1,490 research outputs found
Spatially Resolved PAH Emission Features in Nearby, Low Metallicity, Star-Forming Galaxies
Low-resolution, mid-infrared Spitzer/IRS spectral maps are presented for
three nearby, low-metallicity dwarf galaxies (NGC 55, NGC 3109 and IC 5152) for
the purpose of examining the spatial distribution and variation of polycyclic
aromatic hydrocarbon (PAH) emission. The sample straddles a metallicity of
12+log(O/H)~8.0, a transition point below which PAH intensity empirically drops
and the character of the interstellar medium changes. We derive quantitative
radiances of PAH features and atomic lines on both global and
spatially-resolved scales. The Spitzer spectra, combined with extensive
ancillary data from the UV through the mid-infrared, allow us to examine
changes in the physical environments and in PAH feature radiances down to a
physical scale of 50 pc. We discuss correlations between various PAH emission
feature and atomic line radiances. The (6.2 micron)/(11.3 micron), (7.7
micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron),
and (8.6 micron)/(6.2 micron) PAH radiance ratios are found to be independent
of position across all three galaxies, although the ratios do vary from galaxy
to galaxy. As seen in other galaxies, we find no variation in the grain size
distribution as a function of local radiation field strength. Absolute PAH
feature intensities as measured by a ratio of PAH/(24 micron) radiances are
seen to vary both positionally within a given galaxy, and from one galaxy to
another when integrated over the full observed extent of each system. We
examine direct comparisons of CC mode PAH ratios (7.7 micron)/(6.2 micron) and
(8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7
micron)/(11.3 micron). We find little variation in either mode, and no
difference in trends between modes. While the local conditions change markedly
over the observed regions of these galaxies, the properties of PAH emission
show a remarkable degree of uniformity.Comment: Astrophysical Journal, in pres
Mid-Infrared Evidence for Accelerated Evolution in Compact Group Galaxies
We find evidence for accelerated evolution in compact group galaxies from the
distribution in mid-infrared colorspace of 42 galaxies from 12 Hickson Compact
Groups (HCGs) compared to the the distributions of several other samples
including the LVL+SINGS galaxies, interacting galaxies, and galaxies from the
Coma Cluster. We find that the HCG galaxies are not uniformly distributed in
colorspace, as well as quantitative evidence for a gap. Galaxies in the infall
region of the Coma cluster also exhibit a non-uniform distribution and a less
well defined gap, which may reflect a similarity with the compact group
environment. Neither the Coma Center or interacting samples show evidence of a
gap, leading us to speculate that the gap is unique to the environment of high
galaxy density where gas has not been fully processed or stripped.Comment: To appear in the Proceedings of the Galaxy Wars: Stellar Populations
and Star Formation in Interacting Galaxies Conferenc
A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. II. IC 1613
We present Spitzer Space Telescope IRAC photometry of the Local Group dwarf
irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 micron
photometry with broadband optical photometry and find that the optical data do
not detect 43% and misidentify an additional 11% of the total AGB population,
likely because of extinction caused by circumstellar material. Further, we find
that a narrowband optical carbon star study of IC 1613 detects 50% of the total
AGB population and only considers 18% of this population in calculating the
carbon to M-type AGB ratio. We derive an integrated mass-loss rate from the AGB
stars of 0.2-1.0 x 10^(-3) solar masses per year and find that the distribution
of bolometric luminosities and mass-loss rates are consistent with those for
other nearby metal-poor galaxies. Both the optical completeness fractions and
mass-loss rates in IC 1613 are very similar to those in the Local Group dwarf
irregular, WLM, which is expected given their similar characteristics and
evolutionary histories.Comment: Accepted by ApJ, 26 pages, 10 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
The Spitzer Local Volume Legacy: Survey Description and Infrared Photometry
The survey description and the near-, mid-, and far-infrared flux properties
are presented for the 258 galaxies in the Local Volume Legacy (LVL). LVL is a
Spitzer Space Telescope legacy program that surveys the local universe out to
11 Mpc, built upon a foundation of ultraviolet, H-alpha, and HST imaging from
11HUGS (11 Mpc H-alpha and Ultraviolet Galaxy Survey) and ANGST (ACS Nearby
Galaxy Survey Treasury). LVL covers an unbiased, representative, and
statistically robust sample of nearby star-forming galaxies, exploiting the
highest extragalactic spatial resolution achievable with Spitzer. As a result
of its approximately volume-limited nature, LVL augments previous Spitzer
observations of present-day galaxies with improved sampling of the
low-luminosity galaxy population. The collection of LVL galaxies shows a large
spread in mid-infrared colors, likely due to the conspicuous deficiency of 8um
