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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

The atomic gas sequence and mass–metallicity relation from dwarfs to massive galaxies

Galaxy scaling relations provide insights into the processes that drive galaxy evolution. The extension of these scaling relations into the dwarf galaxy regime is of particular interest. This is because dwarf galaxies represent a crucial stage in galaxy evolution, and understanding them could also shed light on their role in reionizing the early Universe. There is currently no consensus on the processes that dominate the evolution of dwarfs. In this work, we constrain the atomic gas sequence (stellar mass versus atomic gas fraction) and mass–metallicity relation (stellar mass versus gas-phase metallicity) from dwarf (⁠10 6.5 M ⊙ ⁠) to massive (⁠10 11.5 M ⊙ ) galaxies in the local Universe. The combined optical and 21-cm spectroscopic observations of the Dark Energy Spectroscopic Instrument and Arecibo Legacy Fast ALFA surveys allow us to constrain both scaling relations simultaneously. We find a slope change of the atomic gas sequence at a stellar mass of ~10 9 M ⊙ ⁠. We also find that the shape and scatter of the atomic gas sequence and mass–metallicity relation are strongly linked for both dwarfs and more massive galaxies. Consequently, the low-mass slope change of the atomic gas sequence is imprinted onto the mass–metallicity relation of dwarf galaxies. The mass scale of the measured slope change is consistent with a predicted escape velocity threshold below which low-mass galaxies experience significant supernova-driven gas loss, as well as with a reduction in cold gas accretion onto more massive galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Emergent surface resonance from charge density wave symmetry breaking in TiSe 2

Surface confined electronic states provide a fertile ground for discovering emergent phenomena that have no counterpart in the bulk, offering new routes to manipulate correlations, symmetry breaking, and dimensionality at the atomic scale. Here, in this study, we show that charge density wave (CDW) symmetry breaking can yield surface states in 1⁢T−TiSe 2 . Micro–angle-resolved photoemission spectroscopy (µ-ARPES) resolves a sharp, two-dimensional surface resonant state (SRS) that emerges within the CDW reconstructed low energy spectrum. The SRS exhibits notable temperature dependence and its spectral weight collapses around ∼ 160 K, while CDW transition temperature 𝑇 CDW is commonly reported as ≈ 202 K. Slab DFT + 𝑈 calculations reproduce a surface localized resonance when CDW folding brings valence and conduction states into near degeneracy, suggesting a correlation tuned, surface selective origin. These results point to a form of correlation-tuned surface resonance in a layered CDW compound and suggest a framework for engineering low-dimensional quantum states in van der Waals materials via symmetry breaking and electronic structure tuning.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Spatial Decorrelation of Young Stars and Dense Gas as a Probe of the Star Formation–Feedback Cycle in Galaxies

The spatial decorrelation of dense molecular gas and young stars observed on ≲1 kpc scales in nearby galaxies indicates rapid dispersal of star-forming regions by stellar feedback. In this work, we explore the sensitivity of this decorrelation to different processes controlling the structure of the interstellar medium, the abundance of molecular gas, star formation, and feedback in a suite of simulations of an isolated dwarf galaxy with structural properties similar to NGC 300 that self-consistently model radiative transfer and molecular chemistry. Our fiducial simulation reproduces the magnitude of decorrelation and its scale dependence measured in NGC 300, and we show that this agreement is due to different aspects of feedback, including H 2 dissociation, gas heating by the locally variable UV field, early mechanical feedback, and supernovae. In particular, early radiative and mechanical feedback affects the correlation on ≲100 pc scales, while supernovae play a significant role on ≳ 100 pc scales. The correlation is also sensitive to the choice of the local star formation efficiency per free fall time, ϵ ff , which provides a strong observational constraint on ϵ ff when the global star formation rate is independent of its value. Finally, we explicitly show that the degree of correlation between the peaks of molecular gas and star formation density is directly related to the distribution of the lifetimes of star-forming regions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2013

Optical Properties of Iron Silicates in the Infrared to Millimeter as a Function of Wavelength and Temperature

The Optical Properties of Astronomical Silicates with Infrared Techniques program utilizes multiple instruments to provide spectral data over a wide range of temperatures and wavelengths. Experimental methods include Vector Network Analyzer and Fourier transform spectroscopy transmission, and reflection/scattering measurements. From this data, we can determine the optical parameters for the index of refraction, n, and the absorption coefficient, k. The analysis of the laboratory transmittance data for each sample type is based upon different mathematical models, which are applied to each data set according to their degree of coherence. Presented here are results from iron silicate dust grain analogs, in several sample preparations and at temperatures ranging from 5 to 300 K, across the infrared and millimeter portion of the spectrum (from 2.5 to 10,000/micron or 4000 to 1/cm).

