1,694 research outputs found
The molecular spiral arms of NGC 6946
From CO-12(J=1 to 0) observations at 45 seconds resolution Tacconi and Young (1989) have found evidence for enhancements in both the CO emissivity and the massive star formation efficiency (MSFE) on optical spiral arms of the bright spiral galaxy NGC 6946. In the optically luminous and well-defined spiral arm in the NE quadrant, there are enhancements in both the H2 surface density and MSFE relative to the interarm regions. In contrast, a poorly defined arm in the SW shows no arm-interarm contrast in the MSFE. To further investigate the molecular gas content of these two spiral arms, researchers have made CO-12 J=2 to 1 and 3 to 2 observations with the James Clerk Maxwell Telescope. In the J=2 to 1 line, they made observations of the NE and SW spiral arm and interarm regions in 4 x 9 10 seconds spaced grids (36 points per grid). Because of decreased sensitivity in the J=3 to 2 line, they were limited to mapping the two arm regions in 2 x 3 10 seconds spaced grids (6 points per grid). The centers of each of the grids lie 2.4 minutes to the NE and 2.3 minutes to the SW of the nucleus of NGC 6946. With the CO J=2 to 1 data researchers are able to fully resolve the two observed spiral arms in NGC 6946. In both cases the CO emission is largely confined to the optical spiral arm regions with the peak observed T asterisk sub A being up to 4 times higher on the spiral arms than in the interarm regions. Researchers are currently estimating massive star formation efficiencies on and off the spiral arms through direct comparison of the CO maps with an H alpha image. They are also comparing the CO J=2 to 1 data with an HI map made at similar resolution. Thus, they will be able to determine structure in all components of the IS on scales of less than 20 inches
NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1
Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars
in the central 2'x2' of the cluster Westerlund 1 (Wd 1) with a resolution of
R~9000 at a central wavelength of 2.30 micron. This allowed us to determine
radial velocities of ten post-main-sequence stars, and from these values a
velocity dispersion. Assuming virial equilibrium, the dispersion of sigma=8.4
km/s leads to a total dynamical cluster mass of 1.25x10^5 solar masses,
comparable to the photometric mass of the cluster. There is no extra-virial
motion which would have to be interpreted as a signature of cluster expansion
or dissolution.Comment: To appear in the proceedings of IAU 246: "Dynamical Evolution of
Dense Stellar Systems" (E. Vesperini, M. Giersz, A. Sills, eds.
The azimuthal and radial distributions of HI and H2 in NGC 6946
A study was completed of the atomic and molecular components of the ISM in NGC 6946. The distribution of molecular clouds was determined from a fully sampled CO map of the inner disk using the 14-meter telescope of the FCRAO. The distribution of atomic gas was derived from VLA observations at 40" resolution in the D configuration. When comparing the global CO and HI properties with other components of the galaxy, it was found that the azimuthally averaged radial distributions of CO, H-alpha, radio continuum and blue light all exhibit similar roughly exponential falloffs, while the azimuthally averaged HI surface densities vary by only a factor of 2 out to R = 16 kpc. This indicates that while the H-alpha/CO ratio is approximately constant with radius, the CO/HI ratio decreases by a factor of 30 from the center of the galaxy to R = 10 kpc
NIEL Dose Dependence for Solar Cells Irradiated with Electrons and Protons
The investigation of solar cells degradation and the prediction of its
end-of-life performance is of primary importance in the preparation of a space
mission. In the present work, we investigate the reduction of solar-cells'
maximum power resulting from irradiations with electrons and protons. Both GaAs
single junction and GaInP/GaAs/Ge triple junction solar cells were studied. The
results obtained indicate how i) the dominant radiation damaging mechanism is
due to atomic displacements, ii) the relative maximum power degradation is
almost independent of the type of incoming particle, i.e., iii) to a first
approximation, the fitted semi-empirical function expressing the decrease of
maximum power depends only on the absorbed NIEL dose, and iv) the actual
displacement threshold energy value (Ed=21 eV) accounts for annealing
treatments, mostly due to self-annealing induced effects. Thus, for a given
type of solar cell, a unique maximum power degradation curve can be determined
as a function of the absorbed NIEL dose. The latter expression allows one to
predict the performance of those solar cells in space radiation environment.Comment: To appear on the Proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics
Applications, Villa Olmo (Como, Italy), 23--27 October, 2013, to be published
by World Scientific (Singapore
Measures of galaxy dust and gas mass with Herschel photometry and prospects for ALMA
(Abridged) Combining the deepest Herschel extragalactic surveys (PEP,
GOODS-H, HerMES), and Monte Carlo mock catalogs, we explore the robustness of
dust mass estimates based on modeling of broad band spectral energy
distributions (SEDs) with two popular approaches: Draine & Li (2007, DL07) and
a modified black body (MBB). As long as the observed SED extends to at least
160-200 micron in the rest frame, M(dust) can be recovered with a >3 sigma
significance and without the occurrence of systematics. An average offset of a
factor ~1.5 exists between DL07- and MBB-based dust masses, based on consistent
dust properties. At the depth of the deepest Herschel surveys (in the GOODS-S
field) it is possible to retrieve dust masses with a S/N>=3 for galaxies on the
main sequence of star formation (MS) down to M(stars)~1e10 [M(sun)] up to z~1.
