Rychard J. Bouwens

We present uncontaminated rest-frame u - R colors of 78 X-ray-selected AGN hosts at 0.5 < z < 1.5 in the Chandra Deep Fields measured with HST/ACS/NICMOS and VLT/ISAAC imaging. We also present spatially-resolved NUV - R color gradients for a subsample of AGN hosts imaged by HST/WFC3. Integrated, uncorrected photometry is not reliable for comparing the mean properties of soft and hard AGN host galaxies at z ~ 1 due to color contamination from point-source AGN emission. We use a cloning simulation to develop a calibration between concentration and this color contamination and use this to correct host galaxy colors. The mean u - R color of the unobscured/soft hosts beyond ~6 kpc is statistically equivalent to that of the obscured/hard hosts (the soft sources are 0.09 +/- 0.16 magnitudes bluer). Furthermore, the rest-frame V - J colors of the obscured and unobscured hosts beyond ~6 kpc are statistically equivalent, suggesting that the two populations have similar distributions of dust extinction. For the WFC3/IR sample, the mean NUV - R color gradients of unobscured and obscured sources differ by less than ~0.5 magnitudes for r > 1.1 kpc. These three observations imply that AGN obscuration is uncorrelated with the star formation rate beyond ~1 kpc. These observations favor a unification scenario for intermediate-luminosity AGNs in which obscuration is determined geometrically. Scenarios in which the majority of intermediate-luminosity AGN at z ~ 1 are undergoing rapid, galaxy-wide quenching due to AGN-driven feedback processes are disfavored.

Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900-2,000 million years (Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep Field (HUDF09) data have yielded the first reliable detections of z ~ 8 galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200 Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest galaxies.

An ultra-deep search through the full Hubble Ultra Deep Field data set has uncovered a galaxy with a redshift of z ≈ 10, equivalent to an age of only 500 million years after the Big Bang. The data also provide strong constraints on the volume density of galaxies — and hence the star formation rate — at this time. The authors conclude that the star formation rate increased by a factor of ten in the time between z ≈ 10 and z ≈ 8, implying that this period in the heart of the reionization epoch was one in which galaxies were evolving very rapidly. Here, the full two-year Hubble Ultra Deep Field (HUDF09) data are used to conduct an ultra-deep search for z≈10 galaxies in the heart of the reionization epoch, only 500 million years after the Big Bang. One possible z≈10 galaxy candidate is found. It is also shown that regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 million years later at z≈8. The 100–200 million years prior to z≈10 is clearly a crucial phase in the assembly of the earliest galaxies. Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900–2,000 million years (Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep Field (HUDF09) data2,3 have yielded the first reliable detections of z ≈ 8 galaxies3,4,5,6,7,8,9 that, together with reports of a γ-ray burst at z ≈ 8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z ≈ 7–8 galaxies suggest substantial star formation at z > 9–10 (refs 12, 13). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z ≈ 10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z ≈ 10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (∼10%) at this time than it is just ∼200 Myr later at z ≈ 8. This demonstrates how rapid galaxy build-up was at z ≈ 10, as galaxies increased in both luminosity density and volume density from z ≈ 10 to z ≈ 8. The 100–200 Myr before z ≈ 10 is clearly a crucial phase in the assembly of the earliest galaxies.

We report on deep spectroscopy using LRIS on Keck I and FORS2 on the VLT of a sample of 22 candidate z~6 Lyman Break galaxies (LBGs) selected by the i-z> 1.3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5.5 - 6.1, confirming the efficiency of the i-z color selection technique. Six of the confirmed galaxies show Ly-alpha emission. Assuming that the 14 objects without redshifts are z~6 LBGs, but lack detectable Ly-alpha emission lines, we infer that the fraction of Ly-alpha emitting LBGs with Ly-alpha equivalent widths greater than 20 Angstroms among z~6 LBGs is ~30%, similar to that found at z~3. Every Ly-alpha emitting object in our sample is compact with r <= 0.14". Furthermore, all the Ly-alpha emitting objects in our sample are more compact than average relative to the observed size-magnitude relation of a large i-dropout sample (332 candidate z~6 objects). We can reject the hypothesis that the Ly-alpha emitting population is a subset of the rest of the z~6 LBG population at >97% confidence. We speculate the small sizes of Ly-alpha emitting LBGs are due to these objects being less massive than other LBGs at z~6.

