102 research outputs found
Extended Tidal Structure In Two Lyman Alpha-Emitting Starburst Galaxies
We present new VLA C-configuration HI imaging of the Lyman Alpha-emitting
starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution
probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger
than 8.1 kpc in IRAS 08339+6517. Both systems are revealed to be tidally
interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum
separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.4 arcminutes =
56 kpc minimum separation). The HI emission is extended in these systems, with
tidal tails and debris between the target galaxies and their companions. Since
Lyman Alpha emission has been detected from both of these primary systems,
these observations suggest that the geometry of the ISM is one of the factors
affecting the escape fraction of Lyman Alpha emission from starburst
environments. Furthermore, these observations argue for the importance of
interactions in triggering massive star formation events.Comment: ApJ, in press; 11 pages, 2 color figure
Time-resolved Monitoring of Enzyme Activity with Ultrafast Hyper-CEST Spectroscopy
We propose a method to dynamically monitor the progress of an enzymatic reaction using NMR of hyperpolarized ^(129)Xe in a host-guest system. It is based on a displacement assay originally designed for fluorescence experiments that exploits the competitive binding of the enzymatic product on the one hand and a reporter dye on the other hand to a supramolecular host. Recently, this assay has been successfully transferred to NMR, using xenon as a reporter, cucurbit[6]uril as supramolecular host, and Hyper-CEST as detection technique. Its advantage is that the enzyme acts on the unmodified substrate and only the product is detected through immediate inclusion into the host. We here apply a method that drastically accelerates the acquisition of Hyper-CEST spectra in vitro using magnetic field gradients. This allows monitoring the dynamic progress of the conversion of lysine to cadaverine with a temporal resolution of ~30 s. Moreover, the method only requires to sample the very early onset of the reaction (<0.5 % of substrate conversion where the host itself is required only at ÎŒM concentrations) at comparatively low reaction rates, thus saving enzyme material and reducing NMR acquisition time. The obtained value for the specific activity agrees well with previously published results from fluorescence assays. We furthermore outline how the Hyper-CEST results correlate with xenon T_2 measurements performed during the enzymatic reaction. This suggests that ultrafast Hyper-CEST spectroscopy can be used for dynamically monitoring enzymatic activity with NMR
Metal enrichment of the neutral gas of blue compact dwarf galaxies: the compelling case of Pox 36
We present the analysis of the interstellar spectrum of Pox 36 with the Far
Ultraviolet Spectroscopic Explorer (FUSE). Pox 36 was selected because of the
relatively low foreground gas content that makes it possible to detect
absorption-lines weak enough that unseen components should not be saturated.
Interstellar lines of HI, NI, OI, SiII, PII, ArI, and FeII are detected. Column
densities are derived directly from the observed line profiles except for HI,
whose lines are contaminated by stellar absorption. We used the TLUSTY models
to remove the stellar continuum and isolate the interstellar component. The
best fit indicates that the dominant stellar population is B0. The fit of the
interstellar HI line gives a column density of 10^{20.3\pm0.4} cm-2. Chemical
abundances were then computed from the column densities using the dominant
ionization stage in the neutral gas. Our abundances are compared to those
measured from emission-line spectra in the optical. Our results suggest that
the neutral gas of Pox 36 is metal-deficient by a factor ~7 as compared to the
ionized gas, and they agree with a metallicity of ~1/35 Z. Conclusions:
The abundance discontinuity between the neutral and ionized phases implies that
most of the metals released by consecutive star-formation episodes mixes with
the HI gas. The volume extent of the enrichment is so large that the
metallicity of the neutral gas increases only slightly. The star-forming
regions could be enriched only by a small fraction (~1%), but it would greatly
enhance its metallicity. Our results are compared to those of other BCDs. We
confirm the overall underabundance of metals in their neutral gas, with perhaps
only the lowest metallicity BCDs showing no discontinuity.Comment: Accepted for publication in A&
On the Escape of Ionizing Radiation from Starbursts
Far-ultraviolet spectra obtained with show that the strong
1036 interstellar absorption-line is essentially black in five of
the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM
below the Lyman-edge will be significantly larger than in the line, these
