arXiv subjects

M. Trenti

Publications and source records attributed to M. Trenti.

At least 55 records · Page 3Linked to original sources

arXiv2015

Bright galaxies at Hubble's redshift detection frontier: Preliminary results and design from the redshift z~9-10 BoRG pure-parallel HST survey

We present the first results and design from the redshift z~9-10 Brightest of the Reionizing Galaxies {\it Hubble Space Telescope} survey BoRG[z9-10], aimed at searching for intrinsically luminous unlensed galaxies during the first 700 Myr after the Big Bang. BoRG[z9-10] is the continuation of a multi-year pure-parallel near-IR and optical imaging campaign with the Wide Field Camera 3. The ongoing survey uses five filters, optimized for detecting the most distant objects and offering continuous wavelength coverage from λ=0.35μm to λ=1.7μm. We analyze the initial ~130 arcmin$^2$ of area over 28 independent lines of sight (~25% of the total planned) to search for z>7 galaxies using a combination of Lyman break and photometric redshift selections. From an effective comoving volume of (5-25) $times 10^5$ Mpc$^3$ for magnitudes brighter than $m_{AB}=26.5-24.0$ in the $H_{160}$-band respectively, we find five galaxy candidates at z~8.3-10 detected at high confidence (S/N>8), including a source at z~8.4 with mAB=24.5 (S/N~22), which, if confirmed, would be the brightest galaxy identified at such early times (z>8). In addition, BoRG[z9-10] data yield four galaxies with $7.3 \lesssim z \lesssim 8$. These new Lyman break galaxies with m$\lesssim26.5$ are ideal targets for follow-up observations from ground and space based observatories to help investigate the complex interplay between dark matter growth, galaxy assembly, and reionization.

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arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). III. A census of Lyα Emission at $z\gtrsim$7 from HST Spectroscopy

[abbreviated] We present a census of Lyα emission at $z\gtrsim7$ utilizing deep near infrared HST grism spectroscopy from the first six completed clusters of the Grism Lens-Amplified Survey from Space (GLASS). In 24/159 photometrically selected galaxies we detect emission lines consistent with Lyα in the GLASS spectra. Based on the distribution of signal-to-noise ratios and on simulations we expect the completeness and the purity of the sample to be 40-100% and 60-90%, respectively. For the objects without detected emission lines we show that the observed (not corrected for lensing magnification) 1$σ$ flux limits reaches $5\times10^{-18}$erg/s/cm$^{2}$ per position angle over the full wavelength range of GLASS (0.8-1.7$μ$m). Based on the conditional probability of Lyα emission measured from the ground at $z\sim7$ we would have expected 12-18 Lyα emitters. This is consistent with the number of detections, within the uncertainties, confirming the drop in Lyα emission with respect to $z\sim6$. These candidates include a promising source at $z=8.1$. The spatial extent of Lyα in a deep stack of the most convincing Lyα emitters with $\langle z\rangle=7.2$ is consistent with that of the rest-frame UV continuum. Extended Ly$α$ emission, if present, has a surface brightness below our detection limit, consistent with the properties of lower redshift comparison samples. From the stack we estimate upper limits on rest-frame UV emission line ratios and find $f_\textrm{CIV} / f_\textrm{Ly$α$} \lesssim 0.32$ and $f_\textrm{CIII]} / f_\textrm{Ly$α$} \lesssim 0.23$ in good agreement with other values published in the literature.

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arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). I. Survey overview and first data release

We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a large Hubble Space Telescope program aimed at obtaining grism spectroscopy of the fields of ten massive clusters of galaxies at redshift z=0.308-0.686, including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near infrared spectra of the cluster cores, covering the wavelength range 0.81-1.69mum through grisms G102 and G141, while the Advanced Camera for Surveys in parallel mode provides G800L spectra of the infall regions of the clusters. The WFC3 spectra are taken at two almost orthogonal position angles in order to minimize the effects of confusion. After summarizing the scientific drivers of GLASS, we describe the sample selection as well as the observing strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF cluster and the first one observed by GLASS, to illustrate the data quality and the high-level data products. Each spectrum brighter than H_AB=23 is visually inspected by at least two co-authors and a redshift is measured when sufficient information is present in the spectra. Furthermore, we conducted a thorough search for emission lines through all the GLASS WFC3 spectra with the aim of measuring redshifts for sources with continuum fainter than H_AB=23. We provide a catalog of 139 emission-line based spectroscopic redshifts for extragalactic sources, including three new redshifts of multiple image systems (one probable, two tentative). In addition to the data itself we also release software tools that are helpful to navigate the data.

