Gerard M. Williger

STIS Observations of He II Gunn-Peterson Absorption toward Q0302–003*

The ultraviolet spectrum (1145¨1720 of the distant quasar Q0302(003 (z 3.286) was observed at Ae ) 1.8 resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope .A Ae total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson He II absorption trough, produced by He II Lya along the line of sight over the redshift range z 2.78¨3.28. Its interpretation was facilitated by modeling based on Keck HIRES spectra of the H I Lya forest (provided by A. Songaila and by M. Rauch and W. Sargent). We —nd that near the quasar, He II Lya absorption is produced by discrete clouds, with no signi—cant diUuse gas; this is attributed to a He II ii proximity eUect ˇˇ in which the quasar fully ionizes He in the diUuse intergalactic medium, but not the He in denser clouds. By two diUerent methods we calculate that the average He II Lya opacity at z B 3.15 is q 4.8. In the Dobrzycki-Bechtold void in the H I Lya forest near z 3.18, the average He II opacity Such large opacities require the presence of a diUuse gas component as well as a q 4.47 ~0.330.48. soft UV background spectrum, whose softness parameter, de—ned as the ratio of the photoionization rate in H I over the one in He II

A Pair of Compact Red Galaxies at Redshift 2.38, Immersed in a 100 Kiloparsec Scale Lyα Nebula*

We present Hubble Space Telescope (HST) and ground-based observations of a pair of galaxies at redshift 2.38, which are collectively known as 2142 4420 B1 (Francis et al. 1996). The two galaxies are both luminous extremely red objects (EROs), separated by 0.8 ′′ . They are embedded within a 100 kpc scale diffuse

The Hubble Deep Field South: Formulation of the Observing Campaign

Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation. The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys. The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10-3 to 2 × 105 μm. We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field. Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits. Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 2.24 quasar in the STIS field of view. The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 A, and deep imaging of an adjacent field with NICMOS camera 3 at 1.1, 1.6, and 2.2 μm. All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed. Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers.

Hubble Space Telescope Space Telescope Imaging System Observations of the He II Gunn-Peterson Effect toward HE 2347–4342*

We present an HST Space Telescope Imaging System (STIS) spectrum of the He II Gunn-Peterson effect toward HE 2347-4342. Compared to the previous HST Goddard High Resolution Spectrograph data obtained by Reimers et al. the STIS spectrum has a much improved resolution. The two-dimensional detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the He II Gunn-Peterson opacity τ in the high-opacity regions. We use the STIS spectrum together with a Keck-HIRES spectrum covering the corresponding H I Lyα forest to calculate a one-dimensional map of the softness S of the ionization radiation along the line of sight toward HE 2347-4342, where S is the ratio of the H I to He II photoionization rates. We find that S is generally large but presents important variations, from ~30 in the opacity gaps to a 1 σ lower limit of 2300 at z 2.86, in a region that shows an extremely low H I opacity over a 6.5 A spectral range. We note that a large softness parameter naturally accounts for most of the large Si IV/C IV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al., we propose that the large softness observed at z 2.86 is due to the presence of bright soft sources close to the line of sight, i.e., for which the ratio between the number of H I to He II ionizing photons reaching the intergalactic medium is large. We discuss these two models and suggest ways to discriminate between them.

HST imaging of the globular clusters in the Fornax Cluster: NGC 1399 and NGC 1404

A B STR A CT The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope’s Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory 1.5-m telescope. The combination of both data sets provides a unique insight on the spatial and colour distribution of globular clusters. From B—I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e., more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N5700„500, SN11.5„1.0 for NGC 1399, and N725„145, SN2.0„0.5 for NGC 1404. Our results are compared with the expectations of globular cluster formation scenarios.

