This week on HST

HST Programs: October 31 - November 6, 2016

Selected highlights

GO 14096: RELICS: Reionization Lensing Cluster Survey

Hubble image and mass map for the cluster ACT-CL J0102-4915, one of the clusters included in the RELICS program

The overwhelming majority of galaxies in the universe are found in clusters. As such, those 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, and the Frontier Fields program, targeting six clusters for deep multi-colour imaging. RELICS is focused on using massive galaxy clusters as gravitational telescopes, searching for strongly lensed background galaxies drawn from the high redshift universe. Imaging 46 fields in 41 galaxy clusters, this program aims to identify galaxies with redshifts in the range 9 < z < 12. By targeting strongly-lensing clusters, standard models for galaxy evolution suggest that the program can deliver ~100 galaxies in that redshift range, together with more than 150 galaxies at z~8. A significant number of these galaxies should be brighter than H~25.5, and therefore accessible to more detailed follow-up observations. Conversely, the actual number of galaxies detected will set constraints on the galaxy number-redshift distribution, and the overall formation and assembly history.

GO 14594: QSAGE: QSO Sightline And Galaxy Evolution

An HST GHRS spectrum of the bright quasar, HE 2347-4342

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. In particular, the star formation rate appears to have declined significantly between redshifts z~2 and z~1. The present program aims to compile observations that measure the star formation rate in a large sample of galaxies at z~1. This goal will be achieved by using the G141 grism on Wide Field Camera 3 to target fields centred on quasars where spectroscopy indicates the presence of gaseous absorbers 9ie galactic halos) at the appropriate redshift.

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 14709: HST/WFC3 Spectroscopy of < 400 AU Companions to Orion Young Stellar Objects

An image of the Orion Nebula superimposed on the 13CO map of Orion A (from this link ).

The Orion association is the largest nearby star-forming complex, providing a key laboratory for unlocking the secrets of star formation. As such, it has been subject to intense scrutiny at all wavelengths from both ground and space. Surveys at near-infrared and mid-infrared wavelenths, notably by Spitzer, have identified an extensive number of embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas. A follow-up HST proposal focused on more than 250 sources within the Orion A molecular cloud, the complex that includes the Orion Nebula Cluster. Initially,NICMOS was used to survey a subset of the protostars; following SM4, the WFC3-IR camera was applied to the task. The observations provided an excellent complement to Spitzer since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, HST gives a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images. That program resulted in the detection of several very faint companions, with luminosities consistent with planetary mass (5 MJ) objects. The present program is using the G141 grism on WFC3-IR to obtain spectra and determine the true nature of these objects with the overall goal of constraining the mass function of sub-stellar-mass companions.