Hot Halo Research Articles

Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream∗

Abstract We present the first measurements of [Fe/H] and abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31's giant stellar stream (GSS). The spectroscopic observations, obtained at a projected distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H] measurements, including 21 stars with measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31's halo are relatively metal rich compared to stars in the Milky Way’s inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances . The stars in this field are α-enhanced at lower metallicities, while decreases with increasing [Fe/H] above metallicities of [Fe/H] ≳ −0.9. Three kinematical components have been previously identified in this field: the GSS, a second kinematically cold feature of unknown origin, and M31's kinematically hot halo. We compare probabilistic [Fe/H] and distribution functions for each of the components. The GSS and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal poor. Although the current sample sizes are small, a comparison of the abundances of stars in the GSS field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31's existing dSphs or the dEs NGC 147 and NGC 185.

The Impact of Enhanced Halo Resolution on the Simulated Circumgalactic Medium

Traditional cosmological hydrodynamics simulations fail to spatially resolve the circumgalatic medium (CGM), the reservoir of tenuous gas surrounding a galaxy and extending to its virial radius. We introduce the technique of Enhanced Halo Resolution (EHR), enabling more realistic physical modeling of the simulated CGM by consistently forcing gas refinement to smaller scales throughout the virial halo of a simulated galaxy. We investigate the effects of EHR in the Tempest simulations, a suite of Enzo-based cosmological zoom simulations following the evolution of an L* galaxy, resolving spatial scales of 500 comoving pc out to 100 comoving kpc in galactocentric radius. Among its many effects, EHR (1) changes the thermal balance of the CGM, increasing its cool gas content and decreasing its warm/hot gas content; (2) preserves cool gas structures for longer periods; and (3) enables these cool clouds to exist at progressively smaller size scales. Observationally, this results in a boost in "low ions" like H I and a drop in "high ions" like O VI throughout the CGM. These effects of EHR do not converge in the Tempest simulations, but extrapolating these trends suggests that the CGM in reality is a mist consisting of ubiquitous, small, long-lived, cool clouds suspended in a hot medium at the virial temperature of the halo. Additionally, we explore the physical mechanisms to explain why EHR produces the above effects, proposing that it works both by (1) better sampling the distribution of CGM phases enabling runaway cooling in the denser, cooler tail of the phase distribution; and (2) preventing cool gas clouds from artificially mixing with the ambient hot halo and evaporating. Evidence is found for both EHR mechanisms occurring in the Tempest simulations.

Living with Neighbors. I. Observational Clues to Hydrodynamic Impact of Neighboring Galaxies on Star Formation

Abstract Galaxies in pairs show enhanced star formation (SF) compared to their counterparts in isolation, which is often explained by the tidal effect of neighboring galaxies. Recent observations, however, reported that galaxies paired with early-type neighbors do not undergo the SF enhancement. Here we revisit the influence of neighbors using a large sample of paired galaxies from the Sloan Digital Sky Survey and a carefully constructed control sample of isolated counterparts. We find that star-forming neighbors enhance SF, and even more so for more star-forming (and closer) neighbors, which can be attributed to collisions of interstellar medium (ISM) leading to SF. We further find that, contrary to the anticipated tidal effect, quiescent neighbors quench SF, and even more so for more quiescent (and closer) neighbors. This seems to be due to removal of gas reservoirs via ram pressure stripping and gas accretion cut off by hot gas halos of quiescent neighbors, on top of their paucity of ISM to collide to form stars. Our findings, especially the intimate connection of SF to the status and strength of neighbors’ SF, imply that the hydrodynamic mechanisms, along with the tidal effect, play a crucial role during the early phase of galactic interactions.

