22,616 research outputs found
Geomagnetically Trapped Radiation Produced by a High-Altitude Nuclear Explosion on July 9, 1962
Geomagnetically trapped radiation produced by a high altitude nuclear explosio
A new method for obtaining the star formation law in galaxies
We present a new observational method to evaluate the star formation law as
formulated by Schmidt: the power-law expression assumed to relate the rate of
star formation in a volume of space to the local total gas volume density.
Volume densities in the clouds surrounding an OB association are determined
with a simple model which considers atomic hydrogen as a photodissociation
product on cloud surfaces. The photodissociating flux incident on the cloud is
computed from the far-UV luminosity of the OB association and the geometry. We
have applied this "PDR Method" to a sample of star-forming regions in M33 using
VLA 21-cm data for the HI and GALEX imagery in the far-UV. It provides an
estimate of the total volume density of hydrogen (atomic + molecular) in the
gas clouds surrounding the young star cluster. A logarithmic graph of the
cluster UV luminosity versus the surrounding gas density is a direct measure of
the star formation law. However, this plot is severely affected by
observational selection, rendering large areas of the diagram inaccessible to
the data. An ordinary least-squares regression fit therefore gives a strongly
biased result. Its slope primarily reflects the boundary defined when the 21-cm
line becomes optically thick, no longer reliably measuring the HI column
density. We use a maximum-likelihood statistical approach which can deal with
truncated and skewed data, taking into account the large uncertainties in the
derived total gas densities. The exponent we obtain for the Schmidt law in M33
is 1.4 \pm 0.2.Comment: Accepted for publication in Ap
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
A photodissociation model for the morphology of the HI near OB associations in M33
We present an approach for analysing the morphology and physical properties
of Hi features near giant OB asso- ciations in M33, in the context of a model
whereby the Hi excess arises from photodissociation of the molecular gas in
remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented
here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33.
These are the first results of a detailed analysis of the environs of a large
number of OB associations in that galaxy. We present evidence for "diffusion"
of the far-UV radiation from the OB association through a clumpy remnant GMC,
and show further that enhanced CO(1-0) emission appears preferentially
associated with GMCs of higher volume density.Comment: Accepted to Ap
Automating the Synthetic Field Method:Application to Sextans A
We have automated the ``Synthetic Field Method'' developed by Gonzalez et
al.(1998) and used it to measure the opacity of the ISM in the Local Group
dwarf galaxy Sextans A by using the changes in counts of background galaxies
seen through the foreground system. The Sextans A results are consistent with
the observational relation found by Cuillandre et al. (2001) between dust
opacity and HI column density in the outer parts of M31.Comment: 4 pages, 6 figures, submitted for the proceedings of The Dynamics,
Structure and History of Galaxies: A Workshop in Honour of Prof. Ken Freema
The Opacity of Spiral Galaxy Disks IV: Radial Extinction Profiles from Counts of Distant Galaxies seen through Foreground Disks
Dust extinction can be determined from the number of distant field galaxies
seen through a spiral disk. To calibrate this number for the crowding and
confusion introduced by the foreground image, Gonzalez et al.(1998) and
Holwerda et al. (2005) developed the ``Synthetic Field Method'' (SFM), which
analyses synthetic fields constructed by adding various deep exposures of
unobstructed background fields to the candidate foreground galaxy field.
The advantage of the SFM is that it gives the average opacity for area of
galaxy disk without assumptions about either the distribution of absorbers or
of the disk starlight. However it is limited by low statistics of the surviving
field galaxies, hence the need to combine a larger sample of fields. This paper
presents the first results for a sample of 32 deep HST/WFPC2 archival fields of
29 spirals.
The radial profiles of average dust extinction in spiral galaxies based on
calibrated counts of distant field galaxies is presented here, both for
individual galaxies as well as for composites from our sample. The effects of
inclination, spiral arms and Hubble type on the radial extinction profile are
discussed. (Abbreviated)Comment: 43 pages, 16 figures, 3 tables, accepted for publication in the
Astronomical Journal, (typos, table update, updates abstract
- …