58 research outputs found
Zanclean Gilbert-type fan deltas in the Turnu Severin area (Dacian Basin, Romania). A critical analysis
Abstract. The critical analysis of the Gura VÄii ā Turnu Severin ā Izvoru BĆ¢rzii Gilbert-type deltas concentrated on the deltaic architecture and the age of the deltaic deposits, referring especially to the conglomeratic sediments considered the proximal foreset unit of the Gilbert-type fan deltas. It reveals the lack of evidences necessary to demonstrate the existence of the three units (foreset, bottomset and topset beds) which define a Gilbert-type delta. Although there are convincing data for its tectonic
origin, the homoclinal structure of these deltaic deposits was considered, with no argumentation, as a primary structure generated through the deltaic progradation.
The age of the rudaceous deposits in the area Gura VÄii ā Turnu Severin ā Izvoru BĆ¢rzii, representing the core of the two outlined Gilbert deltas, was considered Zanclean (Bosphorian) by Clauzon et al. (2005) and Suc et al. (2011), rejecting the Badenian ā Sarmatian (s.l.) age formerly attributed (Marinescu, 1978 and the references herein). The critical examination of these two concepts pointed out the paleontological and stratigraphic proofs which substantiate the Badenian-Sarmatian (s.l.) age,
in contrast with the lack of evidences in favor of the Zanclean age. The Gilbert-type fan deltas figured by Clauzon et al. (2005) and Suc et al. (2011) in the area of the present-day Danube River course (area Gura VÄii ā Turnu Severin ā Izvoru BĆ¢rzii), closely downstream of the Iron Gates, represents one of the main arguments of these authors for the action of the Messinian crisis in the Dacian Basin. Taking into consideration the importance of this concept for the understanding of the Dacian Basin
evolution, this paper undertakes a critical analysis of the factual data which generated the concept of the Gilbert-type fan deltas in the Turnu Severin area
Galaxy Zoo: Disentangling the Environmental Dependence of Morphology and Colour
We analyze the environmental dependence of galaxy morphology and colour with
two-point clustering statistics, using data from the Galaxy Zoo, the largest
sample of visually classified morphologies yet compiled, extracted from the
Sloan Digital Sky Survey. We present two-point correlation functions of spiral
and early-type galaxies, and we quantify the correlation between morphology and
environment with marked correlation functions. These yield clear and precise
environmental trends across a wide range of scales, analogous to similar
measurements with galaxy colours, indicating that the Galaxy Zoo
classifications themselves are very precise. We measure morphology marked
correlation functions at fixed colour and find that they are relatively weak,
with the only residual correlation being that of red galaxies at small scales,
indicating a morphology gradient within haloes for red galaxies. At fixed
morphology, we find that the environmental dependence of colour remains strong,
and these correlations remain for fixed morphology \textit{and} luminosity. An
implication of this is that much of the morphology--density relation is due to
the relation between colour and density. Our results also have implications for
galaxy evolution: the morphological transformation of galaxies is usually
accompanied by a colour transformation, but not necessarily vice versa. A
spiral galaxy may move onto the red sequence of the colour-magnitude diagram
without quickly becoming an early-type. We analyze the significant population
of red spiral galaxies, and present evidence that they tend to be located in
moderately dense environments and are often satellite galaxies in the outskirts
of haloes. Finally, we combine our results to argue that central and satellite
galaxies tend to follow different evolutionary paths.Comment: 19 pages, 18 figures. Accepted for publication in MNRA
Galaxy Zoo: The large-scale spin statistics of spiral galaxies in the Sloan Digital Sky Survey
We re-examine the evidence for a violation of large-scale statistical
isotropy in the distribution of projected spin vectors of spiral galaxies. We
have a sample of spiral galaxies from the Sloan Digital Sky
Survey, with their line of sight spin direction confidently classified by
members of the public through the online project Galaxy Zoo. After establishing
and correcting for a certain level of bias in our handedness results we find
the winding sense of the galaxies to be consistent with statistical isotropy.
