arXiv subjects

Eliot Quataert

Publications and source records attributed to Eliot Quataert.

At least 163 records · Page 9Linked to original sources

arXiv2021

Exploring the epoch of hydrogen reionization using FRBs

We describe three different methods for exploring the hydrogen reionization epoch using fast radio bursts (FRBs) and provide arguments for the existence of FRBs at high redshift (z). The simplest way, observationally, is to determine the maximum dispersion measure (DM$_{\rm max}$) of FRBs for an ensemble that includes bursts during the reionization. The DM$_{\rm max}$ provides information regarding reionization much like the optical depth of the CMB to Thomson scattering does, and it has the potential to be more accurate than constraints from Planck, if DM$_{\rm max}$ can be measured to a precision better than 500 $\mbox{pc cm}^{-3}$. Another method is to measure redshifts of about 40 FRBs between z of 6-10 with$\sim10\%$ accuracy to obtain the average electron density in 4 different z-bins with $\sim4\%$ accuracy. These two methods don't require knowledge of the FRB luminosity function and its possible redshift evolution. Finally, we show that the reionization history is reflected in the number of FRBs per unit DM, given a fluence limited survey of FRBs that includes bursts during the reionization epoch; we show using FIRE simulations that the contributions to DM from the FRB host galaxy $\&$ CGM during the reionization era is a small fraction of the observed DM. This third method requires no redshift information but does require knowledge of the FRB luminosity function.

astro-ph.CO↗

arXiv2021

A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1

HR 6819 is a bright ($V=5.36$), blue star recently proposed to be a triple containing a detached black hole (BH). We show that the system is a binary and does not contain a BH. Using spectral decomposition, we disentangle the observed composite spectra into two components: a rapidly rotating Be star and a slowly rotating B star with low surface gravity $(\log g \approx 2.75)$. Both stars show periodic radial velocity (RV) variability, but the RV semi-amplitude of the B star's orbit is $K_{\rm B}= (62.7 \pm 1)\,\rm km\,s^{-1}$, while that of the Be star is only $K_{\rm Be} = (4.5\pm 2)\,\rm km\,s^{-1}$. This implies that the B star is less massive by at least a factor of 10. The surface abundances of the B star bear imprints of CNO burning. We argue that the B star is a bloated, recently stripped helium star with mass $\approx 0.5\,M_{\odot}$ that is currently contracting to become a hot subdwarf. The orbital motion of the Be star obviates the need for a BH to explain the B star's motion. A stripped-star model reproduces the observed luminosity of the system, while a normal star with the B star's temperature and gravity would be more than 10 times too luminous. HR 6819 and the binary LB-1 probably formed through similar channels. We use MESA models to investigate their evolutionary history, finding that they likely formed from intermediate-mass ($3-7\,M_{\odot}$) primaries stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB binaries such as $ϕ$ Persei. The lifetime of their current evolutionary phase is on average $2\times 10^5$ years, of order half a percent of the total lifetime of the Be phase. This implies that many Be stars have hot subdwarf and white dwarf companions, and that a substantial fraction ($20-100\%$) of field Be stars form through accretion of material from a binary companion.

astro-ph.SR↗

arXiv2021

The impact of astrophysical dust grains on the confinement of cosmic rays

We argue that charged dust grains could significantly impact the confinement and transport of galactic cosmic rays. For sub-GeV to ~1000GeV cosmic rays, small-scale parallel Alfvén waves, which isotropize cosmic rays through gyro-resonant interactions, are also gyro-resonant with charged grains. If the dust is nearly stationary, as in the bulk of the interstellar medium, Alfvén waves are damped by dust. This will reduce the amplitude of Alfvén waves produced by the cosmic rays through the streaming instability, thus enhancing cosmic-ray transport. In well-ionized regions, the dust damping rate is larger by a factor of ~10 than other mechanisms that damp parallel Alfvén waves at the scales relevant for ~GeV cosmic rays, suggesting that dust could play a key role in regulating cosmic-ray transport. In astrophysical situations in which the dust moves through the gas with super-Alfvénic velocities, Alfvén waves are rendered unstable, which could directly scatter cosmic rays. This interaction has the potential to create a strong feedback mechanism where dust, driven through the gas by radiation pressure, then strongly enhances the confinement of cosmic rays, increasing their capacity to drive outflows. This mechanism may act in the circumgalactic medium around star-forming galaxies and active galactic nuclei.

