532 research outputs found
Dust Abundance Variations in the Magellanic Clouds: Probing the Lifecycle of Metals with All-Sky Surveys
Observations and modeling suggest that the dust abundance (gas-to-dust ratio,
G/D) depends on (surface) density. The variations of the G/D provide
constraints on the timescales for the different processes involved in the
lifecycle of metals in galaxies. Recent G/D measurements based on Herschel data
suggest a factor 5---10 decrease in the dust abundance between the dense and
diffuse interstellar medium (ISM) in the Magellanic Clouds. However, the
relative nature of the Herschel measurements precludes definitive conclusions
on the magnitude of those variations. We investigate the variations of the dust
abundance in the LMC and SMC using all-sky far-infrared surveys, which do not
suffer from the limitations of Herschel on their zero-point calibration. We
stack the dust spectral energy distribution (SED) at 100, 350, 550, and 850
microns from IRAS and Planck in intervals of gas surface density, model the
stacked SEDs to derive the dust surface density, and constrain the relation
between G/D and gas surface density in the range 10---100 \Msu pc on
80 pc scales. We find that G/D decreases by factors of 3 (from 1500 to
500) in the LMC and 7 (from 1.5 to 2000) in the SMC between the
diffuse and dense ISM. The surface density dependence of G/D is consistent with
elemental depletions and with simple modeling of the accretion of gas-phase
metals onto dust grains. This result has important implications for the
sub-grid modeling of galaxy evolution, and for the calibration of dust-based
gas mass estimates, both locally and at high-redshift.Comment: 20 pages, 14 figure
The Turbulence Spectrum of Molecular Clouds in the Galactic Ring Survey: A Density-Dependent PCA Calibration
Turbulence plays a major role in the formation and evolution of molecular
clouds. The problem is that turbulent velocities are convolved with the density
of an observed region. To correct for this convolution, we investigate the
relation between the turbulence spectrum of model clouds, and the statistics of
their synthetic observations obtained from Principal Component Analysis (PCA).
We apply PCA to spectral maps generated from simulated density and velocity
fields, obtained from hydrodynamic simulations of supersonic turbulence, and
from fractional Brownian motion fields with varying velocity, density spectra,
and density dispersion. We examine the dependence of the slope of the PCA
structure function, alpha_PCA, on intermittency, on the turbulence velocity
(beta_v) and density (beta_n) spectral indexes, and on density dispersion. We
find that PCA is insensitive to beta_n and to the log-density dispersion
sigma_s, provided sigma_s 2, alpha_PCA increases with sigma_s
due to the intermittent sampling of the velocity field by the density field.
The PCA calibration also depends on intermittency. We derive a PCA calibration
based on fBms with sigma_s<2 and apply it to 367 CO spectral maps of molecular
clouds in the Galactic Ring Survey. The average slope of the PCA structure
function, =0.62\pm0.2, is consistent with the hydrodynamic
simulations and leads to a turbulence velocity exponent =2.06\pm0.6 for
a non-intermittent, low density dispersion flow. Accounting for intermittency
and density dispersion, the coincidence between the PCA slope of the GRS clouds
and the hydrodynamic simulations suggests beta_v~1.9, consistent with both
Burgers and compressible intermittent turbulence
The location, clustering, and propagation of massive star formation in giant molecular clouds
Massive stars are key players in the evolution of galaxies, yet their
formation pathway remains unclear. In this work, we use data from several
galaxy-wide surveys to build an unbiased dataset of ~700 massive young stellar
objects (MYSOs), ~200 giant molecular clouds (GMCs), and ~100 young (<10 Myr)
optical stellar clusters (SCs) in the Large Magellanic Cloud. We employ this
data to quantitatively study the location and clustering of massive star
formation and its relation to the internal structure of GMCs. We reveal that
massive stars do not typically form at the highest column densities nor centers
of their parent GMCs at the ~6 pc resolution of our observations. Massive star
formation clusters over multiple generations and on size scales much smaller
than the size of the parent GMC. We find that massive star formation is
significantly boosted in clouds near SCs. Yet, whether a cloud is associated
with a SC does not depend on either the cloud's mass or global surface density.
