Status Report

NASA Hubble Space Telescope Daily Report #4037

By SpaceRef Editor
January 27, 2006
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NASA Hubble Space Telescope Daily Report #4037

HUBBLE SPACE TELESCOPE – Continuing to collect World Class Science


PERIOD COVERED: UT January 26, 2006 (DOY 026)


ACS/HRC 10512

Search for Binaries Among Faint Jupiter Trojan Asteroids

We propose an ambitious SNAPSHOT program to survey faint Jupiter Trojan asteroids for binary companions. We target 150 objects, with the expectation of acquiring data on about 50%. These objects span Vmag = 17.5-19.5, a range inaccessible with ground-based adaptive optics. We now have a significant sample from our survey of brighter Trojans to suggest that the binary fraction is similar to that which we find among brighter main-belt asteroids, roughly 2%. However, our observations suggest a higher binary fraction for smaller main-belt asteroids, probably the result of a different formation mechanism {evident also from the physical characteristics of the binaries}. Because the collision environment among the Trojans is similar to that of the Main Belt, while the composition is likely to be very different, sampling the binary fraction among the fainter Trojans should help us understand the collisional and binary formation mechanisms at work in various populations, including the Kuiper Belt, and help us evaluate theories for the origin of the Trojans. Calibration of and constraints on models of binary production and collisional evolution can only be done using these large-scale, real-life physical systems that we are beginning now to find and utilize.

ACS/HRC 10553

Bipolar Scattering Structures in AGN

The Unified Scheme for Seyfert galaxies successfully explains the basic distinctions between Type 1 and Type 2 AGN, including the existence of broad scattered lines in polarized flux spectra of the latter. However, it fails to account for the strongly polarized broad lines often observed in Type 1 AGN. We have discovered an intermediate type red QSO that exhibits polarization properties of both types; indeed it suggests a bipolar scattering geometry similar to that seen in Galactic protoplanetary nebulae. We request a small allocation with the high- resolution camera on ACS to image the object in two key spectral bands. The results will allow an unambiguous interpretation of ground-based data, and enable modeling of the inclination and opening angle that illuminates the scattering clouds. Like NGC 1068, a successful explanation of this object will not only allow further unification of AGN but also aid in unraveling the details of their inner structure.

ACS/HRC 10609

Sizes, Shapes, and SEDs: Searching for Mass Segregation in the Super Star Clusters of Nearby Starburst

We propose to investigate mass segregation and star cluster evolution and dissolution processes in Super Star Cluster {SSC} populations in a small sample of nearby starburst galaxies. ACS/HRC and NICMOS images of these nearby {d < 10 Mpc} starbursts can reveal evidence for mass segregation in the form of variations in size, shape, and color of the SSCs as a function of wavelength. The compactness of the cluster light profiles, and hence the stellar mass distributions, is a critical indicator of the likely fate of an SSC: long life and eventual evolution into a globular-like cluster, or dissolution. These observations will allow us to generate spectral energy distributions {SEDs} for a large sample of the SSCs at all ages and extinctions in each system. We will combine the SEDs with population synthesis models and existing ground- based spectra and Spitzer images to estimate ages, reddenings, and masses thus derive a more complete picture of the star-formation histories of the galaxies. For the brightest and most likely virialized among the SSCs we will also constrain their initial mass functions {IMFs} using high- resolution spectroscopy. Conclusions about IMFs from this technique require detailed information about the SSC concentration, light profiles, and virial status, which are only possible via ACS data. The proposed observations will provide an extensive and comprehensive data set for a large number of SSCs. By addressing the issues of mass segregation, evaporation, and destruction of SSC populations, the proposed observations will provide strong constraints on theories regarding the processes involved in the formation and evolution of SSCs and globular clusters. Given the dire predictions for the lifetime of HST, and its tremendous impact on the study of SSCs, we feel that the proposed observations not only are necessary and timely {even urgent} but will also be a fitting { and possibly final} addition to HST's legacy in the study of starburst SSCs.


