Search

In this work, we investigate the properties of a population of cool clouds in simulated galaxy outflows. Using data from the CGOLS isolated galaxy simulations, we generate catalogues of $\sim 10^5$ clouds. We describe the impact of two different supernova feedback models -- a centrally concentrated starburst and disk-wide distributed star formation -- on the resulting cloud population. In both cases we find that the mass distribution function $dN/dM \propto M^{-2}$, in good agreement with model predictions of turbulent fragmentation. We explore how cloud properties change with distance from the galaxy and find no qualitative distinction between the two feedback modes, although significant quantitative differences exist in attributes such as the total number of clouds, their densities, etc. We further show that both internal cloud velocities and cloud-cloud relative velocities are described well by properties of turbulent motion, despite significant bulk radial velocities. Finally, we investigate the distribution of cloud sizes in the context of recent theoretical arguments about cloud survival in winds. We find that proposed cloud survival criteria are a good predictor of cloud survival, in both the case where clouds are primarily destroyed and the case where cloud growth occurs in the outflow., Comment: 24 pages, 18 figures, submitted to The Astrophysical Journal

We present new JWST observations of the nearby, prototypical edge-on, spiral galaxy NGC 891. The northern half of the disk was observed with NIRCam in its F150W and F277W filters. Absorption is clearly visible in the mid-plane of the F150W image, along with vertical dusty plumes that closely resemble the ones seen in the optical. A $\sim 10 \times 3~{\rm kpc}^2$ area of the lower circumgalactic medium (CGM) was mapped with MIRI F770W at 12 pc scales. Thanks to the sensitivity and resolution of JWST, we detect dust emission out to $\sim 4$ kpc from the disk, in the form of filaments, arcs, and super-bubbles. Some of these filaments can be traced back to regions with recent star formation activity, suggesting that feedback-driven galactic winds play an important role in regulating baryonic cycling. The presence of dust at these altitudes raises questions about the transport mechanisms at play and suggests that small dust grains are able to survive for several tens of million years after having been ejected by galactic winds in the disk-halo interface. We lay out several scenarios that could explain this emission: dust grains may be shielded in the outer layers of cool dense clouds expelled from the galaxy disk, and/or the emission comes from the mixing layers around these cool clumps where material from the hot gas is able to cool down and mix with these cool cloudlets. This first set of data and upcoming spectroscopy will be very helpful to understand the survival of dust grains in energetic environments, and their contribution to recycling baryonic material in the mid-plane of galaxies., Comment: Accepted for publication in Astronomy & Astrophysics; 16 pages, 8 figures

We present a suite of high-resolution numerical simulations to study the evolution and survival of dust in hot galactic winds. We implement a novel dust framework in the Cholla hydrodynamics code and use wind tunnel simulations of cool, dusty clouds to understand how thermal sputtering affects the dust content of galactic winds. Our simulations illustrate how various regimes of cloud evolution impact dust survival, dependent on cloud size, wind properties, and dust grain size. We find that significant amounts of dust can survive in winds in all scenarios, even without shielding from the cool phase of outflows. We present an analytic framework that explains this result, along with an analysis of the impact of cloud evolution on the total fraction of dust survival. Using these results, we estimate that 60 percent of 0.1 micron dust that enters a starburst-driven wind could survive to populate both the hot and cool phases of the halo, based on a simulated distribution of cloud properties. We also investigate how these conclusions depend on grain size, exploring grains from 0.1 micron to 10 Angstrom. Under most circumstances, grains smaller than 0.01 micron cannot withstand hot-phase exposure, suggesting that the small grains observed in the CGM are either formed in situ due to the shattering of larger grains, or must be carried there in the cool phase of outflows. Finally, we show that the dust-to-gas ratio of clouds declines as a function of distance from the galaxy due to cloud-wind mixing and condensation. These results provide an explanation for the vast amounts of dust observed in the CGMs of galaxies and beyond., Comment: Published in ApJ. 23 pages, 17 figures, 2 tables

We present the fifth simulation in the CGOLS project -- a set of isolated starburst galaxy simulations modeled over large scales ($10\kpc$) at uniformly high resolution ($\Delta x \approx 5\pc$). Supernova feedback in this simulation is implemented as a disk-wide distribution of clusters, and we assess the impact of this geometry on several features of the resulting outflow, including radial profiles of various phases; mass, momentum, and energy outflow rates; covering fraction of cool gas; mock absorption-line spectra; and X-ray surface brightness. In general, we find that the outflow generated by this model is cooler, slower, and contains more mass in the cool phase than a more centrally concentrated outflow driven by a similar number of supernovae. In addition, the energy loading factors in the hot phase are an order-of-magnitude lower, indicating much larger losses due to radiative cooling in the outflow. However, coupling between the hot and cool phases is more efficient than in the nuclear burst case, with almost 50\% of the total outflowing energy flux carried by the cool phase at a radial distance of 5 kpc. These physical differences have corresponding signatures in observable quantities: the covering fraction of cool gas is much larger, and there is greater evidence of absorption in low and intermediate ionization-energy lines. Taken together, our simulations indicate that centrally-concentrated starbursts are more effective at driving hot, low-density outflows that will expand far into the halo, while galaxy-wide bursts may be more effective at removing cool gas from the disk., Comment: 22 pages, 13 figures, accepted in ApJ

