arXiv subjects

Alex S. Hill

Publications and source records attributed to Alex S. Hill.

At least 55 records · Page 3Linked to original sources

arXiv2018

Effect of the heating rate on the stability of the three-phase interstellar medium

We investigate the impact of the far ultraviolet (FUV) heating rate on the stability of the three-phase interstellar medium using three-dimensional simulations of a $1$ kpc$^2$, vertically-extended domain. The FUV heating rate sets the range of thermal pressures across which the cold ($\sim10^2$ K) and warm ($\sim10^4$ K) neutral media (CNM and WNM) can coexist in equilibrium. Even absent a variable star formation rate regulating the FUV heating rate, the gas physics keeps the pressure in the two-phase regime: because radiative heating and cooling processes happen on shorter timescales than sound wave propagation, turbulent compressions tend to keep the interstellar medium within the CNM-WNM pressure regime over a wide range of heating rates. The thermal pressure is set primarily by the heating rate with little influence from the hydrostatics. The vertical velocity dispersion adjusts as needed to provide hydrostatic support given the thermal pressure: when the turbulent pressure $\langleρ\rangleσ_z^2$ is calculated over scales $\gtrsim500$ pc, the thermal plus turbulent pressure approximately equals the weight of the gas. The warm gas volume filling fraction is $0.2<f_w<0.8$ over a factor of less than three in heating rate, with $f_w$ near unity at higher heating rates and near zero at lower heating rates. We suggest that cosmological simulations that do not resolve the CNM should maintain an interstellar thermal pressure within the two-phase regime.

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arXiv2017

Warm Ionized Medium Throughout the Sagittarius-Carina Arm

Wisconsin H-Alpha Mapper (WHAM) observations of H-Alpha and [S II]$\lambda6716$ emission are used to trace the vertical distribution and physical conditions of the warm ionized medium (WIM) along the Sagittarius-Carina arm. CO emission, tracing cold molecular gas in the plane of the Galaxy, is used as a guide to isolate H-Alpha and [S II] emission along individual spiral arms. Exponential scale heights of electron density squared (or emission measure) are determined using H-Alpha emission above (below) the midplane to be $330 \pm 80$ pc ( $550 \pm 230$ pc) along the near Sagittarius arm, $300 \pm 100$ pc ($250 \pm 30$ pc) along the near Carina arm, and $>1000$ pc along the far Carina arm. The emission measure scale height tends to increase as a function of Galactocentric radius along the Sagittarius-Carina arm for $R_G > 8$ kpc. Physical conditions of the ionized gas are analyzed using the [S II]/H-Alpha line ratio, which more closely traces H-Alpha Intensity than height above the plane, z, suggesting a stronger relationship with the in-situ electron density. We interpret this result as further evidence for the majority of the observed diffuse emission originating from in-situ ionized gas as opposed to scattered light from classical H II regions in the plane.

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arXiv2015

On the Metallicity and Origin of the Smith High-Velocity Cloud

The Smith Cloud is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in 27 Myr. It is unique among HVCs in having a known distance (12.4+/-1.3 kpc) and a well-constrained 3D velocity (296 km/s), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using HST/COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II 1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives [S/H]=-0.28+/-0.14, or 0.53+0.21-0.15 solar metallicity. The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.

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arXiv2015

Blasting away a dwarf galaxy: The "tail" of ESO 324-G024

We present Australia Telescope Compact Array radio data of the dwarf irregular galaxy ESO 324-G024 which is seen in projection against the giant, northern lobe of the radio galaxy Centaurus A (Cen A, NGC 5128). The distorted morphology and kinematics of ESO 324-G024, as observed in the 21 cm spectral line emission of neutral hydrogen, indicate disruptions by external forces. We investigate whether tidal interactions and/or ram pressure stripping are responsible for the formation of the HI tail stretching to the northeast of ESO 324-G024 with the latter being most probable. Furthermore, we closely analyze the sub-structure of Cen A's polarized radio lobes to ascertain whether ESO 324-G024 is located in front, within or behind the northern lobe. Our multi-wavelength, multi-component approach allows us to determine that ESO 324-G024 is most likely behind the northern radio lobe of Cen A. This result helps to constrain the orientation of the lobe, which is likely inclined to our line of sight by approximately 60 degrees if NGC 5128 and ESO 324-G024 are at the same distance.

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arXiv2015

The Origin of the Hot Gas in the Galactic Halo: Testing Galactic Fountain Models' X-ray Emission

We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission.

