPC2 UV imaging of the central regions of M31; the likely location of SN1885 is marked

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. The Large Magellanic Cloud hosted a Type II supernova some 26 years ago, and Hubble has monitored the evolution of that remnant for almost 20 years. Moving beyond the Milky Way system, the most recent, indeed the only, supernova in the Andromeda galaxy was discovered in mid-August 1885, probably by Ernst Hartwig at Dorpat Observatory (there are unproven claims of prior observations. The supernova, identified as S Andromeda, peaked at magnitude 5.85 on August 21st and was still visible at 14th magnitude in early 1886. S Andromeda's discovery therefore came before the true nature of the Milky Way as an "island universe" had been elucidated, and the object was initially classified as a peculiar nova. Once the Andromeda nebula was a identified as a spiral galaxy it became clear that the variable was much more luminous, and a supernova. Even though the outburst occurred over 125 years ago, SN 1885's location has been pinpointed with the detection of Ca I and Fe II resonance-lice absorption within the expanding gaseous remnant. The present program aims to build on those observations by using STIS to obtain near-UV and optical spectra of the line absorption, probing the spatial and velocity structure of the remnant.