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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,405 records · Page 78
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DOE OSTI2016

Young, star-forming galaxies and their local counterparts: The evolving relationship of mass–SFR–metallicity since z ∼ 2.1

We explore the evolution of the Stellar Mass–Star Formation Rate (SFR)–Metallicity relation using a set of 256 COSMOS and GOODS galaxies in the redshift range 1.90 < z < 2.35. We present the galaxies’ rest-frame optical emission-line fluxes derived from IR-grism spectroscopy with the Hubble Space Telescope and combine these data with SFRs and stellar masses obtained from deep, multi-wavelength (rest-frame UV to IR) photometry. We then compare these measurements to those for a local sample of galaxies carefully matched in stellar mass (7.5≲log(M{sub ∗}/M{sub ⊙})≲10.5) and SFR (−0.5≲log(SFR)≲2.5 in M{sub ⊙} yr{sup −1}). We find that the distribution of z ∼ 2.1 galaxies in stellar mass–SFR–metallicity space is clearly different from that derived for our sample of similarly bright (L{sub Hβ}>3×10{sup 40} erg s{sup −1}) local galaxies, and this offset cannot be explained by simple systematic offsets in the derived quantities. At stellar masses above ∼10{sup 9} M{sub ⊙} and SFRs above ∼10 M{sub ⊙} yr{sup −1}, the z ∼ 2.1 galaxies have higher oxygen abundances than their local counterparts, while the opposite is true for lower-mass, lower-SFR systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

NUSTAR OBSERVATIONS OF THE YOUNG, ENERGETIC RADIO PULSAR PSR B1509–58

We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of the young rotation-powered radio pulsar PSR B1509−59 in the supernova remnant MSH 15−52. We confirm the previously reported curvature in the hard X-ray spectrum, showing that a log parabolic model provides a statistically superior fit to the spectrum compared with the standard power law. The log parabolic model describes the NuSTAR data, as well as previously published γ-ray data obtained with COMPTEL and AGILE, all together spanning 3 keV through 500 MeV. Our spectral modeling allows us to constrain the peak of the broadband high energy spectrum to be at 2.6 ± 0.8 MeV, an improvement of nearly an order of magnitude in precision over previous measurements. In addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm previously reported variations of photon indices with phase. Finally, we measure the pulsed fraction of PSR B1509−58 in the hard X-ray energy band for the first time. Using the energy resolved pulsed fraction results, we estimate that the pulsar’s off-pulse emission has a photon index value between 1.26 and 1.96. Our results support a model in which the pulsar’s lack of GeV emission is due to viewing geometry, with the X-rays originating from synchrotron emission from secondary pairs in the magnetosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Inferences from the distributions of fast radio burst pulse widths, dispersion measures, and fluences

The widths, dispersion measures (DMs), dispersion indices, and fluences of Fast Radio Bursts (FRBs) impose coupled constraints that all models must satisfy. The non-monotonic dependence of burst widths (after deconvolution of instrumental effects) on DMs excludes the intergalactic medium as the location of scattering that broadens the FRBs in time. Temporal broadening far greater than that of pulsars at similar high Galactic latitudes implies that scattering occurs close to the sources where high densities and strong turbulence or heterogeneity are plausible. FRB energetics are consistent with supergiant pulses from young, fast, high-field pulsars at cosmological distances. The distribution of FRB DMs is: (1) inconsistent with that of expanding clouds (such as SNRs); (2) inconsistent with space-limited source populations (such as the local Supercluster); and (3) consistent with intergalactic dispersion of a homogeneous source population at cosmological distances. Finally, the FRB log N–log S relation also indicates a cosmological distribution aside from the anomalously bright Lorimer burst.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Globular cluster populations: Results including S{sup 4}G late-type galaxies

