This week on HST

HST Programs: November 23 - November 29, 2015

Selected highlights

GO 13691: CHP-II: The Carnegie Hubble Program to Measure Ho to 3% Using Population II

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cepheids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions, while stellar population characteristics, such as the location of the tip of the red giant branch (TRGB), also play a role. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. Distances to stellar populations can also be derived from measuring the location of the TRGB, marking the point in intermediate- and low-mass star evolution where core temperatures are raised to the point thatn helium ignites in the triple-alpha reaction, and the star evolves rapidly onto the horizontal branch. The present programs combines deep optical/far-red/near-IR imaging of moderately distant galaxies that have hosted Type Ia supernovae and of nearby galaxies with RR Lyraes with WFC3-IR observations of 4 individual Galactic variables with trigonometric parallaxes to cosmntruct a distance ladder that is independent of the Cepheid calibration. The oevrall goal is to define H0 to 3% accuracy.

GO 13779: The Faint infrared Grism Survey (FIGS)

Grism spectra from the CANDELS program

One of the exciting capabilities offered by the post-SM4 Hubble Telescope is multi-object, low-resolution, near-infrared spectroscopy, using the two grisms available on the IR channel of Wide-Field Camera 3. Those observations provide an important avenue for complementing the various dep imaging surveys undertaken by HST. The 3D-HST program used relatively shallow observations to observe a significant fraction of the area covered by the CANDELS Multi-Cycle Treasury program. The present program, FIGS, targets only 4 fields, split between GOODS South and GOODS North, but with integrations totalling 40 orbits for each field. As a consequence, the observations will have significantly greater sensitivity, with the potential of measuring Lyman-alpha emission from galaxies at redshifts 5.5 < z < 8.5. The spectroscopic data will provide important additional information on the galaxy redshift distribution, on the formation of early-type galaxies at 1 < z < 2 and the evolution of star formation for moderate luminosity galaxies at z > 1.

GO 14138: Absolute Measurement of the Cosmic Near-Infrared Background Using Eclipsed Galilean Satellites as Occulters

Jupiter and the Galilean satellite Ganymede

The Cosmic Infrared Background is generally conjectured to represent the diffuse, redshifted light from star formation early in the post-recombination Universe. It provides an important link between the resolved structure that we see today and the primordial fluctuations measured by the cosmic mcirowave background. Measuring the CIB, however, is not a straightforward task, since there are several other sources of infrared radiation that dominate the measured fluxes, notably stars at near-infrared wavelengths, the zodiacal light at mid-infrared wavelengths and emission from Galactic cirrus in the far infrared. The present program proposes a novel mean of isolating the near-infrared contribution from one of those components, the zodiacal light. The WFC3 IR camera will be used to observe the two of the Galilean satellites during the period when they have entered the jovian shadow, and are therefore under a solar eclipse, but are still visible from Earth. The overwhelming majority of the zodiacal light is contributed by scattered light from dust particles between us and Jupiter; the Galilean satellites obscure any contribution to the near-infrared background from sources that lie beyond Jupiter's orbit, including contributions from the CIB. If the latter contributions are significant, then one would expect to see reduced flux (ie dark spots) in the satellite locations. The present observations target Europa and Ganymede during eclipse.

GO 14241: Cloud Atlas: Vertical Cloud Structure and Gravity in Exoplanet and Brown Dwarf Atmospheres

Ground-based imaging the the very low-mass brown dwarf binary, 2MASS1207

Recent years have seen the discovery of numerous extrasolar planets. Initially, most were discovered through radial velocity monitoring; more recently the superb photometric properties of the Kepler mission have led to the discovery of numerous transiting systems. Lagging behind in numbers, but offering the greatest prospect for probing physical conditions, is direct imaging. A handful of systems with resolved planetary companions have been discovered, most notably the multi-planet system around the nearby A-type star, HR 8799, and 2MASS1207B, the planetary-mass wide companion to the brown dwarf, 2MASS1207A, a member of the TW Hydrae association. These systems are expected to have gross properties that are similar to the Solar System gas giants, particularly atmospheric cloud structure. but they are much more massive and therefore have higher gravities. The present program aims to gain insight into their structure by comparing resolved exoplanets and higher mass, but similar temperature, brown dwarfs. Observations of isolated brown dwarfs have shown evidence for systematic variations in brightness, possibly due to dust within the atmosphere forming clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. Past programs have used both Spitzer and HST to monitor spectral variability in a number of systems. The present program will use Wide-field Camera 3 to obtain time-series near-infrared grism spectra of ten brown dwarfs and high-mass exoplanets. Variations in those spectra map the atmospheric cloud structure in each system, and those variations can be examined for correlations with the mass/gravity of the parent object.