110 research outputs found
Finding Gravitationally Lensed Systems in the VLA Stripe 82 Survey
We describe the selection, observations, and analysis of four potential gravitationally lensed radio active galactic nuclei within the VLA Stripe 82 Survey. Using the Very Large Array, we have obtained high-resolution (0.3\u27\u27) observations of two of the four candidates. We analyze the source morphology and 5-7GHz spectral index of each source to determine whether the sources are indeed lensed. We find that neither of the observed sources are lensed, and instead they are core-jet systems contained within the host galaxies. The radio complexes at 00:42:31.4-00:43:40.6 and 01:24:55.9+00:11:17.4 are compact and extend 16.2 kpc and 19.4 kpc respectively. We also find compact, unresolved radio cores with flat spectral indices (alpha \u3e -0.12) at the center of each complex. These systems, while not the lensed systems we sought, can serve as windows into the growth of radio jets and their propagation through the interstellar medium of the host galaxy
The Evolution of Molecular Gas Fraction Traced by the CO Tully-Fisher Relation
Carbon monoxide (CO) observations show a luminosityline-width correlation
that evolves with redshift. We present a method to use CO measurements alone to
infer the molecular gas fraction () and constrain the COH
conversion factor (). We compile from the literature spatially
integrated low- CO observations of six galaxy populations, including a total
of 449 galaxies between . The CO data of each population
provide an estimate of the -normalized mean molecular gas
fraction (). The redshift evolution of the
luminosityline-width correlation thus indicates an evolution of . We use a Bayesian-based Monte-Carlo Markov Chain sampler
to derive the posterior probability distribution functions of for these galaxy populations, accounting for random
inclination angles and measurement errors in the likelihood function. We find
that the molecular gas fraction evolves rapidly with redshift, with , for both normal star-forming and
starburst galaxies. Furthermore, the evolution trend agrees well with that
inferred from the Kennicutt-Schmidt relation and the star-forming main
sequence. Finally, at normal star-forming galaxies require a
larger than starburst galaxies to match their
molecular gas fractions, but at both star-forming types exhibit
sub-Galactic values and normal star-forming galaxies appear
more gas-rich than starbursts. Future applications of this method include
calibrating Tully-Fisher relations without inclination correction and inferring
the evolution of the atomic gas fraction with HI observations.Comment: 8 pages, 3 figures, 1 table. Accepted by ApJ Letter
BIO-INSPIRED SONAR IN COMPLEX ENVIRONMENTS: ATTENTIVE TRACKING AND VIEW RECOGNITION
Bats are known for their unique ability to sense the world through echolocation. This allows them to perceive the world in a way that few animals do, but not without some difficulties. This dissertation explores two such tasks using a bio-inspired sonar system: tracking a target object in cluttered environments, and echo view recognition. The use of echolocation for navigating in dense, cluttered environments can be a challenge due to the need for rapid sampling of nearby objects in the face of delayed echoes from distant objects. If long-delay echoes from a distant object are received after the next pulse is sent out, these “aliased” echoes appear as close-range phantom objects. This dissertation presents three reactive strategies for a high pulse-rate sonar system to combat aliased echoes: (1) changing the interpulse interval to move the aliased echoes away in time from the tracked target, (2) changing positions to create a geometry without aliasing, and (3) a phase-based, transmission beam-shaping strategy to illuminate the target and not the aliasing object. While this task relates to immediate sensing needs and lower level motor loops, view recognition is involved in higher level navigation and planning. Neurons in the mammalian brain (specifically in the hippocampus formation) named “place cells” are thought to reflect this recognition of place and are involved in implementing a spatial map that can be used for path planning and memory recall. We propose hypothetical “echo view cells” that could contribute (along with odometry) to the creation of place cell representations actually observed in bats. We strive to recognize views over extended regions that are many body lengths in size, reducing the number of places to be remembered for a map. We have successfully demonstrated some of this spatial invariance by training feed-forward neural networks (traditional neural networks and spiking neural networks) to recognize 66 distinct places in a laboratory environment over a limited range of translations and rotations. We further show how the echo view cells respond in between known places and how the population of cell outputs can be combined over time for continuity
The Circumgalactic Medium of Submillimeter Galaxies. II. Unobscured QSOs within Dusty Starbursts and QSO Sightlines with Impact Parameters below 100 Kiloparsec
We present Atacama Large Millimeter/submillimeter Array (ALMA) 870um
observations of 29 bright Herschel sources near high-redshift QSOs. The
observations confirm that 20 of the Herschel sources are submillimeter-bright
galaxies (SMGs) and identify 16 new SMG-QSO pairs that are useful to studies of
the circumgalactic medium (CGM) of SMGs. Eight out of the 20 SMGs are blends of
multiple 870um sources. The angular separations for six of the Herschel-QSO
pairs are less than 10", comparable to the sizes of the Herschel beam and the
ALMA primary beam. We find that four of these six "pairs" are actually QSOs
hosted by SMGs. No additional submillimeter companions are detected around
these QSOs and the rest-frame ultraviolet spectra of the QSOs show no evidence
of significant reddening. Black hole accretion and star formation contribute
almost equally in bolometric luminosity in these galaxies. The SMGs hosting
QSOs show similar source sizes, dust surface densities, and SFR surface
densities as other SMGs in the sample. We find that the black holes are growing
3 faster than the galaxies when compared to the present-day
black-hole-galaxy mass ratio, suggesting a QSO duty cycle of 30% in
SMGs at z ~ 3. The remaining two Herschel-detected QSOs are undetected at 870um
but each has an SMG "companion" only 9" and 12" away (71 and 95 kpc at z = 3).