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arXiv2014

Models of Diffuse Hα in the Interstellar Medium: The Relative Contributions from In Situ Ionisation and Dust Scattering

Using three dimensional Monte Carlo radiation transfer models of photoionisation and dust scattering, we explore different components of the widespread diffuse H$α$ emission observed in the interstellar medium of the Milky Way and other galaxies. We investigate the relative contributions of H$α$ from recombination emission in ionised gas and H$α$ that originates in H{\sc ii} regions near the Galactic midplane and scatters off high altitude dust in the diffuse interstellar medium. For the radiation transfer simulations we consider two geometries for the interstellar medium: a three dimensional fractal geometry that reproduces the average density structure inferred for hydrogen in the Milky Way, and a density structure from a magneto hydrodynamic simulation of a supernovae driven turbulent interstellar medium. Although some sight lines that are close to H{\sc ii} regions can be dominated by scattered light, overall we find that less than $\sim 20\%$ of the total H$α$ intensity in our simulations can be attributed to dust scattering. Our findings on the relative contribution of scattered H$α$ are consistent with previous observational and theoretical analyses. We also investigate the relative contributions of dust scattering and in situ ionisation of high density dust clouds in the diffuse gas. Dust scattering in these partially ionised clouds contribute $\sim 40\%$ to the total intensity of H$α$.

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arXiv2014

Steady-State Hadronic Gamma-Ray Emission from 100-Myr-Old Fermi Bubbles

The Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within $\sim$ few $\times$ Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energising source of the Bubbles. Likewise, both inverse-Compton emission by non-thermal electrons (`leptonic' models) and collisions between non-thermal protons and gas (`hadronic' models) have been advanced as the process supplying the Bubbles' γ-ray emission. An issue for any steady state hadronic model is that the very low density of the Bubbles' plasma seems to require that they accumulate protons over a multi-Gyr timescale, much longer than other natural timescales occurring in the problem. Here we present a hadronic model where the timescale for generating the Bubbles' hadronic γ-ray emission is $\sim$ few $\times 10^8$ years. Our model invokes collapse of the Bubbles' thermally-unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localised, warm ($\sim 10^4$ K), and likely filamentary condensations of higher density gas. Under the condition that these filaments are supported by non-thermal pressure, we can predict the hadronic emission from the Bubbles to be $L_γ\simeq 2 \times 10^{37}$ erg/s $\ \dot{M}_\mathrm{in}/(0.1 \ M_{Sun}/$ year $) \ T_\mathrm{FB}^2/(3.5 \times 10^7 K) ^2 M_{fil}/M_{pls}$ ; precisely their observed luminosity (normalizing to the star-formation-driven mass flux into the Bubbles and their measured plasma temperature and adopting the further result that the mass in the filaments, $M_{fil}$ is approximately equal to that of the Bubbles' plasma, $M_{pls}$).

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arXiv2014

Halpha and [S II] emission from warm ionized gas in the Scutum-Centaurus Arm

We present Wisconsin H-Alpha Mapper [S II] λ6716 and Hα spectroscopic maps of the warm ionized medium (WIM) in the Scutum-Centaurus Arm at Galactic longitudes 310° < l < 345°. Using extinction-corrected Hα intensities (IHαc), we measure an exponential scale height of electron density-squared in the arm of H_ne^2 = 0.30 kpc (assuming a distance of 3.5 kpc), intermediate between that observed in the inner Galaxy and in the Perseus Arm. The [S II]/Hα line ratio is enhanced at large |z| and in sightlines with faint IHαc. We find that the [S II]/Hα line ratio has a power law relationship with IHαc from a value of ~=1.0 at IHαc < 0.2 R (Rayleighs) to a value of ~=0.08 at IHαc >= 100 R. The line ratio is better correlated with Hα intensity than with height above the plane, indicating that the physical conditions within the WIM vary systematically with electron density. We argue that the variation of the line ratio with height is a consequence of the decrease of electron density with height. Our results reinforce the well-established picture in which the diffuse Hα emission is due primarily to emission from in situ photoionized gas, with scattered light only a minor contributor.

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arXiv2014

Photoionisation and Heating of a Supernova Driven, Turbulent, Interstellar Medium

The Diffuse Ionised Gas (DIG) in galaxies traces photoionisation feedback from massive stars. Through three dimensional photoionisation simulations, we study the propagation of ionising photons, photoionisation heating and the resulting distribution of ionised and neutral gas within snapshots of magnetohydrodynamic simulations of a supernova driven turbulent interstellar medium. We also investigate the impact of non-photoionisation heating on observed optical emission line ratios. Inclusion of a heating term which scales less steeply with electron density than photoionisation is required to produce diagnostic emission line ratios similar to those observed with the Wisconsin Hα Mapper. Once such heating terms have been included, we are also able to produce temperatures similar to those inferred from observations of the DIG, with temperatures increasing to above 15000 K at heights |z| > 1 kpc. We find that ionising photons travel through low density regions close to the midplane of the simulations, while travelling through diffuse low density regions at large heights. The majority of photons travel small distances (< 100pc); however some travel kiloparsecs and ionise the DIG.