We present an extension of the massively parallel, GPU native, astrophysical hydrodynamics code Cholla to magnetohydrodynamics (MHD). Cholla solves the ideal MHD equations in their Eulerian form on a static Cartesian mesh utilizing the Van Leer + Constrained Transport integrator, the HLLD Riemann solver, and reconstruction methods at second and third order. Cholla's MHD module can perform $\approx260$ million cell updates per GPU-second on an NVIDIA A100 while using the HLLD Riemann solver and second order reconstruction. The inherently parallel nature of GPUs combined with increased memory in new hardware allows Cholla's MHD module to perform simulations with resolutions $\sim500^3$ cells on a single high end GPU (e.g. an NVIDIA A100 with 80GB of memory). We employ GPU direct MPI to attain excellent weak scaling on the exascale supercomputer \textit{Frontier}, while using 74,088 GPUs and simulating a total grid size of over 7.2 trillion cells. A suite of test problems highlights the accuracy of Cholla's MHD module and demonstrates that zero magnetic divergence in solutions is maintained to round off error. We also present new testing and continuous integration tools using GoogleTest, GitHub Actions, and Jenkins that have made development more robust and accurate and ensure reliability in the future., Comment: 36 pages, 14 figures

We present CloudFlex, a new open-source tool for predicting the absorption-line signatures of cool gas in galaxy halos with complex small-scale structure. Motivated by analyses of cool material in hydrodynamical simulations of turbulent, multiphase media, we model individual cool gas structures as assemblies of cloudlets with a power-law distribution of cloudlet mass $\propto m_{\rm cl}^{-\alpha}$ and relative velocities drawn from a turbulent velocity field. The user may specify $\alpha$, the lower limit of the cloudlet mass distribution ($m_{\rm cl,min}$), and several other parameters that set the total mass, size, and velocity distribution of the complex. We then calculate the MgII 2796 absorption profiles induced by the cloudlets along pencil-beam lines of sight. We demonstrate that at fixed metallicity, the covering fraction of sightlines with equivalent widths $W_{2796} < 0.3$ Ang increases significantly with decreasing $m_{\rm cl,min}$, cool cloudlet number density ($n_{\rm cl}$), and cloudlet complex size. We then present a first application, using this framework to predict the projected $W_{2796}$ distribution around ${\sim}L^*$ galaxies. We show that the observed incidences of $W_{2796}>0.3$ Ang sightlines within 10 kpc < $R_{\perp}$ < 50 kpc are consistent with our model over much of parameter space. However, they are underpredicted by models with $m_{\rm cl,min}\ge100M_{\odot}$ and $n_{\rm cl}\ge0.03$ $\rm cm^{-3}$, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets ($m_{\rm cl}\lesssim100M_{\odot}$) with a volume filling factor ${\lesssim}1\%$. When used to simultaneously model absorption-line datasets built from multi-sightline and/or spatially-extended background probes, CloudFlex will enable detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM., Comment: 22 pages, 7 figures. Submitted to AAS Journals, with minor modifications. Comments welcome. (1) Co-first authors who made equal contributions to this work

The interaction between the supersonic motion of the Large Magellanic Cloud (LMC) and the Circumgalactic Medium (CGM) is expected to result in a bow shock that leads the LMC's gaseous disk. In this letter, we use hydrodynamic simulations of the LMC's recent infall to predict the extent of this shock and its effect on the Milky Way's (MW) CGM. The simulations clearly predict the existence of an asymmetric shock with a present day stand-off radius of $\sim6.7$ kpc and a transverse diameter of $\sim30$ kpc. Over the past 500 Myr, $\sim8\%$ of the MW's CGM in the southern hemisphere should have interacted with the shock front. This interaction may have had the effect of smoothing over inhomogeneities and increasing mixing in the MW CGM. We find observational evidence of the existence of the bow shock in recent $H\alpha$ maps of the LMC, providing a potential explanation for the envelope of ionized gas surrounding the LMC. Furthermore, the interaction of the bow shock with the MW CGM may also explain observations of ionized gas surrounding the Magellanic Stream. Using recent orbital histories of MW satellites, we find that many satellites have likely interacted with the LMC shock. Additionally, the dwarf galaxy Ret2 is currently sitting inside the shock, which may impact the interpretation of reported gamma ray excess in Ret2. This work highlights bow shocks associated with infalling satellites are an under-explored, yet potentially very important dynamical mixing process in the circumgalactic and intracluster media., Comment: Re-uploaded after acceptance to ApJ Letters. 5 figures and 1 table. Comments welcome!

