Low-redshift Interlopers Research Articles

A GEOMETRICALLY SUPPORTED z ∼ 10 CANDIDATE MULTIPLY IMAGED BY THE HUBBLE FRONTIER FIELDS CLUSTER A2744

The deflection angles of lensed sources increase with their distance behind a given lens. We utilize this geometric effect to corroborate the $z_{phot}\simeq9.8$ photometric redshift estimate of a faint near-IR dropout, triply-imaged by the massive galaxy cluster Abell 2744 in deep Hubble Frontier Fields images. The multiple images of this source follow the same symmetry as other nearby sets of multiple images which bracket the critical curves and have well defined redshifts (up to $z_{spec}\simeq3.6$), but with larger deflection angles, indicating that this source must lie at a higher redshift. Similarly, our different parametric and non-parametric lens models all require this object be at $z\gtrsim4$, with at least 95\% confidence, thoroughly excluding the possibility of lower-redshift interlopers. To study the properties of this source we correct the two brighter images for their magnifications, leading to a SFR of $\sim0.3 M_{\odot}$/yr, a stellar mass of $\sim4\times10^{7} M_{\odot}$, and an age of $\lesssim220$ Myr (95\% confidence). The intrinsic apparent magnitude is 29.9 AB (F160W), and the rest-frame UV ($\sim1500 \AA$) absolute magnitude is $M_{UV,AB}=-17.6$. This corresponds to $\sim0.1 L^{*}_{z=8}$ ($\sim0.2 L^{*}_{z=10}$, adopting $dM^{*}/dz\sim0.45$), making this candidate one of the least luminous galaxies discovered at $z\sim10$.

REAL OR INTERLOPER? THE REDSHIFT LIKELIHOODS OFz> 8 GALAXIES IN THE HUDF12

In the absence of spectra, fitting template model spectra to observed photometric fluxes, known as Spectral Energy Distribution (SED) fitting, has become the workhorse for identifying high-z galaxies. In this paper, we present an analysis of the most recent and possibly most distant galaxies discovered in the Hubble Ultra Deep Field using a more robust method of redshift estimation based on Markov Chain Monte Carlo fitting (MCMC), rather than relying on the redshift of "best fit" models obtained using common chi^2 minimization techniques. The advantage of MCMC fitting is the ability to accurately estimate the probability density function of the redshift, as well as any other input model parameters, allowing us to derive accurate credible intervals by properly marginalizing over all other input model parameters. We apply our method to 13 recently identified sources and show that, despite claims based on chi^2 minimization, none of these sources can be securely ruled out as low redshift interlopers given the low signal-to-noise of currently available observations. We estimate that there is an average probability of 21% that these sources are low redshift interlopers.

CLASH: THREE STRONGLY LENSED IMAGES OF A CANDIDATEz≈ 11 GALAXY

We present a candidate for the most distant galaxy known to date with a photometric redshift z = 10.7 +0.6 / -0.4 (95% confidence limits; with z < 9.5 galaxies of known types ruled out at 7.2-sigma). This J-dropout Lyman Break Galaxy, named MACS0647-JD, was discovered as part of the Cluster Lensing and Supernova survey with Hubble (CLASH). We observe three magnified images of this galaxy due to strong gravitational lensing by the galaxy cluster MACSJ0647.7+7015 at z = 0.591. The images are magnified by factors of ~8, 7, and 2, with the brighter two observed at ~26th magnitude AB (~0.15 uJy) in the WFC3/IR F160W filter (~1.4 - 1.7 um) where they are detected at >~ 12-sigma. All three images are also confidently detected at >~ 6-sigma in F140W (~1.2 - 1.6 um), dropping out of detection from 15 lower wavelength HST filters (~0.2 - 1.4 um), and lacking bright detections in Spitzer/IRAC 3.6um and 4.5um imaging (~3.2 - 5.0 um). We rule out a broad range of possible lower redshift interlopers, including some previously published as high redshift candidates. Our high redshift conclusion is more conservative than if we had neglected a Bayesian photometric redshift prior. Given CLASH observations of 17 high mass clusters to date, our discoveries of MACS0647-JD at z ~ 10.8 and MACS1149-JD1 at z ~ 9.6 are consistent with a lensed luminosity function extrapolated from lower redshifts. This would suggest that low luminosity galaxies could have reionized the universe. However given the significant uncertainties based on only two galaxies, we cannot yet rule out the sharp drop off in number counts at z >~ 10 suggested by field searches.