At higher redshift (z<=2) the same result is achieved only for objects at the
tip of the MS or lying above it. Molecular gas masses, obtained converting
M(dust) through the metallicity-dependent gas-to-dust ratio delta(GDR), are
consistent with those based on the scaling of depletion time, and on CO
spectroscopy. Focusing on CO-detected galaxies at z>1, the delta(GDR)
dependence on metallicity is consistent with the local relation. We combine
far-IR Herschel data and sub-mm ALMA expected fluxes to study the advantages of
a full SED coverage.Comment: Accepted for publication in Astronomy and Astrophysics. Some figures
have degraded quality for filesize reason
ALFA & 3D: integral field spectroscopy with adaptive optics
One of the most important techniques for astrophysics with adaptive optics is
the ability to do spectroscopy at diffraction limited scales. The extreme
difficulty of positioning a faint target accurately on a very narrow slit can
be avoided by using an integral field unit, which provides the added benefit of
full spatial coverage. During 1998, working with ALFA and the 3D integral field
spectrometer, we demonstrated the validity of this technique by extracting and
distinguishing spectra from binary stars separated by only 0.26". The
combination of ALFA & 3D is also ideally suited to imaging distant galaxies or
the nuclei of nearby ones, as its field of view can be changed between
1.2"x1.2" and 4"x4", depending on the pixel scale chosen. In this contribution
we present new results both on galactic targets, namely young stellar objects,
as well as extra-galactic objects including a Seyfert and a starburst nucleus.Comment: SPIE meeting 4007 on Adaptive Optical Systems Technology, March 200
Local Swift-BAT active galactic nuclei prefer circumnuclear star formation
We use Herschel data to analyze the size of the far-infrared 70micron
emission for z<0.06 local samples of 277 hosts of Swift-BAT selected active
galactic nuclei (AGN), and 515 comparison galaxies that are not detected by
BAT. For modest far-infrared luminosities 8.5<log(LFIR)<10.5, we find large
scatter of half light radii Re70 for both populations, but a typical Re70 <~ 1
kpc for the BAT hosts that is only half that of comparison galaxies of same
far-infrared luminosity. The result mostly reflects a more compact distribution
of star formation (and hence gas) in the AGN hosts, but compact AGN heated dust
may contribute in some extremely AGN-dominated systems. Our findings are in
support of an AGN-host coevolution where accretion onto the central black hole
and star formation are fed from the same gas reservoir, with more efficient
black hole feeding if that reservoir is more concentrated. The significant
scatter in the far-infrared sizes emphasizes that we are mostly probing spatial
scales much larger than those of actual accretion, and that rapid accretion
variations can smear the distinction between the AGN and comparison categories.
Large samples are hence needed to detect structural differences that favour
feeding of the black hole. No size difference AGN host vs. comparison galaxies
is observed at higher far-infrared luminosities log(LFIR)>10.5 (star formation
rates >~ 6 Msun/yr), possibly because these are typically reached in more
compact regions in the first place.Comment: 7 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
A Compact Starburst Core in the Dusty Lyman Break Galaxy Westphal-MD11
Using the IRAM Plateau de Bure Interferometer, we have searched for CO(3-2)
emission from the dusty Lyman break galaxy Westphal-MD11 at z = 2.98. Our
sensitive upper limit is surprisingly low relative to the system's 850 um flux
density and implies a far-IR/CO luminosity ratio as elevated as those seen in
local ultraluminous mergers. We conclude that the observed dust emission must
originate in a compact structure radiating near its blackbody limit and that a
relatively modest molecular gas reservoir must be fuelling an intense nuclear
starburst (and/or deeply buried active nucleus) that may have been triggered by
a major merger. In this regard, Westphal-MD11 contrasts strikingly with the
lensed Lyman break galaxy MS1512-cB58, which is being observed apparently
midway through an extended episode of more quiescent disk star formation.Comment: 5 pages, 1 figure (emulateapj), accepted by ApJ
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