Merger signatures in low excitation radio galaxies

ABSTRACT While no consensus governs our understanding of the origin of low-redshift radio galaxies, the possibility that mergers may trigger accretion from hot cluster halo gas has spurred a recent search for such signatures. Evidence for mergers is at best tenuous, however, and even when found, generates more questions than answers. With scant evidence for minor mergers, some connection to major mergers is found in isolated environments but not where one would expect, i.e. in clusters. We provide an explanation for these recent results by Gordon et al. on the relevance of major mergers in low excitation radio galaxies (LERGs) at low redshift. While LERGs are not the direct result of a merger, we describe how they form in clusters in only a few million years while that time-scale is an order of magnitude longer in field environments. As a result of these different time-scales, the average lifetime of a cluster LERG is estimated at an order of magnitude greater value than for field LERGs. Observing an LERG in the cluster environment, therefore, will tend to occur when greater time has passed since the major merger event that produced its high excitation radio galaxy ancestor, such that fewer signatures of that event remain visible. We provide simple estimates for the fraction of LERGs as a function of environment that are directly related to these time-scales, obtaining a probability of about 7 per cent that field LERGs will show merger signatures and 3 per cent for clusters, showing that theory and observation match if major merger signatures remain visible for a few hundred million years.

Cosmological Simulations of Satellites around Isolated Dwarf Galaxies

Abstract We trace the cosmological origin of satellites around a dwarf galaxy using a very high-resolution (12 pc/h) cosmological hydrodynamic zoom simulation. To realistically describe the formation and evolution of small-mass stellar satellites, our model includes a full baryonic physics treatment including a recipe for UV self-shielding. We find that the majority of their star-forming gas is accreted after reionization, thus the survival of a mini-halo gas to reionization is not an important factor. Instead, the key factor seems to be the ability for a mini-halo to cool its recently accreted gas, which is more efficient in more massive halos. We find that halos in denser environments suffer more mergers, enabling them to grow their mass such that cooling of accreted gas can occur efficiently. Although the host galaxy is only a dwarf galaxy itself, we find that ram pressure is an efficient means by which accreted mini-halos lose their gas content, both by interacting with hot halo gas but also in direct collisions with the gas disk of the host. The satellites are also disrupted by the tidal forces near the center of the host galaxy. In summary, our results suggest that the characteristics of satellites are mainly determined by their ability to efficiently cool gas that is accreted in the redshift range of z = 3–5, prior to their infall into the host galaxy.

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models (Soderblom 2010) required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting (Gallart et al. 2005), till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission (Brown et al. 2018) have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo (Babusiaux et al. 2018), which blue-sequence has been linked to a major accretion event (Haywood et al. 2018, Helmi et al. 2018). Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation (Brook et al. 2012), these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.

Electron Energy Partition across Interplanetary Shocks. I. Methodology and Data Product.

Analyses of 15,314 electron velocity distribution functions (VDFs) within ±2 hr of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 au are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar VDF, while both the halo and beam/strahl components were best fit to bi-kappa VDF. This is the first statistical study to show that the core electron distribution is better fit to a self-similar VDF than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The ranges of values defined by the lower and upper quartiles for the kappa exponents are κec ~ 5.40–10.2 for the core, κeh ~ 3.58–5.34 for the halo, and κeb ~ 3.40–5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents is sec ~ 2.00–2.04, and those of asymmetric bi-self-similar core exponents are pec ~ 2.20–4.00 for the parallel exponent and qec ~ 2.00–2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.

G.A.S.

