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DOE OSTI2024

The XRISM first-light observation: Velocity structure and thermal properties of the supernova remnant N 132D

Abstract We present an initial analysis of the X-Ray Imaging and Spectroscopy Mission (XRISM) first-light observation of the supernova remnant (SNR) N 132D in the Large Magellanic Cloud. The Resolve microcalorimeter has obtained the first high-resolution spectrum in the 1.6–10 keV band, which contains K-shell emission lines of Si, S, Ar, Ca, and Fe. We find that the Si and S lines are relatively narrow, with a broadening represented by a Gaussian-like velocity dispersion of $\sigma _v \sim 450$ km s$^{-1}$. However, the Fe He$\alpha$ lines are substantially broadened with $\sigma _v \sim 1670$ km s$^{-1}$. This broadening can be explained by a combination of the thermal Doppler effect due to the high ion temperature and the kinematic Doppler effect due to the SNR expansion. Assuming that the Fe He$\alpha$ emission originates predominantly from the supernova ejecta, we estimate the reverse shock velocity at the time when the bulk of the Fe ejecta were shock heated to be $-1000 \lesssim V_{\rm rs}$ (km s$^{-1}$) $\lesssim 3300$ (in the observer frame). We also find that Fe Ly$\alpha$ emission is redshifted with a bulk velocity of $\sim 890$ km s$^{-1}$, substantially larger than the radial velocity of the local interstellar medium surrounding N 132D. These results demonstrate that high-resolution X-ray spectroscopy is capable of providing constraints on the evolutionary stage, geometry, and velocity distribution of SNRs.

individual objects↗
DOE OSTI2015

BLOWIN' IN THE WIND: BOTH ''NEGATIVE'' AND ''POSITIVE'' FEEDBACK IN AN OBSCURED HIGH-z QUASAR

Quasar feedback in the form of powerful outflows is invoked as a key mechanism to quench star formation in galaxies, preventing massive galaxies to overgrow and producing the red colors of ellipticals. On the other hand, some models are also requiring ''positive'' active galactic nucleus feedback, inducing star formation in the host galaxy through enhanced gas pressure in the interstellar medium. However, finding observational evidence of the effects of both types of feedback is still one of the main challenges of extragalactic astronomy, as few observations of energetic and extended radiatively driven winds are available. Here we present SINFONI near infrared integral field spectroscopy of XID2028, an obscured, radio-quiet z = 1.59 QSO detected in the XMM-COSMOS survey, in which we clearly resolve a fast (1500 km s{sup –1}) and extended (up to 13 kpc from the black hole) outflow in the [O III] lines emitting gas, whose large velocity and outflow rate are not sustainable by star formation only. The narrow component of Hα emission and the rest frame U-band flux from Hubble Space Telescope/Advanced Camera for Surveys imaging enable to map the current star formation in the host galaxy: both tracers independently show that the outflow position lies in the center of an empty cavity surrounded by star forming regions on its edge. The outflow is therefore removing the gas from the host galaxy (''negative feedback''), but also triggering star formation by outflow induced pressure at the edges (''positive feedback''). XID2028 represents the first example of a host galaxy showing both types of feedback simultaneously at work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DESTRUCTION OF INTERSTELLAR DUST IN EVOLVING SUPERNOVA REMNANT SHOCK WAVES

Supernova generated shock waves are responsible for most of the destruction of dust grains in the interstellar medium (ISM). Calculations of the dust destruction timescale have so far been carried out using plane parallel steady shocks, however, that approximation breaks down when the destruction timescale becomes longer than that for the evolution of the supernova remnant (SNR) shock. In this paper we present new calculations of grain destruction in evolving, radiative SNRs. To facilitate comparison with the previous study by Jones et al., we adopt the same dust properties as in that paper. We find that the efficiencies of grain destruction are most divergent from those for a steady shock when the thermal history of a shocked gas parcel in the SNR differs significantly from that behind a steady shock. This occurs in shocks with velocities ≳200 km s{sup −1} for which the remnant is just beginning to go radiative. Assuming SNRs evolve in a warm phase dominated ISM, we find dust destruction timescales are increased by a factor of ∼2 compared to those of Jones et al., who assumed a hot gas dominated ISM. Recent estimates of supernova rates and ISM mass lead to another factor of ∼3 increase in the destruction timescales, resulting in a silicate grain destruction timescale of ∼2–3 Gyr. These increases, while not able to resolve the problem of the discrepant timescales for silicate grain destruction and creation, are an important step toward understanding the origin and evolution of dust in the ISM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Evaporation and Survival of Cluster Galaxy Coronae. I. The Effectiveness of Isotropic Thermal Conduction Including Saturation

