A limit on the number density of bright z â 7 galaxies (original) (raw)
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The Astrophysical Journal, 2004
We measure the surface density of i 0-band dropout galaxies at z $ 6 through wide-field Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging and ultradeep Keck DEIMOS spectroscopy. Using deep HST ACS SDSS i 0 (F775W) and SDSS z 0 (F850LP) imaging from the Great Observatories Origins Deep Survey-North (GOODS-N; 200 arcmin 2), we identify nine i 0-drops satisfying (i 0 Àz 0) AB > 1:5 to a depth of z 0 AB ¼ 25:6 (corresponding to L Ã UV at z $ 3). We use HK 0 imaging data to improve the fidelity of our sample, discriminating against lower redshift red galaxies and cool Galactic stars. Three i 0-drops are consistent with M/L/T dwarf stars. We present ultradeep Keck DEIMOS spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i 0-drop sample. We detect Ly emission at z ¼ 5:83 from one object in the GOODS-S field, which lies only 8 0 (i.e., 3 h À1 70 Mpc) away from a previously confirmed z ¼ 5:78 object. One possible Ly emitter at z ¼ 6:24 is found in the GOODS-N field (although identification of this spatially offset emission line is ambiguous). Using the rest-frame UV continuum from our six candidate z $ 6 galaxies from the GOODS-N field, we determine a lower limit to the unobscured volume-averaged global star formation rate at z $ 6 of (5:4 AE 2:2) ; 10 À4 h 70 M yr À1 Mpc À3. We find that the cosmic star formation density in Lyman break galaxies (LBGs) with unobscured star formation rates greater than 15 M yr À1 falls by a factor of 8 between z $ 3 and z $ 6. Hence, the luminosity function of LBGs must evolve in this redshift interval: a constant integrated star formation density at z > 3 requires a much steeper faint-end slope, or a brighter characteristic luminosity. This result is in agreement with our previous measurement from the GOODS-S field, indicating that cosmic variance is not a dominant source of uncertainty.
Optical dropout galaxies lensed by the cluster A2667
Astronomy & Astrophysics, 2011
Context. We investigate the nature and the physical properties of ten z, Y and J−dropout galaxies selected in the field of the lensing cluster A2667. Aims. This cluster is part of our project aimed at obtaining deep photometry at ∼0.8-2.5 microns with ESO/VLT HAWK-I and FORS2 on a representative sample of lensing clusters extracted from our multi-wavelength combined surveys with SPITZER, HST, and Herschel. The goal is to identify a sample of redshift z∼ 7-10 candidates accessible to detailed spectroscopic studies. Methods. The selection function is the usual dropout technique based on deep I, z,Y, J, H and K s-band images (AB∼26-27, 3σ), targeting z 7.5 galaxy candidates. We also include IRAC data between 3.6 and 8 µm, and MIPS 24µm when available. In this paper we concentrate on the complete Y and J−dropout sample among the sources detected with a high S/N ratio in both H and K s bands, as well as the bright z-dropout sources fulfilling the color and magnitude selection criteria adopted by . SED-fitting and photometric redshifts were used to further constrain the nature and the properties of these candidates. Results. 10 photometric candidates are selected within the ∼ 7 ′ × 7 ′ HAWK-I field of view (∼ 33arcmin 2 of effective area once corrected for contamination and lensing dilution at z ∼7-10). All of them are detected in H and K s bands in addition to J and/or IRAC 3.6µm/4.5µm images, with H AB ranging from 23.4 to 25.2, and have modest magnification factors between 1.1 and 1.4. Although best-fit photometric redshifts are obtained at high-z for all these candidates, the contamination by low-z interlopers is expected to range between ∼50-75% based on previous studies, and on comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The same result is obtained when photometric redshifts are computed using a luminosity prior, allowing us to remove half of the original sample. Among the remaining galaxies, two additional sources could be identified as low-z interlopers based on a detection at 24µm and on the HST z 850 band. These low-z interlopers are not well described by current spectral templates given the large break, and cannot be easily identified based on broad-band photometry in the optical and near-IR domains alone. A good fit at z∼1.7-3 is obtained at assuming a young stellar population together with a strong extinction. Given the estimated dust extinction and high SFRs, some of them could be also detected in the IR or sub-mm bands. Conclusions. After correction for contaminants, the observed number counts at z 7.5 seem to be in agreement with expectations for an evolving LF, and inconsistent with a constant LF since z ∼ 4. At least one and up to three candidates in this sample are expected to be genuine high-z, although spectroscopy is still needed to conclude.