The first 900 million years (Myr) to redshift z ≈ 6 (the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z ≈ 6 (refs 1 –4) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old (z ≈ 12–15) to z ≈ 6, just 600 Myr later. Here we report the results of a search for galaxies at z ≈ 7–8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z ≈ 7–8, where ten would be expected if there were no evolution in the galaxy population between z ≈ 7–8 and z ≈ 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z ≈ 7–8 by the hierarchical merging of small galaxies.

Extending the study of star-forming galaxies to z>6 is extremely difficult due to the faintness of the sources and the challenging nature of deep near-infrared observations. Nevertheless, current observations are now just good enough that we can begin drawing some conclusions about the nature of galaxies at z≳7. At present, deep near-infrared observations with NICMOS (reaching ≳27 AB mag at 5σ) cover more than 20arcmin2 of area with deep optical coverage and allow us to identify four strong z≃7–8 candidates. Comparing this sample with dropout samples at later times (z≃4–6), we are able to study evolution in the rest-frame UV LF over the range z≃8 to z≃4. We find strong evidence for significant evolution in the characteristic luminosity with time (brightening by ∼2 mag, from z≃8 to z≃4). The observed evolution appears to be the direct result of hierarchical growth in the galaxy population.

Today's deep high resolution multiwavelength surveys contain a wealth of information about galaxies at different epochs. To fully exploit this information, it is useful to be able to produce highly realistic simulations to compare with the observations. Here, we describe one such simulator for producing imaging data for deep galaxy fields. Based upon a pixel-by-pixel modelling of object SEDs and their selection volumes, this simulator allows users to make realistic multicolor simulations of galaxy fields from galaxy samples at all redshifts. Input samples for this simulator range from z~0 samples selected from the Sloan Digital Sky Survey to z~1-6 samples selected from the Great Observatories Origins Deep Survey. Users can specify the passbands, noise, and PSFs, or equivalently the exposure times on well-known instruments like HST or ground-based telescopes.

We present aperture-matched PSF-corrected BVi'z'JH photometry and Bayesian photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field (UDF), 8,042 of which are detected at the 10-sigma level (e.g., i'<29.01 or z'<28.43). Most of our objects are defined identically to those in the public STScI catalogs, enabling straightforward object-by-object comparison. We have combined detections from i', z', J+H, and B+V+i'+z' images into a single comprehensive segmentation map. Using a new program called SExSeg we are able to force this segmentation map into SExtractor for photometric analysis. The resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro software. We also correct for the ACS z'-band PSF halo. The NIC3 magnitudes are found to be too faint relative to the ACS fluxes. Based on BPZ SED fits to objects of know spectroscopic redshift, we derived corrections of -0.30 +/- 0.03 mag in J and -0.18 +/- 0.04 mag in H. The offsets appear to be supported by a recent recalibration of the UDF NIC3 images combined with non-linearity measured in NICMOS itself. The UDF reveals a large population of faint blue galaxies (presumably young starbursts), bluer than those observed in the original Hubble Deep Fields (HDF). To accommodate these galaxies, we have added two new starburst templates to the SED library used in previous BPZ papers. The resulting photometric redshifts are accurate to within 0.04 * 1+z_spec out to z < 6. Finally, we measure galaxy morphology, including Sersic index and asymmetry. Our full catalog, software packages, and more are available at http://adcam.pha.jhu.edu/~coe/UDF/ (abridged)