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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 631 records · Page 35
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DOE OSTI2016

RESOLVING THE ROTATION MEASURE OF THE M87 JET ON KILOPARSEC SCALES

We investigate the distribution of Faraday rotation measure (RM) in the M87 jet at arcsecond scales by using archival polarimetric Very Large Array data at 8, 15, 22 and 43 GHz. We resolve the structure of the RM in several knots along the jet for the first time. We derive the power spectrum in the arcsecond-scale jet and find indications that the RM cannot be associated with a turbulent magnetic field with a 3D Kolmogorov spectrum. Our analysis indicates that the RM probed on jet scales has a significant contribution of a Faraday screen associated with the vicinity of the jet, in contrast with that on kiloparsec scales, typically assumed to be disconnected from the jet. Comparison with previous RM analyses suggests that the magnetic fields giving rise to the RMs observed in jet scales have different properties and are well less turbulent than those observed in the lobes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A SEARCH FOR SUB-SECOND RADIO VARIABILITY PREDICTED TO ARISE TOWARD 3C 84 FROM INTERGALACTIC DISPERSION

We empirically evaluate the scheme proposed by Lieu and Duan in which the light curve of a time-steady radio source is predicted to exhibit increased variability on a characteristic timescale set by the sightline’s electron column density. Application to extragalactic sources is of significant appeal, as it would enable a unique and reliable probe of cosmic baryons. We examine temporal power spectra for 3C 84, observed at 1.7 GHz with the Karl G. Jansky Very Large Array and the Robert C. Byrd Green Bank Telescope. These data constrain the ratio between standard deviation and mean intensity for 3C 84 to less than 0.05% at temporal frequencies ranging between 0.1 and 200 Hz. This limit is 3 orders of magnitude below the variability predicted by Lieu and Duan and is in accord with theoretical arguments presented by Hirata and McQuinn rebutting electron density dependence. We identify other spectral features in the data consistent with the slow solar wind, a coronal mass ejection, and the ionosphere.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

GAMMA-RAY BURST REVERSE SHOCK EMISSION IN EARLY RADIO AFTERGLOWS

Reverse shock (RS) emission from gamma-ray bursts is an important tool in investigating the nature of the ejecta from the central engine. If the magnetization of the ejecta is not high enough to suppress the RS, a strong RS emission component, usually peaking in the optical/IR band early on, would provide an important contribution to early afterglow light curve. In the radio band, synchrotron self-absorption may suppress early RS emission and also delay the RS peak time. In this paper, we calculate the self-absorbed RS emission in the radio band under different dynamical conditions. In particular, we stress that the RS radio emission is subject to self-absorption in both RSs and forward shocks (FSs). We calculate the ratio between the RS to FS flux at the RS peak time for different frequencies, which is a measure of the detectability of the RS emission component. We then constrain the range of physical parameters for a detectable RS, in particular the role of magnetization. We notice that unlike optical RS emission which is enhanced by moderate magnetization, moderately magnetized ejecta do not necessarily produce a brighter radio RS due to the self-absorption effect. For typical parameters, the RS emission component would not be detectable below 1 GHz unless the medium density is very low (e.g., n < 10{sup −3} cm{sup −3} for the interstellar medium and A {sub *} < 5 × 10{sup −4} for wind). These predictions can be tested using the afterglow observations from current and upcoming radio facilities such as the Karl G. Jansky Very Large Array, the Low-Frequency Array, the Five Hundred Meter Aperture Spherical Telescope, and the Square Kilometer Array.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

NH{sub 3}(3,3) AND CH{sub 3}OH NEAR SUPERNOVA REMNANTS: GBT AND VLA OBSERVATIONS

We report on Green Bank Telescope 23.87 GHz NH{sub 3}(3,3) emission observations in five supernova remnants (SNRs) interacting with molecular clouds (G1.4−0.1, IC 443, W44, W51C, and G5.7−0.0). The observations show a clumpy gas density distribution, and in most cases the narrow line widths of ∼3–4 km s{sup −1} are suggestive of maser emission. Very Large Array observations reveal 36 and/or 44 GHz CH{sub 3}OH maser emission in a majority (72%) of the NH{sub 3} peak positions toward three of these SNRs. This good positional correlation is in agreement with the high densities required for the excitation of each line. Through these observations we have shown that CH{sub 3}OH and NH{sub 3} maser emission can be used as indicators of high-density clumps of gas shocked by SNRs, and provide density estimates thereof. Modeling of the optical depth of the NH{sub 3}(3,3) emission is compared to that of CH{sub 3}OH, constraining the densities of the clumps to a typical density of the order of 10{sup 5} cm{sup −3} for cospatial masers. Regions of gas with this density are found to exist in the post-shocked gas quite close to the SNR shock front, and may be associated with sites where cosmic rays produce gamma-ray emission via neutral pion decay.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Imaging the photoevaporating disk and radio jet of GM Aur

