ALBERT

Description: Galaxies strongly self-regulate their growth via energetic feedback from stars, supernovae, and black holes, but these processes are among the least understood aspects of galaxy formation theory. We present an analytic galaxy evolution model that directly constrains such feedback processes from observed galaxy scaling relations. The equilibrium model, which is broadly valid for star-forming central galaxies that dominate cosmic star formation, is based on the ansatz that galaxies live in a slowly evolving equilibrium between inflows, outflows, and star formation. Using a Bayesian Monte Carlo Markov chain approach, we constrain our model to match observed galaxy scaling relations between stellar mass and halo mass, star formation rate, and metallicity from 0 〈  z  〈 2. A good fit ( 2   1.6) is achieved with eight free parameters. We further show that constraining our model to any two of the three data sets also produces a fit to the third that is within reasonable systematic uncertainties. The resulting best-fitting parameters that describe baryon cycling suggest galactic outflow scalings intermediate between energy and momentum-driven winds, a weak dependence of wind recycling time on mass, and a quenching mass scale that evolves modestly upwards with redshift. This model further predicts a stellar mass–star formation rate relation that is in good agreement with observations to z  ~ 6. Our results suggest that this simple analytic framework captures the basic physical processes required to model the mean evolution of stars and metals in galaxies, despite not incorporating many canonical ingredients of galaxy formation models such as merging or disc formation.

Description: From a large sample of 170 000 local SDSS (Sloan Digital Sky Survey) galaxies, we find that the fundamental metallicity relation (FMR) has an overabundance of outliers, compared to what would be expected from a Gaussian distribution of residuals, with significantly lower metallicities than predicted from their stellar mass and star formation rate (SFR). This low-metallicity population has lower stellar masses, bimodial specific SFRs with enhanced star formation within the aperture and smaller half-light radii than the general sample and is hence a physically distinct population. We show that they are consistent with being galaxies that are merging or have recently merged with a satellite galaxy. In this scenario, low-metallicity gas flows in from large radii, diluting the metallicity of star-forming regions and enhancing the specific SFR until the inflowing gas is processed and the metallicity has recovered. We introduce a simple model in which mergers with a mass ratio larger than a minimum dilute the central galaxy's metallicity by an amount that is proportional to the stellar mass ratio for a constant time, and show that it provides an excellent fit to the distribution of FMR residuals. We find the dilution time-scale to be $\tau =1.568_{-0.027}^{+0.029}$ Gyr, the average metallicity depression caused by a 1:1 merger to be $\alpha =0.2480_{-0.0020}^{+0.0017}$ dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be $\xi _{\rm min}=0.2030_{-0.0095}^{+0.0127}$ (these are statistical errors only). From this we derive that the average metallicity depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.

Description: We explore the impact of incorporating physically motivated ionization and recombination rates on the history and topology of cosmic reionization and the resulting 21 cm power spectrum, by incorporating inputs from small-volume hydrodynamic simulations into our semi-numerical code, simfast21 , that evolves reionization on large scales. We employ radiative hydrodynamic simulations to parametrize the ionization rate R ion and recombination rate R rec as functions of halo mass, overdensity and redshift. We find that R ion scales superlinearly with halo mass ( ${R_{\rm ion}}\propto M_h^{1.41}$ ), in contrast to previous assumptions. Implementing these scalings into simfast21 , we tune our one free parameter, the escape fraction f esc , to simultaneously reproduce recent observations of the Thomson optical depth, ionizing emissivity and volume-averaged neutral fraction by the end of reionization. This yields $f_{\rm esc}=4^{+7}_{-2}\hbox{ per cent}$ averaged over our 0.375 h –1 Mpc cells, independent of halo mass or redshift, increasing to 6 per cent if we also constrain to match the observed z  = 7 star formation rate function. Introducing superlinear R ion increases the duration of reionization and boosts small-scale 21 cm power by two to three times at intermediate phases of reionization, while inhomogeneous recombinations reduce ionized bubble sizes and suppress large-scale 21 cm power by two to three times. Gas clumping on sub-cell scales has a minimal effect on the 21 cm power. Superlinear R ion also significantly increases the median halo mass scale for ionizing photon output to ~ 10 10 M , making the majority of reionizing sources more accessible to next-generation facilities. These results highlight the importance of accurately treating ionizing sources and recombinations for modelling reionization and its 21 cm power spectrum.

Description: Observations suggest that C ii was more abundant than C iv in the intergalactic medium towards the end of the hydrogen reionization epoch ( z  ~ 6). This transition provides a unique opportunity to study the enrichment history of intergalactic gas and the growth of the ionizing ultraviolet background (UVB) at early times. We study how carbon absorption evolves from z  = 10 to 5 using a cosmological hydrodynamic simulation that includes a self-consistent multifrequency UVB as well as a well-constrained model for galactic outflows to disperse metals. Our predicted UVB is within ~2–4 times of that from Haardt & Madau, which is fair agreement given the uncertainties. Nonetheless, we use a calibration in post-processing to account for Lyman α forest measurements while preserving the predicted spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way that it always exceeds the volume average in regions where metals are found. This implies both that a spatially uniform UVB is a poor approximation and that metal absorption is not sensitive to the epoch when H ii regions overlap globally even at column densities of 10 12  cm –2 . We find, consistent with observations, that the C ii mass fraction drops to low redshift while C iv rises owing the combined effects of a growing UVB and continued addition of carbon in low-density regions. This is mimicked in absorption statistics, which broadly agree with observations at z  = 6–3 while predicting that the absorber column density distributions rise steeply to the lowest observable columns. Our model reproduces the large observed scatter in the number of low-ionization absorbers per sightline, implying that the scatter does not indicate a partially neutral Universe at z  ~ 6.

