304 research outputs found
On The Extended Knotted Disks of Galaxies
The stellar disks of many spiral galaxies are twice as large as generally
thought. We use archival data from the Galaxy Evolution Explorer mission
(GALEX) to quantify the statistical properties of young stellar clusters in the
outer, extended disks of a sample of eleven nearby galaxies. We find an excess
of sources between 1.25 and 2 optical radii, R(25), for five of the galaxies,
which statistically implies that at least a quarter of such galaxies have this
cluster population (90% confidence level), and no significant statistical
excess in the sample as a whole beyond 2 optical radii, even though one galaxy
(M 83) individually shows such an excess. Although the excess is typically most
pronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excess
of sources with redder colors. Although from galaxy to galaxy the number of
sources varies significantly, on average, the galaxies with such sources have
75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, the
radial distribution is consistent with the extended dust emission observed in
the far IR and with the properties of H-alpha sources, assuming a constant
cluster formation rate over the last few hundred Myrs. All of these results
suggest that the phenomenon of low-level star formation well outside the
apparent optical edges of disks (R ~ R(25)) is common and long-lasting.Comment: 8 pages, accepted for publication in A
Measuring Outer Disk Warps with Optical Spectroscopy
Warps in the outer gaseous disks of galaxies are a ubiquitous phenomenon, but
it is unclear what generates them. One theory is that warps are generated
internally through spontaneous bending instabilities. Other theories suggest
that they result from the interaction of the outer disk with accreting
extragalactic material. In this case, we expect to find cases where the
circular velocity of the warp gas is poorly correlated with the rotational
velocity of the galaxy disk at the same radius. Optical spectroscopy presents
itself as an interesting alternative to 21-cm observations for testing this
prediction, because (i) separating the kinematics of the warp from those of the
disk requires a spatial resolution that is higher than what is achieved at 21
cm at low HI column density; (ii) optical spectroscopy also provides important
information on star formation rates, gas excitation, and chemical abundances,
which provide clues to the origin of the gas in warps. We present here
preliminary results of a study of the kinematics of gas in the outer-disk warps
of seven edge-on galaxies, using multi-hour VLT/FORS2 spectroscopy.Comment: 7 pages, 7 figures; to appear in the proceedings of IAU Symposium 254
"The Galaxy disk in a cosmological context", Copenhagen, June 200
An Evaluation of Popular Copy-Move Forgery Detection Approaches
A copy-move forgery is created by copying and pasting content within the same
image, and potentially post-processing it. In recent years, the detection of
copy-move forgeries has become one of the most actively researched topics in
blind image forensics. A considerable number of different algorithms have been
proposed focusing on different types of postprocessed copies. In this paper, we
aim to answer which copy-move forgery detection algorithms and processing steps
(e.g., matching, filtering, outlier detection, affine transformation
estimation) perform best in various postprocessing scenarios. The focus of our
analysis is to evaluate the performance of previously proposed feature sets. We
achieve this by casting existing algorithms in a common pipeline. In this
paper, we examined the 15 most prominent feature sets. We analyzed the
detection performance on a per-image basis and on a per-pixel basis. We created
a challenging real-world copy-move dataset, and a software framework for
systematic image manipulation. Experiments show, that the keypoint-based
features SIFT and SURF, as well as the block-based DCT, DWT, KPCA, PCA and
Zernike features perform very well. These feature sets exhibit the best
robustness against various noise sources and downsampling, while reliably
identifying the copied regions.Comment: Main paper: 14 pages, supplemental material: 12 pages, main paper
appeared in IEEE Transaction on Information Forensics and Securit
Galaxy Luminosity Functions from Deep Spectroscopic Samples of Rich Clusters
Using a new spectroscopic sample and methods accounting for spectroscopic
sampling fractions that vary in magnitude and surface brightness, we present
R-band galaxy luminosity functions (GLFs) for six nearby galaxy clusters with
redshifts 4000 < cz < 20000 km/s and velocity dispersions 700 < sigma < 1250
