This week on HST

HST Programs: March 3 - March 9, 2014

Selected highlights

GO 12903: The Evolutionary Link Between Type 2 and Type 1 Quasars

Artist's impression of the black hole and surrounding torus in an AGN

This year (2013) is the fiftieth anniversary of the recognition that QSOs (quasars, quasi-stellar objects) were extremely luminous objects lying at substantial redshifts. The central engine powering these luminous objects is now recognised as a supermassive black hole, marking the central regions of a galaxy. As such, QSOs are clearly related to (and more luminous than) active galactic nuclei (AGN). Like AGNs, QSOs have been segregated into two categories based on their spectral properties: systems with broad lines are characterised as Type 1 QSOs; systems with narrow lines are classed as Type 2. As with AGN, the underlying cause of these differences is generally believed to reside more in our perspective than on the sources themselves: heavily obscured systems, where the central accretion disk lies behind a thick veil of dust, are observed as Type 2 systems; they are expected to evolve to form Type 1 systems as the dust is ablated and destroyed. The present SNAP program aims to test this scenario by coupling mid-infrared Herschel observations, probing the dust environment, with HST near-infrared WFC3 imaging of targets drawn from two matched samples of Type 1 and Type 2 QSOs.

GO 13332: A SNAPSHOT Survey of the Local Interstellar Medium: New NUV Observations of Stars with Archived FUV Observation

A map of the Local Stellar Neighbourhood

Understanding the nature and structure of gas within the interstellar medium is a key step towards understanding how material is recycled and how energetic processes, such as stellar winds and outflows, feed energy into the overall system. UV spectroscopy plays a key role in probing these effects: hot, background objects that produce relatively few intrinsic absorption features serve to map the the velocities and temperatures within the intervening gas along the line of sight. Observations of quasars are used to probe galaxy halos at moderate and high redshift; observations of hot stars provide similar information for gas in the Milky Way. The present program is using STIS to target stars within 100 parsecs of the Sun, studying the nearby interstellar medium. All of these stars have prior observations at far-UV wavelengths; the STIS data will cover the near-UV, surveying Fe II and Mg II absorption.This program builds on observations spanning 36 targets from Cycle 17.

GO 13386: Frontier Field Supernova Search
GO 13389: The Ultraviolet Frontier: Completing the Census of Star Formation at Its Peak Epoch

Pandora's Cluster, Abell 2744: the Chandra X-ray image, tracking hot gas, is plotted in red; the inferred dark matter distribution in blue

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. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs, two of which are highlighted here. Program GO 13386 uses the Frontier Field observations to search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. Program 13389 supplements the visual and near-infrared data in the core Frontier Fields program with deep imaging at near-UV wavelengths using the F275W and F336W filters on WFC3's UVIS camera. At the same time, the ACS-WFC camera is being used to obtain blue (F435W) and red (F606W) data on the associated parallel field from the Frontier Fields program. The UV data will enable investigation of the star formation rates and morphologies of moderate redshift galaxies, 0.5 < z < 3, lying behind the galaxy cluster. Combined with the Frontier Fields photometry, these data will enable more accurate photometric redshift determinations, probe Lyman escape fractions and offer the prospect of mapping the spatial distribution of star formation in lensed systems.

GO 13467: Follow The Water: The Ultimate WFC3 Exoplanet Atmosphere Survey

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered in 1995 through high-precision radial velocity measurements. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 400 in over 300 planetary systems, with the overwhelming majority identified by the Kepler satellite, which has also contributed close to 3,000 additional (very strong) candidates. As these observations have accumulated,the broad diversity of exoplanet systems has become increasingly apparent. Transiting systems are invaluable, since they provide not only unambiguous measurements of mass and diameter, but also an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, while high-precision ground-based observations have succeeded in constraining atmospheric properties in a few systems, the only successful detections of atmospheric features to date have been with HST and Spitzer. HST capabilities have been enhanced in the last few years with addition of spatial scanning, moving the target star over the chip in a controlled fashion during an observation. This allows observers to accumulate images or spectra of substantially higher signal-to-noise, a crucial advantage if one is looking for flux differences of elss than 1 part in 104. Past programs have accumulated observations of over a dozen exoplanets, using STIS at optical wavelengths and WFC3 in the near-infrared. The present program targets eight exoplanet systems with a diverse range of properties: HD 209458b,GL 3470b, HAT-P-26b, WASP-12b, WASP-18b, WASP-43b, WASP-80b and WASP-19b. The WFC3-IR G141 grism will be used to search for the characteristic near-infrared spectral features due to water in the amospheres of these exoplanets.