Mauro Giavalisco

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) to an equivalent limiting flux of μ~26 mag arcsec^-2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo and coworkers. In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us to rule out definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolute magnitude of M~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L and comparable to the brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities ( 0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.

We have measured the counts-in-cells fluctuations of 268 Lyman-break galaxies with spectroscopic redshifts in six 9 arcmin by 9 arcmin fields at z~3. The variance of galaxy counts in cubes of comoving side length 7.7, 11.9, 11.4 h^{-1} Mpc is \sigma_{gal}^2 ~ 1.3\pm0.4 for \Omega_M=1, 0.2 open, 0.3 flat, implying a bias on these scales of \sigma_{gal} / \sigma_{mass} = 6.0\pm1.1, 1.9\pm0.4, 4.0\pm0.7. The bias and abundance of Lyman-break galaxies are surprisingly consistent with a simple model of structure formation which assumes only that galaxies form within dark matter halos, that Lyman-break galaxies' rest-UV luminosities are tightly correlated with their dark masses, and that matter fluctuations are Gaussian and have a linear power-spectrum shape at z~3 similar to that determined locally (\Gamma~0.2). This conclusion is largely independent of cosmology or spectral normalization \sigma_8. A measurement of the masses of Lyman-break galaxies would in principle distinguish between different cosmological scenarios.Comment: Accepted for publication in ApJ, 16 pages including 4 figure

We have measured the angular correlation function w(θ) for a sample of 871 Lyman-break galaxies (LBGs) in five fields at redshift z ~ 3. Fitting the power law A_w θ^-β to a weighted average of w(θ) from the five fields over the range 12'' θ 330'', we find A_w ~ 2 arcsec^β and β ~ 0.9. The slope is, within the errors, the same as for galaxy samples in the local and intermediate-redshift universe, and a slope β = 0.25 or shallower is ruled out by the data at the 99.9% confidence level. Because N(z) of LBGs is well determined from 376 spectroscopic LBG redshifts, the real-space correlation function can be accurately derived from the angular one through the Limber transform. The inversion of w(θ) is rather insensitive to the still relatively large uncertainties on A_ω and β, and the spatial correlation length is much more tightly constrained than either of these parameters. We estimate r₀=3.3^{+ 0.7}_−0.6(2.1^{+ 0.4}_−0.5) h^-1 Mpc (comoving) for q₀ = 0.1 (0.5) at the median redshift of the survey, =3.04 (h is in units of 100 km s^-1 Mpc^-1 throughout this paper). The observed comoving correlation length of LBGs at z ~ 3 is comparable to that of present-day spiral galaxies and is only ~50% smaller than that of present-day ellipticals; it is as large or larger than any measured in recent intermediate-redshift galaxy samples (0.3 z 1). By comparing the observed galaxy correlation length to that of the mass predicted from cold dark matter (CDM) theory, we estimate a linear bias for LBGs of b ~ 1.5 (4.5) for q₀ = 0.1 (0.5), in broad agreement with our previous estimates based on preliminary spectroscopy. The strong clustering and the large bias of the LBGs are consistent with biased galaxy formation theories and provide additional evidence that these systems are associated with massive dark matter halos. The results of the clustering of LBGs at z ~ 3 emphasize that apparent evolution in the clustering properties of galaxies may be due as much to variations in effective light-to-mass bias parameter among different galaxy samples as to evolution in the mass distribution through gravitational instability.