Near-UV Snapshot Survey of Low Luminosity AGNs

Low-luminosity active galactic nuclei {LLAGNs} comprise ~30% of all bright galaxies {B<12.5} and are the most common type of AGN. These include low-luminosity Seyfert galaxies, LINERs, and transition-type objects {TOs, also called weak-[OI] LINERs}. What powers them is still at the forefront of AGN research. To unveil the nature of the central source we propose a near-UV snapshot survey of 50 nearby LLAGNs using ACS/HRC and the filter {F330W}, a configuration which is optimal to detect faint star forming regions around their nuclei. These images will complement optical and near-IR images available in the HST archive, providing a panchromatic atlas of the inner regions of these galaxies, which will be used to study their nuclear stellar population. Our main goals are to: 1} Investigate the presence of nuclear unresolved sources that can be attributed to an AGN; 2} Determine the frequency of nuclear and circumnuclear stellar clusters, and whether they are more common in Transition Objects {TOs} than in LINERs; 3} Characterize the sizes, colors, luminosities, masses and ages of these clusters; 4} Derive the luminosity function of star clusters and study their evaporation over time in the vicinity of AGNs. Finally, the results of this project will be combined with those of a previous similar one for Seyfert galaxies in order to compare the nature of the nuclear sources and investigate if there could be an evolution from Seyferts to TOs and LINERs. By adding UV images to the existing optical and near-IR ones, this project will also create an extremely valuable database for astronomers with a broad range of scientific interests.

ACS/WFC 10491

A Snapshot Survey of the most massive clusters of galaxies

We propose a snapshot survey of a sample of 124 high X-ray luminosity clusters in the redshift range 0.3-0.7. Similarly luminous clusters at these redshifts frequently exhibit strong gravitational lensing. The proposed observations will provide important constraints on the nature of the cluster mass distributions and a set of optically bright, lensed galaxies for further 8-10m spectroscopy. We acknowledge the broad community interest in this sample and waive our data rights for these observations.

ACS/WFC 10520

Resolving the Complex Star Formation History of the Leo I Dwarf Spheroidal Galaxy

Determining the star formation histories {SFHs} and chemical evolution of nearby galaxies gives us powerful constrains on the physical processes that regulate galaxy evolution. The SFHs can be measured most accurately by comparing the observed densities of stars in color-magnitude diagrams {CMDs} to predictions from stellar evolutionary models. WFPC2 imaging of the Leo I dSph shows it is unique because its stellar population is relatively young. Approximately 68% of its stars formed between 1 and 7 Gyr ago and only 12% of its stars formed >~ 10 Gyr ago. We propose to vastly improve the derived SFH of Leo I by exploiting ACS/WFC’s higher quantum efficiency at bluer wavelengths, higher spatial resolution, and larger field-of-view. The figure of merit for our proposed observations, defined as the age resolution times the number of stars detected, will be a factor of 12 higher than existing WFPC2 observations. To surmount the degeneracy of age and metallicity in the CMD, we have independently measured the metallicity distribution of its stars using spectroscopy. Simultaneously modeling the metallicity distribution and CMD, we will firmly constrain the evolution of the Leo I dSph, a unique example of an isolated dwarf galaxy that has not been influenced by interactions with the Milky Way or M31.

ACS/WFC 10543

Microlensing in M87 and the Virgo Cluster

Resolving the nature of dark matter is an urgent problem. The results of the MACHO survey of the Milky Way dark halo toward the LMC indicate that a significant fraction of the halo consists of stellar mass objects. The VATT/Columbia survey of M31 finds a similar lens fraction in the M31 dark halo. We propose a series of observations with ACS that will provide the most thorough search for microlensing toward M87, the central elliptical galaxy of the Virgo cluster. This program is optimized for lenses in the mass range from 0.01 to 1.0 solar masses. By comparing with archival data, we can detect lenses as massive as 100 solar masses, such as the remnants of the first stars. These observations will have at least 15 times more sensitivity to microlensing than any previous survey, e.g. using WFPC2. This is due to the factor of 2 larger area, factor of more than 4 more sensitivity in the I-band, superior pixel scale and longer baseline of observations. Based on the halo microlensing results in the Milky Way and M31, we might expect that galaxy collisions and stripping would populate the overall cluster halo with a large number of stellar mass objects. This program would determine definitively if such objects compose the cluster dark matter at the level seen in the Milky Way. A negative result would indicate that such objects do not populate the intracluster medium, and may indicate that galaxy harassment is not as vigorous as expected. We can measure the level of events due to the M87 halo: this would be the best exploration to date of such a lens population in an elliptical galaxy. Star-star lensing should also be detectable. About 20 erupting classical novae will be seen, allowing to determine the definitive nova rate for this giant elliptical galaxy. We will determine if our recent HST detection of an M87 globular cluster nova was a fluke, or indicative of a 100x higher rate of incidence of cataclysmic variables and nova eruptions in globulars than previously believed. We will examine the populations of variable stars, and will be able to cleanly separate them from microlensing.

