72 research outputs found
A Morphological Method to Determine Co-Rotation Radii in Spiral Galaxies
Shock induced star formation in a stellar density wave scenario produces an
azimuthal gradient of ages across the spiral arms which has opposite signs on
either side of the corotation resonance (CR). We present a method based on the
Fourier analysis of azimuthal profiles, to locate the CR and determine the arm
character (trailing or leading) in spiral galaxies. Basically, we compare the
behavior of the phase angle of the two-armed spiral in blue and infrared colors
which pick out respectively young and older disk stellar population. We
illustrate the method using theoretical leading and trailing, spirals. We have
also applied the method to the spiral galaxies NGC 7479, for which we confirm
the reported leading arms, and NGC 1832. In these galaxies we find two and
three CRs respectively.Comment: 9 pages, accepted for publication in ApJL, figures 4 and 6 avaliables
at ftp://ftp.inaoep.mx/pub/salida/puerari, full paper also avaliable at
http://www.inaoep.mx/~puerar
Formation of rings in galactic discs by infalling small companions
We use N-body simulations to study the formation of rings in a disc galaxy by
the impact of a small spherical companion. Both barred and nonbarred target
discs are considered. We discuss the effect of the properties of the target
disc (distribution of mass in the disc, velocity dispersion, etc.) as well as
of the mass and orbit of the companion on the properties of the rings, such as
their amplitude, width, shape, expansion velocity and lifetime. In particular
the amplitude, width, lifetime and expansion velocity of the first ring
increase considerably with companion mass, and so does the expansion velocity
of the particles in it and the total extent of the disc after the interaction.
We also discuss the formation and evolution of spokes and show that they can be
caused by companions of relatively small mass. In our three examples of oblique
impacts on barred target galaxies we note important transient displacements of
the bar, as well as changes of its pattern speed and size. An asymmetric
pseudoring is formed in each case, and during the first stages of its evolution
the bar forms part of it.Comment: 21 pages Latex, 22 figures, of which 7 as separate gif-files.
Requires mn.sty, accepted for publication in MNRAS. High resolution version
at http://www-obs.cnrs-mrs.fr/dynamique/pap/ring.htm
Colour jumps across the spiral arms of Hubble Ultra Deep Field galaxies
We have measured, at various wavelengths, the spiral arm pitch angles of a
sample of distant spiral galaxies from the Hubble Space Telescope eXtreme Deep
Field (XDF). According to density wave theory, we should detect colour jumps
from red-to-blue across the spiral arms. Colour jumps are a consequence of
large-scale shocks, which also generate the classic blue-to-red age/colour
gradients, and have only been detected until now in nearby spiral galaxies. Our
results indicate that colour jumps and gradients have been occurring in distant
galaxies for at least the last 8 Gyr, in agreement with density wave theory.Comment: Accepted for publication in MNRA
Variation of Galactic Bar Length with Amplitude and Density as Evidence for Bar Growth over a Hubble Time
K_s-band images of 20 barred galaxies show an increase in the peak amplitude
of the normalized m=2 Fourier component with the R_25-normalized radius at this
peak. This implies that longer bars have higher amplitudes. The long bars
also correlate with an increased density in the central parts of the disks, as
measured by the luminosity inside 0.25R_25 divided by the cube of this radius
in kpc. Because denser galaxies evolve faster, these correlations suggest that
bars grow in length and amplitude over a Hubble time with the fastest evolution
occurring in the densest galaxies. All but three of the sample have early-type
flat bars; there is no clear correlation between the correlated quantities and
the Hubble type.Comment: ApJ Letters, 670, L97, preprint is 7 pages, 4 figure
Citizen engagement in spatial planning, shaping places together
This paper explores the roles and practices of collective citizen engagement in spatial planning. Drawing on a selection of core articles in planning scholarship, it investigates how citizens (re-)shape urban places by responding to perceived flaws in how spatial planning addresses societal challenges. Formal planning interventions are often spatially and socially selective, ineffective, or even non-existent due to a lack of institutional capacities and resources. Consequently, citizens take on roles that they consider as missing, underperformed or ineffective. The paper shows that this results in a variety of practices complementary to, independent from, or opposing formal planning actors and interventions. Five dilemmas citizens face are identified, highlighting the tensions that surface on exclusion, participation, and governmental responsibilities when citizens claim their role in urban governance
Keck spectroscopy and Spitzer Space Telescope analysis of the outer disk of the Triangulum Spiral Galaxy M33
In an earlier study of the spiral galaxy M33, we photometrically identified
arcs or outer spiral arms of intermediate age (0.6 Gyr - 2 Gyr) carbon stars
precisely at the commencement of the HI-warp. Stars in the arcs were
unresolved, but were likely thermally-pulsing asymptotic giant branch carbon
stars. Here we present Keck I spectroscopy of seven intrinsically bright and
red target stars in the outer, northern arc in M33. The target stars have
estimated visual magnitudes as faint as V \sim 25 mag. Absorption bands of CN
are seen in all seven spectra reported here, confirming their carbon star
status. In addition, we present Keck II spectra of a small area 0.5 degree away
from the centre of M33; the target stars there are also identified as carbon
stars. We also study the non-stellar PAH dust morphology of M33 secured using
IRAC on board the Spitzer Space Telescope. The Spitzer 8 micron image attests
to a change of spiral phase at the start of the HI warp. The Keck spectra
confirm that carbon stars may safely be identified on the basis of their red
J-K_s colours in the outer, low metallicity disk of M33. We propose that the
enhanced number of carbon stars in the outer arms are an indicator of recent
star formation, fueled by gas accretion from the HI-warp reservoir.Comment: 9 pages, 5 figures, accepted in A&
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