800 research outputs found
A ring galaxy at z=1 lensed by the cluster Abell 370
We present a study of a very peculiar object found in the field of the
cluster-lens Abell 370. This object displays, in HST imaging, a spectacular
morphology comparable to nearby ring-galaxies. From spectroscopic observations
at the CFHT, we measured a redshift of based on the identification of
[O ii] 3727 \AA and [Ne v] 3426 \AA emission lines. These emission lines are
typical of starburst galaxies hosting a central active nucleus and are in good
agreement with the assumption that this object is a ring-galaxy. This object is
also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR)
flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally
distorted by the cluster-lens, and most particularly by a nearby cluster
elliptical galaxy. Using the cluster mass model, we can compute its intrinsic
shape. Requiring that the outer ring follows an ellipse we put constraints on
the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5
0.2. The absolute luminosities of the source are then $L_B = 1.3 \
10^{12} L_{B \odot}\nu_\nu \simeq 4. 10^{10}_\odot$ in the
mid-IR.Comment: 5 pages, 5 figures, uses aa.cls, accepted to A&A Letters. Minor
changes, Figure 1 revisited and typos adde
Dwarf Galaxy Clustering and Missing Satellites
At redshifts around 0.1 the CFHT Legacy Survey Deep fields contain some
6x10^4 galaxies spanning the mass range from 10^5 to 10^12 Msun. We measure the
stellar mass dependence of the two point correlation using angular measurements
to largely bypass the errors, approximately 0.02 in the median, of the
photometric redshifts. Inverting the power-law fits with Limber's equation we
find that the auto-correlation length increases from a very low 0.4hMpc at
10^5.5 Msun to the conventional 4.5hMpc at 10^10.5 Msun. The power law fit to
the correlation function has a slope which increases from gamma approximately
1.6 at high mass to gamma approximately 2.3 at low mass. The spatial
cross-correlation of dwarf galaxies with more massive galaxies shows fairly
similar trends, with a steeper radial dependence at low mass than predicted in
numerical simulations of sub-halos within galaxy halos. To examine the issue of
missing satellites we combine the cross-correlation measurements with our
estimates of the low mass galaxy number density. We find on the average there
are 60+/-20 dwarfs in sub-halos with M(total) > 10^7 Msun for a typical Local
Group M(total)/M(stars)=30, corresponding to M/L_V approximately 100 for a
galaxy with no recent star formation. The number of dwarfs per galaxy is about
a factor of two larger than currently found for the Milky Way. Nevertheless,
the average dwarf counts are about a factor of 30 below LCDM simulation
results. The divergence from LCDM predictions is one of slope of the relation,
approximately dN/dlnM approximately -0.5 rather than the predicted -0.9, not
sudden onset at some characteristic scale. The dwarf galaxy star formation
rates span the range from passive to bursting, which suggests that there are
few completely dark halos.Comment: revised version submitted to Astrophysical Journa
Ultraviolet to infrared emission of z>1 galaxies: Can we derive reliable star formation rates and stellar masses?
We seek to derive star formation rates (SFR) and stellar masses (M_star) in
distant galaxies and to quantify the main uncertainties affecting their
measurement. We explore the impact of the assumptions made in their derivation
with standard calibrations or through a fitting process, as well as the impact
of the available data, focusing on the role of IR emission originating from
dust. We build a sample of galaxies with z>1, all observed from the UV to the
IR (rest frame). The data are fitted with the code CIGALE, which is also used
to build and analyse a catalogue of mock galaxies. Models with different SFHs
are introduced. We define different set of data, with or without a good
sampling of the UV range, NIR, and thermal IR data. The impact of these
different cases on the determination of M_star and SFR are analysed.
