28,942 research outputs found
Colour Gradients in the Optical and Near-IR
For many years broadband colours have been used to obtain insight into the
contents of galaxies, in particular to estimate stellar and dust content.
Broadband colours are easy to obtain for large samples of objects, making them
ideal for statistical studies. In this paper I use the radial distribution of
the colours in galaxies, which gives more insight into the local processes
driving the global colour differences than integrated colours. Almost all
galaxies in my sample of 86 face-on galaxies become systematically bluer with
increasing radius. The radial photometry is compared to new dust extinction
models and stellar population synthesis models. This comparison shows that the
colour gradients in face-on galaxies are best explained by age and metallicity
gradients in the stellar populations and that dust reddening plays a minor
role. The colour gradients imply gradients, making the `missing light'
problem as derived from rotation curve fitting even worse.Comment: 5 pages TeX file, 5 PostScript figures using epsf.sty To appear in
proceedings of the ESO Workshop on "Spiral Galaxies in the Near-IR", eds.: D.
Minniti, H.-W. Rix A gzipped PostScript version with figures in text can be
obtained at http://star-www.dur.ac.uk/~rdejong/papers.htm
Search for the magnetic field of the O7.5 III star xi Persei
Cyclical wind variability is an ubiquitous but as yet unexplained feature
among OB stars. The O7.5 III(n)((f)) star xi Persei is the brightest
representative of this class on the Northern hemisphere. As its prominent
cyclical wind properties vary on a rotational time scale (2 or 4 days) the star
has been already for a long time a serious magnetic candidate. As the cause of
this enigmatic behavior non-radial pulsations and/or a surface magnetic field
are suggested. We present a preliminary report on our attempts to detect a
magnetic field in this star with high-resolution measurements obtained with the
spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in
2006 and 5 nights in 2007. Only upper limits could be obtained, even with the
longest possible exposure times. If the star hosts a magnetic field, its
surface strength should be less than about 300 G. This would still be enough to
disturb the stellar wind significantly. From our new data it seems that the
amplitude of the known non-radial pulsations has changed within less than a
year, which needs further investigation.Comment: 2 pages, 6 figures, contributed poster at IAU Symposium 259 "Cosmic
Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife, Spain,
November 3-7, 200
The stellar populations of spiral galaxies
We have used a large sample of low-inclination spiral galaxies with
radially-resolved optical and near-infrared photometry to investigate trends in
star formation history with radius as a function of galaxy structural
parameters. A maximum likelihood method was used to match all the available
photometry of our sample to the colours predicted by stellar population
synthesis models. The use of simplistic star formation histories, uncertainties
in the stellar population models and regarding the importance of dust all
compromise the absolute ages and metallicities derived in this work, however
our conclusions are robust in a relative sense. We find that most spiral
galaxies have stellar population gradients, in the sense that their inner
regions are older and more metal rich than their outer regions. Our main
conclusion is that the surface density of a galaxy drives its star formation
history, perhaps through a local density dependence in the star formation law.
The mass of a galaxy is a less important parameter; the age of a galaxy is
relatively unaffected by its mass, however the metallicity of galaxies depends
on both surface density and mass. This suggests that galaxy mass-dependent
feedback is an important process in the chemical evolution of galaxies. In
addition, there is significant cosmic scatter suggesting that mass and density
may not be the only parameters affecting the star formation history of a
galaxy.Comment: 25 pages; 17 figures; re-submitted to MNRAS after replying to
referee's repor
Dark and Baryonic Matter in Bright Spiral Galaxies: I.Near-infrared and Optical Broadband Surface Photometry of 30 Galaxies
We present photometrically calibrated images and surface photometry in the B,
V, R, J, H, and K-bands of 25, and in the g, r, and K-bands of 5 nearby bright
(Bo_T<12.5 mag) spiral galaxies with inclinations between 30-65 degrees
spanning the Hubble Sequence from Sa to Scd. Data are from The Ohio State
University Bright Spiral Galaxy Survey, the Two Micron All Sky Survey, and the
Sloan Digital Sky Survey Second Data Release. Radial surface brightness
profiles are extracted, and integrated magnitudes are measured from the
profiles. Axis ratios, position angles, and scale lengths are measured from the
near-infrared images. A 1-dimensional bulge/disk decomposition is performed on
the near-infrared images of galaxies with a non-negligible bulge component, and
an exponential disk is fit to the radial surface brightness profiles of the
remaining galaxies.Comment: 28 page
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