We simulate the evolution of cluster galaxy hot interstellar medium (ISM) gas that is a result of the effects of ram pressure and thermal conduction in the intracluster medium (ICM). At the density and temperature of the ICM, the mean free paths of ICM electrons are comparable to the sizes of galaxies, therefore electrons can efficiently transport heat that is due to thermal conduction from the hot ICM to the cooler ISM. Galaxies consisting of dark matter halos and hot gas coronae are embedded in an ICM-like “wind tunnel” in our simulations. In this paper, we assume that thermal conduction is isotropic and include the effects of saturation. We find that as heat is transferred from the ICM to the ISM, the cooler denser ISM expands and evaporates. This process is significantly faster than gas loss due to ram pressure stripping; for our standard model galaxy, the evaporation time is 160 Myr, while the ram pressure stripping timescale is 2.5 Gyr. Thermal conduction also suppresses the formation of shear instabilities, and there are no stripped ISM tails since the ISM evaporates before tails can form. Observations of long-lived X-ray emitting coronae and ram pressure stripped X-ray tails in galaxies in group and cluster environments therefore require that thermal conduction is suppressed or offset by some additional physical process. The most likely process is anisotropic thermal conduction that is due to magnetic fields in the ISM and ICM, which we simulate and study in the next paper in this series.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Investigating nearby star-forming galaxies in the ultraviolet with HST/COS spectroscopy. I. Spectral analysis and interstellar abundance determinations

This is the first in a series of three papers describing a project with the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure abundances of the neutral interstellar medium (ISM) in a sample of nine nearby star-forming galaxies. The goal is to assess the (in)homogeneities of the multiphase ISM in galaxies where the bulk of metals can be hidden in the neutral phase, yet the metallicity is inferred from the ionized gas in the H II regions. The sample, spanning a wide range in physical properties, is to date the best suited to investigate the metallicity behavior of the neutral gas at redshift z = 0. ISM absorption lines were detected against the far-ultraviolet spectra of the brightest star-forming region(s) within each galaxy. Here we report on the observations, data reduction, and analysis of these spectra. Column densities were measured by a multicomponent line-profile fitting technique, and neutral-gas abundances were obtained for a wide range of elements. Several caveats were considered, including line saturation, ionization corrections, and dust depletion. Ionization effects were quantified with ad hoc CLOUDY models reproducing the complex photoionization structure of the ionized and neutral gas surrounding the UV-bright sources. An 'average spectrum of a redshift z = 0 star-forming galaxy' was obtained from the average column densities of unsaturated profiles of neutral-gas species. This template can be used as a powerful tool for studies of the neutral ISM at both low and high redshift.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STUDYING THE INTERSTELLAR MAGNETIC FIELD FROM ANISOTROPIES IN VELOCITY CHANNELS

Turbulence in the interstellar medium is anisotropic due to the ubiquitous magnetic fields. This anisotropy depends on the strength of the magnetic field and leaves an imprint on observations of spectral line maps. We use a grid of ideal magnetohydrodynamic simulations of driven turbulence and produce synthetic position–position–velocity maps to study the turbulence anisotropy in velocity channels of various resolutions. We found that the average structure function of velocity channels is aligned with the projection of the magnetic field on the plane of the sky. We also found that the degree of such anisotropy increases with the magnitude of the magnetic field. For thick velocity channels (low velocity resolution), the anisotropy is dominated by density, and the degree of anisotropy in these maps allows one to distinguish sub-Alfvénic and super-Alfvénic turbulence regimes, but it also depends strongly on the sonic Mach number. For thin channels (high velocity resolution), we find that the anisotropy depends less on the sonic Mach number. An important limitation of this technique is that it only gives a lower limit on the magnetic field strength because the anisotropy is related only to the magnetic field component on the plane of the sky. It can, and should, be used in combination with other techniques to estimate the magnetic field, such as the Fermi-Chandrasekhar method, anisotropies in centroids, Faraday rotation measurements, or direct line-of-sight determinations of the field from Zeeman effect observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

HIGH-ENERGY ELECTRON IRRADIATION OF INTERSTELLAR CARBONACEOUS DUST ANALOGS: COSMIC-RAY EFFECTS ON THE CARRIERS OF THE 3.4 μ m ABSORPTION BAND

