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Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
Theory and observations of ice particle evolution in cirrus using Doppler radar: evidence for aggregation
Vertically pointing Doppler radar has been used to study the evolution of ice
particles as they sediment through a cirrus cloud. The measured Doppler fall
speeds, together with radar-derived estimates for the altitude of cloud top,
are used to estimate a characteristic fall time tc for the `average' ice
particle. The change in radar reflectivity Z is studied as a function of tc,
and is found to increase exponentially with fall time. We use the idea of
dynamically scaling particle size distributions to show that this behaviour
implies exponential growth of the average particle size, and argue that this
exponential growth is a signature of ice crystal aggregation.Comment: accepted to Geophysical Research Letter
Ellipticals with Kinematically-Distinct Cores: (V-I) Color Images with WFPC2
We have analysed HST/WFPC2 F555W and F814W images for fifteen elliptical
galaxies with kinematically-distinct cores. For each of them we have derived
surface brightness and isophotal parameter profiles in the two bands, color
maps, and radial profiles in (V-I). We have detected photometric evidence for
faint stellar disks, on scales of a few tens to a few arcseconds, in seven
galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494 and 5322. In NGC 1700,
the isophotes are slightly boxy at the scale of the counter-rotating component,
and disky at larger radii. We find no difference in (V-I) color greater than
0.02 mag between these disks and the surrounding galactic regions. Hence the
stellar populations in the kinematically distinct cores are not strongly
deviant from the population of the main body. For one galaxy, NGC 4365, the
innermost region is bluer than the surrounding regions. This area extends to
about 15pc, and contains a luminosity of 2.5x10^6 L. If interpreted as
a stellar population effect, an age difference of 3-4 Gyrs, or an
variation of about 0.2 dex, is derived. The nuclear intensity profiles
show a large variety: some galaxies have steep cusp profiles, others have
shallow cusps and a ``break radius''. The nuclear cusps of galaxies with
kinematically-distinct cores follow the same trends as the nuclei of normal
galaxies. We have not been able to identify a unique, qualifying feature in the
WFPC2 images which distinguish the galaxies with kinematically distinct cores
from the kinematically normal cores. [shortened]Comment: 56 pages, latex, 17 figures; figure 1 available upon request; ApJ,
481 in pres
Imprints of Environment on Cluster and Field Late-type Galaxies at z~1
We present a comparison of late-type galaxies (Sa and later) in intermediate
redshift clusters and the field using ACS imaging of four cluster fields:
CL0152-1357, CL1056-0337 (MS1054), CL1604+4304, and CL1604+4321. Concentration,
asymmetry, and clumpiness parameters are calculated for each galaxy in blue
(F606W or F625W) and red (F775W or F814W) filters. Galaxy half-light radii,
disk scale lengths, color gradients, and overall color are compared. We find
marginally significant differences in the asymmetry distributions of spiral and
irregular galaxies in the X-ray luminous and X-ray faint clusters. The massive
clusters contain fewer galaxies with large asymmetries. The physical sizes of
the cluster and field populations are similar; no significant differences are
found in half-light radii or disk scale lengths. The most significant
difference is in rest-frame color. Late-type cluster galaxies are
significantly redder, magnitudes at rest-frame , than their
field counterparts. Moreover, the intermediate-redshift cluster galaxies tend
to have blue inward color gradients, in contrast to the field galaxies, but
similar to late-type galaxies in low redshift clusters. These blue inward color
gradients are likely to be the result of enhanced nuclear star formation rates
relative to the outer disk. Based on the significant rest-frame color
difference, we conclude that late-type cluster members at are not a
pristine infalling field population; some difference in past and/or current
star formation history is already present. This points to high redshift
``groups'', or filaments with densities similar to present-day groups, as the
sites where the first major effects of environment are imprinted.Comment: updated titl
A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies
We present a morphological study of the galaxy population of the luminous
X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically
confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with
WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L >
L* cluster population. Most of these mergers will likely evolve into luminous
(\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming
the galaxy population in MS1054-03 is typical for its redshift it is estimated
