This Special Issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multi-band imaging data obtained with the Hubble Space Telescope (HST) and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 square arcminutes in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-ray Observatory (CXO) and groundbased facilities are supplemented with new, deep imaging in the optical and

Hubble Space Telescope images of high-redshift galaxies selected via color and photometric redshifts are used to examine the size and axial ratio distribution of galaxies as a function of redshift at look-back times Gyr. t 1 8 These parameters are measured at rest-frame UV wavelengths (1200 ! l ! 2000 ) on images with a rest-Å A frame resolution of less than 0.8 kpc. Galaxy radii are found to scale with redshift approximately as the Hubble parameter . This is in accord with the theoretical expectation that the typical sizes of the luminous parts Ϫ1 H (z) of galaxies should track the expected evolution in the virial radius of dark matter halos. The mean ratio of the semimajor axis to the semiminor axis for a bright well-resolved sample of galaxies at is , z ∼ 4 b/a p 0.65 suggesting that these Lyman break galaxies are not drawn from a spheroidal population. However, the median concentration index of this sample is , which is closer to the typical concentration indices of nearby C p 3.5 elliptical galaxies ( ) than to the values for local disk galaxies of type Sb and later ( ). C ∼ 4 C ! 2

ABSTRACT. We describe the TFIT software package to measure galaxy photometry using prior information from high-resolution observations. Our basic methodology is similar in principle but different in detail from previous procedures for crowded field photometry. We use the spatial positions and morphologies of objects in an image with higher angular resolution to construct object templates, which are then fitted to a lower resolution image, solving for the object fluxes as free parameters. Using extensive experiments on both simulated and real data, we show that this template-fitting method measures accurate object photometry to the limiting sensitivity of the image. In this limit, our method derives robust flux upper limits for objects fainter than the limiting image surface brightness. We describe the challenges encountered in applying this technique to real data, and methods to cope with some of them.

We use extensive multiwavelength photometric data from the Great Observatories Origins Deep Survey to estimate photometric redshifts for a sample of 434 galaxies with spectroscopic redshifts in the Chandra Deep Field-South. Using the Bayesian method, which incorporates redshift/magnitude priors, we estimate photometric redshifts for galaxies in the range , giving an rms scatter . The outlierfraction is less than 10%, with the outlier-clipped rms being 0.047. We examine the accuracy of photometric redshifts for several special subclasses of objects. The results for extremely red objects are more accurate than those for the sample as a whole, with and very few outliers (3%). Photometric redshifts for active j p 0.051 galaxies, identified from their X-ray emission, have a dispersion of , with 10% outlier fraction, similar j p 0.104 to that for normal galaxies. Employing a redshift/magnitude prior in this process seems to be crucial in improving the agreement between photometric and spectroscopic redshifts.

We investigate the biases and uncertainties in estimates of physical parameters of high-redshift Lyman break galaxies (LBGs), such as stellar mass, mean stellar population age, and star formation rate (SFR), obtained from broad-band photometry. These biases arise from the simplifying assumptions often used in fitting the spectral energy distributions (SEDs). By combining ΛCDM hierarchical structure formation theory, semi-analytic treatments of baryonic physics, and stellar population synthesis models, we construct model galaxy catalogs from