HERA Phase I Limits on the Cosmic 21 cm Signal: Constraints on Astrophysics and Cosmology during the Epoch of Reionization

Recently, the Hydrogen Epoch of Reionization Array (HERA) has produced the experiment’s first upper limits on the power spectrum of 21 cm fluctuations at z ∼ 8 and 10. Here, we use several independent theoretical models to infer constraints on the intergalactic medium (IGM) and galaxies during the epoch of reionization from these limits. We find that the IGM must have been heated above the adiabatic-cooling threshold by z ∼ 8, independent of uncertainties about IGM ionization and the radio background. Combining HERA limits with complementary observations constrains the spin temperature of the z ∼ 8 neutral IGM to 27 K 〈 T ¯ S 〉 630 K (2.3 K 〈 T ¯ S 〉 640 K) at 68% (95%) confidence. They therefore also place a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the cosmic microwave background dominates the z ∼ 8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones. The z ∼ 10 limits require even earlier heating if dark-matter interactions cool the hydrogen gas. If an extra radio background is produced by galaxies, we rule out (at 95% confidence) the combination of high radio and low X-ray luminosities of L r,ν /SFR > 4 × 1024 W Hz−1 M ⊙ − 1 yr and L X /SFR < 7.6 × 1039 erg s−1 M ⊙ − 1 yr. The new HERA upper limits neither support nor disfavor a cosmological interpretation of the recent Experiment to Detect the Global EOR Signature (EDGES) measurement. The framework described here provides a foundation for the interpretation of future HERA results.

Finding High-redshift Galaxies with JWST

One of the primary goals for the upcoming James Webb Space Telescope is to observe the first galaxies. Predictions for planned and proposed surveys have typically focused on average galaxy counts, assuming a random distribution of galaxies across the observed field. The first and most-massive galaxies, however, are expected to be tightly clustered, an effect known as cosmic variance. We show that cosmic variance is likely to be the dominant contribution to uncertainty for high-redshift mass and luminosity functions, and that median high-redshift and high-mass galaxy counts for planned observations lie significantly below average counts. Several different strategies are considered for improving our understanding of the first galaxies, including adding depth, area, and independent pointings. Adding independent pointings is shown to be the most efficient both for discovering the single highest-redshift galaxy and also for constraining mass and luminosity functions.

Radiation hydrodynamical simulations of the birth of intermediate-mass black holes in the first galaxies

ABSTRACT The leading contenders for the seeds of z &gt; 6 quasars are direct-collapse black holes (DCBHs) forming in atomically cooled haloes at z ∼ 20. However, the Lyman–Werner (LW) UV background required to form DCBHs of 105 M⊙ are extreme, about 104 J21, and may have been rare in the early universe. Here we investigate the formation of intermediate-mass black holes (IMBHs) under moderate LW backgrounds of 100 and 500 J21, which were much more common at early times. These backgrounds allow haloes to grow to a few 106–107 M⊙ and virial temperatures of nearly 104 K before collapsing, but do not completely sterilize them of H2. Gas collapse then proceeds via Lyα and rapid H2 cooling at rates that are 10–50 times those in normal Pop III star-forming haloes, but less than those in purely atomically cooled haloes. Pop III stars accreting at such rates become blue and hot, and we find that their ionizing UV radiation limits their final masses to 1800–2800 M⊙ at which they later collapse to IMBHs. Moderate LW backgrounds thus produced IMBHs in far greater numbers than DCBHs in the early universe.

