Hubble Space Telescope Ultra Deep Research Articles

Euclid preparation

In modern weak-lensing surveys, the common approach to correct for residual systematic biases in the shear is to calibrate shape measurement algorithms using simulations. These simulations must fully capture the complexity of the observations to avoid introducing any additional bias. In this paper we study the importance of faint galaxies below the observational detection limit of a survey. We simulate simplified Euclid VIS images including and excluding this faint population, and measure the shift in the multiplicative shear bias between the two sets of simulations. We measure the shear with three different algorithms: a moment-based approach, model fitting, and machine learning. We find that for all methods, a spatially uniform random distribution of faint galaxies introduces a shear multiplicative bias of the order of a few times 10−3. This value increases to the order of 10−2 when including the clustering of the faint galaxies, as measured in the Hubble Space Telescope Ultra-Deep Field. The magnification of the faint background galaxies due to the brighter galaxies along the line of sight is found to have a negligible impact on the multiplicative bias. We conclude that the undetected galaxies must be included in the calibration simulations with proper clustering properties down to magnitude 28 in order to reach a residual uncertainty on the multiplicative shear bias calibration of a few times 10−4, in line with the 2 × 10−3 total accuracy budget required by the scientific objectives of the Euclid survey. We propose two complementary methods for including faint galaxy clustering in the calibration simulations.

Flexion measurement in simulations of Hubble Space Telescope data

We present a simulation analysis of weak gravitational lensing flexion and shear measurement using shapelet decomposition, and identify differences between flexion and shear measurement noise in deep survey data. Taking models of galaxies from the Hubble Space Telescope Ultra Deep Field (HUDF) and applying a correction for the HUDF point spread function we generate lensed simulations of deep, optical imaging data from Hubble's Advanced Camera for Surveys (ACS), with realistic galaxy morphologies. We find that flexion and shear estimates differ in our measurement pipeline: whereas intrinsic galaxy shape is typically the dominant contribution to noise in shear estimates, pixel noise due to finite photon counts and detector read noise is a major contributor to uncertainty in flexion estimates, across a broad range of galaxy signal-to-noise. This pixel noise also increases more rapidly as galaxy signal-to-noise decreases than is found for shear estimates. We provide simple power law fitting functions for this behaviour, for both flexion and shear, allowing the effect to be properly accounted for in future forecasts for flexion measurement. Using the simulations we also quantify the systematic biases of our shapelet flexion and shear measurement pipeline for deep Hubble data sets such as Galaxy Evolution from Morphology and SEDs, Space Telescope A901/902 Galaxy Evolution Survey or the Cosmic Evolution Survey. Flexion measurement biases are found to be significant but consistent with previous studies.

ESTIMATING LUMINOSITY FUNCTION CONSTRAINTS FROM HIGH-REDSHIFT GALAXY SURVEYS

The installation of the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) will revolutionize the study of high-redshift galaxy populations. Initial observations of the HST Ultra Deep Field (UDF) have yielded multiple z>~7 dropout candidates. Supplemented by the Great Observatory Origins Deep Survey (GOODS) Early Release Science (ERS) and further UDF pointings, these data will provide crucial information about the most distant known galaxies. However, achieving tight constraints on the z~7 galaxy luminosity function (LF) will require even more ambitious photometric surveys. Using a Fisher matrix approach to fully account for Poisson and cosmic sample variance, as well as covariances in the data, we estimate the uncertainties on LF parameters achieved by surveys of a given area and depth. Applying this method to WFC3 z~7 dropout galaxy samples, we forecast the LF parameter uncertainties for a variety of model surveys. We demonstrate that performing a wide area (~1 deg^2) survey to H_AB~27 depth or increasing the UDF depth to H_AB~30 provides excellent constraints on the high-z LF when combined with the existing UDF GO and GOODS ERS data. We also show that the shape of the matter power spectrum may limit the possible gain of splitting wide area (>~0.5 deg^2) high-redshift surveys into multiple fields to probe statistically independent regions; the increased root-mean-squared density fluctuations in smaller volumes mostly offset the improved variance gained from independent samples.

Resolved Galaxies in the Hubble Ultra Deep Field: Star Formation in Disks at High Redshift

The photometric redshift distributions, spectral types, Sersic indices, and sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are studied in order to understand the environment and nature of star formation in the early Universe. Clumpy disk galaxies that are bright at short wavelengths (restframe <5000 Angstroms) dominate the UDF out to z~5.5. Their uniformity in V/V_max and co-moving volume density suggest they go even further, spanning a total time more than an order of magnitude larger than their instantaneous star formation times. They precede as well as accompany the formation epoch of distant red galaxies and extreme red objects. Those preceding could be the pre-merger objects that combined to make red spheroidal types at z~2 to 3. Clumpy disks that do not undergo mergers are likely to evolve into spirals. The morphology of clumpy disks, the size and separation of the clumps, and the prevalence of this type of structure in the early Universe suggests that most star formation occurs by self-gravitational collapse of disk gas.