In particular we find no significant dipole signal, and thus no evidence for
overall preferred handedness of the Universe. We compare this result to those
of other authors and conclude that these may also be affected and explained by
a bias effect.Comment: Accepted for publication in MNRAS. 8 pages, 5 figure
Galaxy Zoo: Reproducing Galaxy Morphologies Via Machine Learning
We present morphological classifications obtained using machine learning for
objects in SDSS DR6 that have been classified by Galaxy Zoo into three classes,
namely early types, spirals and point sources/artifacts. An artificial neural
network is trained on a subset of objects classified by the human eye and we
test whether the machine learning algorithm can reproduce the human
classifications for the rest of the sample. We find that the success of the
neural network in matching the human classifications depends crucially on the
set of input parameters chosen for the machine-learning algorithm. The colours
and parameters associated with profile-fitting are reasonable in separating the
objects into three classes. However, these results are considerably improved
when adding adaptive shape parameters as well as concentration and texture. The
adaptive moments, concentration and texture parameters alone cannot distinguish
between early type galaxies and the point sources/artifacts. Using a set of
twelve parameters, the neural network is able to reproduce the human
classifications to better than 90% for all three morphological classes. We find
that using a training set that is incomplete in magnitude does not degrade our
results given our particular choice of the input parameters to the network. We
conclude that it is promising to use machine- learning algorithms to perform
morphological classification for the next generation of wide-field imaging
surveys and that the Galaxy Zoo catalogue provides an invaluable training set
for such purposes.Comment: 13 Pages, 5 figures, 10 tables. Accepted for publication in MNRAS.
Revised to match accepted version
Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies
Morphology is a powerful indicator of a galaxy's dynamical and merger
history. It is strongly correlated with many physical parameters, including
mass, star formation history and the distribution of mass. The Galaxy Zoo
project collected simple morphological classifications of nearly 900,000
galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of
thousands of volunteers. This large number of classifications allows us to
exclude classifier error, and measure the influence of subtle biases inherent
in morphological classification. This paper presents the data collected by the
project, alongside measures of classification accuracy and bias. The data are
now publicly available and full catalogues can be downloaded in electronic
format from http://data.galaxyzoo.org.Comment: Accepted by MNRAS, 14 pages. Updated to match final version; problem
with table 7 header fixed. Full tables available at http://data.galaxyzoo.or
Galaxy Zoo: Dust in Spirals
We investigate the effect of dust on spiral galaxies by measuring the
inclination-dependence of optical colours for 24,276 well-resolved SDSS
galaxies visually classified in Galaxy Zoo. We find clear trends of reddening
with inclination which imply a total extinction from face-on to edge-on of 0.7,
0.6, 0.5 and 0.4 magnitudes for the ugri passbands. We split the sample into
"bulgy" (early-type) and "disky" (late-type) spirals using the SDSS fracdeV (or
f_DeV) parameter and show that the average face-on colour of "bulgy" spirals is
redder than the average edge-on colour of "disky" spirals. This shows that the
observed optical colour of a spiral galaxy is determined almost equally by the
spiral type (via the bulge-disk ratio and stellar populations), and reddening
due to dust. We find that both luminosity and spiral type affect the total
amount of extinction, with "disky" spirals at M_r ~ -21.5 mags having the most
reddening. This decrease of reddening for the most luminous spirals has not
been observed before and may be related to their lower levels of recent star
formation. We compare our results with the latest dust attenuation models of
Tuffs et al. We find that the model reproduces the observed trends reasonably
well but overpredicts the amount of u-band attenuation in edge-on galaxies. We
end by discussing the effects of dust on large galaxy surveys and emphasize
that these effects will become important as we push to higher precision
measurements of galaxy properties and their clustering.Comment: MNRAS in press. 25 pages, 22 figures (including an abstract comparing
GZ classifications with common automated methods for selecting disk/early
type galaxies in SDSS data). v2 corrects typos found in proof
Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies
We investigate a class of rapidly growing emission line galaxies, known as āGreen Peasā, first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in Sloan Digital Sky Survey imaging. Their appearance is due to very strong optical emission lines, namely [O iii]Ī»5007 Ć
, with an unusually large equivalent width of up to ā¼1000 Ć
. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some active galactic nuclei interlopers including eight newly discovered narrow-line Seyfert 1 galaxies. The star-forming Peas are low-mass galaxies (Mā¼ 108.5ā1010 Mā) with high star formation rates (ā¼10 Mā yrā1), low metallicities (log[O/H]+ 12 ā¼ 8.7) and low reddening [E(BāV) ā¤ 0.25] and they reside in low-density environments. They have some of the highest specific star formation rates (up to ā¼10ā8 yrā1) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myr. The few star-forming Peas with Hubble Space Telescope imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to luminous blue compact galaxies. They are also similar to high-redshift ultraviolet-luminous galaxies, e.g. Lyman-break galaxies and LyĪ± emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe
Galaxy Zoo: Passive Red Spirals
We study the spectroscopic properties and environments of red spiral galaxies
found by the Galaxy Zoo project. By carefully selecting face-on, disk dominated
spirals we construct a sample of truly passive disks (not dust reddened, nor
dominated by old stellar populations in a bulge). As such, our red spirals
represent an interesting set of possible transition objects between normal blue
spirals and red early types. We use SDSS data to investigate the physical
processes which could have turned these objects red without disturbing their
morphology. Red spirals prefer intermediate density regimes, however there are
no obvious correlations between red spiral properties and environment -
environment alone is not sufficient to determine if a spiral will become red.