Dust↗
NASA NTRS2014

Searching for Dark Matter Annihilation in the Smith High-Velocity Cloud

Recent observations suggest that some high-velocity clouds may be confined by massive dark matter halos. In particular, the proximity and proposed dark matter content of the Smith Cloud make it a tempting target for the indirect detection of dark matter annihilation. We argue that the Smith Cloud may be a better target than some Milky Way dwarf spheroidal satellite galaxies and use gamma-ray observations from the Fermi Large Area Telescope to search for a dark matter annihilation signal. No significant gamma-ray excess is found coincident with the Smith Cloud, and we set strong limits on the dark matter annihilation cross section assuming a spatially extended dark matter profile consistent with dynamical modeling of the Smith Cloud. Notably, these limits exclude the canonical thermal relic cross section (approximately 3 x 10 (sup −26) cubic centimeters per second) for dark matter masses less than or approximately 30 gigaelectronvolts annihilating via the B/B- bar oscillation or tau/antitau channels for certain assumptions of the dark matter density profile; however, uncertainties in the dark matter content of the Smith Cloud may significantly weaken these constraints.

gamma rays: ISM aEuro" ISM: clouds↗
NASA NTRS2015

Probing the Effects of External Irradiation on Low-Mass Protostars Through Unbiased Line Surveys

The envelopes of molecular gas around embedded low-mass protostars show different chemistries, which can be used to trace their formation history and physical conditions. The excitation conditions of some molecular species can also be used to trace these physical conditions, making it possible to constrain for instance sources of heating and excitation. Aims. We study the range of influence of an intermediate-mass Herbig Be protostar. We also study the effect of feedback from the environment on the chemical and physical properties of embedded protostars. Methods. We followed up on an earlier line survey of the Class 0I source R CrA IRS7B in the 0.8 mm window with an unbiased line survey of the same source in the 1.3 mm window using the Atacama Pathfinder Experiment (APEX) telescope. We also studied the excitation of the key species H2CO, CH3OH, and c-C3H2 in a complete sample of the 18 embedded protostars in the Corona Australis star-forming region. Radiative transfer models were employed to establish abundances of the molecular species. Results. We detect line emission from 20 molecular species (32 including isotopologues) in the two surveys. The most complex species detected are CH3OH, CH3CCH, CH3CHO, and CH3CN (the latter two are only tentatively detected). CH3CN and several other complex organic molecules are significantly under-abundant in comparison with what is found towards hot corino protostars. The H2CO rotational temperatures of the sources in the region decrease with the distance to the Herbig Be star R CrA, whereas the c-C3H2 temperatures remain constant across the star-forming region.Conclusions. The high H2CO temperatures observed towards objects close to R CrA suggest that this star has a sphere of influence of several 10 000 AU in which it increases the temperature of the molecular gas to 3050 K through irradiation. The chemistry in the IRS7B envelope differs significantly from many other embedded protostars, which could be an effect of the external irradiation from R CrA.

ISM: individual objects↗
NASA NTRS2017

Asymmetric Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3

The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded approximately 1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011-2015) of 0.58% per yr, but large spatial variations are present. Using the nonparametric 'Demons' method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from 0.''09 to 0.''44 per yr. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities v(sub s) as low as 3600 km per sec (for an assumed distance of 8.5 kpc), much less than v(sub s) = 12,000-13,000 km per sec along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at 1.3% +/- 0.8% per yr. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion.

ISM: individual objects (G1.9+0.3) ISM: individual↗
NASA NTRS2017

The Refined Shock Velocity of the X-Ray Filaments in the RCW 86 Northeast Rim

A precise measurement of shock velocities is crucial for constraining the mechanism and efficiency of cosmic-ray (CR) acceleration at supernova remnant (SNR) shock fronts. The northeastern rim of the SNR RCW 86 is thought to be a particularly efficient CR acceleration site, owing to the recent result in which an extremely high shock velocity of 6000 km s1 was claimed. Here, we revisit the same SNR rim with the Chandra X-ray Observatory, 11 years after the first observation. This longer baseline than previously available allows us to determine a more accurate proper motion of the nonthermal X-ray filament, revealing a much lower velocity of 3000 340 km s1 (and even slower at a brighter region). Although the value has dropped to one-half of that from the previous X-ray measurement, it is still higher than the mean velocity of the H filaments in this region (1200 km s1). This discrepancy implies that the filaments bright in nonthermal X-rays and H emission trace different velocity components, and thus a CR pressure constrained by combining the X-ray kinematics and the H spectroscopy can easily be overestimated. We also measure the proper motion of the thermal X-ray filament immediately to the south of the nonthermal one. The inferred velocity (720 360 km s1) is significantly lower than that of the nonthermal filament, suggesting the presence of denser ambient material, possibly a wall formed by a wind from the progenitor, which has drastically slowed down the shock.

