6,545 research outputs found
Double lenses
The analysis of the shear induced by a single cluster on the images of a
large number of background galaxies is all centered around the curl-free
character of a well-known vector field that can be derived from the data. Such
basic property breaks down when the source galaxies happen to be observed
through two clusters at different redshifts, partially aligned along the line
of sight. In this paper we address the study of double lenses and obtain five
main results. (i) First we generalize the procedure to extract the available
information, contained in the observed shear field, from the case of a single
lens to that of a double lens. (ii) Then we evaluate the possibility of
detecting the signature of double lensing given the known properties of the
distribution of clusters of galaxies. (iii) As a different astrophysical
application, we demonstrate how the method can be used to detect the presence
of a dark cluster that might happen to be partially aligned with a bright
cluster studied in terms of statistical lensing. (iv) In addition, we show that
the redshift distribution of the source galaxies, which in principle might also
contribute to break the curl-free character of the shear field, actually
produces systematic effects typically two orders of magnitude smaller than the
double lensing effects we are focusing on. (v) Remarkably, a discussion of
relevant contributions to the noise of the shear measurement has brought up an
intrinsic limitation of weak lensing analyses, since one specific contribution,
associated with the presence of a non-vanishing two-galaxy correlation
function, turns out not to decrease with the density of source galaxies (and
thus with the depth of the observations).Comment: 40 pages, 15 figures. Accepted for publication in ApJ main journa
Mapping Global Star Formation in the Interacting Galaxy Pair Arp32
A multi-wavelength set of photometric data including UV (GALEX), optical, near-IR, infrared (Spitzer) and radio (VLA 20cm) images and spectroscopic observations are used to map the dust-obscured and unobscured star formation in the galaxy pair Arp 32. The system consists of an actively starforming galaxy and another one with depressed star formation. The most active galaxy has disrupted morphology and different sites of star formation. Spectroscopic data show hints of nuclear activity in its core, intense star formation in limited regions of the galaxy as well as an underlying population of stars witnessing a past episode of star formation. Current star formation rates are estimated from UV and bolometric IR luminosities
Control of Material Damping in High-Q Membrane Microresonators
We study the mechanical quality factors of bilayer aluminum/silicon-nitride
membranes. By coating ultrahigh-Q Si3N4 membranes with a more lossy metal, we
can precisely measure the effect of material loss on Q's of tensioned resonator
modes over a large range of frequencies. We develop a theoretical model that
interprets our results and predicts the damping can be reduced significantly by
patterning the metal film. Using such patterning, we fabricate Al-Si3N4
membranes with ultrahigh Q at room temperature. Our work elucidates the role of
material loss in the Q of membrane resonators and informs the design of hybrid
mechanical oscillators for optical-electrical-mechanical quantum interfaces
HST/ACS weak lensing analysis of the galaxy cluster RDCS 1252.9-2927 at z=1.24
We present a weak lensing analysis of one of the most distant massive galaxy
cluster known, RDCS 1252.9-2927 at z=1.24, using deep images from the Advanced
Camera for Survey (ACS) on board the Hubble Space Telescope (HST). By taking
advantage of the depth and of the angular resolution of the ACS images, we
detect for the first time at z>1 a clear weak lensing signal in both the i
(F775W) and z (F850LP) filters. We measure a 5-\sigma signal in the i band and
a 3-\sigma signal in the shallower z band image. The two radial mass profiles
are found to be in very good agreement with each other, and provide a
measurement of the total mass of the cluster inside a 1Mpc radius of M(<1Mpc) =
(8.0 +/- 1.3) x 10^14 M_\odot in the current cosmological concordance model h
=0.70, \Omega_m=0.3, \Omega_\Lambda=0.7, assuming a redshift distribution of
background galaxies as inferred from the Hubble Deep Fields surveys. A weak
lensing signal is detected out to the boundary of our field (3' radius,
corresponding to 1.5Mpc at the cluster redshift). We detect a small offset
between the centroid of the weak lensing mass map and the brightest cluster
galaxy, and we discuss the possible origin of this discrepancy. The cumulative
weak lensing radial mass profile is found to be in good agreement with the
X-ray mass estimate based on Chandr and XMM-Newton observations, at least out
to R_500=0.5Mpc.Comment: 38 pages, ApJ in press. Full resolution images available at
http://www.eso.org/~prosati/RDCS1252/Lombardi_etal_accepted.pd
Simulations of Damped Lyman-Alpha and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift
We use hydrodynamic cosmological simulations to study damped Lyman-alpha
(DLA) and Lyman limit (LL) absorption at redshifts z=2-4 in five variants of
the cold dark matter scenario. Our standard simulations resolve the formation
of dense concentrations of neutral gas in halos with circular velocity v_c
roughly 140 km/s for Omega_m=1 and 90 km/s for Omega_m=0.4, at z=2; an
additional LCDM simulation resolves halos down to v_c approximately 50 km/s at
z=3. We find a clear relation between HI column density and projected distance
to the center of the nearest galaxy, with DLA absorption usually confined to
galactocentric radii less than 10-15 kpc and LL absorption arising out to
projected separations of 30 kpc or more. Detailed examination provides evidence
of non-equilibrium effects on absorption cross-section. If we consider only
absorption in the halos resolved by our standard simulations, then all five
models fall short of reproducing the observed abundance of DLA and LL systems
at these redshifts. If we extrapolate to lower halo masses, we find all four
models are consistent with the observed abundance of DLA systems if the the
extrapolated behavior extends to circular velocities roughly 50-80 km/s, and
they may produce too much absorption if the relation continues to 40 km/s. Our
results suggest that LL absorption is closely akin to DLA absorption, arising
in less massive halos or at larger galactocentric radii but not caused by
processes acting on a radically different mass scale.Comment: 33 pages with 10 embedded EPS figures. Substantially revised and
updated from original version. Includes new high-resolution simulations.