Photoevaporation is probably the main agent for gas dispersal during the last stages of protoplanetary disk evolution. However, the overall mass-loss rate in the photoevaporative wind and its driving mechanism are still not well understood. Here we report multi-configuration Very Large Array observations at 0.7, 3, and 5 cm toward the transitional disk of GM Aur. Our radio continuum observations allow us to image and spatially resolve, for the first time, the three main components at work in this stage of the disk evolution: the disk of dust, the ionized radio jet perpendicular to it, and the photoevaporative wind arising from the disk. The mass-loss rate inferred from the flux density of the radio jet is consistent with the ratio between ejection and accretion rates found in younger objects, suggesting that transitional disks can power collimated ejections of material apparently following the same physical mechanisms as much younger protostars. Our results indicate that extreme-UV (EUV) radiation is the main ionizing mechanism of the photoevaporative wind traced by the free–free emission. The required low EUV photon luminosity of ∼6 × 10{sup 40} s{sup −1} would produce a photoevaporation rate of only M-dot {sub w,EUV}≃1.3×10{sup −10} M{sub ⊙} yr{sup −1}. Therefore, other mechanisms are required to disperse the disk in the timescale imposed by observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Faraday rotation measure synthesis of intermediate redshift quasars as a probe of intervening matter

There is evidence that magnetized material along the line of sight to distant quasars is detectable in the polarization properties of the background sources. The polarization properties appear to be correlated with the presence of intervening Mg ii absorption, which is thought to arise in outflowing material from star forming galaxies. In order to investigate this further, we have obtained high spectral resolution polarization measurements, with the Very Large Array and the Australia Telescope Compact Array, of a set of 49 unresolved quasars for which we have high quality optical spectra. These enable us to produce a Faraday Depth spectrum for each source, using Rotation Measure (RM) Synthesis. Our new independent radio data confirms that interveners are strongly associated with depolarization. We characterize the complexity of the Faraday Depth spectrum using a number of parameters and show how these are related, or not, to the depolarization and to the presence of Mg ii absorption along the line of sight. We argue that complexity and structure in the Faraday Depth distribution likely arise from both intervening material and intrinsically to the background source and attempt to separate these. We find that the strong radio depolarization effects associated with intervening material at redshifts out to z≈1 arise from inhomogeneous Faraday screens producing a dispersion in RM across individual sources of around 10 rad m{sup −2}. This is likely produced by disordered fields with strengths of at least 3 μG.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Variable radio emission from the young stellar host of a hot jupiter

We report the discovery of variable radio emission associated with the T Tauri star, V830 Tau, which was recently shown to host a hot Jupiter companion. Very Large Array observations at a frequency of 6 GHz reveal a detection on 2011 May 1 with a flux density 919 ± 26 μJy, along with non-detections in two other epochs at <66 and <150 μJy. Additionally, Very Long Baseline Array observations include one detection and one non-detection at comparable sensitivity, demonstrating that the emission is nonthermal in origin. The emission is consistent with the gyro-synchrotron or synchrotron mechanism from a region with a magnetic field ≳30 G and is likely driven by an energetic event such as magnetic reconnection that accelerated electrons. With the limited data we have, we are not able to place any constraint on the relationship between the radio emission and the rotational or orbital properties of V830 Tau. This is the first detection of radio emission from a non-degenerate star known to host an exoplanet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

New constraints on the molecular gas in the prototypical HyLIRGs BRI 1202-0725 and BRI 1335-0417

We present Karl G Jansky Very Large Array observations of CO(J=2→1) line emission and rest-frame 250 GHz continuum emission of the Hyper-Luminous IR Galaxies (HyLIRGs) BRI 1202–0725 (z = 4.69) and BRI 1335–0417 (z = 4.41), with an angular resolution as high as 0.″15. Our low-order CO observations delineate the cool molecular gas, the fuel for star formation in the systems, in unprecedented detail. For BRI 1202–0725, line emission is seen from both extreme starburst galaxies: the quasar host and the optically obscured submm galaxy (SMG), in addition to one of the Lyα emitting galaxies in the group. Line emission from the SMG shows an east–west extension of about 0.″6. For Lyα-2, the CO emission is detected at the same velocity as [C ii] and [N ii], indicating a total gas mass ∼4.0 × 10{sup 10} M {sub ⊙}. The CO emission from BRI 1335–0417 peaks at the nominal quasar position, with a prominent northern extension (∼1″, a possible tidal feature). The gas depletion timescales are ∼10{sup 7} years for the three HyLIRGs, consistent with extreme starbursts, while that of Lyα-2 may be consistent with main sequence galaxies. We interpret these sources as major star formation episodes in the formation of massive galaxies and supermassive black holes via gas-rich mergers in the early universe.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

