We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4 to 8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 ×10 6 Mpc 3 over this epoch, allowing us to perform a robust search for faint (M UV = −18) and bright (M UV < −21) highredshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 < z < 8.5, with >1000 galaxies at z ≈ 6 -8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, as well as a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z ≥ 6. Our best-fit value of the characteristic magnitude M * UV is consistent with −21 at z ≥ 5, different than that inferred based on previous trends at lower redshift, and brighter at ∼2σ significance than previous measures at z = 6 and 7 (Bouwens et al. 2007(Bouwens et al. , 2011b. At z = 8, a single powerlaw provides an equally good fit to the UV luminosity function, while at z = 6 and 7, an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M * UV is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, though a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M UV = −17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1+z) −4.3±0.5 at z > 4, consistent with observations at z ≥ 9. Our observed luminosity functions are consistent with a reionization history that starts at z 10, completes at z > 6, and reaches a midpoint (x HII = 0.5) at 6.7 < z < 9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, though consistent with recent estimates of bright z ∼ 10 galaxies.
We present chemical abundance measurements for 47 damped Lyα (DLA) systems, 30 at z > 4, observed with the Echellette Spectrograph and Imager and the High Resolution Echelle Spectrometer on the Keck telescopes. H i column densities of the DLAs are measured with Voigt profile fits to the Lyα profiles, and we find an increased number of false DLA identifications with Sloan Digital Sky Survey at z > 4 due to the increased density of the Lyα forest. Ionic column densities are determined using the apparent optical depth method, and we combine our new metallicity measurements with 195 from previous surveys to determine the evolution of the cosmic metallicity of neutral gas. We find the metallicity of DLAs decreases with increasing redshift, improving the significance of the trend and extending it to higher redshifts, with a linear fit of −0.22 ± 0.03 dex per unit redshift from z = 0.09-5.06. The metallicity "floor" of ≈1/600 solar continues out to z ∼ 5, despite our sensitivity for finding DLAs with much lower metallicities. However, this floor is not statistically different from a steep tail to the distribution. We also find that the intrinsic scatter of metallicity among DLAs of ∼0.5 dex continues out to z ∼ 5. In addition, the metallicity distribution and the α/Fe ratios of z > 2 DLAs are consistent with being drawn from the same parent population with those of halo stars. It is therefore possible that the halo stars in the Milky Way formed out of gas that commonly exhibits DLA absorption at z > 2.
We present photometry and derived redshifts from up to eleven bandpasses for 9927 galaxies in the Hubble Ultra Deep field (UDF), covering an observed wavelength range from the near-ultraviolet (NUV) to the near-infrared (NIR) with Hubble Space Telescope observations. Our Wide Field Camera 3 (WFC3)/UV F225W, F275W, and F336W image mosaics from the ultra-violet UDF (UVUDF) imaging campaign are newly calibrated to correct for charge transfer inefficiency, and use new dark calibrations to minimize background gradients and pattern noise. Our NIR WFC3/IR image mosaics combine the imaging from the UDF09 and UDF12 campaigns with CANDELS data to provide NIR coverage for the entire UDF field of view. We use aperture-matched point-spread function corrected photometry to measure photometric redshifts in the UDF, sampling both the Lyman break and Balmer break of galaxies atz 0.8-3.4, and one of the breaks over the rest of the redshift range. Our comparison of these results with a compilation of robust spectroscopic redshifts shows an improvement in the galaxy photometric redshifts by a factor of two in scatter and a factor three in outlier fraction (OLF) over previous UDF catalogs. The inclusion of the new NUV data is responsible for a factor of two decrease in the OLF compared to redshifts determined from only the optical and NIR data, and improves the scatter at < z 0.5 and at > z 2. The panchromatic coverage of the UDF from the NUV through the NIR yields robust photometric redshifts of the UDF, with the lowest OLF available.
Out of several dozen z > 7 candidate galaxies observed spectroscopically, only five have been confirmed via Lymanα emission, at z=7.008, 7.045, 7.109, 7.213 and 7.215. [1][2][3][4] The small fraction of confirmed galaxies may indicate that the neutral fraction in the intergalactic medium (IGM) rises quickly at z > 6.5, as Lymanα is resonantly scattered by neutral gas.3, 5-8 However, the small samples and limited depth of previous observations makes these conclusions tentative. Here we report the results of a deep near-infrared spectroscopic survey of 43 z > 6.5 galaxies. We detect only a single galaxy, confirming that some process is making Lymanα difficult to detect. The detected emission line at 1.0343 µm is likely to be Lymanα emission, placing this galaxy at a redshift z = 7.51, an epoch 700 million years after the Big Bang. This galaxy's colors are consistent with significant metal content, implying that galaxies become enriched rapidly. We measure a surprisingly high star formation rate of 330 M⊙ yr −1 , more than a factor of 100 greater than seen in the Milky Way. Such a galaxy is unexpected in a survey of our size 9 , suggesting that the early universe may harbor more intense sites of star-formation than expected.We obtained near-infrared (near-IR) spectroscopy of galaxies originally discovered in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) 10, 11 with the newlycommissioned near-infrared spectrograph MOSFIRE 12 on the Keck I 10 meter telescope. From a parent sample of over 100 galaxy candidates at z > 7 selected via their HST colors through the photometric redshift technique [13][14][15][16] , we observed 43 candidate high-redshift galaxies over two MOSFIRE pointings with exposure times of 5.6 and 4.5 hr, respectively. Our observations covered Lyα emission at redshifts of 7.0 -8.2. We visually inspected the reduced data at the expected slit positions for our 43 observed sources and found plausible emission lines in eight objects, with only one line detected at >5σ significance. The detected emission line is at a wavelength of 1.0343µm with an integrated signal-to-noise (S/N) of 7.8 ( Figure 1) and comes from the object designated z8 GND 5296 in our sample (RA=12:36:37.90; Dec=62:18:08.5 J2000). Based on the arguments outlined below (and discussed extensively in the supplementary material), we identify this line as the Lyα transition of hydrogen at a line-peak redshift of z = 7.5078 ± 0.0004; consistent with our photometric redshift 68% confidence range of 7.5 < z < 7.9 for z8 GND 5296. As expected for a galaxy at z = 7.51, z8 GND 5296 is completely undetected in the HST optical bands, including an extremely deep 0.8 µm image (Figure 2). The galaxy is bright in the HST near-IR bands, becoming brighter with increasing wavelength, implying that the Lyman break lies near 1 µm and that the galaxy has a moderately red rest-frame UV color. The galaxy is well-detected in both Spitzer/IRAC bands and is much brighter at 4.5 µm than at 3.6 µm. The strong break at observed 1 µm res...
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