ON THE DETECTION OF IONIZING RADIATION ARISING FROM STAR-FORMING GALAXIES AT REDSHIFTz∼ 3-4: LOOKING FOR ANALOGS OF “STELLAR RE-IONIZERS”

We use the spatially-resolved, multi-band photometry in the GOODS South field acquired by the CANDELS project to constrain the nature of candidate Lyman continuum (LyC) emitters at redshift z~3.7 identified using ultra-deep imaging below the Lyman limit (1-sigma limit of ~30 AB in a 2" diameter aperture). In 18 candidates, out of a sample of 19 with flux detected at >3-sigma level, the light centroid of the candidate LyC emission is offset from that of the LBG by up to 1.5". We fit the SED of the LyC candidates to spectral population synthesis models to measure photometric redshifts and the stellar population parameters. We also discuss the differences in the UV colors between the LBG and the LyC candidates, and how to estimate the escape fraction of ionizing radiation (f_esc) in cases, like in most of our galaxies, where the LyC emission is spatially offset from the host galaxy. In all but one case we conclude that the candidate LyC emission is most likely due to lower redshift interlopers. Based on these findings, we argue that the majority of similar measurements reported in the literature need further investigation before it can be firmly concluded that LyC emission is detected. Our only surviving LyC candidate is a LBG at z=3.795, which shows the bluest (B-V) color among LBGs at similar redshift, a stellar mass of M~2 x 10^9 Msun, weak interstellar absorption lines and a flat UV spectral slope with no Lya in emission. We estimate its f_esc to be in the range 25%-100%, depending on the dust and intergalactic attenuation.

VERY STRONG EMISSION-LINE GALAXIES IN THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY AND IMPLICATIONS FOR HIGH-REDSHIFT GALAXIES,

The WFC3 Infrared Spectroscopic Parallel Survey (WISP) uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the Universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths higher than 200 A. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin^2 area we analyzed so far. After estimating the AGN fraction in the sample, we show that this population consists of young and low-mass starbursts with higher specific star formation rates than normal star-forming galaxies at any redshift. After spectroscopic follow-up of one of these galaxies with Keck/LRIS, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12+Log(O/H)= 7.47 +- 0.11. The nebular emission-lines can substantially affect the broadband flux density with a median brightening of 0.3 mag, with examples producing brightening of up to 1 mag. The presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ~ 8 dropout surveys. In order to effectively remove low redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their SED. Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

A robust sample of galaxies at redshifts 6.0&lt;z&lt;8.7: stellar populations, star formation rates and stellar masses

We present the results of a photometric redshift analysis designed to identify z>6 galaxies from the near-IR HST imaging in three deep fields (HUDF, HUDF09-2 & ERS). By adopting a rigorous set of criteria for rejecting low-z interlopers, and by employing a deconfusion technique to allow the available IRAC imaging to be included in the candidate selection process, we have derived a robust sample of 70 Lyman-break galaxies (LBGs) spanning the redshift range 6.0<z<8.7. Based on our final sample we investigate the distribution of UV spectral slopes (beta), finding a variance-weighted mean value of <beta>=-2.05 +/- 0.09 which, contrary to some previous results, is not significantly bluer than displayed by lower-redshift starburst galaxies. We confirm the correlation between UV luminosity and stellar mass reported elsewhere, but based on fitting galaxy templates featuring a range of star-formation histories, metallicities and reddening we find that, at z>=6, the range in mass-to-light ratio (M*/L_UV) at a given UV luminosity could span a factor of ~50. Focusing on a sub-sample of twenty-one candidates with IRAC detections at 3.6-microns we find that L* LBGs at z~6.5 have a median stellar mass of M* = (2.1 +/- 1.1) x 10^9 Msun and a median specific star-formation rate of 1.9 +/- 0.8 Gyr^-1. Using the same sub-sample we have investigated the influence of nebular continuum and line emission, finding that for the majority of candidates (16 out of 21) the best-fitting stellar-mass estimates are reduced by less than a factor of 2.5. Finally, a detailed comparison of our final sample with the results of previous studies suggests that, at faint magnitudes, several high-redshift galaxy samples in the literature are significantly contaminated by low-redshift interlopers (abridged).