Context. Star formation in galaxies is inefficient, and understanding how star formation is regulated in galaxies is one of the most fundamental challenges of contemporary astrophysics. Radiative cooling, feedback from supernovae and active galactic nuclei (AGN), and large-scale dynamics and dissipation of turbulent energy act over various time and spatial scales and all regulate star formation in a complex gas cycle. Aims. This paper presents the physics implemented in a new semi-analytical model of galaxy formation and evolution called the Galaxy Assembler from dark-matter Simulation (G.A.S.). Methods. The fundamental underpinning of our new model is the development of a multiphase interstellar medium (ISM) in which energy produced by supernovae and AGN maintains an equilibrium between a diffuse, hot, and stable gas and a cooler, clumpy, and low-volume filling factor gas. The hot gas is susceptible to thermal and dynamical instabilities. We include a description of how turbulence leads to the formation of giant molecular clouds through an inertial turbulent energy cascade, assuming a constant kinetic energy transfer per unit volume. We explicitly modelled the evolution of the velocity dispersion at different scales of the cascade and accounted for thermal instabilities in the hot halo gas. Thermal instabilities effectively reduce the impact of radiative cooling and moderates accretion rates onto galaxies, and in particular, for those residing in massive haloes. Results. We show that rapid and multiple exchanges between diffuse and unstable gas phases strongly regulates star formation rates in galaxies because only a small fraction of the unstable gas is forming stars. We checked that the characteristic timescales describing the gas cycle, gas depletion timescale, and star-forming laws at different scales are in good agreement with observations. For high-mass haloes and galaxies, cooling is naturally regulated by the growth of thermal instabilities, so we do not need to implement strong AGN feedback in this model. Our results are also in good agreement with the observed stellar mass function from z ≃ 6.0 to z ≃ 0.5. Conclusion. Our model offers the flexibility to test the impact of various physical processes on the regulation of star formation on a representative population of galaxies across cosmic times. Thermal instabilities and the cascade of turbulent energy in the dense gas phase introduce a delay between gas accretion and star formation, which keeps galaxy growth inefficient in the early Universe. The main results presented in this paper, such as stellar mass functions, are available in the GALAKSIENN library.

A new method to quantify environment and model ram-pressure stripping in N-body simulations

We introduce a Local Background Environment (LBE) estimator that can be measured in and around every galaxy or its dark matter subhalo in high-resolution cosmological simulations. The LBE is designed to capture the influence of environmental effects such as ram-pressure stripping on the formation and evolution of galaxies in semi-analytical models. We define the LBE directly from the particle data within an adaptive spherical shell, and devise a Gaussian mixture estimator to separate background particles from previously unidentified subhalo particles. Analyzing the LBE properties, we find that the LBE of satellite galaxies is not at rest with respect to their host halo, in contrast to typical assumptions. The orientations of the velocities of a subhalo and its LBE are well aligned in the outer infall regions of haloes, but decorrelated near halo center. Significantly, there is no abrupt change in LBE velocity or density at the halo virial radius. This suggests that stripping should also happen beyond this radius. Therefore, we use the time-evolving LBE of galaxies to develop a method to better account for ram-pressure stripping within the Munich semi-analytical model, L-Galaxies. Overall, our new approach results in a significant increase in gas stripping across cosmic time. Central galaxies, as well as satellites beyond the virial radius, can lose a significant fraction of their hot halo gas. As a result, the gas fractions and star formation rates of satellite galaxies are suppressed relative to the fiducial model, although the stellar masses and global stellar mass functions are largely unchanged.

Cluster induced quenching of galaxies in the massive cluster XMMXCS J2215.9−1738 at z ∼ 1.5 traced by enhanced metallicities inside half R200

Aims. Cluster environments at z < 0.5 were found to increase the gas metallicities of galaxies which enter inner regions of the clusters where the density of the intracluster medium is high enough to remove their hot halo gas by ram-pressure stripping effects and to stop the inflow of pristine gas. To extend these studies to z > 1, the most massive clusters known at these redshifts are the sites where these environmental effects should be more pronounced and more easily observed with present day telescopes. Methods. We explore the massive cluster XMMXCS J2215.9−1738 at z ∼ 1.5 with KMOS spectroscopy of Hα and [N II] λ 6584 covering a region that corresponds to about one virial radius. Using published spectroscopic redshifts of 108 galaxies in and around the cluster we computed the location of galaxies in the projected velocity-versus-position phase-space to separate our cluster sample into a virialized region of objects accreted longer ago (roughly inside half R200) and a region of infalling galaxies. We measured oxygen abundances for ten cluster galaxies with detected [N II] λ 6584 lines in the individual galaxy spectra and compared the mass–metallicity relation of the galaxies inside half R200 with the infalling galaxies and a field sample at similar redshifts. Results. We find that the oxygen abundances of individual z ∼ 1.5 star-forming cluster galaxies inside half R200 are comparable, at the respective stellar mass, to the higher local SDSS metallicity values. We compare our measurements with a field galaxy sample from the KMOS3D survey at similar redshifts. We find that the [N II] λ 6584/Hα line ratios inside half R200 are higher by 0.2 dex and that the resultant metallicities of the galaxies in the inner part of the cluster are higher by about 0.1 dex, at a given mass, than the metallicities of infalling galaxies and of field galaxies at z ∼ 1.5. The enhanced metallicities of cluster galaxies at z ∼ 1.5 inside 0.5R200 indicate that the density of the intracluster medium in this massive cluster becomes high enough toward the cluster center such that the ram pressure exceeds the restoring pressure of the hot gas reservoir of cluster galaxies. This can remove the gas reservoir and initiate quenching; although the galaxies continue to form stars, albeit at slightly lower rates, using the available cold gas in the disk which is not stripped.

