Hubble Space Telescope Point-spread Function Research Articles

SEARCHING FOR THE HR 8799 DEBRIS DISK WITH HST/STIS

ABSTRACT We present a new algorithm for space telescope high contrast imaging of close-to-face-on planetary disks called Optimized Spatially Filtered (OSFi) normalization. This algorithm is used on HR 8799 Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) coronagraphic archival data, showing an over-luminosity after reference star point-spread function (PSF) subtraction that may be from the inner disk and/or planetesimal belt components of this system. The PSF-subtracted radial profiles in two separate epochs from 2011 and 2012 are consistent with one another, and self-subtraction shows no residual in both epochs. We explore a number of possible false-positive scenarios that could explain this residual flux, including telescope breathing, spectral differences between HR 8799 and the reference star, imaging of the known warm inner disk component, OSFi algorithm throughput and consistency with the standard spider normalization HST PSF subtraction technique, and coronagraph misalignment from pointing accuracy. In comparison to another similar STIS data set, we find that the over-luminosity is likely a result of telescope breathing and spectral difference between HR 8799 and the reference star. Thus, assuming a non-detection, we derive upper limits on the HR 8799 dust belt mass in small grains. In this scenario, we find that the flux of these micron-sized dust grains leaving the system due to radiation pressure is small enough to be consistent with measurements of other debris disk halos.

Decomposition of the Host Galaxies of Active Galactic Nuclei UsingHubble Space TelescopeImages

Investigating the link between supermassive black hole and galaxy evolution requires careful measurements of the properties of the host galaxies. We perform simulations to test the reliability of a two-dimensional image-fitting technique to decompose the host galaxy and the active galactic nucleus (AGN), especially on images obtained using cameras on board the Hubble Space Telescope (HST), such as the Wide-Field Planetary Camera 2, the Advanced Camera for Surveys, and the Near-Infrared Camera and Multi-Object Spectrometer. We quantify the relative importance of spatial, temporal, and color variations of the point-spread function (PSF). To estimate uncertainties in AGN-to-host decompositions, we perform extensive simulations that span a wide range in AGN-to-host galaxy luminosity contrast, signal-to-noise ratio, and host galaxy properties (size, luminosity, central concentration). We find that realistic PSF mismatches that typically afflict actual observations systematically lead to an overestimate of the flux of the host galaxy. Part of the problem is caused by the fact that the HST PSFs are undersampled. We demonstrate that this problem can be mitigated by broadening both the science and the PSF images to critical sampling without loss of information. Other practical suggestions are given for optimal analysis of HST images of AGN host galaxies.

Detection of Cosmic Shear with the [ITAL]Hubble Space Telescope[/ITAL] Survey Strip

Weak lensing by large-scale structure provides a unique method to directly measure matter fluctuations in the universe and has recently been detected from the ground. Here we report the first detection of this based on space-based images. The detection was derived from the Hubble Space Telescope (HST) Survey Strip (or Groth Strip), a 4' × 42' set of 28 contiguous Wide Field Planetary Camera 2 (WFPC2) pointings with I < 27. The small size of the HST point-spread function affords both a lower statistical noise and a much weaker sensitivity to systematic effects, a crucial limiting factor of cosmic shear measurements. Our method and treatment of systematic effects were discussed in an earlier paper. We measure an rms shear of 1.8% on the WFPC2 chip scale (127), in agreement with the predictions of cluster-normalized cold dark matter (CDM) models. Using a maximum likelihood analysis, we show that our detection is significant at the 99.5% confidence level (CL) and measure the normalization of the matter power spectrum to be σ8Ω = 0.51, in a ΛCDM universe. These 68% CL errors include (Gaussian) cosmic variance, systematic effects, and the uncertainty in the redshift distribution of the background galaxies. The signal comes primarily from the chip scale (127) with gradually decreasing contributions up to roughly 10'. Our result is consistent with earlier lensing measurements from the ground and with the normalization derived from cluster abundance. We discuss how our measurement can be improved with the analysis of a large number of independent WFPC2 fields.

On the feasibility of detecting extrasolar planets by reflected starlight using the Hubble Space Telescope

Detecting extrasolar planets in reflected stellar radiation is an exceedingly challenging task due to the great faintness of planets as compared with stars. When a telescope focuses two close sources on a detector, the fainter source can be “discovered” only if the signal recorded near the location of its image is convincingly larger than would be expected or observed in its absence. We investigate the implications of this criterion for planet searches using the Hubble Space Telescope (HST). Using the best available metrology data, we estimate the HST point-spread function (PSF) in both the center and the wing. We define the maximum star-planet flux ratio. We introduce a figure of merit, Q, which is the contrast ratio between a best-case planet and the background of scattered star light. For HST, the value of Q is unfavorable for detecting planets.

ST-FOC observations of globular clusters: Simulations

Using a first approximation to the Hubble space telescope point-spread function, the problems concerning observation and reduction of globular clusters in the Andromeda nebula are analysed through a simulation procedure in order to study the feasibility of accurate photometry of individual stars in the clusters.