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arXiv2015

A spectroscopically confirmed z=1.327 galaxy-scale deflector magnifying a z~8 Lyman-Break galaxy in the Brightest of Reionizing Galaxies survey

We present a detailed analysis of an individual case of gravitational lensing of a $z\sim8$ Lyman-Break galaxy (LBG) in a blank field, identified in Hubble Space Telescope imaging obtained as part of the Brightest of Reionizing Galaxies survey. To investigate the close proximity of the bright ($m_{AB}=25.8$) $Y_{098}$-dropout to a small group of foreground galaxies, we obtained deep spectroscopy of the dropout and two foreground galaxies using VLT/X-Shooter. We detect H-$α$, H-$β$, [OIII] and [OII] emission in the brightest two foreground galaxies (unresolved at the natural seeing of $0.8$ arcsec), placing the pair at $z=1.327$. We can rule out emission lines contributing all of the observed broadband flux in $H_{160}$ band at $70σ$, allowing us to exclude the $z\sim8$ candidate as a low redshift interloper with broadband photometry dominated by strong emission lines. The foreground galaxy pair lies at the peak of the luminosity, redshift and separation distributions for deflectors of strongly lensed $z\sim8$ objects, and we make a marginal detection of a demagnified secondary image in the deepest ($J_{125}$) filter. We show that the configuration can be accurately modelled by a singular isothermal ellipsoidal deflector and a Sérsic source magnified by a factor of $μ=4.3\pm0.2$. The reconstructed source in the best-fitting model is consistent with luminosities and morphologies of $z\sim8$ LBGs in the literature. The lens model yields a group mass of $9.62\pm0.31\times10^{11} M_{\odot}$ and a stellar mass-to-light ratio for the brightest deflector galaxy of $M_{\star}/L_{B}=2.3^{+0.8}_{-0.6} M_{\odot}/L_{\odot}$ within its effective radius. The foreground galaxies' redshifts would make this one of the few strong lensing deflectors discovered at $z>1$.

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arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). IV. Mass reconstruction of the lensing cluster Abell 2744 from frontier field imaging and GLASS spectroscopy

We present a strong and weak lensing reconstruction of the massive cluster Abell 2744, the first cluster for which deep Hubble Frontier Field (HFF) images and spectroscopy from the Grism Lens-Amplified Survey from Space (GLASS) are available. By performing a targeted search for emission lines in multiply imaged sources using the GLASS spectra, we obtain 5 high-confidence spectroscopic redshifts and 2 tentative ones. We confirm 1 strongly lensed system by detecting the same emission lines in all 3 multiple images. We also search for additional line emitters blindly and use the full GLASS spectroscopic catalog to test reliability of photometric redshifts for faint line emitters. We see a reasonable agreement between our photometric and spectroscopic redshift measurements, when including nebular emission in photometric redshift estimations. We introduce a stringent procedure to identify only secure multiple image sets based on colors, morphology, and spectroscopy. By combining 7 multiple image systems with secure spectroscopic redshifts (at 5 distinct redshift planes) with 18 multiple image systems with secure photometric redshifts, we reconstruct the gravitational potential of the cluster pixellated on an adaptive grid, using a total of 72 images. The resulting mass map is compared with a stellar mass map obtained from the deep Spitzer Frontier Fields data to study the relative distribution of stars and dark matter in the cluster. We find that the stellar to total mass ratio varies substantially across the cluster field, suggesting that stars do not trace exactly the total mass in this interacting system. The maps of convergence, shear, and magnification are made available in the standard HFF format.

