Evolution of C iv Absorbers. II. Where Does C iv Live?

We use the observed cumulative statistics of C iv absorbers and dark matter halos to infer the distribution of C iv-absorbing gas relative to galaxies at redshifts 0 ≤ z ≤ 5. We compare the cosmic incidence dN/dX of C iv absorber populations and galaxy halos, finding that massive L ≥ L ⋆ halos alone cannot account for all the observed W r ≥ 0.05 Å absorbers. However, the dN/dX of lower-mass halos exceeds that of W r ≥ 0.05 Å absorbers. We also estimate the characteristic gas radius of absorbing structures required for the observed C iv dN/dX, assuming each absorber is associated with a single galaxy halo. The W r ≥ 0.3 Å and W r ≥ 0.6 Å C iv gas radii are ∼30%–70% (∼20%–40%) of the virial radius of L ⋆ (0.1L ⋆) galaxies, and the W r ≥ 0.05 Å gas radius is ∼100%–150% (∼60%–100%) of the virial radius of L ⋆ (0.1L ⋆) galaxies. For stronger absorbers, the gas radius relative to the virial radius rises across Cosmic Noon and falls afterwards, while for weaker absorbers, the relative gas radius declines across Cosmic Noon and then dramatically rises at z < 1. A strong luminosity-dependence of the gas radius implies highly extended C iv envelopes around massive galaxies before Cosmic Noon, while a luminosity-independent gas radius implies highly extended envelopes around dwarf galaxies after Cosmic Noon. From available absorber-galaxy and C iv evolution data, we favor a scenario in which low-mass galaxies enrich the volume around massive galaxies at early epochs and propose that the outer halo gas (>0.5 R v ) was produced primarily in ancient satellite dwarf galaxy outflows, while the inner halo gas (<0.5 R v ) originated from the central galaxy and persists as recycled accreting gas.

Massive Molecular Outflow and 100 kpc Extended Cold Halo Gas in the Enormous Lyα Nebula of QSO 1228+3128

The link between the circumgalactic medium (CGM) and the stellar growth of massive galaxies at high-z depends on the properties of the widespread cold molecular gas. As part of the SUPERCOLD-CGM survey (Survey of Protocluster ELANe Revealing CO/[C i] in the Lyα-Detected CGM), we present the radio-loud QSO Q1228+3128 at z = 2.2218, which is embedded in an enormous Lyα nebula. ALMA+ACA observations of CO(4–3) reveal both a massive molecular outflow, and a more extended molecular gas reservoir across ∼100 kpc in the CGM, each containing a mass of M H2 ∼ 4–5 × 1010 M ⊙. The outflow and molecular CGM are aligned spatially, along the direction of an inner radio jet. After reanalysis of Lyα data of Q1228+3128 from the Keck Cosmic Web Imager, we found that the velocity of the extended CO agrees with the redshift derived from the Lyα nebula and the bulk velocity of the massive outflow. We propose a scenario where the radio source in Q1228+3128 is driving the molecular outflow and perhaps also enriching or cooling the CGM. In addition, we found that the extended CO emission is nearly perpendicular to the extended Lyα nebula spatially, indicating that the two gas phases are not well mixed, and possibly even represent different phenomena (e.g., outflow versus infall). Our results provide crucial evidence in support of predicted baryonic recycling processes that drive the early evolution of massive galaxies.

Figuring Out Gas & Galaxies In Enzo (FOGGIE). V. The Virial Temperature Does Not Describe Gas in a Virialized Galaxy Halo

