ALBERT

Description: Massive early-type galaxies (ETGs) commonly have gas discs which are kinematically misaligned with the stellar component. These discs feel a torque from the stars and the angular momentum vectors are expected to align quickly. We present results on the evolution of a misaligned gas disc in a cosmological simulation of a massive ETG from the feedback in realistic environments project. This galaxy experiences a merger which, together with a strong galactic wind, removes most of the original gas disc. The galaxy subsequently reforms a gas disc through accretion of cold gas, but it is initially 120° misaligned with the stellar rotation axis. This misalignment persists for about 2 Gyr before the gas–star misalignment angle drops below 20°. The time it takes for the gaseous and stellar components to align is much longer than previously thought, because the gas disc is accreting a significant amount of mass for about 1.5 Gyr after the merger, during which the angular momentum change induced by accreted gas dominates over that induced by stellar torques. Once the gas accretion rate has decreased sufficiently, the gas disc decouples from the surrounding halo gas and realigns with the stellar component in about six dynamical times. During the late evolution of the misaligned gas disc, the centre aligns faster than the outskirts, resulting in a warped disc. We discuss the observational consequences of the long survival of our misaligned gas disc and how our results can be used to calibrate merger rate estimates from observed gas misalignments.

Description: We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for M *  ~ 10 4–11 M galaxies in M h  ~ 10 9–12 M haloes. FIRE incorporates explicit stellar feedback in the multiphase interstellar medium, with energetics from stellar population models. We find that stellar feedback, without ‘fine-tuned’ parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile (α) shows a strong mass dependence: profiles are shallow at M h  ~ 10 10 –10 11 M and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of α, and relatively late growth of cores. Because the star formation efficiency M * / M h is strongly halo mass dependent, a rapid change in α occurs around M h  ~ 10 10 M ( M *  ~ 10 6 –10 7 M ), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of haloes because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid build-up has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the ‘Too Big To Fail’ problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass haloes produce DM profiles slightly shallower than the Navarro–Frenk–White profile, consistent with the normalization of the observed Tully–Fisher relation.

Description: We present an analysis of the galaxy-scale gaseous outflows from the Feedback in Realistic Environments (FIRE) simulations. This suite of hydrodynamic cosmological zoom simulations resolves formation of star-forming giant molecular clouds to z = 0, and features an explicit stellar feedback model on small scales. Our simulations reveal that high-redshift galaxies undergo bursts of star formation followed by powerful gusts of galactic outflows that eject much of the interstellar medium and temporarily suppress star formation. At low redshift, however, sufficiently massive galaxies corresponding to L* progenitors develop stable discs and switch into a continuous and quiescent mode of star formation that does not drive outflows far into the halo. Mass-loading factors for winds in L* progenitors are 10 at high redshift, but decrease to 〈〈 1 at low redshift. Although lower values of are expected as haloes grow in mass over time, we show that the strong suppression of outflows with decreasing redshift cannot be explained by mass evolution alone. Circumgalactic outflow velocities are variable and broadly distributed, but typically range between one and three times the circular velocity of the halo. Much of the ejected material builds a reservoir of enriched gas within the circumgalactic medium, some of which could be later recycled to fuel further star formation. However, a fraction of the gas that leaves the virial radius through galactic winds is never regained, causing most haloes with mass M h ≤ 10 12 M to be deficient in baryons compared to the cosmic mean by z = 0.

Description: We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M * ~= 10 4–6.3 M dwarf galaxies that form within two M vir = 10 9.5–10 M dark matter halo initial conditions. Our simulations rely on the Feedback in Realistic Environments ( fire ) implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve structure on the ~200 pc scales. The resultant galaxies sit on the M * versus M vir relation required to match the Local Group stellar mass function via abundance matching. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. We demonstrate that it is possible to create a large (~1 kpc) constant-density dark matter core in a cosmological simulation of an M * ~= 10 6.3 M dwarf galaxy within a typical M vir = 10 10 M halo – precisely the scale of interest for resolving the ‘too big to fail’ problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late ( z 2), after the early epoch of cusp building mergers has ended. Our M * ~= 10 4 M dwarf retains a cuspy dark matter halo density profile that matches that of a dark-matter-only run of the same system. Though ancient, most of the stars in our ultrafaint form after reionization; the ultraviolet field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the protodwarf.