data provide strong constraints on the escape of ionizing radiation from these
starbursts. Interpreted as a a uniform absorbing slab, the implied optical
depth at the Lyman edge is huge (). Alternatively, the areal
covering factor of opaque material is typically 94%. Thus, the fraction
of ionizing stellar photons that escape the ISM of each galaxy is small: our
conservative estimates typically yield . Inclusion of
extinction due to dust will further decrease . An analogous analysis
of the rest-UV spectrum of the star-forming galaxy at =2.7
leads to similar constraints on . These new results agree with the
constraints provided by direct observations below the Lyman edge in a few other
local starbursts. However, they differ from the recently reported properties of
star-forming galaxies at 3. We assess the idea that the strong
galactic winds seen in many powerful starbursts clear channels through their
neutral ISM. We show empirically that such outflows may be a necessary - but
not sufficient - part of the process for creating a relatively porous ISM. We
note that observations will soon document the cosmic evolution in the
contribution of star-forming galaxies to the metagalactic ionizing background,
with important implications for the evolution of the IGM.Comment: 17 pages; ApJ, in pres
Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow
The kinematics of neutral gas and warm ionized gas have been mapped in
one-dimension across ultraluminous starburst galaxies using interstellar
absorption and emission lines, in Keck II ESI spectra. Blue-shifted absorption
is found along more of the slit than anticipated, exceeding scales of 15 kpc
across several systems. The large velocity gradient measured across some of
these outflows is inconsistent with a flow diverging from the central starburst
-- angular momentum conservation reduces the rotational velocity of an outflow
as it expands. More widespread star formation, likely triggered by the merger,
probably drives these outflows, although some models suggest the collision
itself could generate a wind by shock heating interstellar gas throughout the
disk. Young mergers with separated nuclei present the highest outflow masses,
due mainly to the larger area over which the cool gas can be detected. In a
typical ULIG, the mass carried by the cool phase of the outflow is around
10^8Msun, or a few percent of the total dynamical mass. Assuming the starburst
activity has proceeded at the observed rate for the past 10 Myr, the kinetic
energy of the cool outflows is a few percent of the supernova energy,
consistent with starbursts powering the outflows. The cool wind is expected to
be accelerated by momentum deposition, possibly from radiation pressure as well
as supernovae. Such models imply turn-around radii for the cool outflows of at
least 30 to 90 kpc. This cross-section presents a significant NaI absorption
cross section. If most L > 0.1L* galaxies pass through a luminous starburst
phase, then relics of cool outflows will create a significant redshift-path
density. Galaxy formation models should include this cool phase of the outflow
in addition to a hot wind in feedback models.Comment: Accepted to ApJ. Text and figures available as a single document at
http://www.physics.ucsb.edu/~cmartin/publications.htm
Interstellar abundances in the neutral and ionized gas of NGC604
We present FUSE spectra of the giant HII region NGC604 in the spiral galaxy
M33. Chemical abundances are derived from far-UV absorption lines and are
compared to those derived from optical emission lines. We derived the column
densities of HI, NI, OI, SiII, PII, ArI, and FeII, fitting the line profiles
with either a single component or several components. Our net results, assuming
a single component, show that N, O, Si, and Ar are apparently underabundant in
the neutral phase by a factor of 10 or more with respect to the ionized phase,
while Fe is the same. However, we discuss the possibility that the absorption
lines are made of individual unresolved components, and find that only PII,
ArI, and FeII lines should not be affected by the presence of hidden saturated
components, while NI, OI, and SiII might be much more affected. If N, O, and Si
are actually underabundant in the neutral gas of NGC604 with respect to the
ionized gas, this would confirm earlier results obtained for the blue compact
dwarfs. However, a deeper analysis focused on P, Ar, and Fe mitigates the above
conclusion and indicates that the neutral gas and ionized gas could have
similar abundances.Comment: Accepted for publication in A&
Continuous star formation in IZw18
We study the dynamical and chemical evolution of a galaxy similar to IZw18
under the assumption of a continuous star formation during bursts. We adopt a
2-D hydrocode coupled with detailed chemical yields originating from SNeII,