RADIO CONSTRAINTS ON LONG-LIVED MAGNETAR REMNANTS IN SHORT GAMMA-RAY BURSTS

The merger of a neutron star (NS) binary may result in the formation of a rapidly spinning magnetar. The magnetar can potentially survive for seconds or longer as a supramassive NS before collapsing to a black hole if, indeed, it collapses at all. During this process, a fraction of the magnetar’s rotational energy of ∼10{sup 53} erg is transferred via magnetic spin-down to the surrounding ejecta. The resulting interaction between the ejecta and the surrounding circumburst medium powers a year-long or greater synchrotron radio transient. We present a search for radio emission with the Very Large Array following nine short-duration gamma-ray bursts (GRBs) at rest-frame times of ≈1.3–7.6 yr after the bursts, focusing on those events that exhibit early-time excess X-ray emission that may signify the presence of magnetars. We place upper limits of ≲18–32 μ Jy on the 6.0 GHz radio emission, corresponding to spectral luminosities of ≲(0.05–8.3) × 10{sup 39} erg s{sup -1}. Comparing these limits to the predicted radio emission from a long-lived remnant and incorporating measurements of the circumburst densities from broadband modeling of short GRB afterglows, we rule out a stable magnetar with an energy of 10{sup 53} erg for half of the events in our sample. A supramassive remnant that injects a lower rotational energy of 10{sup 52} erg is ruled out for a single event, GRB 050724A. This study represents the deepest and most extensive search for long-term radio emission following short GRBs to date, and thus the most stringent limits placed on the physical properties of magnetars associated with short GRBs from radio observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

A VLA SURVEY FOR FAINT COMPACT RADIO SOURCES IN THE ORION NEBULA CLUSTER

We present Karl G. Jansky Very Large Array 1.3, 3.6, and 6 cm continuum maps of compact radio sources in the Orion Nebular Cluster (ONC). We mosaicked 34 arcmin{sup 2} at 1.3 cm, 70 arcmin{sup 2} at 3.6 cm and 109 arcmin{sup 2} at 6 cm, containing 778 near-infrared detected young stellar objects and 190 Hubble Space Telescope -identified proplyds (with significant overlap between those characterizations). We detected radio emission from 175 compact radio sources in the ONC, including 26 sources that were detected for the first time at these wavelengths. For each detected source, we fitted a simple free–free and dust emission model to characterize the radio emission. We extrapolate the free–free emission spectrum model for each source to ALMA bands to illustrate how these measurements could be used to correctly measure protoplanetary disk dust masses from submillimeter flux measurements. Finally, we compare the fluxes measured in this survey with previously measured fluxes for our targets, as well as four separate epochs of 1.3 cm data, to search for and quantify the variability of our sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Low gas fractions connect compact star-forming galaxies to their z ∼ 2 quiescent descendants

Early quiescent galaxies at z∼2 are known to be remarkably compact compared to their nearby counterparts. Possible progenitors of these systems include galaxies that are structurally similar, but are still rapidly forming stars. Here, we present Karl G. Jansky Very Large Array (VLA) observations of the CO(1–0) line toward three such compact, star-forming galaxies (SFGs) at z∼2.3, significantly detecting one. The VLA observations indicate baryonic gas fractions ≳5 times lower and gas depletion timescales ≳10 times shorter than normal, extended massive SFGs at these redshifts. At their current star formation rates, all three objects will deplete their gas reservoirs within 100 Myr. These objects are among the most gas-poor objects observed at z>2, and are outliers from standard gas scaling relations, a result that remains true regardless of assumptions about the CO–H{sub 2} conversion factor. Our observations are consistent with the idea that compact, SFGs are in a rapid state of transition to quiescence in tandem with the buildup of the z∼2 quenched population. In the detected compact galaxy, we see no evidence of rotation or that the CO-emitting gas is spatially extended relative to the stellar light. This casts doubt on recent suggestions that the gas in these compact galaxies is rotating and significantly extended compared to the stars. Instead, we suggest that, at least for this object, the gas is centrally concentrated, and only traces a small fraction of the total galaxy dynamical mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