acceleration of particles - ISM: individuals objec↗
NASA NTRS2016

X-Ray Burst Oscillations: From Flame Spreading to the Cooling Wake

Type I X-ray bursts are thermonuclear flashes observed from the surfaces of accreting neutron stars (NSs) in low mass X-ray binaries. Oscillations have been observed during the rise and/or decay of some of these X-ray bursts. Those seen during the rise can be well explained by a spreading hot spot model, but large amplitude oscillations in the decay phase remain mysterious because of the absence of a clear-cut source of asymmetry. To date there have not been any quantitative studies that consistently track the oscillation amplitude both during the rise and decay (cooling tail) of bursts. Here we compute the light curves and amplitudes of oscillations in X-ray burst models that realistically account for both flame spreading and subsequent cooling. We present results for several such "cooling wake" models, a "canonical" cooling model where each patch on the NS surface heats and cools identically, or with a latitude-dependent cooling timescale set by the local effective gravity, and an "asymmetric" model where parts of the star cool at significantly different rates. We show that while the canonical cooling models can generate oscillations in the tails of bursts, they cannot easily produce the highest observed modulation amplitudes. Alternatively, a simple phenomenological model with asymmetric cooling can achieve higher amplitudes consistent with the observations.

ISM: individual objects (G32.8-0.1↗
NASA NTRS2017

The Origin of the Iron-Rich Knot in Tycho's Supernova Remnant

X-ray observations of supernova remnants (SNRs) allow us to investigate the chemical inhomogeneity of ejecta, offering unique insight into the nucleosynthesis in supernova explosions. Here we present detailed imaging and spectroscopic studies of the Fe knot located along the eastern rim of the Type Ia SNR Tycho ( SN 1572) using Suzaku and Chandra long-exposure data. Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn, or Ni, which is unusual for the Fe-rich regions in this SNR. Within the framework of the canonical delayed-detonation models for SN Ia, the observed mass ratios M(sub Cr)/M(sub Fe) is less than 0.023, M(sub Mn)/M(sub Fe) is less than 0.012, and M(sub Ni)/M(sub Fe) is less than 0.029 (at 90% confidence) can only be achieved for a peak temperature of (5.3 - 5.7) x 10(exp. 9) K and a neutron excess of approximately less than 2.0 x 10(exp. -3). These constraints rule out the deep, dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot and favor either incomplete Si burning or an Alpha-rich freeze-out regime, probably close to the boundary. An explosive He burning regime is a possible alternative, although this hypothesis is in conflict with the main properties of this SNR.

ISM supernova remnants↗
NASA NTRS2017

The Three-Dimensional Expansion of the Ejecta from Tycho's Supernova Remnant

We present the first 3D measurements of the velocity of various ejecta knots in Tycho's supernova remnant, known to result from a Type Ia explosion. Chandra X-ray observations over a 12 yr baseline from 2003 to 2015 allow us to measure the proper motion of nearly 60 tufts of Si-rich ejecta, giving us the velocity in the plane of the sky. For the line-of-sight velocity, we use two different methods: a nonequilibrium ionization model fit to the strong Si and S lines in the 1.22.8 keV regime, and a fit consisting of a series of Gaussian lines. These methods give consistent results, allowing us to determine the redshift or blueshift of each of the knots. Assuming a distance of 3.5 kpc, we find total velocities that range from 2400 to 6600 km/s, with a mean of 4430 km/s. We find several regions where the ejecta knots have overtaken the forward shock. These regions have proper motions in excess of 6000 km/s. Some SN Ia explosion models predict a velocity asymmetry in the ejecta. We find no such velocity asymmetries in Tycho, and we discuss our findings in light of various explosion models, favoring those delayed-detonation models with relatively vigorous and symmetrical deflagrations. Finally, we compare measurements with models of the remnant's evolution that include both smooth and clumpy ejecta profiles, finding that both ejecta profiles can be accommodated by the observations.