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arXiv2015

Ultradeep IRAC Imaging Over The HUDF And GOODS-South: Survey Design And Imaging Data Release

The IRAC ultradeep field (IUDF) and IRAC Legacy over GOODS (IGOODS) programs are two ultradeep imaging surveys at 3.6μm and 4.5μm with the Spitzer Infrared Array Camera (IRAC). The primary aim is to directly detect the infrared light of reionization epoch galaxies at z > 7 and to constrain their stellar populations. The observations cover the Hubble Ultra Deep Field (HUDF), including the two HUDF parallel fields, and the CANDELS/GOODS-South, and are combined with archival data from all previous deep programs into one ultradeep dataset. The resulting imaging reaches unprecedented coverage in IRAC 3.6μm and 4.5μm ranging from > 50 hour over 150 arcmin^2, > 100 hour over 60 sq arcmin2, to 200 hour over 5 - 10 arcmin$^2$. This paper presents the survey description, data reduction, and public release of reduced mosaics on the same astrometric system as the CANDELS/GOODS-South WFC3 data. To facilitate prior-based WFC3+IRAC photometry, we introduce a new method to create high signal-to-noise PSFs from the IRAC data and reconstruct the complex spatial variation due to survey geometry. The PSF maps are included in the release, as are registered maps of subsets of the data to enable reliability and variability studies. Simulations show that the noise in the ultradeep IRAC images decreases approximately as the square root of integration time over the range 20 - 200 hours, well below the classical confusion limit, reaching 1σ point source sensitivities as faint as of 15 nJy (28.5 AB) at 3.6μm and 18 nJy (28.3 AB) at 4.5μm. The value of such ultradeep IRAC data is illustrated by direct detections of z = 7 - 8 galaxies as faint as HAB = 28.

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arXiv2015

First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples

We search the complete Hubble Frontier Field dataset of Abell 2744 and its parallel field for z~10 sources to further refine the evolution of the cosmic star-formation rate density (SFRD) at z>8. We independently confirm two images of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014) and set an upper limit for similar z~10 galaxies with red colors of J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive simulations to derive the effective selection volume of Lyman-break galaxies at z~10, both in the lensed cluster field and in the adjacent parallel field. Particular care is taken to include position-dependent lensing shear to accurately account for the expected sizes and morphologies of highly-magnified sources. We show that both source blending and shear reduce the completeness at a given observed magnitude in the cluster, particularly near the critical curves. These effects have a significant, but largely overlooked, impact on the detectability of high-redshift sources behind clusters, and substantially reduce the expected number of highly-magnified sources. The detections and limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex than previous estimates at z~10 from blank fields. Nevertheless, the combination of these new results with all other estimates remain consistent with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by cosmological simulations and dark-matter halo evolution in LambdaCDM. Once biases introduced by magnification-dependent completeness are accounted for, the full six cluster and parallel Frontier Field program will be an extremely powerful new dataset to probe the evolution of the galaxy population at z>8 before the advent of the JWST.

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arXiv2015

The Impact of Strong Gravitational Lensing on Observed Lyman-Break Galaxy Numbers at 4<z<8 in the GOODS and the XDF Blank Fields

Detection of Lyman-Break Galaxies (LBGs) at high-redshift can be affected by gravitational lensing induced by foreground deflectors not only in galaxy clusters, but also in blank fields. We quantify the impact of strong magnification in the samples of $B$, $V$, $i$, $z$ $\&$ $Y$ LBGs ($4\lesssim z \lesssim8$) observed in the XDF and GOODS/CANDELS fields, by investigating the proximity of dropouts to foreground objects. We find that $\sim6\%$ of bright LBGs ($m_{H_{160}}<26$) at $z\sim7$ have been strongly lensed ($μ>2$) by foreground objects. This fraction decreases from $\sim 3.5\%$ at $z\sim6$ to $\sim1.5\%$ at $z\sim4$. Since the observed fraction of strongly lensed galaxies is a function of the shape of the luminosity function (LF), it can be used to derive Schechter parameters, $α$ and $M_{\star}$, independently from galaxy number counts. Our magnification bias analysis yields Schechter-function parameters in close agreement with those determined from galaxy counts albeit with larger uncertainties. Extrapolation of our analysis to $z\gtrsim 8$ suggests that future surveys with JSWT, WFIRST and EUCLID should find excess LBGs at the bright-end, even if there is an intrinsic exponential cutoff of number counts. Finally, we highlight how the magnification bias measurement near the detection limit can be used as probe of the population of galaxies too faint to be detected. Preliminary results using this novel idea suggest that the magnification bias at $M_{UV}\sim -18$ is not as strong as expected if $α\lesssim -1.7$ extends well below the current detection limits in the XDF. At face value this implies a flattening of the LF at $M_{UV}\gtrsim-16.5$. However, selection effects and completeness estimates are difficult to quantify precisely. Thus, we do not rule out a steep LF extending to $M_{UV}\gtrsim -15$.

