Gamma-ray bursts are the most luminous explosive events known, acting as beacons to the high redshift universe. Long duration GRBs have their origin in the collapse of massive stars and thus select star forming galaxies across a wide range of redshift. Due to their bright afterglows we can study the details of GRB host galaxies via absorption spectroscopy, providing redshifts, column densities and metallicities for galaxies far too faint to be accessible directly with current technology. We have already obtained deep ground based observations for many hosts and here propose ACS/WFC3 and WFC3 observations of the fields of bursts at z>3 which are undetected in deep ground based images. These observations will study the hosts in emission, providing luminosities and morphologies and will enable the construction of a sample of high-z galaxies with more detailed physical properties than has ever been possible before.
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 Dark Current Monitor
Analyses of ground test data showed that dark current signals are more reliably removed from science data using darks taken with the same exposure sequences as the science data, than with a single dark current image scaled by desired exposure time. Therefore, dark current images must be collected using all sample sequences that will be used in science observations. These observations will be used to monitor changes in the dark current of the WFC3-IR channel on a day-to-day basis, and to build calibration dark current ramps for each of the sample sequences to be used by Gos in Cycle 17. For each sample sequence/array size combination, a median ramp will be created and delivered to the calibration database system (CDBS).
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.
Multiple Stellar Populations in Galactic Globular Clusters
This is a proposal to bring the unique new properties of WFC3 to bear on the most exciting recent development in stellar populations: multiple generations of stars in globular clusters. From our vantage point in the midst of these developments, we feel that the present-day situation merits a concentration on increasing the depth of knowledge in clusters that are already known to have multiple populations, rather than merely increasing the list of clusters with perplexing peculiarities. We are therefore proposing to look for a clear splitting of the main sequence (and other sequences) of 47 Tuc, M4, M22, NGC 1851, and NGC 6752, and quantify them. The main-sequence study will cast particular light on the question of helium enrichment. Coupling the requested F275W data with F814W images available from the archive will allow us to follow the multiple sequences in the color magnitude diagram from the main sequence to the horizontal branch and asymptotic giant branch, and therefore constrain the effects of enhanced He and CNO on their evolution.
The Ultraviolet and Optical Counterparts of the Intermediate Mass Black Hole Candidate ESO 243-49 HLX-1
We request imaging observations of the record breaking hyper-luminous X-ray source and intermediate mass black hole candidate HLX-1 in the galaxy ESO 243-49, in order to investigate the nature of recent detections of UV emission and an optical counterpart. HLX-1 is currently the best candidate for an intermediate mass black hole, the possible building blocks of super-massive black holes found in the centres of galaxies. UV emission possibly associated with the X-ray source position was recently detected in lower resolution observations with the Swift and GALEX satellites. If this emission can be tied to HLX-1 and is point-like in nature, it will likely be dominated by emission from a hot accretion disc. By obtaining UV photometry we will be able to place constraints upon the temperature of the disc and therefore the mass of the black hole. The optical counterpart may be related to disc emission, though it is also possible that it is associated with a globular cluster or nucleated dwarf galaxy. By obtaining photometry of the counterpart in near-infrared to UV wavelengths we will be able to construct a broad-band SED, which will allow us to place firm constraints on the environment around this intriguing object.
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.
Search for Very High-z Galaxies with WFC3 Pure Parallel
WFC3 will provide an unprecedented probe to the early universe beyond the current redshift frontier. Here we propose a pure parallel program using this new instrument to search for Lyman-break galaxies at 6.520deg) that last for 4 orbits and longer, resulting a total survey area of about 140~230 square arcminute. Based on our understanding of the new HST parallel observation scheduling process, we believe that the total number of long-duration pure parallel visits in Cycle 17 should be sufficient to accommodate our program. We waive all proprietary rights to our data, and will also make the enhanced data products public in a timely manner.
(1) We will use both the UVIS and the IR channels, and do not need to seek optical data from elsewhere.
(2) Our program will likely triple the size of the probable candidate samples at z~7 and z~8, and will complement other targeted programs aiming at the similar redshift range.
(3) Being a pure parallel program, our survey will only make very limited demand on the scarce HST resources. More importantly, as the pure parallel pointings will be at random sight-lines, our program will be least affected by the bias due to the large scale structure ("cosmic variance").
(4) We aim at the most luminous LBG population, and will address the bright-end of the luminosity function at z~8 and z~7. We will constrain the value of L* in particular, which is critical for understanding the star formation process and the stellar mass assembly history in the first few hundred million years of the universe.
(5) The candidates from our survey, most of which will be the brightest ones that any surveys would be able to find, will have the best chance to be spectroscopically confirmed at the current 8--10m telescopes.
(6) We will also find a large number of extremely red, old galaxies at intermediate redshifts, and the fine spatial resolution offered by the WFC3 will enable us constrain their formation history based on the study of their morphology, and hence shed light on their connection to the very early galaxies in the universe.