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arXiv2013

Magnetized Gas in the Smith High Velocity Cloud

We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures towards extragalactic radio sources behind the Smith Cloud, new HI observations from the Green Bank Telescope, and a spectroscopic map of Hα from the Wisconsin H-Alpha Mapper Northern Sky Survey. There are enhancements in rotation measure of approximately 100 rad m^(-2) which are generally well correlated with decelerated Hα emission. We estimate a lower limit on the line-of-sight component of the field of approximately 8 μG along a decelerated filament; this is a lower limit due to our assumptions about the geometry. No RM excess is evident in sightlines dominated by HI or Hα at the velocity of the Smith Cloud. The smooth Hα morphology of the emission at the Smith Cloud velocity suggests photoionization by the Galactic ionizing radiation field as the dominant ionization mechanism, while the filamentary morphology and high (approximately 1 Rayleigh) Hα intensity of the lower-velocity magnetized ionized gas suggests an ionization process associated with shocks due to interaction with the Galactic interstellar medium. The presence of the magnetic field may contribute to the survival of high velocity clouds like the Smith Cloud as they move from the Galactic halo to the disk. We expect these data to provide a test for magnetohydrodynamic simulations of infalling gas.

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arXiv2013

Present-day Galactic Evolution: Low-metallicity, Warm, Ionized Gas Inflow Associated with High-Velocity Cloud Complex A

The high-velocity cloud (HVC) Complex A is a probe of the physical conditions in the Galactic halo. The kinematics, morphology, distance, and metallicity of Complex A indicate that it represents new material that is accreting onto the Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved observations of Complex A over the velocity range of -250 to -50 km/s in the local standard of rest reference frame. These observations include the first full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53) and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O I]6300 towards regions with high H I column densities, background quasars, and stars. The H-alpha data imply that the masses of neutral and ionized material in the cloud are similar, both being greater than a million solar masses. We find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of the ionizing radiation near the Milky Way is consistent with the known distance of the high-latitude part of Complex A and an assumed cloud geometry that puts the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This compatibility implies a 5% ionizing photon escape fraction from the Galactic disk. We also provide the nitrogen and sulfur upper abundance solutions for a series of temperatures, metallicities, and cloud configurations for purely photoionized gas; these solutions are consistent with the sub-solar abundances found by previous studies, especially for temperatures above 10,000 K or for gas with a high fraction of singly-ionized nitrogen and sulfur.

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arXiv2012

Vertical structure of a supernova-driven turbulent magnetized ISM

Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (\approx 90 %) of the mass is contained in thermally-stable temperature regimes of cold molecular and atomic gas at T < 200 K or warm atomic and ionized gas at 5000 K < T < 10^{4.2} K, with strong peaks in probability distribution functions of temperature in both the cold and warm regimes. The 200 - 10^{4.2} K gas fills 50-60 % of the volume near the plane, with hotter gas associated with supernova remnants (30-40 %) and cold clouds (< 10 %) embedded within. At |z| ~ 1-2 kpc, transition-temperature (10^5 K) gas accounts for most of the mass and volume, while hot gas dominates at |z| > 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (< 200 K) regime with a corresponding increase in the fraction of warm (~ 10^4 K) gas. However, our models lack rotational shear and thus have no large-scale dynamo, which reduces the role of the field in the models compared to reality. The supernovae drive oscillations in the vertical distribution of halo gas, with the period of the oscillations ranging from ~ 30 Myr in the T < 200 K gas to ~ 100 Myr in the 10^6 K gas, in line with predictions by Walters & Cox.

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arXiv2012

GSH 006-15+7: A local Galactic supershell featuring transition from HI emission to absorption

We report on the discovery of a new Galactic supershell, GSH 006-15+7, from the Galactic All Sky Survey data. Observed and derived properties are presented and we find that GSH 006-15+7 is one of the nearest physically large supershells known, with dimensions of ~ 780 x 520 pc at a distance of ~ 1.5 kpc. The shell wall appears in HI emission at b <~ -6.5 deg and in HI self-absorption (HISA) at b >~ -6.5 deg. We use this feature along with HISA diagnostics to estimate an optical depth of tau ~ 3, a spin temperature of ~ 40 K and a swept-up mass of M ~ 3e6 solar masses. We also investigate the origin of GSH 006-15+7, assessing the energy contribution of candidate powering sources and finding evidence in favour of a formation energy of ~ 1e52 ergs. We find that this structure provides evidence for the transfer of mass and energy from the Galactic disk into the halo.

