14,233 research outputs found
The Visibility of Galactic Bars and Spiral Structure At High Redshifts
We investigate the visibility of galactic bars and spiral structure in the
distant Universe by artificially redshifting 101 B-band CCD images of local
spiral galaxies from the Ohio State University Bright Spiral Galaxy Survey. Our
artificially redshifted images correspond to Hubble Space Telescope I-band
observations of the local galaxy sample seen at z=0.7, with integration times
matching those of both the very deep Northern Hubble Deep Field data, and the
much shallower Flanking Field observations. The expected visibility of galactic
bars is probed in two ways: (1) using traditional visual classification, and
(2) by charting the changing shape of the galaxy distribution in "Hubble
space", a quantitative two-parameter description of galactic structure that
maps closely on to Hubble's original tuning fork. Both analyses suggest that
over 2/3 of strongly barred luminous local spirals i.e. objects classified as
SB in the Third Reference Catalog) would still be classified as strongly barred
at z=0.7 in the Hubble Deep Field data. Under the same conditions, most weakly
barred spirals (classified SAB in the Third Reference Catalog) would be
classified as regular spirals. The corresponding visibility of spiral structure
is assessed visually, by comparing luminosity classifications for the
artificially redshifted sample with the corresponding luminosity
classifications from the Revised Shapley Ames Catalog. We find that for
exposures times similar to that of the Hubble Deep Field spiral structure
should be detectable in most luminous low-inclination spiral galaxies at z=0.7
in which it is present. [ABRIDGED]Comment: Accepted for publication in The Astronomical Journa
Extent of regretted sexual intercourse among young teenagers in Scotland: a cross sectional survey
No abstract available
FERENGI: Redshifting galaxies from SDSS to GEMS, STAGES and COSMOS
We describe the creation of a set of artificially "redshifted" galaxies in
the range 0.1<z<1.1 using a set of ~100 SDSS low redshift (v<7000 km/s) images
as input. The intention is to generate a training set of realistic images of
galaxies of diverse morphologies and a large range of redshifts for the GEMS
and COSMOS galaxy evolution projects. This training set allows other studies to
investigate and quantify the effects of cosmological redshift on the
determination of galaxy morphologies, distortions and other galaxy properties
that are potentially sensitive to resolution, surface brightness and bandpass
issues. We use galaxy images from the SDSS in the u, g, r, i, z filter bands as
input, and computed new galaxy images from these data, resembling the same
galaxies as located at redshifts 0.1<z<1.1 and viewed with the Hubble Space
Telescope Advanced Camera for Surveys (HST ACS). In this process we take into
account angular size change, cosmological surface brightness dimming, and
spectral change. The latter is achieved by interpolating a spectral energy
distribution that is fit to the input images on a pixel-to-pixel basis. The
output images are created for the specific HST ACS point spread function and
the filters used for GEMS (F606W and F850LP) and COSMOS (F814W). All images are
binned onto the desired pixel grids (0.03" for GEMS and 0.05" for COSMOS) and
corrected to an appropriate point spread function. Noise is added corresponding
to the data quality of the two projects and the images are added onto empty sky
pieces of real data images. We make these datasets available from our website,
as well as the code - FERENGI: "Full and Efficient Redshifting of Ensembles of
Nearby Galaxy Images" - to produce datasets for other redshifts and/or
instruments.Comment: 11 pages, 10 figures, 3 table
Janis-Newman-Winicour and Wyman solutions are the same
We show that the well-known most general static and spherically symmetric
exact solution to the Einstein-massless scalar equations given by Wyman is the
same as one found by Janis, Newman and Winicour several years ago. We obtain
the energy associated with this spacetime and find that the total energy for
the case of the purely scalar field is zero.Comment: 9 pages, LaTex, no figures, misprints corrected, to appear in Int. J.
Mod. Phys.
Non-LTE, Relativistic Accretion Disk Fits to 3C~273 and the Origin of the Lyman Limit Spectral Break
We fit general relativistic, geometrically thin accretion disk models with
non-LTE atmospheres to near simultaneous multiwavelength data of 3C~273,
extending from the optical to the far ultraviolet. Our model fits show no flux
discontinuity associated with a hydrogen Lyman edge, but they do exhibit a
spectral break which qualitatively resembles that seen in the data. This break
arises from relativistic smearing of Lyman emission edges which are produced
locally at tens of gravitational radii in the disk. We discuss the possible
effects of metal line blanketing on the model spectra, as well as the
substantial Comptonization required to explain the observed soft X-ray excess.