astro-ph.HE↗

arXiv2020

Virialization of the inner CGM in the FIRE simulations and implications for galaxy discs, star formation and feedback

We use the FIRE-2 cosmological simulations to study the formation of a quasi-static, virial-temperature gas phase in the circumgalactic medium (CGM) at redshifts 0<z<5, and how the formation of this virialized phase affects the evolution of galactic discs. We demonstrate that when the halo mass crosses ~10^12 M_sun, the cooling time of shocked gas in the inner CGM (~0.1 R_vir, where R_vir is the virial radius) exceeds the local free-fall time. The inner CGM then experiences a transition from on average sub-virial temperatures (T<<T_vir), large pressure fluctuations and supersonic inflow/outflow velocities, to virial temperatures (T~T_vir), uniform pressures and subsonic velocities. This transition occurs when the outer CGM (~0.5 R_vir) is already subsonic and has a temperature ~T_vir, indicating that the longer cooling times at large radii allow the outer CGM to virialize at lower halo masses than the inner CGM. This outside-in CGM virialization scenario is in contrast with inside-out scenarios commonly envisioned based on more idealized simulations. We demonstrate that inner CGM virialization coincides with abrupt changes in the central galaxy and its stellar feedback: the galaxy settles into a stable rotating disc, star formation transitions from `bursty' to `steady,' and stellar-driven galaxy-scale outflows are suppressed. Our results thus suggest that CGM virialization is initially associated with the formation of rotation-dominated thin galactic discs, rather than with the quenching of star formation as often assumed.

astro-ph.GA↗

arXiv2020

The Effect of Jet-Ejecta Interaction on the Viewing Angle Dependence of Kilonova Light Curves

The merger of two neutron stars produces an outflow of radioactive heavy nuclei. Within a second of merger, the central remnant is expected to also launch a relativistic jet, which shock-heats and disrupts a portion of the radioactive ejecta. Within a few hours, emission from the radioactive material gives rise to an ultraviolet, optical, and infrared transient (a kilonova). We use the endstates of a suite of 2D relativistic hydrodynamic simulations of jet-ejecta interaction as initial conditions for multi-dimensional Monte Carlo radiation transport simulations of the resulting viewing angle-dependent light curves and spectra starting at 1.5h after merger. We find that on this timescale, jet shock heating does not affect the kilonova emission. However, the jet disruption to the density structure of the ejecta does change the light curves. The jet carves a channel into the otherwise spheroidal ejecta, revealing the hot, inner regions. As seen from near ($\lesssim 30 °$) the jet axis, the kilonova is brighter by a factor of a few and bluer. The strength of this effect depends on the jet parameters, since the light curves of more heavily disrupted ejecta are more strongly affected. The light curves and spectra are also more heavily modified in the ultraviolet than in the optical.

astro-ph.HE↗

arXiv2020

Black widow formation by pulsar irradiation and sustained magnetic braking

Black widows are millisecond pulsars with low-mass companions, a few per cent the mass of the sun, on orbits of several hours. These companions are presumably the remnants of main sequence stars that lost their mass through a combination of Roche-lobe overflow and ablation by the host pulsar's high-energy radiation. While ablation itself is too weak to significantly reduce the mass of the companion star, the ablated wind couples to its magnetic field, removes orbital angular momentum, and thus maintains stable Roche-lobe overflow. We use the MESA stellar evolution code, complemented by analytic estimates, to track initially main sequence companions as they are reduced to a fraction of their original mass by this ablation-driven magnetic braking. We argue that magnetic braking remains effective even for low-mass companions. A key ingredient of our model is that the irradiating luminosity of the pulsar $L_{\rm irr}$ deposits energy in the companion's atmosphere and thereby slows down its Kelvin-Helmholtz cooling. We find that the high-energy luminosities measured by Fermi $L_{\rm irr}=0.1-3\,{\rm L}_\odot$ can explain the span of black widow orbital periods. The same $L_{\rm irr}$ range reproduces the companions' night-side temperatures, which cluster around 3000 K, as inferred from optical light curves.