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arXiv2011

MHD simulations of a supernova-driven ISM and the warm ionized medium using a positivity preserving ideal MHD scheme

We present new 3D magnetohydrodynamic (MHD) simulations of a supernova-driven, stratified interstellar medium. These simulations were run using the Waagan (2009) positivity preserving scheme for ideal MHD implemented in the Flash code. The scheme is stable even for the Mach numbers approaching 100 found in this problem. We have previously shown that the density distribution arising from hydrodynamical versions of these simulations creates low-density pathways through which Lyman continuum photons can travel to heights |z| > 1 kpc. This naturally produces the warm ionized medium through photoionization due primarily to O stars near the plane. However, our earlier models reproduce the peak but not the width of the observed emission measure distribution. Here, we examine whether inclusion of magnetic fields and a greater vertical extent to the simulation domain produce a gas distribution that better matches the observations. We further study the change of magnetic energy over time in our models, showing that it appears to reach a steady state after a few hundred megayears, presumably supported by a turbulent dynamo driven by the supernova explosions.

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arXiv2010

Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium

We investigate models for the photoionization of the widespread diffuse ionized gas in galaxies. In particular we address the long standing question of the penetration of Lyman continuum photons from sources close to the galactic midplane to large heights in the galactic halo. We find that recent hydrodynamical simulations of a supernova-driven interstellar medium have low density paths and voids that allow for ionizing photons from midplane OB stars to reach and ionize gas many kiloparsecs above the midplane. We find ionizing fluxes throughout our simulation grids are larger than predicted by one dimensional slab models, thus allowing for photoionization by O stars of low altitude neutral clouds in the Galaxy that are also detected in Halpha. In previous studies of such clouds the photoionization scenario had been rejected and the Halpha had been attributed to enhanced cosmic ray ionization or scattered light from midplane H II regions. We do find that the emission measure distributions in our simulations are wider than those derived from Halpha observations in the Milky Way. In addition, the horizontally averaged height dependence of the gas density in the hydrodynamical models is lower than inferred in the Galaxy. These discrepancies are likely due to the absence of magnetic fields in the hydrodynamic simulations and we discuss how magnetohydrodynamic effects may reconcile models and observations. Nevertheless, we anticipate that the inclusion of magnetic fields in the dynamical simulations will not alter our primary finding that midplane OB stars are capable of producing high altitude diffuse ionized gas in a realistic three-dimensional interstellar medium.

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arXiv2010

The role of turbulence in the warm ionized medium

We discuss the role of turbulence in establishing the observed emission measures and ionization of the warm ionized medium. A Monte Carlo radiative transfer code applied to a snapshot of a simulation of a supernova-driven, multi-temperature, stratified ISM reproduces the essential observed features of the WIM.

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arXiv2009

Ionized Gas in the Smith Cloud

We present Wisconsin H-Alpha Mapper observations of ionized gas in the Smith Cloud, a high velocity cloud which Lockman et al. have recently suggested is interacting with the Galactic disk. Our H-alpha map shows the brightest H-alpha emission, 0.43 \pm 0.04 R, coincident with the brightest H I, while slightly fainter H-alpha emission (0.25 \pm 0.02 R) is observed in a region with H I intensities < 0.1 times as bright as the brightest H I. We derive an ionized mass of \gtrsim 3 \times 10^6 M_\odot, comparable to the H I mass, with the H^+ mass spread over a considerably larger area than the H I. An estimated Galactic extinction correction could adjust these values upwards by 40 %. H-alpha and [S II] line widths towards the region of brightest emission constrain the electron temperature of the gas to be between 8000 K and 23000 K. A detection of [N II] λ6583 in the same direction with a line ratio [N II] / H-alpha = 0.32 \pm 0.05 constrains the metallicity of the cloud: for typical photoionization temperatures of 8000-12000 K, the nitrogen abundance is 0.15-0.44 times solar. These results lend further support to the claim that the Smith Cloud is new material accreting onto the Galaxy.

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arXiv2009

Ionized Gas in the Smith Cloud

We present WHAM observations of Halpha, [N II], and [S II] in the Smith Cloud. A map of Halpha emission from the cloud shows ionized gas coincident with the brightest H I emission, but nearly-as-bright Halpha in some regions with faint H I. The ionized mass of the cloud is at least as large as the neutral mass, > 10^6 M_sun. Ionized gas in the core of the Smith Cloud has an electron temperature 6000 K < T < 16000 K. The observed ratio [N II] / Halpha = 0.39 \pm 0.09 shows that the cloud has a non-primordial nitrogen abundance, 0.1 - 1 times solar.

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