arXiv subjects

Evan Scannapieco

Publications and source records attributed to Evan Scannapieco.

At least 55 records · Page 3Linked to original sources

arXiv2019

Catastrophic Cooling in Superwinds: Line Emission and Non-equilibrium Ionization

Outflows are a pervasive feature of mechanical feedback from super star clusters (SSC) in starburst galaxies, playing a fundamental role in galaxy evolution. Observations are now starting to confirm that outflows can undergo catastrophic cooling, suppressing adiabatic superwinds. Here we present a suite of one-dimensional, hydrodynamic simulations that study the ionization structure of these outflows and the resulting line emission generated by the cooling gas. We use the non-equilibrium atomic chemistry package within MAIHEM, our modified version of FLASH, which evolves the ionization state of the gas and computes the total cooling rate on an ion-by-ion basis. We find that catastrophically cooling models produce strong nebular line emission compared to adiabatic outflows. We also show that such models exhibit non-equilibrium conditions, thereby generating more highly ionized states than equivalent equilibrium models. When including photoionization from the parent SSC, catastrophically cooling models show strong C IV λ1549 and O VI λ1037 emission. For density bounded photoionization, He II λ1640, λ4686, C III] λ1908, Si IV λ1206, and Si III λ1400 are also strongly enhanced. These lines are seen in extreme starbursts where catastrophic cooling is likely to occur, suggesting that they may serve as diagnostics of such conditions. The higher ionization generated by these flows may help to explain line emission that cannot be attributed to SSC photoionization alone.

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arXiv2019

Measuring the delay time distribution of binary neutron stars. II. Using the redshift distribution from third-generation gravitational wave detectors network

We investigate the ability of current and third-generation gravitational wave (GW) detectors to determine the delay time distribution (DTD) of binary neutron stars (BNS) through a direct measurement of the BNS merger rate as a function of redshift. We assume that the DTD follows a power law distribution with a slope $Γ$ and a minimum merger time $t_{\rm min}$, and also allow the overall BNS formation efficiency per unit stellar mass to vary. By convolving the DTD and mass efficiency with the cosmic star formation history, and then with the GW detector capabilities, we explore two relevant regimes. First, for the current generation of GW detectors, which are only sensitive to the local universe, but can lead to precise redshift determinations via the identification of electromagnetic counterparts and host galaxies, we show that the DTD parameters are strongly degenerate with the unknown mass efficiency and therefore cannot be determined uniquely. Second, for third-generation detectors such as Einstein Telescope (ET) and Cosmic Explorer (CE), which will detect BNS mergers at cosmological distances, but with a redshift uncertainty inherent to GW-only detections ($δ(z)/z\approx 0.1z$), we show that the DTD and mass efficiency can be well-constrained to better than 10\% with a year of observations. This long-term approach to determining the DTD through a direct mapping of the BNS merger redshift distribution will be supplemented by more near term studies of the DTD through the properties of BNS merger host galaxies at $z\approx 0$ (Safarzadeh & Berger 2019).

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arXiv2019

On Neutron Star Mergers as the Source of r-process Enhanced Metal Poor Stars in the Milky Way

We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way (MW) type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to to better compute the properties of metal poor (MP) and carbon enhanced metal poor (CEMP) stars, along with statistics of their r-process enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars ($t_{\rm min}$) is set to 1 Myr. No r-process enhanced stars form if $t_{\rm min}=100$ Myr. Our results show that even when we adopt the r-process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r-process enhanced stars if either the birth rate of double neutron stars per unit mass of stars is boosted to $\approx10^{-4} M_\odot^{-1}$ or the Europium yield of each NSM event is boosted to $\approx 10^{-4} M_{\odot}$.

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arXiv2019

Non-equilibrium Ionization States Within Galactic Outflows: Explaining Their O VI and N V Column Densities

We present a suite of one-dimensional spherically-symmetric hydrodynamic simulations that study the atomic ionization structure of galactic outflows. We track the ionization state of the outflowing gas with a non-equilibrium atomic chemistry network that includes photoionization, photo-heating, and ion-by-ion cooling. Each simulation describes a steady-state outflow that is defined by its mass and energy input rates, sonic radius, metallicity, and UV flux from both the host galaxy and meta-galactic background. We find that for a large range of parameter choices, the ionization state of the material departs strongly from what it would be in photo-ionization equilibrium, in conflict with what is commonly assumed in the analysis of observations. In addition, nearly all the models reproduce the low N V to O VI column density ratios and the relatively high O VI column densities that are observed.