The classical definition of the virial temperature of a galaxy halo excludes a fundamental contribution to the energy partition of the halo: the kinetic energy of nonthermal gas motions. Using simulations of low-redshift, ∼L* galaxies from the Figuring Out Gas & Galaxies In Enzo (FOGGIE) project that are optimized to resolve low-density gas, we show that the kinetic energy of nonthermal motions is roughly equal to the energy of thermal motions. The simulated FOGGIE halos have ∼2× lower bulk temperatures than expected from a classical virial equilibrium, owing to significant nonthermal kinetic energy that is formally excluded from the definition of T vir. We explicitly derive a modified virial temperature including nonthermal gas motions that provides a more accurate description of gas temperatures for simulated halos in virial equilibrium. Strong bursts of stellar feedback drive the simulated FOGGIE halos out of virial equilibrium, but the halo gas cannot be accurately described by the standard virial temperature even when in virial equilibrium. Compared to the standard virial temperature, the cooler modified virial temperature implies other effects on halo gas: (i) the thermal gas pressure is lower, (ii) radiative cooling is more efficient, (iii) O vi absorbing gas that traces the virial temperature may be prevalent in halos of a higher mass than expected, (iv) gas mass estimates from X-ray surface brightness profiles may be incorrect, and (v) turbulent motions make an important contribution to the energy balance of a galaxy halo.

Characterizing mass, momentum, energy and metal outflow rates of multi-phase galactic winds in the FIRE-2 cosmological simulations

We characterize mass, momentum, energy and metal outflow rates of multi-phase galactic winds in a suite of FIRE-2 cosmological ‘zoom-in’ simulations from the Feedback in Realistic Environments (FIRE) project. We analyse simulations of low-mass dwarfs, intermediate-mass dwarfs, Milky Way-mass haloes, and high-redshift massive haloes. Consistent with previous work, we find that dwarfs eject about 100 times more gas from their interstellar medium (ISM) than they form in stars, while this mass ‘loading factor’ drops below one in massive galaxies. Most of the mass is carried by the hot phase (&gt;105 K) in massive haloes and the warm phase (103 − 105 K) in dwarfs; cold outflows (&lt;103 K) are negligible except in high-redshift dwarfs. Energy, momentum and metal loading factors from the ISM are of order unity in dwarfs and significantly lower in more massive haloes. Hot outflows have 2 − 5 × higher specific energy than needed to escape from the gravitational potential of dwarf haloes; indeed, in dwarfs, the mass, momentum, and metal outflow rates increase with radius whereas energy is roughly conserved, indicating swept up halo gas. Burst-averaged mass loading factors tend to be larger during more powerful star formation episodes and when the inner halo is not virialized, but we see effectively no trend with the dense ISM gas fraction. We discuss how our results can guide future controlled numerical experiments that aim to elucidate the key parameters governing galactic winds and the resulting associated preventative feedback.

Galaxy assembly bias and large-scale distribution: a comparison between IllustrisTNG and a semi-analytic model

In this work, we compare large scale structure observables for stellar mass selected samples at z = 0, as predicted by two galaxy models, the hydrodynamical simulation IllustrisTNG and the Santa-Cruz semi-analytic model (SC-SAM). Although both models have been independently calibrated to match observations, rather than each other, we find good agreement between the two models for two-point clustering and galaxy assembly bias signatures. The models also show a qualitatively similar response of occupancy and clustering to secondary halo paramaters other than mass, such as formation history and concentration, although with some quantitative differences. Thus, our results demonstrate that the galaxy-halo relationships in SC-SAM and TNG are quite similar to first order. However, we also find areas in which the models differ. For example, we note a strong correlation between halo gas content and environment in TNG, which is lacking in the SC-SAM, as well as differences in the occupancy predictions for low-mass haloes. Moreover, we show that higher-order statistics, such as cumulants of the density field, help to accurately describe the galaxy distribution and discriminate between models that show degenerate behaviour for two-point statistics. Our results suggest that SAMs are a promising cost-effective and intuitive method for generating mock catalogues for next generation cosmological surveys.

Feedback from Active Galactic Nuclei in Galaxy Groups

The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. Outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source preventing runaway cooling by carving cavities and driving shocks across the medium. The AGN feedback loop is a key feature of all modern galaxy evolution models. Here, we review our knowledge of the AGN feedback process in the specific context of galaxy groups. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling–heating balance. Unlike in more massive halos, the energy that is supplied by the central AGN to the hot intragroup medium can exceed the gravitational binding energy of halo gas particles. We report on the state-of-the-art in observations of the feedback phenomenon and in theoretical models of the heating-cooling balance in galaxy groups. We also describe how our knowledge of the AGN feedback process impacts galaxy evolution models and large-scale baryon distributions. Finally, we discuss how new instrumentation will answer key open questions on the topic.

