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Register a new userUV Diagnostics of Galaxies from the Peak of Star-Formation to the Epoch of Reionization
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Casey Papovich
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The rest-frame UV emission from massive stars contains a wealth of information about the physical nature and conditions of star formation in galaxies. Using studies of the rest-frame UV, the past decade has witnessed the beginning of knowledge about the existence and properties of galaxies during the first few billion years after the Big Bang. This period of history corresponds to the formation of the first stars, the rapid formation of galaxy stellar populations, the reionization of the IGM, the production and dissemination of heavy elements, and the formation of the first black holes. Massive stars in these galaxies drive all of these events, and their light dominates the spectral energy distributions of galaxies. As we look to the 2020s, fundamental questions remain about the nature of these stellar populations and their evolution, from just before the peak of the cosmic star formation density (z~3), up to the epoch of reionization (z > 6). This next decade will provide transformative gains both in our ability to identify star-forming galaxies and accreting supermassive black holes at these early epochs with imaging surveys in the rest-frame UV (e.g., LSST, WFIRST). Ground-based, rest-frame UV spectroscopy on >20 m-class telescopes (e.g., GMT/TMT) offers the ability to investigate the astrophysical conditions in galaxies at the earliest cosmic times. This includes studies of the evolution in galaxy stellar populations, gas ionization (temperature, pressure), metallicity, and interstellar (and circumgalactic) gas kinematics and covering fractions. In this white paper, we describe the scientific prospects and the requirements for research in this area.
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[Abridged] We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at 1<z<3. We use our deep near ultraviolet imaging from WFC3/UVIS on the Hubble Space Telescope (HST) and existing ACS/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACSJ0717 from the Hubble Frontier Field survey and Abell 1689. We use photometric redshifts to identify 780 ultra-faint galaxies with $M_{UV}$<-12.5 AB mag at 1<z<3. From these samples, we identified 5 new, faint, multiply imaged systems in A1689. We compute the rest-frame UV LF and find the best-fit faint-end slopes of $alpha=-1.56pm0.04$, $alpha=-1.72pm0.04$ and $alpha=-1.94pm0.06$ at 1.0<z<1.6, 1.6<z<2.2 and 2.2<z<3.0, respectively. Our results demonstrate that the UV LF becomes steeper from zsim1.3 to zsim2.6 with no sign of a turnover down to $M_{UV}=-14$ AB mag. We further derive the UV LFs using the Lyman break dropout selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large, and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example), likely dominate. If we restrict our analysis to galaxies and volumes above > 50% completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values ($alpha=-1.55pm0.06$, $-1.69pm0.07$ and $-1.79pm0.08$ for $zsim1.3$, 1.9 and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of $-18.5<M_{UV}<-12.5$ covered in this study, produce the majority (55%-60%) of the unobscured UV luminosity density at 1<z<3.
We present a model for the evolution of the galaxy ultraviolet (UV) luminosity function (LF) across cosmic time where star formation is linked to the assembly of dark matter halos under the assumption of a mass dependent, but redshift independent, efficiency. We introduce a new self-consistent treatment of the halo star formation history, which allows us to make predictions at $z>10$ (lookback time $lesssim500$ Myr), when growth is rapid. With a calibration at a single redshift to set the stellar-to-halo mass ratio, and no further degrees of freedom, our model captures the evolution of the UV LF over all available observations ($0lesssim zlesssim10$). The significant drop in luminosity density of currently detectable galaxies beyond $zsim8$ is explained by a shift of star formation toward less massive, fainter galaxies. Assuming that star formation proceeds down to atomic cooling halos, we derive a reionization optical depth $tau = 0.056^{+0.007}_{-0.010}$, fully consistent with the latest Planck measurement, implying that the universe is fully reionized at $z=7.84^{+0.65}_{-0.98}$. In addition, our model naturally produces smoothly rising star formation histories for galaxies with $Llesssim L_*$ in agreement with observations and hydrodynamical simulations. Before the epoch of reionization at $z>10$ we predict the LF to remain well-described by a Schechter function, but with an increasingly steep faint-end slope ($alphasim-3.5$ at $zsim16$). Finally, we construct forecasts for surveys with JWST~and WFIRST and predict that galaxies out to $zsim14$ will be observed. Galaxies at $z>15$ will likely be accessible to JWST and WFIRST only through the assistance of strong lensing magnification.