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arXiv2019

r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars

The recent aLIGO/aVirgo discovery of gravitational waves from the neutron star merger (NSM) GW170817 and the follow up kilonova observations have shown that NSMs produce copious amount of r-process material. However, it is difficult to reconcile the large natal kicks and long average merging times of Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming that such dwarf systems have lost a significant fraction of their stellar mass through tidal stripping, we conclude that contrary to most current models, it is the DNSs with rather large natal kicks but very short merging timescales that can enrich UFD-type galaxies. These binaries are either on highly eccentric orbits, or form with very short separations due to an additional mass-transfer between the first-born neutron star and a naked helium star, progenitor of the second-born neutron star. These DNSs are born with a frequency that agrees with the statistics of the r-process UFDs, and merge well within the virial radius of their host halos, therefore contributing significantly to their r-process enrichment.

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arXiv2019

Following the Cosmic Evolution of Pristine Gas III: The Observational Consequences of the Unknown Properties of Population III Stars

We study the observational consequences of several unknown properties of Population III (Pop III) stars using large-scale cosmological simulations that include a subgrid model to track the unresolved mixing of pollutants. Varying the value of the critical metallicity that marks the boundary between Pop III and Population II (Pop II) star formation across 2 dex has a negligible effect on the fraction of Pop III stars formed and the subsequent fraction of Pop III flux from high-redshift galaxies. However, adopting a log normal initial mass function (IMF) for Pop III stars, in place of a baseline Salpeter IMF, results in a Pop III star formation rate density (SFRD) that is 1/4 of the baseline rate. The flux from high-redshift galaxies modeled with this IMF is highly bimodal, resulting in a tiny fraction of $z \leq 8$ galaxies with more than 75\% of their flux coming from Pop III stars. However, at $z=9$, right before reionization in our simulations, $\approx$ 20\% of galaxies are Pop III-bright with $m_{\rm UV} \le 31.4$ mag and at least 75\% of their flux generated by Pop III stars . Additionally, the log normal Pop III IMF results in a population of carbon enhanced, metal poor stars in reasonable agreement with MW halo observations. Our analysis supports the conclusion that the Pop III IMF was dominated by stars in the 20-120$M_{\odot}$ range that generate SN with carbon-enhanced ejecta.

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arXiv2018

Modeling Star Formation as a Markov Process in a Supersonic Gravoturbulent Medium

Molecular clouds exhibit lognormal probability density functions (PDF) of mass densities, which are thought to arise as a consequence of isothermal, supersonic turbulence. Star formation is then widely assumed to occur in perturbations in which gravitational collapse is faster than the rate of change due to turbulent motions. Here we use direct numerical simulations to measure this rate as a function of density for a range of turbulent Mach numbers, and show it is faster at high densities than at low densities. Furthermore, we show that both the density PDF and rate of change arise naturally in a simple model of turbulence as a continuous Markov process. The one-dimensional Langevin equation that describes this evolution depends on only two parameters, yet it captures the full evolution seen in direct three-dimensional simulations. If it is modified to include gravity, the Langevin equation also reproduces the rate of material collapsing to high densities seen in turbulent simulations including self-gravity. When generalized to include both temperature and density, similar analyses are likely applicable throughout astrophysics.

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arXiv2018

Modeling Photoionized Turbulent Material in the Circumgalactic Medium

The circumgalactic medium (CGM) of nearby star-forming galaxies show clear indications of \ion{O}{6} absorption accompanied by little to no \ion{N}{5} absorption. This unusual spectral signature, accompanied by absorption from lower ionization state species whose columns vary by orders of magnitude along \st{difference} \textbf{different} sightlines, indicates that the CGM must be viewed as a dynamic, multiphase medium, such as occurs in the presence of turbulence. To explore this possibility, we carry out a series of chemodynamical simulations of a isotropic turbulent media, using the MAIHEM package. The simulations assume a metallicity of $0.3\ Z_{\odot}$ and a redshift zero metagalatic UV background, and they track ionizations, recombinations, and species-by-species radiative cooling for a wide range of elements. We find that turbulence with a one-dimensional velocity dispersion of $σ_{1D} \approx 60$ km/s replicates many of the observed features within the CGM, such as clumping of low ionization-state ions and the existence of \ion{O}{6} at moderate ionization parameters. However, unlike observations, \ion{N}{5} often arises in our simulations with derived column densities of a similar magnitude to those of \ion{O}{6}. While higher values of $σ_{1D}$ lead to a thermal runaway in our isotropic simulations, this would not be the case in stratified media, and thus we speculate that more complex models of the turbulence may well match the absence of \ion{N}{5} in the CGM of star-forming galaxies.