The effects of cosmic rays on the carriers of the interstellar 3.4 μ m absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH{sub 3} and CH{sub 2} in carbonaceous dust. It is widely observed in the diffuse interstellar medium, but disappears in dense clouds. Destruction of CH{sub 3} and CH{sub 2} by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, samples of hydrogenated amorphous carbon (a-C:H) have been irradiated with 5 keV electrons. The decay of the band intensity versus electron fluence reflects a-C:H dehydrogenation, which is well described by a model assuming that H{sub 2} molecules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron bombardment has allowed the use of higher-energy doses than in the ion experiments. The effects of cosmic rays on the aliphatic components of cosmic dust are found to be small. The estimated cosmic-ray destruction times for the 3.4 μ m band carriers lie in the 10{sup 8} yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3 × 10{sup 7} yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1997

Interstellar and Cometary Dust

'Interstellar dust' forms a continuum of materials with differing properties which I divide into three classes on the basis of observations: (a) diffuse dust, in the low-density interstellar medium; (b) outer-cloud dust, observed in stars close enough to the outer edges of molecular clouds to be observed in the optical and ultraviolet regions of the spectrum, and (c) inner-cloud dust, deep within the cores of molecular clouds, and observed only in the infrared by means of absorption bands of C-H, C=O, 0-H, C(triple bond)N, etc. There is a surprising regularity of the extinction laws between diffuse- and outer-cloud dust. The entire mean extinction law from infrared through the observable ultraviolet spectrum can be characterized by a single parameter. There are real deviations from this mean law, larger than observational uncertainties, but they are much smaller than differences of the mean laws in diffuse- and outer-cloud dust. This fact shows that there are processes which operate over the entire distribution of grain sizes, and which change size distributions extremely efficiently. There is no evidence for mantles on grains in local diffuse and outer-cloud dust. The only published spectra of the star VI Cyg 12, the best candidate for showing mantles, does not show the 3.4 micro-m band which appreciable mantles would produce. Grains are larger in outer-cloud dust than diffuse dust because of coagulation, not accretion of extensive mantles. Core-mantle grains favored by J. M. Greenberg and collaborators, and composite grains of Mathis and Whiffen (1989), are discussed more extensively (naturally, I prefer the latter). The composite grains are fluffy and consist of silicates, amorphous carbon, and some graphite in the same grain. Grains deep within molecular clouds but before any processing within the solar system are presumably formed from the accretion of icy mantles on and within the coagulated outer-cloud grains. They should contain a mineral/carbonaceous matrix, without organic refractory mantles, in between the ices. Unfortunately, they may be significantly processed by chemical processes accompanying the warming (over the 10 K of the dark cloud cores) which occurs in the outer solar system. Evidence of this processing is the chemical anomalies present in interplanetary dust particles collected in the stratosphere, which may be the most primitive materials we have obtained to date. The comet return mission would greatly clarify the situation, and probably provide samples of genuine interstellar grains.

Mathis, John S.↗
DOE OSTI2016

THE EFFECTS OF GRAIN SIZE AND TEMPERATURE DISTRIBUTIONS ON THE FORMATION OF INTERSTELLAR ICE MANTLES

Computational models of interstellar gas-grain chemistry have historically adopted a single dust-grain size of 0.1 micron, assumed to be representative of the size distribution present in the interstellar medium. Here, we investigate the effects of a broad grain-size distribution on the chemistry of dust-grain surfaces and the subsequent build-up of molecular ices on the grains, using a three-phase gas-grain chemical model of a quiescent dark cloud. We include an explicit treatment of the grain temperatures, governed both by the visual extinction of the cloud and the size of each individual grain-size population. We find that the temperature difference plays a significant role in determining the total bulk ice composition across the grain-size distribution, while the effects of geometrical differences between size populations appear marginal. We also consider collapse from a diffuse to a dark cloud, allowing dust temperatures to fall. Under the initial diffuse conditions, small grains are too warm to promote grain-mantle build-up, with most ices forming on the mid-sized grains. As collapse proceeds, the more abundant, smallest grains cool and become the dominant ice carriers; the large population of small grains means that this ice is distributed across many grains, with perhaps no more than 40 monolayers of ice each (versus several hundred assuming a single grain size). This effect may be important for the subsequent processing and desorption of the ice during the hot-core phase of star formation, exposing a significant proportion of the ice to the gas phase, increasing the importance of ice-surface chemistry and surface–gas interactions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Three-dimensional hydrodynamic simulations of multiphase galactic disks with star formation feedback. II. Synthetic H I 21 cm line observations