A critical analysis of the UV luminosity function at redshift ~7 from deep WFC3 data

The study of the Luminosity Function (LF) of Lyman Break Galaxies (LBGs) at z=7 is important for ascertaining their role in the reionization of the Universe. We perform a detailed and critical analysis of the statistical and systematic errors in the z~7 LF determination: we have assembled a large sample of candidate LBGs at z~7 from different surveys, spanning a large variety of areas and depths. In particular, we have combined data from the deep (J<27.4) and ultradeep (J<29.2) surveys recently acquired with the new WFC3 NIR camera on HST, over the GOODS-ERS and the HUDF fields, with ground based surveys in wide and shallow areas from VLT and Subaru. We have used public ACS images in the z-band to select z-dropout galaxies, and other public data both in the blue (BVI) and in the red bands to reject possible low-redshift interlopers. We have compared our results with extensive simulations to quantify the observational effects of our selection criteria as well as the effects of photometric scatter, color selections or the morphology of the candidates. We have found that the number density of faint LBGs at z~7 is only marginally sensitive to the color selection adopted, but it is strongly dependent from the assumption made on the half light distributions of the simulated galaxies, used to correct the observed sample for incompleteness. The slope of the faint end of the LBGs LF has thus a rather large uncertainty, due to the unknown distribution of physical sizes of the z~7 LBGs. We conclude that galaxies at z~7 are unable to reionize the Universe unless there is a significant evolution in the clumpiness of the IGM or in the escape fraction of ionising photons or, alternatively, there is a large population of z~7 LBGs with large physical dimensions but still not detected by the present observations.

Contamination on Lyman continuum emission at : implication on the ionizing radiation evolution

We investigate the possibility of contamination by lower-redshift interlopers in the measure of the ionising radiation escaping from high redshift galaxies. Taking advantage of the new ultra-deep VLT/VIMOS U-band number counts in the GOODS-S field,we calculate the expected probability of contamination by low-z interlopers as a function of the U-mag and the image spatial resolution (PSF). Assuming that ground-based observations can not resolve objects lying within a 0.5" radius of each other, then each z>=3 galaxy has a 2.1 and 3.2% chance of foreground contamination, adopting surface density U-band number counts down to 27.5 and 28.5,respectively. Those probabilities increase to 8.5 and 12.6%, assuming 1.0" radius. If applied to the estimates reported in the literature at z~3 for which a Lyman continuum has been observed directly,the probability that at least 1/3 of them are affected by foreground contamination is larger than 50%.From a Monte-Carlo simulation we estimate the median integrated contribution of foreground sources to the Lyman continuum flux (f900). Current estimations from stacked data are >2 sigma of the median integrated pollution by foreground sources.The spatial cross-correlation between the U-band catalog and a sample of galaxies at z>=3.4 in the GOODS-S field,produces a number of U-band detected systems fully consistent with the expected superposition statistics. Indeed, each of them shows the presence of at least one offset contaminant in the ACS images. An exemplary case of a foreground contamination in the HUDF at redshift 3.797 by a foreground blue compact source (U=28.63 +/- 0.2) is reported; if observed with a seeing larger than 0.5" the polluting source would mimic an observed (f1500/f900)_OBS ~ 38, erroneously ascribed to the source at higher redshift. [Abridged]

Probing ∼L* Lyman-break galaxies at z≈ 7 in GOODS-South with WFC3 on Hubble Space Telescope

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the GOODS-South field to search for star forming galaxies at z~7.0. By comparing WFC 3 0.98 micron Y-band images with ACS z-band (0.85 micron) images, we identify objects with colours consistent with Lyman break galaxies at z~6.4-7.4. This new data covers an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field, and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B, V and i-bands from GOODS v2.0 and the WFC3 J-band we attempt to remove any low-redshift interlopers. Our selection criteria yields 6 candidates brighter than Y_AB = 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all 6 candidates are at z~7 this implies a surface density of objects brighter than Y_AB = 27.0 of 0.30\pm0.12 arcmin-2, a value significantly smaller than the prediction from the z ~ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function between z = 6 to 7, with number densities lower at higher redshift.

