61 research outputs found
Un exercice de transposition incongru : le mythe de Salomé revu par le décadent Jules Laforgue
International audienc
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
The role of thermal and non-thermal processes in the ISM of the Magellanic Clouds
Radio continuum emission is a dust-unbiased tracer of both thermal and non-thermal processes in the interstellar medium. We present new maps of the free–free and synchrotron emission in the Magellanic Clouds (MCs) at 0.166, 1.4, and 4.8 GHz with no prior assumption about the radio non-thermal spectrum. The maps were constructed using a de-reddened H α map as a template for the thermal radio emission, which we subtract from maps of the total radio continuum emission. To de-redden the H α emission, it is important to know the fraction of dust surface density that attenuates the H α emission along the line of sight, fd. This fraction is obtained by comparing the dust opacity obtained through the infrared emission spectrum and the Balmer decrement method. In star-forming regions, the median fd is about 0.1, which is lower than that in diffuse regions by a factor of three. We obtain a global thermal radio fraction, fth, of 30 per cent (35 per cent) in the LMC (SMC) at 1.4 GHz. Furthermore, we present maps of the equipartition magnetic field strength with average values of ≃10.1μG in the LMC and ≃5.5μG in the SMC. The magnetic field is proportional to the star-formation rate to the power of 0.24 and 0.20 for the LMC and SMC, respectively. This study shows that non-thermal processes control the interstellar medium in the MCs
Photoemission Beyond the Sudden Approximation
The many-body theory of photoemission in solids is reviewed with emphasis on
methods based on response theory. The classification of diagrams into loss and
no-loss diagrams is discussed and related to Keldysh path-ordering
book-keeping. Some new results on energy losses in valence-electron
photoemission from free-electron-like metal surfaces are presented. A way to
group diagrams is presented in which spectral intensities acquire a
Golden-Rule-like form which guarantees positiveness. This way of regrouping
should be useful also in other problems involving spectral intensities, such as
the problem of improving the one-electron spectral function away from the
quasiparticle peak.Comment: 18 pages, 11 figure
Far-infrared Polarization of the Supernova Remnant Cassiopeia A with SOFIA HAWC+
We present polarization observations of the young supernova remnant (SNR) Cas
A using the High-resolution Airborne Wideband Camera-Plus (HAWC+) instrument
onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The
polarization map at 154 microns reveals dust grains with strong polarization
fractions (5 - 30 percent), supporting previous measurements made over a
smaller region of the remnant at 850 microns. The 154 microns emission and the
polarization signal is coincident with a region of cold dust observed in the
southeastern shell and in the unshocked central ejecta. The highly polarized
far-IR emission implies the grains are large (greater than 0.14 microns) and
silicate-dominated. The polarization level varies across the SNR, with an
inverse correlation between the polarization degree and the intensity and
smaller polarization angle dispersion for brighter SNR emission. Stronger
polarization is detected between the bright structures. This may result from a
higher collision rate between the gas and dust producing a lower grain
alignment efficiency where the gas density is higher. We use the dust emission
to provide an estimate of the magnetic field strength in Cas A using the
Davis-Chandrasekhar-Fermi method. The high polarization level is direct
evidence that grains are highly elongated and strongly aligned with the
magnetic field of the SNR. The dust mass from the polarized region is
0.14+-0.04 Msun, a lower limit of the amount of dust present within the ejecta
of Cas A. This result strengthens the hypothesis that core-collapse SNe are an
important contributor to the dust mass in high redshift galaxies.Comment: MNRAS, accepted (18 pages with 14 figures
Wide-field CO isotopologue emission and the CO-to-H factor across the nearby spiral galaxy M101
Carbon monoxide (CO) emission is the most widely used tracer of the bulk
molecular gas in the interstellar medium (ISM) in extragalactic studies. The
CO-to-H conversion factor, , links the observed CO
emission to the total molecular gas mass. However, no single prescription
perfectly describes the variation of across all environments
across galaxies as a function of metallicity, molecular gas opacity, line
excitation, and other factors. Using resolved spectral line observations of CO
and its isotopologues, we can constrain the molecular gas conditions and link
them to a variation in the conversion factor. We present new IRAM 30-m 1mm and
3mm line observations of CO, CO, and CO} across the nearby
galaxy M101. Based on the CO isotopologue line ratios, we find that selective
nucleosynthesis and opacity changes are the main drivers of the variation in
