arXiv subjects

Gerard M. Williger

Publications and source records attributed to Gerard M. Williger.

25 records · Page 2Linked to original sources

arXiv2004

Low-Metallicity Gas Clouds in a Galaxy Proto-Cluster at Redshift 2.38

We present high resolution spectroscopy of a QSO whose sight-line passes through the halo of a pair of elliptical galaxies at redshift 2.38. This pair of galaxies probably lies at the center of a galaxy proto-cluster, and is embedded in a luminous extended Ly-alpha nebula. The QSO sight-line intersects two small gas clouds within this halo. These clouds have properties similar to those of high velocity clouds (HVCs) seen in the halo of the Milky Way. The gas is in a cool (< 2 x 10^4 K) and at least 20% neutral phase, with metallicities in the range -3.0 < [Fe/H] < -1.1 and neutral hydrogen column densities of ~10^19.5 /cm^2. The origin of these clouds is unclear. The presence of low metallicity gas within this possible proto-cluster implies either that the intra-cluster medium has not been enriched with metals at this redshift, or the clouds are embedded within a hot, ionized, metal-rich gas phase.

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arXiv2003

The Distribution of Lya-Emitting Galaxies at z=2.3

We present the detection of 34 Ly-alpha emission-line galaxy candidates in a 80x80x60 co-moving Mpc region surrounding the known z=2.38 galaxy cluster J2143-4423. The space density of Ly-alpha emitters is comparable to that found by Steidel et al. when targeting a cluster at redshift 3.09, but is a factor of 5.8 +/- 2.5 greater than that found by field samples at similar redshifts. The distribution of these galaxy candidates contains several 5-10 Mpc scale voids. We compare our observations with mock catalogs derived from the VIRGO consortium Lambda-CDM n-body simulations. Fewer than 1% of the mock catalogues contain voids as large as we observe. Our observations thus tentatively suggest that the galaxy distribution at redshift 2.38 contains larger voids than predicted by current models. Three of the candidate galaxies and one previously discovered galaxy have the large luminosities and extended morphologies of "Ly-alpha blobs".

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arXiv2001

HST/STIS observations of the HeII Gunn-Peterson effect towards HE 2347-4342

We present an HST STIS spectrum of the HeII Gunn-Peterson effect towards HE2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al. (1997), the STIS spectrum has a much improved resolution. The 2-D 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 HeII G-P opacity in the high opacity regions. We use the STIS spectrum together with a Keck--HIRES spectrum covering the corresponding HI Lya forest to calculate a 1-D map of the softness S of the ionization radiation along the line of sight towards HE 2347-4342, where S is the ratio of the HI to HeII photoionization rates. We find that S is generally large but presents important variations, from S ~ 30 in the opacity gaps to a 1 sigma lower limit of 2300 at z~2.86, in a region which shows an extremely low HI opacity over a 6.5 A range. We note that a large S naturally accounts for most of the large SiIV to CIV 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. (2000) 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. (1997), 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 HI to HeII ionizing photons reaching the IGM is large. We discuss these two models and suggest ways to discriminate between them.

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arXiv2000

Wide Field Imaging of the Hubble Deep Field-South Region I: Quasar Candidates

We present candidate quasars from a multi-color (uBVRI + narrow-band) imaging survey of 1/2 square degree around the Hubble Deep Field - South. We identify 154 candidate quasars with B < 23 using color selection, consistent with previously measured QSO number counts if we assume a 60% selection efficiency. The narrow-band filter (NB) was centered at 3958 A to detect Ly-alpha at the redshift of J2233-6033, the HDF-S QSO. We confirm the presence of Ly-alpha nebulosity extending ~12'' around the HDF-S QSO, reported by Bergeron et al. (1999). We detect 10 point-like objects in emission through the NB filter. Of these, 7 satisfy our QSO color selection criteria. One of emission-line objects is a B~20 radio-quiet quasar at z=1.56, 6.7' from the line of sight to the HDF-S QSO and ~12'' from the western edge of the WFPC2 deep field.

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arXiv1999

Limits on the star formation rates of z>2 damped Ly-alpha systems from H-alpha spectroscopy

We present the results of a long-slit K-band spectroscopic search for H-alpha emission from eight damped Ly-alpha absorbers (DLAs) at z>2 with the goal of measuring the star-formation rates in these systems. For each system we searched for compact sources of H-alpha emission within a solid angle 11x2.5arcsec^2 (44x10 kpc^2/h^2, for q_0=0.5, H_0=100 h km/s/Mpc). No H-alpha emission was detected above 3-sigma limits in the range (6.5 - 16)x1E-20 W/m^2, equivalent to star formation rates of (5.6 - 18) M_sun/yr/h^2, for a standard IMF, assuming the lines are spectrally unresolved (<650 km/s FWHM). We compare these results against the predictions of the models of Pei & Fall (1995) of the global history of star formation, under two different simplifying hypotheses: i.) the space density of DLAs at z=2.3 is equal to the space density of spiral galaxies today (implying DLA disks were larger in the past, the `large-disk' hypothesis); ii.) the sizes of DLAs at z=2.3 were the same as the gas sizes of spiral galaxies today (implying DLA disks were more common in the past, the `hierarchical' hypothesis). Compared to the previous most sensitive spectroscopic search our sample is twice as large, our limits are a factor greater than two deeper, and the solid angle surveyed is over three times as great. Despite this our results are not in conflict with either the large-disk hypothesis, because of the limited solid angle covered by the slit, or the hierarchical hypothesis, because of the limited sensitivity.

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arXiv1998

Limits on the star formation rates of z>2 damped Ly-alpha systems from H-alpha spectroscopy

We present the results of a long-slit K-band spectroscopic search with CGS4 on UKIRT for H-alpha emission from the objects responsible for high-redshift (z > 2) damped Ly-alpha absorption systems. The objective was to measure the star-formation rates in these systems. However, no H-alpha emission was detected above our 3-sigma limits of f < 10E-19 W/m**2, corresponding to star formation rates < 10 M_sun/yr/h**2 (q_0=0.5). These upper limits are more meaningful than those from searches for Ly-alpha emission because the H-alpha line is unaffected by resonant scattering. For q_0=0.5 our limits are in conflict with the star formation rates predicted under the assumption that the high-z DLAs are the fully-formed galactic-disk counterparts of today's massive spiral galaxies. Deeper spectroscopy is needed to test this picture for q_0=0.0. A programme of NICMOS imaging observations currently underway, combined with VLT spectroscopy, will provide a detailed picture of the link between DLAs and young galaxies.

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arXiv1997

Optical Light Curves of the Type Ia Supernovae 1990N and 1991T

We present UBVRI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN 1991T in NGC 4527 based on photometry gathered in the course of the Calan/Tololo supernova program. Both objects have well-sampled light curves starting several days before maximum light and spanning well through the exponential tail. These data supercede the preliminary photometry published by Leibundgut et al (1991) and Phillips et al (1992). The host galaxies for these supernovae have (or will have) accurate distances based on the Cepheid period-luminosity relationship. The photometric data in this paper provide template curves for the study of general population of Type Ia supernova and accurate photometric indices needed for the Cepheid-supernova distance scale.

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