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
α
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. Lastly, we review a number of promising observational projects that are expected to make significant impact in this growing, exciting field.
Abstract
Observations of luminous quasars and their supermassive black holes at
z
≳ 6 suggest that they formed at dense matter peaks in the early universe. However, few studies have found definitive ...evidence that the quasars lie at cosmic density peaks, in clear contrast with theory predictions. Here we present new evidence that the radio-loud quasar SDSS J0836+0054 at
z
= 5.8 could be part of a surprisingly rich structure of galaxies. This conclusion is reached by combining a number of findings previously reported in the literature. Bosman et al. obtained the redshifts of three companion galaxies, confirming an overdensity of
i
775
dropouts found by Zheng et al. By comparing this structure with those found near other quasars and large overdense regions in the field at
z
∼ 6–7, we show that the SDSS J0836+0054 field is among the densest structures known at these redshifts. One of the spectroscopic companions is a very massive star-forming galaxy (
log
10
(
⋆
/
M
⊙
)
=
10.3
−
0.2
+
0.3
) based on its unambiguous detection in a Spitzer 3.6
μ
m image. This suggests that the quasar field hosts not one, but at least two rare, massive dark matter halos (
log
10
(
h
/
M
⊙
)
≳
12
), corresponding to a galaxy overdensity of at least 20. We discuss the properties of the young radio source. We conclude that the environment of SDSS J0836+0054 resembles, at least qualitatively, the type of conditions that may have spurred the direct collapse of a massive black hole seed according to recent theory.
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.
Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. We present a possible local analog that has an escape fraction ...of ionizing flux of 21%. Our detection confirms the existence of gaps in the neutral gas enveloping the starburst region. The candidate contains a massive yet highly compact star-forming region. The gaps are most likely created by the unusually strong winds and intense ionizing radiation produced by this extreme object. Our study also validates the indirect technique of using the residual flux in saturated low-ionization interstellar absorption lines for identifying such leaky galaxies. Because direct detection of ionizing flux is impossible at the epoch of reionization, this represents a highly valuable technique for future studies.
A growing number of galaxy clusters at z = 1-2 is being discovered as part of deep optical, IR, X-ray, and Sunyaev-Zel'dovich effect surveys. For a complete picture of cluster formation, however, it ...is important that we also start probing the much earlier epoch, between redshifts of about 2 and 7, during which these clusters and their galaxies first began to form. We use the Millennium Simulations to track the evolution of dark matter and galaxies in about 3000 clusters from the earliest times to z = 0. We define an effective radius R sub(e) for proto-clusters and characterize their growth in size and mass with cosmic time. This paper provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e.g., the upcoming HETDEX and Hyper Suprime-Cam surveys.
The source responsible for the reionization of the universe is believed to be the population of star-forming galaxies at z ∼ 6 to 12. The biggest uncertainty concerns the fraction of Lyman-continuum ...photons that actually escape from the galaxies. In recent years, several relatively small samples of "leaky" galaxies have been discovered, and clues have begun to emerge as to both the indirect signposts of leakiness and of the conditions/processes that enable the escape of ionizing radiation. In this paper we present the results of a pilot program to test a new technique for finding leaky galaxies, using the weakness of the S ii nebular emission lines relative to typical star-forming galaxies as evidence that the interstellar medium (ISM) is optically thin to the Lyman continuum. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to detect significant emerging flux below the Lyman edge in two out of three S ii-weak star-forming galaxies at z ∼ 0.3. We show that these galaxies differ markedly in their properties from the class of leaky "Green-Pea" galaxies at similar redshifts: our sample galaxies are more massive, more metal-rich, and less extreme in terms of their stellar population and the ionization state of the ISM. Like the Green Peas, they have exceptionally high star formation rates per unit area. They also share some properties with the known leaky galaxies at z ∼ 3, but are significantly dustier. Our results validate a new method to identify local laboratories for exploring the processes that made it possible for galaxies to reionize the universe.
We used Very Large Telescope/X-Shooter to target a sample of nearby analogs of Lyman break galaxies (LBGs). These Lyman break analogs are similar to the LBGs in many of their physical properties. We ...determine electron temperatures using the weak O iii λ4363 emission line and determine the oxygen abundance (O/H) using the direct and strong-line methods. We show that the direct and strong-line abundances are consistent with established relations within ∼0.2 dex. The analogs have nitrogen-to-oxygen ratios (N/O) and ionization parameters (q) that are, on average, offset with respect to typical local galaxies but similar to galaxies at z ∼ 2 and other analogs. The N/O and q excesses correlate with the offsets observed in the strong-line ratios, again similar to z ∼ 2. The star formation rate surface densities are consistent with the high electron density and ionization, indicating that the interstellar medium (ISM) pressure is set by feedback from the starbursts. For a given O/H, the apparent N/O excess arises owing to the offset in O/H with respect to the local mass-metallicity relation. This can be explained by recent inflow of relatively metal-poor gas that lowers O/H while leaving N/O unchanged. The difficulties in determining even basic ISM parameters in these nearby analogs illustrate some of the challenges we face at much higher redshifts, where similar rest-frame optical diagnostics for large samples of galaxies can be accessed with the James Webb Space Telescope.
