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Register a new userThe realm of the galaxy protoclusters
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R. A. Overzier
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The study of galaxy protoclusters is beginning to fill in unknown details of the important phase of the assembly of clusters and cluster galaxies. This review describes the current status of this field and highlights promising recent findings related to galaxy formation in the densest regions of the early universe. We discuss the main search techniques and the characteristic properties of protoclusters in observations and simulations, and show that protoclusters will have present-day masses similar to galaxy clusters when fully collapsed. We discuss the physical properties of galaxies in protoclusters, including (proto-)brightest cluster galaxies, and the forming red sequence. We highlight the fact that the most massive halos at high redshift are found in protoclusters, making these objects uniquely suited for testing important recent models of galaxy formation. We show that galaxies in protoclusters should be among the first galaxies at high redshift making the transition from a gas cooling regime dominated by cold streams to a regime dominated by hot intracluster gas, which could be tested observationally. We also discuss the possible connections between protoclusters and radio galaxies, quasars, and Ly-alpha blobs. Because of their early formation, large spatial sizes and high total star formation rates, protoclusters have also likely played a crucial role during the epoch of reionization, which can be tested with future experiments that will map the neutral and ionized cosmic web. Last, we review a number of promising observational projects that are expected to make significant impact in this growing, exciting field.
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Present-day clusters are massive halos containing mostly quiescent galaxies, while distant protoclusters are extended structures containing numerous star-forming galaxies. We investigate the implications of this fundamental change in a cosmological context using a set of N-body simulations and semi-analytic models. We find that the fraction of the cosmic volume occupied by all (proto)clusters increases by nearly three orders of magnitude from z=0 to z=7. We show that (proto)cluster galaxies are an important, and even dominant population at high redshift, as their expected contribution to the cosmic star-formation rate density rises (from 1% at z=0) to 20% at z=2 and 50% at z=10. Protoclusters thus provide a significant fraction of the cosmic ionizing photons, and may have been crucial in driving the timing and topology of cosmic reionization. Internally, the average history of cluster formation can be described by three distinct phases: at z~10-5, galaxy growth in protoclusters proceeded in an inside-out manner, with centrally dominant halos that are among the most active regions in the Universe; at z~5-1.5, rapid star formation occurred within the entire 10-20 Mpc structures, forming most of their present-day stellar mass; at z<~1.5, violent gravitational collapse drove these stellar contents into single cluster halos, largely erasing the details of cluster galaxy formation due to relaxation and virialization. Our results motivate observations of distant protoclusters in order to understand the rapid, extended stellar growth during Cosmic Noon, and their connection to reionization during Cosmic Dawn.
We use the GALFORM semi-analytical model to study high density regions traced by radio galaxies and quasars at high redshifts. We explore the impact that baryonic physics has upon the properties of galaxies in these environments. Star-forming emission-line galaxies (Ly{alpha} and H{alpha} emitters) are used to probe the environments at high redshifts. Radio galaxies are predicted to be hosted by more massive haloes than quasars, and this is imprinted on the amplitude of galaxy overdensities and cross-correlation functions. We find that Ly{alpha} radiative transfer and AGN feedback indirectly affect the clustering on small scales and also the stellar masses, star- formation rates and gas metallicities of galaxies in dense environments. We also investigate the relation between protoclusters associated with radio galaxies and quasars, and their present- day cluster descendants. The progenitors of massive clusters associated with radio galaxies and quasars allow us to determine an average protocluster size in a simple way. Overdensities within the protoclusters are found to correlate with the halo descendant masses. We present scaling relations that can be applied to observational data. By computing projection effects due to the wavelength resolution of modern spectrographs and narrow-band filters we show that the former have enough spectral resolution to map the structure of protoclusters, whereas the latter can be used to measure the clustering around radio galaxies and quasars over larger scales to determine the mass of dark matter haloes hosting them.
We present the Obelisk project, a cosmological radiation-hydrodynamics simulation following the assembly and reionization of a protocluster progenitor during the first two billions of years from the big bang, down to $z = 3.5$. The simulation resolves haloes down to the atomic cooling limit, and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The Obelisk project is designed specifically to study the coevolution of high redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to high redshift protoclusters. We find that hydrogen reionization happens inside-out and is completed by $z sim 6$ in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at $z sim 4$.
We present spectroscopic confirmation of two new massive galaxy protoclusters at $z=2.24pm0.02$, BOSS1244 and BOSS1542, traced by groups of Coherently Strong Ly$alpha$ Absorption (CoSLA) systems imprinted in the absorption spectra of a number of quasars from the SDSS III and identified as overdensities of narrowband-selected H$alpha$ emitters (HAEs). Using MMT/MMIRS and LBT/LUCI near-infrared (NIR) spectroscopy, we confirm 46 and 36 HAEs in the BOSS1244 and BOSS1542 fields, respectively. BOSS1244 displays a South-West (SW) component at $z=2.230pm0.002$ and another North-East (NE) component at $z=2.246pm0.001$ with the line-of-sight velocity dispersions of $405pm202$ km s$^{-1}$ and $377pm99$ km s$^{-1}$, respectively. Interestingly, we find that the SW region of BOSS1244 contains two substructures in redshift space, likely merging to form a larger system. In contrast, BOSS1542 exhibits an extended filamentary structure with a low velocity dispersion of $247pm32$ km s$^{-1}$ at $z=2.241pm0.001$, providing a direct confirmation of a large-scale cosmic web in the early Universe. The galaxy overdensities $delta_{rm g}$ on the scale of 15 cMpc are $22.9pm4.9$, $10.9pm2.5$, and $20.5pm3.9$ for the BOSS1244 SW, BOSS1244 NE, and BOSS1542 filament, respectively. They are the most overdense galaxy protoclusters ($delta_{rm g}>20$) discovered to date at $z>2$. These systems are expected to become virialized at $zsim0$ with a total mass of $M_{rm SW}=(1.59pm0.20)times10^{15}$ $M_{odot}$, $M_{rm NE} =(0.83pm0.11)times10^{15}$ $M_{odot}$ and $M_{rm filament}=(1.42pm0.18)times10^{15}$ $M_{odot}$, respectively. Together with BOSS1441 described in Cai et al. (2017a), these extremely massive overdensities at $z=2-3$ exhibit different morphologies, indicating that they are in different assembly stages in the formation of early galaxy clusters.
Gaia is an ambitious ESA space mission which will provide photometric and astrometric measurements with the accuracies needed to produce a kinematic census of almost one billion stars in our Galaxy. These data will revolutionize our understanding of the dynamics of the Milky Way, and our knowledge of its detailed gravitational potential and mass distribution, including the putative dark matter component and the non-axisymmetric features such as spiral arms. The Gaia mission will help to answer various currently unsettled questions by using kinematic information on both disk and halo stellar populations. Among many others: what does the rotation curve of the outer Galaxy look like? How far from axisymmetry and equilibrium is the Galaxy? What are the respective roles of hierarchical formation and secular evolution in shaping the Galaxy and its various components? Are the properties of the Galaxy in accordance with expectations from the standard model of cosmology?
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