We use three-dimensional numerical hydrodynamic simulations of the turbulent, multiphase atomic interstellar medium (ISM) to construct and analyze synthetic H I 21 cm emission and absorption lines. Our analysis provides detailed tests of 21 cm observables as physical diagnostics of the atomic ISM. In particular, we construct (1) the 'observed' spin temperature, T{sub s,obs}(v{sub ch})≡T{sub B}(v{sub ch})/[1−e{sup −τ(v{sub c}{sub h})}], and its optical-depth weighted mean T {sub s,} {sub obs}; (2) the absorption-corrected ''observed'' column density, N{sub H,obs}∝∫dv{sub ch}T{sub B}(v{sub ch})τ(v{sub ch})/[1−e{sup −τ(v{sub c}{sub h})}]; and (3) the 'observed' fraction of cold neutral medium (CNM), f {sub c,} {sub obs} ≡ T{sub c} /T {sub s,} {sub obs} for T{sub c} the CNM temperature; we compare each observed parameter with true values obtained from line-of-sight (LOS) averages in the simulation. Within individual velocity channels, T {sub s,} {sub obs}(v {sub ch}) is within a factor 1.5 of the true value up to τ(v {sub ch}) ∼ 10. As a consequence, N {sub H,} {sub obs} and T{sub s,} {sub obs} are, respectively, within 5% and 12% of the true values for 90% and 99% of LOSs. The optically thin approximation significantly underestimates N {sub H} for τ > 1. Provided that T{sub c} is constrained, an accurate observational estimate of the CNM mass fraction can be obtained down to 20%. We show that T{sub s,} {sub obs} cannot be used to distinguish the relative proportions of warm and thermally unstable atomic gas, although the presence of thermally unstable gas can be discerned from 21 cm lines with 200 K ≲ T{sub s,} {sub obs}(v {sub ch}) ≲ 1000 K. Our mock observations successfully reproduce and explain the observed distribution of the brightness temperature, optical depth, and spin temperature in Roy et al. The threshold column density for CNM seen in observations is also reproduced by our mock observations. We explain this observed threshold behavior in terms of vertical equilibrium in the local Milky Way's ISM disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PULSAR OBSERVATIONS OF EXTREME SCATTERING EVENTS

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a “diverging plasma lens” across the line of sight (LOS). Modeling the refraction of such a lens indicates that the structure size must be of the order of AU and the electron density of the order of 10s of cm{sup −3}. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs, showing increases in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array. These allow us to make more complete models of the ESE, including an estimate of the “outer-scale” of the turbulence in the plasma lens. These observations clearly show that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the LOS, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures that would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

DETECTION OF CCN (X {sup 2}Π{sub r}) IN IRC+10216: CONSTRAINING CARBON-CHAIN CHEMISTRY

The CCN radical (X {sup 2}Π{sub r}) has been detected for the first time in the interstellar medium. The molecule was observed at the 1-2 mK level toward the circumstellar envelope of IRC+10216, using the facilities of the Arizona Radio Observatory (ARO). Lambda doublets of the J = 4.5 → 3.5 and the J = 6.5 → 5.5 transitions at 106 GHz and 154 GHz in the Ω = 1/2 ladder were measured with the ARO 12 m telescope, as well as the J = 9.5 → 8.5 lines near 225 GHz, using the ARO Sub-Millimeter Telescope (SMT). In the two lower frequency transitions, the e component of the doublet was partially blended with other lines, while emission from the f component was uncontaminated. The lambda doublet of the 1 mm transition is collapsed and appears adjacent to an unidentified line. Modeling of the spectra, using the new radiative transfer code ESCAPADE, indicates that CCN arises from a shell in the outer envelope centered at r ∼ 3 × 10{sup 16} cm, with a peak fractional abundance of f {sub 0} ∼ 6 × 10{sup –8}. The column density of the radical is ∼4 × 10{sup 13} cm{sup –2}, in agreement with that derived from a rotational diagram analysis. Modeling was also conducted for spectra of CN, C{sub 3}N, and HC{sub 3}N, obtained with the SMT in the ARO 1 mm survey. CCN, C{sub 3}N, and HC{sub 3}N appear to originate from adjacent shells in the envelope, suggesting they are chemically related. The [CN]/[CCN]/[C{sub 3}N] abundance ratio was found to be ∼500:1:50.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ORION’S VEIL. IV. H{sub 2} EXCITATION AND GEOMETRY