We use the HiZELS narrow-band H-alpha survey in combination with CANDELS, UKIDSS and WIRDS near-infrared imaging, to investigate the morphologies, merger rates and sizes of a sample of H-alpha emitting galaxies in the redshift range z=0.40 - 2.23, an epoch encompassing the rise to the peak of the star formation rate density. Merger rates are estimated from space- and ground-based imaging using the M20 coefficient. To account for the increase in the specific star-formation rate (sSFR) of the star forming `main-sequence with redshift, we normalise the star-formation rate of galaxies at each epoch to the typical value derived from the H-alpha luminosity function. Once this trend in sSFR is removed we see no evidence for an increase in the number density of star-forming galaxies or the merger rate with redshift. We thus conclude that neither is the main driver of the enhanced star-formation rate density at z=1-2, with secular processes such as instabilities within efficiently fuelled, gas-rich discs or multiple minor mergers the most likely alternatives. However, we find that 40-50% of starburst galaxies, those with enhanced specific star formation at their epoch, are major mergers and this fraction is redshift independent. Finally, we find the surprising result that the typical size of a star-forming galaxy of a given mass does not evolve across the redshift range considered, suggesting a universal size-mass relation. Taken in combination, these results indicate a star-forming galaxy population that is statistically similar in physical size, merger rate and mass over the 6 Gyr covered in this study, despite the increase in typical sSFR.
We present James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2 (SCUBA-2) 850 & 450 $mu$m observations ($sigma_{850}sim0.5$ mJy, $sigma_{450}sim5$ mJy) of the HS1549+19 and HS1700+64 survey fields containing two of the largest known galaxy over-densities at $z=2.85$ and $2.30$, respectively. We detect 56 sub-millimetre galaxies (SMGs) with SNR $> 4$ over $sim 50$ arcmin$^2$ at 850 $mu$m with flux densities of 3 - 17 mJy. The number counts indicate over-densities in the $3$-arcmin diameter core region ($sim 1.5$ Mpc at $z=2.5$) of $6^{+4}_{-2}times$ (HS1549) and $4^{+6}_{-2}times$ (HS1700) compared to blank field surveys. Within these core regions, we spectroscopically confirm that approximately one third of the SMGs lie at the protocluster redshifts for both HS1549 and HS1700. We use statistical identifications of other SMGs in the wider fields to constrain an additional four candidate protocluster members in each system. We combine multi wavelength estimates of the star-formation rates (SFRs) from Lyman-break dropout- and narrowband-selected galaxies, and the SCUBA-2 SMGs, to estimate total SFRs of 12,$500pm2800$ M$_odot$ yr$^{-1}$ ($4900pm1200$ M$_odot$ yr$^{-1}$) in HS1549 (HS1700), and SFR densities (SFRDs) within the central 1.5-Mpc diameter of each protocluster to be $3000pm900$ M$_odot$ yr$^{-1}$ Mpc$^{-3}$ ($1300pm400$ M$_odot$ yr$^{-1}$ Mpc$^{-3}$) in the HS1549 (HS1700) protocluster, $sim10^4times$ larger than the global SFRDs found at their respective epochs, due to the concentration of star-forming galaxies in the small volume of the dense cluster cores. Our results suggest centrally concentrated starbursts within protoclusters may be a relatively common scenario for the build up of mass in rich clusters assembling at $zgtrsim2$.
Recent observations of galaxies at $z gtrsim 7$, along with the low value of the electron scattering optical depth measured by the Planck mission, make galaxies plausible as dominant sources of ionizing photons during the epoch of reionization. However, scenarios of galaxy-driven reionization hinge on the assumption that the average escape fraction of ionizing photons is significantly higher for galaxies in the reionization epoch than in the local Universe. The NIRSpec instrument on the James Webb Space Telescope (JWST) will enable spectroscopic observations of large samples of reionization-epoch galaxies. While the leakage of ionizing photons will not be directly measurable from these spectra, the leakage is predicted to have an indirect effect on the spectral slope and the strength of nebular emission lines in the rest-frame ultraviolet and optical. Here, we apply a machine learning technique known as lasso regression on mock JWST/NIRSpec observations of simulated $z=7$ galaxies in order to obtain a model that can predict the escape fraction from JWST/NIRSpec data. Barring systematic biases in the simulated spectra, our method is able to retrieve the escape fraction with a mean absolute error of $Delta f_{mathrm{esc}} approx 0.12$ for spectra with $S/Napprox 5$ at a rest-frame wavelength of 1500 {AA} for our fiducial simulation. This prediction accuracy represents a significant improvement over previous similar approaches.
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