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arXiv2018

Using Real and Simulated Measurements of the Thermal Sunyaev-Zel'dovich Effect to Constrain Models of AGN Feedback

Energetic feedback from active galactic nuclei (AGNs) is often used in simulations to resolve several outstanding issues in galaxy formation, but its impact is still not fully understood. Here we derive new constraints on AGN feedback by comparing observations and simulations of the thermal Sunyaev-Zel'dovich (tSZ) effect. We draw on observational results presented in Spacek et al. (2016, 2017) who used data from the South Pole Telescope (SPT) and Atacama Cosmology Telescope (ACT) to measure the tSZ signal from >= 10^11 M_Sun and >= 1 Gyr galaxies at z=0.5-1.0 (low-z) and z=1.0-1.5 (high-z). Using the large-scale cosmological hydrodynamical simulations Horizon-AGN and Horizon-NoAGN, which include and omit AGN feedback, we extract simulated tSZ measurements around galaxies equivalent to the observational work. We find that the Horizon-AGN results only differ from the SPT measurements at levels of 0.4 sigma (low-z) and 0.6 sigma (high-z), but differ from the ACT measurements by 3.4 sigma (low-z) and 2.3 sigma (high-z). The Horizon-NoAGN results provide a slightly better fit to the SPT measurements by differing by 0.2 sigma (low-z) and 0.4 sigma (high-z), but a significantly better match to the ACT measurements by differing by only 0.5 sigma (low-z) and 1.4 sigma (high-z). We conclude that, while the lower-mass (<~ 5 x 10^11 M_Sun) SPT results allow for the presence AGN feedback energy, the higher-mass (>~ 5 x 10^11 M_Sun) ACT results show significantly less energy than predicted in the simulation including AGN feedback, while more closely matching the simulation without AGN feedback, indicating that AGN feedback may be milder than often predicted in simulations.

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arXiv2018

Column Density Profiles of Cold Clouds Driven by Galactic Outflows

Absorption line studies are essential to understanding the origin, nature, and impact of starburst-driven galactic outflows. Such studies have revealed a multiphase medium with a number of poorly-understood features leading to a need to study the ionization mechanism of this gas. To better interpret these observations, we make use of a suite of adaptive mesh refinement hydrodynamic simulations of cold, atomic clouds driven by hot, supersonic outflows, including the effect of radiative cooling, thermal conduction, and an ionizing background characteristic of a starbursting galaxy. Using a new analysis tool, Trident, we estimate the equilibrium column density distributions for ten different ions: H I, Mg II, C II, C III, C IV, Si III, Si IV, N V, O VI, and Ne VIII. These are fit to model profiles with two parameters describing the maximum column density and coverage, and for each ion we provide a table of these fit parameters, along with average velocities and line widths. Our results are most sensitive to Mach number and conduction efficiency, with higher Mach numbers and more efficient conduction leading to more compact, high column density clouds. We use our results to interpret down-the-barrel observations of outflows and find that the adopted ionization equilibrium model overpredicts column densities of ions such as Si IV and does not adequately capture the observed trends for N V and O VI, implying the presence of strong non equilibrium ionization effects.

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arXiv2018

A limit on the warm dark matter particle mass from the redshifted 21 cm absorption line

The recent EDGES collaboration detection of an absorption signal at a central frequency of $ν= 78 \pm 1$ MHz points to the presence of a significant Lyman-$α$ background by a redshift of $z=18$. The timing of this signal constrains the dark matter particle mass ($m_χ$) in the warm dark matter (WDM) cosmological model. WDM delays the formation of small-scale structures, and therefore a stringent lower limit can be placed on $m_χ,$ based on the presence of a sufficiently strong Ly-$α$ background due to star formation at $z=18$. Our results show that the coupling the spin temperature to the gas through Ly-$α$ pumping requires a minimum mass of $m_χ>3$ keV if atomic cooling halos dominate the star formation rate at $z=18,$ and $m_χ>2$ keV if ${\rm\,H_2}$ cooling halos also form stars efficiently at this redshift. These limits match or exceed the most stringent limits cited to date in the literature, even in the face of the many uncertainties regarding star-formation at high redshift.