Evidence of a fast evolution of the UV luminosity function beyond redshift 6 from a deep HAWK-I survey of the GOODS-S field

We perform a deep search for galaxies in the redshift range 6.5<z<7.5, to measure the evolution of the number density of luminous galaxies in this redshift range and derive useful constraints on the evolution of their Luminosity Function. We present here the first results of an ESO Large Program, that exploits the unique combination of area and sensitivity provided in the near-IR by the camera Hawk-I at the VLT. We have obtained two Hawk-I pointings on the GOODS South field for a total of 32 observing hours, covering ~90 arcmin2. The images reach Y=26.7 mags for the two fields. We have used public ACS images in the z band to select z-dropout galaxies with the colour criteria Z-Y>1, Y-J<1.5 and Y-K<2. The other public data in the UBVRIJHK bands are used to reject possible low redshift interlopers. The output has been compared with extensive Monte Carlo simulations to quantify the observational effects of our selection criteria as well as the effects of photometric errors. We detect 7 high quality candidates in the magnitude range Y=25.5-26.7. This interval samples the critical range for M* at z>6 (M_1500 ~- 19.5 to -21.5). After accounting for the expected incompleteness, we rule out at a 99% confidence level a Luminosity Function constant from z=6 to z=7, even including the effects of cosmic variance. For galaxies brighter than M_1500=-19.0 we derive a luminosity density rho_UV=1.5^{+2.0}_{-0.9} 10^25 erg/s/Hz/Mpc3, implying a decrease by a factor 3.5 from z=6 to z~6.8. On the basis of our findings, we make predictions for the surface densities expected in future surveys surveys, based on ULTRA-VISTA, HST-WFC3 or JWST-NIRCam, evaluating the best observational strategy to maximise their impact.

BRIGHT STRONGLY LENSED GALAXIES AT REDSHIFTz∼ 6-7 BEHIND THE CLUSTERS ABELL 1703 AND CL0024+16

We report on the discovery of three bright, strongly lensed objects behind Abell 1703 and CL0024+16 from a dropout search over 25 arcmin2 of deep NICMOS data, with deep ACS optical coverage. They are undetected in the deep ACS images below 8500 Å and have clear detections in the J and H bands. Fits to the ACS, NICMOS, and IRAC data yield robust photometric redshifts in the range z ∼ 6–7 and largely rule out the possibility that they are low-redshift interlopers. All three objects are extended, and resolved into a pair of bright knots. The bright i-band dropout in Abell 1703 has an H-band AB magnitude of 23.9, which makes it one of the brightest known galaxy candidates at z > 5.5. Our model fits suggest a young, massive galaxy only ∼60 million years old with a mass of ∼1010 M☉. The dropout galaxy candidates behind CL0024+16 are separated by 25 (∼2 kpc in the source plane), and have H-band AB magnitudes of 25.0 and 25.6. Lensing models of CL0024+16 suggest that the objects have comparable intrinsic magnitudes of AB ∼27.3, approximately one magnitude fainter than L* at z ∼ 6.5. Their similar redshifts, spectral energy distribution, and luminosities, coupled with their very close proximity on the sky, suggest that they are spatially associated, and plausibly are physically bound. Combining this sample with two previously reported, similarly magnified galaxy candidates at z ∼ 6–8, we find that complex systems with dual nuclei may be a common feature of high-redshift galaxies.

Z∼ 7-10 GALAXIES BEHIND LENSING CLUSTERS: CONTRAST WITH FIELD SEARCH RESULTS

We conduct a search for z 7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin2 of deep Hubble Space Telescope NICMOS, ACS, and WFPC2 image data. In total, over this entire area, we find only one robust z ~ 7 z-dropout candidate (previously reported around Abell 1689). Four less robust z-dropout and J-dropout candidates are also found. The nature of the four weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all four are likely to be low-redshift interlopers. By contrast, we estimate that our robust candidate A1689-zD1 has <0.2% probability of being a low-redshift interloper. We compare these numbers with what we might expect using the z ~ 7 UV luminosity function (LF) determined from field searches. We predict 2.7 z ~ 7 z-dropouts and 0.3 z ~ 9 J-dropouts over our cluster search area, in reasonable agreement with our observational results, given the small numbers. The number of z 7 candidates we find in the present search is much lower than that which has been reported in several previous studies of the prevalence of z 7 galaxies behind lensing clusters. To understand these differences, we examined z 7 candidates in other studies and conclude that only a small fraction are likely to be z 7 galaxies. Our findings support models that show that gravitational lensing from clusters is of the most value for detecting galaxies at magnitudes brighter than L* (H 27) where the LF is expected to be very steep. Use of these clusters to constrain the faint-end slope or determine the full LF is likely of less value due to the shallower effective slope measured for the LF at fainter magnitudes, as well as significant uncertainties introduced from modeling both the gravitational lensing and incompleteness.