The incidence of active galactic nuclei (AGNs) within the local environment is a potentially valuable probe of the mechanisms that trigger and provide fuel for accretion onto supermassive black ...holes. While the correlation between AGN fraction and environment has been well-studied in the local universe, AGN fractions have been measured for relatively few dense environments at high redshift. In this paper, we present a measurement of the X-ray AGN fraction in the USS 1558−003 protocluster associated with the z = 2.53 radio galaxy 4C-00.62. Our measurement is based on a 100-ks Chandra observation, follow-up spectroscopy from the Multi-Object Double Spectrograph on the Large Binocular Telescope, and broad and narrowband photometry. These data are sensitive to AGNs more luminous than LX > 2 × 1043 erg s−1 in the rest-frame hard X-ray band (2-10 keV). We have identified two X-ray AGNs at the redshift of USS 1558−003, one of which is the radio galaxy. We have determined that of the H emitters in the protocluster are X-ray AGNs. Unlike most other high-redshift cluster progenitors studied with similar techniques, USS 1558−003 does not have a significantly higher fraction of AGNs than field galaxies at similar redshifts. This lower AGN fraction is inconsistent with the expectation that the higher gas fractions at high redshift, combined with the high galaxy densities and modest relative velocities in protoclusters, should produce higher AGN fractions.
ABSTRACT A population of early star-forming galaxies is the leading candidate for the re-ionization of the universe. It is still unclear, however, what conditions and physical processes would enable ...a significant fraction of the ionizing (Lyman continuum) photons to escape from these gas-rich galaxies. In this paper we present the results of the analysis of Hubble Space Telescope Cosmic Origins Spectrograph far-UV (FUV) spectroscopy plus ancillary multi-waveband data of a sample of 22 low-redshift galaxies that are good analogs to typical star-forming galaxies at high redshift. We measure three parameters that provide indirect evidence of the escape of ionizing radiation (leakiness): (1) the residual intensity in the cores of saturated interstellar low-ionization absorption lines, which indicates incomplete covering by that gas in the galaxy; (2) the relative amount of blueshifted Ly line emission, which can indicate the existence of holes in the neutral hydrogen on the front-side of the galaxy outflow, and (3) the relative weakness of the S ii optical emission lines that trace matter-bounded H ii regions. We show that our residual intensity measures are only negligibly affected by infilling from resonance emission lines. We find all three diagnostics agree well with one another. We use these diagnostics to rank-order our sample in terms of likely leakiness, noting that a direct measure of escaping Lyman continuum has recently been made for one of the leakiest members of our sample. We then examine the correlations between our ranking and other proposed diagnostics of leakiness. We find a good correlation with the equivalent width of the Ly emission line, but no significant correlations with either the flux ratio of the O iii/O ii emission lines or the ratio of star-formation rates derived from the (dust-corrected) FUV and H luminosities. Turning to galaxy properties, we find the strongest correlations with leakiness are with the compactness of the star-forming region (Star formation rate/area) and the speed of the galactic outflow. This suggests that extreme feedback-a high intensity of ionizing radiation and strong pressure from both radiation and a hot galactic wind-combines to create significant holes in the neutral gas. These results not only shed new light on the physical mechanisms that can allow ionizing radiation to escape from intensely star-forming galaxies, they also provide indirect observational indicators that can be used at high redshift where direct measurements of escaping Lyman continuum radiation are impossible.
To demonstrate the feasibility of studying the epoch of massive galaxy cluster formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample ...of protocluster candidates in the 1.62 deg super(2) COSMOS/UltraVISTA field traced by optical/infrared selected galaxies using photometric redshifts. By comparing properly smoothed three-dimensional galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed ~15 comoving Mpc-scale galaxy clustering is consistent with ?CDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6 < z < 3.1, showing overdensities consistent with the progenitors of M sub(z=0) ~ 10 super(15) M sub(middot in circle) clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M sub(z=0) ~ 10 super(14) M sub(middot in circle) are ~70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true protocluster members that will merge into a cluster-scale halo by z = 0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly selected protocluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster formation. Meanwhile, it demonstrates the potential for probing early cluster formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.
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