The Sizes of Candidate z∼9−10z\sim9-10 Galaxies: confirmation of the bright CANDELS sample and relation with luminosity and mass

Recently, a small sample of six $z\sim9-10$ candidates was discovered in CANDELS that are $\sim10-20\times$ more luminous than any of the previous $z\sim9-10$ galaxies identified over the HUDF/XDF and CLASH fields. We measure the sizes of these candidates to map out the size evolution of galaxies from the earliest observable times. Their sizes are also used to provide a valuable constraint on whether these unusual galaxy candidates are at high redshift. Using galfit to derive sizes from the CANDELS F160W images of these candidates, we find a mean size of 0.13$\pm$0.02" (or 0.5$\pm$0.1 kpc at $z\sim9-10$). This handsomely matches the 0.6 kpc size expected extrapolating lower redshift measurements to $z\sim9-10$, while being much smaller than the 0.59" mean size for lower-redshift interlopers to $z\sim9-10$ photometric selections lacking the blue IRAC color criterion. This suggests that source size may be an effective constraint on contaminants from $z\sim9-10$ selections lacking IRAC data. Assuming on the basis of the strong photometric evidence that the Oesch et al. 2014 sample is entirely at $z\sim9-10$, we can use this sample to extend current constraints on the size-luminosity, size-mass relation, and size evolution of galaxies to $z\sim10$. We find that the $z\sim9-10$ candidate galaxies have broadly similar sizes and luminosities as $z\sim6$-8 counterparts with star-formation-rate surface densities in the range of $\rm \Sigma_{SFR}=1-20\, M_\odot~ yr^{-1}\, kpc^{-2}$. The stellar mass-size relation is uncertain, but shallower than those inferred for lower-redshift galaxies. In combination with previous size measurements at z=4-7, we find a size evolution of $(1+z)^{-m}$ with $m=1.0\pm0.1$ for $>0.3L^*_{z=3}$ galaxies, consistent with the evolution previously derived from $2 < z < 8$ galaxies.Comment: 9 figures, 5 tables, accepted by Ap

Characterization and modeling of contamination for Lyman break galaxy samples at high redshift

The selection of high redshift sources from broad-band photometry using the Lyman-break galaxy (LBG) technique is a well established methodology, but the characterization of its contamination for the faintest sources is still incomplete. We use the optical and near-IR data from four (ultra)deep Hubble Space Telescope legacy fields to investigate the contamination fraction of LBG samples at z~5-8 selected using a colour-colour method. Our approach is based on characterizing the number count distribution of interloper sources, that is galaxies with colors similar to those of LBGs, but showing detection at wavelengths shorter than the spectral break. Without sufficient sensitivity at bluer wavelengths, a subset of interlopers may not be properly classified, and contaminate the LBG selection. The surface density of interlopers in the sky gets steeper with increasing redshift of LBG selections. Since the intrinsic number of dropouts decreases significantly with increasing redshift, this implies increasing contamination from misclassified interlopers with increasing redshift, primarily by intermediate redshift sources with unremarkable properties (intermediate ages, lack of ongoing star formation and low/moderate dust content). Using Monte Carlo simulations, we estimate that the CANDELS deep data have contamination induced by photometric scatter increasing from ~2% at z~5 to ~6% at z~8 for a typical dropout color >1 mag, with contamination naturally decreasing for a more stringent dropout selection. Contaminants are expected to be located preferentially near the detection limit of surveys, ranging from 0.1 to 0.4 contaminants per arcmin2 at J=30, depending on the field considered. This analysis suggests that the impact of contamination in future studies of z>10 galaxies needs to be carefully considered.Comment: 17 pages, 13 figures, ApJ in pres