Exponentially decreasing models with a redshift formation of the stellar
population z ~8 cannot fit the data correctly. The other models fit the data
correctly at the price of unrealistically young ages when the age of the single
stellar population is taken to be a free parameter. The best fits are obtained
with two stellar populations. As long as one measurement of the dust emission
continuum is available, SFR are robustly estimated whatever the chosen model
is, including standard recipes. M_star measurement is more subject to
uncertainty, depending on the chosen model and the presence of NIR data, with
an impact on the SFR-M_star scatter plot. Conversely, when thermal IR data from
dust emission are missing, the uncertainty on SFR measurements largely exceeds
that of stellar mass. Among all physical properties investigated here, the
stellar ages are found to be the most difficult to constrain and this
uncertainty acts as a second parameter in SFR measurements and as the most
important parameter for M_star measurements.Comment: 14 pages, 14 figures, accepted for publication A&
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Invariant densities for dynamical systems with random switching
We consider a non-autonomous ordinary differential equation on a smooth
manifold, with right-hand side that randomly switches between the elements of a
finite family of smooth vector fields. For the resulting random dynamical
system, we show that H\"ormander type hypoellipticity conditions are sufficient
for uniqueness and absolute continuity of an invariant measure.Comment: 16 pages; we replaced our original article to point out and close a
gap in the discussion of the Lorenz system in Section 7 (see Remark 2); this
gap is only present in the journal version of this article --- it wasn't
present in the previous arxiv versio
Non-Fickian dispersion in porous media : 1. Multiscale measurements using single-well injection withdrawal tracer tests
International audienceWe present a set of single-well injection withdrawal tracer tests in a paleoreef porous reservoir displaying important small-scale heterogeneity. An improved dual-packer probe was designed to perform dirac-like tracer injection and accurate downhole automatic measurements of the tracer concentration during the recovery phase. By flushing the tracer, at constant flow rate, for increasing time duration, we can probe distinctly different reservoir volumes and test the multiscale predictability of the (non-Fickian) dispersion models. First we describe the characteristics, from microscale to meter scale, of the reservoir rock. Second, the specificity of the tracer test setup and the results obtained using two different tracers and measurement methods (salinity-conductivity and fluorescent dyeoptical measurement, respectively) are presented. All the tracer tests display strongly tailed breakthrough curves (BTC) consistent with diffusion in immobile regions. Conductivity results, measured over 3 orders of magnitude only, could have been easily interpreted by the conventional mobile-immobile (MIM) diffusive mass transfer model of asymptotic log-log slope of 2. However, the fluorescent dye sensor, which allows exploring much lower concentration values, shows that a change in the log-log slope occurs at larger time with an asymptotic value of 1.5, corresponding to the double-porosity model. These results suggest that the conventional, one-slope MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity of the diffusion-dominant fraction of the reservoir
The Star Formation Epoch of the Most Massive Early-Type Galaxies
We present new Keck spectroscopy of early-type galaxies in three galaxy
clusters at z~0.5. We focus on the fundamental plane (FP) relation, and combine
the kinematics with structural parameters determined from HST images. The
galaxies obey clear FP relations, which are offset from the FP of the nearby
Coma cluster due to passive evolution of the stellar populations. The z~0.5
data are combined with published data for 11 additional clusters at
0.18<z<1.28, to determine the evolution of the mean M/L(B) ratio of cluster
galaxies with masses M>10^11 M_sun, as implied by the FP. We find
dlog(M/L(B))/dz = -0.555+-0.042, stronger evolution than was previously
inferred from smaller samples. The observed evolution depends on the
luminosity-weighted mean age of the stars in the galaxies, the initial mass
function (IMF), selection effects due to progenitor bias, and other parameters.
Assuming a normal IMF but allowing for various other sources of uncertainty we
find z* = 2.01+-0.20 for the luminosity-weighted mean star formation epoch. The
main uncertainty is the slope of the IMF in the range 1-2 Solar masses: we find
z* = 4.0 for a top-heavy IMF with slope x=0. The M/L(B) ratios of the cluster
galaxies are compared to those of recently published samples of field
early-type galaxies at 0.32<z<1.14. Assuming that progenitor bias and the IMF
do not depend on environment we find that the present-day age of stars in
massive field galaxies is 4.1 +- 2.0 % (~0.4 Gyr) less than that of stars in
massive cluster galaxies, consistent with most, but not all, previous studies
of local and distant early-type galaxies. This relatively small age difference
is surprising in the context of expectations from ``standard'' hierarchical
galaxy formation models. [ABRIDGED]Comment: Accepted for publication in ApJ. Minor corrections to match published
versio
Optimal Principal Component Analysis in Distributed and Streaming Models
We study the Principal Component Analysis (PCA) problem in the distributed
and streaming models of computation. Given a matrix a
rank parameter , and an accuracy parameter , we
want to output an orthonormal matrix for which where is the best rank- approximation to .
This paper provides improved algorithms for distributed PCA and streaming
PCA.Comment: STOC2016 full versio
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