We have measured the angular correlation function w(theta) for a sample of 871 Lyman-break galaxies (LBGs) at z=3. Fitting a power-law to a weighted average of w(theta) from 5 fields, we find the amplitude to be A_w=2 arcsec^{\beta} and the slope \beta=0.9. The slope is the same as in the local and moderate redshift universe. A slope \beta=0.25 or shallower is ruled out by the data at the 99.9% confidence level. Because N(z) of LBGs is well determined from 376 spectroscopic redshifts, the real-space correlation function can be derived from the angular one through the Limber transform. This inversion is rather insensitive to the still relatively large errors on A_w and \beta, and the spatial correlation length is much more tightly constrained than either of these parameters. We estimate r_0=3.3 -0.6 +0.7 (2.1 -0.5 +0.4) h^{-1} Mpc (comoving) for q_0=0.1 (0.5) at z=3.04 (h is in units of 100 km/s/Mpc). The correlation length of LBGs at z=3 is comparable to that of present-day spiral galaxies and is only about 50% smaller than that of the ellipticals; it is as large or larger than any measured in galaxy samples at 0.3<z<1. By comparing the observed galaxy correlation length to that of the mass predicted from CDM theory, we estimate a bias for LBGs of b\sim 1.5 (4.5) for q_0=0.1 (0.5), in agreement with our previous estimates based on preliminary spectroscopy. The strong clustering and the large bias of the LBGs are consistent with biased galaxy formation theories and provide additional evidence that these systems are associated with massive dark matter halos. The clustering of LBGs at z=3 emphasizes that apparent evolution of galaxy clustering may be due as much to variations in effective bias parameter among different samples as to evolution in the mass distribution through gravitational instability.

We present high-resolution images of three BL Lac objects (0814+425, 1823+568, 2254+074) taken with the WFPC-2 camera on Hubble Space Telescope (HST) as part of a study of BL Lacs from a complete radio-selected sample. In two cases we clearly detect the host galaxies while the third BL Lac (0814+425) is unresolved. The host galaxy of 2254+074 is a moderately bright elliptical, while the inferred luminosity of the host galaxy for 1823+568 is at the high end of the range for previously known BL Lac hosts. The lack of detection of a host galaxy for 0814+425 calls into question the validity of its low reported redshift. Comparison of the HST data with subarcsecond ground-based R-band images shows excellent agreement on the largest scales, while on smaller scales only HST detects the galaxy significantly above the nuclear light. In the case of 2254+074, the high resolution of HST allows us to establish that the host galaxy is an elliptical.

We report on the initial results of a spectroscopic investigation of galaxies in the Hubble Deep Field which exhibit spectral discontinuities between the F450W and F300W passbands, indicative of the presence of the Lyman continuum break in the redshift range 2.4 < z < 3.4. We have employed color selection criteria similar to those we have used for selecting high redshift galaxy candidates from ground--based images. We find that, as for the ground--based color selection, the criteria are very successful in selecting high redshift objects. Of the 8 galaxies observed (selected from a list of 23 candidates with magnitudes equivalent to R <= 25.3, five have confirmed redshifts in the range 2.59 <= z <= 3.22, with the remaining 3 being indeterminate because of contamination from nearby brighter objects. As expected, the HST filter system is sensitive to a somewhat broader range of redshifts than our ground--based U_nGR filter system, and therefore the surveyed volume per unit area on the sky is correspondingly larger. The distribution of candidates on the plane of the sky is clearly non--uniform, consistent with the available ground--based data on the high redshift galaxies. Most Lyman break objects in the Hubble Deep Field exhibit a similar range of morphological properties to the z > 3 galaxies we have previously identified in other fields, characterized by very compact cores (some with multiple components) with half--light radii of 0.2-0.3 arcseconds, often surrounded by more diffuse and asymmetric ``halos''. A few of the brighter HDF Lyman break galaxies, however, have particularly unusual morphologies.

We present HST images of star-forming galaxies at redshifts z>3. These galaxies have been color selected for having a Lyman discontinuity in the otherwise flat (in f_\nu units) UV spectra of unreddened star formation. The spectroscopic confirmation of these z>3 galaxies is reported in a companion paper (Steidel et al. 1996). The HST images probe the rest-frame UV at 1400--1900 Ang and show that the morphologies of the z>3 galaxies are generally compact, although we find a few cases of more diffuse light profiles and objects comprised of multiple compact structures. Overall, the dispersion of morphologies is relatively narrow, in contrast to the variety found in star-forming galaxies at intermediate redshifts (z~1). The galaxies with compact morphology are typically characterized by a small but resolved ``core'', approximately 3 galaxies are characterized by a relatively high degree of spherical symmetry. Morphology, space density, star-formation rates, masses, and epoch of the star-formation phase all support the hypothesis that we have identified the progenitors of present-day luminous galaxies at the epoch when they were forming the stars of