THE CIRCUMGALACTIC MEDIUM OF SUBMILLIMETER GALAXIES. I. FIRST RESULTS FROM A RADIO-IDENTIFIED SAMPLE

We present the first results from an ongoing survey to characterize the circumgalactic medium (CGM) of massive high-redshift galaxies detected as submillimeter galaxies (SMGs). We constructed a parent sample of 163 SMG–QSO pairs with separations less than ∼36″ by cross-matching far-infrared-selected galaxies from Herschel with spectroscopically confirmed QSOs. The Herschel sources were selected to match the properties of the SMGs. We determined the sub-arcsecond positions of six Herschel sources with the Very Large Array and obtained secure redshift identification for three of those with near-infrared spectroscopy. The QSO sightlines probe transverse proper distances of 112, 157, and 198 kpc at foreground redshifts of 2.043, 2.515, and 2.184, respectively, which are comparable to the virial radius of the ∼10{sup 13} M {sub ⊙} halos expected to host SMGs. High-quality absorption-line spectroscopy of the QSOs reveals systematically strong H i Ly α absorption around all three SMGs, with rest-frame equivalent widths of ∼2–3 Å. However, none of the three absorbers exhibit compelling evidence for optically thick H i gas or metal absorption, in contrast to the dominance of strong neutral absorbers in the CGM of luminous z ∼ 2 QSOs. The low covering factor of optically thick H i gas around SMGs tentatively indicates that SMGs may not have as prominent cool gas reservoirs in their halos as the coeval QSOs and that they may inhabit less massive halos than previously thought.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SHIELD: neutral gas kinematics and dynamics

We present kinematic analyses of the 12 galaxies in the “Survey of H i in Extremely Low-mass Dwarfs” (SHIELD). We use multi-configuration interferometric observations of the H i 21 cm emission line from the Karl G. Jansky Very Large Array (VLA){sup 22} to produce image cubes at a variety of spatial and spectral resolutions. Both two- and three-dimensional fitting techniques are employed in an attempt to derive inclination-corrected rotation curves for each galaxy. In most cases, the comparable magnitudes of velocity dispersion and projected rotation result in degeneracies that prohibit unambiguous circular velocity solutions. We thus make spatially resolved position–velocity cuts, corrected for inclination using the stellar components, to estimate the circular rotation velocities. We find v{sub circ} ⩽ 30 km s{sup −1} for the entire survey population. Baryonic masses are calculated using single-dish H i fluxes from Arecibo and stellar masses derived from HST and Spitzer imaging. Comparison is made with total dynamical masses estimated from the position–velocity analysis. The SHIELD galaxies are then placed on the baryonic Tully–Fisher relation. There exists an empirical threshold rotational velocity, V {sub rot} < 15 km s{sup −1}, below which current observations cannot differentiate coherent rotation from pressure support. The SHIELD galaxies are representative of an important population of galaxies whose properties cannot be described by current models of rotationally dominated galaxy dynamics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ON THE RELATIONSHIP OF UC H ii REGIONS AND CLASS II METHANOL MASERS. I. SOURCE CATALOGS

We conducted Very Large Array C-configuration observations to measure positions and luminosities of Galactic Class II 6.7 GHz methanol masers and their associated ultra-compact H ii regions. The spectral resolution was 3.90625 kHz and the continuum sensitivity reached 45 μ Jy beam{sup −1}. We mapped 372 methanol masers with peak flux densities of more than 2 Jy selected from the literature. Absolute positions have nominal uncertainties of 0.″3. In this first paper on the data analysis, we present three catalogs; the first gives information on the strongest feature of 367 methanol maser sources, and the second provides information on all detected maser spots. The third catalog presents derived data of the 127 radio continuum counterparts associated with maser sources. Our detection rate of radio continuum counterparts toward methanol masers is approximately one-third. Our catalogs list properties including distance, flux density, luminosity, and the distribution in the Galactic plane. We found no significant relationship between luminosities of masers and their associated radio continuum counterparts, however, the detection rate of radio continuum emission toward maser sources increases statistically with the maser luminosities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