The Astrophysical Journal, 2011
We present three bright z + dropout candidates selected from deep Near-Infrared (NIR) imaging of the COSMOS 2 square degree field. All three objects match the 0.8 − 8µm colors of other published z > 7 candidates but are three magnitudes brighter, facilitating further study. Deep spectroscopy of two of the candidates covering 0.64 − 1.02µm with Keck-DEIMOS and all three covering 0.94-1.10µm and 1.52-1.80µm with Keck-NIRSPEC detects weak spectral features tentatively identified as Ly-α at z = 6.95 and z = 7.69 in two of the objects. The third object is placed at z ∼ 1.6 based on a 24µm and weak optical detection. A comparison with the spectral energy distributions of known z < 7 galaxies, including objects with strong spectral lines, large extinction, and large systematic uncertainties in the photometry yields no objects with similar colors. However, the λ > 1µm properties of all three objects can be matched to optically detected sources with photometric redshifts at z ∼ 1.8, so the non-detection in the i + and z + bands are the primary factors which favors a z > 7 solution. If any of these objects are at z ∼ 7 the bright end of the luminosity function is significantly higher at z > 7 than suggested by previous studies, but consistent within the statistical uncertainty and the dark matter halo distribution. If these objects are at low redshift, the Lyman-Break selection must be contaminated by a previously unknown population of low redshift objects with very strong breaks in their broad band spectral energy distributions and blue NIR colors. The implications of this result on luminosity function evolution at high redshift is discussed. We show that the primary limitation of z > 7 galaxy searches with broad filters is the depth of the available optical data.
The Astrophysical …, 2008
We present the results of a systematic search for gravitationally-lensed continuum Lyman break 'drop-outs' beyond a redshift 7 conducted via very deep imaging through six foreground clusters undertaken with the Hubble and Spitzer Space Telescopes. The survey has yielded 10 z-band and 2 J-band drop-out candidates to photometric limits of J 110 ≃26.2 AB (5σ). Taking into account the magnifications afforded by our clusters (1-4 magnitudes), we probe the presence of z >7 sources to unlensed limits of J 110 ≃30 AB, fainter than those charted in the Hubble Ultradeep Field. To verify the fidelity of our candidates we conduct a number of tests for instrumental effects which would lead to spurious detections, and carefully evaluate the likelihood of foreground contamination by considering photometric uncertainties in the drop-out signature, the upper limits from stacked IRAC data and the statistics of multiply-imaged sources. Overall, we conclude that we can expect about half of our sample of z-band drop-outs to be at high redshift. An ambitious infrared spectroscopic campaign undertaken with the NIRSPEC spectrograph at the WM Keck Observatory for seven of the most promising candidates failed to detect any Lyman α emission highlighting the challenge of making further progress in this field. While the volume density of high redshift sources will likely remain uncertain until more powerful facilities -2are available, our data provides the first potentially interesting constraints on the UV luminosity function at z ≃7.5 at intrinsically faint limits. We discuss the implications of our results in the context of the hypothesis that the bulk of the reionizing photons in the era 7 < z < 12 arise in low luminosity galaxies undetected by conventional surveys.
The Stellar Population of Lyα‐emitting Galaxies at z ∼ 5.7
The Astrophysical Journal, 2007
We present a study of three Lyα emitting galaxies (LAEs), selected via a narrow-band survey in the GOODS northern field, and spectroscopically confirmed to have redshifts of z ∼ 5.65. Using HST ACS and Spitzer IRAC data, we constrain the rest-frame UV-to-optical spectral energy distributions (SEDs) of the galaxies. Fitting stellar population synthesis models to the observed SEDs, we find best-fit stellar populations with masses between ∼ 10 9 − 10 10 M ⊙ and ages between ∼ 5 − 100 Myr, assuming a simple starburst star formation history. However, stellar populations as old as 700 Myr are admissible if a constant star formation rate model is considered. Very deep near-IR observations may help to narrow the range of allowed models by providing extra constraints on the rest-frame UV spectral slope. Our narrow-band selected objects and other IRAC-detected z ∼ 6 i ′-dropout galaxies have similar 3.6 µm magnitudes and z ′ − [3.6] colors, suggesting that they posses stellar populations of similar masses and ages. This similarity may be the result of a selection bias, since the IRAC-detected LAEs and i ′-dropouts probably only sample the bright end of the luminosity function. On the other hand, our LAEs have blue i ′ − z ′ colors compared to the i ′-dropouts, and would have been missed by the i ′-dropout selection criterion. A better understanding of the overlap between the LAE and the i ′-dropout populations is necessary in order to constrain the properties of the overall high-redshift galaxy population, such as the total stellar mass density at z ∼ 6.
Astrophysical Journal, 2006
We investigate the luminosity functions of Lyman-break galaxies (LBG) at z ∼ 4 and 5 based on the optical imaging data obtained in the Subaru Deep Field (SDF) Project, a program conducted by Subaru Observatory to carry out a deep and wide survey of distant galaxies. Three samples of LBGs in a contiguous 875 arcmin 2 area are constructed. One consists of LBGs at z ∼ 4 down to i ′ = 26.85 selected with the B −R vs R−i ′ diagram (BRi ′ -LBGs). The other two consist of LBGs at z ∼ 5 down to z ′ = 26.05 selected with two kinds of two-color diagrams:
No Evidence for Lyman-alpha Emission in Spectroscopy of z > 7 Candidate Galaxies
2012
We present Gemini/GNIRS spectroscopic observations of 4 z−band (z ≈ 7) dropout galaxies and VLT/XSHOOTER observations of one z−band dropout and 3 Y −band (z ≈ 8 − 9) dropout galaxies in the Hubble Ultra Deep Field, which were selected with Wide Field Camera 3 imaging on the Hubble Space Telescope. We find no evidence of Lyman-α emission with a typical 5σ sensitivity of 5 × 10 −18 erg cm −2 s −1 , and we use the upper limits on Lyman-α flux and the broad-band magnitudes to constrain the rest-frame equivalent widths for this line emission. Accounting for incomplete spectral coverage, we survey 3.0 z-band dropouts and 2.9 Y -band dropouts to a Lyman-α rest-frame equivalent width limit > 120Å (for an unresolved emission line); for an equivalent width limit of 50Å the effective numbers of drop-outs surveyed fall to 1.2 z-band drop-outs and 1.5 Y -band drop-outs. A simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3−6.5 is inconsistent with our non-detections at z = 7 − 9 at the ≈ 1σ level for spectrally unresolved lines, which may indicate that a significant neutral HI fraction in the intergalactic medium suppresses the Lyman-α line in z-drop and Y -drop galaxies at z > 7.