HALO7D I. The Line-of-sight Velocities of Distant Main-sequence Stars in the Milky Way Halo

Abstract The Halo Assembly in Lambda-CDM: Observations in 7 Dimensions (HALO7D) data set consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope–measured proper motions of Milky Way halo main-sequence turnoff stars in the CANDELS fields. In this paper, we present the spectroscopic component of this data set and discuss target selection, observing strategy, and survey properties. We present a new method of measuring line-of-sight (LOS) velocities by combining multiple spectroscopic observations of a given star, utilizing Bayesian hierarchical modeling. We present the LOS velocity distributions of the four HALO7D fields and estimate their means and dispersions. All of the LOS distributions are dominated by the “hot halo”: none of our fields are dominated by substructure that is kinematically cold in the LOS velocity component. Our estimates of the LOS velocity dispersions are consistent across the different fields, and these estimates are consistent with studies using other types of tracers. To complement our observations, we perform mock HALO7D surveys using the synthetic survey software Galaxia to “observe” the Bullock & Johnston accreted stellar halos. Based on these simulated data sets, the consistent LOS velocity distributions across the four HALO7D fields indicate that the HALO7D sample is dominated by stars from the same massive (or few relatively massive) accretion event(s).

The evolution of SMBH spin and AGN luminosities for z < 6 within a semi-analytic model of galaxy formation

Understanding how Active Galactic Nuclei (AGN) evolve through cosmic time allows us to probe the physical processes that control their evolution. We use an updated model for the evolution of masses and spins of supermassive black holes (SMBHs), coupled to the latest version of the semi-analytical model of galaxy formation GALFORM using the Planck cosmology and a high resolution Millennium style dark matter simulation to make predictions for AGN and SMBH properties for $0 < z < 6$. We compare the model to the observed black hole mass function and the SMBH versus galaxy bulge mass relation at $z=0$, and compare the predicted bolometric, hard X-ray, soft X-ray and optical AGN luminosity functions to observations at $z < 6$, and find that the model is in good agreement with the observations. The model predicts that at $z<2$ and $L_{\mathrm{bol}} < 10^{43} \mathrm{ergs^{-1}}$, the AGN luminosity function is dominated by objects accreting in an Advection Dominated Accretion Flow (ADAF) disc state, while at higher redshifts and higher luminosities the dominant contribution is from objects accreting via a thin disc or at super-Eddington rates. The model also predicts that the AGN luminosity function at $z<3$ and $L_{\mathrm{bol}} < 10^{44} \mathrm{ergs^{-1}}$ is dominated by the contribution from AGN fuelled by quiescent hot halo accretion, while at higher luminosities and higher redshifts, the AGN luminosity function is dominated by the contribution from AGN fuelled by starbursts triggered by disc instabilities. We employ this model to predict the evolution of SMBH masses, Eddington ratios, and spins, finding that the median SMBH spin evolves very little for $0<z<6$.

