This week on HST

HST Programs: March 27 - April 2, 2017

Selected highlights

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.

GO 14648: A New Threshold of Precision, 30 micro-arcsecond Parallaxes and Beyond

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 Cepheids in the external galaxies that serve as the basis for the distance scale. Observations have been obtained for more than 20 such stars. The present program aims to refine the distance estimates by obtaining four additional epochs for 9 core Cepheids (Z Sct, DD Cas, VX Per, SZ Cyg, SS CMa, XY Car, S Vul, X Pup and WZ Sgr). These data will improve the precision of the final parallaxes by identifying and eliminating binaries among ther eference stars, providing a longer baselien for proper motion determination, and providing direct overlap with Gaia observations.

GO 14666: Astrometric Light Deflection Test of General Relativity for Non-spherical Bodies: Close Approach to Jupiter

Gravitational light deflection by a massive body

Einstein's theory of General Relativity unifies gravity and space-time. GR contributes in a major fashion cosmological studies through gravitational lensing, where the spacetime distortions produced by massive objects deflect light from background objects. Indeed, one of the first observational tests of GR was made during the total solar eclipse of May 29, 1919; accurate astrometry of background stars in the vicinity of the Sun, visible during totality, confirmed the expected deflection. The present program aims to carry out exactly the same test, but with Jupiter, rather than the Sun, as the gravitational lens. Wide Field Camera 3 will be used to obtain images of stars in close proximity to Jupiter. WFC3 will be used in spatial scanning mode, enabling the relative positions of the stars to be determined to better than 1 microarcsecond. Images of the same field will be obtained when Jupiter has moved well away from the field. The relative separations of the stars will change as the gravitatonal deflections diminish. The expectation is that these measurements can not only test GR deflections predictions to better than 0.04%, but also probe the differential deflection produced by Jupiter's non-sphericity.

GO 14725: Hunting for Brown Dwarfs in Globular Clusters: Second Epoch Deep IR observations of the Globular Clusters M4

The globular cluster, M4

The globular cluster M4 lies at a distance of ~2.2 kiloparsecs towards the Galactic bulge. It is a relatively metal-rich system, with an average abundance of [Fe/H}~-1.07, or slightly below one tenth solar. As one of the nearest systems, M4 has been studied extensively with a wide range of ground- and space-based facilties, including Hubble. In particular, M4 was the target of a 123 orbit campaign with Wide-Field Planetary Camera 2 (WFPC2) in Cycle 9 (GO 8679), and a 120 orbit program using the Advanced camera for Surveys (GO 12911) in Cycle 20. Analyses of those data have revealed that, like many other clusters, M4 is more complex than previously thought, with multiple populations evident on the main sequence. The ACS observatons are extremely deep, reaching the lower extremes of the main-sequence marked by the hydrogren burning limit. indeed, those data offered the particularly intriguing possibility of detecting the tip of the brown dwarf sequence - that is, the most massive star-like objects that just failed to maintain long-term hydrogen burning, and have been cooling for the last ~12 Gyrs. The present program aims to confirm or deny that hypothesis through deep near-IR imaging with Wide Field Camera 3. Brown dwarfs are extremely cool, and will therefore have colours that are significantly redder than any background objects.