PAH emission from low-metallicity, low-luminosity galaxies. Conversely, the
far-infrared emission tightly tracks the total infrared emission, with a
dispersion in their flux ratio of only 0.1 dex. In terms of the relation
between infrared-to-ultraviolet ratio and ultraviolet spectral slope, the LVL
sample shows redder colors and/or lower infrared-to-ultraviolet ratios than
starburst galaxies, suggesting that reprocessing by dust is less important in
the lower mass systems that dominate the LVL sample. Comparisons with
theoretical models suggest that the amplitude of deviations from the relation
found for starburst galaxies correlates with the age of the stellar populations
that dominate the ultraviolet/optical luminosities.Comment: Accepted for publication in ApJ; Figures 1,8,9 provided as jpeg
Continuous manufacturing of lentiviral vectors
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Continuous production of lentiviral vectors using a fixed-bed bioreactor
Lentiviral vectors (LVs) have emerged as indispensable tools for mediating stable transfer of large transgenes in mammalian cells, which has resulted in the widespread application of LVs for the manufacture of gene-modified cell therapies, particularly chimeric antigen receptor T-cell (CAR-T-cell) therapies.
The typical manufacture of LVs through chemical transfection of adherent embryonic kidney 293T-cells with plasmid DNA is highly versatile and enables manufacturing to pivot to produce vectors with alternative transgenes and envelope proteins. However, transient transfection presents several challenges, including batch-to-batch variability, limited scalability and high costs attributed to the plasmid DNA and transfection reagent. Therefore, the transition towards continuous LV production using producer cell lines would allow for cost effective and scalable manufacturing along with enhanced safety and reproducibility. Specifically, continuous production using stable producer cell lines could extend vector expression by counteracting the limited post-induction expression time and eliminating the requirement for removal of the inducing agent during downstream processing.
In this study, a continuous LV manufacturing process was established using a stable producer cell line, which constitutively expresses third-generation LVs pseudotyped the RDpro envelope protein (WinPacRDpro). Production of LVs in typical 2D culture vessels was performed, however, these systems lack the control of culture parameters, such as pH and dissolved oxygen, and require laborious handling, which introduce risks of contamination. Therefore, the transition to LV production in bioreactors is a necessary step to achieve sufficient LV supply, e.g. for late-stage clinical trials.
This work demonstrates the establishment of a quasi-perfusion process using repetitive batch strategy with medium exchange, first, in 2D culture vessels and then, to an iCELLis fixed-bed bioreactor to continuously produce RDpro-pseudotyped LVs. This initial work using a continuous process-mimic in a fixed-bed bioreactor enables cell expansion in a controlled environment and the bioreactor characterization was used to gain insights for the development of a perfusion strategy. Growth, substrate utilization, lactate generation and LV production were evaluated under different medium exchange strategies. Finally, the established bioreactor process was then transformed to perfusion culture with studies aimed at understanding the impact of perfusion conditions on LV production
The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators
(Abridged) Spitzer data at 24, 70, and 160 micron and ground-based H-alpha
images are analyzed for a sample of 189 nearby star-forming and starburst
galaxies to investigate whether reliable star formation rate (SFR) indicators
can be defined using the monochromatic infrared dust emission centered at 70
and 160 micron. We compare recently published recipes for SFR measures using
combinations of the 24 micron and observed H-alpha luminosities with those
using 24 micron luminosity alone. From these comparisons, we derive a reference
SFR indicator for use in our analysis. Linear correlations between SFR and the
70 and 160 micron luminosity are found for L(70)>=1.4x10^{42} erg/s and
L(160)>=2x10^{42} erg/s, corresponding to SFR>=0.1-0.3 M_sun/yr. Below those
two luminosity limits, the relation between SFR and 70 micron (160 micron)
luminosity is non-linear and SFR calibrations become problematic. The
dispersion of the data around the mean trend increases for increasing
wavelength, becoming about 25% (factor ~2) larger at 70 (160) micron than at 24
micron. The increasing dispersion is likely an effect of the increasing
contribution to the infrared emission of dust heated by stellar populations not
associated with the current star formation. The non-linear relation between SFR
and the 70 and 160 micron emission at faint galaxy luminosities suggests that
the increasing transparency of the interstellar medium, decreasing effective
dust temperature, and decreasing filling factor of star forming regions across
the galaxy become important factors for decreasing luminosity. The SFR
calibrations are provided for galaxies with oxygen abundance 12+Log(O/H)>8.1.
