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Register a new userTwo sub-millimetre bright protoclusters bounding the epoch of peak star-formation activity
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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 $zgtrsim2$.
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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.
We use ALMA observations of four sub-millimetre galaxies (SMGs) at $zsim2-3$ to investigate the spatially resolved properties of the inter-stellar medium (ISM) at scales of 1--5 kpc (0.1--0.6$$). The velocity fields of our sources, traced by the $^{12}$CO($J$=3-2) emission, are consistent with disk rotation to first order, implying average dynamical masses of $sim$3$times10^{11}$M$_{odot}$ within two half-light radii. Through a Bayesian approach we investigate the uncertainties inherent to dynamically constraining total gas masses. We explore the covariance between the stellar mass-to-light ratio and CO-to-H$_{2}$ conversion factor, $alpha_{rm CO}$, finding values of $alpha_{rm CO}=1.1^{+0.8}_{-0.7}$ for dark matter fractions of 15 %. We show that the resolved spatial distribution of the gas and dust continuum can be uncorrelated to the stellar emission, challenging energy balance assumptions in global SED fitting. Through a stacking analysis of the resolved radial profiles of the CO(3-2), stellar and dust continuum emission in SMG samples, we find that the cool molecular gas emission in these sources (radii $sim$5--14 kpc) is clearly more extended than the rest-frame $sim$250 $mu$m dust continuum by a factor $>2$. We propose that assuming a constant dust-to-gas ratio, this apparent difference in sizes can be explained by temperature and optical-depth gradients alone. Our results suggest that caution must be exercised when extrapolating morphological properties of dust continuum observations to conclusions about the molecular gas phase of the ISM.
As progenitors of the most massive objects, protoclusters are key to tracing the evolution and star-formation history of the Universe, and are responsible for ${gtrsim},20$ per cent of the cosmic star formation at $z,{>},2$. Using a combination of state-of-the-art hydrodynamical simulations and empirical models, we show that current galaxy-formation models do not produce enough star formation in protoclusters to match observations. We find that the star-formation rates (SFRs) predicted from the models are an order of magnitude lower than what is seen in observations, despite the relatively good agreement found for their mass-accretion histories, specifically that they lie on an evolutionary path to become Coma-like clusters at $z,{simeq}, 0$. Using a well-studied protocluster core at $z,{=},4.3$ as a test case, we find that star-formation efficiency of protocluster galaxies is higher than predicted by the models. We show that a large part of the discrepancy can be attributed to a dependence of SFR on the numerical resolution of the simulations, with a roughly factor of 3 drop in SFR when the spatial resolution decreases by a factor of 4. We also present predictions up to $z,{simeq},7$. Compared to lower redshifts, we find that centrals (the most massive member galaxies) are more distinct from the other galaxies, while protocluster galaxies are less distinct from field galaxies. All these results suggest that, as a rare and extreme population at high-$z$, protoclusters can help constrain galaxy formation models tuned to match the average population at $z,{simeq},0$.
We perform a stacking analysis of {it Planck}, {it AKARI}, Infrared Astronomical Satellite ($IRAS$), Wide-field Infrared Survey Eplorer ($WISE$), and {it Herschel} images of the largest number of (candidate) protoclusters at $zsim3.8$ selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Stacking the images of the $179$ candidate protoclusters, the combined infrared (IR) emission of the protocluster galaxies in the observed $12-850~mu$m wavelength range is successfully detected with $>5sigma$ significance (at $Planck$). This is the first time that the average IR spectral energy distribution (SED) of a protocluster has been constrained at $zsim4$. The observed IR SEDs of the protoclusters exhibit significant excess emission in the mid-IR compared to that expected from typical star-forming galaxies (SFGs). They are reproduced well using SED models of intense starburst galaxies with warm/hot dust heated by young stars, or by a population of active galactic nuclei (AGN)/SFG composites. For the pure star-forming model, a total IR (from 8 to 1000 $mu$m) luminosity of $19.3_{-4.2}^{+0.6}times10^{13}~L_{odot}$ and a star formation rate (SFR) of $16.3_{-7.8}^{+1.0}times10^3~M_{odot}$ yr$^{-1}$ are found whereas for the AGN/SFG composite model, $5.1_{-2.5}^{+2.5}times10^{13}~L_{odot}$ and $2.1^{+6.3}_{-1.7}times10^3~M_{odot}$ yr$^{-1}$ are found. Uncertainty remaining in the total SFRs; however, the IR luminosities of the most massive protoclusters are likely to continue increasing up to $zsim4$. Meanwhile, no significant IR flux excess is observed around optically selected QSOs at similar redshifts, which confirms previous results. Our results suggest that the $zsim4$ protoclusters trace dense, intensely star-forming environments that may also host obscured AGNs missed by the selection in the optical.
[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.
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