John G. Hoessel

THE PERFORMANCE AND CALIBRATION OF WFPC2 ON THE HUBBLE SPACE TELESCOPE

The WFPC2 was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant amount of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 °C). The operating temperature of the WFPC2 CCDs was changed to -88 °C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the PSF, the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and fiat fields.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XIII. The Metallicity Dependence of the Cepheid Distance Scale

Uncertainty in the metal-abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and in the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) to observe Cepheids that span a range in oxygen abundance of 0.7 ± 0.15 dex in two fields in the nearby spiral galaxy M101. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ(m-M)0/δ[O/H] = -0.24 ± 0.16 mag dex-1. The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisons of Cepheid distances with those derived from three other distance indicators, in particular from the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.

Far-Ultraviolet Imaging of Jupiter's Aurora and the Io “Footprint”

Far-ultraviolet images of Jupiter from the Hubble Space Telescope Wide Field Planetary Camera 2 reveal polar auroral emissions at 300 kilometer resolution and three times higher sensitivity than previously achieved. Persistent features include a main oval containing most of the emission and magnetically connected to the middle magnetosphere, diffuse and variable emissions poleward of the main oval, and discrete emission from Ios magnetic footprint equatorward of the oval. The auroral emissions are variable, exhibit magnetic conjugacy, and are visible above the planet limb. All emissions approximately co-rotate with Jupiter except the Io “footprint,” which is fixed along Ios magnetic flux tube.

The Extragalactic Distance Scale Key Project. IV. The Discovery of Cepheids and a New Distance to M100 Using the Hubble Space Telescope

This paper presents initial observations, including the discovery of 30 Cepheids in the nearby galaxy M81, made using the Wide Field Camera (WFC).

Hubble Space Telescope Observations of the SN 1987A Triple Ring Nebula

We have observed SN 1987A with the optically corrected WFPC2 on the Hubble Space Telescope both in emission lines and in the UV and optical continuum. The previously observed outer nebular structure is shown to be part of two closed unresolved loops. These loops were flash-ionized by the supernova itself. They are not caused by limb brightening of an hourglass shell produced by the interaction of the winds from the progenitor. The inner ring is seen to be extended and may be connected to the new outer rings by sheets of material. However, beyond the outer rings, emission is not seen, implying a very low density (n 1000. This density contrast of at least 100 is difficult to reconcile with the conventional picture of the progenitor evolution. Two models for the rings are presented, but each is deficient in important respects. A proper understanding of this system will require new physical insight.

The Extragalactic Distance Scale Key Project. XVI. Cepheid Variables in an Inner Field of M101

We report on the identification of 255 candidate variable stars in a field located some 17 from the center of the late-type spiral galaxy M101 = NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. Photometric measurements in the F555W and F814W filters—analyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suites—have been transformed to the Johnson V and Kron-Cousins I standard magnitude systems. Periods and intensity-averaged mean magnitudes for 61 carefully selected candidate Cepheid variables with periods in the range 10-48 days indicate a reddening-corrected mean distance modulus (m - M)0 = 29.05 ± 0.14 (if the true modulus of the Large Magellanic Cloud is 18.50 ± 0.10, and if there is no dependence of the period-luminosity relation on metal abundance); results consistent with this are obtained whether or not the sample is expanded to include a larger fraction of the candidates. Applying a metallicity-dependent correction of +0.16 ± 0.10 mag would increase this estimate to (m - M)0 = 29.21 ± 0.17 mag.

A systematic survey for distant galaxy clusters

A photographic survey for faint clusters of galaxies has been carried out with fine-grained photographic emulsions using the 1.2 m Schmidt and 5 m Hale telescopes, as well as the 4 m Mayall telescope. A total of 418 clusters have been found with redshifts mostly in the range from 0.15 to 0.92. The survey was planned to minimize distance-dependent selection effects in the resulting catalog. In areas of sky where the deepest search was made, the sample is complete to about z = 0.50; there are 11 clusters per square degree at this limit. At a redshift of 1.0 there should be 63 or 45 clusters per square degree depending on whether q0 is 0.0 or 0.5, provided there is no evolution.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications

Using the Hubble Space Telescope, 37 long-period Cepheid variables have been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of ?0=31.35?0.07 mag, which corresponds to a distance of 18.6?0.6 Mpc. This measurement provides several routes for estimating the Hubble constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble constant of 70?18 (random) ?7 (systematic) km s?1 Mpc?1. (2) Nine Cepheid-based distances to groups of galaxies out to and including the Fornax and Virgo Clusters yield H0=73?16 (random) ?7 (systematic) km s?1 Mpc?1. (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galaxy clusters out to 100 Mpc, give H0=76?3 (random) ?8 (systematic) km s?1 Mpc?1. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H0=72?3 (random) ?6 (systematic) km s?1 Mpc?1. Finally, (5) assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SN Ia calibration (correcting for light-curve shape) gives H0=67?6 (random) ?7 (systematic) km s?1 Mpc?1 out to a distance in excess of 500 Mpc. All five of these H0 determinations agree to within their statistical errors. The resulting estimate of the Hubble constant, combining all of these determinations, is H0=72?5 (random) ?7 (systematic) km s?1 Mpc?1. An extensive tabulation of identified systematic and statistical errors, and their propagation, is given.

Deep Hubble Space Telescope Imaging of IC 1613. I. Variable Stars and Distance

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyrae stars and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyrae stars) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 ± 0.20 mag per dex for the RR Lyrae stars and a shallow slope of -0.07 ± 0.16 mag per dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is μ0 = 24.31 ± 0.06, corresponding to a distance of 730 ± 20 kpc. This distance produces an RR Lyrae absolute magnitude of 0.61 ± 0.08.

Detection of the Tip of Red Giant Branc in NGC 5128

We present a color-magnitude diagram of more than 10,000 stars in the halo of the galaxy NGC 5128 (Centaurus A), based on WFPC2 images through V-band and I-band filters. The position of the red giant branch (RGB) stars is compared with the loci of the red giant branch in six well-studied globular clusters and in the dwarf elliptical galaxy NGC 185; the tip of the RGB is signalled by an observed turn-up in the luminosity function at I≃24.1 ± 0.1 mag; this yields a distance modulus (m - M)_0 = 27.8 ± 0.2 for NGC 5128 (i.e., a distance of 3.6 ± 0.2 Mpc), in agreement with previous determinations based on the planetary nebulae luminosity function and on the surface brightness fluctuations technique. The presence of an intermediate-age stellar population (~5 Gyr) is suggested by the luminosity function of the asymptotic giant branch stars, extending up to I= 22.6 mag (for V - I > 2) and M_(bot) ~ -5 mag; however, the number of these stars constrains the intermediate-age stellar population in the halo of NGC 5128 to be less than ~ 10% of the total. The color distribution at constant I magnitude, albeit affected by the completeness level of our sample, strongly suggests a mean value of [Fe/H] > -0.9 dex, possibly similar to the value found in M31 and higher than that observed in NGC 185. Like the M31 halo, the halo of NGC 5128 exhibits a broad range of levels of chemical enrichment.