Automated Selection and Characterization of Emission-Line Sources in Advanced Camera for Surveys Wide Field Camera Grism Data

We present complementary techniques to find emission-line targets and measure their properties in a semi-automated fashion from grism observations obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST). The first technique is to find all likely sources in a direct image, extract their spectra, and search them for emission lines. The second method is to look for emission-line sources as compact structures in an unsharp masked version of the grism image. Using these methods we identify 46 emission-line targets in the Hubble Deep Field-North using a modest (three-orbit) expenditure of HST observing time. Grism spectroscopy is a powerful tool for efficiently identifying interesting low-luminosity, moderate-redshift emission-line field galaxies. The sources found here have a median i-band (F775W) flux 1.5 mag fainter than the spectroscopic redshift catalog of Cohen et al. They have redshift z ≤ 1.42, and high equivalent widths (typically >100 A) and are usually less luminous than the characteristic luminosity at the same redshift. The chief obstacle in interpreting the results is line identification, since the majority of sources have a single emission line and the spectral resolution is low. Photometric redshifts are useful for providing a first-guess redshift. However, even at the depth of the state-of-the-art ground-based and HST data used here, photometric errors can result in uncertainties in line identifications, especially for sources with i magnitudes fainter than 24.5 ABmag. Reliable line identification for the faintest emission-line galaxies requires additional ground-based spectroscopy for confirmation. Of particular concern are the faint high-EW [O II] emitters, which could represent a strongly evolving galaxy population if the possibility that they are misidentified lower redshift interlopers can be ruled out.

Populations of candidate black holes at redshift 7 or above

AbstractI will describe recent results on constructing samples of candidate active galactic nuclei (AGN) at or beyond redshift 7, probing several orders of magnitude fainter than the top end of the quasar luminosity function at redshift 6. These advances have been made possible by the advent of deep, wide multi-waveband surveys that enable the selection of samples of sources that are detected at radio or X-ray wavelengths but completely undetected at optical wavelengths to very deep limits. A variety of multi-band selection criteria are used to identify the high-redshift candidates and eliminate lower-redshift interlopers by means of extensive spectral energy distribution modelling. The resulting constraints on the numbers of high-redshift AGN at or above redshift 7 are used to examine the evolution of the AGN luminosity function at high redshift, and help understand the properties of the first supermassive black holes in the universe.

Optical and Infrared Nondetection of thez= 10 Galaxy behind Abell 1835

Gravitational lensing by massive galaxy clusters is a powerful tool for the discovery and study of high-redshift galaxies, including those at z ≥ 6 likely responsible for cosmic reionization. Pello et al. recently used this technique to discover a candidate gravitationally magnified galaxy at z = 10 behind the massive cluster lens Abell 1835 (z = 0.25). We present new Keck and Spitzer Space Telescope observations of the z = 10 candidate (hereafter #1916, following Pello et al.'s nomenclature) together with a reanalysis of archival optical and near-infrared imaging from the Hubble Space Telescope and Very Large Telescope, respectively. Our analysis extends from the atmospheric cutoff at λobs 0.35 μm out to λobs 5 μm. The z = 10 galaxy is not detected in any of these data, including an independent reduction of Pello et al.'s discovery H- and K-band imaging. We conclude that there is no statistically reliable evidence for the existence of #1916. We also assess the implications of our results for ground-based near-infrared searches for gravitationally magnified galaxies at z 7. The broad conclusion is that such experiments remain feasible, assuming that space-based optical and mid-infrared imaging are available to break the degeneracy with low-redshift interlopers (e.g., z ~ 2-3) when fitting spectral templates to the photometric data.

Constraints on z ≈ 10 Galaxies from the Deepest Hubble Sp a ce Telescope NICMOS Fields