When cool clouds are ram-pressure accelerated by a hot supersonic galactic wind, some of the clouds may be shredded by hydrodynamical instabilities and incorporated into the hot flow. Recent one-dimensional steady-state calculations show how cool cloud entrainment directly affects the bulk thermodynamics, kinematics, and observational characteristics of the hot gas. In particular, mass-loading decelerates the hot flow and changes its entropy. Here, we investigate the stability of planar and spherical mass-loaded hot supersonic flows using both perturbation analysis and three-dimensional time-dependent radiative hydrodynamical simulations. We show that mass-loading is stable over a broad range of parameters and that the 1D time-steady analytic solutions exactly reproduce the 3D time-dependent calculations, provided that the flow does not decelerate sufficiently to become subsonic. For higher values of the mass-loading, the flow develops a sonic point and becomes thermally unstable, rapidly cooling and forming elongated dense cometary filaments. We explore the mass-loading parameters required to reach a sonic point and the radiative formation of these filaments. For certain approximations, we can derive simple analytic criteria. In general a mass-loading rate similar to the initial mass outflow rate is required. In this sense, the destruction of small cool clouds by a hot flow may ultimately spontaneously generate fast cool filaments, as observed in starburst superwinds. Lastly, we find that the kinematics of filaments is sensitive to the slope of the mass-loading function. Filaments move faster than the surrounding wind if mass-loading is over long distances whereas filaments move slower than their surroundings if mass-loading is abrupt., Comment: 12 pages, 15 figures, submitted to MNRAS (21 July 2023)

The analytic galactic wind model derived by Chevalier and Clegg in 1985 (CC85) assumes $\textit{uniform}$ energy and mass-injection within the starburst galaxy nucleus. However, the structure of nuclear star clusters, bulges, and star-forming knots are non-uniform. We generalize to cases with spherically-symmetric energy/mass injection that scale as $r^{-\Delta}$ within the starburst volume $R$, providing solutions for $\Delta = 0$, 1/2, 1, 3/2, and 2. In marked contrast with the CC85 model ($\Delta=0$), which predicts zero velocity at the center, for a singular isothermal sphere profile ($\Delta=2$), we find that the flow maintains a $\textit{constant}$ Mach number of $\mathcal{M}=\sqrt{3/5} \simeq 0.77$ throughout the volume. The fast interior flow can be written as $v_{r < R} = (\dot{E}_T/3\dot{M}_T)^{1/2} \simeq 0.41 \, v_\infty$, where $v_\infty$ is the asymptotic velocity, and $\dot{E}_T$ and $\dot{M}_T$ are the total energy and mass injection rates. For $v_\infty \simeq 2000 \, \mathrm{km \, s^{-1}}$, $v_{r

The forest of Lyman-$\alpha$ absorption lines detected in the spectra of distant quasars encodes information on the nature and properties of dark matter and the thermodynamics of diffuse baryonic material. Its main observable -- the 1D flux power spectrum (FPS) -- should exhibit a suppression on small scales and an enhancement on large scales in warm dark matter (WDM) cosmologies compared to standard $\Lambda$CDM. Here, we present an unprecedented suite of 1080 high-resolution cosmological hydrodynamical simulations run with the Graphics Processing Unit-accelerated code Cholla to study the evolution of the Lyman-$\alpha$ forest under a wide range of physically-motivated gas thermal histories along with different free-streaming lengths of WDM thermal relics in the early Universe. A statistical comparison of synthetic data with the forest FPS measured down to the smallest velocity scales ever probed at redshifts $4.0\lesssim z\lesssim 5.2$ (Boera et al. 2019) yields a lower limit $m_{\rm WDM}>3.1$ keV (95 percent CL) for the WDM particle mass and constrains the amplitude and spectrum of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei at these redshifts. Interestingly, our Bayesian inference analysis appears to weakly favor WDM models with a peak likelihood value at the thermal relic mass of $m_{\rm WDM}=4.5$ keV. We find that the suppression of the FPS from free-streaming saturates at $k\gtrsim 0.1\,\mathrm{s}\,\mathrm{km}^{-1}$ because of peculiar velocity smearing, and this saturated suppression combined with a slightly lower gas temperature provides a moderately better fit to the observed small-scale FPS for WDM cosmologies., Comment: Version accepted to Phys. Rev. D

Understanding the connections between galaxy stellar mass, star formation rate, and dark matter halo mass represents a key goal of the theory of galaxy formation. Cosmological simulations that include hydrodynamics, physical treatments of star formation, feedback from supernovae, and the radiative transfer of ionizing photons can capture the processes relevant for establishing these connections. The complexity of these physics can prove difficult to disentangle and obfuscate how mass-dependent trends in the galaxy population originate. Here, we train a machine learning method called Explainable Boosting Machines (EBMs) to infer how the stellar mass and star formation rate of nearly 6 million galaxies simulated by the Cosmic Reionization on Computers (CROC) project depend on the physical properties of halo mass, the peak circular velocity of the galaxy during its formation history $v_\mathrm{peak}$, cosmic environment, and redshift. The resulting EBM models reveal the relative importance of these properties in setting galaxy stellar mass and star formation rate, with $v_\mathrm{peak}$ providing the most dominant contribution. Environmental properties provide substantial improvements for modeling the stellar mass and star formation rate in only $\lesssim10\%$ of the simulated galaxies. We also provide alternative formulations of EBM models that enable low-resolution simulations, which cannot track the interior structure of dark matter halos, to predict the stellar mass and star formation rate of galaxies computed by high-resolution simulations with detailed baryonic physics., Comment: 27 pages, 18 figures, Submitted to AAS Journals

