Hubble Investigation of 103P/Hartley 2 in Support of NASA's DIXI Mission
Comet 103P/Hartley 2 is a small but highly active comet that will pass unusually close to the Earth (0.12 AU) during the fall of 2010, when it will also be visited by NASA's DIXI spacecraft. We propose a 15-orbit spectroscopic observing campaign with Hubble, comprised of three 5-orbit visits spanning a 2-month period, to measure the abundances of several key volatiles (CO, CO2, S2) and their possible seasonal variations. CO has not yet been detected in 103P, and Hubble may be the only facility capable of doing it. Hubble is also uniquely capable of providing confirmation of DIXI's measurements of the CO2 abundance. The DIXI flyby is an exceptional opportunity to study the nature of comets, and Hubble will contribute important and unique data to the international campaign supporting this mission.
SWELLS: Doubling the Number of Disk-dominated Edge-on Spiral Lens Galaxies
The formation of realistic disk galaxies within the LCDM cosmology is still largely an unsolved problem. Theory is now beginning to make predictions for how dark matter halos respond to galaxy formation, and for the properties of disk galaxies. Measuring the density profiles of dark matter halos on galaxy scales is therefore a strong test for the standard paradigm of galaxy formation, offering great potential for discovery. However, the degeneracy between the stellar and dark matter contributions to galaxy rotation curves remains a major obstacle. Strong gravitational lensing, when combined with spatially resolved kinematics and stellar population models, can solve this long-standing problem. Unfortunately, this joint methodology could not be exploited until recently due to the paucity of known edge-on spiral lenses. We have developed and demonstrated an efficient technique to find exactly these systems. During supplemental cycle-16 we discovered five new spiral lens galaxies, suitable for rotation curve measurements. We propose multi-color HST imaging of 16 candidates and 2 partially-imaged confirmed systems, to measure a sample of eight new edge-on spiral lenses. This program will at least double the number of known disk-dominated systems. This is crucial for constraining the relative contribution of the disk, bulge and dark halo to the total density profile.
Spectroscopy of Faint T Dwarf Calibrators: Understanding the Substellar Mass Function and the Coolest Brown Dwarfs
More than 100 methane brown dwarfs, or T dwarfs, have now been discovered in the local field with 2MASS, SLOAN and UKIDSS, opening up a new area of physics describing objects at 450-1400 K. However, very few calibrator objects exist with well established ages and metallicities. A very surprising result from the UKIDSS sample (supported by 2MASS and SLOAN) is that the substellar mass function in the local field appears to decline to lower masses, in marked contrast to the rising initial mass function (IMF) observed in young clusters. Given that such a difference between the present day IMF and the Galactic time-averaged IMF is unlikely, it is very possible that the apparently falling IMF is an artifact of serious errors in either T model atmospheres or the evolutionary isochrones. We propose WFC3 spectroscopy of 4 faint T dwarf calibrators with well established ages and metallicities in the Pleiades and Sigma Ori clusters, and 2 faint field T dwarfs from UKIDSS for comparison. These spectra will constitute vital calibration data for T dwarf atmospheres with a wide range of surface gravities, which will be used to test and improve the model atmospheres. They will also aid preparation for future spectroscopy of the much larger numbers of field T dwarfs to soon be found by VISTA and WISE. These new surveys will permit a more precise measurement of the mass function and detection of even cooler objects.
We seek to measure Pluto's albedo below 2100, to better constrain surface composition. COS observations will provide a substantial improvement in the S/N of Pluto spectra from <1800 to 2100. Accumulation of past HST/FOS spectra yields extremely low S/N below 2000 (S/N of only 1-3 in 100 bins; Krasnopolsky 2001). We expect to achieve S/N=5 at 1950 with 10 binning. In addition to spectrally broad albedo measurements, these observations could reveal line or molecular band emission, such as C I 1931 or CO 1993.
Ultra-Luminous X-Ray Sources in the Most Metal-Poor Galaxies
There is growing observational and theoretical evidence to suggest that Ultra-Luminous X-ray sources (ULX) form preferentially in low metallicity environments. Here we propose a survey of 27 nearby (< 30Mpc) star-forming Extremely Metal Poor Galaxies (Z<5% solar). There are almost no X-ray observations of such low abundance galaxies (3 in the Chandra archive). These are the most metal-deficient galaxies known, and a logical place to find ULX if they favor metal-poor systems. We plan to test recent population synthesis models which predict that ULX should be very numerous in metal-poor galaxies. We will also test the hypothesis that ULX form in massive young star clusters, and ask for HST time to obtain the necessary imaging data.