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arXiv2018

Selecting ultra-faint dwarf candidate progenitors in cosmological N-body simulations at high redshifts

The smallest satellites of the Milky Way ceased forming stars during the epoch of reionization and thus provide archaeological access to galaxy formation at $z>6$. Numerical studies of these ultra-faint dwarf galaxies (UFDs) require expensive cosmological simulations with high mass resolution that are carried out down to $z=0$. However, if we are able to statistically identify UFD host progenitors at high redshifts \emph{with relatively high probabilities}, we can avoid this high computational cost. To find such candidates, we analyze the merger trees of Milky Way type halos from the high-resolution ${\it Caterpillar}$ suite of dark matter only simulations. Satellite UFD hosts at $z=0 $ are identified based on four different abundance matching (AM) techniques. All the halos at high redshifts are traced forward in time in order to compute the probability of surviving as satellite UFDs today. Our results show that selecting potential UFD progenitors based solely on their mass at z=12 (8) results in a 10\% (20\%) chance of obtaining a surviving UFD at $z=0$ in three of the AM techniques we adopted. We find that the progenitors of surviving satellite UFDs have lower virial ratios ($η$), and are preferentially located at large distances from the main MW progenitor, while they show no correlation with concentration parameter. Halos with favorable locations and virial ratios are $\approx 3$ times more likely to survive as satellite UFD candidates at $z=0.$

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arXiv2018

Following the Cosmic Evolution of Pristine Gas II: The search for Pop III-Bright Galaxies

Direct observational searches for Population III (Pop III) stars at high redshift are faced with the question of how to select the most promising targets for spectroscopic follow-up. To help answer this, we use a large-scale cosmological simulation, augmented with a new subgrid model that tracks the fraction of pristine gas, to follow the evolution of high-redshift galaxies and the Pop III stars they contain. We generate rest-frame ultraviolet (UV) luminosity functions for our galaxies and find that they are consistent with current $z \ge 7 $ observations. Throughout the redshift range $7 \le z \le 15$ we identify "Pop III-bright" galaxies as those with at least 75% of their flux coming from Pop III stars. While less than 1% of galaxies brighter than $m_{\rm UV, AB} = 31.4$ mag are Pop III--bright in the range $7\leq z \leq8$, roughly 17% of such galaxies are Pop III--bright at $z=9$, immediately before reionization occurs in our simulation. Moving to $z=10$, $m_{\rm UV, AB} = 31.4$ mag corresponds to larger, more luminous galaxies and the Pop III-bright fraction falls off to 5%. Finally, at the highest redshifts, a large fraction (29% at $z=14$ and 41% at $z=15)$ of all galaxies are Pop III-bright regardless of magnitude. While $m_{\rm UV, AB} = 31.4$ mag galaxies are extremely rare during this epoch, we find that 13% of galaxies at $z = 14$ are Pop III-bright with $m_{\rm UV, AB} \le 33$ mag, an intrisic magnitude within reach of the James Webb Space Telescope using lensing. Thus, we predict that the best redshift to search for luminous Pop III--bright galaxies is just before reionization, while lensing surveys for fainter galaxies should push to the highest redshifts possible.

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arXiv2017

The Fate of Gas rich Satellites in Clusters

We investigate the stellar mass loss of gas rich galaxies falling into clusters due to the change in the gravitational potential caused by the ram pressure stripping of their gas. We model the satellites with exponential stellar and gas disk profiles, assume rapid ram pressure stripping, and follow the stellar orbits in the shocked potential. Due to the change of the potential, the stars move from circular orbits to elliptical orbits with apocenters that are often outside the tidal radius, causing those stars to be stripped. We explore the impact of the redshift of infall, gas fraction, satellite halo mass and cluster mass on this process. The puffing of the satellites makes them appear as ultra diffuse galaxies, and the stripped stars contribute to the intracluster light. Our results show these effects are most significant for less-massive satellites, which have larger gas fractions when they are accreted into clusters. The preferential destruction of low mass systems causes the red fraction of cluster galaxies to be smaller at lower masses, an observation that is otherwise difficult to explain.

