Lambda Cold Dark Matter (ΛCDM) is now the standard theory of structure formation in the Universe. We present the first results from the new Bolshoi dissipationless cosmological ΛCDM simulation that uses cosmological parameters favored by current observations. The Bolshoi simulation was run in a volume 250 h −1 Mpc on a side using ∼8 billion particles with mass and force resolution adequate to follow subhalos down to the completeness limit of V circ = 50 km s −1 maximum circular velocity. Using merger trees derived from analysis of 180 stored time-steps we find the circular velocities of satellites before they fall into their host halos. Using excellent statistics of halos and subhalos (∼10 million at every moment and ∼50 million over the whole history) we present accurate approximations for statistics such as the halo mass function, the concentrations for distinct halos and subhalos, abundance of halos as a function of their circular velocity, the abundance and the spatial distribution of subhalos. We find that at high redshifts the concentration falls to a minimum value of about 4.0 and then rises for higher values of halo mass, a new result. We present approximations for the velocity and mass functions of distinct halos as a function of redshift. We find that while the Sheth-Tormen approximation for the mass function of halos found by spherical overdensity is quite accurate at low redshifts, the ST formula over-predicts the abundance of halos by nearly an order of magnitude by z = 10. We find that the number of subhalos scales with the circular velocity of the host halo as V 1/2 host , and that subhalos have nearly the same radial distribution as dark matter particles at radii 0.3-2 times the host halo virial radius. The subhalo velocity function N (> V sub ) scales as V −3 circ . Combining the results of Bolshoi and Via Lactea-II simulations, we find that inside the virial radius of halos with V circ = 200 km s −1 the number of satellites is N (> V sub ) = (V sub /58 km s −1 ) −3 for satellite circular velocities in the range 4 km s −1 < V sub < 150 km s −1 .

It has long been regarded as difficult if not impossible for a cosmological model to account simultaneously for the galaxy luminosity, mass, and velocity distributions. We revisit this issue using a modern compilation of observational data along with the best available large-scale cosmological simulation of dark matter. We find that the standard cosmological model, used in conjunction with halo abundance matching (HAM) and simple dynamical corrections, fits -at least on average -all basic statistics of galaxies with circular velocities V circ > 80 km s −1 calculated at a radius of ∼10 kpc. Our primary observational constraint is the luminosity-velocity relation -which generalizes the Tully-Fisher and Faber-Jackson relations in allowing all types of galaxies to be included, and provides a fundamental benchmark to be reproduced by any theory of galaxy formation. We have compiled data for a variety of galaxies ranging from dwarf irregulars to giant ellipticals. The data present a clear monotonic luminosity-velocity relation from ∼50 km s −1 to ∼500 km s −1 , with a bend below ∼ 80 km s −1 and a systematic offset between late-and early-type galaxies. For comparison to theory, we employ our new ΛCDM "Bolshoi" simulation of dark matter, which has unprecedented mass and force resolution over a large cosmological volume, while using an up-to-date set of cosmological parameters. We use halo abundance matching to assign rank-ordered galaxy luminosities to the dark matter halos, a procedure that automatically fits the empirical luminosity function and provides a predicted luminosity-velocity relation that can be checked against observations. The adiabatic contraction of dark matter halos in response to the infall of the baryons is included as an optional model ingredient. The resulting predictions for the luminosity-velocity relation are in excellent agreement with the available data on both early-type and late-type galaxies for the luminosity range from M r = −14 to M r = −22. We also compare our predictions for the "cold" baryon mass (i.e., stars and cold gas) of galaxies as a function of circular velocity with the available observations, again finding a very good agreement. The predicted circular velocity function is also in agreement with the galaxy velocity function from 80 to 400 km s −1 , using the HIPASS survey for late-type and SDSS for early-type galaxies. However, in accord with other recent results, we find that the dark matter halos with V circ < 80 km s −1 are much more abundant than observed galaxies with the same V circ . Finally, we find that the two-point correlation function of bright galaxies in our model matches very well the results from the final data release of the SDSS, especially when a small amount of scatter is included in the HAM prescription.

Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions, suggested by current abundance matching models. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modeled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. We also include photoionization and photoheating by massive stars. The full photoionization of hydrogen reduces the radiative cooling in the 10 4−4.5 K regime. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. This maintains a low star formation rate of ∼ 1 M ⊙ yr −1 in halos with masses about 10 11 M ⊙ at z ≃ 3. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The galaxies residing in these low-mass halos contain only ∼ 0.6% of the total virial mass in stars, roughly consistent with abundance matching. Radiative feedback maintains an extended galaxy with a rising circular velocity profile.

Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: Gaia-Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered, ‘low-energy’ GCs, which provide an excellent match to the predicted properties of the enigmatic galaxy ‘Kraken’. The five galaxies cover a narrow stellar mass range [M⋆ = (0.6 − 4.6) × 108 M⊙], but have widely different accretion redshifts (${z_{\rm acc}}=0.57{-}2.65$). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with stellar and virial mass ratios of ${r_{M_\star }}=1$:$31^{+34}_{-16}$ and ${r_{M_{\rm h}}}=1$:$7^{+4}_{-2}$, respectively. The progenitors match the z = 0 relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. Even though these progenitors likely were the Milky Way’s most massive accretion events, they contributed a total mass of only log (M⋆, tot/M⊙) = 9.0 ± 0.1, similar to the stellar halo. This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. We conclude by organising these accretion events into the most detailed reconstruction to date of the Milky Way’s merger tree.

We present a detailed analysis of a large-scale galactic outflow in the CGM of a massive (M h ∼ 10 12.5 M ⊙ ), star forming (∼ 6.9 M ⊙ yr −1 ), sub-L * (∼ 0.5L * B ) galaxy at z = 0.39853 that exhibits a wealth of metal-line absorption in the spectra of the background quasar Q 0122 − 003 at an impact parameter of 163 kpc. The galaxy inclination angle (i = 63 • ) and the azimuthal angle (Φ = 73 • ) imply that the QSO sightline is passing through the projected minor-axis of the galaxy. The absorption system shows a multiphase, multicomponent structure with ultra-strong, wide velocity spread O VI (log N = 15.16 ± 0.04, ∆v 90 = 419 km s −1 ) and N V (log N = 14.69 ± 0.07, ∆v 90 = 285 km s −1 ) lines that are extremely rare in the literature. The highly ionized absorption components are well explained as arising in a low density (∼ 10 −4.2 cm −3 ), diffuse (∼ 10 kpc), cool (∼ 10 4 K) photoionized gas with a super-solar metallicity ([X/H] 0.3). From the observed narrowness of the Lyβ profile, the non-detection of S IV absorption, and the presence of strong C IV absorption in the low-resolution FOS spectrum we rule out equilibrium/non-equilibrium collisional ionization models. The lowionization photoionized gas with a density of ∼ 10 −2.5 cm −3 and a metallicity of [X/H] −1.4 is possibly tracing recycled halo gas. We estimate an outflow mass of ∼ 2 × 10 10 M ⊙ , a mass-flow rate of ∼ 54 M ⊙ yr −1 , a kinetic luminosity of ∼ 9 × 10 41 erg s −1 , and a mass loading factor of ∼ 8 for the outflowing high-ionization gas. These are consistent with the properties of "down-the-barrel" outflows from infrared-luminous starbursts as studied by Rupke et al. Such powerful, large-scale, metal-rich outflows are the primary means of sufficient mechanical and chemical feedback as invoked in theoretical models of galaxy formation and evolution.