They could be either merging or projected pairs. If the former, they would
represent a rare class of "wet-dry" mergers. If the latter, the QSOs would, for
the first time, probe the CGM of SMGs at impact parameters below 100 kpc.Comment: ApJ accepte
Diversifying bioenergy crops increases yield and yield stability by reducing weed abundance
Relationships between species diversity, productivity, temporal stability of productivity, and plant invasion have been well documented in grasslands, and these relationships could translate to improved agricultural sustainability. However, few studies have explored these relationships in agricultural contexts where fertility and weeds are managed. Using 7 years of biomass yield and species composition data from 12 species mixture treatments varying in native species diversity, we found that species richness increased yield and interannual yield stability by reducing weed abundance. Stability was driven by yield as opposed to temporal variability of yield. Nitrogen fertilization increased yield but at the expense of yield stability. We show how relationships between diversity, species asynchrony, invasion, productivity, and stability observed in natural grasslands can extend into managed agricultural systems. Increasing bioenergy crop diversity can improve farmer economics via increased yield, reduced yield variability, and reduced inputs for weed control, thus promoting perennial vegetation on agricultural lands
The Subarcsecond Mid-Infrared View of Local Active Galactic Nuclei. IV. The L- and M-band Imaging Atlas
We present the largest currently existing subarcsecond 3-5 m atlas of
119 local () active galactic nuclei (AGN). This atlas includes AGN of
5 subtypes: 22 are Seyfert 1; 5 are intermediate Seyferts; 46 are Seyfert 2; 26
are LINERs; and 20 are composites/starbursts. Each AGN was observed with VLT
ISAAC in the - and/or -bands between 2000 and 2013. We detect at
3 confidence 92 sources in the -band and 83 sources in the -band.
We separate the flux into unresolved nuclear flux and resolved flux through
two-Gaussian fitting. We report the nuclear flux, extended flux, apparent size,
and position angle of each source, giving upper-limits for sources
which are undetected. Using WISE W1- and W2-band photometry we derive relations
predicting the nuclear and fluxes for Sy1 and Sy2 AGN based on their
W1-W2 color and WISE fluxes. Lastly, we compare the measured mid-infrared
colors to those predicted by dusty torus models SKIRTOR, CLUMPY, CAT3D, and
CAT3D-WIND, finding best agreement with the latter. We find that models
including polar winds best reproduce the 3-5m colors, indicating that
winds are an important component of dusty torus models. We find that several
AGN are bluer than models predict. We discuss several explanations for this and
find that it is most plausibly stellar light contamination within the ISAAC
-band nuclear fluxes.Comment: Main Text: 22 pages, 9 figures, 3 tables. Accepted by Ap
Why Chromatic Imaging Matters
During the last two decades, the first generation of beam combiners at the
Very Large Telescope Interferometer has proved the importance of optical
interferometry for high-angular resolution astrophysical studies in the near-
and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u-v
coverage per pointing increases significantly, providing an opportunity to use
reconstructed images as powerful scientific tools. Therefore, interferometric
imaging is already a key feature of the new generation of VLTI instruments, as
well as for other interferometric facilities like CHARA and JWST. It is thus
imperative to account for the current image reconstruction capabilities and
their expected evolutions in the coming years. Here, we present a general
overview of the current situation of optical interferometric image
reconstruction with a focus on new wavelength-dependent information,
highlighting its main advantages and limitations. As an Appendix we include
several cookbooks describing the usage and installation of several state-of-the
art image reconstruction packages. To illustrate the current capabilities of
the software available to the community, we recovered chromatic images, from
simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we
aim at showing the importance of selecting good regularization functions and
their impact on the reconstruction.Comment: Accepted for publication in Experimental Astronomy as part of the
topical collection: Future of Optical-infrared Interferometry in Europ
Flat Rotation Curves Found in Merging Dusty Starbursts at z = 2.3 through Tilted-ring Modeling
The brightest 500m source in the XMM field, HXMM01, is a rare merger
of luminous starburst galaxies at with a dust-obscured star-formation
rate of 2,000. Here we present high-resolution
spectroscopic observations of HXMM01 with the Atacama Large
Millimeter/submillimeter Array (ALMA). We detect line emission from , [C I], and p- and continuum emission at GHz. At a
spatial resolution of 0.2" and a spectral resolution of 40,
the source is resolved into three distinct components, which are spatially and
dynamically associated within a projected radius of 20kpc and a radial
velocity range of 2,000. For two major components, our
Bayesian-based tilted-ring modeling of the ALMA spectral cubes shows almost
flat rotation curves peaking at at galactocentric
distances between 2 and 5kpc. Each of them has a dynamical mass of
. The combination of the dynamical masses and the
archival data places strong upper limits on the
COH conversion factor of . These
limits are significantly below the Galactic inner disk value
of but are consistent with
those of local starbursts. Therefore, the previously estimated short gas
depletion timescale of Myr remains unchanged.Comment: Accepted for publication in ApJ
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