Using 3.6 and 4.5 μm images of 73 late-type, edge-on galaxies from the S{sup 4}G survey, we compare the richness of the globular cluster populations of these galaxies to those of early-type galaxies that we measured previously. In general, the galaxies presented here fill in the distribution for galaxies with lower stellar mass, M{sub *}, specifically log(M{sub ∗}/M{sub ⊙})<10, overlap the results for early-type galaxies of similar masses, and, by doing so, strengthen the case for a dependence of the number of globular clusters per 10{sup 9}M{sub ⊙} of galaxy stellar mass, T{sub N}, on M{sub *}. For 8.5<log(M{sub ∗}/M{sub ⊙})<10.5 we find the relationship can be satisfactorily described as T{sub N}=(M{sub ∗}/10{sup 6.7}){sup −0.56} when M{sub *} is expressed in solar masses. The functional form of the relationship is only weakly constrained, and extrapolation outside this range is not advised. Our late-type galaxies, in contrast to our early types, do not show the tendency for low-mass galaxies to split into two T{sub N} families. Using these results and a galaxy stellar mass function from the literature, we calculate that, in a volume-limited, local universe sample, clusters are most likely to be found around fairly massive galaxies (M{sub *} ∼ 10{sup 10.8}M{sub ⊙}) and present a fitting function for the volume number density of clusters as a function of parent-galaxy stellar mass. We find no correlation between T{sub N} and large-scale environment, but we do find a tendency for galaxies of fixed M{sub *} to have larger T{sub N} if they have converted a larger proportion of their baryons into stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Star formation black hole growth and dusty tori in the most luminous AGNS at Z = 2–3.5

We report Herschel/SPIRE observations of 100 very luminous, optically selected active galactic nuclei (AGNs) at z=2--3.5 with log L{sub 1350} (erg s{sup −1}) ≥ 46.5 , where L{sub 1350} is λL{sub λ} at 1350 Å. The distribution in L{sub 1350} is similar to the general distribution of Sloan Digital Sky Survey AGNs in this redshift and luminosity interval. We measured star-formation (SF) luminosity, L{sub SF}, and SF rate (SFR) in 34 detected sources by fitting combined SF and torus templates, where the torus emission is based on Wide Field Infrared Survey Explorer observations. We also obtained statistically significant stacks for the undetected sources in two luminosity groups. The sample properties are compared with those of very luminous AGNs at z>4.5. The main findings are: (1) The mean and the median SFRs of the detected sources are 1176{sub −339}{sup +476} and 1010{sub −503}{sup +706} M{sub ⊙} yr{sup −1}, respectively. The mean SFR of the undetected sources is 148 M{sub ⊙} yr{sup −1}. The ratio of SFR to the black hole accretion rate is ≈80 for the detected sources and less than 10 for the undetected sources. Unlike a sample of sources at z ≃ 4.8 that we studied recently, there is no difference in L{sub AGN} and only a very small difference in L{sub torus} between the detected and undetected sources. (2) The redshift distribution of L{sub SF} and L{sub AGN} for the most luminous, redshift 2–7 AGNs are different. Similar to previous studies, the highest L{sub AGN} are found at z ≈ 3. However, the L{sub SF} of such sources peaks at z ≈ 5. Assuming the objects in our sample are hosted by the most massive galaxies at those redshifts, we find that approximately 2/3 of the hosts are already below the main sequence of SF galaxies at z = 2−3.5. (3) The spectral energy distributions (SEDs) of dusty tori at high redshift are similar to the shapes found in low redshift, low luminosity AGNs. Herschel upper limits put strong constraints on the long wavelength shape of the SED, ruling out several earlier suggested torus templates as applicable for this sample. (4) We find no evidence for a luminosity dependence of the torus covering factor in sources with log L{sub AGN} (erg s{sup −1}) = 44−47.5. This conclusion is based on the recognition that the estimated L{sub AGN} in several earlier studies is highly uncertain and non-uniformally treated. The median covering factors over this range are 0.68 for isotropic dust emission and 0.4 for anisotropic emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

HIERARCHICAL FORMATION IN ACTION: CHARACTERIZING ACCELERATED GALAXY EVOLUTION IN COMPACT GROUPS USING WHOLE-SKY WISE DATA