The foreground Veil of material that lies in front of the Orion Nebula is the best studied sample of the interstellar medium because we know where it is located, how it is illuminated, and the balance of thermal and magnetic energy. In this work, we present high-resolution STIS observations toward the Trapezium, with the goal of better understanding the chemistry and geometry of the two primary Veil layers, along with ionized gas along the line of sight. The most complete characterization of the rotational/vibrational column densities of H{sub 2} in the almost purely atomic components of the Veil are presented, including updates to the Cloudy model for H{sub 2} formation on grain surfaces. The observed H{sub 2} is found to correlate almost exclusively with Component B. The observed H{sub 2}, observations of CI, CI*, and CI**, and theoretical calculations using Cloudy allow us to place the tightest constraints yet on the distance, density, temperature, and other physical characteristics for each cloud component. We find the H{sub 2} excitation spectrum observed in the Veil is incompatible with a recent study that argued that the Veil was quite close to the Trapezium. The nature of a layer of ionized gas lying between the Veil and the Trapezium is characterized through the emission and absorption lines it produces, which we find to be the blueshifted component observed in S iii and P iii absorption. We deduce that, within the next 30–60 thousand years, the blueshifted ionized layer and Component B will merge, which will subsequently merge with Component A in the next one million years.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

MILLISECOND PULSAR SCINTILLATION STUDIES WITH LOFAR: INITIAL RESULTS

High-precision timing of millisecond pulsars (MSPs) over years to decades is a promising technique for direct detection of gravitational waves at nanohertz frequencies. Time-variable, multi-path scattering in the interstellar medium is a significant source of noise for this detector, particularly as timing precision approaches 10 ns or better for MSPs in the pulsar timing array. For many MSPs, the scattering delay above 1 GHz is at the limit of detectability; therefore, we study it at lower frequencies. Using the LOw-Frequency ARray (LOFAR) radio telescope, we have analyzed short (5-20 minutes) observations of 3 MSPs in order to estimate the scattering delay at 110-190 MHz, where the number of scintles is large and, hence, the statistical uncertainty in the scattering delay is small. We used cyclic spectroscopy, still relatively novel in radio astronomy, on baseband-sampled data to achieve unprecedented frequency resolution while retaining adequate pulse-phase resolution. We detected scintillation structure in the spectra of the MSPs PSR B1257+12, PSR J1810+1744, and PSR J2317+1439 with diffractive bandwidths of 6 ± 3, 2.0 ± 0.3, and ∼7 kHz, respectively, where the estimate for PSR J2317+1439 is reliable to about a factor of two. For the brightest of the three pulsars, PSR J1810+1744, we found that the diffractive bandwidth has a power-law behavior Δν{sub d}∝ν{sup α}, where ν is the observing frequency and α = 4.5 ± 0.5, consistent with a Kolmogorov inhomogeneity spectrum. We conclude that this technique holds promise for monitoring the scattering delay of MSPs with LOFAR and other high-sensitivity, low-frequency arrays like the low-frequency component of the Square Kilometre Array.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ULTRAVIOLET ESCAPE FRACTIONS FROM GIANT MOLECULAR CLOUDS DURING EARLY CLUSTER FORMATION

The UV photon escape fraction from molecular clouds is a key parameter for understanding the ionization of the interstellar medium and extragalactic processes such as cosmic reionization. We present the ionizing photon flux and the corresponding photon escape fraction ( f {sub esc}) arising as a consequence of star cluster formation in a turbulent, 10{sup 6} M {sub ⊙} giant molecular cloud, simulated using the code FLASH. We make use of sink particles to represent young, star-forming clusters coupled with a radiative transfer scheme to calculate the emergent UV flux. We find that the ionizing photon flux across the cloud boundary is highly variable in time and space due to the turbulent nature of the intervening gas. The escaping photon fraction remains at ∼5% for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr with a maximum f {sub esc} of 30% and 37%, respectively. These peaks are due to the formation of large H ii regions that expand into regions of lower density, some of which reaching the cloud surface. However, these phases are short-lived, and f {sub esc} drops sharply as the H ii regions are quenched by the central cluster passing through high-density material due to the turbulent nature of the cloud. We find an average f {sub esc} of 15% with factor of two variations over 1 Myr timescales. Our results suggest that assuming a single value for f {sub esc} from a molecular cloud is in general a poor approximation, and that the dynamical evolution of the system leads to large temporal variation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SUPERNOVA DRIVING. III. SYNTHETIC MOLECULAR CLOUD OBSERVATIONS