that \sim 50% of present-day cluster ellipticals experienced a major merger at
z < 1. The mergers are preferentially found in the outskirts of the cluster,
and probably occur in small infalling clumps. Morphologies, spectra, and colors
of the mergers show that their progenitors were typically E/S0s or early-type
spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of
the merger remnants are consistent with the low scatter in the color-magnitude
relation in rich clusters at lower redshift. The discovery of a high fraction
of mergers in this young cluster is direct evidence against formation of
ellipticals in a single ``monolithic'' collapse at high redshift, and in
qualitative agreement with predictions of hierarchical models for structure
formation.Comment: Added GIF version of Figure 1. At
http://www.astro.rug.nl/~dokkum/preprints/merger_fig1.eps.gz the PS file is
available. Accepted for publication in ApJ Letter
Weak Lensing Analysis of the z~0.8 cluster CL 0152-1357 with the Advanced Camera for Surveys
We present a weak lensing analysis of the X-ray luminous cluster CL 0152-1357
at z~0.84 using HST/ACS observations. The unparalleled resolution and
sensitivity of ACS enable us to measure weakly distorted, faint background
galaxies to the extent that the number density reaches ~175 arcmin^-2. The PSF
of ACS has a complicated shape that also varies across the field. We construct
a PSF model for ACS from an extensive investigation of 47 Tuc stars in a
modestly crowded region. We show that this model PSF excellently describes the
PSF variation pattern in the cluster observation when a slight adjustment of
ellipticity is applied. The high number density of source galaxies and the
accurate removal of the PSF effect through moment-based deconvolution allow us
to restore the dark matter distribution of the cluster in great detail. The
direct comparison of the mass map with the X-ray morphology from Chandra
observations shows that the two peaks of intracluster medium traced by X-ray
emission are lagging behind the corresponding dark matter clumps, indicative of
an on-going merger. The overall mass profile of the cluster can be well
described by an NFW profile with a scale radius of r_s =309+-45 kpc and a
concentration parameter of c=3.7+-0.5. The mass estimates from the lensing
analysis are consistent with those from X-ray and Sunyaev-Zeldovich analyses.
The predicted velocity dispersion is also in good agreement with the
spectroscopic measurement from VLT observations. In the adopted WMAP cosmology,
the total projected mass and the mass-to-light ratio within 1 Mpc are estimated
to be 4.92+-0.44 10^14 solar mass and 95+-8 solar mass/solar luminosity,
respectively.Comment: Accepted for publication in Astrophysical Journal. 58 pages, 26
figures. Figures have been degraded to meet size limit; a higher resolution
version available at http://acs.pha.jhu.edu/~mkjee/ms_cl0152.pd
Hubble and Spitzer Space Telescope Observations of the Debris Disk around the Nearby K Dwarf HD 92945
[ABRIDGED] We present the first resolved images of the debris disk around the
nearby K dwarf HD 92945. Our F606W (V) and F814W (I) HST/ACS coronagraphic
images reveal an inclined, axisymmetric disk consisting of an inner ring
2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface
brightness declines slowly with increasing radius 3".0-5".1 (65-110 AU) from
the star. A precipitous drop in the surface brightness beyond 110 AU suggests
that the outer disk is truncated at that distance. The radial surface-density
profile is peaked at both the inner ring and the outer edge of the disk. The
dust in the outer disk scatters neutrally but isotropically, and it has a low
V-band albedo of 0.1. We also present new Spitzer MIPS photometry and IRS
spectra of HD 92945. These data reveal no infrared excess from the disk
shortward of 30 micron and constrain the width of the 70 micron source to < 180
AU. Assuming the dust comprises compact grains of astronomical silicate with a
surface-density profile described by our scattered-light model of the disk, we
successfully model the 24-350 micron emission with a minimum grain size of
a_min = 4.5 micron and a size distribution proportional to a^-3.7 throughout
the disk, but with a maximum grain size of 900 micron in the inner ring and 50
micron in the outer disk. Our observations indicate a total dust mass of ~0.001
M_earth. However, they provide contradictory evidence of the dust's physical
characteristics: its neutral V-I color and lack of 24 micron emission imply
grains larger than a few microns, but its isotropic scattering and low albedo
suggest a large population of submicron-sized grains. The dynamical causes of
the disk's morphology are unclear, but recent models of dust creation and
transport in the presence of migrating planets indicate an advanced state of
planet formation around HD 92945.Comment: 29 pages, 10 figures; to be published in The Astronomical Journa
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