The filamentary network of intergalactic medium (IGM) gas that gives origin to the Lyman-alpha forest in the spectra of distant quasars encodes information on the physics of structure formation and the early thermodynamics of diffuse baryonic material. Here, we use a massive suite of more than 400 high-resolution cosmological hydrodynamical simulations run with the Graphics Processing Unit-accelerated code Cholla to study the IGM at high spatial resolution maintained over the entire computational volume. The simulations capture a wide range of possible thermal histories of intergalactic gas by varying the amplitude and timing of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei. A statistical comparison of synthetic spectra with the observed 1D flux power spectra of hydrogen in 14 redshift bins over the full range 2.2 <= z <= 5.0 and with the Lyman-alpha opacity of helium in 5 redshift bins over the range 2.4 < z < 2.9 tightly constrains the photoionization and photoheating history of the IGM. By leveraging the constraining power of the available Lyman-alpha forest data to break model degeneracies, we find that the IGM experienced two main reheating events from the non-equilibrium ionization of hydrogen and helium over 1.2 Gyr of cosmic time. For our best-fit model, hydrogen reionization completes by z~6.0 with a first IGM temperature peak T_0 ~ 1.3x10^4 K, and is followed by the reionization of HeII that completes by z~3.0 and yields a second temperature peak of T_0 ~ 1.4x10^4 K. We discuss how our results can be used to obtain information on the timing and the sources of hydrogen and helium reionization., Comment: 33 pages, 22 figures. Accepted for publication in AAS Journal. Revised version (V3)

Supernova-driven galactic winds are multiphase streams of gas that are often observed flowing at a range of velocities out of star-forming regions in galaxies. In this study, we use high resolution 3D simulations of multiphase galactic winds modeled with the hydrodynamics code Cholla to investigate the connection between numerical studies and observations. Using a simulated interaction between a hot $T\sim 10^{7}\,\rm K$ supernova-driven wind and a cool $T\sim 10^{4}\,\rm K$ cloud of interstellar material, we create mock observables, including the optical depth and covering fraction of six commonly observed ions (Si II, C II, Si IV, C IV, N V, and O VI) as a function of gas velocity. We compare our mock observables to surveys of galactic winds in the literature, finding good agreement with velocities and profiles of the low ions. We then compute "empirical" values for the optical depth and covering fraction following observational techniques, and compare them to the values calculated directly from the simulation data. We find that the empirically computed values tend to underestimate the "true" value of $\tau$ for ions with high optical depth, and overestimate the "true" value of $\tau$ for ions with low optical depth, relative to the simulated data.

Energy injection by supernovae may drive hot supersonic galactic winds in rapidly star-forming galaxies, driving metal-enriched gas into the circumgalactic medium and potentially accelerating cool gas. If sufficiently mass-loaded, such flows become radiative within the wind-driving region, reducing the overall mass outflow rate from the host galaxy. We show that this sets a maximum on the total outflow momentum for hot energy-driven winds. For a spherical wind of Solar metallicity driven by continuous star formation, $\dot p_\rm{max} \simeq 1.9\times10^4\ M_\odot/\rm{yr\ km/s}\ (\alpha/0.9)^{0.86}(R_\star/300\ \rm{pc})^{0.14}(\dot M_\star/20\ M_\odot/\rm{yr})^{0.86}$, where $\alpha$ is the fraction of supernova energy that thermalizes the wind, and $\dot M_\star$ and $R_\star$ are the star formation rate and radius of the wind-driving region. This maximum momentum for hot winds can also apply to cool, ionized outflows that are typically observed in starburst galaxies, if the hot wind undergoes bulk radiative cooling or if the hot wind transfers mass and momentum to cool clouds within the flow. We show that requiring the hot wind to undergo single-phase cooling on large scales sets a minimum on the total outflow momentum rate. These maximum and minimum outflow momenta have similar values, setting a characteristic momentum rate of hot galactic winds that can become radiative on large scales. We find that most observations of photoionized outflow wind momentum fall below the theoretical maximum and thus may be signatures of cooling hot flows. On the other hand, many systems fall below the minimum momentum required for bulk cooling, indicating that perhaps the cool material observed has instead been entrained in or mixed with the hot flow., Comment: 14 pages, 7 figures, accepted by MNRAS