May Bad Luck Be Without You: The Effect of CEO Luck on Strategic Risk-taking

We investigate how luck, namely, changes in a firm's performance beyond the CEO's control, affects strategic risk-taking. Fusing upper echelons theory with insights from psychology and behavioral strategy research, we hypothesize that there is a positive association between luck and strategic risk-taking and that this effect is stronger for bad luck than for good luck. We further argue that these effects vary depending on whether CEOs have experienced negative events earlier in their professional careers. Measuring luck as the exogenous component of recent firm performance, we show empirically that CEOs react to bad luck by adopting more conservative risk-taking policies while showing no reactions to good luck. This effect predictably varies with the strength of bad luck signals, and it is stronger for CEOs who have experienced negative events during their professional careers. We contribute to the literature by providing the first evidence on the role of luck in corporate strategic risk-taking

Quantifying the UV-continuum slopes of galaxies to z ˜ 10 using deep Hubble+Spitzer/IRAC observations

Measurements of the UV-continuum slopes β provide valuable information on the physical properties of galaxies forming in the early universe, probing the dust reddening, age, metal content, and even the escape fraction. While constraints on these slopes generally become more challenging at higher redshifts as the UV-continuum shifts out of the Hubble Space Telescope bands (particularly at z > 7), such a characterization actually becomes abruptly easier for galaxies in the redshift window z = 9.5-10.5 due to the Spitzer/Infrared Array Camera 3.6 μm-band probing the rest-UV continuum and the long wavelength baseline between this Spitzer band and the Hubble Hf160w band. Higher S/N constraints on β are possible at z ˜ 10 than at z = 8. Here, we take advantage of this opportunity and five recently discovered bright z = 9.5-10.5 galaxies to present the first measurements of the mean β for a multi-object sample of galaxy candidates at z ˜ 10. We find the measured βobs's of these candidates are -2.1 ± 0.3 ± 0.2 (random and systematic), only slightly bluer than the measured β's (βobs ≈ -1.7) at 3.5 < z < 7.5 for galaxies of similar luminosities. Small increases in the stellar ages, metallicities, and dust content of the galaxy population from z ˜ 10 to z ˜ 7 could easily explain the apparent evolution in β

Spectroscopic Constraints on UV Metal Line Emission at z~6-9: The Nature of Lyman-alpha Emitting Galaxies in the Reionization-Era

Recent studies have revealed intense UV metal emission lines in a modest sample of z>7 Lyman-alpha emitters, indicating a hard ionizing spectrum is present. If such high ionization features are shown to be common, it may indicate that extreme radiation fields play a role in regulating the visibility of Lyman-alpha in the reionization era. Here we present deep near-infrared spectra of seven galaxies with Lyman-alpha emission at 5.4<z<8.7 (including a newly-confirmed lensed galaxy at z=6.031) and three bright z>7 photometric targets. In nine sources we do not detect UV metal lines. However in the z=8.683 galaxy EGSY8p7, we detect a 4.6 sigma emission line in the narrow spectral window expected for NV 1243. The feature is unresolved (FWHM<90 km/s) and is likely nebular in origin. A deep H-band spectrum of EGSY8p7 reveals non-detections of CIV, He II, and OIII]. The presence of NV requires a substantial flux of photons above 77 eV, pointing to a hard ionizing spectrum powered by an AGN or fast radiative shocks. Regardless of its origin, the intense radiation field of EGSY8p7 may aid the transmission of Lyman-alpha through what is likely a partially neutral IGM. With this new detection, five of thirteen known Lyman-alpha emitters at z>7 have now been shown to have intense UV line emission, suggesting that extreme radiation fields are commonplace among the Lyman-alpha population. Future observations with JWST will eventually clarify the origin of these features and explain their role in the visibility of Lyman-alpha in the reionization era.Comment: 15 pages, 3 tables and 7 figures. Accepted for publication in MNRA

Slow Evolution of the Specific Star Formation Rate at z>2: The Impact of Dust, Emission Lines, and A Rising Star Formation History

