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.
What are the Locations and Kinematics of Mass Outflows in AGN?
Mass outflows of ionized gas in AGN, first revealed through blueshifted UV and X-ray absorption lines, are likely important feedback mechanisms for the enrichment of the IGM, self-regulation of black-hole growth, and formation of structure in the early Universe. To understand the origin, dynamics, and impact of the outflowing absorbers on their surroundings, we need to know their locations (radial positions and polar angles with respect to the AGN rotation axes) and kinematics (radial and transverse velocities). We will use COS high-resolution spectra of 11 Seyfert 1 galaxies to derive velocity-dependent covering factors, ionic column densities, number densities (via metastable lines or variability), and ionization parameters (via photoionization models) of the UV absorbers, and thereby determine their radial locations as we have done for NGC 4151. We will use absorption variability over time scales of up to ~20 years, to determine transverse velocities and detect changes in radial velocities. We will use STIS G430M long-slit spectra and WFC3 [OIII] images to resolve the kinematics of the narrow-line region (NLR) and determine the inclinations of the AGN, to investigate the connection between nuclear absorption and NLR emission outflows and their dependence on polar angle.
How Dwarf Galaxies Got That Way: Mapping Multiphase Gaseous Halos and Galactic Winds Below L*
One of the most vexing problems in galaxy formation concerns how gas accretion and feedback influence the evolution of galaxies. In high mass galaxies, numerical simulations predict the initial fuel is accreted through 'cold' streams, after which AGN suppress star formation to leave galaxies red and gas-poor. In the shallow potential wells that host dwarf galaxies, gas accretion can be very efficient, and "superwinds" driven either by hot gas expelled by SNe or momentum imparted by SNe and hot-star radiation are regarded as the likely source(s) of feedback. However, major doubts persist about the physics of gas accretion, and particularly about SN-driven feedback, including their scalings with halo mass and their influence on the evolution of the galaxies. While "superwinds" are visible in X-rays near the point of their departure, they generally drop below detectable surface-brightness limits at ~ 10 kpc. Cold clumps in winds can be detected as blue-shifted absorption against the galaxy's own starlight, but the radial extent of these winds are difficult to constrain, leaving their energy, momentum, and ultimate fate uncertain. Wind prescriptions in hydrodynamical simulations are uncertain and at present are constrained only by indirect observations, e.g. by their influence on the stellar masses of galaxies and IGM metallicity. All these doubts lead to one conclusion: we do not understand gas accretion and feedback because we generally do not observe the infall and winds directly, in the extended gaseous halos of galaxies, when it is happening. To do this effectively, we must harness the power of absorption-line spectroscopy to measure the density, temperature, metallicity, and kinematics of small quantities of diffuse gas in galaxy halos. The most important physical diagnostics lie in the FUV, so this is uniquely a problem for HST and COS. We propose new COS G130M and G160M observations of 41 QSOs that probe the gaseous halos of 44 SDSS dwarf galaxies well inside their virial radii. Using sensitive absorption-line measurements of the multiphase gas diagnostics Lya, CII/IV, Si II/III/IV, and other species, supplemented by optical data from SDSS and Keck, we will map the halos of galaxies with L = 0.02 - 0.3 L*, stellar masses M* = 10^(8-10) Msun, over impact parameter from 15 - 150 kpc. These observations will directly constrain the content and kinematics of accreting and outflowing material, provide a concrete target for simulations to hit, and statistically test proposed galactic superwind models. These observations will also inform the study of galaxies at high z, where the shallow halo potentials that host dwarf galaxies today were the norm. These observations are low-risk and routine for COS, easily schedulable, and promise a major advance in our understanding of how dwarf galaxies came to be.
Hubble Infrared Pure Parallel Imaging Extragalactic Survey (HIPPIES)
WFC3 has demonstrated its unprecedented power in probing the early universe. Here we propose to continue our pure parallel program with this instrument to search for LBGs at z~6--8. Our program, dubbed as the Hubble Infrared Pure Parallel Imaging Extragalactic Survey ("HIPPIES"), will carry on the HST pure parallel legacy in the new decade. We request 205 orbits in Cycle-18, which will spread over ~ 50 high Galactic latitude visits (|b|>20deg) that last for 3 orbits and longer, resulting a total survey area of ~230 square arcmin. Combining the WFC3 pure parallel observations in Cycle-17, HIPPIES will complement other existing and forthcoming WFC3 surveys, and will make unique contributions to the study in the new redshift frontier because of the randomness of the survey fields. To make full use of the parallel opportunities, HIPPIES will also take ACS parallels to study LBGs at z~5--6. Being a pure parallel program, HIPPIES will only make very limited demand on the scarce HST resources, but will have potentially large scientific returns. As in previous cycle, we waive all proprietary data rights, and will make the enhanced data products public in a timely manner.
(1) The WFC3 part of HIPPIES aims at the most luminous LBG population at z~8 and z~7. As its survey fields are random and completely uncorrelated, the number counts of the bright LBGs from HIPPIES will be least affected by the "cosmic variance", and hence we will be able to obtain the best constraint on the bright-end of the LBG luminosity function at z~8 and 7. Comparing the result from HIPPIES to the hydrodynamic simulations will test the input physics and provide insight into the nature of the early galaxies. (2) The z~7--8 candidates from HIPPIES, 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. (3) The ACS part of HIPPIES will produce a significant number of candidate LBGs at z~5 and z~6 per ACS field. Combining with the existing, suitable ACS fields in the HST archive, we will be able to utilize the random nature of the survey to quantify the cosmic variance and to measure the galaxy bias at z~5--6, and therefore the galaxy halo masses at these redshifts. (4) 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.
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.
Cycle 18: 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.
UVIS Long Darks Test
Darks during SMOV showed a systematically lower global dark rate as well as lower scatter when compared to the Cycle 17 darks. Those two sets of exposures differ in exposure time - 1800 sec during SMOV and 900 sec during Cycle 17. Hypothetically, the effect could be caused by short-duration stray light, say ~500-sec in duration. During the latter part of Cycle 17, operation of WFC3 was changed to additionally block the light path to the detector with the CSM. This program acquires a small number of darks at the longer SMOV exposure times (1800 sec) in order to check whether the effect repeats in the new operating mode.
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.