1,708 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
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
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
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
Spatially Resolved Spitzer-IRS Spectroscopy of the Central Region of M82
We present high spatial resolution (~ 35 parsec) 5-38 um spectra of the
central region of M82, taken with the Spitzer Infrared Spectrograph. From these
spectra we determined the fluxes and equivalent widths of key diagnostic
features, such as the [NeII]12.8um, [NeIII]15.5um, and H_2 S(1)17.03um lines,
and the broad mid-IR polycyclic aromatic hydrocarbon (PAH) emission features in
six representative regions and analysed the spatial distribution of these lines
and their ratios across the central region. We find a good correlation of the
dust extinction with the CO 1-0 emission. The PAH emission follows closely the
ionization structure along the galactic disk. The observed variations of the
diagnostic PAH ratios across M82 can be explained by extinction effects, within
systematic uncertainties. The 16-18um PAH complex is very prominent, and its
equivalent width is enhanced outwards from the galactic plane. We interpret
this as a consequence of the variation of the UV radiation field. The EWs of
the 11.3um PAH feature and the H_2 S(1) line correlate closely, and we conclude
that shocks in the outflow regions have no measurable influence on the H_2
emission. The [NeIII]/[NeII] ratio is on average low at ~0.18, and shows little
variations across the plane, indicating that the dominant stellar population is
evolved (5 - 6 Myr) and well distributed. There is a slight increase of the
ratio with distance from the galactic plane of M82 which we attribute to a
decrease in gas density. Our observations indicate that the star formation rate
has decreased significantly in the last 5 Myr. The quantities of dust and
molecular gas in the central area of the galaxy argue against starvation and
for negative feedback processes, observable through the strong extra-planar
outflows.Comment: 15 pages, 12 figures, 3 tables, ApJ, emulateap
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
SMA Imaging of CO(3-2) Line and 860 micron Continuum of Arp 220 : Tracing the Spatial Distribution of Luminosity
We used the Submillimeter Array (SMA) to image 860 micron continuum and
CO(3-2) line emission in the ultraluminous merging galaxy Arp 220, achieving a
resolution of 0.23" (80 pc) for the continuum and 0.33" (120 pc) for the line.
The CO emission peaks around the two merger nuclei with a velocity signature of
gas rotation around each nucleus, and is also detected in a kpc-size disk
encompassing the binary nucleus. The dust continuum, in contrast, is mostly
from the two nuclei. The beam-averaged brightness temperature of both line and
continuum emission exceeds 50 K at and around the nuclei, revealing the
presence of warm molecular gas and dust. The dust emission morphologically
agrees with the distribution of radio supernova features in the east nucleus,
as expected when a starburst heats the nucleus. In the brighter west nucleus,
however, the submillimeter dust emission is more compact than the supernova
distribution. The 860 micron core, after deconvolution, has a size of 50-80 pc,
consistent with recent 1.3 mm observations, and a peak brightness temperature
of (0.9-1.6)x10^2 K. Its bolometric luminosity is at least 2x10^{11} Lsun and
could be ~10^{12} Lsun depending on source structure and 860 micron opacity,
which we estimate to be of the order of tau_{860} ~ 1 (i.e., N_{H_2} ~ 10^{25}
cm^{-2}). The starbursting west nuclear disk must have in its center a dust
enshrouded AGN or a very young starburst equivalent to hundreds of super star
clusters. Further spatial mapping of bolometric luminosity through
submillimeter imaging is a promising way to identify the heavily obscured
heating sources in Arp 220 and other luminous infrared galaxies.Comment: ApJ. in press. 26 pages, 10 figure
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