ISM: individual objects (Tycho’s SNR) – ISM: s↗
NASA NTRS2018

Interstellar Ices and Radiation-Induced Oxidations of Alcohols

Infrared spectra of ices containing alcohols that are known or potential interstellar molecules are examined before and after irradiation with 1 MeV protons at ~20 K. The low-temperature oxidation (hydrogen loss) of six alcohols is followed, and conclusions are drawn based on the results. The formation of reaction products is discussed in terms of the literature on the radiation chemistry of alcohols and a systematic variation in their structures. The results from these new laboratory measurements are then applied to a recent study of propargyl alcohol. Connections are drawn between known interstellar molecules, and several new reaction products in interstellar ices are predicted.

astrochemistry – infrared: ISM – ISM: molecule↗
NASA NTRS2018

Interstellar Ices and Radiation-induced Oxidations of Alcohols

Infrared spectra of ices containing alcohols that are known or potential interstellar molecules are examined before and after irradiation with 1 MeV protons at ~20 K. The low-temperature oxidation (hydrogen loss) of six alcohols is followed, and conclusions are drawn based on the results. The formation of reaction products is discussed in terms of the literature on the radiation chemistry of alcohols and a systematic variation in their structures. The results from these new laboratory measurements are then applied to a recent study of propargyl alcohol. Connections are drawn between known interstellar molecules, and several new reaction products in interstellar ices are predicted.

astrochemistry – infrared: ISM – ISM: molecule↗
NASA NTRS2018

N2 Chemistry in Interstellar and Planetary Ices: Radiation-Driven Oxidation

As part of our work on nitrogen-rich ices, the IR spectra and band strengths used in a recent paper to identify and quantify radiation-induced changes in an N2+H2O ice near 15 K are examined, along with reports of (i) a chemical tracer for N2+H2O ices, (ii) a new IR feature of solid N2, and (iii) a striking 15N isotopic enrichment. Problems are found for each IR band strength used and for each of the three claims made, to the extent that none are supported by the results presented to date. In contrast, new work presented here, combined with several older investigations, strongly supports the formation of di- and triatomic nitrogen oxides in irradiated N2-rich ices. Observations and trends in the chemistry of N2-rich icy solids are described, and conclusions are drawn. A considerable amount of material from previous chemical studies of N2-rich systems, spanning more than a century, is brought together for the rst time and used to examine the chemistry of N2-rich ices in extraterrestrial environments. Needs are identied and suggestions made for future studies of N2-rich interstellar and planetary ice analogs.

astrobiology↗
NASA NTRS2018

Infrared Spectra and Interstellar Sulfur: New Laboratory Results for H2S and Four Malodorous Thiol Ices

New infrared spectra are presented for H2S and four other sulfur-containing compounds, all thiols, at 10–140 K to aid in the study of interstellar and solar system ices. Infrared spectral changes on warming H2S and each thiol are described with an emphasis on the S–H stretching vibration at 2550–2525 cm(exp -1) (λ=3.92–3.96 μm) as it is in a relatively unobscured part of the infrared spectra of interstellar and planetary ices. Infrared positions and band strengths for each thiol’s S–H and C–H stretching vibrations are reported, along with the S–H band strength of H2S. Two band strengths of near-infrared features of CH3SH are included. Results for these compounds are compared, and some areas of agreement and disagreement with the literature are described.

Astrobiology↗
NASA NTRS2018

N2 Chemistry in Interstellar and Planetary Ices: Radiation-Driven Oxidation

As part of our work on nitrogen-rich ices, the IR spectra and band strengths used in a recent paper to identify and quantify radiation-induced changes in an N2+H2O ice near 15 K are examined, along with reports of (i) a chemical tracer for N2+H2O ices, (ii) a new IR feature of solid N2, and (iii) a striking 15N isotopic enrichment. Problems are found for each IR band strength used and for each of the three claims made, to the extent that none are supported by the results presented to date. In contrast, new work presented here, combined with several older investigations, strongly supports the formation of di- and triatomic nitrogen oxides in irradiated N2-rich ices. Observations and trends in the chemistry of N2-rich icy solids are described, and conclusions are drawn. A considerable amount of material from previous chemical studies of N2-rich systems, spanning more than a century, is brought together for the first time and used to examine the chemistry of N2-rich ices in extraterrestrial environments. Needs are identified and suggestions made for future studies of N2-rich interstellar and planetary ice analogs

astrobiology – astrochemistry – infrared: ISM – IS↗
NASA NTRS2018

Infrared Spectra and Interstellar Sulfur: New Laboratory Results for H2S and Four Malodorous Thiol Ices

New infrared spectra are presented for H2S and four other sulfur-containing compounds, all thiols, at 10–140 K to aid in the study of interstellar and solar system ices. Infrared spectral changes on warming H2S and each thiol are described with an emphasis on the S–H stretching vibration at 2550–2525 cm(exp −1) (λ = 3.92–3.96 μm) as it is in a relatively unobscured part of the infrared spectra of interstellar and planetary ices. Infrared positions and band strengths for each thiol’s S–H and C–H stretching vibrations are reported, along with the S–H band strength of H2S. Two band strengths of near-infrared features of CH3SH are included. Results for these compounds are compared, and some areas of agreement and disagreement with the literature are described.

Astrochemistry; infrared: ISM; ISM: molecules; mol↗