Satellite dwarf galaxies: stripped but not quenched

In the Local Group, quenched gas-poor dwarfs galaxies are most often found close to the Milky Way and Andromeda, while star forming gas-rich ones are located at greater distances. This so-called morphology-density relation is often interpreted as the consequence of the ram pressure stripping of the satellites during their interaction with the Milky Way hot halo gas. While this process has been often investigated, self-consistent high resolution simulations were still missing. In this study, we have analysed the impact of both the ram pressure and tidal forces induced by a host galaxy on dwarf models as realistic as possible emerging from cosmological simulations. These models were re-simulated using both a wind tunnel and a moving box technique. The secular mass growth of the central host galaxy, as well as the gas density and temperature profiles of its hot halo have been taken into account. We show that while ram pressure is very efficient at stripping the hot and diffuse gas of the dwarf galaxies, it can remove their cold gas (T &lt; 103 K) only in very specific conditions. Depending on the infall time of the satellites relatively to the build-up stage of the massive host, star formation can thus be prolonged instead of being quenched. This is the direct consequence of the clumpy nature of the cold gas and the thermal pressure the hot gas exerts onto it. We discuss the possibility that the variety in satellite populations among Milky Way-like galaxies reflects their accretion histories.

A high baryon fraction in massive haloes at z ∼ 3

We investigate the baryon content of the circumgalactic medium (CGM) within the virial radius of $M_h \sim 10^{12} \; M_\odot$ haloes at z ~ 3, by modelling the surface brightness profile of the giant Ly$\alpha$ nebulae recently discovered by MUSE around bright QSOs at this redshift. We initially assume fluorescent emission from cold photo-ionized gas confined by the pressure of a hot halo. Acceptable CGM baryon fractions (equal or smaller than the cosmological value) require that the cold gas occupies $\lesssim$ 1% of the volume, but is about as massive as the hot gas. CGM baryon fractions as low as 30% of the cosmic value, as predicted by some strongly ejective feedback models at this redshift, are not easy to reconcile with observations, under our assumptions, unless both the QSO-hosting haloes at $z\sim3$ are more massive than recent BOSS estimates based on clustering and the photo-ionized gas is colder than expected in a standard QSO ionizing radiation field. We also consider the option that the emission is dominated by photons scattered from the QSO broad line region. In this scenario, a very stringent lower limit to the baryon fraction can be obtained under the extreme assumption of optically thin scattering. We infer in this case a baryon fraction of at least 70% of the cosmic value, for fiducial parameters. Lower values require halo masses or gas temperatures different than expected, or that some mechanism keeps the cold gas systematically over-pressured with respect to the ambient medium.

Exploring the physical properties of the cool circumgalactic medium with a semi-analytic model

We develop a semi-analytic model to explore the physical properties of cool pressure-confined circumgalactic clouds with mass ranging from $10$ to $10^{8} \, \rm M_{\odot}$ in a hot diffuse halo. We consider physical effects that control the motion and mass loss of the clouds, and estimate the lifetime and the observed properties of individual cool gas clouds inferred from the CLOUDY simulation. Our results show that the cool pressure-confined gas clouds have physical properties consistent with absorption line systems with neutral hydrogen column densities $N_{\rm HI}\geq10^{18.5} \rm cm^{-2}$ such as strong metal absorbers, sub-DLAs, and DLAs. The cool circumgalactic clouds are transient due to evaporation and recycling and therefore a constant replenishment is needed to maintain the cool CGM. We further model the ensemble properties of the cool CGM with clouds originated from outflows, inflows, or/and in-situ formation with a range of initial cloud mass function and velocity distribution. We find that only with a certain combination of parameters, an outflow model can broadly reproduce three cool gas properties around star-forming galaxies simultaneously: the spatial distribution, down-the-barrel outflow absorption, and gas velocity dispersion. Both a constant insitu model and gas inflow model can reproduce the observed covering fractions of high $\rm N_{HI}$ gas around passive galaxies but they fail to reproduce sufficient number of low $\rm N_{HI}$ systems. The limitations and the failures of the current models are discussed. Our results illustrate that semi-analytic modeling is a promising tool to understand the physics of the cool CGM which is usually unresolved by state-of-the-art cosmological hydrodynamic simulations.