We measure the evolution of the specific star formation rate (sSFR = SFR / Mstellar) between redshift 4 and 6 to investigate the previous reports of "constant" sSFR at z>2. We obtain photometry on a large sample of galaxies at z~4-6 located in the GOODS-S field that have high quality imaging from HST and Spitzer. We have derived stellar masses and star formation rates (SFRs) through stellar population modeling of their spectral energy distributions (SEDs). We estimate the dust extinction from the observed UV colors. In the SED fitting process we have studied the effects of assuming a star formation history (SFH) both with constant SFR and one where the SFR rises exponentially with time. The latter SFH is chosen to match the observed evolution of the UV luminosity function. We find that neither the mean SFRs nor the mean stellar masses change significantly when the rising SFR (RSF) model is assumed instead of the constant SFR model. When focusing on galaxies with Mstar ~ 5x10^9 Msun, we find that the sSFR evolves weakly with redshift (sSFR(z) \propto (1+z)^(0.6+/-0.1) Gyr^-1), consistent with previous results and with recent estimates of the sSFR at z~2-3 using similar assumptions. We have also investigated the impact of optical emission lines on our results. We estimate that the contribution of emission lines to the rest-frame optical fluxes is only modest at z~4 and 5 but it could reach ~50% at z~6. When emission lines of this strength are taken into account, the sSFR shows somewhat higher values at high redshifts, according to the relation sSFR(z) \propto (1+z)^(1.0+/-0.1) Gyr^-1, i.e., ~2.3x higher at z~6 than at z~2. However, the observed evolution is substantially weaker than that found at z<2 or that expected from current models (which corresponds to sSFR(z) \propto (1+z)^(2.5) Gyr^-1). -abridged-Comment: 15 pages, 10 figures, 2 tables. Update from v1: after second referee report, improved (larger) sample at z~

Discovery of a dark, massive, ALMA-only galaxy at z~5-6 in a tiny 3-millimeter survey

We report the serendipitous detection of two 3 mm continuum sources found in deep ALMA Band 3 observations to study intermediate redshift galaxies in the COSMOS field. One is near a foreground galaxy at 1.3", but is a previously unknown dust-obscured star-forming galaxy (DSFG) at probable $z_{CO}=3.329$, illustrating the risk of misidentifying shorter wavelength counterparts. The optical-to-mm spectral energy distribution (SED) favors a grey $\lambda^{-0.4}$ attenuation curve and results in significantly larger stellar mass and SFR compared to a Calzetti starburst law, suggesting caution when relating progenitors and descendants based on these quantities. The other source is missing from all previous optical/near-infrared/sub-mm/radio catalogs ("ALMA-only"), and remains undetected even in stacked ultradeep optical ($>29.6$ AB) and near-infrared ($>27.9$ AB) images. Using the ALMA position as a prior reveals faint $SNR\sim3$ measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850$\mu$m, and VLA 3GHz, indicating the source is real. The SED is robustly reproduced by a massive $M^*=10^{10.8}$M$_\odot$ and $M_{gas}=10^{11}$M$_\odot$, highly obscured $A_V\sim4$, star forming $SFR\sim300$ M$_{\odot}$yr$^{-1}$ galaxy at redshift $z=5.5\pm$1.1. The ultrasmall 8 arcmin$^{2}$ survey area implies a large yet uncertain contribution to the cosmic star formation rate density CSFRD(z=5) $\sim0.9\times10^{-2}$ M$_{\odot}$ yr$^{-1}$ Mpc$^{-3}$, comparable to all ultraviolet-selected galaxies combined. These results indicate the existence of a prominent population of DSFGs at $z>4$, below the typical detection limit of bright galaxies found in single-dish sub-mm surveys, but with larger space densities $\sim3 \times 10^{-5}$ Mpc$^{-3}$, higher duty cycles $50-100\%$, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function.Comment: Accepted for publication in ApJ. 2 galaxies, too many pages, 8 figures, 2 table