Compact groups provide an environment to study the growth of galaxies amid multiple prolonged interactions. With their dense galaxy concentrations and relatively low velocity dispersions, compact groups mimic the conditions of hierarchical galaxy assembly. Compact group galaxies are known to show a bimodality in Spitzer IRAC infrared color space: galaxies are preferentially either quiescent with low specific star formation rates (SSFRs) or prolifically forming stars—galaxies with moderate levels of specific star formation are rare. Previous Spitzer IRAC studies identifying this “canyon” have been limited by small number statistics. We utilize whole-sky Wide-field Infrared Survey Explorer (WISE) data to study 163 compact groups, thereby tripling our previous sample and including more galaxies with intermediate mid-IR colors indicative of moderate SSFRs. We define a distinct WISE mid-IR color space (log[((f{sub 12})/(f{sub 4.6}))]) versus (log[((f{sub 22})/(f{sub 3.4}))]) that we use to identify canyon galaxies from the larger sample. We confirm that compact group galaxies show a bimodal distribution in the mid-infrared and identify 37 canyon galaxies with reliable photometry and intermediate mid-IR colors. Morphologically, we find that the canyon harbors a large population of both Sa–Sbc and E/S0 type galaxies, and that they fall on the optical red sequence rather than the green valley. Finally, we provide a catalog of WISE photometry for 567 of 652 galaxies selected from the sample of 163 compact groups.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

STELLAR MASS–GAS-PHASE METALLICITY RELATION AT 0.5 ≤ z ≤ 0.7: A POWER LAW WITH INCREASING SCATTER TOWARD THE LOW-MASS REGIME

We present the stellar mass (M{sub ∗})–gas-phase metallicity relation (MZR) and its scatter at intermediate redshifts (0.5⩽z⩽0.7) for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given M{sub ∗} of this magnitude-limited (R≲24 AB) sample are representative of normal star-forming galaxies. For masses below 10{sup 9} M{sub ⊙}, our sample of 237 galaxies is ∼10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at 10{sup 8} M{sub ⊙} < M{sub ∗} < 10{sup 11} M{sub ⊙}: 12+log(O/H)=(5.83±0.19) +(0.30±0.02)log(M{sub ∗}/M{sub ⊙}). At 10{sup 9} M{sub ⊙} < M{sub ∗} < 10{sup 10.5} M{sub ⊙}, our MZR shows agreement with others measured at similar redshifts in the literature. Our power-law slope is, however, shallower than the extrapolation of the MZRs of others to masses below 10{sup 9} M{sub ⊙}. The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the fundamental metallicity relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as M{sub ∗} decreases. Our result implies that either the scatter of the baryonic accretion rate (σ{sub M-dot }) or the scatter of the M{sub ∗}–M{sub halo} relation (σ{sub SHMR}) increases as M{sub ∗} decreases. Moreover, our measure of scatter at z=0.7 appears consistent with that found for local galaxies. This lack of redshift evolution constrains models of galaxy evolution to have both σ{sub M-dot }and σ{sub SHMR} remain unchanged from z=0.7 to z = 0.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

PROPERTIES OF CARBON–OXYGEN WHITE DWARFS FROM MONTE CARLO STELLAR MODELS

We investigate properties of carbon–oxygen white dwarfs with respect to the composite uncertainties in the reaction rates using the stellar evolution toolkit, Modules for Experiments in Stellar Astrophysics (MESA) and the probability density functions in the reaction rate library STARLIB. These are the first Monte Carlo stellar evolution studies that use complete stellar models. Focusing on 3 M{sub ⊙} models evolved from the pre main-sequence to the first thermal pulse, we survey the remnant core mass, composition, and structure properties as a function of 26 STARLIB reaction rates covering hydrogen and helium burning using a Principal Component Analysis and Spearman Rank-Order Correlation. Relative to the arithmetic mean value, we find the width of the 95% confidence interval to be ΔM{sub 1TP} ≈ 0.019 M{sub ⊙} for the core mass at the first thermal pulse, Δt{sub 1TP} ≈ 12.50 Myr for the age, Δlog(T{sub c}/K) ≈ 0.013 for the central temperature, Δlog(ρ{sub c}/g cm{sup −3}) ≈ 0.060 for the central density, ΔY{sub e,c} ≈ 2.6 × 10{sup −5} for the central electron fraction, ΔX{sub c}({sup 22}Ne) ≈ 5.8 × 10{sup −4}, ΔX{sub c}({sup 12}C) ≈ 0.392, and ΔX{sub c}({sup 16}O) ≈ 0.392. Uncertainties in the experimental {sup 12}C(α,γ){sup 16}O, triple-α, and {sup 14}N(p,γ){sup 15}O reaction rates dominate these variations. We also consider a grid of 1–6 M{sub ⊙} models evolved from the pre main-sequence to the final white dwarf to probe the sensitivity of the initial–final mass relation to experimental uncertainties in the hydrogen and helium reaction rates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A Chandra study of the image power spectra of 41 cool core and non-cool core galaxy clusters