Lyman-alpha absorption in the spectrum of the Z = 4.5 QSO BR 1033-0327

We present the results of echelle spectroscopy of the Lyɑ forest region in the spectrum of the bright (m_R = 18.5) optically selected z = 4.5 QSO BR 1033-0327. These results, which extend Lyɑ forest absorption line studies up to a redshift of z= 4.5 at a resolution of 12 km s^(-1), represent the first high-resolution study of Lyɑ forest systems at such high redshifts. We compare the properties of the Lyɑ forest systems in the redshift range 3.7 < than z < than 4.3 with those inferred from similar observations at 1.86 < than z < than 3.27 and find some clear indications of redshift dependence in these. First, the ionizing flux from the quasar appears to affect the Lyɑ line density further from the QSO than at lower redshifts, implying that the Lyman limit background flux at redshift z ~ 4.2 is J_v ~ 1 to 3 x 10^(-22) ergs cm^(-2) s^(-1) sr^(-1). This is about a factor of 10 lower than estimates at z ~ 2.5. Second, the Doppler parameter distribution for systems where the effect of the QSO ionizing flux is small (3.7 < than z < than 4.3) has an excess at values ~ 20 km s^(-1), compared with lower redshifts; we suggest this may be due to the lower ionizing flux at z ~ 4.2, which would reduce the heating of the gas. Finally, while there is no evidence that the shape of the H I column density distribution changes significantly, the redshift dependence of the total number of systems is stronger than that found at lower redshifts, with the number of systems per unit redshift ~ (1+ z)^(4.6). Also, over the entire redshift range there is no significant correlation between Doppler parameter and H I column density and no signal in the two-point correlation function down to velocity separations of Δv ~ 100 km s^(-1). There is no evidence for continuously distributed HI absorption (the Gunn-Peterson effect) at these redshifts.

The proximity effect in a close group of QSOs

We present an analysis of the proximity effect in a sample of 10 2-A-resolution QSO spectra of the Lyα forest at . Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the >xa099 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of . This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by . We find evidence at a level of 2.6σ that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1σ. n n n nThe QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Lyα forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000xa0kmxa0s−1 of the redshift of the foreground QSO.

Testing Cosmological Variability of the Proton-to-Electron Mass Ratio Using the Spectrum of PKS 0528–250

Multidimensional cosmologies allow for variations of fundamental physical constants over the course of cosmological evolution, and different versions of the theories predict different time dependences. In particular, such variations could manifest themselves as changes of the proton-to-electron mass ratio μ = mp/me over the period of ~1010 yr since the moment of formation of high-redshift QSO spectra. Here we analyze a new high-resolution spectrum of the z = 2.81080 molecular hydrogen absorption system toward the QSO PKS 0528-250 to derive a new observational constraint to the time-averaged variation rate of the proton-to-electron mass ratio. We find | /μ |<1.5×10−14 yr-1, which is much tighter than previously measured limits.

Evidence for large-scale structure in the Ly alpha forest at z > 2.6

We present a search for spatial and redshift correlations in a 2 A resolution spectroscopic survey of the Lyα forest at 2.15 < z < 3.37 toward 10 QSOs concentrated within a 1° diameter field. We find a signal at 2.7 σ significance for correlations of the Lyα absorption-line wavelengths between different lines of sight over the whole redshift range. The significance rises to 3.2 σ if we restrict the redshift range to 2.60 < z < 3.37, and to 4.0 σ if we further restrict the sample to lines with rest equivalent widths 0.1 ≤ W0 < 0.9 A. We conclude that a significant fraction of the Lyα forest arises in structures whose correlation length extends at least over 30 (~26 h-1 comoving Mpc at z = 2.6 for H0 ≡ 100 h km s-1 Mpc-1, Ω = 1.0, Λ = 0). We have also calculated the three-dimensional two-point correlation function for Lyα absorbers; we do not detect any significant signal in the data. However, we note that line blending prevents us from detecting the signal produced by a 100% overdensity of Lyα absorbers in simulated data. We find that the Lyα forest redshift distribution provides a more sensitive test for such clustering than the three-dimensional two-point correlation function.

Infrared Emission-Line Galaxies Associated with Damped Lyα and Strong Metal Absorber Redshifts

Eighteen candidates for emission-line galaxies were discovered in a narrowband infrared survey that targeted the redshifts of damped Lyα or metal lines in the spectra of quasars. The presence of emission lines is inferred from the photometric magnitudes in narrowband and broadband interference filters, corresponding to Hα at redshifts of 0.89 (six objects) and 2.4 (10 objects) and to [O II] λ3727 at a redshift of 2.3 (two objects). Most of the candidates are small, resolved objects that are compatible with galaxies at the redshifts of the damped Lyα absorbers. Because a similar survey, targeted at the redshifts of quasars themselves, uncovered only one emission-line galaxy in a larger volume, the results imply substantial clustering of young galaxies or a formation within filaments or sheets whose locations are indicated by the redshifts of strong Lyα line absorption along the lines of sight to more distant quasars.