340 research outputs found
Constitutional Law - Due Process of Law - Statute Denying Passports to all Communist Party Members Violative of Due Process Clause
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
Rapid formation of exponential disks and bulges at high redshift from the dynamical evolution of clump cluster and chain galaxies
Many galaxies at high redshift have peculiar morphologies dominated by
10^8-10^9 Mo kpc-sized clumps. Using numerical simulations, we show that these
"clump clusters" can result from fragmentation in gravitationally unstable
primordial disks. They appear as "chain galaxies" when observed edge-on. In
less than 1 Gyr, clump formation, migration, disruption, and interaction with
the disk cause these systems to evolve from initially uniform disks into
regular spiral galaxies with an exponential or double-exponential disk profile
and a central bulge. The inner exponential is the initial disk size and the
outer exponential is from material flung out by spiral arms and clump torques.
A nuclear black hole may form at the same time as the bulge from smaller black
holes that grow inside the dense cores of each clump. The properties and
lifetimes of the clumps in our models are consistent with observations of the
clumps in high redshift galaxies, and the stellar motions in our models are
consistent with the observed velocity dispersions and lack of organized
rotation in chain galaxies. We suggest that violently unstable disks are the
first step in spiral galaxy formation. The associated starburst activity gives
a short timescale for the initial stellar disk to form.Comment: ApJ Accepted, 13 pages, 9 figure
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
Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies
Gas-rich disks in the early universe are highly turbulent and have giant
star-forming clumps. Models suggest the clumps form by gravitational
instabilities, and if they resist disruption by star formation, then they
interact, lose angular momentum, and migrate to the center to form a bulge.
Here we study the properties of the bulges formed by this mechanism. They are
all thick, slowly rotating, and have a high Sersic index, like classical
bulges. Their rapid formation should also give them relatively high
alpha-element abundances. We consider fourteen low-resolution models and four
high-resolution models, three of which have supernova feedback. All models have
an active halo, stellar disk, and gaseous disk, three of the models have a
pre-existing bulge and three others have a cuspy dark matter halo. All show the
same basic result except the one with the highest feedback, in which the clumps
are quickly destroyed and the disk thickens too much. The coalescence of
massive disk clumps in the center of a galaxy is like a major merger in terms
of orbital mixing. It differs by leaving a bulge with no specific dark matter
component, unlike the merger of individual galaxies. Normal supernova feedback
has little effect because the high turbulent speed in the gas produces tightly
bound clumps. A variety of indirect observations support the model, including
clumpy disks with young bulges at high redshift and bulges with relatively
little dark matter.Comment: 21 pages, 9 figures, ApJ 688, November 20 2008, in pres
Hierarchical Star Formation in Nearby LEGUS Galaxies
Hierarchical structure in ultraviolet images of 12 late-type LEGUS galaxies
is studied by determining the numbers and fluxes of nested regions as a
function of size from ~1 to ~200 pc, and the number as a function of flux. Two
starburst dwarfs, NGC 1705 and NGC 5253, have steeper number-size and flux-size
distributions than the others, indicating high fractions of the projected areas
filled with star formation. Nine subregions in 7 galaxies have similarly steep
number-size slopes, even when the whole galaxies have shallower slopes. The
results suggest that hierarchically structured star-forming regions several
hundred parsecs or larger represent common unit structures. Small galaxies
dominated by only a few of these units tend to be starbursts. The
self-similarity of young stellar structures down to parsec scales suggests that
star clusters form in the densest parts of a turbulent medium that also forms
loose stellar groupings on larger scales. The presence of super star clusters
in two of our starburst dwarfs would follow from the observed structure if
cloud and stellar subregions more readily coalesce when self-gravity in the
unit cell contributes more to the total gravitational potential.Comment: 9 pages, 4 figures, accepted for ApJ
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