In this work we propose a new diagnostic to segregate cool core (CC) clusters from non-CC (NCC) clusters by studying the two-dimensional power spectra of the X-ray images observed with the Chandra X-ray observatory. Our sample contains 41 members (z=0.01--0.54) which are selected from the Chandra archive when a high photon count, an adequate angular resolution, a relatively complete detector coverage, and coincident CC–NCC classifications derived with three traditional diagnostics are simultaneously guaranteed. We find that in the log–log space the derived image power spectra can be well represented by a constant model component at large wavenumbers, while at small wavenumbers a power excess beyond the constant component appears in all clusters, with a clear tendency that the excess is stronger in CC clusters. By introducing a new CC diagnostic parameter, i.e., the power excess index (PEI), we classify the clusters in our sample and compare the results with those obtained with three traditional CC diagnostics. We find that the results agree with each other very well. By calculating the PEI values of the simulated clusters, we find that the new diagnostic works well at redshifts up to 0.5 for intermediately sized and massive clusters with a typical Chandra or XMM-Newton pointing observation. The new CC diagnostic has several advantages over its counterparts, e.g., it is free of the effects of the commonly seen centroid shift of the X-ray halo caused by merger event, and the corresponding calculation is straightforward, almost irrelevant to the complicated spectral analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The energy budget of GRBs based on a large sample of prompt and afterglow observations

We compare the isotropic equivalent 15–2000 keV γ-ray energy, E {sub γ}, emitted by a sample of 91 swift Gamma-Ray Bursts with known redshifts, with the isotropic equivalent fireball energy, E {sub fb}, as estimated within the fireball model framework from X-ray afterglow observations of these bursts. The uncertainty in E {sub γ}, which spans the range of ∼10{sup 51} to ∼10{sup 53.5} erg, is ≈25% on average, due mainly to the extrapolation from the BAT detector band to the 15–2000 keV band. The uncertainty in E {sub fb} is approximately a factor of 2, due mainly to the X-ray measurements’ scatter. We find E {sub γ} and E {sub fb} to be tightly correlated. The average(std) of η{sub γ}{sup 11 hr}≡log{sub 10}(E{sub γ}/(3ε{sub e}E{sub fb}{sup 11 hr})) are −0.34(0.60), and the upper limit on the intrinsic spread of η {sub γ} is approximately 0.5 (ε{sub e} is the fraction of energy carried by electrons and E{sub fb}{sup x hr} is inferred from the X-ray flux at x hours). E{sub fb}{sup 3 hr} and E{sub fb}{sup 11 hr} are similar, with an average(std) of log{sub 10}(E{sub fb}{sup 3 hr}/E{sub fb}{sup 11 hr}) of 0.04(0.28). The small variance of η {sub γ} implies that burst-to-burst variations in ε{sub e} and in the efficiency of fireball energy conversion to γ-rays are small, and suggests that both are of order unity. The small variance of η {sub γ} and the similarity of E{sub fb}{sup 3 hr} and E{sub fb}{sup 11 hr} further imply that deviations from a simple fireball model description, if present, are small. This puts stringent constraints on models incorporating such modifications (due e.g., to radiative losses, energy injection, off-axis viewing).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The effects of the local environment and stellar mass on galaxy quenching to z ∼ 3