Description: The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at z  ~ 6 versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially inhomogeneous ‘galaxies+QSOs’ UVB; a homogeneous ‘galaxies+QSOs’ UVB, and a ‘QSOs-only’ model. All UVBs reproduce the observed column density distributions of $\mathrm{C\,\small {ii}}$ , $\mathrm{Si\,\small {iv}}$ , and $\mathrm{C\,\small {iv}}$ reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed $\mathrm{Si\,\small {iv}}/\mathrm{C\,\small {iv}}$ and $\mathrm{C\,\small {ii}}/\mathrm{C\,\small {iv}}$ distributions. The QSOs-only UVB overpredicts both $\mathrm{C\,\small {iv}}/\mathrm{C\,\small {ii}}$ and $\mathrm{C\,\small {iv}}/\mathrm{Si\,\small {iv}}$ , indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts $\mathrm{C\,\small {iv}}/\mathrm{Si\,\small {iv}}$ , suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of 2 deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.

Description: Recent analysis of strongly lensed sources in the Hubble Frontier Fields indicates that the rest-frame UV luminosity function of galaxies at z  = 6–8 rises as a power law down to M UV  = –15, and possibly as faint as –12.5. We use predictions from a cosmological radiation hydrodynamic simulation to map these luminosities on to physical space, constraining the minimum dark matter halo mass and stellar mass that the Frontier Fields probe. While previously published theoretical studies have suggested or assumed that early star formation was suppressed in haloes less massive than 10 9 –10 11 M , we find that recent observations demand vigorous star formation in haloes at least as massive as (3.1, 5.6, 10.5) x 10 9 M at z  = (6, 7, 8). Likewise, we find that Frontier Fields observations probe down to stellar masses of (8.1, 18, 32) x 10 6 M : that is, they are observing the likely progenitors of analogues to Local Group dwarfs such as Pegasus and M32. Our simulations yield somewhat different constraints than two complementary models that have been invoked in similar analyses, emphasizing the need for further observational constraints on the galaxy–halo connection.

Description: We use a radiation hydrodynamic simulation of the hydrogen reionization epoch to study O i absorbers at z  ~ 6. The intergalactic medium (IGM) is reionized before it is enriched; hence, O i absorption originates within dark matter haloes. The predicted abundance of O i absorbers is in reasonable agreement with observations. At z  = 10, 70 per cent of sightlines through atomically cooled haloes encounter a visible (N O I 〉 10 14 cm –2 ) column. Reionization ionizes and removes gas from haloes less massive than 10 8.4 M , but 20 per cent of sightlines through more massive haloes encounter visible columns even at z  = 5. The mass scale of absorber host haloes is 10–100 times smaller than the haloes of Lyman-break galaxies and Lyman α emitters, hence absorption probes the dominant ionizing sources more directly. O i absorbers have neutral hydrogen columns of 10 19 –10 21  cm –2 , suggesting a close resemblance between objects selected in O i and H i absorption. Finally, the absorption in the foreground of the z  = 7.085 quasar ULAS J1120+0641 cannot originate in a dark matter halo because halo gas at the observed H i column density is enriched enough to violate the upper limits on the O i column. By contrast, gas at less than one-third the cosmic mean density satisfies the constraints. Hence, the foreground absorption likely originates in the IGM.

Description: The analytic ‘equilibrium model’ for galaxy evolution using a mass balance equation is able to reproduce mean observed galaxy scaling relations between stellar mass, halo mass, star formation rate (SFR), and metallicity across the majority of cosmic time with a small number of parameters related to feedback. Here, we aim to test this data-constrained model to quantify deviations from the mean relation between stellar mass and SFR, i.e. the star-forming galaxy main sequence (MS). We implement fluctuation in halo accretion rates parametrized from merger-based simulations, and quantify the intrinsic scatter introduced into the MS under the assumption that fluctuations in star formation follow baryonic inflow fluctuations. We predict the 1 MS scatter to be ~0.2–0.25 dex over the stellar mass range 10 8 –10 11 M and a redshift range 0.5 z 3 for SFRs averaged over 100 Myr. The scatter increases modestly at z 3, as well as by averaging over shorter time-scales. The contribution from merger-induced star formation is generally small, around 5 per cent today and 10–15 per cent during the peak epoch of cosmic star formation. These results are generally consistent with available observations, suggesting that deviations from the MS primarily reflect stochasticity in the inflow rate owing to halo mergers.