Space Project for Astrophysical and Cosmological Exploration (SPACE), an ESA stand-alone mission and a possible contribution to the Origins Space Telescope

We propose a new mission called Space Project for Astrophysical and Cosmological Exploration (SPACE) as part of the ESA long term planning Voyage 2050 programme. SPACE will study galaxy evolution at the earliest times, with the key goals of charting the formation of the heavy elements, measuring the evolution of the galaxy luminosity function, tracing the build-up of stellar mass in galaxies over cosmic time, and finding the first super-massive black holes (SMBHs) to form. The mission will exploit a unique region of the parameter space, between the narrow ultra-deep surveys with HST and JWST, and shallow wide-field surveys such as the Roman Space Telescope and EUCLID, and should yield by far the largest sample of any current or planned mission of very high redshift galaxies at z > 10 which are sufficiently bright for detailed follow-up spectroscopy. Crucially, we propose a wide-field spectroscopic near-IR + mid-IR capability which will greatly enhance our understanding of the first galaxies by detecting and identifying a statistical sample of the first galaxies and the first supermassive black holes, and to chart the metal enrichment history of galaxies in the early Universe – potentially finding signatures of the very first stars to form from metal-free primordial gas. The wide-field and wavelength range of SPACE will also provide us a unique opportunity to study star formation by performing a wide survey of the Milky Way in the near-IR + mid-IR. This science project can be enabled either by a stand-alone ESA-led M mission or by an instrument for an L mission (with ESA and/or NASA, JAXA and other international space agencies) with a wide-field (sub-)millimetre capability at λ > 500 μm.

Magnetogenesis around the first galaxies: the impact of different field seeding processes on galaxy formation

We study the evolution of magnetic fields generated by charge segregation ahead of ionization fronts during the Epoch of Reionization, and their effects on galaxy formation. We compare this magnetic seeding process with the Biermann battery, injection from supernovae, and an imposed seed field at redshift z ≳ 127. Using a suite of self-consistent cosmological and zoom-in simulations based on the Auriga galaxy-formation model, we determine that all mechanisms produce galactic magnetic fields that equally affect galaxy formation, and are nearly indistinguishable at z ≲ 1.5. The former is compatible with observed values, while the latter is correlated with the gas metallicity below a seed-dependent redshift. Low-density gas and haloes below a seed-dependent mass threshold retain memory of the initial magnetic field. We produce synthetic Faraday rotation measure maps, showing that they have the potential to constrain the seeding process, although current observations are not yet sensitive enough. Our results imply that the ad-hoc assumption of a primordial seed field – widely used in galaxy formation simulations but of uncertain physical origin – can be replaced by physically-motivated mechanisms for magnetogenesis with negligible impact on galactic properties. Additionally, magnetic fields generated ahead of ionization fronts appear very similar but weaker than those produced by the Biermann battery. Hence, in a realistic scenario where both mechanisms are active, the former will be negligible compared to the latter. Finally, our results highlight that the high-redshift Universe is a fruitful testing ground for our understanding of magnetic fields generation.

The detailed structure and the onset of galaxy formation in low-mass gaseous dark matter haloes

ABSTRACT We present a model for the formation of the first galaxies before and after the reionization of hydrogen in the early universe. In this model, galaxy formation can only take place in dark matter haloes whose mass exceeds a redshift-dependent critical value, which, before reionization, is equal (in the simplest case) to the mass at which atomic hydrogen cooling becomes effective and, after reionization, is equal to the mass above which gas cannot remain in hydrostatic equilibrium. We define the Halo Occupation Fraction (HOF) as the fraction of haloes that host a luminous galaxy as a function of halo mass. The HOF is established by the interplay between the evolution of the critical mass and the assembly history of haloes and depends on three factors: the minimum halo mass for galaxy formation before reionization, the redshift of reionization, and the intensity of the (evolving) external photoheating rate. Our fiducial model predicts a cutoff in the galaxy mass function at a present-day halo mass, $M_{200} \sim 3\times 10^{8} \, \mathrm{M}_{\odot }$; 100 per cent occupation at $M_{200} \gt 5\times 10^9 \, \mathrm{M}_{\odot }$; and a population of starless gaseous haloes of present-day mass in the range 106 ≲ M200/M⊙ ≲ 5 × 109, in which the gas is in thermal equilibrium with the ultraviolet background radiation and in hydrostatic equilibrium in the gravitational potential of the halo. The transition between HOF = 0 and HOF = 1 reflects the stochastic nature of halo mass growth. We explore how these characteristic masses vary with model assumptions and parameter values. The results of our model are in excellent agreement with cosmological hydrodynamic simulations of galaxy formation.