The density and temperature properties of the intergalactic medium (IGM) reflect the heating and ionization history during cosmological structure formation, and are primarily probed by the Lyman-alpha forest of neutral hydrogen absorption features in the observed spectra of background sources (Gunn & Peterson 1965). We present the methodology and initial results from the Cholla IGM Photoheating Simulation (CHIPS) suite performed with the Graphics Process Unit-accelerated Cholla code to study the IGM at high, uniform spatial resolution maintained over large volumes. In this first paper, we examine the IGM structure in CHIPS cosmological simulations that include IGM uniform photoheating and photoionization models where hydrogen reionization completes early (Haardt & Madau 2012) or by redshift z~6 (Puchwein et al. 2019). Comparing with observations of the large- and small-scale Lyman-alpha transmitted flux power spectra P(k) at redshifts 2 <~ z <~ 5.5, the relative agreement of the models depends on scale, with the self-consistent Puchwein et al. (2019) IGM photoheating and photoionization model in good agreement with the flux P(k) at k >~ 0.01 s/km at redshifts 2 <~ z <~ 3.5. On larger scales the P(k) measurements increase in amplitude from z~4.6 to z~2.2 faster than the models, and lie in between the model predictions at 2.2 <~ z <~ 4.6 for k~= 0.002-0.01 s/km. We argue the models could improve by changing the HeII photoheating rate associated with active galactic nuclei to reduce the IGM temperature at z~3. At higher redshifts z>~4.5 the observed flux P(k) amplitude increases at a rate intermediate between the models, and we argue that for models where hydrogen reionization completes late (z~5.5 - 6) resolving this disagreement will require inhomogeneous or 'patchy' reionization. (Abridged), Comment: 30 pages, 19 figures, 2 tables. Accepted for publication at ApJ

We examine the properties of the circumgalactic medium (CGM) at low redshift in a range of simulated Milky Way mass halos. The sample is comprised of seven idealized simulations, an adaptive mesh refinement cosmological zoom-in simulation, and two groups of 50 halos with star forming or quiescent galaxies taken from the IllustrisTNG100 simulation. The simulations have very different setups, resolution, and feedback models, but are analyzed in a uniform manner. By comparing median radial profiles and mass distributions of CGM properties, we isolate key similarities and differences. In doing so, we advance the efforts of the SMAUG (Simulating Multiscale Astrophysics to Understand Galaxies) project that aims to understand the inherently multiscale galaxy formation process. In the cosmological simulations, the CGM exhibits nearly flat temperature distributions, and broad pressure and radial velocity distributions. In the idealized simulations, similar distributions are found in the inner CGM ($\lesssim 0.5 \, r_{\rm 200c}$) when strong galactic feedback models are employed, but the outer CGM ($\gtrsim 0.5 \, r_{\rm 200c}$) has a much less prominent cold phase, and narrower pressure and velocity distributions even in models with strong feedback. This comparative analysis demonstrates the dominant role feedback plays in shaping the inner CGM and the increased importance of cosmological effects, such as nonspherical accretion and satellite galaxies, in the outer CGM. Furthermore, our findings highlight that while cosmological simulations are required to capture the multiphase structure of the CGM at large radii, idealized simulations provide a robust framework to study how galactic feedback interacts with the inner CGM and thereby provide a reliable avenue to constrain feedback prescriptions., Comment: More information on the SMAUG project and the other first result papers can be found here: https://www.simonsfoundation.org/flatiron/center-for-computational-astrophysics/galaxy-formation/smaug/papersplash1

We present the fourth of the Cholla Galactic OutfLow Simulations suite (CGOLS). Using a physically-motivated prescription for clustered supernova feedback, we successfully drive a multiphase outflow from a disk galaxy. The high resolution ($< 5\,\mathrm{pc}$) across a relatively large domain ($20\,\mathrm{kpc}$) allows us to capture the hydrodynamic mixing and dynamical interactions between the hot and cool ($T \sim 10^4\,\mathrm{K}$) phases in the outflow, which in turn leads to direct evidence of a qualitatively new mechanism for cool gas acceleration in galactic winds. We show that mixing of momentum from the hot phase to the cool phase accelerates the cool gas to $800\,\mathrm{km}\,\mathrm{s}^{-1}$ on kpc scales, with properties inconsistent with the physical models of ram pressure acceleration or with bulk cooling from the hot phase. The mixing process also affects the hot phase, modifying its radial profiles of temperature, density, and velocity from the expectations of radial supersonic flow. This mechanism provides a physical explanation for the high velocity, blue shifted, low ionization absorption lines often observed in the spectra of starburst and high redshift galaxies., Comment: 29 pages, 21 figures. Submitted to ApJ. Comments welcome

The current disparity in computational knowledge is a critical hindrance to the diversity and success of the field. Recommendations are outlined for policies and funding models to enable the growth and retention of a new generation of computational researchers that reflect the demographics of the undergraduate population in Astronomy and Physics., Comment: Astro2020 APC White Paper: State of the Profession Consideration

