Rich Disk Research Articles

Inefficient volatile loss from the Moon-forming disk: Reconciling the giant impact hypothesis and a wet Moon

The Earth's Moon is thought to have formed from a circumterrestrial disk generated by a giant impact between the proto-Earth and an impactor approximately 4.5 billion years ago. Since this impact was energetic, the disk would have been hot (4000–6000 K) and partially vaporized (20–100% by mass). This formation process is thought to be responsible for the geochemical observation that the Moon is depleted in volatiles (e.g., K and Na). To explain this volatile depletion, some studies suggest the Moon-forming disk was rich in hydrogen, which was dissociated from water, and it escaped from the disk as a hydrodynamic wind accompanying heavier volatiles (hydrodynamic escape). This model predicts that the Moon should be significantly depleted in water, but this appears to contradict some of the recently measured lunar water abundances and D/H ratios that suggest that the Moon is more water-rich than previously thought. Alternatively, the Moon could have retained its water if the upper parts (low pressure regions) of the disk were dominated by heavier species because hydrogen would have had to diffuse out from the heavy-element rich disk, and therefore the escape rate would have been limited by this slow diffusion process (diffusion-limited escape). To identify which escape the disk would have experienced and to quantify volatiles loss from the disk, we compute the thermal structure of the Moon-forming disk considering various bulk water abundances (100–1000 ppm) and mid-plane disk temperatures (2500–4000 K). Assuming that the disk consists of silicate (SiO2 or Mg2SiO4) and water and that the disk is in the chemical equilibrium, our calculations show that the upper parts of the Moon-forming disk are dominated by heavy atoms or molecules (SiO and O at Tmid>2500–2800 K and H2O at Tmid<2500–2800 K) and hydrogen is a minor species. This indicates that hydrogen escape would have been diffusion-limited, and therefore the amount of lost water and hydrogen would have been small compared to the initial abundance assumed. This result indicates that the giant impact hypothesis can be consistent with the water-rich Moon. Furthermore, since the hydrogen wind would have been weak, the other volatiles would not have escaped either. Thus, the observed volatile depletion of the Moon requires another mechanism.

UGC 3672: an unusual merging triplet of gas-rich galaxies in the Lynx–Cancer void

We present HI 21cm and optical observations of UGC 3672 which is located near the centre of the nearby Lynx-Cancer void. We find that UGC 3672 consists of an approximately linearly aligned triplet of gas rich dwarfs with large scale velocity continuity along the triplet axis. The faintest component of the triplet is extremely gas-rich (MHI/LB ~ 17) and also extremely metal deficient (12+log(O/H) ~ 7.0). The metallicity of this dwarf is close to the 'floor' observed in star forming galaxies. Low resolution HI images show that the galaxy triplet is located inside a common HI envelope, with fairly regular, disk like kinematics. At high angular resolution however, the gas is found to be confined to several filamentary tidal tails and bridges. The linear alignment of the galaxies, along with the velocity continuity that we observe, is consistent with the galaxies lying along a filament. We argue that the location of this highly unusual system in an extremely low density environment is not a coincidence, but is a consequence of structure formation proceeding more slowly and also probing smaller scales than in regions with average density. Our observations also indicate that wet mergers of galaxies flowing along filaments is a possible pathway for the formation of gas rich disks. The UGC 3672 system provides an interesting opportunity to study the kind of interactions typical between high redshift extremely gas rich unevolved small systems that lie at base of the hierarchical galaxy formation model.

THE ACCRETION DISK WIND IN THE BLACK HOLE GRS 1915+105

ABSTRACT We report on a 120 ks Chandra/HETG spectrum of the black hole GRS 1915+105. The observation was made during an extended and bright soft state in 2015 June. An extremely rich disk wind absorption spectrum is detected, similar to that observed at lower sensitivity in 2007. The very high resolution of the third-order spectrum reveals four components to the disk wind in the Fe K band alone; the fastest has a blueshift of v = 0.03 c . Broadened re-emission from the wind is also detected in the first-order spectrum, giving rise to clear accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum gives wind launching radii of r ≃ 10 2 − 4 GM / c 2 . Wind density values of n ≃ 10 13 − 16 cm − 3 are then required by the ionization parameter formalism. The small launching radii, high density values, and inferred high mass outflow rates signal a role for magnetic driving. With simple, reasonable assumptions, the wind properties constrain the magnitude of the emergent magnetic field to be B ≃ 10 3 − 4 G if the wind is driven via magnetohydrodynamic (MHD) pressure from within the disk and B ≃ 10 4 − 5 G if the wind is driven by magnetocentrifugal acceleration. The MHD estimates are below upper limits predicted by the canonical α-disk model. We discuss these results in terms of fundamental disk physics and black hole accretion modes.