Red spirals are a small fraction of spirals at low masses, but are a
significant fraction at large stellar masses - massive galaxies are red
independent of morphology. We confirm that red spirals have older stellar popns
and less recent star formation than the main spiral population. While the
presence of spiral arms suggests that major star formation cannot have ceased
long ago, we show that these are not recent post-starbursts, so star formation
must have ceased gradually. Intriguingly, red spirals are ~4 times more likely
than normal spirals to host optically identified Seyfert or LINER, with most of
the difference coming from LINERs. We find a curiously large bar fraction in
the red spirals suggesting that the cessation of star formation and bar
instabilities are strongly correlated. We conclude by discussing the possible
origins. We suggest they may represent the very oldest spiral galaxies which
have already used up their reserves of gas - probably aided by strangulation,
and perhaps bar instabilities moving material around in the disk.Comment: MNRAS in press, 20 pages, 15 figures (v3
Galaxy Zoo: Chiral correlation function of galaxy spins
Galaxy Zoo is the first study of nearby galaxies that contains reliable
information about the spiral sense of rotation of galaxy arms for a sizeable
number of galaxies. We measure the correlation function of spin chirality (the
sense in which galaxies appear to be spinning) of face-on spiral galaxies in
angular, real and projected spaces. Our results indicate a hint of positive
correlation at separations less than ~0.5 Mpc at a statistical significance of
2-3 sigma. This is the first experimental evidence for chiral correlation of
spins. Within tidal torque theory it indicates that the inertia tensors of
nearby galaxies are correlated. This is complementary to the studies of nearby
spin axis correlations that probe the correlations of the tidal field.
Theoretical interpretation is made difficult by the small distances at which
the correlations are detected, implying that substructure might play a
significant role, and our necessary selection of face-on spiral galaxies,
rather than a general volume-limited sample.Comment: 9 pages, 5 figures; v2: minor changes, matches version accepted by
MNRA
Galaxy Zoo: the dependence of morphology and colour on environment
We analyse the relationships between galaxy morphology, colour, environment
and stellar mass using data for over 100,000 objects from Galaxy Zoo, the
largest sample of visually classified morphologies yet compiled. We
conclusively show that colour and morphology fractions are very different
functions of environment. Both are sensitive to stellar mass; however, at fixed
stellar mass, while colour is also highly sensitive to environment, morphology
displays much weaker environmental trends. Only a small part of both relations
can be attributed to variation in the stellar mass function with environment.
Galaxies with high stellar masses are mostly red, in all environments and
irrespective of their morphology. Low stellar-mass galaxies are mostly blue in
low-density environments, but mostly red in high-density environments, again
irrespective of their morphology. The colour-density relation is primarily
driven by variations in colour fractions at fixed morphology, in particular the
fraction of spiral galaxies that have red colours, and especially at low
stellar masses. We demonstrate that our red spirals primarily include galaxies
with true spiral morphology. We clearly show there is an environmental
dependence for colour beyond that for morphology. Before using the Galaxy Zoo
morphologies to produce the above results, we first quantify a luminosity-,
size- and redshift-dependent classification bias that affects this dataset, and
probably most other studies of galaxy population morphology. A correction for
this bias is derived and applied to produce a sample of galaxies with reliable
morphological type likelihoods, on which we base our analysis.Comment: 25 pages, 20 figures (+ 6 pages, 11 figures in appendices);
moderately revised following referee's comments; accepted by MNRA
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