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ABSTRACT Anisotropies of the Sunyaev–Zel’dovich (SZ) effect serve as a powerful probe of the thermal history of the universe. At high redshift, hot galactic outflows driven by supernovae (SNe) can inject a significant amount of thermal energy into the intergalactic medium, causing a strong y-type distortion of the cosmic microwave background (CMB) spectrum through inverse Compton scattering. The resulting anisotropies of the y-type distortion are sensitive to key physical properties of high-z galaxies pertaining to the launch of energetic SNe-driven outflows, such as the efficiency and the spatio-temporal clustering of star formation. We develop a simple analytic framework to calculate anisotropies of y-type distortion associated with SNe-powered outflows of galaxies at $$z\gt 6$$. We show that galactic outflows are likely the dominant source of thermal energy injection, compared to contributions from reionized bubbles and gravitational heating. We further show that next-generation CMB experiments such as LiteBIRD are likely to detect the contribution to y anisotropies from high-z galactic outflows through the cross-correlation with surveys of Lyman-break galaxies by e.g. the Roman Space Telescope. Our analysis and forecasts demonstrate that thermal SZ anisotropies can be a promising probe of SN feedback and outflows in early star-forming galaxies. 

ABSTRACT New JWST observations are revealing the first galaxies to be prolific producers of ionizing photons, which we argue gives rise to a tension between different probes of reionization. Over the last two decades, a consensus has emerged where star-forming galaxies are able to generate enough photons to drive reionization, given reasonable values for their number densities, ionizing efficiencies $$\xi _{\rm ion}$$ (per unit ultraviolet luminosity), and escape fractions $$f_{\rm esc}$$. However, some new JWST observations infer high values of $$\xi _{\rm ion}$$ during reionization and an enhanced abundance of earlier ($$z\gtrsim 9$$) galaxies, dramatically increasing the number of ionizing photons produced at high z. Simultaneously, recent low-z studies predict significant escape fractions for faint reionization-era galaxies. Put together, we show that the galaxies we have directly observed ($$M_{\rm UV} < -15$$) not only can drive reionization, but would end it too early. That is, our current galaxy observations, taken at face value, imply an excess of ionizing photons and thus a process of reionization in tension with the cosmic microwave background and Lyman-$$\alpha$$ forest. Considering galaxies down to $$M_{\rm UV}\approx -11$$, below current observational limits, only worsens this tension. We discuss possible avenues to resolve this photon budget crisis, including systematics in either theory or observations. 

Abstract Bursty star formation—a key prediction for high-redshift galaxies from cosmological simulations explicitly resolving stellar feedback in the interstellar medium—has recently been observed to prevail among galaxies at redshiftz≳ 6. Line intensity mapping (LIM) of the 158μm [Cii] line as a star formation rate (SFR) indicator offers unique opportunities to tomographically constrain cosmic star formation at high redshift, in a way complementary to observations of individually detected galaxies. To understand the effects of bursty star formation on [Cii] LIM, which have remained unexplored in previous studies, we present an analytic modeling framework for high-zgalaxy formation and [Cii] LIM signals that accounts for bursty star formation histories induced by delayed supernova feedback. We use it to explore and characterize how bursty star formation can impact and thus complicate the interpretation of the [Cii] luminosity function and power spectrum. Our simple analytic model indicates that bursty star formation mainly affects low-mass galaxies by boosting their average SFR and [Cii] luminosity, and in the [Cii] power spectrum it can create a substantial excess in the large-scale clustering term. This distortion results in a power spectrum shape that cannot be explained by invoking a mass-independent logarithmic scatter. We conclude that burstiness must be accounted for when modeling and analyzing [Cii] data sets from the early Universe, and that in the extreme, the signature of burstiness may be detectable with first-generation experiments such as TIME, CONCERTO, and CCAT-DSS. 

ABSTRACT The formation of the first stars marks a watershed moment in the history of our Universe. As the first luminous structures, these stars (also known as Population III, or Pop III stars) seed the first galaxies and begin the process of reionization. We construct an analytic model to self-consistently trace the formation of Pop III stars inside minihaloes in the presence of the fluctuating ultraviolet background, relic dark matter (DM)-baryon relative velocities from the early universe, and an X-ray background, which largely work to suppress cooling of gas and delay the formation of this first generation of stars. We demonstrate the utility of this framework in a semi-analytic model for early star formation that also follows the transition between Pop III and Pop II star formation inside these haloes. Using our new prescription for the criteria allowing Pop III star formation, we follow a population of DM haloes from z = 50 through z = 6 and examine the global star formation history, finding that each process defines its own key epoch: (i) the stream velocity dominates at the highest redshifts (z ≳ 30), (ii) the UV background sets the tone at intermediate times (30 ≳ z ≳ 15), and (iii) X-rays control the end of Pop III star formation at the latest times (z ≲ 15). In all of our models, Pop III stars continue to form down to z ∼ 7–10, when their supernovae will be potentially observable with forthcoming instruments. Finally, we identify the signatures of variations in the Pop III physics in the global 21-cm spin–flip signal of atomic hydrogen. 

ABSTRACT Over the last three decades, photometric galaxy selection using the Lyman-break technique has transformed our understanding of the high-z Universe, providing large samples of galaxies at $$3 \lesssim z \lesssim 8$$ with relatively small contamination. With the advent of the JWST, the Lyman-break technique has now been extended to z ∼ 17. However, the purity of the resulting samples has not been tested. Here, we use a simple model, built on the robust foundation of the dark matter halo mass function, to show that the expected level of contamination rises dramatically at $$z \gtrsim 10$$, especially for luminous galaxies, placing stringent requirements on the selection process. The most luminous sources at $$z \gtrsim 12$$ are likely at least 10 000 times rarer than potential contaminants, so extensive spectroscopic follow-up campaigns may be required to identify a small number of target sources. 

ABSTRACT Lyman α emitters (LAEs) are excellent probes of the reionization process, as they must be surrounded by large ionized bubbles in order to be visible during the reionization era. Large ionized regions are thought to correspond to overdense regions and may be protoclusters, making them interesting test-beds for early massive structures. Close associations containing several LAEs are often assumed to mark overdense, ionized bubbles. Here, we develop the first framework to quantify the ionization and density fields of high-z galaxy associations. We explore the interplay between (i) the large-scale density of a survey field, (ii) Poisson noise due to the small number density of bright sources at high redshifts (z ∼ 7), and (iii) the effects of the ionized fraction on the observation of LAEs. We use Bayesian statistics, a simple model of reionization, and a Monte Carlo simulation to construct a more comprehensive method for calculating the large-scale density of LAE regions than previous works. We find that Poisson noise has a strong effect on the inferred density of a region and show how the ionized fraction can be inferred. We then apply our framework to the strongest association yet identified: Hu et al. found 14 LAEs in a volume of ∼50 000 cMpc3 inside the COSMOS field at z ∼ 7. We show that this is most likely a 2.4σ overdensity inside of an ionized or nearly ionized bubble. We also show that this LAE association implies that the global ionized fraction is $$\bar{Q} = 0.59^{+0.10}_{-0.11}$$, within the context of a simple reionization model.