The Chemistry of Nearby Disks

AbstractThe gas and dust rich disks around young stars are the formation sites of planets. Observations of molecular trace species have great potential as probes of the disk structures and volatile compositions that together regulate planet formation. The disk around young star TW Hya has become a template for disk molecular studies due to a combination of proximity, a simple face-on geometry and richness in volatiles. It is unclear, however, how typical the chemistry of the TW disk is. In this proceeding, we review lessons learnt from exploring the TW Hya disk chemistry, focusing on the CO snowline, and on deuterium fractionation chemistry. We compare these results with new ALMA observations toward more distant, younger disks. We find that while all disks have some chemical structures in common, there are also substantial differences between the disks, which may be due to different initial conditions, structural or chemical evolutionary stages, or a combination of all three.

EFFECT OF SIC SUBSTRATE PROPERTIES ON STRUCTURAL PERFECTION AND ELECTRICAL PARAMETERS OF ALGAN/G LAYERS

We have analyzed the effect of volume and surface defects SiC substrates on structure and some electrophysical parameters of AlGaN/GaN epitaxial layer heterostructures grown on them. Regions with internal stresses usually induced by carbon rich disk−shaped inclusions were detected in the initial substrates. We show experimentally that the presence of internal stresses in SiC could affect the microroughness of epitaxial films in regions above stressed areas. Abrupt deterioration of electrophysical parameters was observed in regions of epitaxial films growing above areas with internal stresses in the substrate. AlGaN/GaN layers contain impurities delivered to their bulk during epitaxy or preparatory operations.

Mapping and resolving galaxy formation at its peak epoch with Mahalo-Subaru and Gracias-ALMA

AbstractMAHALO-Subaru (MApping HAlpha and Lines of Oxygen with Subaru) project aims to investigate how the star forming activities in galaxies are propagated as a function of time, mass, and environment. It employs a unique set of narrow-band filters on MOIRCS/Subaru to search for Ha emitters associated to the proto-clusters or in narrow redshift slices in the general field. We have shown not only filamentary/clumpy structures of all the proto-clusters but also very high star formation activities therein especially at z &gt; 2. HST images from the CANDELS survey have revealed that nearly half of the Hα emitters in the field at z ∼ 2 have clumpy structures. Among them, “red dusty clumps” are preferentially found at or near the mass center of galaxies. Therefore, they are probably linked to the formation of bulge component. To explore physical states and the mode of star formation of those forming galaxies, we have started Gracias-ALMA project in full coordination with the Mahalo-Subaru project. We will resolve molecular gas contents and dusty star formation within these galaxies, and tell whether clumps are formed by gravitational instability of gas rich disks, and whether bulges are formed by clump migration or through galaxy-galaxy mergers.

EXPANDED VERY LARGE ARRAY OBSERVATIONS OF A PROTO-CLUSTER OF MOLECULAR GAS-RICH GALAXIES AT z = 4.05

We present observations of the molecular gas in the GN20 proto-cluster of galaxies at $z =4.05$ using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the Big Bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H$_2$) between $10^{10}$ and $10^{11} \times (\alpha/0.8)$ M$_\odot$. The emission from the brightest source, GN20, is resolved with a size $\sim 2"$, and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas mass in GN20 is comparable to the stellar mass ($1.3\times 10^{11} \times (\alpha/0.8)$ M$_\odot$ and $2.3\times 10^{11}$ M$_\odot$, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system ($\sim 3.4\times 10^{11} [sin(i)/sin(45^o)]^{-2}$ M$_\odot$), within a 5kpc radius. There is also evidence for a tidal tail extending another $2"$ north of the galaxy with a narrow velocity dispersion. GN20 may be a massive, gas rich disk that is gravitationally disturbed, but not completely disrupted. There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with an optical spectroscopic redshift within our search volume, and we set a 3$\sigma$ limit to the molecular gas mass of this galaxy of $1.1\times 10^{10} \times (\alpha/0.8)$ M$_\odot$.

