213 research outputs found
A study of probability distributions of DCT coefficients in JPEG compression
The Discrete Cosine Transform (DCT) used in JPEG compression has shown excellent energy compaction properties that rival that of the ideal Karhunen-Loève Transform. Lossy compression in JPEG is achieved by distorting 8x8 block DCT coefficients through quantization. It has been shown in literature that DC block DCT coefficients are Gaussian probability distributed and AC block DCT coefficients are Generalized Normal probability distributed.
In this investigation, three probability density models for individual modes of non- quantized AC block DCT coefficients are evaluated and are used as basis for the derivation of probability distributions for quantized block DCT coefficients. The suitability of each of the three derived models is evaluated using the Kolmogorov-Smirnov and Ď2 goodness-of-fit tests, and the moments of the best-fit model are derived. The best-fit model is applied to detect the presence and extent of JPEG compression history in bitmap images. A model for all quantized AC block DCT coefficients is derived using mixtures of individual quantized block DCT modes, and the model hence developed is used to validate the Generalized Benford\u27s Law for leading digit distributions of quantized AC block DCT coefficients
Kinematics of Protostellar Objects in the Ď Ophiuchus A Region
We present the detection of infall, rotation, and outflow kinematic signatures toward both a protostellar source, VLA 1623, and what was initially thought to be a pre-protostellar core, SM 1N, in the Ď Ophiuchus A region. The kinematic signatures of early star formation were detected in the dense molecular gas surrounding the embedded sources using high signal-to-noise ratio millimeter and submillimeter data. Centroid velocity maps made with HCO+ J = 4 â 3 and J = 1 â 0 line emission exhibit the blue bulge signature of infall, which is predicted to be seen when infall motion dominates over rotational motion. Further evidence for infalling gas is found in the HCO+ blue asymmetric line profiles and red asymmetric opacity profiles. We also performed CO J = 3 â 2 and J = 1 â 0 observations to determine the direction, orientation, and extent of molecular outflows, and we report the discovery of a new bipolar outflow possibly driven by SM 1N
Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Linear Dynamic Range Spectral Line Mapping
We report the results of a 100 square degree survey of the Taurus Molecular
Cloud region in the J = 1-0 transition of 12CO and 13CO. The image of the cloud
in each velocity channel includes ~ 3 million Nyquist sampled pixels on a 20"
grid. The high sensitivity and large linear dynamic range of the maps in both
isotopologues reveal a very complex, highly structured cloud morphology. There
are large scale correlated structures evident in 13CO emission having very fine
dimensions, including filaments, cavities, and rings. The 12CO emission shows a
quite different structure, with particularly complex interfaces between regions
of greater and smaller column density defining the boundaries of the
largest-scale cloud structures. The axes of the striations seen in the 12CO
emission from relatively diffuse gas are aligned with the direction of the
magnetic field. Using a column density-dependent model for the CO fractional
abundance, we derive the mass of the region mapped to be 24,000 solar masses, a
factor of three greater than would be obtained with canonical CO abundance
restricted to the high column density regions. We determine that half the mass
of the cloud is in regions having column density below 2.1x10^{21} per square
cm. The distribution of young stars in the region covered is highly nonuniform,
with the probability of finding a star in a pixel with a specified column
density rising sharply for N(H2) = 6x10^{21} cm^{-2}. We determine a relatively
low star formation efficiency (mass of young stars/mass of molecular gas),
between 0.3 and 1.2 %, and an average star formation rate during the past 3 Myr
of 8x10^{-5} stars yr^{-1}.Comment: 53 pages, 21 figure
Discovery Of A Molecular Outflow in the Haro 6-10 Star-Forming Region
We present high sensitivity 12CO and 13CO (1-0) molecular line maps covering
the full extent of the parsec scale Haro~6-10 Herbig-Haro (HH) flow. We report
the discovery of a molecular CO outflow along the axis of parsec-scale HH flow.
Previous molecular studies missed the identification of the outflow probably
due to their smaller mapping area and the confusing spectral features present
towards the object. Our detailed molecular line study of the full 1.6 pc extent
of the optical flow shows evidence for both blueshifted and redshifted gas set
in motion by Haro~6-10 activity. The molecular outflow is centered at
Haro~6-10, with redshifted gas being clumpy and directed towards the northeast,
while blueshifted gas is in the southwest direction. The molecular gas
terminates well within the cloud, short of the most distant HH objects of the
optical flow. Contamination from an unrelated cloud along the same line of
sight prevents a thorough study of the blueshifted outflow lobe and the mass
distribution at the lowest velocities in both lobes. The cloud core in which
Haro~6-10 is embedded is filamentary and flattened in the east-west direction.