astro-ph.HE↗

arXiv2020

Pressure balance in the multiphase ISM of cosmologically simulated disk galaxies

Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyze how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the midplane. Bulk flows (e.g., inflows and fountains) are important at a few disk scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total midplane pressure is well-predicted by the weight of the disk gas, and we show that it also scales linearly with the star formation rate surface density (Sigma_SFR). These results support the notion that the Kennicutt-Schmidt relation arises because Sigma_SFR and the gas surface density (Sigma_g) are connected via the ISM midplane pressure.

astro-ph.GA↗

arXiv2020

Virial shocks are suppressed in cosmic ray-dominated galaxy halos

We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In massive ($M_{\rm halo} \gtrsim 10^{11}\,M_{\odot}$), low-redshift ($z\lesssim 1-2$) halos, which are expected to form "hot halos" with slowly-cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure-confined to over-dense clumps, embedded in low-density, volume-filling hot gas whose cooling time is much longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius ($\approx R_{\rm vir}$), and the shock is quasi-spherical. With CRs, we previously argued that halos in this particular mass and redshift range build up CR-pressure-dominated gaseous halos. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond $R_\mathrm{vir}$. Rapid cooling also maintains sub-virial temperatures in the inflowing gas within $\sim R_\mathrm{vir}$.

astro-ph.GA↗

arXiv2020

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

We present the escape fraction of hydrogen ionizing photons (f_esc) from a sample of 34 high-resolution cosmological zoom-in simulations of galaxies at z>5 in the Feedback in Realistic Environments project, post-processed with a Monte Carlo radiative transfer code for ionizing radiation. Our sample consists of 8500 halos in M_vir~10^8--10^{12} M_sun (M_star~10^4--10^{10} M_sun) at z=5--12. We find the sample average increases with halo mass for M_vir~10^8--10^{9.5} M_sun, becomes nearly constant for M_vir~10^{9.5}--10^{11} M_sun, and decreases at M_vir>10^{11} M_sun. Equivalently, increases with stellar mass up to M_star~10^8 M_sun and decreases at higher masses. Even applying single-star stellar population synthesis models, we find a moderate ~0.2 for galaxies at M_star~10^8 M_sun. Nearly half of the escaped ionizing photons come from stars 1--3 Myr old and the rest from stars 3--10 Myr old. Binaries only have a modest effect, boosting by ~25--35% and the number of escaped photons by 60--80%. Most leaked ionizing photons are from vigorously star-forming regions that usually contain a feedback-driven kpc-scale superbubble surrounded by a dense shell. The shell is forming stars while accelerated, so new stars formed earlier in the shell are already inside the shell. Young stars in the bubble and near the edge of the shell can fully ionize some low-column-density paths pre-cleared by feedback, allowing a large fraction of their ionizing photons to escape. The decrease of at the high-mass end is due to dust attenuation, while at the low-mass end, decreases owing to inefficient star formation (and hence feedback). At fixed mass, tends to increase with redshift. Our simulations produce sufficient ionizing photons for cosmic reionization.

astro-ph.GA↗

arXiv2020

A model for the formation of stellar associations and clusters from giant molecular clouds

We present a large suite of MHD simulations of turbulent, star-forming giant molecular clouds(GMCs) with stellar feedback, extending previous work by simulating 10 different random realizations for each point in the parameter space of cloud mass and size. It is found that oncethe clouds disperse due to stellar feedback, both self-gravitating star clusters and unbound stars generally remain, which arise from the same underlying continuum of substructured stellar density, ie. the hierarchical cluster formation scenario. The fraction of stars that are born within gravitationally-bound star clusters is related to the overall cloud star formation efficiency set by stellar feedback, but has significant scatter due to stochastic variations in the small-scale details of the star-forming gas flow. We use our numerical results to calibrate a model for mapping the bulk properties (mass, size, and metallicity) of self-gravitating GMCs onto the star cluster populations they form, expressed statistically in terms of cloud-level distributions. Synthesizing cluster catalogues from an observed GMC catalogue in M83, we find that this model predicts initial star cluster masses and sizes that are in good agreement with observations, using only standard IMF and stellar evolution models as inputs for feedback. Within our model, the ratio of the strength of gravity to stellar feedback is the key parameter setting the masses of star clusters, and of the various feedback channels direct stellar radiation(photon momentum and photoionization) is the most important on GMC scales.