Thermal stability dependence on Tb content in TbFeCo recording media

Thermal stability of magnetic domain size in magneto-optical recording media TbFeCo is discussed. Shrinkage in domain size under accelerated conditions using irradiating laser light is found more significant for rare earth (RE) rich disk than transition metal (TM) rich disk. The reason for this difference is because the coercivity Hc of the RE-rich type is higher than the TM-rich type at the time of laser irradiation, which is effective against a wall field for minimizing domain size shrinkage.

Calorimetry of Gamma-Ray Bursts: Echoes in Gravitational Waves

Black holes surrounded by a disk or torus may drive the enigmatic cosmological gamma-ray bursts (GRBs). Equivalence in poloidal topology to pulsar magnetospheres shows a high incidence of the black hole-luminosity $L_H$ into the surrounding magnetized matter. We argue that this emission is re-radiated into gravitational waves at $L_{GW}\simeq L_H/3$ in frequencies of order 1kHz, winds and, potentially, MeV neutrinos. The total energy budget and input to the GRB from baryon poor jets are expected to be standard in this scenario, consistent with recent analysis of afterglow data. Collimation of these outflows by baryon rich disk or torus winds may account for the observed spread in opening angles up to about $35^o$. This model may be tested by future LIGO/VIRGO observations.

Superdisks in Radio Galaxies

We present evidence for a striking new feature of powerful radio galaxies: gigantic disklike structures of thermal gas and dust oriented roughly perpendicular to the radio jets and situated around the elliptical host galaxy. These superdisks, or fat pancakes, appear to have a typical diameter of at least 75 kpc and a width of ~25 kpc. Quite plausibly, they are tidally stretched remnants of gas rich disk galaxies, or damped Lyman-a clouds, captured by the massive elliptical, and heated by its hot coronal gas. Observational manifestations of the superdisks include the sharp, quasi-linear, edges of the radio lobes on the side facing the central elliptical, detected in at least a dozen well mapped radio galaxies at low to moderate redshifts. For radio galaxies with z > 1.8, evidence for superdisks is based on the apparent asymmetry of diffuse Lya emission associated with the radio lobes, which is a very sensitive tracer of dust. This asymmetry can be understood if the brighter Lya emission is associated with the radio lobe on the near side of the nucleus and hence not obscured by the dust in the superdisk. Superdisks can provide more consistent explanations for some of the best established correlations among the radio source properties: namely, the Laing-Garrington effect and the correlated radio-optical asymmetry. They are also likely to be responsible for part of the low-frequency flux variability of compact radio sources. We argue they may provide an improved explanation of Lya absorption dips in the emission profiles of distant radio galaxies. Additional methods of searching for superdisks and testing this scenario are proposed.

Infrared Emission from AGN

Infrared observations of complete samples of active galactic nuclei (AGN) have shown that a substantial fraction of their bolometric luminosity is emitted at wavelengths ˜8–1000μm. In radio-loud and Blazar-like objects much of this emission appears to be direct non-thermal synchrotron radiation. However, in the much larger numbers of radio-quiet AGN it is now clear that thermal dust emission is responsible for the bulk of radiation from the near-infrared through submillimeter wavelengths. Luminous infrared-selected AGN are often surrounded by powerful nuclear starbursts, both of which appear to be fueled by enormous supplies of molecular gas and dust funneled into the nuclear region during the strong interaction/merger of gas rich disks. All-sky surveys in the infrared show that luminous infrared AGN are at least as numerous as optically-selected AGN of comparable bolometric luminosity, suggesting that AGN may spend a substantial fraction of their lifetime in a dust-enshrouded phase. The space density of luminous infrared AGN at high redshift may be sufficient to account for much of the X-Ray background, and for a substantial fraction of the far-infrared background as well. These objects plausibly represent a major epoch in the formation of spheroids and massive black holes (MBH).

Active Galactic Nucleus Activity in Giant, Low Surface Brightness Galaxies

A search of large, H I–rich disk galaxies finds a significantly higher fraction of low-luminosity active galactic nucleus (AGN) signatures compared with other late-type galaxies. Approximately half the galaxies selected in this sample have AGN-like behavior in their cores; the rest have H II nuclei resulting from simple star formation. Since AGN behavior is not evident in all the sample galaxies, which where selected by high gas mass, we speculate that it is the fuel flow rate that is the common feature between late-type low surface brightness disks and other active nuclear galaxies.