The total cloud mass is calculated from 13CO(1-0) to be ~200Msun. The lower
limit of the mass associated with the outflow is ~0.25Msun.Comment: ApJ Accepted; 9 pages, 8 figures. For high resolution ps file use:
http://www.astro.umass.edu/~irena/haro.p
Entrainment Mechanisms for Outflows in the L1551 Star-Forming Region
We present high sensitivity 12CO and 13CO J=1!0 molecular line maps covering the full extent of the parsec scale L1551 molecular outflow, including the redshifted east-west (EW) flow. We also present 12CO J=3!2 data that extends over a good fraction of the area mapped in the J=1!0 transition. We compare the molecular data to widefield, narrow-band optical emission in H. While there are multiple outflows in the L1551 cloud, the main outflow is oriented at 50⌠position angle and appears to be driven by embedded source(s) in the central IRS 5 region. The blueshifted outflowing molecular gas extends to the edge of the molecular cloud and beyond the last HH object, HH 256. On the contrary, the redshifted molecular gas terminates within the cloud, short of the most distant HH object, HH 286, which lies well beyond the cloud boundary. The J=3!2 data indicate that there may be molecular emission associated with the L1551 NE jet, within the redshifted lobe of main outflow. We have also better defined the previously known EW flow and believe we have identified its blueshifted counterpart. We further speculate that the origin of the EW outflow lies near HH 102. We use velocity dependent opacity correction to estimate the mass and the energy of the outflow. The resulting mass spectral indices from our analysis, are systematically lower (less steep) than the power law indices obtained towards other outflows in several recent studies that use a similar opacity correction method. We show that systematic errors and biases in the analysis procedures for deriving mass spectra could result in errors in the determination of the power-law indices. The mass spectral indices, the morphological appearance of the position-velocity plots and integrated intensity emission maps of the molecular data, compared with the optical, suggest that jet-driven bow-shock entrainment is the best explanation for the driving mechanism of outflows in L1551. The kinetic energy of the outflows is found to be comparable to the binding energy of the cloud and sufficient to maintain the turbulence in the L1551 cloud
Spectra of Nearby Galaxies Measured with a New Very Broadband Receiver
Three-millimeter-wavelength spectra of a number of nearby galaxies have been
obtained at the Five College Radio Astronomy Observatory (FCRAO) using a new,
very broadband receiver. This instrument, which we call the Redshift Search
Receiver, has an instantaneous bandwidth of 36 GHz and operates from 74 to
110.5 GHz. The receiver has been built at UMass/FCRAO to be part of the initial
instrumentation for the Large Millimeter Telescope (LMT) and is intended
primarily for determination of the redshift of distant, dust-obscured galaxies.
It is being tested on the FCRAO 14m by measuring the 3mm spectra of a number of
nearby galaxies. There are interesting differences in the chemistry of these
galaxies.Comment: published in the Proceedings of the International Astronomical Union
(2008), 4. Vol 251, pp 251-256 Cambridge University Pres
Star Formation in Bright Rimmed Clouds. I. Millimeter and Submillimeter Molecular Line Surveys
We present the results of the first detailed millimeter and submillimeter
molecular line survey of bright rimmed clouds, observed at FCRAO in the CO
(J=1-0), C18O (J=1-0), HCO+ (J=1-0), H13CO+ (J=1-0), and N2H+ (J=1-0)
transitions, and at the HHT in the CO (J=2-1), HCO+ (J=3-2), HCO+ (J=4-3),
H13CO+ (J=3-2), and H13CO+ (J=4-3) molecular line transitions. The source list
is composed of a selection of bright rimmed clouds from the catalog of such
objects compiled by Sugitani et al. (1991). We also present observations of
three Bok globules done for comparison with the bright rimmed clouds. We find
that the appearance of the millimeter CO and HCO+ emission is dominated by the
morphology of the shock front in the bright rimmed clouds. The HCO+ (J=1-0)
emission tends to trace the swept up gas ridge and overdense regions which may
be triggered to collapse as a result of sequential star formation. Five of the
seven bright rimmed clouds we observe seem to have an outflow, however only one
shows the spectral line blue-asymmetric signature that is indicative of infall,
in the optically thick HCO+ emission. We also present evidence that in bright
rimmed clouds the nearby shock front may heat the core from outside-in thereby
washing out the normally observed line infall signatures seen in isolated star
forming regions. We find that the derived core masses of these bright rimmed
clouds are similar to other low and intermediate mass star forming regions.Comment: 67 pages, including 35 figures and 6 tables. Accepted for publication
in ApJ. Version with embedded full-resolution figures available at
http://www.astro.umass.edu/~devries/brc1
SuperCam, a 64-pixel heterodyne imaging array for the 870 micron atmospheric window
We report on the development of SuperCam, a 64 pixel, superheterodyne camera
designed for operation in the astrophysically important 870 micron atmospheric
window. SuperCam will be used to answer fundamental questions about the physics
and chemistry of molecular clouds in the Galaxy and their direct relation to
star and planet formation. The advent of such a system will provide an order of
magnitude increase in mapping speed over what is now available and
revolutionize how observational astronomy is performed in this important
wavelength regime. Unlike the situation with bolometric detectors, heterodyne
receiver systems are coherent, retaining information about both the amplitude
and phase of the incident photon stream. From this information a high
resolution spectrum of the incident light can be obtained without multiplexing.
