FLASH REPORT: ACS Suspend Recovery Side 1 was successfully completed at 2010/273/21:03 UTC, returning ACS to its nominal WFHROper state in readiness to intercept the on-going science timeline.
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.
The Stellar Origins of Supernovae
Supernovae (SNe) have a profound effect on galaxies, and have been used recently as precise cosmological probes, resulting in the discovery of the accelerating Universe. They are clearly very important events deserving of intense study. Yet, even with nearly 4000 known SNe, we know relatively little about the stars which give rise to these powerful explosions. The main limitation has been the lack of spatial resolution in pre-SN imaging data. However, since 1999 our team has been at the vanguard of directly identifying SN progenitor stars in HST images. From this exciting new line of study, the emerging trend from 5 detections for Type II- Plateau SNe is that their progenitors appear to be relatively low mass (8 to 20 Msun) red supergiants, although more cases are needed. Nonetheless, the nature of the progenitors of Type Ib/c SNe, a subset of which are associated with the amazing gamma-ray bursts, remains ambiguous. Furthermore, we remain in the continually embarrassing situation that we still do not yet know which progenitor systems explode as Type Ia SNe, which are currently being used for precision cosmology. In Cycle 16 we have triggered on the Type Ic SN 2007gr and Type IIb SN 2008ax so far. We propose to determine the identities of the progenitors of 4 SNe within 17 Mpc, which we expect to occur during Cycle 17, through ToO observations using ACS/HRC.
COS-GTO: Sampling the Local ISM with Hot White Dwarfs
We shall use hot white dwarf stars located within 150pc of the Sun to probe the absorption properties of the interstellar gas associated with the local cavity. There is still much debate concerning the ionization state of the local gas, since previously detected highly ionized lines (such as CIV and SiIV) could be associated with the circumstellar environments of hot white dwarfs. By using a priori knowledge of the velocity structure of the interstellar sight-lines to these targets (gained from high spectral resolution ground-based observations) in conjunction with the UV absorption data gained with HST-COS, we shall be able to better determine both the physical and chemical state of the numerous diffuse interstellar clouds present within the local cavity.
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.
The Impact of Starbursts on the Gaseous Halos of Galaxies
Perhaps the most important (yet uncertain) aspects of galaxy evolution are the processes by which galaxies accrete gas and by which the resulting star formation and black hole growth affects this accreting gas. It is believed that both the form of the accretion and the nature of the feedback change as a function of the galaxy mass. At low mass the gas comes in cold and the feedback is provided by massive stars. At high mass, the gas comes in hot, and the feedback is from an AGN. The changeover occurs near the mass where the galaxy population transitions from star-forming galaxies to red and dead ones. The population of red and dead galaxies is building with cosmic time, and it is believed that feedback plays an important role in this process: shutting down star formation by heating and/or expelling the reservoir of cold halo gas. To investigate these ideas, we propose to use COS far-UV spectra of background QSOs to measure the properties of the halo gas in a sample of galaxies near the transition mass that have undergone starbursts within the past 100 Myr to 1 Gyr. The galactic wind associated with the starburst is predicted to have affected the properties of the gaseous halo. To test this, we will compare the properties of the halos of the post-starburst galaxies to those of a control sample of galaxies matched in mass and QSO impact parameter. Do the halos of the post-starburst galaxies show a higher incidence rate of Ly-Alpha and metal absorption-lines? Are the kinematics of the halo gas more disturbed in the post-starbursts? Has the wind affected the ionization state and/or the metallicity of the halo? These data will provide fresh new insights into the role of feedback from massive stars on the evolution of galaxies, and may also offer clues about the properties of the QSO metal absorption-line systems at high-redshift .
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.