Description: Observations reveal that quasar host haloes at z ~ 2 have large covering fractions of cool dense gas (60 per cent for Lyman limit systems within a projected virial radius). Most simulations have so far failed to explain these large observed covering fractions. We analyse a new set of 15 simulated massive haloes with explicit stellar feedback from the FIRE project, covering the halo mass range M h 2 x 10 12 – 10 13 M at z = 2. This extends our previous analysis of the circum-galactic medium of high-redshift galaxies to more massive haloes. Active galactic nuclei (AGN) feedback is not included in these simulations. We find Lyman limit system covering fractions consistent with those observed around quasars. The large H  i covering fractions arise from star formation-driven galactic winds, including winds from low-mass satellite galaxies that interact with cosmological filaments. We show that it is necessary to resolve these satellite galaxies and their winds to reproduce the large Lyman limit system covering fractions observed in quasar-mass haloes. Our simulations predict that galaxies occupying dark matter haloes of mass similar to quasars but without a luminous AGN should have Lyman limit system covering fractions comparable to quasars.

Description: The cosmic noon ( z  ~ 1.5–3) marked a period of vigorous star formation for most galaxies. However, about a third of the more massive galaxies at those times were quiescent in the sense that their observed stellar populations are inconsistent with rapid star formation. The reduced star formation activity is often attributed to gaseous outflows driven by feedback from supermassive black holes, but the impact of black hole feedback on galaxies in the young Universe is not yet definitively established. We analyse the origin of quiescent galaxies with the help of ultrahigh resolution, cosmological simulations that include feedback from stars but do not model the uncertain consequences of black hole feedback. We show that dark matter haloes with specific accretion rates below ~0.25–0.4 Gyr –1 preferentially host galaxies with reduced star formation rates and red broad-band colours. The fraction of such haloes in large dark matter only simulations matches the observed fraction of massive quiescent galaxies (~10 10 –10 11 M ). This strongly suggests that halo accretion rate is the key parameter determining which massive galaxies at z  ~ 1.5–3 become quiescent. Empirical models that connect galaxy and halo evolution, such as halo occupation distribution or abundance matching models, assume a tight link between galaxy properties and the masses of their parent haloes. These models will benefit from adding the specific accretion rate of haloes as a second model parameter.

Description: Rapid accretion of cold intergalactic gas plays a crucial role in getting gas into galaxies. It has been suggested that this gas accretion proceeds along narrow streams that might also directly drive the turbulence in galactic gas, dynamical disturbances and bulge formation. In cosmological simulations, however, it is impossible to isolate and hence disentangle the effect of cold stream accretion from internal instabilities and mergers. Moreover, in most current cosmological simulations, the phase structure and turbulence in the interstellar medium (ISM) arising from stellar feedback are treated in an approximate (subgrid) manner, so that the feedback cannot generate turbulence in the ISM. In this paper we therefore test the effects of cold streams in extremely high-resolution simulations of otherwise isolated galaxy discs using detailed models for star formation and stellar feedback; we then include or exclude mock cold flows falling on to the galaxies, with mass accretion rates, velocities and flow geometry set to maximize their effect on the gaseous disc. We find (1) turbulent velocity dispersions in gas discs are identical with or without the presence of the cold flow; the energy injected by the flow is efficiently dissipated where it meets the disc. (2) In runs without stellar feedback, the presence of a cold flow has essentially no effect on runaway fragmentation (local collapse), resulting in star formation rates (SFRs) that are an order-of-magnitude too large. (3) Model discs in runs with both explicit feedback and cold flows have higher SFRs, but only insofar as they have more gas. (4) Because the flows are extended, relative to the size of the disc, they do not trigger strong resonant responses and so induce weak gross morphological perturbation (bulge formation via instabilities/fragmentation is not accelerated). (5) However, flows can thicken the disc by direct contribution of out-of-plane or misaligned star-forming streams/filaments. We conclude that while inflows are critical over cosmological time-scales to determine the supply and angular momentum of gas discs, they have weak instantaneous dynamical effects on galaxies.

