This program comprises basic tests for measuring the read noise and dark current of the ACS WFC and for tracking the growth of hot pixels. The recorded frames are used to create bias and dark reference images for science data reduction and calibration. This program will be executed four days per week (Mon, Wed, Fri, Sun) for the duration of Cycle 17. To facilitate scheduling, this program is split into three proposals. This proposal covers 308 orbits (19.25 weeks) from 21 June 2010 to 1 November 2010.
SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii
Strong gravitational lensing provides the most accurate possible measurement of mass in the central regions of early-type galaxies (ETGs). We propose to continue the highly productive Sloan Lens ACS (SLACS) Survey for strong gravitational lens galaxies by observing a substantial fraction of 135 new ETG gravitational-lens candidates with HST-ACS WFC F814W Snapshot imaging. The proposed target sample has been selected from the seventh and final data release of the Sloan Digital Sky Survey, and is designed to complement the distribution of previously confirmed SLACS lenses in lens-galaxy mass and in the ratio of Einstein radius to optical half-light radius. The observations we propose will lead to a combined SLACS sample covering nearly two decades in mass, with dense mapping of enclosed mass as a function of radius out to the half-light radius and beyond. With this longer mass baseline, we will extend our lensing and dynamical analysis of the mass structure and scaling relations of ETGs to galaxies of significantly lower mass, and directly test for a transition in structural and dark-matter content trends at intermediate galaxy mass. The broader mass coverage will also enable us to make a direct connection to the structure of well-studied nearby ETGs as deduced from dynamical modeling of their line-of-sight velocity distribution fields. Finally, the combined sample will allow a more conclusive test of the current SLACS result that the intrinsic scatter in ETG mass-density structure is not significantly correlated with any other galaxy observables. The final SLACS sample at the conclusion of this program will comprise approximately 130 lenses with known foreground and background redshifts, and is likely to be the largest confirmed sample of strong-lens galaxies for many years to come.
Lmaging the Crab Nebula while it is Flaring in Gamma-rays
The high energy gamma-ray flux from the Crab nebula doubled in a couple of days (from September 19 to September 21). Such enhancement, never seen before, is clearly detected both by Agile and Fermi, the two gamma-ray telescopes currently in operation. While such huge brightening is exciting the entire astrophysical community, TOOs are being scheduled by high energy space observatories such as Integral, Swift and Chandra. By imaging the status of the knots and wisps in the inner Crab nebula, HST could provide a unique piece of information, which may yield a clue to understand the source behavior.
The Stellar Halo Profiles of Massive Disk Galaxies
Stellar halos surrounding massive galaxies are of prime interest in hierarchical galaxy formation models: most of the halo is formed by the very early accretion of small, metal poor satellite galaxies each with their independent evolution history. As such, halos contain the fossil remnants of the earliest star formation and accretion phases of a galaxy in formation. The resulting size, shape, age, and metallicity of stellar halos provide therefore a direct test of the basic ingredients (reionization, feedback from star formation, density fluctuation power spectrum) of hierarchical galaxy formation models.
In our GHOSTS survey we have sampled the principle axes of a sample of 11 nearby galaxies with Vrot>100 km/s. Our detection of resolved stellar halo populations ~1.5 mag below the tip of the Red Giant Branch has revealed halos that extend as far as 30 kpc around the most massive galaxies in our sample. Those extended stellar halos seem more compact than current model predictions, they have unexpectedly high metallicity up to the last detected point, and have a luminosity that is more closely related to the bulge luminosity than to the galaxy mass. We propose to extend the light profiles of 4 massive galaxies with a range in bulge-to-disk ratio to the background limit at ~70 kpc. This will enable us to:
- confirm the stellar halo shape (compactness) and assess with confidence any conflict with models using these very extended and accurate halo profile characterizations;
- establish whether stellar envelopes beyond 30 kpc are still morphologically connected to inner bulges, or whether a break occurs at larger radii revealing a distinct new component;
- determine whether every massive galaxy has an old, metal-poor halo at large radius like the Milky Way and M31; if not, constrain for the first time the range of stellar metallicity gradients in extended stellar halos.
