P. G. van Dokkum

UV Luminosity Functions from 113 z~7 and z~8 Lyman-Break Galaxies in the ultra-deep HUDF09 and wide-area ERS WFC3/IR Observations

We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to <-18 AB mag and the star formation rate density at z~7 and z~8. The galaxy sample is derived using a sophisticated Lyman-Break technique on the full two-year WFC3/IR and ACS data available over the HUDF09 (~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and the wider area ERS (~27.5 AB mag) ~40 arcmin**2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, AGN, spurious sources, low mass stars, and transients (e.g., SNe). From careful modelling of the selection volumes for each of our search fields we derive luminosity functions for galaxies at z~7 and z~8 to <-18 AB mag. The faint-end slopes alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05. With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The star formation rate densities derived from these new z~7 and z~8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency, with the SFR densities implied from reported stellar mass densities, being only ~40% higher at z<7. This suggests that (1) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be very different from that at later times.

UV LUMINOSITY FUNCTIONS AT REDSHIFTS z ∼ 4 TO z ∼ 10: 10,000 GALAXIES FROM HST LEGACY FIELDS* **

The remarkable Hubble Space Telescope?(HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) from to . We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at , , , , , and , respectively, from the ?1000 arcmin2 area covered by these data sets. This sample of >10,000 galaxy candidates at is by far the largest assembled to date with HST. The selection of 4?8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at . Our new LF determinations at and span a 6 mag baseline and reach to ?16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to significance for a steeper faint-end slope of the UV LF at , with ? evolving from at to at (and at ), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M* from to the observed evolution in the LF is now largely represented by changes in . No evidence for a non-Schechter-like form to the z ? 4?8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio of halos provides a good representation of the observed evolution.

A significant population of red, near-infrared-selected high-redshift galaxies

We use very deep near-infrared photometry of the Hubble Deep Field-South taken with ISAAC on the Very Large Telescope to identify a population of high-redshift galaxies with rest-frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxies with redshifts above 2. When applied to our data set, we find 14 galaxies with Ks < 22.5, corresponding to a surface density of 3 ± 0.8 arcmin-2. The photometric redshifts all lie above 1.9, with a median of 2.6 and an rms of 0.7. The spectral energy distributions of these galaxies show a wide range. One is very blue in the rest-frame UV and satisfies the normal Lyman break criteria for high-redshift, star-forming galaxies. Others are quite red throughout the observed spectral range and are extremely faint in the optical, with a median V = 26.6. Hence, these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or the 4000 A break. The median age is 1 Gyr when fitted with a constant star formation model with dust or 0.7 Gyr when fitted with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to Ks = 22.5 is half that of Lyman break galaxies at z ≈ 3. Since the mass-to-light ratios of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman break galaxies. These red galaxies may be the descendants of galaxies that started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Long-lived emission, known as afterglow, has now been detected from about a dozen γ-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ∼1 to 3. The energy required to produce these bright γ-ray flashes is enormous: up to ∼10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z ⩾ 1.6. This is to date the brightest γ-ray burst with a well-localized position and if the γ-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr

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 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.

z ~ 7 Galaxies in the HUDF: First Epoch WFC3/IR Results

We present a sample of 16 robust z ~ 7 z 850-drop galaxies detected by the newly installed Wide Field Camera 3 (WFC3)/IR on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y 105, J 125, and H 160 observations. These remarkable data cover 4.7 arcmin2 and are the deepest near infrared images ever taken, reaching to ~29 mag AB (5?). The 16 z ~ 6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z 850 ? Y 105) versus (Y 105 ? J 125) colors. They have magnitudes J 125=26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (1 kpc), and show very blue colors (some even ? ?2.5), in particular at low luminosities. The WFC3/IR data confirm previous Near Infrared Camera and Multi-Object Spectrometer detections indicating that the dropout selection at z ~ 7 is very reliable. Our data allow a first determination of the faint-end slope of the z ~ 7 luminosity function, reaching down to M UV ~ ?18, a full magnitude fainter than previous measurements. When fixing * = 1.4 ? 10?3 Mpc?3 mag?1 to the value previously measured at z ~ 6, we find a best-fit value of ? = ?1.77 ? 0.20, with a characteristic luminosity of M * = ?19.91 ? 0.09. This steep slope is similar to what is seen at z ~ 2-6 and indicates that low-luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of the James Webb Space Telescope.

Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at z ≥ 5-8 from the HUDF09 WFC3/IR Observations

The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LFs) from these new HUDF09 data for 132 z ~ 7 and z ~ 8 galaxies are combined with new LFs for z ~ 5-6 galaxies and the earlier z ~ 4 LF to reach to very faint limits ( 8, taking α to be –1.87 ± 0.13 (the mean value at z ~ 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061+0.009 – 0.006. However, this result will change if the faint-end slope α is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dα/dz ~ –0.05 ± 0.04) that would increase the Thomson optical depths to 0.079+0.063 – 0.017, consistent with recent WMAP estimates (τ = 0.088 ± 0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies.

Probing the Dawn of Galaxies at z ~ 9-12: New Constraints from HUDF12/XDF and CANDELS data

We present a comprehensive analysis of z {gt} 8 galaxies based on ultra-deep WFC3/IR data. We exploit all the WFC3/IR imaging over the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in addition to the HUDF09 parallel field data, as well as wider area imaging over GOODS-South. Galaxies are selected based on the Lyman break technique in three samples centered around z ~{} 9, z ~{} 10, and z ~{} 11, with seven z ~{} 9 galaxy candidates, and one each at z ~{} 10 and z ~{} 11. We confirm a new z ~{} 10 candidate (with z = 9.8 {plusmn} 0.6) that was not convincingly identified in our first z ~{} 10 sample. Using these candidates, we perform one of the first estimates of the z ~{} 9 UV luminosity function (LF) and improve our previous constraints at z ~{} 10. Extrapolating the lower redshift UV LF evolution should have revealed 17 z ~{} 9 and 9 z ~{} 10 sources, i.e., a factor ~{}3 { imes} and 9{ imes} larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0.7 M

NICMOS Imaging of DRGs in the HDF-S: A Relation between Star Formation and Size at z ~ 2.5

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The KMOS3D survey: design, first results, and the evolution of galaxy kinematics from 0.7 ≤ z ≤ 2.7

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