The Neutral Hydrogen Disk of ARP 10 (=VV 362): A Nonequilibrium Disk Associated with a Galaxy with Rings and Ripples

We present VLA \ion{H}{1} and optical spectra of the peculiar galaxy Arp~10. Originally believed to be an example of a classical colliding ring galaxy with multiple rings, the new observations show a large disturbed neutral hydrogen disk extending 2.7 times the radius of the bright optical ring. We also present evidence for optical shells or ripples in the outer isophotes of the galaxy reminiscent of the ripples seen in some early type systems. The small elliptical originally believed to be the companion is shown to be a background galaxy. The \ion{H}{1} disk consists of two main parts: a very irregular outer structure, and a more regular inner disk associated with the main bright optical ring. In both cases, the \ion{H}{1} structures do not exactly trace the optical morphology. In the outer parts, the \ion{H}{1} distribution does not correlate well with the optical ripples. Even the inner \ion{H}{1} disk does not correspond well morphologically nor kinematically to the optical rings. These peculiarities lead us to believe that the potential in which the \ion{H}{1} disk resides is significantly out of equilibrium --- a situation which would inherently produce rings of star formation. We suggest that Arp~10 is the result of the intermediate stage of a merger between a large \ion{H}{1} rich disk and a gas-poor disk system. As such, it may represent an example of a class of mergers which lies intermediate between the ``ripple and shell'' accretion systems and the head-on collisional ring galaxies.

TbFeCo SINGLE LAYER DISKS FOR MAGNETO-OPTICAL HIGH DENSITY RECORDING

TbFeCo magneto-optical single layer disks with different compositions have been investigated for recording of small size domains. Using a 830 nm wavelength laser, circular shaped domains with a diameter of 0.25 μm diameter could successfully be written. A low writing stability, dependent on the applied field, was found when recording domains smaller than 0.5 μm on a transition metal rich disk. On the contrary, a high writing stability was achieved on a disk with a highly rare earth rich composition. On this disk two 0.25 μm domains could succesfully be written as close as 0.1 μm to each other. It was also possible to use this single layer TbFeCo disk for recording of domains with widths of 0.1 μm.

Write/erase cyclability of TbFeCo for mark edge recording

Domain size changes due to thermal relaxation after high laser power irradiation on amorphous TbFeCo films were studied. In order to achieve mark edge recording for high-density magneto-optical memory, accurate domain size recording is required even after a million erase/write cycles. Highly accelerated erase/write test was performed on typical recording media, rare-earth (RE) -rich and transition-metal (TM) -rich disks. The elongation of the domain size written on the TM-rich film after erase/write cycles was clearly evident; however, the change in the domain size of RE-rich films was small. The small domain size change of the RE-rich disk can be explained in terms of the rapid increase in the coercive force with decreasing temperature and the smaller domain-wall driving force at recording temperature than those for the TM-rich disk, even though both have the same level of structural relaxation.

Modeling the Spatial Distribution of Star Formation in Interacting Disk Galaxies

Models of star-forming interacting galaxies are combined with observational data to investigate the nature of star formation in interacting and merging disk galaxies. Detailed models of specific interacting systems are created using the observed morphology and kinematics of the system to constrain model parameters describing the interaction. The models employ an N-body code to calculate the gravitational dynamics, a discrete cloud model to govern the ISM dynamics, and a modified Schmidt law to - describe star formation. To compare the models to the observed systems, new observations of three interacting systems NGC 4676, IC 1908, and NGC 6872/IC 4970 -- are presented, along with observations of NGC 4038/39 and NGC 7252 culled from the literature. The resulting modeled star-forming properties of each system are then compared to observed star formation tracers such as the spatial distribution of Hα and the total far-infrared luminosity. <P />We find that the models describe the star-forming properties of several observed systems reasonably well in terms of the relative intensity and morphology of induced star formation. In several cases, however, the models underestimate the star formation rate in regions where the ISM is presumed to be experiencing a strong collision, suggesting that collisionally induced star formation may play an important role in such regions. Furthermore, strong star formation is observed in regions of high-velocity dispersion and velocity gradients in Fabry-Perot observations of both IC 1908 and NGC 6872. Finally, the relatively mild modeled starbursts in the merging systems again show the difficulty in triggering starbursts intense enough to drive the emission from ultraluminous infrared galaxies. In order to trigger such activity via merger-induced star formation, the merger must occur rapidly and involve extremely gas rich disks.