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arXiv2015

UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields

The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF from z~10 to z~4. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z~4, z~5, z~6, z~7, z~8, and z~10, respectively from the ~1000 arcmin**2 area probed. The selection of z~4-8 galaxies over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag baseline (-22.5 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 >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4 sigma significance for a steeper faint-end slope to the UV LF at z>4, with alpha evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7 (and alpha = -2.02+/-0.23 at z~8), consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M* over the redshift range z~4 to z~7; the observed evolution in the LF is now largely represented by changes in phi*. 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 ((1+z)**-1.5) provides a good representation of the observed evolution.

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arXiv2014

UV-Continuum Slopes of >4000 z~4-8 Galaxies from the HUDF/XDF, HUDF09, ERS, CANDELS-South, and CANDELS-North Fields

We measure the UV-continuum slope beta for over 4000 high-redshift galaxies over a wide range of redshifts z~4-8 and luminosities from the HST HUDF/XDF, HUDF09-1, HUDF09-2, ERS, CANDELS-N, and CANDELS-S data sets. Our new beta results reach very faint levels at z~4 (-15.5 mag: 0.006 L*(z=3)), z~5 (-16.5 mag: 0.014L*(z=3)), and z~6 and z~7 (-17 mag: 0.025 L*(z=3)). Inconsistencies between previous studies led us to conduct a comprehensive review of systematic errors and develop a new technique for measuring beta that is robust against biases that arise from the impact of noise. We demonstrate, by object-by-object comparisons, that all previous studies, including our own and those done on the latest HUDF12 dataset, suffer from small systematic errors in beta. We find that after correcting for the systematic errors (typically d(beta) ~0.1-0.2) all beta results at z~7 from different groups are in excellent agreement. The mean beta we measure for faint (-18 mag: 0.1L*(z=3)) z~4, z~5, z~6, and z~7 galaxies is -2.03+/-0.03+/-0.06 (random and systematic errors), -2.14+/-0.06+/-0.06, -2.24+/-0.11+/-0.08, and -2.30+/-0.18+/-0.13, respectively. Our new beta values are redder than we have reported in the past, but bluer than other recent results. Our previously reported trend of bluer beta's at lower luminosities is confirmed, as is the evolution to bluer beta's at high redshifts. beta appears to show only a mild luminosity dependence faintward of M(UV,AB) ~ -19 mag, suggesting that the mean beta asymptotes to ~ -2.2 to -2.4 for faint z>~4 galaxies. At z~7, the observed beta's suggest non-zero, but low dust extinction, and they agree well with values predicted in cosmological hydrodynamical simulations.

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arXiv2014

Measurement of Galaxy Clustering at z~7.2 and the Evolution of Galaxy Bias from 3.8<z<8 in the XDF, GOODS-S AND GOODS-N

