This week on HST

HST Programs: March 30 - April 5, 2015

Selected highlights

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. 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. 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 absolute 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 13679: Europa's Water Vapor Plumes: Systematically Constraining their Abundance and Variability

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter.The present program is conducting a methodical series of observations designed to image Europa at a range of orbital locations. STIS is being used to search for H and O auroral emissions at UV wavelengths and will aim to map the distribution of emission at different phases of the Europan orbit.

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 13868: Are Compton-Thick AGN the Missing Link Between Mergers and Black Hole Growth?

Composite optical/radio image of CenA, the elltipical merger that harbours the nearest AGN

AGN (Active Galactic Nuclei) systems are galaxies that exhibit strong non-thermal emission within their core regions. The energy is generally believed to stem from gas accretion onto a central supermassive black hole, with M > 107 MSun - similar in form, but less extreme in luminosity, to the energy source in QSOs. One of the key issues in such systems is understanding where the fuel comes from, since AGN can be found within galaxies that are morphologically similar to elliptical galaxies, which are predominantly gas poor at the present epoch. One possibility is through mergers, with the host galaxy assimilating smaller, gas-rich neighbours. Many nearby ellipticals are known to exhibit characteristics signatures of mergers - tidal tails, dust lanes and shells. Cen A is the classic example, which also happens to support a weak AGN and mild star formation within the dust lane. However, most previous surveys of AGN at moderate redshift (z < 2) have failed to find strong evidence for ongoing mergers. One possibility is that the very act of merging can hide AGN activity, as gas from the accreted satellite galaxy obscures the central black hole. The present program aims to test that hypothesis by using WFC3 to obtain near-infrared (F160W - H-band) imaging of 25 "Compton-thick" AGN systems at z~2 - systems that show evidence for significant line-of-sight obscuration by dust and gas. By working at near-infrared wavelengths, WFC3 will obtain detailed images of the galaxy morphology at rest-frame optical wavelengths, and will probe for the presence of merger signatures.