arXiv subjects

Anton Koekemoer

Publications and source records attributed to Anton Koekemoer.

136 records · Page 8Linked to original sources

arXiv2007

COSMOS: 3D weak lensing and the growth of structure

We present a three dimensional cosmic shear analysis of the Hubble Space Telescope COSMOS survey, the largest ever optical imaging program performed in space. We have measured the shapes of galaxies for the tell-tale distortions caused by weak gravitational lensing, and traced the growth of that signal as a function of redshift. Using both 2D and 3D analyses, we measure cosmological parameters Omega_m, the density of matter in the universe, and sigma_8, the normalization of the matter power spectrum. The introduction of redshift information tightens the constraints by a factor of three, and also reduces the relative sampling (or "cosmic") variance compared to recent surveys that may be larger but are only two dimensional. From the 3D analysis, we find sigma_8*(Omega_m/0.3)^0.44=0.866+^0.085_-0.068 at 68% confidence limits, including both statistical and potential systematic sources of error in the total budget. Indeed, the absolute calibration of shear measurement methods is now the dominant source of uncertainty. Assuming instead a baseline cosmology to fix the geometry of the universe, we have measured the growth of structure on both linear and non-linear physical scales. Our results thus demonstrate a proof of concept for tomographic analysis techniques that have been proposed for future weak lensing surveys by a dedicated wide-field telescope in space.

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arXiv2007

Dark matter maps reveal cosmic scaffolding

Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious "dark matter" component, which does not interact via electromagnetism and thus neither emits nor reflects light. As dark matter cannot be seen directly using traditional observations, very little is currently known about its properties. It does interact via gravity, and is most effectively probed through gravitational lensing: the deflection of light from distant galaxies by the gravitational attraction of foreground mass concentrations. This is a purely geometrical effect that is free of astrophysical assumptions and sensitive to all matter -- whether baryonic or dark. Here we show high fidelity maps of the large-scale distribution of dark matter, resolved in both angle and depth. We find a loose network of filaments, growing over time, which intersect in massive structures at the locations of clusters of galaxies. Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built.

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arXiv2006

Modelling and Correcting the Time-Dependent ACS PSF

The ability to accurately measure the shapes of faint objects in images taken with the Advanced Camera for Surveys(ACS) on the Hubble Space Telescope (HST) depends upon detailed knowledge of the Point Spread Function (PSF). We show that thermal fluctuations cause the PSF of the ACS Wide Field Camera (WFC) to vary over time. We describe a modified version of the TinyTim PSF modeling software to create artificial grids of stars across the ACS field of view at a range of telescope focus values. These models closely resemble the stars in real ACS images. Using ~10 bright stars in a real image, we have been able to measure HST's apparent focus at the time of the exposure. TinyTim can then be used to model the PSF at any position on the ACS field of view. This obviates the need for images of dense stellar fields at different focus values, or interpolation between the few observed stars. We show that residual differences between our TinyTim models and real data are likely due to the effects of Charge Transfer Efficiency (CTE) degradation. Furthermore, we discuss stochastic noise that is added to the shape of point sources when distortion is removed, and we present Multidrizzle parameters that are optimal for weak lensing science. Specifically, we find that reducing the Multidrizzle output pixel scale and choosing a Gaussian kernel significantly stabilizes the resulting PSF after image combination, while still eliminating cosmic rays/bad pixels, and correcting the large geometric distortion in the ACS. We discuss future plans, which include more detailed study of the effects of CTE degradation on object shapes and releasing our TinyTim models to the astronomical community.

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arXiv2005

The WFPC2 Archival Parallels Project

We describe the methods and procedures developed to obtain a near-automatic combination of WFPC2 images obtained as part of the WFPC2 Archival Pure Parallels program. Several techniques have been developed or refined to ensure proper alignment, registration, and combination of overlapping images that can be obtained at different times and with different orientations. We quantify the success rate and the accuracy of the registration of images of different types, and we develop techniques suitable to equalize the sky background without unduly affecting extended emission. About 600 combined images of the 1,500 eventually planned have already been publicly released through the STScI Archive. The images released to date are especially suited to study star formation in the Magellanic Clouds, the stellar population in the halo of nearby galaxies, and the properties of star-forming galaxies at $ z \sim 3 $.

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arXiv2005

Resolving the Shocks in Radio Galaxy Nebulae: HST and Radio Imaging of 3C 171, 3C277.3 and PKS2250-41

We present the results of HST/WFPC2 medium and narrow band imaging and VLA and MERLIN2 radio imaging of three powerful radio galaxies: 3C 171, 3C 277.3, and PKS 2250-41. We obtained images of the rest frame [OIII]$λ$5007 and [OII]$λ$3727 line emission using the Linear Ramp Filters on WFPC2. The correlations between the emission line morphology and the [OIII]/[OII] line ratios with the radio emission seen in ground based observations are clarified by the HST imaging. We confirm that the radio lobes and hot-spots are preferentially associated with lower ionization gas. 3C 171 exhibits high surface brightness emission line gas mainly along the radio source axis. The lowest ionization gas is seen at the Eastern hot spot. In 3C 277.3 there is bright high ionization gas (and continuum) offset just to the east of the radio knot K1. Our observations are consistent with previous work suggesting that this emission is produced by precursor gas ionized by the shock being driven into the cloud by the deflected radio jet. In PKS 2250-41 we resolve the emission line arc which wraps around the outer rim of the western lobe. The lower ionization [OII] emission is nested just interior to the higher ionization [OIII] emission suggesting that we have resolved the cooling region behind the bow shock. We also detect possible continuum emission from the secondary hot-spot. Thus, our observations support the hypothesis that in these sources, the interaction between the expanding radio source and the ambient gas strongly influences the morphology, kinematics, and ionization of the gas.