astro-ph.GA↗

arXiv2020

Properties of the Circumgalactic Medium in Cosmic Ray-Dominated Galaxy Halos

We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass halos hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, collisional and streaming losses, with constant parallel diffusivity $κ\sim3\times10^{29}\,\mathrm{cm^2\ s^{-1}}$ chosen to match $γ$-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass halos at $z\lesssim 1-2$. The gas in these "CR-dominated" halos differs significantly from runs without CRs: the gas is primarily cool (a few $\sim10^{4}\,$K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the "low" and "mid" ions in this diffuse cool gas is dominated by photo-ionization, with O VI columns $\gtrsim 10^{14.5}\,\mathrm{cm^{-2}}$ at distances $\gtrsim 150\,\mathrm{kpc}$. CR and thermal gas pressure are locally anti-correlated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same halos are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermal pressure balancing gravity, collisional ionization dominates, O VI columns are lower and Ne VIII higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.

astro-ph.GA↗

arXiv2020

The Impact of Type Ia Supernovae in Quiescent Galaxies: I. Formation of the Multiphase Interstellar medium

A cool phase of the interstellar medium has been observed in many giant elliptical galaxies, but its origin remains unclear. We propose that uneven heating from Type Ia supernovae (SNe Ia), together with radiative cooling, can lead to the formation of the cool phase. The basic idea is that since SNe Ia explode randomly, gas parcels which are not directly heated by SN shocks will cool, forming multiphase gas. We run a series of idealized high-resolution numerical simulations, and find that cool gas develops even when the overall SNe heating rate $H$ exceeds the cooling rate $C$ by a factor as large as 1.4. We also find that the time for multiphase gas development depends on the gas temperature. When the medium has a temperature $T = 3\times 10^6$ K, the cool phase forms within one cooling time \tc; however, the cool phase formation is delayed to a few times \tc\ for higher temperatures. The main reason for the delay is turbulent mixing. Cool gas formed this way would naturally have a metallicity lower than that of the hot medium. For constant $H/C$, there is more turbulent mixing for higher temperature gas. We note that this mechanism of producing cool gas cannot be captured in cosmological simulations, which usually fail to resolve individual SN remnants.

astro-ph.GA↗

arXiv2020

The Impact of Type Ia Supernovae in Quiescent Galaxies: II. Energetics and Turbulence

Type Ia supernovae (SNe Ia) provide unique and important feedback in quiescent galaxies, but their impact has been underappreciated. In this paper, we analyze a series of high-resolution simulations to examine the energetics and turbulence of the medium under SNe Ia. We find that when SN remnants are resolved, their effects differ distinctly from a volumetric heating term, as is commonly assumed in unresolved simulations. First, the net heating is significantly higher than expected, by 30$\pm$10\% per cooling time. This is because a large fraction of the medium is pushed into lower densities which cool inefficiently. Second, the medium is turbulent; the root-mean-squared (RMS) velocity of the gas to 20-50 km s$^{-1}$ on a driving scale of tens of parsec. The velocity field of the medium is dominated by compressional modes, which are larger than the solenoidal components by a factor of 3-7. Third, the hot gas has a very broad density distribution. The ratio between the density fluctuations and the RMS Mach number, parameterized as $b$, is 2-20. This is in contrast to previous simulations of turbulent media, which have found $b\lesssim$ 1. The reason for the difference is mainly caused by the \textit{localized} heating of SNe Ia, which creates a large density contrast. Last, the typical length scale of a density fluctuation grows with time, forming increasingly larger bubbles and filamentary ridges. These underlying density fluctuations need to be included when X-ray observations are interpreted.

astro-ph.GA↗

arXiv2020

Ab Initio Horizon-Scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds

We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) stellar winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR stellar winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.

astro-ph.HE↗

arXiv2020

The Zwicky Transient Facility Census of the Local Universe I: Systematic search for Calcium rich gap transients reveal three related spectroscopic sub-classes