Infrared Survey of Star Formation Across Cosmic Time
We propose to use the unique power of WFC3 slitless spectroscopy to measure the evolution of cosmic star formation from the end of the reionization epoch at z>6 to the close of the galaxy-building era at z~0.3.Pure parallel observations with the grisms have proven to be efficient for identifying line emission from galaxies across a broad range of redshifts. The G102 grism on WFC3 was designed to extend this capability to search for Ly-alpha emission from the first galaxies. Using up to 250 orbits of pure parallel WFC3 spectroscopy, we will observe about 40 deep (4-5 orbit) fields with the combination of G102 and G141, and about 20 shallow (2-3 orbit) fields with G141 alone.
Our primary science goals at the highest redshifts are: (1) Detect Lya in ~100 galaxies with z>5.6 and measure the evolution of the Lya luminosity function, independent of of cosmic variance; 2) Determine the connection between emission line selected and continuum-break selected galaxies at these high redshifts, and 3) Search for the proposed signature of neutral hydrogen absorption at re-ionization. At intermediate redshifts we will (4) Detect more than 1000 galaxies in Halpha at 0.5< z<1.8 to measure the evolution of the extinction-corrected star formation density across the peak epoch of star formation. This is over an order-of-magnitude improvement in the current statistics, from the NICMOS Parallel grism survey. (5) Trace ``cosmic downsizing" from 0.5< z<2.2; and (6) Estimate the evolution in reddening and metallicty in star-forming galaxies and measure the evolution of the Seyfert population. For hundreds of spectra we will be able to measure one or even two line pair ratios -- in particular, the Balmer decrement and [OII]/[OIII] are sensitive to gas reddening and metallicity. As a bonus, the G102 grism offers the possibility of detecting Lya emission at z=7-8.8.
To identify single-line Lya emitters, we will exploit the wide 0.8--1.9um wavelength coverage of the combined G102+G141 spectra. All [OII] and [OIII] interlopers detected in G102 will be reliably separated from true LAEs by the detection of at least one strong line in the G141 spectrum, without the need for any ancillary data. We waive all proprietary rights to our data and will make high-level data products available through the ST/ECF.
Determining the Physical Nature of a Unique Giant Lya Emitter at z=6.595
We propose deep WFC3/IR imaging for a giant Lya emitter (LAE) with a Keck spectroscopic redshift of z=6.595 discovered by extensive narrow-band imaging with Subaru in the SXDS-UKIDSS/UDS field. This remarkable object is unique in many respects including its large stellar mass and luminous nebula which extends over 17 kpc; no equivalent source has been found in other surveys. The nature of this rare object is unclear. Fundamental to progress is determining the origin of star formation in such an early massive object; if the age of the stellar population is short we are likely witnessing a special moment in the formation history of a massive galaxy. The heating source for the nebula is also unclear; options include intense star formation, the infall of cold gas onto a dark halo or shock heating from a merger. We will take deep broad-band (F125W and F160W) images and an intermediate-band (F098M) image which will be analyzed in conjunction with ultra-deep IRAC 3.6 and 4.5 micron data being taken by the Spitzer/SEDS project. These data will enable us to constrain the star formation rate and stellar age. Moreover, the UV continuum morphology and Lya-line distribution will be investigated for evidence of a major merger, cold accretion, or hot bubbles associated with outflows. We will address the physical origin of the remarkable object observed at an epoch where massive galaxies are thought to begin their assembly.
A public SNAPSHOT Survey of Gamma-ray Burst Host Galaxies
We propose to conduct a public infrared survey of the host galaxies of Swift selected gamma-ray bursts (GRBs) at z<3. By obtaining deep, diffraction limited imaging in the IR we will complete detections for the host galaxies, and in concert with our extensive ground based afterglow and host programmes will compile a detailed catalog of the properties of high-z galaxies selected by GRBs. In particular these observations will enable us to study the colours, luminosities and morphologies of the galaxies. This in turn informs studies of the nature of the progenitors and the role of GRBs as probes of star formation across cosmic history. Ultimately it provides a product of legacy value which will greatly complement further studies with next generation facilities such as ALMA and JWST.
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).
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.
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).
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.