We use all available fields with deep NICMOS imaging to search for J110-dropouts (H160,AB 28) at z ≈ 10. Our primary data set for this search is the two J110 + H160 NICMOS fields taken in parallel with the Advanced Camera for Surveys (ACS) Hubble Ultra Deep Field (UDF). The 5 σ limiting magnitudes were ~28.6 in J110 and ~28.5 in H160 (06 apertures). Several shallower fields were also used: J110 + H160 NICMOS frames available over the Hubble Deep Field (HDF) North, the HDF-South NICMOS parallel, and the ACS UDF (with 5 σ limiting magnitudes in J110 and H160 ranging from 27.0 to 28.2). The primary selection criterion was (J110-H160)AB > 1.8. Eleven such sources were found in all search fields using this criterion. Eight of these are clearly ruled out as credible z ≈ 10 sources, either as a result of detections (>2 σ) blueward of J110 or their colors redward of the break (H160-K ~ 1.5) (redder than 98% of lower redshift dropouts). The nature of the three remaining sources could not be determined from the data. This number appears consistent with the expected contamination from low-redshift interlopers. Analysis of the stacked images for the three candidates also suggests some contamination. Regardless of their true redshifts, the actual number of z ≈ 10 sources must be three or fewer. To assess the significance of these results, two lower redshift samples (a z ~ 3.8 B-dropout and z ~ 6 i-dropout sample) were projected to z ~ 7-13 using a (1 + z)-1 size scaling (for fixed luminosity). They were added to the image frames and the selection was repeated, giving 15.6 and 4.8 J110-dropouts, respectively. This suggests that to the limit of this probe (≈0.3L), there has been evolution from z ~ 3.8 and possibly from z ~ 6. This is consistent with the strong evolution already noted at z ~ 6 and z ~ 7.5 relative to z ~ 3-4. Even assuming that three sources from this probe are at z ≈ 10, the rest-frame continuum UV (~1500 A) luminosity density at z ~ 10 (integrated down to 0.3L) is just 0.19 times that at z ~ 3.8 (or 0.19 times, including the small effect from cosmic variance). However, if none of our sources are at z ≈ 10, this ratio has a 1 σ upper limit of 0.07.

Redshift Estimation from Low‐Resolution Prism Spectral Energy Distributions with a Next Generation Space Telescope Multiobject Spectrograph

ABSTRACTWe discuss the utility of a low‐resolution prism as a component of a multiobject spectrometer for NASA’s proposed Next Generation Space Telescope (NGST). Low‐resolution prism spectroscopy permits simultaneous observation of the 0.6–5 μm wavelength regime at R≲50. Such data can take advantage of modern techniques in spectral energy distribution (SED) fitting to determine source redshifts, sometimes called “photometric redshifts.” We compare simulated prism observations with filter imaging for this purpose with NGST. Low‐resolution prism observations of galaxy SEDs provide a significant advantage over multifilter observations for any realistic observing strategy. For an ideal prism in sky background–limited observing, the prism has a signal‐to‐noise ratio advantage of the square root of the resolution over serial observations by filters with similar spatial and spectral resolution in equal integration time. For a realistic case the advantage is slightly less, and we have performed extensive simulations to quantify it. We define strict criteria for the recovery of input redshifts, such that to be considered a success, redshift residuals must be δz<0.03 + 0.1log z. The simulations suggest that in 105 s, a realistic prism will recover (by our definition of success) the redshift of ∼70% of measured objects (subject to multiobject spectrograph selection) at KAB<32, compared to less than 45% of the objects with serial filter observations. The advantage of the prism is larger in the regime of faint (KAB>30) objects at high redshift (z>4), where the prism recovers 80% of redshifts, while the filters recover barely 35% to similar accuracy. The primary discovery space of NGST will be at the faintest magnitudes and the highest redshifts. Many important objects will be too faint for follow‐up at higher spectral resolution, so prism observations are the optimal technique to study them. Prism observations also reduce the contamination of high‐redshift samples by lower redshift interlopers.

Candidate high-redshift and primeval galaxies in Hubble Deep Field South

We present the results of colour selection of candidate high redshift galaxies in Hubble Deep Field South (HDF-S) using the Lyman dropout scheme. The HDF-S data we discuss were taken in a number of different filters extending from the near--UV (F300W) to the infrared (F222M) in two different fields. This allows us to select candidates with redshifts from z~3 to z~12. We find 15 candidate z~3 objects (F300W dropouts), 1 candidate z~4 object (F450W dropout) and 16 candidate z$\sim$5 objects (F606W dropouts) in the ~ 4.7 arcmin^2 WFPC-2 field, 4 candidate z~6 (optical dropouts) and 1 candidate z~8 (F110W dropout) in the 0.84 arcmin^2 NICMOS-3 field. No F160W dropouts are found (z~12). We compare our selection technique with existing data for HDF-North and discuss alternative interpretations of the objects. We conclude that there are a number of lower redshift interlopers in the selections, including one previously identified object (Treu et al. 1998), and reject those objects most likely to be foreground contaminants. Even after this we conclude that the F606W dropout list is likely to still contain substantial foreground contamination. The lack of candidate very high redshift UV-luminous galaxies supports earlier conclusions by Lanzetta et al. (1998). We discuss the morphologies and luminosity functions of the high redshift objects, and their cosmological implications.