(Abridged) Using the Zwicky Transient Facility alert stream, we are conducting a large campaign to spectroscopically classify all transients occurring in galaxies in the Census of the Local Universe (CLU) catalog. The aim of the experiment is to construct a spectroscopically complete, volume-limited sample of transients coincident within 100" of CLU galaxies out to 200 Mpc, and to a depth of 20 mag. We describe the survey design and spectroscopic completeness from the first 16 months of operations. We present results from a systematic search for Calcium rich gap transients in the sample of 22 low luminosity (peak absolute magnitude $M > -17$), hydrogen poor events found in the experiment (out of 754 spectroscopically classified SNe). We report the detection of eight Calcium rich gap transients, and constrain their volumetric rate to be at least $\approx 15\pm5$% of the SN Ia rate. Combining this sample with ten events from the literature, we find a likely continuum of spectroscopic properties ranging from events with SN Ia-like features (Ca-Ia objects) to SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within the Ca-Ib/c events, we find two populations of events distinguished by their red ($g - r \approx 1.5$ mag) or green ($g - r \approx 0.5$ mag) spectral colors at $r$-band peak, wherein redder events show strong line blanketing signatures, slower light curves, weaker He lines and lower [Ca II]/[O I] in the nebular phase. Together, we find that the spectroscopic continuum, volumetric rates and striking old environments are consistent with the explosive burning of He shells on low mass white dwarfs. We posit that Ca-Ia and red Ca-Ib/c objects are consistent with the double detonation of He shells with high He burning efficiency, while green Ca-Ib/c objects could arise from less efficient He burning scenarios such as detonations in low density He shells or He shell deflagrations.

astro-ph.HE↗

arXiv2020

The Effects of Tilt on the Images of Black Hole Accretion Flows

We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent difference is that the central shadow in the image is noticeably eccentric in tilted models, with the ring of emission relatively unchanged. Applying this procedure to M87 with a viewing angle based on the large-scale jet, we find that adding tilt increases the angular size of the ring for fixed black hole mass and distance, while at the same time increasing the number of bright spots in the image. Our findings illustrate observable features that can distinguish tilted from aligned flows. They also show that tilted models can be viable for M87, and that not accounting for tilt can bias inferences of physical parameters. Future modeling of horizon-scale observations should account for potential angular momentum misalignment, which is likely generic at the low accretion rates appropriate for EHT targets.

astro-ph.HE↗

arXiv2020

Synthetic Gaia surveys from the FIRE cosmological simulations of Milky Way-mass galaxies

With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing observations to simulations of Milky-Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and use it to generate a suite of synthetic surveys resembling Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky-Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments (FIRE) project, which feature self-consistent clustering of star formation in dense molecular clouds and thin stellar/gaseous disks in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We sample synthetic stars by assuming each star particle (of mass 7070 $M_{\odot}$) represents a single stellar population. At each viewpoint, we compute dust extinction from the simulated gas metallicity distribution and apply a simple error model to produce a synthetic Gaia-like survey that includes both observational properties and a pointer to the generating star particle. We provide the complete simulation snapshot at $z = 0$ for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data.

astro-ph.GA↗

arXiv2020

Thermal Instability of Halo Gas Heated by Streaming Cosmic Rays

Heating of virialized gas by streaming cosmic rays (CRs) may be energetically important in galaxy halos, groups and clusters. We present a linear thermal stability analysis of plasmas heated by streaming CRs. We separately treat equilibria with and without background gradients, and with and without gravity. We include both CR streaming and diffusion along the magnetic-field direction. Thermal stability depends strongly on the ratio of CR pressure to gas pressure, which determines whether modes are isobaric or isochoric. Modes with $\mathbf{k \cdot B }\neq 0$ are strongly affected by CR diffusion. When the streaming time is shorter than the CR diffusion time, thermally unstable modes (with $\mathbf{k \cdot B }\neq 0$) are waves propagating at a speed $\propto$ the Alfvén speed. Halo gas in photoionization equilibrium is thermally stable independent of CR pressure, while gas in collisional ionization equilibrium is unstable for physically realistic parameters. In gravitationally stratified plasmas, the oscillation frequency of thermally overstable modes can be higher in the presence of CR streaming than the buoyancy/free-fall frequency. This may modify the critical $t_{\rm cool}/t_{\rm ff}$ at which multiphase gas is present. The criterion for convective instability of a stratified, CR-heated medium can be written in the familiar Schwarzschild form $d s_{\rm eff} / d z < 0$, where $s_{\rm eff}$ is an effective entropy involving the gas and CR pressures. We discuss the implications of our results for the thermal evolution and multiphase structure of galaxy halos, groups and clusters.

astro-ph.GA↗