Description: We study galaxy superwinds driven in major mergers, using pc-scale resolution simulations with detailed models for stellar feedback that can self-consistently follow the generation of winds. The models include molecular cooling, star formation at high densities in giant molecular clouds, and gas recycling and feedback from supernovae (I and II), stellar winds and radiation pressure. We study mergers of systems from Small-Magellanic-Cloud-like dwarfs and Milky Way analogues to z  ~ 2 starburst discs. Multiphase superwinds are generated in all passages, with outflow rates up to ~1000 M yr –1 . However, the wind mass-loading efficiency (outflow rate divided by star formation rate, SFR) is similar to that in the isolated galaxy counterparts of each merger: it depends more on global galaxy properties (mass, size and escape velocity) than on the dynamical state or orbital parameters of the merger. Winds tend to be bi- or unipolar, but multiple ‘events’ build up complex morphologies with overlapping, differently oriented bubbles and shells at a range of radii. The winds have complex velocity and phase structure, with material at a range of speeds up to ~1000 km s –1 (forming a Hubble-like flow), and a mix of molecular, ionized and hot gas that depends on galaxy properties. We examine how these different phases are connected to different feedback mechanisms. These simulations resolve a problem in some ‘subgrid’ models, where simple wind prescriptions can dramatically suppress merger-induced starbursts, often making it impossible to form Ultra Luminous Infrared Galaxies (ULIRGs). Despite large mass-loading factors (10–20) in the winds simulated here, the peak SFRs are comparable to those in ‘no wind’ simulations. Wind acceleration does not act equally, so cold dense gas can still lose angular momentum and form stars, while these stars blow out gas that would not have participated in the starburst in the first place. Considerable wind material is not unbound, and falls back on the disc at later times post-merger, leading to higher post-starburst SFRs in the presence of stellar feedback. We consider different simulation numerical methods and their effects on the wind phase structure; while most results are converged, we find that the existence of small clumps in the outflow at large distances from the galaxy is quite sensitive to the methodology.

Description: We present Feedback in Realistic Environment (FIRE)/G izmo hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies ( M vir ~= 10 10 M ) and ultra-faint galaxies ( M vir ~= 10 9 M ), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M * ~= 10 6 to 10 4 M without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M * ~= 10 6 M has 1–2 well-resolved satellites with M * = 3-200 x 10 3 M . Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting ~ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ~35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations ( M * ~= 3-30 x 10 3 M ) form within M peak ~= 0.5-3 x 10 9 M haloes. Each has a uniformly ancient stellar population ( 〉 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that M halo ~= 5 x 10 9 M is a probable dividing line between haloes hosting reionization ‘fossils’ and those hosting dwarfs that can continue to form stars in isolation after reionization.

Description: We present a series of high-resolution (20–2000 M , 0.1–4 pc) cosmological zoom-in simulations at z   6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10 9 –10 11 M and rest-frame ultraviolet magnitude M UV  = –9 to –19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z  = 0–6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction ( f esc ) of hydrogen ionizing photons. We find that the instantaneous f esc has large time variability (0.01–20 per cent), while the time-averaged f esc over long time-scales generally remains  5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of f esc on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3 and 10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that f esc can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g. models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance f esc by only a small fraction. We show that ‘sub-grid’ star formation models, which do not explicitly resolve star formation in dense clouds with n  〉〉 1 cm –3 , will dramatically overpredict f esc .