3,540 research outputs found
A forming disk at z~0.6: Collapse of a gaseous disk or major merger remnant?
[Abridged] We present and analyze observations of J033241.88-274853.9 at
z=0.6679, using multi-wavelength photometry and imaging with FLAMES/GIRAFFE 3D
spectroscopy. J033241.88-274853.9 is found to be a blue, young (~320Myr)
stellar disk embedded in a very gas-rich (fgas=73-82% with
log(Mstellar/Mo)=9.45) and turbulent phase that is found to be rotating on
large spatial scales. We identified two unusual properties of
J033241.88-274853.9. (1) The spatial distributions of the ionized gaseous and
young stars show a strong decoupling; while almost no stars can be detected in
the southern part down to the very deep detection limit of ACS/UDF images,
significant emission from the [OII] ionized gas is detected. (2) We detect an
excess of velocity dispersion in the southern part of J033241.88-274853.9 in
comparison to expectations from a rotating disk model. We considered two disk
formation scenarios, depending on the gaseous phase geometry. In the first one,
we examined whether J033241.88-274853.9 could be a young rotating disk that has
been recently collapsed from a pre-existing, very gas-rich rotating disk. This
scenario requires two (unknown) additional assumptions to explain the
decoupling between the distribution of stars and gas and the excess of velocity
dispersion in the same region. In a second scenario, we examine whether
J033241.88-274853.9 could be a merger remnant of two gas-rich disks. In this
case, the asymmetry observed between the gas and star distributions, as well as
the excess of velocity dispersion, find a common explanation. Shocks produced
during the merger in this region can be ionized easily and heat the gas while
preventing star formation. This makes this scenario more satisfactory than the
collapse of a pre-existing, gas-rich rotating disk.Comment: Accepted for publication in A&A. 8 pages & 5 figure
IMAGES I. Strong evolution of galaxy kinematics since z=1
(abbreviated) We present the first results of the ESO large program,
``IMAGES'' which aims at obtaining robust measurements of the kinematics of
distant galaxies using the multi-IFU mode of GIRAFFE on the VLT. 3D
spectroscopy is essential to robustly measure the often distorted kinematics of
distant galaxies (e.g., Flores et al. 2006). We derive the velocity fields and
-maps of 36 galaxies at 0.4<z<0.75 from the kinematics of the [OII]
emission line doublet, and generate a robust technique to identify the nature
of the velocity fields based on the pixels of the highest signal-to-noise
ratios (S/N). We have gathered a unique sample of 63 velocity fields of
emission line galaxies (W0([OII]) > 15 A) at z=0.4-0.75, which are a
representative subsample of the population of M_stellar>1.5x10^{10} M_sun
emission line galaxies in this redshift range, and are largely unaffected by
cosmic variance. Taking into account all galaxies -with or without emission
lines- in that redshift range, we find that at least 41+/-7% of them have
anomalous kinematics, i.e., they are not dynamically relaxed. This includes
26+/-7% of distant galaxies with complex kinematics, i.e., they are not simply
pressure or rotationally supported. Our result implies that galaxy kinematics
are among the most rapidly evolving properties, because locally, only a few
percent of the galaxies in this mass range have complex kinematics.Comment: 17 pages, 6 figures, Accepted by A&
Batch kernel SOM and related Laplacian methods for social network analysis
Large graphs are natural mathematical models for describing the structure of
the data in a wide variety of fields, such as web mining, social networks,
information retrieval, biological networks, etc. For all these applications,
automatic tools are required to get a synthetic view of the graph and to reach
a good understanding of the underlying problem. In particular, discovering
groups of tightly connected vertices and understanding the relations between
those groups is very important in practice. This paper shows how a kernel
version of the batch Self Organizing Map can be used to achieve these goals via
kernels derived from the Laplacian matrix of the graph, especially when it is
used in conjunction with more classical methods based on the spectral analysis
of the graph. The proposed method is used to explore the structure of a
medieval social network modeled through a weighted graph that has been directly
built from a large corpus of agrarian contracts
IMAGES-III: The evolution of the Near-Infrared Tully-Fisher relation over the last 6 Gyr
Using the multi-integral field spectrograph GIRAFFE at VLT, we have derived
the K-band Tully-Fisher relation (TFR) at z~0.6 for a representative sample of
65 galaxies with emission lines. We confirm that the scatter in the z~0.6 TFR
is caused by galaxies with anomalous kinematics, and find a positive and strong
correlation between the complexity of the kinematics and the scatter that they
contribute to the TFR. Considering only relaxed-rotating disks, the scatter,
and possibly also the slope of the TFR, do not appear to evolve with z. We
detect an evolution of the K-band TFR zero point between z~0.6 and z=0, which,
if interpreted as an evolution of the K-band luminosity of rotating disks,
would imply that a brightening of 0.66+/-0.14 mag occurs between z~0.6 and z=0.