Lyman-Break Galaxy (LBG) samples observed during reionization ($z\gtrsim6$) with Hubble Space Telescope's Wide Field Camera 3 are reaching sizes sufficient to characterize their clustering properties. Using a combined catalog from the Hubble eXtreme Deep Field and CANDELS surveys, containing $N=743$ LBG candidates at z>6.5 at a mean redshift of $z=7.2$, we detect a clear clustering signal in the angular correlation function (ACF) at $\sim4σ$, corresponding to a real-space correlation length $r_{0}=6.7^{+0.9}_{-1.0}h^{-1}$cMpc. The derived galaxy bias $b=8.6^{+0.9}_{-1.0}$ is that of dark-matter halos of $M=10^{11.1^{+0.2}_{-0.3}}$M$_{\odot}$ at $z=7.2$, and highlights that galaxies below the current detection limit ($M_{AB}\sim-17.7$) are expected in lower-mass halos ($M\sim10^{8}-10^{10.5}$M$_{\odot}$). We compute the ACF of LBGs at $z\sim3.8-z\sim5.9$ in the same surveys. A trend of increasing bias is found from $z=3.8$ ($b\sim3.0$) to $z=7.2$ ($b\sim8.6$), broadly consistent with galaxies at fixed luminosity being hosted in dark-matter halos of similar mass at $4<z<6$, followed by a slight rise in halo masses at $z\sim7$ ($\sim2σ$ confidence). Separating the data at the median luminosity of the $z=7.2$ sample ($M_{UV}=-19.4$) shows higher clustering at $z=5.9$ for bright galaxies ($r_{0}=5.5^{+1.4}_{-1.5}h^{-1}$cMpc, $b=6.2^{+1.2}_{-1.5}$) compared to faint galaxies ($r_{0}=1.9^{+1.1}_{-1.0}h^{-1}$cMpc, $b=2.7\pm1.2$) implying a constant mass-to-light ratio $\frac{dlogM}{dlogL}\sim1.2^{+1.8}_{-0.8}$. A similar trend is present in the $z=7.2$ sample with larger uncertainty. Finally, our bias measurements allow us to investigate the fraction of dark-matter halos hosting UV-bright galaxies (the duty-cycle, $ε_{DC}$). At $z=7.2$ values near unity are preferred, which may be explained by the shortened halo assembly time at high-redshift.

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arXiv2014

Through the looking GLASS: HST spectroscopy of faint galaxies lensed by the Frontier Fields cluster MACS0717.5+3745

The Grism Lens-Amplified Survey from Space (GLASS) is a Hubble Space Telescope (HST) Large Program, which will obtain 140 orbits of grism spectroscopy of the core and infall regions of 10 galaxy clusters, selected to be among the very best cosmic telescopes. Extensive HST imaging is available from many sources including the CLASH and Frontier Fields programs. We introduce the survey by analyzing spectra of faint multiply-imaged galaxies and $z\gtrsim6$ galaxy candidates obtained from the first seven orbits out of fourteen targeting the core of the Frontier Fields cluster MACS0717.5+3745. Using the G102 and G141 grisms to cover the wavelength range 0.8-1.7$μ$m, we confirm 4 strongly lensed systems by detecting emission lines in each of the images. For the 9 $z\gtrsim6$ galaxy candidates clear from contamination, we do not detect any emission lines down to a seven-orbit 1$σ$ noise level of $\sim$5$\times$10$^{-18}$erg s$^{-1}$cm$^{-2}$. Taking lensing magnification into account, our flux sensitivity reaches $\sim$0.2-5$\times$10$^{-18}$erg s$^{-1}$cm$^{-2}$. These limits over an uninterrupted wavelength range rule out the possibility that the high-$z$ galaxy candidates are instead strong line emitters at lower redshift. These results show that by means of careful modeling of the background - and with the assistance of lensing magnification - interesting flux limits can be reached for large numbers of objects, avoiding pre-selection and the wavelength restrictions inherent to ground-based multi-slit spectroscopy. These observations confirm the power of slitless HST spectroscopy even in fields as crowded as a cluster core.

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arXiv2014

The Most Luminous z~9-10 Galaxy Candidates yet Found: The Luminosity Function, Cosmic Star-Formation Rate, and the First Mass Density Estimate at 500 Myr

