3,321 research outputs found
Coupled Vlasov and two-fluid codes on GPUs
We present a way to combine Vlasov and two-fluid codes for the simulation of
a collisionless plasma in large domains while keeping full information of the
velocity distribution in localized areas of interest. This is made possible by
solving the full Vlasov equation in one region while the remaining area is
treated by a 5-moment two-fluid code. In such a treatment, the main challenge
of coupling kinetic and fluid descriptions is the interchange of physically
correct boundary conditions between the different plasma models. In contrast to
other treatments, we do not rely on any specific form of the distribution
function, e.g. a Maxwellian type. Instead, we combine an extrapolation of the
distribution function and a correction of the moments based on the fluid data.
Thus, throughout the simulation both codes provide the necessary boundary
conditions for each other. A speed-up factor of around 20 is achieved by using
GPUs for the computationally expensive solution of the Vlasov equation and an
overall factor of at least 60 using the coupling strategy combined with the GPU
computation. The coupled codes were then tested on the GEM reconnection
challenge
Search for IR Emission from Intracluster Dust in A2029
We have searched for IR emission from the intracluster dust (ICD) in the
galaxy cluster A2029. Weak signals of enhanced extended emission in the cluster
are detected at both 24 and 70 micron. However, the signals are
indistinguishable from the foreground fluctuations. The 24 versus 70 micron
color map does not discriminate the dust emission in the cluster from the
cirrus emission. After excluding the contamination from the point sources, we
obtain upper limits for the extended ICD emission in A2029, 5 x 10^3 Jy/sr at
24 micron and 5 x 10^4 Jy/sr at 70 micron. The upper limits are generally
consistent with the expectation from theoretical calculations and support a
dust deficiency in the cluster compared to the ISM in our galaxy. Our results
suggest that even with the much improved sensitivity of current IR telescopes,
a clear detection of the IR emission from ICD may be difficult due to cirrus
noise.Comment: 5 pages, 4 figures, accepted by ApJ
The potential for high performance HgCdTe arrays at 4 microns
The potential of existing technology at Rockwell International in terms of the goals for astronomical detector arrays in the 3 to 5 micron interval is evaluated. Measurements have been obtained for a number of samples of HgCdTe diodes manufactured by Rockwell International. All the diodes reported on here had cutoff wavelengths at high temperatures of 4.6 to 4.7 microns. Although no confirming measurements were made, the cutoff wavelength is expected to move to 5 microns or beyond at the low temperatures of our tests. Diode sizes ranged from 20 to 150 microns. The test program yielded full diode curves and relative response at 3.4 microns for the sample diodes as a function of temperature. Dark currents are quoted below as the current passing through the diode with a back bias of 50 mV. The various diode types showed a wide range of behavior, both with regard to dark current and responsibility. The test results for one of the best diode types are illustrated. This detector has a size of 148 microns and a cutoff wavelength of 4.61 microns
Optical and near infrared photometry of Butcher-Oemler clusters
Rich clusters of galaxies at moderate redshifts (z approx. .3) have a larger proportion of optically blue galaxies than their low redshift counterparts. Spectroscopic examination of the blue galaxies by various authors has shown that the blue galaxies are generally Seyferts, show evidence for recent star formation, or are foreground objects. Unfortunately, spectroscopy is too time consuming to be used on large samples. Thus, we have looked for a way to separate Seyferts, starbursts, ellipticals and nonmembers using photometry alone. Five moderate redshift clusters, Abell numbers 777, 963, 1758, 1961 and 2218, have been observed in the V, R and K bands. We model the spectral energy distributions of various kinds of galaxies found in clusters and derive observed colors. We have modeled the spectral energy distributions (SED) of several kinds of galaxies and compute their colors as a function of redshift. We expect to see ellipticals, spirals, starbursts, post-starburst and Seyfert galaxies. The SED of elliptical and Sbc galaxies was observed by Rieke and Rieke. The SEDs for the starburst galaxies was created by adding a reddened 10(exp 8) year old burst to a spiral galaxy SED. The post-starburst (E+A) galaxy SEDs are composed of a slightly reddened 10(exp 9) year old burst and elliptical