Using the Spitzer Space Telescope and Herschel Space Observatory, we have conducted a survey of infrared galaxies in the field of the galaxy cluster Abell S1063 (AS1063) at $z=0.347$, which is one of the most massive clusters known and a target of the HST CLASH and Frontier-Field surveys. The Spitzer/MIPS 24 $\mu$m and Herschel/PACS & SPIRE images revealed that the core of AS1063 is surprisingly devoid of infrared sources, showing only a few detectable sources within the central r$\sim1^{\prime}$. There is, however, one particularly bright source (2.3 mJy at 24 $\mu$m; 106 mJy at 160 $\mu$m), which corresponds to a background galaxy at $z=0.61$. The modest magnification factor (4.0$\times$) implies that this galaxy is intrinsically IR-luminous (L$_{\rm IR}=3.1\times10^{11}\ \rm L_{\odot}$). What is particularly interesting about this galaxy is that HST optical/near-infrared images show a remarkably bright and large (1 kpc) clump at one edge of the disk. Our follow-up optical/near-infrared spectroscopy shows Balmer (H$\alpha$-H8) and forbidden emission from this clump ([OII] $\lambda$3727, [OIII] $\lambda\lambda$4959,5007, [NII] $\lambda\lambda$6548,6583), indicating that it is a HII region. The HII region appears to have formed in-situ, as kinematically it is part of a rotating disk, and there is no evidence of nearby interacting galaxies. With an extinction correction of A$_{\rm V}=1.5$ mag, the star formation rate of this giant HII region is $\sim$10 M$_{\odot}$ yr$^{-1}$, which is exceptionally large, even for high redshift HII regions. Such a large and luminous HII region is often seen at $z\sim2$ but quite rare in the nearby Universe., Comment: Accepted for publication in the Astrophysical Journal. 29 pages, 15 figures

There is wide consensus that galaxy outflows are one of the most important processes determining the evolution of galaxies through cosmic time, for example playing a key role in shaping the galaxy mass function. Our understanding of outflows and their drivers, however, is in its infancy --- this is particularly true for the cold (neutral atomic and molecular) phases of outflows, which present observational and modeling challenges. Here we outline several key open questions, briefly discussing the requirements of the observations necessary to make progress, and the relevance of several existing and planned facilities. It is clear that galaxy outflows, and particularly cold outflows, will remain a topic of active research for the next decade and beyond., Comment: Submitted as a Science White Paper to the Astro2020 Decadal Survey. arXiv admin note: substantial text overlap with arXiv:1810.06737

The past decade began with the first light of ALMA and will end at the start of the new era of hyperdimensional astrophysics. Our community-wide movement toward highly multiwavelength and multidimensional datasets has enabled immense progress in each science frontier identified by the 2010 Decadal Survey, particularly with regard to black hole feedback and the cycle of baryons in galaxies. Facilities like ALMA and the next generation of integral field unit (IFU) spectrographs together enable mapping the physical conditions and kinematics of warm ionized and cold molecular gas in galaxies in unprecedented detail (Fig. 1). JWST's launch at the start of the coming decade will push this capability to the rest-frame UV at redshifts z > 6, mapping the birth of stars in the first galaxies at cosmic dawn. Understanding of their subsequent evolution, however, now awaits an ability to map the processes that transform galaxies directly, rather than the consequences of those processes in isolation. In this paper, we argue that doing so requires an equivalent revolution in spatially resolved spectroscopy for the hot plasma that pervades galaxies, the atmospheres in which they reside, and the winds that are the engines of their evolution., Comment: A Science White Paper submitted to the Astro2020 Decadal Survey (7 papers, 4 figures)

We present the Cholla Galactic OutfLow Simulations (CGOLS) suite, a set of extremely high resolution global simulations of isolated disk galaxies designed to clarify the nature of multiphase structure in galactic winds. Using the GPU-based code Cholla, we achieve unprecedented resolution in these simulations, modeling galaxies over a 20 kpc region at a constant resolution of 5 pc. The simulations include a feedback model designed to test the effects of different mass- and energy-loading factors on galactic outflows over kiloparsec scales. In addition to describing the simulation methodology in detail, we also present the results from an adiabatic simulation that tests the frequently-adopted analytic galactic wind model of Chevalier & Clegg. Our results indicate that the Chevalier & Clegg model is a good fit to nuclear starburst winds in the non-radiative region of parameter space. Lastly, we investigate the role of resolution and convergence in large-scale simulations of multiphase galactic winds. While our largest-scale simulations do show convergence of observable features like soft X-ray emission, our tests demonstrate that simulations of this kind with resolutions greater than 10 pc are not yet converged, confirming the need for extreme resolution in order to study the structure of winds and their effects on the circumgalactic medium., Comment: 18 pages, 15 figures; version accepted in ApJ