ACS/WFC 10618

The Light Echoes around V838 Monocerotis: MHD in 3 Dimensions, Circumstellar Mapping, and Dust

V838 Monocerotis, which burst upon the astronomical scene in early 2002, is a completely unanticipated new object. It underwent a large-amplitude and very luminous outburst, during which its spectrum remained that of an extremely cool supergiant. A rapidly evolving set of light echoes around V838 Mon was discovered soon after the outburst, and quickly became the most spectacular display of the phenomenon ever seen. The light echoes, which were imaged by us with HST during 2002, provide the means to accomplish four unique types of measurements based on continued HST imaging during the event: {1} Study effects of MHD turbulence at high resolution and in 3 dimensions; {2} Construct the first unambiguous and fully 3-D map of a circumstellar dust envelope in the Milky Way; {3} Study dust physics in a unique setting where the spectrum and light curve of the illumination, and the scattering angle, are unambiguously known; and {4} Determine the distance to V838 Mon through two independent direct geometric techniques {polarimetry and angular expansion rates}. Because of the extreme rarity of light echoes, this is almost certainly the only opportunity to achieve such results during the lifetime of HST. We propose a campaign during Cycle 14 of imaging the echoes every 8 days for a total of 6 epochs, in order to fully map a thin slab through the dust shell and achieve the other goals listed above.


Probing Evolution And Reionization Spectroscopically {PEARS}

While imaging with HST has gone deep enough to probe the highest redshifts, e.g. the GOODS survey and the Ultra Deep Field, spectroscopic identifications have not kept up. We propose an ACS grism survey to get slitless spectra of all sources in a wide survey region {8 ACS fields} up to z =27.0 magnitude, and an ultradeep field in the HUDF reaching sources up to z =28 magnitude. The PEARS survey will: {1} Find and spectrocopically confirm all galaxies between z=4-7. {2} Probe the reionization epoch by robustly determining the luminosity function of galaxies and low luminosity AGNs at z = 4 – 6. With known redshifts, we can get a local measure of star formation and ionization rate in case reionization is inhomogeneous. {3} Study galaxy formation and evolution by finding galaxies in a contiguous redshift range between 4 < z < 7, and black hole evolution through a census of low-luminosity AGNs. {4} Get a robust census of galaxies with old stellar populations at 1 < z < 2.5, invaluable for checking consistency with heirarchical models of galaxy formation. Fitting these galaxies' spectra will yield age and metallicity estimates. {5} Study star-formation and galaxy assembly at its peak at 1< z < 2 by identifying emission lines in star-forming galaxies, old populations showing the 4000A break, and any combination of the two. {6} Constrain faint white dwarfs in the Galactic halo and thus measure their contribution to the dark matter halo. {7} Derive spectro-photometric redshifts by using the grism spectra along with broadband data. This will be the deepest unbiased spectroscopy yet, and will enhance the value of the multiwavelength data in UDF and the GOODS fields to the astronomical community. To this end we will deliver reduced spectra to the HST archives.


NICMOS Post-SAA calibration – CR Persistence Part 1.

A new procedure proposed to alleviate the CR-persistence problem of NICMOS. Dark frames will be obtained immediately upon exiting the SAA contour 23, and every time a NICMOS exposure is scheduled within 50 minutes of coming out of the SAA. The darks will be obtained in parallel in all three NICMOS Cameras. The POST-SAA darks will be non-standard reference files available to users with a USEAFTER date/time mark.


Significant Spacecraft Anomalies: (The following are preliminary reports of potential non-nominal performance that will be investigated.)


10107 – REacq(2,1,1) failed due to search radius limit exceeded. @ 027/09:45:12z

Reacq(2,1,1) scheduled at 027/09:40:28 failed due to search radius limit exceeded on FGS 2 at 09:45:12. A ESB a05 was received. The Map at 09:47:21 showed errors of V1=2.67, V2=86.92, V3=5.13, RSS=87.11. OBAD2 before the reacq showed errors of V1=10.58, V2=48.40, V3=10.99, RSS=50.74.



                          SCHEDULED      SUCCESSFUL      FAILURE TIMES
FGS GSacq               10                      10
FGS REacq                04                     03                  Hstar # 
OBAD with Maneuver   24                     24


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