2012
The recent Hubble Space Telescope near-infrared imaging with the Wide-Field Camera #3 (WFC 3) of the GOODS-South field in the CANDELS program covering nearly 100 arcmin 2 , along with already existing Advanced Camera for Surveys optical data, makes possible the search for bright galaxy candidates at redshift z ≈ 7 − 9 using the Lyman-break technique. We present the first analysis of z ′ -drop z ≈ 7 candidate galaxies in this area, finding 19 objects. We also analyse Y -drops at z ≈ 8, trebling the number of bright (H AB < 27 mag) Y -drops from our previous work, and compare our results with those of other groups based on the same data. The bright high redshift galaxy candidates we find serve to better constrain the bright end of the luminosity function at those redshift, and may also be more amenable to spectroscopic confirmation than the fainter ones presented in various previous work on the smaller fields (the Hubble Ultra Deep Field and the WFC 3 Early Release Science observations). We also look at the agreement with previous luminosity functions derived from WFC 3 dropout counts, finding a generally good agreement, except for the luminosity function of at z ≈ 8, which is strongly ruled out.
The Astrophysical Journal, 2012
Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most massive and luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with Hubble Space Telescope observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z ∼ 8 candidates as Y 098 -dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y 098dropouts to fainter luminosities (M * galaxies with M AB ∼ −20), with detections at 5σ confidence (compared to the 8σ confidence threshold adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y 098 -dropouts at 99.84% confidence. Field BoRG58, which contains the best bright z ∼ 8 candidate (M AB = −21.3), has the most significant overdensity of faint Y 098 -dropouts. Four new sources are located within 70 ′′ (corresponding to 3.1 comoving Mpc at z = 8) from the previously known brighter z ∼ 8 candidate. The overdensity of Y 098 -dropouts in this field has a physical origin to very high confidence (p > 99.975%), independent of completeness and contamination rate of the Y 098 -dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M h ≈ (4 − 7) × 10 11 M ⊙ (∼ 5σ density peak) and is surrounded by several M h ≈ 10 11 M ⊙ halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M h > 2 × 10 14 M ⊙ galaxy cluster by z = 0. Follow-up observations with ground and space 2 Trenti et al.
Shels: Optical spectral properties of wise 22 μm selected galaxies
Astrophysical Journal, 2012
We use a dense, complete redshift survey, the Smithsonian Hectospec Lensing Survey (SHELS), covering a 4 square degree region of a deep imaging survey, the Deep Lens Survey (DLS), to study the optical spectral properties of Wide-field Infrared Survey Explorer (WISE) 22 µm-selected galaxies. Among 507 WISE 22 µm-selected sources with (S/N) 22µm ≥ 3 (≈ S 22µm 2.5 mJy), we identify the optical counterparts of 481 sources (∼ 98%) at R < 25.2 in the very deep, DLS R-band source catalog. Among them, 337 galaxies at R < 21 have SHELS spectroscopic data. Most of these objects are at z < 0.8. The infrared (IR) luminosities are in the range 4.5 × 10 8 (L ⊙ ) L IR 5.4 × 10 12 (L ⊙ ). Most 22 µm-selected galaxies are dusty star-forming galaxies with a small (<1.5) 4000Å break. The stacked spectra of the 22 µm-selected galaxies binned in IR luminosity show that the strength of the [O III] line relative to Hβ grows with increasing IR luminosity. The optical spectra of the 22 µm-selected galaxies also show that there are some (∼ 2.8%) unusual galaxies with very strong [Ne III] λ3869, 3968 emission lines that require hard ionizing radiation such as AGN or extremely young massive stars. The specific star formation rates (sSFRs) derived from the 3.6 and 22 µm flux densities are enhanced if the 22 µm-selected galaxies have close late-type neighbors. The sSFR distribution of the 22 µm-selected galaxies containing active galactic nuclei (AGNs) is similar to the distribution for star-forming galaxies without AGNs. We identify 48 dust-obscured galaxy (DOG) candidates with large ( 1000) mid-IR to optical flux density ratio. The combination of deep photometric and spectroscopic data with WISE data suggests that WISE can probe the universe to z ∼ 2.