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arXiv2004

A Redshift z=5.4 Lyman alpha Emitting Galaxy with Linear Morphology in the GRAPES/UDF Field

We have discovered an extended Lyman alpha plume associated with a compact source at redshift 5.4 in slitless spectroscopic data from the Grism ACS Program for Extragalactic Science (GRAPES) project. The spatial extent of the emission is about 6 x 1.5 kpc (1 x 0.25 arcsec). Combining our grism data and the broadband images from the Hubble UltraDeep Field (UDF) images, we find a Lyman alpha line flux of 2e-17 erg/cm2/s and surface brightness 7e-17 erg/cm2/s/arcsec2. The UDF images show diffuse continuum emission associated with the Lyman alpha plume (hereafter UDF 5225), with three embedded knots. The morphology of UDF 5225 is highly suggestive of a galaxy in assembly. It is moreover possible that the prominent Lyman alpha emission from this object is due to an active nucleus, and that we are seeing the simultaneous growth through accretion of a galaxy and its central black hole. Followup observations at higher spectral resolution could test this hypothesis.

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arXiv2003

Identification of Type Ia Supernovae at Redshift 1.3 and Beyond with the Advanced Camera for Surveys on HST

We present discoveries of SNe Ia at z > 1 and the photometric diagnostic used to discriminate them from other types of SNe detected during the GOODS Hubble Space Telescope Treasury program with the Advanced Camera for Surveys (ACS). Photometric redshift measurements of the hosts combined with deep f606w, f775w, and f850lp imaging discriminates hydrogen-rich SNe II from SNe I at z > 1 by exploiting the ultraviolet (UV) deficit in the energy distributions of SNe Ia. This sorting allows rapid follow-up of space-based discoveries. Subsequent spectroscopy of 11 GOODS SNe Ia obtained from the ground and with the grism on ACS confirmed the reliability of our photometric screening. We present the highest-redshift spectrum of any supernova published to date, SN Ia 2002fw at z = 1.3 observed near maximum brightness with the ACS grism. The lack of UV flux for true SNe Ia provides an effective tool for our ongoing efforts to build a sample of SNe Ia at 1.2 < z < 1.8 which will extend the useful range of the magnitude-redshift relation of SNe Ia.

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arXiv2001

Ultraviolet HST Snapshot Survey of 3CR Radio Source Counterparts at Low Redshift

We present ultraviolet images of 27 3CR radio galaxies with redshifts z<0.1 that have been imaged with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The observations employed the NUV-MAMA and broad-band filters with peak sensitivity at 2200A . We find that the UV luminosities show approximately a factor of 10 to 100 higher dispersion than the optical. We compare the UV morphologies with optical V- and R-band WFPC2 snapshot survey images. We have found dramatic, complex and extended ultraviolet emission from radio galaxies even at zero redshift. We find a diverse range of UV morphologies, some completely divergent from their visual morphology, which are reminiscent of the chaotic high-z radio galaxies structures seen in rest-frame UV. The UV morphologies show regions of star formation, jets, and possible scattered AGN continuum. The UV emission is generally not aligned with the radio structure. We also detect the diffuse UV emission of the host galaxy. We propose that these are the same physical phenomena as observed at high redshift, but on a smaller spatial scale.

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arXiv2000

Peering through the dust: Evidence for a supermassive Black Hole at the nucleus of Centaurus A from VLT IR spectroscopy

We used the near infrared spectrometer ISAAC at the ESO 'Very Large Telescope' to map the velocity field of Centaurus A (NGC 5128) at several position angles and locations in the central 20" of the galaxy. The high spatial resolution (~0.5") velocity fields from both ionized and molecular gas (PaBeta, [FeII], BrGamma, and H2) are not compromised by either excitation effects or obscuration. We identify three distinct kinematical systems: (i) a rotating 'nuclear disk' of ionized gas, confined to the inner 2", the counterpart of the PaAlpha feature previously revealed by HST/NICMOS imaging; (ii) a ring-like system with a ~6" inner radius detected only in H2, likely the counterpart of the 100pc-scale structure detected in CO by other authors; (iii) a normal extended component of gas rotating in the galactic potential. The nuclear disk is in keplerian rotation around a central mass concentration, dark (M/L>20 Msun/LsunK) and point-like at the spatial resolution of the data (R<0.25" ~4pc). We interpret this mass concentration as a supermassive black hole. Its dynamical mass based on the line velocities and disk inclination (i>15deg) is M=2(+3.0;-1.4) 10^8 Msun. The ring-like system is probably characterized by non-circular motions; a 'figure-of-8' pattern observed in the H2 position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.

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arXiv1999

Unveiling the Active Nucleus of Centaurus A

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near-IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by A_V~14mag, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII] 1.64mic line and presents a morphology similar to that observed in Pa alpha with a [FeII]/Pa alpha ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Pa alpha observations in a 50"x50" circumnuclear region suggest enhanced star formation (~0.3Msun/yr) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ~4" and suggests a black--hole mass of ~10^9 Msun. A linear blue structure aligned with the radio/X-ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.

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