The MASSIVE survey – XI. What drives the molecular gas properties of early-type galaxies

ABSTRACT In this paper, we study the molecular gas content of a representative sample of 67 of the most massive early-type galaxies (ETGs) in the local universe, drawn uniformly from the MASSIVE survey. We present new Institut de Radioastronomie Millimétrique (IRAM) 30-m telescope observations of 30 of these galaxies, allowing us to probe the molecular gas content of the entire sample to a fixed molecular-to-stellar mass fraction of 0.1 per cent. The total detection rate in this representative sample is 25$^{+5.9}_{-4.4}$ per cent, and by combining the MASSIVE and atlas3D molecular gas surveys, we find a joint detection rate of 22.4$^{+2.4}_{-2.1}$ per cent. This detection rate seems to be independent of galaxy mass, size, position on the Fundamental Plane, and local environment. We show here for the first time that true slow rotators can host molecular gas reservoirs, but the rate at which they do so is significantly lower than for fast rotators. Objects with a higher velocity dispersion at fixed mass (a higher kinematic bulge fraction) are less likely to have detectable molecular gas, and where gas does exist, have lower molecular gas fractions. In addition, satellite galaxies in dense environments have ≈0.6 dex lower molecular gas-to-stellar mass ratios than isolated objects. In order to interpret these results, we created a toy model, which we use to constrain the origin of the gas in these systems. We are able to derive an independent estimate of the gas-rich merger rate in the low-redshift universe. These gas-rich mergers appear to dominate the supply of gas to ETGs, but stellar mass loss, hot halo cooling, and transformation of spiral galaxies also play a secondary role.

A comparison between semi-analytical gas cooling models and cosmological hydrodynamical simulations

We compare the mass cooling rates and cumulative cooled-down masses predicted by several semi-analytical (SA) cooling models with cosmological hydrodynamical simulations performed using the AREPO code (ignoring processes such as feedback and chemical enrichment). The SA cooling models are the new GALFORM cooling model introduced in Hou et al. (2017), along with two earlier GALFORM cooling models and the L-GALAXIES and MORGANA cooling models. We find that the predictions of the new GALFORM cooling model are generally in best agreement with the simulations. For halos with $M_{\rm halo}\lesssim 3\times 10^{11}\,{\rm M}_{\odot}$, the SA models predict that the timescale for radiative cooling is shorter than or comparable to the gravitational infall timescale. Even though SA models assume that gas falls onto galaxies from a spherical gas halo, while the simulations show that the cold gas is accreted through filaments, both methods predict similar mass cooling rates, because in both cases the gas accretion occurs on similar timescales. For halos with $M_{\rm halo}\gtrsim 10^{12}\,{\rm M}_{\odot}$, gas in the simulations typically cools from a roughly spherical hot gas halo, as assumed in the SA models, but the halo gas gradually contracts during cooling, leading to compressional heating. SA models ignore this heating, and so overestimate mass cooling rates by factors of a few. At low redshifts halo major mergers or a sequence of successive smaller mergers are seen in the simulations to strongly heat the halo gas and suppress cooling, while mergers at high redshifts do not suppress cooling, because the gas filaments are difficult to heat up. The new SA cooling model best captures these effects.

The origin of the mass scales for maximal star formation efficiency and quenching: the critical role of supernovae

We use the Henriques et al. (2015) version of the Munich galaxy formation model (L-GALAXIES) to investigate why the halo and stellar mass scales above which galaxies are quenched are constant with redshift and coincide with the scale where baryons are most efficiently converted into stars. This model assumes that central galaxies are quenched by AGN feedback when hot halo gas accretes onto a supermassive black hole. Nevertheless, we find that supernova (SN) feedback sets both mass scales. As haloes grow above a threshold mass, SNe can no longer eject material so their hot gas content increases, enhancing the cooling rate onto the central galaxy, its cold gas content, its star formation rate and the growth rate of its central black hole. Strong AGN feedback terminates this short-lived phase by suppressing the fuel supply for star formation. Despite strong evolution of the halo mass - temperature relation, quenching occurs at a redshift-independent halo and stellar mass which coincides with the mass where baryons have been converted into stars with maximal efficiency. These regularities and coincidences are a result of the specific parameters selected by MCMC tuning of the model to fit the observed abundance and passive fraction of galaxies over the redshift range 0<z<3. Thus they are required by the observed evolution of the galaxy population, at least in the context of models of this type.