Any disagreement with the results of Flores et al. (2006) are attributed to
both an improvement of the local TFR and the more detailed accurate measurement
of the rotation velocities in the distant sample. Most of the uncertainty can
be explained by the relatively coarse spatial-resolution of the kinematical
data. Because most rotating disks at z~0.6 are unlikely to experience further
merging events, one may assume that their rotational velocity does not evolve
dramatically. If true, our result implies that rotating disks observed at z~0.6
are rapidly transforming their gas into stars, to be able to double their
stellar masses and be observed on the TFR at z=0. The rotating disks observed
are indeed emission-line galaxies that are either starbursts or LIRGs, which
implies that they are forming stars at a high rate. Thus, a significant
fraction of the rotating disks are forming the bulk of their stars within 6 to
8 Gyr, in good agreement with former studies of the evolution of the M-Z
relation.Comment: 17 pages, 11 figures, accepted for publication in A&A. v2 taking into
account comments from language edito
Clumpy galaxies at z~0.6: kinematics, stability, and comparison with analogs at other redshifts
Distant clumpy galaxies are thought to be Jeans-unstable disks, and an
important channel for the formation of local galaxies, as suggested by recent
spatially-resolved kinematic observations of z~2 galaxies. I study the
kinematics of clumpy galaxies at z~0.6, and compare their properties with those
of counterparts at higher and lower redshifts. I selected a sample of 11 clumpy
galaxies at z~0.6 from the representative sample of emission line,
intermediate-mass galaxies IMAGES. Selection was based on rest-frame UV
morphology from HST/ACS images, mimicking the selection criteria commonly used
at higher redshifts. Their spatially-resolved kinematics were derived in the
frame of the IMAGES survey, using the VLT/FLAMES-GIRAFFE multi-integral field
spectrograph. For those showing large-scale rotation, I derived the Toomre Q
parameter, which characterizes the stability of their gaseous and stellar
phases. I find that the fraction of UV-selected clumpy galaxies at z~0.6 is
20+/-12%. Roughly half of them (45+/-30%) have complex kinematics inconsistent
with Jeans-unstable disks, while those in the remaining half (55+/-30%) show
large-scale rotations. The latter reveal a stable gaseous phase, but the
contribution of their stellar phase makes them globally unstable to clump
formation. Clumpy galaxies appear to be less unstable at z~0.6 than at z~2,
which could explain why the UV clumps tend to vanish in rest-frame optical
images of z~0.6 clumpy galaxies, conversely to z~2 clumpy galaxies, in which
the stellar phase can substantially fragment. This suggests that the former
correspond to patchy star-formation regions superimposed on a smoother mass
distribution. A possible and widespread scenario for driving clump formation
relies on instabilities by cold streams penetrating the dark matter halos where
clumpy galaxies inhabit. While such a gas accretion process is predicted to be
significant in massive, z~2 haloes, it is also predicted to be strongly
suppressed in similar, z~0.6 haloes, which could explain why lowest-z clumpy
galaxies appear to be driven by a different mechanism. Instead, I found that
interactions are probably the dominant driver leading to the formation of
clumpy galaxies at z1 clumpy galaxies remains
more uncertain. While cold flows could be an important driver at z~2, I also
argue that the observed and cumulative merger fraction between z=2 and z=3 is
large enough so that every z~2 galaxy might be the result of a merger that
occurred within their past 1 Gyr. I conclude that it is premature to rule out
mergers as a universal driver for galaxy evolution from z~2 down to z=0.Comment: Accepted for publication in MNRAS. 14 pages, 2 figures
531 new spectroscopic redshifts from the CDFS and a test on the cosmological relevance of the GOODS-South field
(Abbrev.) This paper prepares a series of papers analysing the Intermediate
MAss Galaxy Evolution Sequence (IMAGES) up to z=1. Intermediate mass galaxies
(MJ <=-20.3) are selected from the Chandra Deep Field South (CDFS) for which we
identify a serious lack of spectroscopically determined redshifts..... We have
spectroscopically identified 691 objects including 580 gal., 7 QSOs, and 104
stars. This study provides 531 new redshifts in the CDFS. It confirms the
presence of several large scale structures in the CDFS. To test the impact of
these structures in the GOODS-South field, we ... compare the evolution of
rest-frame U, B, V and K galaxy luminosity densities to that derived from the
CFRS. The CDFS field shows a significant excess of luminosity densities in the
z=0.5-0.75 range, which increases with the wavelength, reaching up to 0.5 dex
at 2.1 um. Stellar mass and specific star formation evolutions might be
significantly affected by the presence of the peculiar large scale structures
at z= 0.668 and at z= 0.735, that contain a significant excess of evolved,
massive galaxies when compared to other fields. This leads to a clear warning
to results based on the CDFS/GOODS South fields, especially those related to