Observations of Thick Disks in theHubble Space TelescopeUltra Deep Field

The vertical profiles of chain and spiral galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are fit to sech^2(z/z_0) functions convolved with stellar profiles in order to measure the disk scale heights z_0 in four passbands. The bulge regions of the spirals are avoided. Photometric redshifts give absolute scales. The rms heights of the giant clumps in these galaxies are also measured. The results indicate that UDF disks are thick with an average z_0 of 1.0\pm0.4 kpc. The ratio of radial exponential scale length to z_0 is \~3\pm1.5. The scale heights are only 20% larger than the radii of the giant star-forming clumps and a factor of ~10 larger than the rms clump deviations around the midplanes. This suggests the clumps formed from midplane gas and dissolved to make the thick disks. Redshifted stellar population models suggest ages of ~1 Gy and mass column densities from 4 to 40 Msun pc^{-2}. The UDF disks look like young versions of modern thick disks. This resemblance is difficult to understand if galaxies grow over time or if subsequent accretion of thin disks gravitationally shrinks the observed thick disks. More likely, high redshift disks are thick because their mass column densities are low; a velocity dispersion of only 14 km/s reproduces the observed thickness. Modern thick disks require more heating at high redshift. This is possible if the gas that eventually makes the thin disk is in place before the youngest age of a modern thick disk, and if the existing stars are heated during the delivery of this gas.

On the Origin of Exponential Disks at High Redshift

The major axis and ellipse-fit intensity profiles of spiral galaxies larger than 0.3'' in the Hubble Space Telescope Ultra Deep Field (UDF) are generally exponential, whereas the major axis profiles in irregular disk galaxies, called clump-clusters in our previous studies, are clearly not. Here we show that the deprojected positions of star-forming clumps in both galaxy types are exponential, as are the deprojected luminosity profiles of the total emissions from these clumps. These exponentials are the same for both types when normalized to the outer isophotal radii. The results imply that clumps form or accrete in exponential radial distributions, and when they disperse they form smooth exponential disks. The exponential scale lengths for UDF spirals average 1.5 kpc for a standard cosmology. This length is smaller than the average for local spirals by a factor of 2. Selection effects that may account for this size difference among spirals are discussed. Regardless of these effects, the mere existence of small UDF galaxies with grand-design spiral arms differs significantly from the situation in local fields, where equally small disks are usually dwarf Irregulars that rarely have spiral arms. Spiral arms require a disk mass comparable to the halo mass in the visible region -- something local spirals have but local dwarfs Irregulars do not. Our UDF result then implies that galaxy disks grow from the inside out, starting with a dense halo and dense disk that can form spiral arms, and then adding lower density halo and disk material over time. Bars that form early in such small, dense, gas-rich disks should disperse more quickly than bars that form later in fully developed disks.

Candidates of z ≃ 5.5-7 Galaxies in the Hubble Space Telescope Ultra Deep Field

We report results from our z 5.5-7 galaxy search in the Hubble Space Telescope Ultra Deep Field (UDF). Using the 400 orbit of Advanced Camera for Surveys data, we found 108 plausible 5.5 ≤ z ≤ 6.5 (or z 6 for short) candidates to mAB(z850) = 30.0 mag. The contamination to the sample, due to either image artifacts or known types of astronomical objects, is likely negligible. The inferred surface densities of z 6 galaxies are consistent with our earlier predictions from mAB(z850) = 26.5 to 28.5 mag. After correcting for detection incompleteness, the counts of z 6 candidates to mAB(z850) = 29.2 mag suggest that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between α = -1.8 and -1.9, which is sufficient to account for the entire Lyman photon budget necessary to complete the reionization of the universe at z 6. We also searched for z 6.5-7 candidates using the UDF Near-Infrared Camera and Multi-Object Spectrometer data and have found four candidates to a magnitude of J110 = 27.2. However, the infrared colors of three candidates cannot be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at z 7 of 0.36 arcmin-2 to a magnitude of J110 = 26.6, which suggests a noticeable drop in the LF amplitude from z 6 to z 7.

Advanced Camera for Surveys Observations of a Strongly Lensed Arc in a Field Elliptical Galaxy

We report the discovery of a strongly lensed arc system around a field elliptical galaxy in Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images of a parallel field observed during NICMOS observations of the HST Ultra Deep Field. The ACS parallel data comprise deep imaging in the F435W, F606W, F775W, and F850LP bandpasses. The main arc is at a radius of 16 from the galaxy center and subtends about 120°. Spectroscopic follow-up at Magellan Observatory yields a redshift z = 0.6174 for the lensing galaxy, and we photometrically estimate zphot = 2.4 ± 0.3 for the arc. We also identify a likely counterarc at a radius of 06 that shows structure similar to that seen in the main arc. We model this system and find a good fit to an elliptical isothermal potential of velocity dispersion σ ≈ 300 km s-1, the value expected from the fundamental plane, and some external shear. Several other galaxies in the field have colors similar to the lensing galaxy and likely make up a small group.

The Major Sources of the Cosmic Reionizing Background at z 6

In this Letter, we address which sources contributed most of the reionizing photons. Our argument assumes that the reionization ended around z 6 and that it was a relatively quick process, i.e., that there was a nonnegligible fraction of neutral hydrogen in the universe at somewhat earlier epochs. Starting from our earlier estimate of the luminosity function (LF) of galaxies at z 6, we quantitatively show that the major sources of reionization are most likely galaxies with L < L*. Our approach allows us to put stronger constraints to the LF of galaxies at z 6. To have the universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than α = -1.6, which is the value measured at lower redshifts (z 3), unless either the normalization (Φ*) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Φ* is actually lower than what we assumed by a factor of 2, a steep slope close to α = -2.0 is required. Our LF predicts a total of 50-80 z 6 galaxies in the Hubble Space Telescope Ultra Deep Field to a depth of AB = 28.4 mag, which can be used to constraint both Φ* and α. We conclude that the least luminous galaxies existing at this redshift should reach as low as some critical luminosity in order to accumulate the entire reionizing photon budget. On the other hand, the existence of significant amounts of neutral hydrogen at slightly earlier epochs, e.g., z 7, requires that the least luminous galaxies should not be fainter than another critical value (i.e., the LF should cut off at this point).