GMRT Low-frequency Imaging of an Extended Sample of X-shaped Radio Galaxies

We present a low-frequency imaging study of an extended sample of X-shaped radio sources using the Giant Metrewave Radio Telescope (GMRT) at two frequencies (610 and 240 MHz). The sources were drawn from a Very Large Array Radio Images of the Sky at Twenty-Centimeters (FIRST) selected sample and extend an initial GMRT study at the same frequencies of 12 X-shaped radio galaxies predominantly from the 3CR catalog (Lal and Rao 2007). Both the intensity maps and spectral index maps of the 16 newly observed sources are presented. With the combined sample of 28 X-shaped radio sources, we found no systematic differences in the spectral properties of the higher surface brightness, active lobes versus the lower surface brightness, or off-axis emission. The properties of the combined sample are discussed, including the possible role of a twin active galactic nuclei model in the formation of such objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

A Multiwavelength Study of the Cool Core Cluster MACS J1447.4+0827

Clusters of galaxies are outstanding laboratories for understanding the physics of supermassive black hole (SMBH) feedback. Here we present the first Chandra, Karl G. Jansky Very Large Array, and Hubble Space Telescope analysis of MACS J1447.4+0827 (z = 0.3755), one of the strongest cool core clusters known, in which extreme feedback from its central SMBH is needed to prevent the hot intracluster gas from cooling. Using this multiwavelength approach, including 70 ks of Chandra X-ray observations, we detect the presence of collimated jetted outflows that coincide with a southern and a northern X-ray cavity. The total mechanical power associated with these outflows (P {sub cav} ≈ 6 × 10{sup 44} erg s{sup −1}) is roughly consistent with the energy required to prevent catastrophic cooling of the hot intracluster gas (L {sub cool} = 1.71 ± 0.01 × 10{sup 45} erg s{sup −1} for t {sub cool} = 7.7 Gyr), implying that powerful SMBH feedback was in place several Gyr ago in MACS J1447.7+0827. In addition, we detect the presence of a radio minihalo that extends over 300 kpc in diameter (P {sub 1.4GHz} = 3.0 ± 0.3 × 10{sup 24} W Hz{sup −1}). The X-ray observations also reveal an ∼20 kpc plumelike structure that coincides with optical dusty filaments that surround the central galaxy. Overall, this study demonstrates that the various physical phenomena occurring in the most nearby clusters of galaxies are also occurring in their more distant analogs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Time-variable Radio Recombination Line Emission in W49A

We present new Jansky Very Large Array (VLA) images of the central region of the W49A star-forming region at 3.6 cm and at 7 mm at resolutions of 0.″15 (1650 au) and 0.″04 (440 au), respectively. The 3.6 cm data reveal new morphological detail in the ultracompact H ii region population, as well as several previously unknown and unresolved sources. In particular, source A shows elongated, edge-brightened bipolar lobes, indicative of a collimated outflow, and source E is resolved into three spherical components. We also present VLA observations of radio recombination lines at 3.6 cm and 7 mm, and IRAM Northern Extended Millimeter Array (NOEMA) observations at 1.2 mm. Three of the smallest ultracompact H ii regions (sources A, B2, and G2) all show broad kinematic linewidths, with ΔV {sub FWHM} ≳ 40 km s{sup −1}. A multi-line analysis indicates that broad linewidths remain after correcting for pressure broadening effects, suggesting the presence of supersonic flows. Substantial changes in linewidth over the 21 yr time baseline at both 3.6 cm and 7 mm are found for source G2. At 3.6 cm, the linewidth of G2 changed from 31.7 ± 1.8 km s{sup −1} to 55.6 ± 2.7 km s{sup −1}, an increase of +23.9 ± 3.4 km s{sup −1}. The G2 source was previously reported to have shown a 3.6 cm continuum flux density decrease of 40% between 1994 and 2015. This source sits near the center of a very young bipolar outflow whose variability may have produced these changes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Ground Vibrational State SiO Emission in the VLA BAaDE Survey

Using a subsample of the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey of stellar SiO masers, we explore the prevalence and characteristics of {sup 28}SiO J = 1 − 0, v = 0 emission. We identify 90 detections of maser, thermal, or composite {sup 28}SiO J = 1 − 0, v = 0 emission out of approximately 13,000 candidate spectra from the National Science Foundation’s Karl G. Jansky Very Large Array (VLA). We find that the detected sources are likely asymptotic giant branch stars belonging to a bright, foreground Milky Way stellar disk population. For the 32 sources showing thermal components, we extract values for outflow velocity by fitting thermal line profiles. We find a range of circumstellar envelope expansion velocities, and compare to previously recorded OH and CO expansion velocities. This preliminary survey is already the largest study of stellar ground vibrational state SiO masers to date, and will be expanded to include the entire VLA BAaDE data set when data reduction for the 18,988 target sources is completed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