Galactic outflows commonly contain multiphase gas, and its physical origin requires explanation. Using the CGOLS (Cholla Galactic OutfLow Simulations) suite of high-resolution isolated galaxy models, we demonstrate the viability of rapid radiative cooling as a source of fast-moving ($v \sim 1000$ km/s), cool ($10^4$ K) gas observed in absorption line studies of outflows around some star-forming galaxies. By varying the mass-loading and geometry of the simulated winds, we identify a region of parameter space that leads to cool gas in outflows. In particular, when using an analytically-motivated central feedback model, we find that cooling flows can be produced with reasonable mass-loading rates ($\dot{M}_{wind} / \dot{M}_{SFR} \sim 0.5$), provided the star formation rate surface density is high. When a more realistic clustered feedback model is applied, destruction of high density clouds near the disk and interactions between different outflow regions indicate that lower mass-loading rates of the hot gas within the feedback region may still produce multiphase outflows. These results suggest an origin for fast-moving cool gas in outflows that does not rely on directly accelerating cool gas from the interstellar medium. These cooling flows may additionally provide an explanation for the multiphase gas ubiquitously observed in the halos of star-forming galaxies at low redshift., Comment: 19 pages, 13 figures; version accepted to ApJ

Using a set of high resolution hydrodynamical simulations run with the Cholla code, we investigate how mass and momentum couple to the multiphase components of galactic winds. The simulations model the interaction between a hot wind driven by supernova explosions and a cooler, denser cloud of interstellar or circumgalactic media. By resolving scales of $\Delta x<0.1$ pc over $>100$ pc distances our calculations capture how the cloud disruption leads to a distribution of densities and temperatures in the resulting multiphase outflow, and quantify the mass and momentum associated with each phase. We find the multiphase wind contains comparable mass and momenta in phases over a wide range of densities and temperatures extending from the hot wind ($n \approx 10^{-2.5}$ $\mathrm{cm}^{-3}$, $T \approx 10^{6.5}$ K) to the coldest components ($n \approx 10^2$ $\mathrm{cm}^{-3}$, $T \approx 10^2$ K). We further find that the momentum distributes roughly in proportion to the mass in each phase, and the mass-loading of the hot phase by the destruction of cold, dense material is an efficient process. These results provide new insight into the physical origin of observed multiphase galactic outflows, and inform galaxy formation models that include coarser treatments of galactic winds. Our results confirm that cool gas observed in outflows at large distances from the galaxy ($\gtrsim1$kpc) likely does not originate through the entrainment of cold material near the central starburst., Comment: 28 pages, 20 figures. Version accepted by ApJ

We present Cholla (Computational Hydrodynamics On ParaLLel Architectures), a new three-dimensional hydrodynamics code that harnesses the power of graphics processing units (GPUs) to accelerate astrophysical simulations. Cholla models the Euler equations on a static mesh using state-of-the-art techniques, including the unsplit Corner Transport Upwind (CTU) algorithm, a variety of exact and approximate Riemann solvers, and multiple spatial reconstruction techniques including the piecewise parabolic method (PPM). Using GPUs, Cholla evolves the fluid properties of thousands of cells simultaneously and can update over ten million cells per GPU-second while using an exact Riemann solver and PPM reconstruction. Owing to the massively-parallel architecture of GPUs and the design of the Cholla code, astrophysical simulations with physically interesting grid resolutions (> 256^3) can easily be computed on a single device. We use the Message Passing Interface library to extend calculations onto multiple devices and demonstrate nearly ideal scaling beyond 64 GPUs. A suite of test problems highlights the physical accuracy of our modeling and provides a useful comparison to other codes. We then use Cholla to simulate the interaction of a shock wave with a gas cloud in the interstellar medium, showing that the evolution of the cloud is highly dependent on its density structure. We reconcile the computed mixing time of a turbulent cloud with a realistic density distribution destroyed by a strong shock with the existing analytic theory for spherical cloud destruction by describing the system in terms of its median gas density., Comment: 38 pages, 20 figures. Version accepted to ApJS

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicates reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the new UDF12 results with the most recent constraints from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. Under reasonable assumptions about the escape fraction of hydrogen ionizing photons and the intergalactic medium clumping factor, we find that to fully reionize the universe by redshift z~6 the population of star-forming galaxies at redshifts z~7-9 likely must extend in luminosity below the UDF12 limits to absolute UV magnitudes of M_UV\sim -13 or fainter. Moreover, low levels of star formation extending to redshifts z~15-25, as suggested by the normal UV colors of z\simeq7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z\simeq10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations., Comment: Version accepted by ApJ (originally submitted Jan 5, 2013). The UDF12 website can be found at http://udf12.arizona.edu