CCD Daily Monitor (Part 3)
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.
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).
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.
Cycle 17: UVIS Bowtie Monitor
Ground testing revealed an intermittent hysteresis type effect in the UVIS detector (both CCDs) at the level of ~1%, lasting hours to days. Initially found via an unexpected bowtie- shaped feature in flatfield ratios, subsequent lab tests on similar e2v devices have since shown that it is also present as simply an overall offset across the entire CCD, i.e., a QE offset without any discernable pattern. These lab tests have further revealed that overexposing the detector to count levels several times full well fills the traps and effectively neutralizes the bowtie. Each visit in this proposal acquires a set of three 3x3 binned internal flatfields: the first unsaturated image will be used to detect any bowtie, the second, highly exposed image will neutralize the bowtie if it is present, and the final image will allow for verification that the bowtie is gone.
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).
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.
CCD Dark Monitor Part 2
Monitor the darks for the STIS CCD.
Dynamical Masses and Radii of Four White Dwarf Stars
This proposal uses the FGS1R in Trans mode to resolve a pair of double degenerate binary systems (WD1639+153 and WD 1818+26) in order to determine their orbital elements. In addition, the binaries and several nearby field stars are observed by FGS1R in Pos mode to establish the local inertial reference frame of each binary, as well as its parallax and proper motion. This will allow for a direct measurement of the distance, which yields the intrinsic luminosity, and when combined with the spectroscopic estimates of the T_eff, the radius of each of the four WD stars. When combined with the orbital elements, this leads to a dynamical mass measurement for each WD, and a four calibration points of the WD mass-radius relation.
The LSD Project: Dynamics, Merging and Stellar Populations of a Sample of Well-Studied LBGs at z~3
A large observational effort with the ground-based ESO/VLT TELESCOPEs allowed us to obtain deep, spatially-resolved, near-IR spectra of complete sample of 11 Lyman-Break Galaxies at z~3.1. These observations were used to obtain, for the first time, the metallicity and the dynamical properties of a sample of objects that, albeit small, is representative of the total population of the LBGs. We propose to use HST to obtain high-resolution optical and near-IR images of this sample of LBGs in order to study the broad-band morphology and the stellar light distribution of these galaxies. These images, exploiting the superior spatial resolution of HST images and the low-background : 1- will allow a precise measure of the dynamical mass from the velocity field derived with spectroscopy; 2- will permit a comparison of the distribution of star formation (from the line emission) with the underlying stellar population, and, 3- will be used to check if the complex velocity field and the multiple line-emitting regions detected in most targets can be ascribed to on-going mergers. This accurate study will shed light on a number of unsolved problems still affecting the knowledge of the LBGs.
The Hosts of High Redshift Gamma-Ray Bursts
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.
Definitive ISM Abundances through Low-mass X-ray Binaries as Lighthouses
We propose observations of the UV spectra of two low-mass X-ray binaries (Sco X-1 and Cyg X-2) with existing Chandra X-Ray Observatory (CXO) data. From the X-ray data we will measure total (phase-independent) column densities of O, Ne, and Fe. From the UV data we will determine gas-phase column densities of H and O. The data in conjunction will allow us to make unique measurements of the total interstellar abundances of oxygen, neon, and iron, and direct measurements of the dust-phase abundances of O and Fe.
Obtaining the Missing Links in the Test of Very Low Mass Evolutionary Models with HST
We are proposing for spatially resolved ACS+HRC observations of 11 very low mass binaries spanning late-M, L and T spectral types in order to obtain precise effective temperature measurements for each component. All of our targets are part of a program in which we are measuring dynamical masses of very low-mass binaries to an unprecedented precision of 10% (or better). However, without precise temperature measurements, the full scientific value of these mass measurements cannot be realized. Together, mass and temperature measurements will allow us to distinguish between brown dwarf evolutionary models that make different assumptions about the interior and atmospheric structure of these ultra-cool objects. While dynamical masses can be obtained from the ground in the near-IR, obtaining precise temperatures require access to optical data which, for these sub-arcsecond binaries, can only be obtained from space with Hubble.