Reionization inference from the CMB optical depth and E-mode polarization power spectra

ABSTRACT The Epoch of Reionization (EoR) depends on the complex astrophysics governing the birth and evolution of the first galaxies and structures in the intergalactic medium. EoR models rely on cosmic microwave background (CMB) observations, and in particular the large-scale E-mode polarization power spectra (EE PS), to help constrain their highly uncertain parameters. However, rather than directly forward-modelling the EE PS, most EoR models are constrained using a summary statistic – the Thompson scattering optical depth, τe. Compressing CMB observations to τe requires adopting a basis set for the EoR history. The common choice is the unphysical, redshift-symmetric hyperbolic tangent (tanh) function, which differs in shape from physical EoR models based on hierarchical structure formation. Combining public EoR and CMB codes, 21cmfast and class, here we quantify how inference using the τe summary statistic impacts the resulting constraints on galaxy properties and EoR histories. Using the last Planck 2018 data release, we show that the marginalized constraints on the EoR history are more sensitive to the choice of the basis set (tanh versus physical model) than to the CMB likelihood statistic (τe versus PS). For example, EoR histories implied by the growth of structure show a small tail of partial reionization extending to higher redshifts. However, biases in inference using τe are negligible for the Planck 2018 data. Using EoR constraints from high-redshift observations including the quasar dark fraction, galaxy UV luminosity functions, and CMB EE PS, our physical model recovers $\tau _\mathrm{ e} = 0.0569_{-0.0066}^{+0.0081}$.

Cosmic variance of z > 7 galaxies: prediction from bluetides

ABSTRACT In the coming decade, a new generation of telescopes, including JWST and WFIRST, will probe the period of the formation of first galaxies and quasars, and open up the last frontier for structure formation. Recent simulations and observations have suggested that these galaxies are strongly clustered (with large-scale bias ≳6), and therefore have significant cosmic variance. In this work, we use bluetides, the largest volume cosmological simulation of galaxy formation, to directly estimate the cosmic variance for current and upcoming surveys. Given its resolution and volume, bluetides can probe the bias and cosmic variance of z &gt; 7 galaxies between magnitude MUV ∼ −16 and MUV ∼ −22 over survey areas ∼0.1 arcmin2 to ∼10 deg2. Within this regime, the cosmic variance decreases with survey area/ volume as a power law with exponents between ∼−0.25 and ∼−0.45. For the planned 10 deg2 field of WFIRST, the cosmic variance is between $3{{\ \rm per\ cent}}$ and $10{{\ \rm per\ cent}}$. Upcoming JWST medium/ deep surveys with areas up to A ∼ 100 arcmin2 will have cosmic variance ranging from ${\sim}20\,\mathrm{ to}\,50{{\ \rm per\ cent}}$. Lensed surveys have the highest cosmic variance ${\gtrsim}40{{\ \rm per\ cent}}$; the cosmic variance of MUV ≲ −16 galaxies is ${\lesssim}100{{\ \rm per\ cent}}$ up to z ∼ 11. At higher redshifts such as z ∼ 12 (14), effective volumes of ≳ (8 Mpc h−1)3 (≳(12 Mpc h−1)3) are required to limit the cosmic variance to within $100{{\ \rm per\ cent}}$. Finally, we find that cosmic variance is larger than Poisson variance and forms the dominant component of the overall uncertainty in all current and upcoming surveys. We present our calculations in the form of simple fitting functions and an online cosmic variance calculator (CV_AT_COSMIC_DAWN) that we publicly release.