701 research outputs found
Densitometry and Thermometry of Starburst Galaxies
With a goal toward deriving the physical conditions in external galaxies, we
present a survey of formaldehyde (H2CO) and ammonia (NH3) emission and
absorption in a sample of starburst galaxies using the Green Bank Telescope. By
extending well-established techniques used to derive the spatial density in
star formation regions in our own Galaxy, we show how the relative intensity of
the 1(10)-1(11) and 2(11)-2(12) K-doublet transitions of H2CO can provide an
accurate densitometer for the active star formation environments found in
starburst galaxies (c.f. Mangum et al. 2008). Similarly, we employ the
well-established technique of using the relative intensities of the (1,1),
(2,2), and (4,4) transitions of NH3 to derive the kinetic temperature in
starburst galaxies. Our measurements of the kinetic temperature constrained
spatial density in our starburst galaxy sample represent the first mean density
measurements made toward starburst galaxies. We note a disparity between
kinetic temperature measurements derived assuming direct coupling to dust and
those derived from our NH3 measurements which points to the absolute need for
direct gas kinetic temperature measurements using an appropriate molecular
probe. Finally, our spatial density measurements point to a rough constancy to
the spatial density (10^{4.5} to 10^{5.5} cm^{-3}) in our starburst galaxy
sample. This implies that the Schmidt-Kennicutt relation between L_{IR} and
M_{dense}: (1) Is a measure of the dense gas mass reservoir available to form
stars, and (2) Is not directly dependent upon a higher average density driving
the star formation process in the most luminous starburst galaxies.Comment: 4 pages, to appear in proceedings of The 5th Zermatt ISM Symposiu
Near-Field Radio Holography of Large Reflector Antennas
We summarise the mathematical foundation of the holographic method of
measuring the reflector profile of an antenna or radio telescope. In
particular, we treat the case, where the signal source is located at a finite
distance from the antenna under test, necessitating the inclusion of the
so-called Fresnel field terms in the radiation integrals. We assume a ``full
phase'' system with reference receiver to provide the reference phase. We
describe in some detail the hardware and software implementation of the system
used for the holographic measurement of the 12m ALMA prototype submillimeter
antennas. We include a description of the practicalities of a measurement and
surface setting. The results for both the VertexRSI and AEC
(Alcatel-EIE-Consortium) prototype ALMA antennas are presented.Comment: 14 pages, 14 figures, to appear in IEEE Antennas and Propagation
Magazine, Vol. 49, No. 5, October 2007. Version 2 includes nice mug-shots of
the author
The On The Fly Imaging Technique
The On-The-Fly (OTF) imaging technique enables single-dish radio telescopes
to construct images of small areas of the sky with greater efficiency and
accuracy. This paper describes the practical application of the OTF imaging
technique. By way of example the implementation of the OTF imaging technique at
the NRAO 12 Meter Telescope is described. Specific requirements for data
sampling, image formation, and Doppler correction are discussed.Comment: 10 pages, 13 figures, accepted A&
Oligomer Composition and Oxygen Binding of the Hemocyanin of the Blue Crab Callinectes sapidus
In the blue crab, the ratio of hexamers to dodecamers of the O2 carrier hemocyanin varies in natural populations. Isolated dodecamers have a lower O2 affinity and greater cooperativity than isolated hexamers. The difference in O2 binding can also be resolved in native mixtures that differ in polymer composition. A high content of dodecamers in native mixtures is, in fact, correlated with the presence of an invariant polypeptide chain that is believed to link two hexamers to form dodecamers. On the other hand, the content of a variable chain that has been postulated to play a role in hexamer pairing is correlated with a low content of dodecamers. The variable, but not the invariant, monomers can be present in levels so low that they must not be represented in all dodecamers in the blood
On the history of the interplay between HD 56925 and NGC 2359
NGC 2359 is an optical nebula excited by the powerful wind and the radiation of the Wolf-Rayet star HD 56925. We have investigated the interaction between this massive star and the surrounding neutral gas by analyzing the large-scale 21cm-HI emission and by mapping the nebula in the J = 1-0 and the J = 2-1 lines of CO. We found a conspicuous (70 x 37 pc) HI shell, expanding at 12 km/s, likely produced during the main-sequence phase of the star. The molecular gas towards NGC 2359 shows three velocity components. Two of these components, A1 and A2, have narrow linewidths (1-2 km/s) and radial velocities of 35-38 and 64-68 km/s, respectively. The third component is detected at radial velocities between 50 and 58 km/s and has a broader profile (up to 5.5 km/s). Furthermore, this component is morphologicaly related with the nebula and has a velocity gradient of a few km/s. We have also estimated the physical parameters of the molecular gas by means of a LVG modelling of the CO emission. The gas projected onto the southern HII region of the nebula has low CO column density and is rather hot, probably up to 80 K. Several profiles of the 13CO J = 1-0 line near the peak of the emission, together with a weak emission bridge between the broad and one of the narrow components (component A2), suggest the presence of a shock front acting in the southern part of the nebula. This shock was likely produced in a previous RSG/LBV phase of HD 56925
Molecular Carbon Chains and Rings in TMC-1
We present mapping results in several rotational transitions of HC3N, C6H,
both cyclic and linear C3H2 and C3H, towards the cyanopolyyne peak of the
filamentary dense cloud TMC-1 using the IRAM 30m and MPIfR 100m telescopes. The
spatial distribution of the cumulene carbon chain propadienylidene H2C3
(hereafter l-C3H2) is found to deviate significantly from the distributions of
the cyclic isomer c-C3H2, HC3N, and C6H which in turn look very similar. The
cyclic over linear abundance ratio of C3H2 increases by a factor of 3 across
the filament, with a value of 28 at the cyanopolyyne peak. This abundance ratio
is an order of magnitude larger than the range (3 to 5) we observed in the
diffuse interstellar medium. The cyclic over linear abundance ratio of C3H also
varies by ~2.5 in TMC-1, reaching a maximum value (13) close to the
cyanopolyyne peak. These behaviors might be related to competitive processes
between ion-neutral and neutral-neutral reactions for cyclic and linear
species.Comment: Accepted for publication in The Astrophysical Journal, part I. 24
pages, including 4 tables, 7 figures, and figure caption
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