At lower metallicity the infrared luminosity no longer reliably traces the SFR
because galaxies are less dusty and more transparent.Comment: 69 pages, 19 figures, 2 tables; accepted for publication on Ap
'The Brick' is not a brick : A comprehensive study of the structure and dynamics of the Central Molecular Zone cloud G0.253+0.016
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO data at 3 mm, using two new pieces of software which we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line of sight velocity dispersion in this cloud, kms, which is somewhat larger than predicted by velocity dispersion-size relations for the Central Molecular Zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding . This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling we conclude that G0.253+0.016 is not a single, coherent, and centrally-condensed molecular cloud; 'the Brick' is not a \emph{brick}. Instead, G0.253+0.016 is a dynamically complex and hierarchically-structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.Peer reviewedFinal Accepted Versio
The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses
In spectroscopy, it is conventional to treat pulses much stronger than the
linewidth as delta-functions. In NMR, this assumption leads to the prediction
that pi pulses do not refocus the dipolar coupling. However, NMR spin echo
measurements in dipolar solids defy these conventional expectations when more
than one pi pulse is used. Observed effects include a long tail in the CPMG
echo train for short delays between pi pulses, an even-odd asymmetry in the
echo amplitudes for long delays, an unusual fingerprint pattern for
intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments
that set limits on possible extrinsic causes for the phenomena are reported. We
find that the action of the system's internal Hamiltonian during any real pulse
is sufficient to cause the effects. Exact numerical calculations, combined with
average Hamiltonian theory, identify novel terms that are sensitive to
parameters such as pulse phase, dipolar coupling, and system size.
Visualization of the entire density matrix shows a unique flow of quantum
coherence from non-observable to observable channels when applying repeated pi
pulses.Comment: 24 pages, 27 figures. Revised from helpful referee comments. Added
new Table IV, new paragraphs on pages 3 and 1
An Aromatic Inventory of the Local Volume
Using infrared photometry from the Spitzer Space Telescope, we perform the
first inventory of aromatic feature emission (AFE, but also commonly referred
to as PAH emission) for a statistically complete sample of star-forming
galaxies in the local volume. The photometric methodology involved is
calibrated and demonstrated to recover the aromatic fraction of the IRAC 8
micron flux with a standard deviation of 6% for a training set of 40 SINGS
galaxies (ranging from stellar to dust dominated) with both suitable
mid-infrared Spitzer IRS spectra and equivalent photometry. A potential factor
of two improvement could be realized with suitable 5.5 and 10 micron
photometry, such as what may be provided in the future by JWST. The resulting
technique is then applied to mid-infrared photometry for the 258 galaxies from
the Local Volume Legacy (LVL) survey, a large sample dominated in number by
low-luminosity dwarf galaxies for which obtaining comparable mid-infrared
spectroscopy is not feasible. We find the total LVL luminosity due to five
strong aromatic features in the 8 micron complex to be 2.47E10 solar
luminosities with a mean volume density of 8.8E6 solar luminosities per cubic
Megaparsec. Twenty-four of the LVL galaxies, corresponding to a luminosity cut
at M = -18.22 in the B band, account for 90% of the aromatic luminosity. Using
oxygen abundances compiled from the literature for 129 of the 258 LVL galaxies,
we find a correlation between metallicity and the aromatic to total infrared
emission ratio but not the aromatic to total 8 micron dust emission ratio. A
possible explanation is that metallicity plays a role in the abundance of
aromatic molecules relative to the total dust content, but other factors such
as star formation and/or the local radiation field affect the excitation of
those molecules.Comment: ApJ in press; 29 pages, 14 figures, 3 tables; emulateapj forma
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