[abridged] We present the discovery of four surprisingly bright (H_160 ~ 26 - 27 mag AB) galaxy candidates at z~9-10 in the complete HST CANDELS WFC3/IR GOODS-N imaging data, doubling the number of z~10 galaxy candidates that are known, just ~500 Myr after the Big Bang. Two similarly bright sources are also detected in a systematic re-analysis of the GOODS-S data set. Three of the four galaxies in GOODS-N are significantly detected at 4.5-6.2sigma in the very deep Spitzer/IRAC 4.5 micron data, as is one of the GOODS-S candidates. Furthermore, the brightest of our candidates (at z=10.2+-0.4) is robustly detected also at 3.6 micron (6.9sigma), revealing a flat UV spectral energy distribution with a slope beta=-2.0+-0.2, consistent with demonstrated trends with luminosity at high redshift. The abundance of such luminous candidates suggests that the luminosity function evolves more significantly in phi_* than in L_* at z>~8 with a higher number density of bright sources than previously expected. Despite the discovery of these luminous candidates, the cosmic star formation rate density for galaxies with SFR >0.7 M_sun/yr shows an order-of-magnitude increase in only 170 Myr from z ~ 10 to z ~ 8, consistent with previous results. Based on the IRAC detections, we derive galaxy stellar masses at z~10, finding that these luminous objects are typically 10^9 M_sun. The cosmic stellar mass density at z~10 is log10 rho_* = 4.7^+0.5_-0.8 M_sun Mpc^-3 for galaxies brighter than M_UV~-18. The remarkable brightness, and hence luminosity, of these z~9-10 candidates highlights the opportunity for deep spectroscopy to determine their redshift and nature, demonstrates the value of additional search fields covering a wider area to understand star-formation in the very early universe, and highlights the opportunities for JWST to map the buildup of galaxies at redshifts much earlier than z~10.

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arXiv2013

The Spectral Energy Distributions Of z~8 Galaxies From The IRAC Ultra Deep Fields: Emission Lines, Stellar Masses, And Specific Star Formation Rates At 650 Myr

Using new ultradeep Spitzer/IRAC photometry from the IRAC Ultradeep Field program (IUDF), we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z~8, only 650Myr after the Big Bang. The sources are selected from HST/ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer/IRAC data increase the coverage at 3.6 micron and 4.5 micron to ~120h over the HUDF reaching depths of ~28 (AB,1 sigma). The improved depth and inclusion of brighter candidates result in direct >3 sigma IRAC detections of 20/63 sources, of which 11/63 are detected at > 5 sigma. The average [3.6]-[4.5] colors of IRAC detected galaxies at z~8 are markedly redder than those at z~7, observed only 130Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [OIII]4959,5007+Hbeta) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z~7 and z~8 we estimate a rest-frame equivalent width of W([OIII]4959,5007+Hbeta) = 670 (+260,-170) Angstrom contributing 0.56 (+0.16,-0.11) mag to the 4.5 micron filter at z~8. The corresponding W(Halpha) = 430 (+160,-110) Angstrom implies an average specific star formation rate of sSFR = 11 (+11,-5) Gyr^-1 and a stellar population age of 100 (+100,-50) Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by x3, decreasing the integrated stellar mass density to rho*(z=8,MUV<-18)=0.6 (+0.4,-0.3) x 10^6 Msun Mpc^-3.

astro-ph.CO↗

arXiv2013

The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever

The eXtreme Deep Field (XDF) combines data from ten years of observations with the HST Advanced Camera for Surveys (ACS) and the Wide-Field Camera 3 Infra-Red (WFC3/IR) into the deepest image of the sky ever in the optical/near-IR. Since the initial observations on the Hubble Ultra-Deep Field (HUDF) in 2003, numerous surveys and programs, including supernova followup, HUDF09, CANDELS, and HUDF12 have contributed additional imaging data across the HUDF region. Yet these have never been combined and made available as one complete ultra-deep optical and near-infrared image dataset. We do so now for the eXtreme Deep Field (XDF) program. Our new and improved processing techniques provide higher quality reductions of the total dataset. All WFC3 near-IR and optical ACS data sets have been fully combined and accurately matched, resulting in the deepest imaging ever taken at these wavelengths ranging from 29.1 to 30.3 AB mag (5sigma in a 0.35" diameter aperture) in 9 filters. The gains in the optical for the four filters done in the original ACS HUDF correspond to a typical improvement of 0.15 mag, with gains of 0.25 mag in the deepest areas. Such gains are equivalent to adding ~130 to ~240 orbits of ACS data to the HUDF. Improved processing alone results in a typical gain of ~0.1 mag. Our 5sigma (optical+near-IR) SExtractor catalogs reveal about 14140 sources in the full field and about 7121 galaxies in the deepest part of the XDF (the HUDF09 region). The XDF is the deepest image of the universe ever taken, reaching, in the combined image for a flat f_nu source, to 31.2 AB mag 5sigma (32.9 at 1sigma) in a 0.35" diameter aperture.