These results reveal a connection between different generations of massive
stars on timescales up to 10 Myr. We compare our work with Galactic studies and
discuss our findings in terms of GMC collapse, triggered star formation, and a
potential dichotomy between low- and high-mass star formation.Comment: 13 pages, 7 figures, in pres
Dust Destruction Rates and Lifetimes in the Magellanic Clouds
The nature, composition, abundance, and size distribution of dust in galaxies
is determined by the rate at which it is created in the different stellar
sources and destroyed by interstellar shocks. Because of their extensive
wavelength coverage, proximity, and nearly face-on geometry, the Magellanic
Clouds (MCs) provide a unique opportunity to study these processes in great
detail. In this paper we use the complete sample of supernova remnants (SNRs)
in the MCs to calculate the lifetime and destruction efficiencies of silicate
and carbon dust in these galaxies. We find dust lifetimes of 22 +- 13 Myr (30
+- 17 Myr) for silicate (carbon) grains in the LMC, and 54 +- 32 Myr (72 +- 43
Myr) for silicate (carbon) grains in the SMC. The significantly shorter
lifetimes in the MCs, as compared to the Milky Way, are explained as the
combined effect of their lower total dust mass, and the fact that the
dust-destroying isolated SNe in the MCs seem to be preferentially occurring in
regions with higher than average dust-to-gas (D2G) mass ratios. We also
calculate the supernova rate and the current star formation rate in the MCs,
and use them to derive maximum dust injection rates by asymptotic giant branch
stars and core collapse supernovae. We find that the injection rates are an
order of magnitude lower than the dust destruction rates by the SNRs. This
supports the conclusion that, unless the dust destruction rates have been
considerably overestimated, most of the dust must be reconstituted from
surviving grains in dense molecular clouds. More generally, we also discuss the
dependence of the dust destruction rate on the local D2G mass ratio, the
ambient gas density and metallicity, as well as the application of our results
to other galaxies and dust evolution models.Comment: 15 pages, 8 figures, 5 tables, accepted to Ap
Principal Component Analysis of Molecular Clouds: Can CO reveal the dynamics?
We use Principal Component Analysis (PCA) to study the gas dynamics in
numerical simulations of typical MCs. Our simulations account for the
non-isothermal nature of the gas and include a simplified treatment of the
time-dependent gas chemistry. We model the CO line emission in a
post-processing step using a 3D radiative transfer code. We consider mean
number densities n_0 = 30, 100, 300 cm^{-3} that span the range of values
typical for MCs in the solar neighbourhood and investigate the slope
\alpha_{PCA} of the pseudo structure function computed by PCA for several
components: the total density, H2 density, 12CO density, 12CO J = 1 -> 0
intensity and 13CO J = 1 -> 0 intensity. We estimate power-law indices
\alpha_{PCA} for different chemical species that range from 0.5 to 0.9, in good
agreement with observations, and demonstrate that optical depth effects can
influence the PCA. We show that when the PCA succeeds, the combination of
chemical inhomogeneity and radiative transfer effects can influence the
observed PCA slopes by as much as ~ +/- 0.1. The method can fail if the CO
distribution is very intermittent, e.g. in low-density clouds where CO is
confined to small fragments.Comment: 12 pages, 8 figures, accepted for publication in MNRA
The Effect of Spiral Arms on Star Formation in the Galaxy
We have examined the ratio between the integrated luminosity of massive young
stellar objects detected by the Red MSX Source (RMS) survey and the mass of
molecular clouds in the Galactic Ring Survey region, as a function of
Galactocentric radius. The results indicate that 60--80% of the observed
increases in the star-formation rate density associated with spiral-arm
features are due to source crowding within the arms. Of the remainder, most of
the increase in the inner Sagittarius arm is due to an enhancement in the
simple star-formation efficiency, i.e. in the number of RMS sources per unit
molecular gas mass. In the inner Perseus arm, the residual increase is due to a
higher than average mean source luminosity, which implies a top-heavy IMF, and
this is entirely due to the presence, in the GRS region, of the W49
star-forming complex, which appears to be exceptional in its nature. The
results also suggest that there is little or no increase in the star-formation
efficiency on kiloparsec scales in the Scutum tangent region which includes
W43. We discuss the possible role played by the spiral arms in influencing the
star-formation efficiency and conclude that the most likely mechanisms are
related to orbit crowding within the arms.Comment: 7 pages, 7 figures, accepted by MNRA
Distribution and mass of diffuse and dense CO gas in the Milky Way
This is the final version of the article. Available from American Astronomical Society and IOP Publishing via the DOI in this record.Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star-forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in 13CO and 12CO, and a low column density component detected in 12CO, but not in 13CO. The “diffuse” and “dense” components are identified using a combination of smoothing, masking, and erosion/dilation procedures, making use of three large-scale 12CO and 13CO surveys of the inner and outer Milky Way. The diffuse component, which globally represents 25% (1.5 × 108M⊙) of the total molecular gas mass (6.5 × {10}8 M⊙), is more extended perpendicular to the Galactic plane. The fraction of diffuse gas increases from ∼10%–20% at a galactocentric radius of 3–4 kpc to 50% at 15 kpc, and increases with decreasing surface density. In the inner Galaxy, a yet denser component traced by CS emission represents 14% of the total molecular gas mass traced by 12CO emission. Only 14% of the molecular gas mass traced by 12CO emission is identified as part of molecular clouds in 13CO surveys by cloud identification algorithms. This study indicates that CO emission not only traces star-forming clouds, but also a significant diffuse molecular ISM component.R.S. and R.S.K. acknowledge support from the Deutsche
Forschungsgemeinschaft (DFG) for funding through the SPP
1573 “The Physics of the Interstellar Medium” as well as via
SFB 881 “The Milky Way System” (sub-projects B12, and
B8). R.S.K. also receives funding from the European Research
Council under the European Communitys Seventh Framework
Program (FP7/2007-2013) via the ERC Advanced Grant
“STARLIGHT” (project number 339177)
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