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  1. ABSTRACT

    We use the simba cosmological galaxy formation simulation to investigate the relationship between major mergers ($\lesssim$4:1), starbursts, and galaxy quenching. Mergers are identified via sudden jumps in stellar mass M* well above that expected from in situ star formation, while quenching is defined as going from specific star formation rate (sSFR) $\gt t_{\rm H}^{-1}$ to $\lt 0.2t_{\rm H}^{-1}$, where tH is the Hubble time. At z ≈ 0–3, mergers show ∼2–3× higher SFR than a mass-matched sample of star-forming galaxies, but globally represent $\lesssim 1{{\ \rm per\ cent}}$ of the cosmic SF budget. At low masses, the increase in SFR in mergers is mostly attributed to an increase in the H2 content, but for $M_*\gtrsim 10^{10.5} \,\mathrm{ M}_{\odot }$ mergers also show an elevated star formation efficiency suggesting denser gas within merging galaxies. The merger rate for star-forming galaxies shows a rapid increase with redshift, ∝(1 + z)3.5, but the quenching rate evolves much more slowly, ∝(1 + z)0.9; there are insufficient mergers to explain the quenching rate at $z\lesssim 1.5$. simba first quenches galaxies at $z\gtrsim 3$, with a number density in good agreement with observations. The quenching time-scales τq are strongly bimodal, with ‘slow’ quenchings (τq ∼ 0.1tH) dominating overall, but ‘fast’ quenchings (τq ∼ 0.01tH) dominating in M* ∼ 1010–1010.5 M$\odot$ galaxies, likely induced by simba’s jet-mode black hole feedback. The delay time distribution between mergers and quenching events suggests no physical connection to either fast or slow quenching. Hence, simba predicts that major mergers induce starbursts, but are unrelated to quenching in either fast or slow mode.

     
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    ABSTRACT The relation between infrared excess (IRX) and UV spectral slope (βUV) is an empirical probe of dust properties of galaxies. The shape, scatter, and redshift evolution of this relation are not well understood, however, leading to uncertainties in estimating the dust content and star formation rates (SFRs) of galaxies at high redshift. In this study, we explore the nature and properties of the IRX–βUV relation with a sample of z = 2–6 galaxies ($M_*\approx 10^9\!-\!10^{12}\, \mathrm{M}_\odot$) extracted from high-resolution cosmological simulations (MassiveFIRE) of the Feedback in Realistic Environments (FIRE) project. The galaxies in our sample show an IRX–βUV relation that is in good agreement with the observed relation in nearby galaxies. IRX is tightly coupled to the UV optical depth, and is mainly determined by the dust-to-star geometry instead of total dust mass, while βUV is set both by stellar properties, UV optical depth, and the dust extinction law. Overall, much of the scatter in the IRX–βUV relation of our sample is found to be driven by variations of the intrinsic UV spectral slope. We further assess how the IRX–βUV relation depends on viewing direction, dust-to-metal ratio, birth-cloud structures, and the dust extinction law and we present a simple model that encapsulates most of the found dependencies. Consequently, we argue that the reported ‘deficit’ of the infrared/sub-millimetre bright objects at z ≳ 5 does not necessarily imply a non-standard dust extinction law at those epochs. 
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  4. ABSTRACT Matching the number counts of high-z submillimetre-selected galaxies (SMGs) has been a long-standing problem for galaxy formation models. In this paper, we use 3D dust radiative transfer to model the submm emission from galaxies in the simba cosmological hydrodynamic simulations, and compare predictions to the latest single-dish observational constraints on the abundance of 850 μm-selected sources. We find good agreement with the shape of the integrated 850 μm luminosity function, and the normalization is within 0.25 dex at >3 mJy, unprecedented for a fully cosmological hydrodynamic simulation, along with good agreement in the redshift distribution of bright SMGs. The agreement is driven primarily by simba’s good match to infrared measures of the star formation rate (SFR) function between z = 2 and 4 at high SFRs. Also important is the self-consistent on-the-fly dust model in simba, which predicts, on average, higher dust masses (by up to a factor of 2.5) compared to using a fixed dust-to-metals ratio of 0.3. We construct a light-cone to investigate the effect of far-field blending, and find that 52 per cent of sources are blends of multiple components, which makes a small contribution to the normalization of the bright end of the number counts. We provide new fits to the 850 μm luminosity as a function of SFR and dust mass. Our results demonstrate that solutions to the discrepancy between submm counts in simulations and observations, such as a top-heavy initial mass function, are unnecessary, and that submillimetre-bright phases are a natural consequence of massive galaxy evolution. 