km/s. In the case of the nearest cluster, Abell 1060, our sample extends to
M_R=-14 (7 magnitudes below M*), making this the deepest spectroscopic
determination of the cluster GLF to date. Our methods also yield composite GLFs
for cluster and field galaxies to M_R=-17 (M*+4), including the GLFs of
subsamples of star forming and quiescent galaxies. The composite GLFs are
consistent with Schechter functions (M*_R=-21.14^{+0.17}_{-0.17},
alpha=-1.21^{+0.08}_{-0.07} for the clusters, M*_R=-21.15^{+0.16}_{-0.16},
alpha=-1.28^{+0.12}_{-0.11} for the field). All six cluster samples are
individually consistent with the composite GLF down to their respective
absolute magnitude limits, but the GLF of the quiescent population in clusters
is not universal. There are also significant variations in the GLF of quiescent
galaxies between the field and clusters that can be described as a steepening
of the faint end slope. The overall GLF in clusters is consistent with that of
field galaxies, except for the most luminous tip, which is enhanced in clusters
versus the field. The star formation properties of giant galaxies are more
strongly correlated with the environment than those of fainter galaxies.Comment: 53 pages, 8 figures, 1 ASCII table; accepted for publication in Ap
Automatic Dating of Historical Documents
With the growing number of digitized documents available to researchers it is becoming possible to answer scientific questions by simply analyzing the image content. In this article, a new approach for the automatic dating of historical documents is proposed. It is based on an approach only recently proposed for scribe identification. It uses local RootSIFT descriptors which are encoded using VLAD. The method is evaluated using a dataset consisting of context areas of medieval papal charters covering around 150 years from 1049 to 1198 AD. Experimental results show very promising mean absolute errors of about 17 years
The Dependence of the Galaxy Luminosity Function on Environment
We present luminosity functions for galaxies in loose groups in the Las
Campanas Redshift Survey, differentiated by their environment (defined by the
line-of-sight velocity dispersion sigma of the host groups) and also by their
spectral type (emission or non-emission, defined by the equivalent width of the
3727-Angstrom [OII] line).
We find systematic variations in the Schechter parameters alpha and M* for
non-emission line galaxies over a range of 0 < sigma < 800 km/s. Alpha varies
from 0.20 to -0.91, indicating an increase in the steepness of the faint end
slope with increasing sigma. The accompanying variation in M* appears to be
accounted for by the intrinsic correlation with alpha and does not indicate a
significant physical variation in the bright end of the luminosity function.
For emission line galaxies, we find no significant systematic variation of the
luminosity function with the environment. Our results show that emission and
non-emission galaxies generally occupy two distinct regions in the alpha-M*
parameter space. From our luminosity functions, we derive the number ratios of
emission to non-emission galaxies as a function of environment and absolute
magnitude, showing that the relative abundance of non-emission line galaxies
generally increases for all magnitudes -23 < M_R < -17.5 towards high-sigma
environments, from ~80% to >90% at M_R = -22 and from ~10% to >50% at M_R = -18
(H_0 = 100 km s^{-1} Mpc^{-1} and q_0 = 0.5).Comment: 34 pages, 10 figures; accepted for publication in the Ap
Convolution and deconvolution based estimates of galaxy scaling relations from photometric redshift surveys
In addition to the maximum likelihood approach, there are two other methods
which are commonly used to reconstruct the true redshift distribution from
photometric redshift datasets: one uses a deconvolution method, and the other a
convolution. We show how these two techniques are related, and how this
relationship can be extended to include the study of galaxy scaling relations
in photometric datasets. We then show what additional information photometric
redshift algorithms must output so that they too can be used to study galaxy
scaling relations, rather than just redshift distributions. We also argue that
the convolution based approach may permit a more efficient selection of the
objects for which calibration spectra are required.Comment: 7 pages, 4 figures, accepted for publication in MNRAS; v2 includes a
new section and other minor change
Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution
We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap
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