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Register a new userDirectly Observing the Galaxies Likely Responsible for Reionization
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We report a new analysis of the Hubble Frontier Fields clusters Abell 2744 and MACS 0416 using wavelet decomposition to remove the cluster light, enabling the detection of highly magnified (>50x) galaxies a factor of 10x fainter in luminosity than previous studies. We find 167 galaxies at z > 6, and with this sample we are able to characterize the UV luminosity function to M_UV = -12.5 at z ~ 6, -14 at z ~ 7 and -15 at z ~ 8. We find a steep faint-end slope (alpha <-2), and with our improved statistics at the faint end we reduce the fractional uncertainty on alpha to <2% at z ~ 6 - 7 and 4% at z ~ 8. We also investigate the systematic uncertainty due to the lens modelling by using every available lens model individually and comparing the results; this systematic fractional uncertainty on alpha is <4% at all redshifts. We now directly observe galaxies in the luminosity regime where simulations predict a change in the faint-end slope of the luminosity function, yet our results provide statistically very strong evidence against any turnover in the luminosity range probed, more consistent with simulations in which stars form in lower-mass halos. Thus we find strong support for the extension of the steep luminosity function to M_UV = -13 at z > 6, consistent with the number of faint galaxies needed to reionize the Universe under standard assumptions.
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The Cosmic Microwave Background (CMB) is a relict of the early universe. Its perfect 2.725K blackbody spectrum demonstrates that the universe underwent a hot, ionized early phase; its anisotropy (about 80 mu K rms) provides strong evidence for the presence of photon-matter oscillations in the primeval plasma, shaping the initial phase of the formation of structures; its polarization state (about 3 mu K rms), and in particular its rotational component (less than 0.1 mu K rms) might allow to study the inflation process in the very early universe, and the physics of extremely high energies, impossible to reach with accelerators. The CMB is observed by means of microwave and mm-wave telescopes, and its measurements drove the development of ultra-sensitive bolometric detectors, sophisticated modulators, and advanced cryogenic and space technologies. Here we focus on the new frontiers of CMB research: the precision measurements of its linear polarization state, at large and intermediate angular scales, and the measurement of the inverse-Compton effect of CMB photons crossing clusters of Galaxies. In this framework, we will describe the formidable experimental challenges faced by ground-based, near-space and space experiments, using large arrays of detectors. We will show that sensitivity and mapping speed improvement obtained with these arrays must be accompanied by a corresponding reduction of systematic effects (especially for CMB polarimeters), and by improved knowledge of foreground emission, to fully exploit the huge scientific potential of these missions.
The [OIII] 88 $mu$m fine structure emission line has been detected into the Epoch of Reionization (EoR) from star-forming galaxies at redshifts $6 < z lesssim 9$ with ALMA. These measurements provide valuable information regarding the properties of the interstellar medium (ISM) in the highest redshift galaxies discovered thus far. The [OIII] 88 $mu$m line observations leave, however, a degeneracy between the gas density and metallicity in these systems. Here we quantify the prospects for breaking this degeneracy using future ALMA observations of the [oiii] 52 $mu$m line. Among the current set of ten [OIII] 88 $mu$m emitters at $6 < z lesssim 9$, we forecast 52 $mu$m detections (at 6-$sigma$) in SXDF-NB1006-2, B14-6566, J0217-0208, and J1211-0118 within on-source observing times of 2-10 hours, provided their gas densities are larger than about $n_{mathrm{H}} gtrsim 10^2-10^3$ cm$^{-3}$. Other targets generally require much longer integration times for a 6-$sigma$ detection. Either successful detections of the 52 $mu$m line, or reliable upper limits, will lead to significantly tighter constraints on ISM parameters. The forecasted improvements are as large as $sim 3$ dex in gas density and $sim 1$ dex in metallicity for some regions of parameter space. We suggest SXDF-NB1006-2 as a promising first target for 52 $mu$m line measurements. We discuss how such measurements will help in understanding the mass metallicity relationship during the EoR.
Though half of cosmic starlight is absorbed by dust and reradiated at long wavelengths (3$mu$m-3mm), constraints on the infrared through millimeter galaxy luminosity function (the `IRLF) are poor in comparison to the rest-frame ultraviolet and optical galaxy luminosity function, particularly at z>2.5. Here we present a backward evolution model for interpreting number counts, redshift distributions, and cross-band flux density correlations in the infrared and submillimeter sky, from 70$mu$m-2mm, using a model for the IRLF out to the epoch of reionization. Mock submillimeter maps are generated by injecting sources according to the prescribed IRLF and flux densities drawn from model spectral energy distributions that mirror the distribution of SEDs observed in $0<z<5$ dusty star-forming galaxies (DSFGs). We explore two extreme hypothetical case-studies: a dust-poor early Universe model, where DSFGs contribute negligibly ($<$10%) to the integrated star-formation rate density at $z>4$, and an alternate dust-rich early Universe model, where DSFGs dominate $sim$90% of $z>4$ star-formation. We find that current submm/mm datasets do not clearly rule out either of these extreme models. We suggest that future surveys at 2mm will be crucial to measuring the IRLF beyond $zsim4$. The model framework developed in this paper serves as a unique tool for the interpretation of multiwavelength IR/submm extragalactic datasets and will enable more refined constraints on the IRLF than can be made from direct measurements of individual galaxies integrated dust emission.
Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; ${rm M}_{rm V}$ $sim -2$ or ${rm M}_{star}$ $sim 10^{2}$ at $z=0$) had ultra-violet (UV) luminosities of ${rm M}_{rm UV}$ $sim -3$ to $-6$ during reionization ($zsim6-10$). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ($alphalesssim-2$) to ${rm M}_{rm UV}$ $sim -3$, then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are $>$2 times lower than currently-adopted values; (iii) direct HST and JWST observations may detect only $sim10-50$% of the UV light from galaxies; (iv) the cosmic star formation history increases by $gtrsim4-6$ at $zgtrsim6$. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to ${rm M}_{rm UV}$ $sim -3$ during reionization.
We study the morphologies and sizes of galaxies at z>5 using high-resolution cosmological zoom-in simulations from the Feedback In Realistic Environments project. The galaxies show a variety of morphologies, from compact to clumpy to irregular. The simulated galaxies have more extended morphologies and larger sizes when measured using rest-frame optical B-band light than rest-frame UV light; sizes measured from stellar mass surface density are even larger. The UV morphologies are usually dominated by several small, bright young stellar clumps that are not always associated with significant stellar mass. The B-band light traces stellar mass better than the UV, but it can also be biased by the bright clumps. At all redshifts, galaxy size correlates with stellar mass/luminosity with large scatter. The half-light radii range from 0.01 to 0.2 arcsec (0.05-1 kpc physical) at fixed magnitude. At z>5, the size of galaxies at fixed stellar mass/luminosity evolves as (1+z)^{-m}, with m~1-2. For galaxies less massive than M_star~10^8 M_sun, the ratio of the half-mass radius to the halo virial radius is ~10% and does not evolve significantly at z=5-10; this ratio is typically 1-5% for more massive galaxies. A galaxys observed size decreases dramatically at shallower surface brightness limits. This effect may account for the extremely small sizes of z>5 galaxies measured in the Hubble Frontier Fields. We provide predictions for the cumulative light distribution as a function of surface brightness for typical galaxies at z=6.
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