We study the effects of the local environment and stellar mass on galaxy properties using a mass complete sample of quiescent and star-forming systems in the COSMOS field at z≲3. We show that at z≲1 the median star formation rate (SFR) and specific SFR (sSFR) of all galaxies depend on the environment, but they become independent of the environment at z ≳ 1. However, we find that only for star-forming galaxies, the median SFR and sSFR are similar in different environments regardless of redshift and stellar mass. We find that the quiescent fraction depends on the environment at z ≲ 1 and on stellar mass out to z ∼ 3. We show that at z ≲ 1 galaxies become quiescent faster in denser environments and that the overall environmental quenching efficiency increases with cosmic time. Environmental and mass quenching processes depend on each other. At z ≲ 1 denser environments more efficiently quench galaxies with higher masses (log(M/M{sub ⊙}) ≳ 10.7), possibly due to a higher merger rate of massive galaxies in denser environments. We also show that mass quenching is more efficient in denser regions. We show that the overall mass quenching efficiency (ϵ{sub mass}) for more massive galaxies (log(M/M{sub ⊙}) ≳ 10.2) rises with cosmic time until z ∼ 1 and then flattens out. However, for less massive galaxies, the rise in ϵ{sub mass} continues to the present time. Our results suggest that environmental quenching is only relevant at z ≲ 1 and is likely a fast process, whereas mass quenching is the dominant mechanism at z ≳ 1 with a possible stellar feedback physics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

NuSTAR HARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. II. X-RAY POINT SOURCES

We present the first survey results of hard X-ray point sources in the Galactic Center (GC) region by NuSTAR. We have discovered 70 hard (3–79 keV) X-ray point sources in a 0.6 deg 2 region around Sgr A* with a total exposure of 1.7 Ms, and 7 sources in the Sgr B2 field with 300 ks. We identify clear Chandra counterparts for 58 NuSTAR sources and assign candidate counterparts for the remaining 19. The NuSTAR survey reaches X-ray luminosities of ~4x and ~8 x 10 32 erg s -1 at the GC (8 kpc) in the 3–10 and 10–40 keV bands, respectively. The source list includes three persistent luminous X-ray binaries (XBs) and the likely run-away pulsar called the Cannonball. New source-detection significance maps reveal a cluster of hard (>10 keV) X-ray sources near the Sgr A diffuse complex with no clear soft X-ray counterparts. The severe extinction observed in the Chandra spectra indicates that all the NuSTAR sources are in the central bulge or are of extragalactic origin. Spectral analysis of relatively bright NuSTAR sources suggests that magnetic cataclysmic variables constitute a large fraction (>40%–60%). Both spectral analysis and logN–logS distributions of the NuSTAR sources indicate that the X-ray spectra of the NuSTAR sources should have kT > 20 keV on average for a single temperature thermal plasma model or an average photon index of Γ = 1.5–2 for a power-law model. These findings suggest that the GC X-ray source population may contain a larger fraction of XBs with high plasma temperatures than the field population.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TESTING THE PROPAGATING FLUCTUATIONS MODEL WITH A LONG, GLOBAL ACCRETION DISK SIMULATION

The broadband variability of many accreting systems displays characteristic structures; log-normal flux distributions, root-mean square (rms)-flux relations, and long inter-band lags. These characteristics are usually interpreted as inward propagating fluctuations of the mass accretion rate in an accretion disk driven by stochasticity of the angular momentum transport mechanism. We present the first analysis of propagating fluctuations in a long-duration, high-resolution, global three-dimensional magnetohydrodynamic (MHD) simulation of a geometrically thin ( h / r ≈ 0.1) accretion disk around a black hole. While the dynamical-timescale turbulent fluctuations in the Maxwell stresses are too rapid to drive radially coherent fluctuations in the accretion rate, we find that the low-frequency quasi-periodic dynamo action introduces low-frequency fluctuations in the Maxwell stresses, which then drive the propagating fluctuations. Examining both the mass accretion rate and emission proxies, we recover log-normality, linear rms-flux relations, and radial coherence that would produce inter-band lags. Hence, we successfully relate and connect the phenomenology of propagating fluctuations to modern MHD accretion disk theory.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Aperture effects on the oxygen abundance determinations from CALIFA data