COS-GTO: Io Atmosphere/STIS
We will use six HST orbits with COS to observe the disk-integrated longitudinal distribution of Io's atmosphere, and ten HST orbits with STIS to provide complementary disk-resolved information at key locations. We will use the COS G225M grating to observe four SO2 absorption bands, which can be used to determine SO2 atmospheric density. Disk-integrated 19 micron observations of the atmosphere indicate that the anti-Jupiter hemisphere of Io has an atmospheric density roughly ten times greater than the Jupiter-facing side (Spencer et al. 2005), and mm-wave observations suggest a similar pattern. However the infrared and mm-wave observations cannot easily separate atmospheric density from atmospheric temperature, so these results are model-dependent. Sparse 2100 2300 disk-resolved observations (McGrath et al. 2000, Jessup et al. 2004) tell a consistent story, but do not cover enough of Io's surface to provide full confirmation of the long-wavelength result. We will therefore observe Io's disk-integrated atmospheric density at six longitudes, roughly 30, 90, 150, 210, 270, and 330 W, to confirm the 19 micron results and improve our ability to model the 19-micron data. With STIS, we plan disk-resolved 2000-3200 spectroscopy of Io's SO2 atmosphere. Our observations will target low-latitude regions away from active plumes (in contrast to our Cycle 10 observations (Jessup et al. 2004) which targeted the Prometheus plume), to look for the effect of plumes on the atmosphere. We will also look at the variation of low-latitude atmospheric abundance with terrain type, to look for explanations for the large longitudinal variations in atmospheric pressure to be studied with COS. Finally, we will look at a variety of regions at two different times of day to determine the extent of diurnal variations in the atmosphere, which are expected if the atmosphere is dominantly supported by frost sublimation.
CCD Dark Monitor Part 2
Monitor the darks for the STIS CCD.
CCD Bias Monitor-Part 2
Monitor the bias in the 1x1, 1x2, 2x1, and 2x2 bin settings at gain=1, and 1x1 at gain = 4, to build up high-S/N superbiases and track the evolution of hot columns.
IR Zero Points
We will measure and monitor the zeropoints through the IR filters using observations of the white dwarf standard stars, GD153, GD71 and GD191B2B and the solar analog standard star, P330E. Data will be taken monthly during Cycle 17. Observations of the star cluster, NGC 104, are made twice to check color transformations. We expect an accuracy of 2% in the wide filter zeropoints relative to the HST photometric system, and 5% in the medium- and narrow-band filters.
The Atmospheric Structure of Giant Hot Exoplanets
Characterization of close-in giant exoplanets has proceeded rapidly over the past few years, due largely to Spitzer and HST observations in transiting systems. Low resolution thermal emission spectra of over two dozen planets have been measured by Spitzer, and HST observations of a few key planets have indicated unusual molecular abundances via transmission spectroscopy. However, current models for the atmospheric structure of these worlds exhibit degeneracies wherein different combinations of temperature and molecular abundance profiles can fit the same Spitzer data for each planet. Fortunately, the advent of the IR capability on HST/WFC3 allows us to solve this major problem in exoplanet science. We propose to inaugurate a Large HST program that is scientifically complementary to Spitzer, Kepler, and CoRoT exoplanet results.
We will obtain transmission spectroscopy of the 1.4-micron water band in a sample of 13 planets, using the G141 grism on WFC3. Among the abundant molecules, only water absorbs at this wavelength, and our measurement of water abundance will enable us to break the degeneracies in the Spitzer results with minimal model assumptions. We will also use the G141 grism to observe secondary eclipses for 7 very hot giant exoplanets at 1.5-microns, including several bright systems in the Kepler and CoRoT fields. The strong temperature sensitivity of the thermal continuum at 1.5-microns provides high leverage on atmospheric temperature for these worlds, again helping to break degeneracies in interpreting the Spitzer data. Moreover, our precise eclipse photometry, in combination with extant Spitzer data, will enable us to extrapolate the thermal continuum to optical wavelengths. Kepler and CoRoT teams will be thereby able to subtract the thermal contribution from their increasingly precise measurements of optical eclipses, and measure, or place extremely stringent limits on, the albedo of these exotic worlds.
