This week on HST

HST Programs: September 22 - September 28, 2014

Selected highlights

GO 13375: Deep photometry of two accreted families of globular clusters in the remote M31 halo

Hubble WFPC2 image of the brightest M31 globular cluster, Mayall II/G1

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the installation on the ACS on HST at Servicing Mission 3B and then Wide-Field camera 3 in Servicing Mission 4, it became possible to resolve main sequence late-F and G dwarfs in M31, permitting observations that probe stars with luminosities significantly below the turnoff of the halo population. Over the past decade, the Panchromatic Hubble Andromeda Treasury (PHAT) program has imaged approximately one quarter of the galaxy's disk, while several programs coupling ground-based and HST observations have probed the overall structure of the M31 halo. One such project is the Pan-Andromeda Archaeological Survey (PAndAS), based primarily on imaging data from the Canada-France Hawaii telescope. Those observations have resulted in the discovery of numerous candidate globular cluster systems at projected distances up to 85 kpc from Andromeda's nucleus. In particular, the PAndAS group have identified two sets of clusters that appear to trace stellar debris streams within the halo, potentially tidally-stripped remnants of accreted satellite galaxies. The present HST program will use ACS to obtain deep imaging of 14 clusters, characterising their overall properties and enabling analysis for systemic similarities among members of the same families.

GO 13472: The Hubble Constant to 1%? STAGE 4: Calibrating the RR Lyrae PL relation at H-Band using HST and Gaia Parallax Stars

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. Cephids 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. 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. The absoltue calibration of that relationship, however, rests on only 4 nearby RR Lyraes with trigonometric parallax measurements. The present program aims to add to the sample of astrometricall well-observed RR Lyraes by using spatial scanning on WFC3 to determine accurate parallaxes for a sample of Galactic variables lying at distances up to several kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual stars, and an overall distance scale calibration accurate to better than 3%%.

GO 13498: HST Frontier Fields - Observations of MACSJ0717.5+3745

The Frontier Fields cluster, MACSJ0717.5 +3745

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 4-6 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. MACSJ0717.5+3745 at z=0.545 is the third target: at this first epoch of observation, the cluster is being imaged with WFC3-IR, with ACS being used to obtain optical data on the nearby blank field; the second epoch observations switch cameras, with ACS on the cluster and WFC3-IR on the parallel field.

GO 13726: Scattered light imaging of Fomalhaut's ice line belt to understand dynamical upheavals in planetary systems

HST-ACS image of the planetary-mass companion of Fomalhaut

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 that planet, the first direct imaging of a "conventional" exoplanet. The detection was confirmed in 2010 by HST observations with STIS, although further observations have shown evidence for variability at optical wavelengths and indicate that the orbit is highly eccentric. The latter characteristic might reflect interactions with a larger body in the system. The current prorgam aims to use STIC to obtain more detailed observations of the structure of the outer debris disk that might be used to constrain the presence of other planetary bodies.