This paper aims to provide aperture corrections for emission lines in a sample of spiral galaxies from the Calar Alto Legacy Integral Field Area Survey (CALIFA) database. In particular, we explore the behavior of the log([O iii] λ5007/Hβ)/([N ii] λ6583/Hα) (O3N2) and log[N ii] λ6583/Hα (N2) flux ratios since they are closely connected to different empirical calibrations of the oxygen abundances in star-forming galaxies. We compute the median growth curves of Hα, Hα/Hβ, O3N2, and N2 up to 2.5R {sub 50} and 1.5 disk R{sub eff}. These distances cover most of the optical spatial extent of the CALIFA galaxies. The growth curves simulate the effect of observing galaxies through apertures of varying radii. We split these growth curves by morphological types and stellar masses to check if there is any dependence on these properties. The median growth curve of the Hα flux shows a monotonous increase with radius with no strong dependence on galaxy inclination, morphological type, and stellar mass. The median growth curve of the Hα/Hβ ratio monotonically decreases from the center toward larger radii, showing for small apertures a maximum value of ≈10% larger than the integrated one. It does not show any dependence on inclination, morphological type, and stellar mass. The median growth curve of N2 shows a similar behavior, decreasing from the center toward larger radii. No strong dependence is seen on the inclination, morphological type, and stellar mass. Finally, the median growth curve of O3N2 increases monotonically with radius, and it does not show dependence on the inclination. However, at small radii it shows systematically higher values for galaxies of earlier morphological types and for high stellar mass galaxies. Applying our aperture corrections to a sample of galaxies from the SDSS survey at 0.02 ≤ z ≤ 0.3 shows that the average difference between fiber-based and aperture-corrected oxygen abundances, for different galaxy stellar mass and redshift ranges, reaches typically to ≈11%, depending on the abundance calibration used. This average difference is found to be systematically biased, though still within the typical uncertainties of oxygen abundances derived from empirical calibrations. Caution must be exercised when using observations of galaxies for small radii (e.g., below 0.5 R{sub eff}) given the high dispersion shown around the median growth curves. Thus, the application of these median aperture corrections to derive abundances for individual galaxies is not recommended when their fluxes come from radii much smaller than either R {sub 50} or R{sub eff}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

State-specific dissociation rates for H{sub 2}(v, j) + H{sub 2}(v′, j′)

State-specific rate coefficients for the dissociation of H{sub 2} as the result of collisions with H{sub 2} were calculated for all combinations of (v, j) with an internal energy below 1 eV. Full-dimensional quasiclassical trajectories were calculated using the BMKP2 interaction potential with a minimum of 80,000 trajectories at each translational energy. Additional large batches of trajectories were carried out to calculate the cross sections near the threshold to dissociation to attain the desired precision of the rate coefficients. A piecewise linear excitation function was used to calculate the rate coefficient between 100 and 100,000 K. The resulting state-specific rate coefficients, γ, were parametrized as a function of temperature over the range 600–10,000 K using: log{sub 10} γ(t)=a+bz+cz{sup 2}−d((1/t)−1) where t=T/4500 K and z=log{sub 10} t. The values of the resulting rate coefficients were sensitive to the internal energy of both molecules, with initial vibrational energy having a slightly greater effect than rotational energy. This effect diminished as temperature increased.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Carbon and oxygen abundances in low metallicity dwarf galaxies