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arXiv2017

The Effect of Turbulence on Nebular Emission Line Ratios

Motivated by the observed differences in the nebular emission of nearby and high-redshift galaxies, we carry out a set of direct numerical simulations of turbulent astrophysical media exposed to a UV background. The simulations assume a metallicity of $Z/Z_{\odot}$=0.5 and explicitly track ionization, recombination, charge transfer, and ion-by-ion radiative cooling for several astrophysically important elements. Each model is run to a global steady state that depends on the ionization parameter $U$, and the one-dimensional turbulent velocity dispersion, $σ_{\rm 1D}$, and the turbulent driving scale. We carry out a suite of models with a T=42,000K blackbody spectrum, $n_e$ = 100 cm$^{-3}$ and $σ_{\rm 1D}$ ranging between 0.7 to 42 km s$^{-1},$ corresponding to turbulent Mach numbers varying between 0.05 and 2.6. We report our results as several nebular diagnostic diagrams and compare them to observations of star-forming galaxies at a redshift of $z\approx$2.5, whose higher surface densities may also lead to more turbulent interstellar media. We find that subsonic, transsonic turbulence, and turbulence driven on scales of 1 parsec or greater, have little or no effect on the line ratios. Supersonic, small-scale turbulence, on the other hand, generally increases the computed line emission. In fact with a driving scale $\approx 0.1$ pc, a moderate amount of turbulence, $σ_{\rm 1D}$=21-28 km s$^{-1},$ can reproduce many of the differences between high and low redshift observations without resorting to harder spectral shapes.

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arXiv2017

Simulating Neutron Star Mergers as r-process Sources in Ultra Faint Dwarf Galaxies

To explain the high observed abundances of r-process elements in local ultra- faint dwarf (UFD) galaxies, we perform cosmological zoom simulations that include r-process production from neutron star mergers (NSMs). We model star-formation stochastically and simulate two different halos with total masses $\approx 10^8 M_{\odot}$ at z = 6. We find that the final distribution of [Eu/H] vs. [Fe/H] is relatively insensitive to the energy by which the r-process material is ejected into the interstellar medium, but strongly sensitive to the environment in which the NSM event occurs. In one halo the NSM event takes place at the center of the stellar distribution, leading to high-levels of r-process enrichment such as seen in a local UFD, Reticulum II (Ret II). In a second halo, the NSM event takes place outside of the densest part of the galaxy, leading to a more extended r-process distribution. The subsequent star formation occurs in an interstellar medium with shallow levels of r-process enrichment which results in stars with low levels of [Eu/H] compared to Ret II stars even when the maximum possible r-process mass is assumed to be ejected. This suggests that the natal kicks of neutron stars may also play an important role in determining the r-process abundances in UFD galaxies, a topic that warrants further theoretical investigation.

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arXiv2017

The Production of Cold Gas Within Galaxy Outflows

I present a suite of three-dimensional simulations of the evolution of initially-hot material ejected by starburst-driven galaxy outflows. The simulations are conducted in a comoving frame that moves with the material, tracking atomic/ionic cooling, Compton cooling, and dust cooling and destruction. Compton cooling is most efficient of these processes, while the main role of atomic/ionic cooling is to enhance density inhomogeneities. Dust, on the other hand, has little effect on the outflow evolution, and is rapidly destroyed in all the simulations except the case with the smallest mass flux. I use the results to construct a simple steady-state model of the observed UV/optical emission from each outflow. The velocity profiles in this case are dominated by geometric effects, and the overall luminosities are extremely strong functions of the properties of the host system, as observed in ultra-luminous infrared galaxies (ULIRGs). Furthermore the luminosities and maximum velocities in several models are consistent with emission-line observations of ULIRGs, although the velocities are significantly greater than observed in absorption-line studies. It may be that absorption line observations of galaxy outflows probe entrained cold material at small radii, while emission-line observations probe cold material condensing from the initially hot medium at larger distances.

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arXiv2016

Numerical Simulation of Star Formation by the Bow Shock of the Centaurus A Jet

Recent Hubble Space Telescope (HST) observations of the extragalactic radio source Centaurus A (Cen A) display a young stellar population around the southwest tip of the inner filament 8.5 kpc from the Cen A galactic center, with ages in the range of 1-3 Myr. Crockett et al. (2012) argue that the transverse bow shock of the Cen A jet triggered this star formation as it impacted dense molecular cores of clouds in the filament. To test this hypothesis, we perform three-dimensional numerical simulations of induced star formation by the jet bow shock in the inner filament of Cen A, using a positivity preserving WENO method to solve the equations of gas dynamics with radiative cooling. We find that star clusters form inside a bow-shocked molecular cloud when the maximum initial density of the cloud is > 40 H2 molecules/cm^3. In a typical molecular cloud of mass 10^6 M_sun and diameter 200 pc, approximately 20 star clusters of mass 10^4 M_sun are formed, matching the HST images.

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