Accepted for publication in the Ap
NICMOS Snapshot Survey of Damped Lyman Alpha Quasars
We image 19 quasars with 22 damped Lyman alpha (DLA) systems using the F160W
filter and the Near-Infrared Camera and Multiobject Spectrograph aboard the
Hubble Space Telescope, in both direct and coronagraphic modes. We reach 5
sigma detection limits of ~H=22 in the majority of our images. We compare our
observations to the observed Lyman-break population of high-redshift galaxies,
as well as Bruzual & Charlot evolutionary models of present-day galaxies
redshifted to the distances of the absorption systems. We predict H magnitudes
for our DLAs, assuming they are producing stars like an L* Lyman-break galaxy
(LBG) at their redshift. Comparing these predictions to our sensitivity, we
find that we should be able to detect a galaxy around 0.5-1.0 L* (LBG) for most
of our observations. We find only one new possible candidate, that near
LBQS0010-0012. This scarcity of candidates leads us to the conclusion that most
DLA systems are not drawn from a normal LBG luminosity function nor a local
galaxy luminosity function placed at these high redshifts.Comment: 31 pages, 8 figures, Accepted for Feb. 10 issue of Ap
The Star Formation Rate Intensity Distribution Function--Implications for the Cosmic Star Formation Rate History of the Universe
We address the effects of cosmological surface brightness dimming on
observations of faint galaxies by examining the distribution of "unobscured"
star formation rate intensities versus redshift. We use the star formation rate
intensity distribution function to assess the ultraviolet luminosity density
versus redshift, based on our photometry and photometric redshift measurements
of faint galaxies in the HDF and the HDF--S WFPC2 and NICMOS fields. We find
that (1) previous measurements have missed a dominant fraction of the
ultraviolet luminosity density of the universe at high redshifts by neglecting
cosmological surface brightness dimming effects, which are important at
redshifts larger than z = 2, (2) the incidence of the highest intensity star
forming regions increases monotonically with redshift, and (3) the ultraviolet
luminosity density plausibly increases monotonically with redshift through the
highest redshifts observed. By measuring the spectrum of the luminosity density
versus redshift, we also find that (4) previous measurements of the ultraviolet
luminosity density at redshifts z < 2 must be reduced by a factor 2 to allow
for the spectrum of the luminosity density between rest-frame wavelengths 1500
and 2800 A. And by comparing with observations of high-redshift damped
Lyman-alpha absorption systems detected toward background QSOs, we further find
that (5) the distribution of star formation rate intensities matches the
distribution of neutral hydrogen column densities at redshifts z = 2 through 5,
which establishes a quantitative connection between high-redshift galaxies and
high column density gas and suggests that high-redshift damped Lyman-alpha
absorption systems trace lower star formation rate intensity regions of the
same galaxies detected in star light in the HDF and HDF--S.Comment: 28 pages, 9 figures; accepted for publication in the Astrophysical
Journa
Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density
We report results on the incidence and cosmological mass density of damped
Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an
efficient non-traditional (but unbiased) survey technique to discover DLA
systems at redshifts z<1.65, where we observe the Lyman-alpha line in known
MgII absorption-line systems. We uncovered 14 DLA lines including 2
serendipitously. We find that (1) The DLA absorbers are drawn almost
exclusively from the population of MgII absorbers which have rest equivalent
widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift,
n(DLA), is observed to decrease with decreasing redshift. (3) On the other
hand, the cosmological mass density of neutral gas in low-redshift DLA
absorbers, Omega(DLA), is observed to be comparable to that observed at high
redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger
fraction of very high column density systems in comparison to determinations at
both high redshift and locally.Comment: 47 pages in LaTeX - emulateapj style with included tables and
encapsulated postscript figures. Accepted for Publication in Astrophysical
Journal Supplements. Results unchanged, text revise
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