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  7. ABSTRACT We present a comparison of galaxy atomic and molecular gas properties in three recent cosmological hydrodynamic simulations, namely SIMBA, EAGLE, and IllustrisTNG, versus observations from z ∼ 0 to 2. These simulations all rely on similar subresolution prescriptions to model cold interstellar gas that they cannot represent directly, and qualitatively reproduce the observed z ≈ 0 H i and H2 mass functions (HIMFs and H2MFs, respectively), CO(1–0) luminosity functions (COLFs), and gas scaling relations versus stellar mass, specific star formation rate, and stellar surface density μ*, with some quantitative differences. To compare to the COLF, we apply an H2-to-CO conversion factor to the simulated galaxies based on their average molecular surface density and metallicity, yielding substantial variations in αCO and significant differences between models. Using this, predicted z = 0 COLFs agree better with data than predicted H2MFs. Out to z ∼ 2, EAGLE’s and SIMBA’s HIMFs and COLFs strongly increase, while IllustrisTNG’s HIMF declines and COLF evolves slowly. EAGLE and simba reproduce high-LCO(1–0) galaxies at z ∼ 1–2 as observed, owing partly to a median αCO(z = 2) ∼ 1 versus αCO(z = 0) ∼ 3. Examining H i, H2, and CO scaling relations, their trends with M* are broadly reproduced in all models, but EAGLE yields too little H i in green valley galaxies, IllustrisTNG and SIMBA overproduce cold gas in massive galaxies, and SIMBA overproduces molecular gas in small systems. Using SIMBA variants that exclude individual active galactic nucleus (AGN) feedback modules, we find that SIMBA’s AGN jet feedback is primarily responsible by lowering cold gas contents from z ∼ 1 → 0 by suppressing cold gas in $M_*\gtrsim 10^{10}{\rm \,M}_\odot$ galaxies, while X-ray feedback suppresses the formation of high-μ* systems. 
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    Understanding the properties of dust attenuation curves in galaxies and the physical mechanisms that shape them are among the fundamental questions of extragalactic astrophysics, with great practical significance for deriving the physical properties of galaxies. Attenuation curves result from a combination of dust grain properties, dust content, and the spatial arrangement of dust and different populations of stars. In this review, we assess the state of the field, paying particular attention to extinction curves as the building blocks of attenuation laws. We introduce a quantitative framework to characterize extinction and attenuation curves, present a theoretical foundation for interpreting empirical results, overview an array of observational methods, and review observational results at low and high redshifts. Our main conclusions include the following: ▪  Attenuation curves exhibit a wide range of UV-through-optical slopes, from curves with shallow (Milky Way–like) slopes to those exceeding the slope of the Small Magellanic Cloud extinction curve. ▪  The slopes of the curves correlate strongly with the effective optical opacities, in the sense that galaxies with lower dust column density (lower visual attenuation) tend to have steeper slopes, whereas the galaxies with higher dust column density have shallower (grayer) slopes. ▪  Galaxies exhibit a range of 2175-Å UV bump strengths, including no bump, but, on average, are suppressed compared with the average Milky Way extinction curve. ▪  Theoretical studies indicate that both the correlation between the slope and the dust column as well as variations in bump strength may result from geometric and radiative transfer effects. 
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  9. ABSTRACT We study specific star formation rate (sSFR) and gas profiles of star-forming (SF) and green valley (GV) galaxies in the simba cosmological hydrodynamic simulation. SF galaxy half-light radii (Rhalf) at z = 0 and their evolution (∝(1 + z)−0.78) agree with observations. Passive galaxy Rhalf agree with observations at high redshift, but by z = 0 are too large, owing to numerical heating. We compare simbaz = 0 sSFR radial profiles for SF and GV galaxies to observations. simba shows strong central depressions in star formation rate (SFR), sSFR, and gas fraction in GV galaxies and massive SF systems, qualitatively as observed, owing to black hole X-ray feedback, which pushes central gas outwards. Turning off X-ray feedback leads to centrally peaked sSFR profiles as in other simulations. In conflict with observations, simba yields GV galaxies with strongly dropping sSFR profiles beyond ≳Rhalf, regardless of active galactic nucleus feedback. The central depression owes to lowering molecular gas content; the drop in the outskirts owes to reduced star formation efficiency. simba’s satellites have higher central sSFR and lower outskirts sSFR than centrals, in qualitative agreement with observations. At z = 2, simba does not show central depressions in massive SF galaxies, suggesting simba’s X-ray feedback should be more active at high-z. High-resolution tests indicate central sSFR suppression is not sensitive to numerical resolution. Reproducing the central sSFR depression in z = 0 GV galaxies represents a unique success of simba. The remaining discrepancies highlight the importance of SFR and gas profiles in constraining quenching mechanisms. 
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