the line emission across the galaxy. Furthermore, we estimated using different approaches, including (i) the dust mass surface
density derived from far-IR emission as an independent tracer of the total gas
surface density and (ii) LTE-based measurements using the optically thin
CO(1-0) intensity. We find an average value of across the galaxy,
with a decrease by a factor of 10 toward the 2 kpc central region. In contrast,
we find LTE-based values are lower by a factor of 2-3 across the disk relative
to the dust-based result. Accounting for variations, we found
significantly reduced molecular gas depletion time by a factor 10 in the
galaxy's center. In conclusion, our result suggests implications for commonly
derived scaling relations, such as an underestimation of the slope of the
Kennicutt Schmidt law, if variations are not accounted for.Comment: Accepted for publication in A&A, 25 pages, 15 figure
A z=0 Multi-wavelength Galaxy Synthesis I: A WISE and GALEX Atlas of Local Galaxies
We present an atlas of ultraviolet and infrared images of ~15,750 local (d <
50 Mpc) galaxies, as observed by NASA's WISE and GALEX missions. These maps
have matched resolution (FWHM 7.5'' and 15''), matched astrometry, and a common
procedure for background removal. We demonstrate that they agree well with
resolved intensity measurements and integrated photometry from previous
surveys. This atlas represents the first part of a program (the z=0
Multi-wavelength Galaxy Synthesis) to create a large, uniform database of
resolved measurements of gas and dust in nearby galaxies. The images and
associated catalogs are publicly available at the NASA/IPAC Infrared Science
Archive. This atlas allows us estimate local and integrated star formation
rates (SFRs) and stellar masses (M) across the local galaxy population
in a uniform way. In the appendix, we use the population synthesis fits of
Salim et al. (2016, 2018) to calibrate integrated M and SFR estimators
based on GALEX and WISE. Because they leverage an SDSS-base training set of
>100,000 galaxies, these calibrations have high precision and allow us to
rigorously compare local galaxies to Sloan Digital Sky Survey results. We
provide these SFR and M estimates for all galaxies in our sample and
show that our results yield a "main sequence" of star forming galaxies
comparable to previous work. We also show the distribution of intensities from
resolved galaxies in NUV-to-WISE1 vs. WISE1-to-WISE3 space, which captures much
of the key physics accessed by these bands.Comment: 46 pages, 27 figures, published in ApJS
(https://ui.adsabs.harvard.edu/abs/2019ApJS..244...24L/abstract ). See that
version for full resolution figures and machine readable tables. Go download
data for your favorite nearby galaxy here:
https://irsa.ipac.caltech.edu/data/WISE/z0MGS/overview.html . The appendix
presents detailed analysis of translations to physical quantitie
CI and CO in Nearby Spiral Galaxies -- I. Line Ratio and Abundance Variations at ~ 200 pc Scales
We present new neutral atomic carbon [CI](3P1-3P0) mapping observations
within the inner ~7 kpc and ~4 kpc of the disks of NGC3627 and NGC4321 at a
spatial resolution of 190 pc and 270 pc, respectively, using the ALMA Atacama
Compact Array (ACA). We combine these with the CO(2-1) data from PHANGS-ALMA,
and literature [CI] and CO data for two other starburst and/or active galactic
nucleus (AGN) galaxies (NGC1808, NGC7469), to study: a) the spatial
distributions of CI and CO emission; b) the observed line ratio RCICO =
I_[CI](1-0)/I_CO(2-1) as a function of various galactic properties; and c) the
abundance ratio of [CI/CO]. We find excellent spatial correspondence between CI
and CO emission and nearly uniform RCICO ~0.1 across the majority of the
star-forming disks of NGC3627 and NGC4321. However, RCICO strongly varies from
~0.05 at the centre of NGC4321 to >0.2-0.5 in NGC1808's starburst centre and
NGC7469's centre with an X-ray AGN. Meanwhile, RCICO does not obviously vary
with , similar to the prediction of PDR models. We also find a mildly
decreasing RCICO with an increasing metallicity over 0.7-0.85 solar
metallicity, consistent with the literature. Assuming various typical ISM
conditions representing GMCs, active star-forming regions and strong
starbursting environments, we calculate the LTE radiative transfer and estimate
the [CI/CO] abundance ratio to be ~0.1 across the disks of NGC3627 and NGC4321,
similar to previous large-scale findings in Galactic studies. However, this
abundance ratio likely has a substantial increase to ~1 and >1-5 in NGC1808's
starburst and NGC7469's strong AGN environments, respectively, in line with the
expectations for cosmic-ray dominated region (CRDR) and X-ray dominated region
(XDR) chemistry. Finally, we do not find a robust evidence for a generally
CO-dark, CI-bright gas in the disk areas we probed. (abbreviated)Comment: 23 pages, 13 figures and one table in total (17 pages and 9 figures
in main text). Accepted for publication in A&A. For associated data cubes and
moment maps, see
https://www.canfar.net/storage/vault/list/phangs/RELEASES/DZLIU_etal_202
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