Searching for the Missing Low-Mass Companions of Massive Stars
Recent results on binary companions of massive O stars appear to indicate that the distribution of secondary masses is truncated at low masses. It thus mimics the distribution of companions of G dwarfs and also the Initial Mass Function (IMF), except that it is shifted upward by a factor of 20 in mass. These results, if correct, provide a distribution of mass ratios that hints at a strong constraint on the star-formation process. However, this intriguing result is derived from a complex simulation of data which suffer from observational incompleteness at the low-mass end.
We propose a snapshot survey to test this result in a very direct way. HST WFC3 images of a sample of the nearest Cepheids (which were formerly B stars of ~5 Msun) will search for low-mass companions down to M dwarfs. We will confirm any companions as young stars, and thus true physical companions, through follow-up Chandra X-ray images. Our survey will show clearly whether the companion mass distribution is truncated at low masses, but at a mass much higher than that of the IMF or G dwarfs.
WFC3/UVIS Charge Injection Test
In preparation for making charge injection (CI) available to observers, this proposal will 1) confirm that the CI performs on-orbit as it did on the ground, 2) provide an initial assessment of which CI mode is most effective (10, 17, 25 line or continuous), and 3) obtain a baseline calibration for each mode.
Photometric Metallicity Calibration with WFC3 Specialty Filters
The community has chosen to include several filters in the WFC3 filter complement that have been designed to allow fairly precise estimates of stellar metallicities, and many science programs are enabled by this capability. Since these filters do not exactly match those used for this purpose on the ground, however, the mapping of stellar colors to stellar metallicities needs to be calibrated. We propose to achieve this calibration through observations of five stellar clusters with well known metallicities. We will calibrate several different filter calibrations which will allow future users to determine what filter combination best meets their science needs.
WFC3 UVIS CCD Daily Monitor
The behavior of the WFC3 UVIS CCD will be monitored daily with a set of full-frame, four-amp bias and dark frames. A smaller set of 2Kx4K subarray biases are acquired at less frequent intervals throughout the cycle to support subarray science observations. The internals from this proposal, along with those from the anneal procedure (Proposal 11909), will be used to generate the necessary superbias and superdark reference files for the calibration pipeline (CDBS).
UVIS Earth Flats
This program is an experimental path finder for Cycle 18 calibration. Visible-wavelength flat fields will be obtained by observing the dark side of the Earth during periods of full moon illumination. The observations will consist of full-frame streaked WFC3 UVIS imagery: per 22- min total exposure time in a single "dark-sky" orbit, we anticipate collecting 7000 e/pix in F606W or 4500 e/pix in F814W. To achieve Poisson S/N > 100 per pixel, we require at least 2 orbits of F606W and 3 orbits of F814W.
For UVIS narrowband filters, exposures of 1 sec typically do not saturate on the sunlit Earth, so we will take sunlit Earth flats for three of the more-commonly used narrowband filters in Cycle 17 plus the also-popular long-wavelength quad filters, for which we get four filters at once.
Why not use the Sunlit Earth for the wideband visible-light filters? It is too bright in the visible for WFC3 UVIS minimum exposure time of 0.5 sec. Similarly, for NICMOS the sunlit-Earth is too bright which saturates the detector too quickly and/or induces abnormal behaviors such as super-shading (Gilmore 1998, NIC 098-011). In the narrowband visible and broadband near- UV its not too bright (predictions in Cox et al. 1987 "Standard Astronomical Sources for HST: 6. Spatially Flat Fields." and observations in ACS Program 10050).
Other possibilities? Cox et al.'s Section II.D addresses many other possible sources for flat fields, rejecting them for a variety of reasons. A remaining possibility would be the totally eclipsed moon. Such eclipses provide approximately 2 hours (1 HST orbit) of opportunity per year, so they are too rare to be generically useful. An advantage of the moon over the Earth is that the moon subtends less than 0.25 square degree, whereas the Earth subtends a steradian or more, so scattered light and light potentially leaking around the shutter presents additional problems for the Earth. Also, we're unsure if HST can point 180 deg from the Sun.