SNeIa and from single intermediate-mass stars. Different nucleosynthetic yields
and different IMF slopes are tested. In most of the explored cases, a galactic
wind develops, mostly carrying out of the galaxy the metal-enriched gas
produced by the burst itself. The chemical species with the largest escape
probabilities are Fe and N. Consequently, we predict that the [/Fe] and
[/N] ratios outside the galaxy are lower than inside. In order to
reproduce the chemical composition of IZw18, the best choice seems to be the
adoption of the yields of Meynet & Maeder (2002) which take into account
stellar rotation, although these authors do not follow the whole evolution of
all the stars. Models with a flat IMF (x=0.5) seem to be able to better
reproduce the chemical properties of IZw18, but they inject in the gas a much
larger amount of energy and the resulting galactic wind is very strong, at
variance with observations. We also predict the evolution of the abundances in
the \hi medium and compare them with recent {\sl FUSE} observations.Comment: 17 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
LINER/H II "Transition" Nuclei and the Nature of NGC 4569
Motivated by the discovery of young, massive stars in the nuclei of some
LINER/H II ``transition'' nuclei such as NGC 4569, we have computed
photoionization models to determine whether some of these objects may be
powered solely by young star clusters rather than by accretion-powered active
nuclei. The models were calculated with the photoionization code CLOUDY, using
evolving starburst continua generated by the the STARBURST99 code of Leitherer
et al. (1999). We find that the models are able to reproduce the emission-line
spectra of transition nuclei, but only for instantaneous bursts of solar or
higher metallicity, and only for ages of ~3-5 Myr, the period when the
extreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars.
For clusters younger than 3 Myr or older than 6 Myr, and for models with a
constant star-formation rate, the softer ionizing continuum results in an
emission spectrum more typical of H II regions. This model predicts that
Wolf-Rayet emission features should appear in the spectra of transition nuclei.
While such features have not generally been detected to date, they could be
revealed in observations having higher spatial resolution. Demographic
arguments suggest that this starburst model may not apply to the majority of
transition nuclei, particularly those in early-type host galaxies, but it could
account for some members of the transition class in hosts of type Sa and later.
The starburst models during the Wolf-Rayet-dominated phase can also reproduce
the narrow-line spectra of some LINERs, but only under conditions of
above-solar metallicity and only if high-density gas is present (n_e >~ 10^5
cm^{-3}). This scenario could be applicable to some ``Type 2'' LINERs which do
not show any clear signs of nonstellar activity.Comment: To appear in PASP. 22 pages, includes 9 figures, uses AASTeX v5.
Protein Nanostructures Produce Self-Adjusting Hyperpolarized Magnetic Resonance Imaging Contrast through Physical Gas Partitioning
Signal amplification strategies are critical for overcoming the intrinsically poor sensitivity of nuclear magnetic resonance (NMR) reporters in noninvasive molecular detection. A mechanism widely used for signal enhancement is chemical exchange saturation transfer (CEST) of nuclei between a dilute sensing pool and an abundant detection pool. However, the dependence of CEST amplification on the relative size of these spin pools confounds quantitative molecular detection with a larger detection pool typically making saturation transfer less efficient. Here we show that a recently discovered class of genetically encoded nanoscale reporters for ^(129)Xe magnetic resonance overcomes this fundamental limitation through an elastic binding capacity for NMR-active nuclei. This approach pairs high signal amplification from hyperpolarized spins with ideal, self-adjusting saturation transfer behavior as the overall spin ensemble changes in size. These reporters are based on gas vesicles, i.e., microbe-derived, gas-filled protein nanostructures. We show that the xenon fraction that partitions into gas vesicles follows the ideal gas law, allowing the signal transfer under hyperpolarized xenon chemical exchange saturation transfer (Hyper-CEST) imaging to scale linearly with the total xenon ensemble. This conceptually distinct elastic response allows the production of quantitative signal contrast that is robust to variability in the concentration of xenon, enabling virtually unlimited improvement in absolute contrast with increased xenon delivery, and establishing a unique principle of operation for contrast agent development in emerging biochemical and in vivo applications of hyperpolarized NMR and magnetic resonance imaging
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