Star Formation, Extinction, and Metallicity at 0.7
The global star formation rate (SFR) is ~10x higher at z=1 than today. This could be due to drastically elevated SFR in some fraction of galaxies, such as mergers with central bursts, or a higher SFR across the board. Either means that the conditions in z=1 star forming galaxies could be quite different from local objects. The next step beyond measuring the global SFR is to determine the dependence of SFR, obscuration, metallicity, and size of the star-forming region on galaxy mass and redshift. However, SFR indicators at z=1 typically apply local calibrations for UV, [O II] and far-IR, and do not agree with each other on a galaxy-by-galaxy basis. Extinction, metallicity, and dust properties cause uncontrolled offsets in SFR calibrations. The great missing link is Balmer H-alpha, the most sensitive probe of SFR. We propose a slitless WFC3/G141 IR grism survey of GOODS-N, at 2 orbits/pointing. It will detect Ha+[N II] emission from 0.7 600 galaxies, and a small number of higher-redshift emitters. This will produce: an emission-line redshift survey unbiased by magnitude and color selection; star formation rates as a function of galaxy properties, e.g. stellar mass and morphology/mergers measured by ACS; comparisons of SFRs from H-alpha to UV and far-IR indicators; calibrations of line ratios of H-alpha to important nebular lines such as [O II] and H-beta, measuring variations in metallicity and extinction and their effect on SFR estimates; and the first measurement of scale lengths of the H-alpha emitting, star-forming region in a large sample of z~1 sources.
Dynamical Masses of the Coolest Brown Dwarfs
T dwarfs are excellent laboratories to study the evolution and the atmospheric physics of both brown dwarfs and extrasolar planets. To date, only a single T dwarf binary has a dynamical mass determination, and more are sorely needed. The prospects of measuring more dynamical masses over the next decade are limited to 6 known short-period T dwarf binaries. We propose here to obtain Long-Term HST/ACS monitoring for the 3 of the 6 binaries which cannot be resolved with AO from the ground. Upon completion, our program will substantially increase the number of T dwarf dynamical mass measurements and thereby provide key benchmarks for testing theoretical models of ultracool objects.
Are Low-Luminosity Galaxies Responsible for Cosmic Reionization?
Our group has demonstrated that massive clusters, acting as powerful cosmic lenses, can constrain the abundance and properties of low-luminosity star-forming sources beyond z~6; such sources are thought to be responsible for ending cosmic reionization. The large magnification possible in the critical regions of well-constrained clusters brings sources into view that lie at or beyond the limits of conventional exposures such as the UDF. We have shown that the combination of HST and Spitzer is particularly effective in delivering the physical properties of these distant sources, constraining their mass, age and past star formation history. Indirectly, we therefore gain a valuable glimpse to yet earlier epochs. Recognizing the result (and limitations) of blank field surveys, we propose a systematic search through 10 lensing clusters with ACS/F814W and WFC3/[F110W+F160W] (in conjunction with existing deep IRAC data). Our goal is to measure with great accuracy the luminosity function at z~7 over a range of at least 3 magnitude, based on the identification of about 50 lensed galaxies at 6.5
The Difference Between Neutral- and Ionized-Gas Metal Abundances in Local Star-Forming Galaxies with COS
The metallicity of galaxies and its evolution with redshift is of paramount importance for understanding galaxy formation. Abundances in the interstellar medium (ISM) are typically determined using emission-line spectroscopy of HII regions. However, since HII regions are associated with recent SF they may not have abundances typical for the galaxy as a whole. This is true in particular for star-forming galaxies (SFGs), in which the bulk of the metals may be contained in the neutral gas. It is therefore important to directly probe the metal abundances in the neutral gas. This can be done using absorption lines in the Far UV. We have developed techniques to do this in SFGs, where the absorption is measured for sightlines toward bright SF regions within the galaxy itself. We have successfully applied this technique to a sample of galaxies observed with FUSE. The results have been very promising, suggesting in I Zw 18 that abundances in the neutral gas may be up to 0.5 dex lower than in the ionized gas. However, the interpretation of the FUSE data is complicated by the very large FUSE aperture (30 arcsec), the modest S/N, and the limited selection of species available in the FUSE bandpass. The advent of COS on HST now allows a significant advance in all of these areas. We will therefore obtain absorption line spectroscopy with G130M in the same sample for which we already have crude constraints from FUSE. We will obtain ACS/SBC images to select the few optimal sightlines to target in each galaxy. The results will be interpreted through line-profile fitting to determine the metal abundances constrained by the available lines. The results will provide important new insights into the metallicities of galaxies, and into outstanding problems at high redshift such as the observed offset between the metallicities of Lyman Break Galaxies and Damped Lyman Alpha systems.