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.
Determining the Sub-stellar IMF in the Most Massive Young Milky Way Cluster, Westerlund 1
Despite over 50 years of active research, a key question in galactic astronomy remains unanswered: is the initial mass function (IMF) of stars and sub-stellar objects universal, or does it depend on initial conditions? The answer has profound consequences for the evolution of galaxies as well as a predictive theory of star formation. Work to date suggests that certain environments (high densities, e.g. Elmegreen 2004; low metallicity, e.g. Larson 2005) should produce a top-heavy IMF, and there are hints from unresolved star-bursts that this might be the case. Yet, there is no clear evidence for an IMF that differs from that characterizing the Galactic field stars in a resolved stellar population down to one solar mass. Westerlund 1 is the most massive young star cluster known in the Milky Way. With an estimated mass of 5x10^4 Msun, an age of 3-5 Myr, and located at a distance of 3-4 kpc, it presents a unique opportunity to test whether the IMF in such a cluster deviates from the norm well down into the brown dwarf regime. We propose WFC3 near-IR imaging to probe the IMF down to 40 Jupiter masses. The data will enable use to: 1) provide a stringent test of the universality of the IMF under conditions approximating those of star-bursts; 2) search for primordial or dynamic mass segregation in the clusters; and 3) assess whether the cluster is likely to remain bound (as a massive open cluster) or disperse into the field. We will obtain images in the F125W, F160W, and F139M filters. The F139M filter covers a strong water absorption feature and the color F125W/F139M is a powerful temperature diagnostic in the range 2800-4000 K. This information will enable us to: a) confirm membership for low mass stars suspected on the basis of their position in the color-magnitude diagram; b) place the members in the HR diagram; and c) estimate the masses and ages of cluster members for low-mass stars and sub-stellar objects. This new capability offered with the WFC3 (through a novel combination of filter complement, high spatial resolution, and large field of view) will enable us to make a fundamental test of whether the IMF is universal on a unique resolved stellar population, as well as assess the clusters structure, dynamics, and ultimate fate.
Illuminating the HI Structure of a Proto-cluster Region at z=2.84
We propose very deep intermediate-band Lyman alpha imaging in the field of a newly-discovered proto-cluster region surrounding the extremely luminous QSO HS1549+19 at z=2.844. The large structure, initially discovered in a spectroscopic survey of galaxies in fields surrounding the brightest QSOs at z=2.5-2.8, represents an ideal laboratory for studying the response of the intergalactic medium to a source of ionizing photons that exceeds the UV background by factors >1000. Within a single pointing of WFC3-UVIS there are already more
than 45 known Lyman alpha emitters, most of which are already spectroscopically confirmed, and at least 3 of which are giant ?Lyman alpha blobs''. Many of the objects have properties similar to those expected from the process of fluorescence, in which Lyman alpha emission is induced by the UV radiation field of the QSO in any HI gas that dense enough to remain partially self-shielded. Fortuitously, the F467M filter (Stromgren "b") in WFC3-UVIS is a perfect match to Lyman alpha at z=2.844. In combination with an equally deep broad-band continuum image, the observations will allow the construction of a Lyman alpha map tracing dense gas throughout the inner parts of a proto-cluster region at sub-kpc resolution. The ability to measure the spatial sub-structure and surface brightness distribution of Lya emission, relative to known protocluster galaxies and AGN, will illuminate the "cosmic web'' in a dense region caught in a violent stage of formation.
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).