the evolution of red luminosity densities, i.e. stellar mass density and
specific star formation rate. Photometric redshift techniques, when applied to
that field, are producing quantities which are apparently less affected by
cosmic variance (0.25 dex at 2.1 um), however at the cost of the difficulty in
disentangling between evolutionary and cosmic variance effects.Comment: Accepted for publication in A&A, 19 pages, 13 figure
Images IV: Strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies since z=0.8
Intermediate mass galaxies (logM(Msun)>10) at z~0.6 are the likeliest
progenitors of the present-day numerous population of spirals. There is growing
evidence that they have evolved rapidly since the last 6 to 8 Gyr ago, and
likely have formed a significant fraction of their stellar mass, often showing
perturbed morphologies and kinematics. We have gathered a representative sample
of 88 such galaxies and have provided robust estimates of their gas phase
metallicity. For doing so, we have used moderate spectral resolution
spectroscopy at VLT/FORS2 with unprecedented high S/N allowing to remove biases
coming from interstellar absorption lines and extinction to establish robust
values of R23=([OII]3727 + [OIII]4959,5007)/Hbeta. We definitively confirm that
the predominant population of z~0.6 starbursts and luminous IR galaxies (LIRGs)
are on average, two times less metal rich than the local galaxies at a given
stellar mass. We do find that the metal abundance of the gaseous phase of
galaxies is evolving linearly with time, from z=1 to z=0 and after comparing
with other studies, from z=3 to z=0. Combining our results with the reported
evolution of the Tully Fisher relation, we do find that such an evolution
requires that ~30% of the stellar mass of local galaxies have been formed
through an external supply of gas, thus excluding the close box model. Distant
starbursts & LIRGs have properties (metal abundance, star formation efficiency
& morphologies) similar to those of local LIRGs. Their underlying physics is
likely dominated by gas infall probably through merging or interactions. Our
study further supports the rapid evolution of z~0.4-1 galaxies. Gas exchanges
between galaxies is likely the main cause of this evolution.Comment: 21 pages, 12 figures, A&A, In pres
Molecular transport calculations with Wannier functions
We present a scheme for calculating coherent electron transport in
atomic-scale contacts. The method combines a formally exact Green's function
formalism with a mean-field description of the electronic structure based on
the Kohn-Sham scheme of density functional theory. We use an accurate
plane-wave electronic structure method to calculate the eigenstates which are
subsequently transformed into a set of localized Wannier functions (WFs). The
WFs provide a highly efficient basis set which at the same time is well suited
for analysis due to the chemical information contained in the WFs. The method
is applied to a hydrogen molecule in an infinite Pt wire and a benzene-dithiol
(BDT) molecule between Au(111) surfaces. We show that the transmission function
of BDT in a wide energy window around the Fermi level can be completely
accounted for by only two molecular orbitals.Comment: 15 pages, 12 figures, submitted to Chemical Physic
IMAGES II. A surprisingly low fraction of undisturbed rotating spiral disks at z~0.6: The morpho-kinematical relation 6 Gyrs ago
We present a first combined analysis of the morphological and dynamical
properties for the Intermediate MAss Galaxy Evolution Sequence (IMAGES) sample.
It is a representative sample of 52 z~0.6 galaxies with Mstell from 1.5 to 15
10^10Msun and possessing 3D resolved kinematics and HST deep imaging in at
least two broad band filters. We aim at evaluate robustly the evolution of
rotating spirals since z~0.6, as well as to test the different schemes for
classifying galaxies morphologically. We used all the information provided by
multi-band images, color maps and 2 dimensional light fitting to assign to each
object a morphological class. We divided our sample between spiral disks,
peculiar objects, compact objects and mergers. Using our morphological
classification scheme, 4/5 of identified spirals are rotating disks and more
than 4/5 of identified peculiar galaxies show complex kinematics, while
automatic classification methods such as Concentration-Asymmetry and GINI-M20
severely overestimate the fraction of relaxed disk galaxies. Using this
methodology, we find that the fraction of rotating spirals has increased by a
factor ~ 2 during the last 6 Gyrs, a much higher fraction that found previously
based on morphologies alone. These rotating spiral disks are forming stars very
rapidly, doubling their stellar masses over the last 6 Gyrs, while most of
their stars have been formed few Gyrs earlier, which reveals the presence of a
large gas supply. Because they are likely the progenitors of local spirals, we
can conjecture how their properties are evolving. Their disks show some
evidence for an inside-out growth and the gas supply/accretion is not made
randomly as the disk need to be stable in order to match the local disk
properties.Comment: Typos corrected, reference adde
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