1,941 research outputs found
Star Formation Rate from Dust Infrared Emission
We examine what types of galaxies the conversion formula from dust infrared
(IR) luminosity into the star formation rate (SFR) derived by
Kennicutt (1998) is applicable to. The ratio of the observed IR luminosity,
, to the intrinsic bolometric luminosity of the newly (\la 10
Myr) formed stars,
, of a galaxy can be determined by a mean dust opacity in the
interstellar medium and the activity of the current star formation. We find
that these parameters area being is very large, and many nearby normal and active star-forming
galaxies really fall in this area. It results from offsetting two effects of a
small dust opacity and a large cirrus contribution of normal galaxies relative
to starburst galaxies on the conversion of the stellar emission into the dust
IR emission. In conclusion, the SFR determined from the IR luminosity under the
assumption of like Kennicutt (1998) is reliable within
a factor of 2 for all galaxies except for dust rich but quiescent galaxies and
extremely dust poor galaxies.Comment: Accepted by ApJL: 6 pages (emulateapj5), 2 figures (one is an extra
figure not appeared in ApJL
New insights on the dense molecular gas in NGC253 as traced by HCN and HCO+
We have imaged the central ~1kpc of the circumnuclear starburst disk in the
galaxy NGC253 in the HCN(1-0), HCO+(1-0), and CO(1-0) transitions at 60pc
resolution using the Owens Valley Radio Observatory Millimeter-Wavelength Array
(OVRO). We have also obtained Atacama Pathfinder Experiment (APEX) observations
of the HCN(4-3) and the HCO+(4-3) lines of the starburst disk. We find that the
emission from the HCN(1-0) and HCO+(1-0) transitions, both indicators of dense
molecular gas, trace regions which are non-distinguishable within the
uncertainties of our observations. Even though the continuum flux varies by
more than a factor 10 across the starburst disk, the HCN/HCO+ ratio is constant
throughout the disk, and we derive an average ratio of 1.1+/-0.2. From an
excitation analysis we find that all lines from both molecules are subthermally
excited and that they are optically thick. This subthermal excitation implies
that the observed HCN/HCO+ line ratio is sensitive to the underlying chemistry.
The constant line ratio thus implies that there are no strong abundance
gradients across the starburst disk of NGC253. This finding may also explain
the variations in L'(HCN)/L'(HCO+) between different star forming galaxies both
nearby and at high redshifts.Comment: 9 pages, 12 figures, ApJ in press (volume 666 September
Efficient routing of single photons by one atom and a microtoroidal cavity
Single photons from a coherent input are efficiently redirected to a separate
output by way of a fiber-coupled microtoroidal cavity interacting with
individual Cesium atoms. By operating in an overcoupled regime for the
input-output to a tapered fiber, our system functions as a quantum router with
high efficiency for photon sorting. Single photons are reflected and excess
photons transmitted, as confirmed by observations of photon antibunching
(bunching) for the reflected (transmitted) light. Our photon router is robust
against large variations of atomic position and input power, with the observed
photon antibunching persisting for intracavity photon number 0.03 \lesssim n
\lesssim 0.7
Demonstration of a state-insensitive, compensated nanofiber trap
We report the experimental realization of an optical trap that localizes single Cs atoms ≃ 215
nm from surface of a dielectric nanober. By operating at magic wavelengths for pairs of counterpropagating
red- and blue-detuned trapping beams, dierential scalar light shifts are eliminated, and
vector shifts are suppressed by ≈ 250. We thereby measure an absorption linewidth Γ/2π = 5.7 ± 0.1
MHz for the Cs 6S_(1/2), F = 4 → 6P_(3/2), F' = 5 transition, where Γ_0/2π = 5.2 MHz in free space.
Optical depth d ≃ 66 is observed, corresponding to an optical depth per atom d_1 ≃ 0.08. These
advances provide an important capability for the implementation of functional quantum optical
networks and precision atomic spectroscopy near dielectric surfaces
Effect of Dust Extinction on Estimating Star Formation Rate of Galaxies: Lyman Continuum Extinction
We re-examine the effect of Lyman continuum ( \AA)
extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it
affects the estimation of the global star formation rate (SFR) of galaxies. To
clarify the first issue, we establish two independent methods for estimating a
parameter of LCE (), which is defined as the fraction of Lyman continuum
photons contributing to hydrogen ionization in an H {\sc ii} region. One of
those methods determines from the set of Lyman continuum flux, electron
density and metallicity. In the framework of this method, as the metallicity
and/or the Lyman photon flux increase, is found to decrease. The other
method determines from the ratio of infrared flux to Lyman continuum flux.