Our best fit accretion disk model underpredicts the near ultraviolet emission
in this source, and also has an optical spectrum which is too red. We discuss
some of the remaining physical uncertainties, and suggest in particular that an
extension of our models to the slim disk regime and/or including nonzero
magnetic torques across the innermost stable circular orbit may help resolve
these discrepancies.Comment: Accepted for publication in Ap
The Star Formation History of the Hubble Sequence: Spatially Resolved Colour Distributions of Intermediate Redshift Galaxies in the Hubble Deep Field
We analyse the spatially resolved colours of distant galaxies of known
redshift in the Hubble Deep Field, using a new technique based on matching
resolved four-band internal colour data to the predictions of evolutionary
synthesis models. We quantify the relative age, dispersion in age, ongoing
star-formation rate, star-formation history, and dust content of these
galaxies. To demonstrate the potential of the method, we study the
near-complete sample of 32 I ~ 0.5 studied by
Bouwens et al (1997). The dispersion of the internal colours of a sample of
0.4<z<1 early-type field galaxies in the HDF indicates that ~40% [4/11] show
evidence of star formation which must have occurred within the past third of
their ages at the epoch of observation. For a sample of well-defined spirals,
we similarly exploit the dispersion in colour to analyse the relative histories
of bulge and disc stars, in order to resolve the current controversy regarding
the ages of galactic bulges. Dust and metallicity gradients are ruled out as
major contributors to the colour dispersions we observe in these systems. The
median ages of bulge stars are found to be signicantly older than those in
galactic discs, and exhibit markedly different star-formation histories. This
result is inconsistent with a secular growth of bulges from disc instabilities,
but consistent with gradual disc formation by accretion of gas onto bulges, as
predicted by hierarchical theories. We extend our technique in order to discuss
the star formation history of the entire Bouwens et al sample in the context of
earlier studies concerned with global star formation histories.Comment: 8 colour postscript figures plus LaTeX source; submitted to MNRAS.
Uses the mnras.sty LaTeX style fil
Galaxy Morphology from NICMOS Parallel Imaging
We present high resolution NICMOS images of random fields obtained in
parallel to other HST observations. We present galaxy number counts reaching
H=24. The H-band galaxy counts show good agreement with the deepest I- and
K-band counts obtained from ground-based data. We present the distribution of
galaxies with morphological type to H<23. We find relatively fewer irregular
galaxies compared to an I-band sample from the Hubble Deep Field, which we
attribute to their blue color, rather than to morphological K-corrections. We
conclude that the irregulars are intrinsically faint blue galaxies at z<1.Comment: 13 pages, including 4 figures. Accepted for publication in ApJ
Letter
The connection between the peaks in velocity dispersion and star-forming clumps of turbulent galaxies
We present Keck/OSIRIS adaptive optics observations with 150-400 pc spatial
sampling of 7 turbulent, clumpy disc galaxies from the DYNAMO sample
(). DYNAMO galaxies have previously been shown to be well matched
in properties to main sequence galaxies at . Integral field
spectroscopy observations using adaptive optics are subject to a number of
systematics including a variable PSF and spatial sampling, which we account for
in our analysis. We present gas velocity dispersion maps corrected for these
effects, and confirm that DYNAMO galaxies do have high gas velocity dispersion
(\kms), even at high spatial sampling. We find statistically
significant structure in 6 out of 7 galaxies. The most common distance between
the peaks in velocity dispersion and emission line peaks is ~kpc, we
note this is very similar to the average size of a clump measured with HST
H maps. This could suggest that the peaks in velocity dispersion in
clumpy galaxies likely arise due to some interaction between the clump and the
surrounding ISM of the galaxy, though our observations cannot distinguish
between outflows, inflows or velocity shear. Observations covering a wider area
of the galaxies will be needed to confirm this result.Comment: Accepted for publication in MNRA
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