1,941 research outputs found
The Apparent Morphology of Peculiar Galaxies at Intermediate to High Redshifts
We use rest frame ultraviolet (UV), B, and V band images of five nearby
(z<0.02) interacting and/or starbursting galaxies to simulate deep HST
observations of peculiar galaxies at medium to high redshifts. In particular,
we simulate Hubble Deep Field (HDF) observations in the F606W and F814W filters
of starburst galaxies in the redshift range z~0.5---2.5 by explicitly account
for the combined effects of band-shifting and surface brightness dimming. We
find that extended morphological features remain readily visible in the long
exposures typical of the HDF out to redshifts of ~ 1. For systems above z~1.5,
the simulated morphologies look remarkably similar to those of the faint
objects found in the HDF and other deep HST fields. Peculiar starburst galaxies
therefore appear to be the best local analogs to the highest redshift galaxies
in terms of morphology, star formation rates, and spectral energy
distributions. Nevertheless, photometric measurements of the z>1.5 images fail
to recover the true global properties of the underlying systems. This is
because the high-z observations are sensitive to the rest-frame UV emission,
which is dominated by the most active star forming regions. The extended
distribution of starlight from more evolved populations would not be detected.
We conclude that imaging observations in the restframe UV alone cannot reveal
whether high-z systems (z>1.5) are proto-galaxies, proto-bulges, or starbursts
within a pre-existing population. Definitive statements regarding the global
properties and dynamical states of these objects require deep imaging
observations at longer wavelengths.Comment: 15 pages, AAS LaTex macros v4.0, 6 Figs. To appear in The
Astronomical Journal. 1200 kB gzipped encapsulated postscript file of paper
and high-resolution figures is available at
http://www.ifa.hawaii.edu/~hibbard/highZ/ or
http://www.ifa.hawaii.edu/~vacca/highz.htm
The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies
We have mapped the neutral atomic gas (HI) in the three disk-disk merger
systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority
of the mergers mapped in HI, in that their stellar and gaseous tidal features
do not coincide. In particular, they exhibit large stellar tidal features with
little if any accompanying neutral gas and large gas-rich tidal features with
little if any accompanying starlight. On smaller scales, there are striking
anti-correlations where the gaseous and stellar tidal features appear to cross.
We explore several possible causes for these differences, including dust
obscuration, ram pressure stripping, and ionization effects. No single
explanation can account for all of the observed differences. The fact that each
of these systems shows evidence for a starburst driven superwind expanding in
the direction of the most striking anti-correlations leads us to suggest that
the superwind is primarily responsible for the observed differences, either by
sweeping the features clear of gas via ram pressure, or by excavating a clear
sightline towards the starburst and allowing UV photons to ionize regions of
the tails.Comment: 16 pages, 5 figures, uses emulateapj.sty. To appear in the March 2000
issue of AJ. Version with full resolution figures is available via
http://www.cv.nrao.edu/~jhibbard/HIdisp/HIdisp.htm
C^+ distribution around S1 in rho Ophiuchi
We analyze a [C II] 158 micron map obtained with the L2 GREAT receiver on
SOFIA of the emission/reflection nebula illuminated by the early B star S1 in
the rho-OphA cloud core. This data set has been complemented with maps of
CO(3-2), 13CO(3-2) and C18O(3-2), observed as a part of the JCMT Gould Belt
Survey, with archival HCO^+(4-3) JCMT data, as well as with [O I] 63 and 145
micron imaging with Herschel/PACS. The [C II] emission is completely dominated
by the strong PDR emission from the nebula surrounding S1 expanding into the
dense Oph A molecular cloud west and south of S1. The [C II] emission is
significantly blue shifted relative to the CO spectra and also relative to the
systemic velocity, particularly in the northwestern part of the nebula. The [C
II] lines are broader towards the center of the S1 nebula and narrower towards
the PDR shell. The [C II] lines are strongly self-absorbed over an extended
region in the S1 PDR. Based on the strength of the [13C II] F = 2-1 hyperfine
component, [C II] is significantly optically thick over most of the nebula. CO
and 13CO(3-2) spectra are strongly self-absorbed, while C18O(3-2) is single
peaked and centered in the middle of the self-absorption. We have used a simple
two-layer LTE model to characterize the background and foreground cloud
contributing to the [C II] emission. From this analysis we estimate the
extinction due to the foreground cloud to be ~9.9 mag, which is slightly less
than the reddening estimated towards S1. Since some of the hot gas in the PDR
is not traced by low J CO emission, this result appears quite plausible. Using
a plane parallel PDR model with the observed [OI(145)]/[C II] brightness ratio
and an estimated FUV intensity of 3100-5000 G0 suggests that the density of the
[C II] emitting gas is ~3-4x10^3 cm^-3.Comment: Accepted for publication in Astronomy & Astrophysic
On the nature of the flux variability during an expansion stage of a type I X-ray burst: Constraints on Neutron Star Parameters for 4U 1820-30
Powerful Type I X-ray burst with strong radial expansion was observed from
the low mass X-ray binary 4U 1820-30 with Rossi X-ray Timing Explorer on May 2,
1997. We investigate closely the flux profile during the burst expansion
stage. Applying a semi-analytical model we are able to uncover the behavior of
a photospheric radius and to simulate the evolution of neutron star
(NS)-accretion disk system. The bottom flux L_{bot} is a few times the
Eddington limit L_{Edd} for outer layers, because the electron cross-section is
a few times less than the Thomson cross-section at such a high temperatures.
The surplus of energy flux with respect to the Eddington, ,
goes into the potential energy of the expanded envelope. As cooling of the
burning zone starts the surplus decreases and thus the envelope shrinks while
the emergent photon flux stays the same . At a certain moment the NS
low-hemisphere, previously screened by the disk, becomes visible to the
observer. Consequently, the flux detected by the observer increases. Indeed, we
observe to the paradoxical situation when the burning zone cools, but the
apparent flux increases because of the NS-accretion disk geometry. We
demonstrate a strong observational evidence of NS-accretion disk occultation in
the behavior of the observed bolometric flux. We estimate the anisotropy due to
geometry and find that the system should have a high inclination angle.
Finally, we apply an analytical model of X-ray spectral formation in the
neutron star atmosphere during burst decay stage to infer the neutron star (NS)
mass-radius relation.Comment: 15 pages, 3 figures, accepted to ApJ
Design of a Novel Actuation System for Variable Displacement Gear Machine
External spur gear pumps are exceedingly useful components in common hydraulic systems. The main issue with external gear pumps is that all current models are limited to only fixed-displacement. This means that for every revolution of the gears inside the pump, a set amount of fluid will always be displaced. Consequently, external gear pumps are limited in their use because they can only operate at full throttle causing inefficiencies when reduced displacement is needed. The successful procurement of a variable displacement gear pump will allow a more efficient use of hydraulic systems, such as in the displacement controlled systems used in some heavy construction equipment. In this research approach, the displacement is varied by changing the timing of the gears inside the pump. More specifically, changing the timing of the gearsâ connections with the inlet and outlet grooves alters the available volume in the gearsâ spaces, subsequently varying the displacement. With this in mind, additional research was conducted to develop an actuation system for the aforementioned process. This includes the design of new gears, bearing blocks, and additional hydraulics for use in a modified existing commercial pump. Simulations have shown the implementation of the design allowed for a 67% reduction in displacement, and thus successful introduction of variable displacement into an external gear pump. Additional research is needed to determine the optimal designs for the actuation system and variable displacement concept used in the gear pump
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