astro-ph.CO↗

arXiv2013

The changing Lya optical depth in the range 6<z<9 from MOSFIRE spectroscopy of Y-dropouts

We present MOSFIRE spectroscopy of 13 candidate z~8 galaxies selected as Y-dropouts as part of the BoRG pure parallel survey. We detect no significant lya emission (our median 1-sigma rest frame equivalent width sensitivity is in the range 2-16 AA). Using the Bayesian framework derived in a previous paper, we perform a rigorous analysis of a statistical subsample of non-detections for ten Y-dropouts, including data from the literature, to study the cosmic evolution of the lya emission of Lyman Break Galaxies. We find that lya emission is suppressed at z~8 by at least a factor of three with respect to z~6 continuing the downward trend found by previous studies of z-dropouts at z~7. This finding suggests a dramatic evolution in the conditions of the intergalactic or circumgalactic media in just 300 Myrs, consistent with the onset of reionization or changes in the physical conditions of the first generations of starforming regions.

astro-ph.CO↗

arXiv2013

No Energy Equipartition in Globular Clusters

It is widely believed that globular clusters evolve over many two-body relaxation times toward a state of energy equipartition, so that velocity dispersion scales with stellar mass as σ~ m^{-η} with η= 0.5. We show that this is incorrect, using direct N-body simulations with a variety of realistic IMFs and initial conditions. No simulated system ever reaches a state close to equipartition. Near the center, the luminous main-sequence stars reach a maximum η_{max} ~ 0.15 \pm 0.03. At large times, all radial bins convergence on an asymptotic value η_{\infty} ~ 0.08 \pm 0.02. The development of this "partial equipartition" is strikingly similar across our simulations, despite the range of initial conditions employed. Compact remnants tend to have higher ηthan main-sequence stars (but still η< 0.5), due to their steeper (evolved) mass function. The presence of an intermediate-mass black hole (IMBH) decreases η, consistent with our previous findings of a quenching of mass segregation under these conditions. All these results can be understood as a consequence of the Spitzer instability for two-component systems, extended by Vishniac to a continuous mass spectrum. Mass segregation (the tendency of heavier stars to sink toward the core) has often been studied observationally, but energy equipartition has not. Due to the advent of high-quality proper motion datasets from the Hubble Space Telescope, it is now possible to measure η. Detailed data-model comparisons open up a new observational window on globular cluster dynamics and evolution. Comparison of our simulations to Omega Cen observations yields good agreement, confirming that globular clusters are not generally in energy equipartition. Modeling techniques that assume equipartition by construction (e.g., multi-mass Michie-King models) are approximate at best.

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arXiv2013

Gamma Ray Burst and star formation rates: The physical origin for the redshift evolution of their ratio

Gamma Ray Bursts (GRBs) and galaxies at high redshift represent complementary probes of the star formation history of the Universe. In fact, both the GRB rate and the galaxy luminosity density are connected to the underlying star formation. Here, we combine a star formation model for the evolution of the galaxy luminosity function from z=0 to z=10 with a metallicity-dependent efficiency for GRB formation to simultaneously predict the comoving GRB rate. Our model sheds light on the physical origin of the empirical relation often assumed between GRB rate and luminosity density-derived star formation rate: Rgrb(z) = ε(z)*SFR_{obs}(z), with ε(z) (1+z)^{1.2}. At z<4, ε(z) is dominated by the effects of metallicity evolution in the GRB efficiency. Our best-fitting model only requires a moderate preference for low-metallicity, that is a GRB rate per unit stellar mass about four times higher for log(Z/Zsun)<-3 compared to log(Z/Zsun)>0. Models with total suppression of GRB formation at log(Z/Zsun)>0 are disfavored. At z>4, most of the star formation happens in low-metallicity hosts with nearly saturated efficiency of GRB production per unit stellar mass. However at the same epoch, galaxy surveys miss an increasing fraction of the predicted luminosity density because of flux limits, driving an accelerated evolution of ε(z) compared to the empirical power-law fit from lower z. Our findings are consistent with the non-detections of GRB hosts in ultradeep imaging at z>5, and point toward current galaxy surveys at z>8 only observing the top 15-20 % of the total luminosity density.

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