SuperCam will be constructed by stacking eight, 1x8 rows of fixed tuned, SIS
mixers. The IF output of each mixer will be connected to a low-noise, broadband
MMIC amplifier integrated into the mixer block. The instantaneous IF bandwidth
of each pixel will be ~2 GHz, with a center frequency of 5 GHz. A spectrum of
the central 500 MHz of each IF band will be provided by the array spectrometer.
Local oscillator power is provided by a frequency multiplier whose output is
divided between the pixels by using a matrix of waveguide power dividers. The
mixer array will be cooled to 4K by a closed-cycle refrigeration system.
SuperCam will reside at the Cassegrain focus of the 10m Heinrich Hertz
telescope (HHT). A prototype single row of the array will be tested on the HHT
in 2006, with the first engineering run of the full array in late 2007. The
array is designed and constructed so that it may be readily scaled to higher
frequencies.Comment: 12 pages, 14 figures, to be published in the Proceedings of SPIE Vol.
6275, "Astronomical Telescopes and Instrumentation, Millimeter and
Submillimeter Detectors and Instrumentation for Astronomy III
Incidence of Mg II absorption systems towards flat-spectrum radio quasars
The conventional wisdom that the rate of incidence of Mg II absorption
systems, dN/dz (excluding `associated systems' having velocity beta*c relative
to the AGN of less than ~5000 km/s) is totally independent of the background
AGN, has been challenged by a recent finding that dN/dz for strong Mg II
absorption systems towards distant blazars is 2.2 \pm_{0.6}^{0.8} times the
value known for normal optically-selected quasars (QSOs). This has led to the
suggestion that a significant fraction of even the absorption systems with beta
as high as 0.1 may have been ejected by the relativistic jets in the blazars,
which are expected to be pointed close to our direction. Here we investigate
this scenario using a large sample of 115 flat-spectrum radio-loud quasars
(FSRQs) which too possess powerful jets, but are only weakly polarized. We
show, for the first time, that dN/dz towards FSRQs is, on the whole, quite
similar to that known for QSOs and the comparative excess of strong \mgii
absorption systems seen towards blazars is mainly confined to beta< 0.15. The
excess relative to FSRQs can probably result from a likely closer alignment of
blazar jets with our direction and hence any gas clouds accelerated by them are
more likely to be on the line of sight to the active quasar nucleus.Comment: 6 pages (emulateapj style), 2 figures, 4 tables, accepted for
publication in Ap
Early Science with the Large Millimeter Telescope: Exploring the Effect of AGN Activity on the Relationships Between Molecular Gas, Dust, and Star Formation
The molecular gas, H, that fuels star formation in galaxies is difficult
to observe directly. As such, the ratio of to
is an observational estimation of the star formation rate compared with the
amount of molecular gas available to form stars, which is related to the star
formation efficiency and the inverse of the gas consumption timescale. We test
what effect an IR luminous AGN has on the ratio
in a sample of 24 intermediate redshift galaxies from the 5 mJy Unbiased
Spitzer Extragalactic Survey (5MUSES). We obtain new CO(1-0) observations with
the Redshift Search Receiver on the Large Millimeter Telescope. We diagnose the
presence and strength of an AGN using Spitzer IRS spectroscopy. We find that
removing the AGN contribution to results in a mean
for our entire sample consistent with
the mean derived for a large sample of star
forming galaxies from . We also include in our comparison the
relative amount of polycyclic aromatic hydrocarbon emission for our sample and
a literature sample of local and high redshift Ultra Luminous Infrared Galaxies
and find a consistent trend between and , such that small dust grain emission decreases
with increasing for both local and high
redshift dusty galaxies.Comment: Accepted for publication in ApJ (to appear on December 10
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