We present a comparison of molecular clouds (MCs) from a simulation of supernova (SN) driven interstellar medium (ISM) turbulence with real MCs from the Outer Galaxy Survey. The radiative transfer calculations to compute synthetic CO spectra are carried out assuming that the CO relative abundance depends only on gas density, according to four different models. Synthetic MCs are selected above a threshold brightness temperature value, T {sub B,min} = 1.4 K, of the J = 1 − 0 {sup 12}CO line, generating 16 synthetic catalogs (four different spatial resolutions and four CO abundance models), each containing up to several thousands MCs. The comparison with the observations focuses on the mass and size distributions and on the velocity–size and mass–size Larson relations. The mass and size distributions are found to be consistent with the observations, with no significant variations with spatial resolution or chemical model, except in the case of the unrealistic model with constant CO abundance. The velocity–size relation is slightly too steep for some of the models, while the mass–size relation is a bit too shallow for all models only at a spatial resolution dx ≈ 1 pc. The normalizations of the Larson relations show a clear dependence on spatial resolution, for both the synthetic and the real MCs. The comparison of the velocity–size normalization suggests that the SN rate in the Perseus arm is approximately 70% or less of the rate adopted in the simulation. Overall, the realistic properties of the synthetic clouds confirm that SN-driven turbulence can explain the origin and dynamics of MCs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

MAGNETIC FIELDS IN COSMOLOGICAL SIMULATIONS OF DISK GALAXIES

Observationally, magnetic fields reach equipartition with thermal energy and cosmic rays in the interstellar medium of disk galaxies such as the Milky Way. However, thus far cosmological simulations of the formation and evolution of galaxies have usually neglected magnetic fields. We employ the moving-mesh code AREPO to follow for the first time the formation and evolution of a Milky Way-like disk galaxy in its full cosmological context while taking into account magnetic fields. We find that a prescribed tiny magnetic seed field grows exponentially by a small-scale dynamo until it saturates around z = 4 with a magnetic energy of about 10% of the kinetic energy in the center of the galaxy's main progenitor halo. By z = 2, a well-defined gaseous disk forms in which the magnetic field is further amplified by differential rotation, until it saturates at an average field strength of ∼6 μG in the disk plane. In this phase, the magnetic field is transformed from a chaotic small-scale field to an ordered large-scale field coherent on scales comparable to the disk radius. The final magnetic field strength, its radial profile, and the stellar structure of the disk compare well with observational data. A minor merger temporarily increases the magnetic field strength by about a factor of two, before it quickly decays back to its saturation value. Our results are highly insensitive to the initial seed field strength and suggest that the large-scale magnetic field in spiral galaxies can be explained as a result of the cosmic structure formation process.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

CO and [C ii] line emission of molecular clouds: the impact of stellar feedback and non-equilibrium chemistry

We analyse synthetic 12 CO, 13 CO, and [C ii] emission maps of molecular cloud (MC) simulations from the SILCC-Zoom project. We present radiation, magnetohydrodynamic zoom-in simulations of individual clouds, both with and without radiative stellar feedback, forming in a turbulent multiphase interstellar medium following on-the-fly the evolution of e.g. H 2 , CO, and C + . We introduce a novel post-processing routine based on cloudy which accounts for higher ionization states of carbon due to stellar radiation in H ii regions. Synthetic emission maps of [C ii] in and around feedback bubbles show that the bubbles are largely devoid of [C ii], as recently found in observations, which we attribute to the further ionization of C+ into C 2+ . For both 12 CO and 13 CO, the cloud-averaged luminosity ratio, $L_\rm {CO}/L_\rm {[C\, \small {II}]}$, can neither be used as a reliable measure of the H 2 mass fraction nor of the evolutionary stage of the clouds. We note a relation between the $I_\rm {CO}/I_\rm {[C\, \small {II}]}$ intensity ratio and the H 2 mass fraction for individual pixels of our synthetic maps. The scatter, however, is too large to reliably infer the H 2 mass fraction. Finally, the assumption of chemical equilibrium overestimates H2 and CO masses by up to 150 and 50 per cent, respectively, and $L_\rm {CO}$ by up to 60 per cent. The masses of H and C + would be underestimated by 65 and 30 per cent, respectively, and $L_\rm {[C\, \small {II}]}$ by up to 35 per cent. Hence, the assumption of chemical equilibrium in MC simulations introduces intrinsic errors of a factor of 2 in chemical abundances, luminosities, and luminosity ratios.

79 ASTRONOMY AND ASTROPHYSICS↗