Constraints on precipitation-limited hot haloes from massive galaxies to galaxy clusters

ABSTRACT We present constraints on a simple analytical model for hot diffuse halo gas, derived from a fit spanning two orders of magnitude in halo mass ($M_{500} \sim 10^{12.5}\!-\!10^{14.5} \, \mathrm{M}_{\odot }$). The model is motivated by the observed prevalence of a precipitation limit, and its main free parameter is the central ratio of gas cooling time-scale to free-fall time-scale (tcool/tff). We use integrated X-ray and thermal Sunyaev–Zel’dovich observations of the environments around massive galaxies, galaxy groups, and clusters, averaged in halo mass bins, and obtain the best-fitting model parameters. We find tcool/tff ∼ 50–110, depending on the model extrapolation beyond the halo virial radius and possibly on biases present in the data sets used in the fitting analysis. The model adequately describes the entire mass range, except for intermediate mass haloes ($M_{500} \sim 10^{13.5} \, \mathrm{M}_{\odot }$) that systematically fall below the model predictions. However, the best fits for tcool/tff substantially exceed the values typically derived from X-ray observations of individual systems (tcool/tff ∼ 10–30). We consider several explanations for those discrepancies, including X-ray selection biases and a potential anticorrelation between X-ray luminosity and the central galaxy’s stellar mass.

Thermodynamic Constraints on the Non-Baryonic Dark Matter Gas Composing Galactic Halos

To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous bodies, their postulated attributes can be tested against classical thermodynamics and the kinetic theory of gas. Macroscopic models are appropriate because these make few assumptions. NBDM–NBDM collisions must be elastic to avoid the generation of light, but this does not permit halo gas temperature to evolve. If no such collisions exist, then the impossible limit of absolute zero would be attainable since the other available energy source, radiation, does not provide energy to NBDM. The alternative possibility, an undefined temperature, is also inconsistent with basic thermodynamic principles. However, a definable temperature could be attained via collisions with baryons in the intergalactic medium since these deliver kinetic energy to NBDM. In this case, light would be produced since some proportion of baryon collisions are inelastic, thereby rendering the halo detectable. Collisions with baryons are unavoidable, even if NBDM particles are essentially point masses. Note that <0.0001 × the size of a proton is needed to avoid scattering with γ-rays, the shortest wavelength used to study halos. If only elastic collisions exist, NBDM gas would collapse to a tiny, dense volume (zero volume for point masses) during a disturbance—e.g., cosmic rays. NBDM gas should occupy central galactic regions, not halos, since self-gravitating objects are density stratified. In summary, properties of NBDM halos as postulated would result in violations of thermodynamic laws and in a universe unlike that observed.

MUSE Analysis of Gas around Galaxies (MAGG) – II: metal-enriched halo gas around z ∼ 1 galaxies

ABSTRACT We present a study of the metal-enriched cool halo gas traced by Mg ii absorption around 228 galaxies at z ∼ 0.8–1.5 within 28 quasar fields from the MUSE Analysis of Gas around Galaxies survey. We observe no significant evolution in the Mg ii equivalent width versus impact parameter relation and in the Mg ii covering fraction compared to surveys at z ≲ 0.5. The stellar mass, along with distance from galaxy centre, appears to be the dominant factor influencing the Mg ii absorption around galaxies. With a sample that is 90 per cent complete down to a star formation rate of ≈0.1 $\rm M_\odot yr^{-1}$ and up to impact parameters ≈250–350 kpc from quasars, we find that the majority ($67^{+12}_{-15}$ per cent or 14/21) of the Mg ii absorption systems are associated with more than one galaxy. The complex distribution of metals in these richer environments adds substantial scatter to previously reported correlations. Multiple galaxy associations show on average five times stronger absorption and three times higher covering fraction within twice the virial radius than isolated galaxies. The dependence of Mg ii absorption on galaxy properties disfavours the scenario in which a widespread intragroup medium dominates the observed absorption. This leaves instead gravitational interactions among group members or hydrodynamic interactions of the galaxy haloes with the intragroup medium as favoured mechanisms to explain the observed enhancement in the Mg ii absorption strength and cross-section in rich environments.