This week on HST

HST Programs: October 19 - October 25, 2015

Selected highlights

GO 13434: Transmission spectroscopy through the debris disk of Fomalhaut

HST-ACS image of Fomalhaut's debris disk and Fomalhaut b

Fomalhaut, or alpha Piscis Austrini, is one of the Sun's closest neighbours, an A-type star with a mass approximately twice that of the Sun and an age between 100 and 300 million years, lying at a distance of only ~7.7 parsecs. Observations with the IRAS satellite in the early 1980s revealed the presence of significant excess radiation at mid-infrared wavelengths, indicating the presence of substantial dust within a disk that is being irradiated by the luminous central star. Since then, observations of Fomalhaut and nearby stars of that ilk have led to a much more detailed characterisation of the debris disk phase. In particular, Spitzer has mapped warm dust at radii of 8-12 AU in these systems, while HST imaging has provided exquisite resolution of the structure of the outer (~100 AU) debris disks in reflected light. It is now recogised that debris disks are the evolutionary stage where planet formation has likely run to completion, the gas has fully dissipated but the disk remains well populated with dusty material spanning a wide range of sizes. Indeed, it is likely that this phase coincides with the heavy bombardment epoch within the Solar System. ACS imaging of Fomalhaut reveals extensive structure in the disk, notably a sharply-defined, eccentric inner edge to the disk, which led to the prediction of a ~Saturn-mass planet at that location. Subsequent ACS/HRC observations led to the identification of a candidate planet. The present program is focuesed more on probing the residual gas content of the debris disk. Fomalhaut's high proper motion carries the disk across a background white dwarf star. At 18th magnitude, the star is bright enough for spectroscopy with STIS and COS, allowing the proposers to search for characteristic signatures of refractory elements in the debris disk. Cycle 21 observations probed diffuse regions of the debris disk; the current Cycle 23 observations will probe the optically thick regime.

GO 14037: HST Frontier Fields - Observations of Abell S1063

The Frontier Fields cluster, Abell S1063

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. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. Abell S1063 at z=0.348 is the fifth target. The first epoch of observation are currently being obtained, with the cluster imaged with ACS and WFC3-IR obtaining obtain optical data on the nearby blank field. The second epoch observations, in April-June 2016, switch cameras, with ACS on the cluster and WFC3-IR on the parallel field.

GO 14174: Probing Extended Star Formation in the Young Massive Cluster NGC 1850

A Hubble image of the LMC cluster, NGC 1850

NGC 1850 is one of several massive star clusters in the Large Magellanic Cloud that rival globular clusters in total mass, but, unlike Galactic globulars, are significantly younger than 11-13 Gyrs. Also like globular clusters, these systems were initially believed to be the product of a songle star-forming event, but more detailed observations have shown that there is evidence for multiple epochs of star formation. In order for those evnts to occur, the cluster has to be sufficiently massive that it can retain gas after the initial star burst - in low mass systems, stellar winds would be expected to clear the system of residual gas, removing fuel for the next epoch of star formation. NGC 1850 is a ~90 Myr-old cluster in the LMC that appears to be sufficiently massive to retain gas, but the system has not yet been subjected to imaging sufficiently deep and of sufficient angular resolution to probe the detailed structure of the colour-magnitude diagram. The present program aims to use Wide-Field Camera 3 on Hubble to obtain deep imaging in the F275W and F814W filters. The broad wavelength span covered by those filters will be extremely effective in determining whether the cluster has undergone multiple star-forming events.

GO 14327: See Change: Testing time-varying dark energy with z>1 supernovae and their massive cluster hosts

HST/ACS images of a supernova in a galazy at z=1.2

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. The overall significance of this result has been recognised through the award of the Nobel prize and, most recently, the Fundamental Physics Breakthrough Prize to Perlmutter, Riess and Schmidt and their respective teams. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts. The seminal work in this field was carried out with ground-based telescopes, but Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Many previous HST supernovae programs have concentrated on field galaxies, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems, while the overall SNe detection rates are relatively low at high redshifts. The present program takes a different tack, and aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters. The expectation is that the majority of detections lie within dust-poor elliptical galaxies; moreover, supernova rates may be higher. The program will obtain ACS observations of ten of the most massive galaxy clusters lying at redshifts 1.1 < z < 1.75.