CHANG-ES

Context. The observation of total and linearly polarized synchrotron radiation of spiral galaxies in the radio continuum reveals the distribution and structure of their magnetic fields. By observing these, information about the proposed dynamo processes that preserve the large-scale magnetic fields in spiral galaxies can be gained. Additionally, by analyzing the synchrotron intensity, the transport processes of cosmic rays into the halo of edge-on spiral galaxies can be investigated. Aims. We analyze the magnetic field geometry and the transport processes of the cosmic rays of the edge-on spiral starburst galaxy NGC 4666 from CHANG-ES radio data in two frequencies; 6 GHz (C-band) and 1.5 GHz (L-band). Supplementary X-ray data are used to investigate the hot gas in NGC 4666. Methods. We determine the radio scale heights of total power emission at both frequencies for this galaxy. We show the magnetic field orientations derived from the polarization data. Using rotation measure (RM) synthesis we further study the behavior of the RM values along the disk in C-band to investigate the large-scale magnetic-field pattern. We use the revised equipartition formula to calculate a map of the magnetic field strength. Furthermore, we model the processes of cosmic-ray transport into the halo with the 1D SPINNAKER model. Results. The extended radio halo of NGC 4666 is box-shaped and is probably produced by the previously observed supernova-driven superwind. This is supported by our finding of an advective cosmic-ray transport such as that expected for a galactic wind. The scale-height analysis revealed an asymmetric halo above and below the disk as well as between the two sides of the major axis. A central point source as well as a bubble structure is seen in the radio data for the first time. Our X-ray data show a box-shaped hot halo around NGC 4666 and furthermore confirm the AGN nature of the central source. NGC 4666 has a large-scale X-shaped magnetic field in the halo, as has been observed in other edge-on galaxies. The analysis furthermore revealed that the disk of NGC 4666 shows hints of field reversals along its radius, which is the first detection of this phenomenon in an external galaxy.

An Absence of Radio-loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-mode Feedback Models

Abstract Maintenance-mode feedback from low-accretion-rate active galactic nuclei (AGNs), manifesting itself observationally through radio-loudness, is invoked in all cosmological galaxy formation models as a mechanism that prevents excessive star formation in massive galaxies (M * ≳ 3 × 1010 M ⊙). We demonstrate that at a fixed mass the incidence of radio-loud (RL) AGNs (L &gt; 1023 W Hz−1) identified in the Faint Images of the Radio Sky at Twenty centimeter and NRAO Very Large Array Sky Survey radio surveys among a large sample of quiescent (non-star-forming) galaxies selected from the Sloan Digital Sky Survey is much higher in geometrically round galaxies than in geometrically flat, disk-like galaxies. As found previously, the RL AGN fraction increases steeply with stellar velocity dispersion σ * and stellar mass, but even at a fixed velocity dispersion of 200–250 km s−1 this fraction increases from 0.3% for flat galaxies (projected axis ratio of q &lt; 0.4) to 5% for round galaxies (q &gt; 0.8). We rule out the hypothesis that this strong trend is due to projection effects in the measured velocity dispersion. The large fraction of RL AGNs in massive, round galaxies is consistent with the hypothesis that such AGNs deposit energy into their hot gaseous halos, preventing cooling and star formation. However, the absence of such AGNs in disk-like quiescent galaxies—most of which are not satellites in massive clusters, raises important questions. Is maintenance-mode feedback a generally valid explanation for quiescence? If so, how does that feedback avoid manifesting at least occasionally as an RL galaxy?