We present a catalog of high redshift star-forming galaxies selected to lie within the redshift range z ~ 7-8 using the Ultra Deep Field 2012 (UDF12), the deepest near-infrared (near-IR) exposures yet taken with the Hubble Space Telescope. As a result of the increased near-infrared exposure time compared to previous HST imaging in this field, we probe 0.65 (0.25) mag fainter in absolute UV magnitude, at z ~ 7 (8), which increases confidence in a measurement of the faint end slope of the galaxy luminosity function. Through a 0.7 mag deeper limit in the key F105W filter that encompasses or lies just longward of the Lyman break, we also achieve a much-refined color-color selection that balances high redshift completeness and a low expected contamination fraction. We improve the number of drop-out selected UDF sources to 47 at z ~ 7 and 27 at z ~ 8. Incorporating brighter archival and ground-based samples, we measure the z ~ 7 UV luminosity function to an absolute magnitude limit of M_UV = -17 and find a faint end Schechter slope of \alpha = -1.87+/- 0.18. Using a similar color-color selection at z ~ 8 that takes account of our newly-added imaging in the F140W filter, and incorporating archival data from the HIPPIES and BoRG campaigns, we provide a robust estimate of the faint end slope at z ~ 8, \alpha = -1.94 +/- 0.23. We briefly discuss our results in the context of earlier work and that derived using the same UDF12 data but with an independent photometric redshift technique (McLure et al 2012)., Comment: 15 pages, 5 figures, To be submitted to ApJ

We analyze the redshift- and luminosity-dependent sizes of dropout galaxy candidates in the redshift range z~7-12 using deep images from the UDF12 campaign, data which offers two distinct advantages over that used in earlier work. Firstly, we utilize the increased S/N ratio offered by the UDF12 imaging to provide improved size measurements for known galaxies at z=6.5-8 in the HUDF. Specifically, we stack the new deep F140W image with the existing F125W data in order to provide improved measurements of the half-light radii of z-dropouts. Similarly we stack this image with the new deep UDF12 F160W image to obtain new size measurements for a sample of Y-dropouts. Secondly, because the UDF12 data have allowed the construction of the first robust galaxy sample in the HUDF at z>8, we have been able to extend the measurement of average galaxy size out to significantly higher redshifts. Restricting our size measurements to sources which are now detected at >15sigma, we confirm earlier indications that the average half-light radii of z~7-12 galaxies are extremely small, 0.3-0.4 kpc, comparable to the sizes of giant molecular associations in local star-forming galaxies. We also confirm that there is a clear trend of decreasing half-light radius with increasing redshift, and provide the first evidence that this trend continues beyond z~8. Modeling the evolution of the average half-light radius as a power-law (1+z)^s, we obtain a best-fit index of s=-1.28+/-0.13 over the redshift range z~4-12, mid-way between the physically expected evolution for baryons embedded in dark halos of constant mass (s=-1) and constant velocity (s=-1.5). A clear size-luminosity relation, such as that found at lower redshift, is also evident in both our z- and Y-dropout sample. This relation can be interpreted in terms of a constant surface density of star formation over a range in luminosity of 0.05-1.0L*_z=3.(abridged), Comment: 15 pages, 13 figures, submitted to ApJ

We present the 2012 Hubble Ultra Deep Field campaign (UDF12), a large 128-orbit Cycle 19 \HST\ program aimed at extending previous WFC3/IR observations of the UDF by quadrupling the exposure time in the F105W filter, imaging in an additional F140W filter, and extending the F160W exposure time by 50%. The principal scientific goal of this project is to determine whether galaxies reionized the universe; our observations are designed to provide a robust determination of the star formation density at $z$$\,\gtrsim\,$8, improve measurements of the ultraviolet continuum slope at $z$$\,\sim\,7\,-\,$8, facilitate the construction of new samples of $z$$\,\sim\,9\,-\,$10 candidates, and enable the detection of sources up to $z$$\,\sim\,$12. For this project we committed to combining these and other WFC3/IR imaging observations of the UDF area into a single homogeneous dataset, to provide the deepest near-infrared observations of the sky currently achievable. In this paper we present the observational overview of the project, motivated by its scientific goals, and describe the procedures used in reducing the data as well as the final products that are produced. We have used the most up up-to-date methods for calibrating and combining the images, in particular paying attention to correcting several instrumental effects. We release the full combined mosaics, comprising a single, unified set of mosaics of the UDF, providing the deepest near-infrared blank-field view of the universe obtained to date, reaching magnitudes as deep as AB$\,\sim\,$30 in the near-infrared, and yielding a legacy dataset on this field of lasting scientific value to the community., Comment: 16 pages, 9 figures, submitted to ApJS

We present the results of the deepest search to date for star-forming galaxies beyond a redshift z~8.5 utilizing a new sequence of near-infrared Wide Field Camera 3 images of the Hubble Ultra Deep Field. This `UDF12' campaign completed in September 2012 doubles the earlier exposures with WFC3/IR in this field and quadruples the exposure in the key F105W filter used to locate such distant galaxies. Combined with additional imaging in the F140W filter, the fidelity of high redshift candidates is greatly improved. Using spectral energy distribution fitting techniques on objects selected from a deep multi-band near-infrared stack we find 7 promising z>8.5 candidates. As none of the previously claimed UDF candidates with 8.510 galaxies with JWST., Comment: Submitted to Astrophys. J. Lett. on Nov. 7, 2012. Resubmitted after responding to referee report. UDF12 public webpage available at http://udf12.arizona.edu