Importantly, we show that f \la 0.5 via both methods in many H {\sc ii}
regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}
regions absorbs significant amount of Lyman continuum photons directly. To
examine the second issue, we approximate to a function of only the
dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the
Galactic H {\sc ii} regions. We find that a characteristic , which is
defined as averaged over a galaxy-wide scale, is 0.3 for the nearby spiral
galaxies. This relatively small indicates that a typical increment
factor due to LCE for estimating the global SFR () is large () for the nearby spiral galaxies. Therefore, we conclude that the effect of
LCE is not negligible relative to other uncertainties of estimating the SFR of
galaxies.Comment: 18 papges, 11 figures, accepted by Ap
Absorption-Line Probes of Gas and Dust in Galactic Superwinds
We discuss moderate resolution spectra of the NaD absorption-line in a sample
of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced
primarily by interstellar gas, and in 12 of these it is blueshifted by over 100
km/s relative to the galaxy systemic velocity. The absorption-line profiles in
these outflow sources span the range from near the galaxy systemic velocity to
a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies
systematically viewed more nearly face-on than the others. We therefore argue
that the absorbing material consists of ambient interstellar gas accelerated
along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD
lines are optically-thick, but indirect arguments imply total Hydrogen column
densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is
expelling matter at a rate comparable to the star-formation rate. This
outflowing material is very dusty: we find a strong correlation between the
depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1
over regions several-to-ten kpc in size). The estimated terminal velocities of
superwinds inferred from these data and extant X-ray data are typically 400 to
800 km/s, are independent of the galaxy rotation speed, and are comparable to
(substantially exceed) the escape velocities for (dwarf) galaxies. The
resulting loss of metals can establish the mass-metallicity relation in
spheroids, produce the observed metallicity in the ICM, and enrich a general
IGM to 10 solar metallicity. If the outflowing dust grains survive their
journey into the IGM, their effect on observations of cosmologically-distant
objects is significant.Comment: 65 pages, including 16 figures. ApJ, in pres
Cystic fibrosis mice carrying the missense mutation G551D replicate human genotype phenotype correlations
We have generated a mouse carrying the human G551D mutation in the cystic fibrosis transmembrane conductance regulator gene (CFTR) by a one-step gene targeting procedure. These mutant mice show cystic fibrosis pathology but have a reduced risk of fatal intestinal blockage compared with 'null' mutants, in keeping with the reduced incidence of meconium ileus in G551D patients. The G551D mutant mice show greatly reduced CFTR-related chloride transport, displaying activity intermediate between that of cftr(mlUNC) replacement ('null') and cftr(mlHGU) insertional (residual activity) mutants and equivalent to approximately 4% of wild-type CFTR activity. The long-term survival of these animals should provide an excellent model with which to study cystic fibrosis, and they illustrate the value of mouse models carrying relevant mutations for examining genotype-phenotype correlations
A deep X-ray observation of M82 with XMM-Newton
We report on the analysis of a deep (100 ks) observation of the starburst
galaxy M82 with the EPIC and RGS instruments on board the X-ray telescope
XMM-Newton. The broad-band (0.5-10 keV) emission is due to at least three
spectral components: i) continuum emission from point sources; ii) thermal
plasma emission from hot gas; iii) charge exchange emission from neutral metals
(Mg and Si). The plasma emission has a double-peaked differential emission
measure, with the peaks at ~0.5 keV and ~7 keV. Spatially resolved spectroscopy
has shown that the chemical absolute abundances are not uniformly distributed
in the outflow, but are larger in the outskirts and smaller close to the galaxy
centre. The abundance ratios also show spatial variations. The X-ray derived
Oxygen abundance is lower than that measured in the atmospheres of red
supergiant stars, leading to the hypothesis that a significant fraction of
Oxygen ions have already cooled off and no longer emit at energies > ~0.5 keV.Comment: Accepted for publication in MNRAS. 20 pages, 15 figures, LaTeX2
- …