galaxy SED. SEDs for the Seyferts were created by adding a v(exp -1.1) power law, and a hot dust thermal spectrum to the Sbc. From the SEDs the colors of galaxies at various redshifts with assorted filters were computed. Lilly & Gunn (1985) have optical and infrared photometry for a sample of galaxies in CL0024+1654 observed spectroscopically by Dressler, Gunn and Schneider (1985). We have used this data to choose the most appropriate SEDs for our starburst and post-starburst models. The most likely explanation for the optically blue colors in most cluster galaxies is star formation. Very few galaxies lie in the Seyfert locus. Abel 1758 has more Seyfert candidates than the other clusters, we observed. It seems possible to roughly sort types of galaxies in clusters by color alone. The cluster population seems to vary considerably between clusters, but our K selected sample has few Seyferts in any cluster
Infrared observations of faint comets
Infrared observations of the periodic comets Encke, Stephan-Oterma and Chernykh indicate that the dusty component in this class of comets is not radically different from the dusty component found in nonperiodic comets. The differences in the infrared behavior among these three comets suggest that a range of behaviors rather than a single behavior typifies the cometary activity. The range in albedo (0.02 to 0.10) of the dust calculated for the periodic comets is similar to the range in albedos seen among the asteroids
Disc galaxies with multiple triaxial structures. II. JHK surface photometry and numerical simulations
We present detailed JHK surface photometry with ellipse fits of 13 galaxies
selected from previous optical observations as likely candidates for having a
secondary bar or a triaxial bulge within the primary bar. We have found 7
double-barred galaxies, 3 double-barred galaxies with an additional
intermediate structure with twisted isophotes, and 3 galaxies with a bar and
central twisted isophotes. A global analysis of the structural parameter
characteristics in the I- and K-bands is presented. Various numerical models of
galaxies with bars within bars are also analysed using the ellipse fitting
technique and compared to the observations. A thorough review of the possible
hypotheses able to explain this phenomenon is given with emphasis on the most
likely ones.Comment: 12 pages, AATEX. Accepted for publication in A&A. Large color
postscript figures omitted (Figs. 1), figures 2-9 included; gzip'ed
postscript files of the paper and Figs. 1 available via anonymous ftp at
ftp://obsftp.unige.ch/pub/fri/aasjhk/ , files fri_aasjhk.ps.gz and ngc*.ps.g
Molecular hydrogen in the young starburst in NGC 253
Shocked molecular hydrogen has been observed around the nucleus of the nearby galaxy, NGC 253. This galaxy has a relatively modest luminosity (approx. 3 x 10 to the 10th power solar luminosities) and appears to have no distortions or companions that would indicate a possible interaction. The energy of the galaxy appears to be derived primarily from a starburst. Thus, our observations have caused us to examine the starburst process in some detail to identify how the molecular hydrogen is excited. It is proposed that the molecular hydrogen emission is produced by collisions of dense molecular clouds accelerated by supernovae explosions. Within the nucleus, this process occurs early in the life of the starbust. This suggest a sequence of nuclear starburst development; examples along this sequence from young to old would include NGC 253, M82, NGC 1097, and M31
Molecular Hydrogen and Paschen-alpha Emission in Cooling Flow Galaxies
We present near-infrared spectra obtained to search for Pa-alpha and
molecular hydrogen lines in edge-darkened (FR I-type) radio galaxies with
bright Halpha emission in the redshift range 0.0535<z<0.15. We find that all
three galaxies in our sample (PKS 0745-191, PKS 1346+26, & PKS2322-12) which
are associated with strong cooling flows also have strong Pa-alpha and H_2
(1-0) S(1) through S(5) emission, while other radio galaxies do not. Together
with earlier observations this confirms claims that cooling flow galaxies are
copious emitters of molecular hydrogen with large H_2 (1-0) S(3)/Pa-alpha
ratios in the range 0.5 to 2. The emission is centrally concentrated within the
inner few kiloparsec and could come from warm (T ~ 1000-1500 K) molecular
material which is being deposited by the cooling flow. We speculate that the
H_2 emission could be related to the interaction between the jets and this
molecular gas.Comment: ApJ Letters, in press, AAS LaTex, preprint also available at
http://www.astro.umd.edu/~hfalcke/publications.html#nirga
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