The study of carbon and oxygen abundances yields information on the time evolution and nucleosynthetic origins of these elements, yet they remain relatively unexplored. At low metallicities, (12+log(O/H) < 8.0), nebular carbon measurements are limited to rest-frame UV collisionally excited emission lines. Therefore, we present the UV spectrophotometry of 12 nearby low-metallicity high-ionization H ii regions in dwarf galaxies obtained using the Cosmic Origins Spectrograph on the Hubble Space Telescope. We present the first analysis of the C/O ratio in local galaxies based solely on simultaneous significant detections of the UV O{sup +2} and C{sup +2} collisionally excited lines in seven of our targets and five objects from the literature to create a final sample of 12 significant detections. Our sample is complemented by optical SDSS spectra, from which we measured the nebular physical conditions and oxygen abundances using the direct method. At low metallicity, (12+log(O/H) < 8.0), no clear trend is evident in C/O versus O/H for the present sample given the large dispersion observed. When combined with recombination line observations at higher values of O/H, a general trend of increasing C/O with increasing O/H is also viable but with some significant outliers. Additionally, we find the C/N ratio appears to be constant (but with significant scatter) over a large range in oxygen abundance, indicating that carbon is predominantly produced by similar nucleosynthetic mechanisms as nitrogen. If true, and our current understanding of nitrogen production is correct, this would indicate that primary production of carbon (a flat trend) dominates at low metallicity, but quasi-secondary production (an increasing trend) becomes prominent at higher metallicities. A larger sample will be needed to determine the true nature and dispersion of the relation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The metal abundances across cosmic time (MACT) survey. II. Evolution of the mass-metallicity relation over 8 billion years, using [O {sub III}] λ4363 Å based metallicities

We present the first results from MMT and Keck spectroscopy for a large sample of 0.1⩽z⩽1 emission-line galaxies selected from our narrowband imaging in the Subaru Deep Field. We measured the weak [O iii] λ4363 emission line for 164 galaxies (66 with at least 3σ detections, and 98 with significant upper limits). The strength of this line is set by the electron temperature for the ionized gas. Because the gas temperature is regulated by the metal content, the gas-phase oxygen abundance is inversely correlated with [O iii] λ4363 line strength. Our temperature-based metallicity study is the first to span ≈8 Gyr of cosmic time and ≈3 dex in stellar mass for low-mass galaxies, log(M{sub ⋆}/M{sub ⊙})≈6.0–9.0. Using extensive multi-wavelength photometry, we measure the evolution of the stellar mass–gas metallicity relation and its dependence on dust-corrected star formation rate (SFR). The latter is obtained from high signal-to-noise Balmer emission-line measurements. Our mass–metallicity relation is consistent with Andrews and Martini at z⩽0.3, and evolves toward lower abundances at a given stellar mass, log(O/H)∝(1+z){sup −2.32{sub −}{sub 0}{sub .}{sub 2}{sub 6}{sup +{sup 0{sup .{sup 5{sup 2}}}}}}. We find that galaxies with lower metallicities have higher SFRs at a given stellar mass and redshift, although the scatter is large (≈0.3 dex) and the trend is weaker than seen in local studies. We also compare our mass–metallicity relation against predictions from high-resolution galaxy formation simulations, and find good agreement with models that adopt energy- and momentum-driven stellar feedback. We identified 16 extremely metal-poor galaxies with abundances of less than a tenth of solar; our most metal-poor galaxy at z≈0.84 is similar to I Zw 18.

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DOE OSTI2016

A NANOFLARE-BASED CELLULAR AUTOMATON MODEL AND THE OBSERVED PROPERTIES OF THE CORONAL PLASMA

We use the cellular automaton model described in López Fuentes and Klimchuk to study the evolution of coronal loop plasmas. The model, based on the idea of a critical misalignment angle in tangled magnetic fields, produces nanoflares of varying frequency with respect to the plasma cooling time. We compare the results of the model with active region (AR) observations obtained with the Hinode /XRT and SDO /AIA instruments. The comparison is based on the statistical properties of synthetic and observed loop light curves. Our results show that the model reproduces the main observational characteristics of the evolution of the plasma in AR coronal loops. The typical intensity fluctuations have amplitudes of 10%–15% both for the model and the observations. The sign of the skewness of the intensity distributions indicates the presence of cooling plasma in the loops. We also study the emission measure (EM) distribution predicted by the model and obtain slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with published observational values.

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