79 research outputs found
IX. Observations on an Autograph of Shakspere, and the orthography of his Name; in a letter from Sir Frederic Madden, K.H. F.R.S. and S.A., to John Gage, Esq. F.R.S., Director
n/
Probing the Dust Properties of Galaxies up to Submillimetre Wavelengths I. The Spectral Energy Distribution of dwarf galaxies using LABOCA
We present 870 micron images of four low metallicity galaxies (NGC1705,
Haro11, Mrk1089 and UM311) observed with the Large APEX BOlometer CAmera
(LABOCA). We model their spectral energy distributions combining the submm
observations of LABOCA, 2MASS, IRAS, Spitzer photometric data and the IRS data
for Haro11. We find that a significant mass of dust is revealed when using
submm constraints compared to that measured with only mid-IR to far-IR
observations extending only to 160 microns. For NGC1705 and Haro11, an excess
in submillimeter wavelengths is detected and we rerun our SED procedure adding
a cold dust component (10K) to better describe the high 870 micron flux derived
from LABOCA observations, which significantly improves the fit. We find that at
least 70% of the dust mass of these two galaxies can reside in a cold dust
component. We also show that the subsequent dust-to-gas mass ratios,
considering HI and CO observations, can be strikingly high for Haro11 in
comparison with what is usually expected for these low-metallicity
environments. Furthermore, we derive the SFR of our galaxies and compare them
to the Schmidt law. Haro11 falls anomalously far from the Schmidt relation.
These results may suggest that a reservoir of hidden gas could be present in
molecular form not traced by the current CO observations. We also derive the
total IR luminosities derived from our models and compare them with relations
that derive this luminosity from Spitzer bands. We find that the Draine & Li
(2007) formula compares well to our direct IR determinations.Comment: 22 pages, 7 figures, 10 tables, accepted for publication in A&
ALMA Resolves 30 Doradus: Sub-parsec Molecular Cloud Structure Near the Closest Super-Star Cluster
We present ALMA observations of 30 Doradus -- the highest resolution view of
molecular gas in an extragalactic star formation region to date (~0.4pc x
0.6pc). The 30Dor-10 cloud north of R136 was mapped in 12CO 2-1, 13CO 2-1, C18O
2-1, 1.3mm continuum, the H30alpha recombination line, and two H2CO 3-2
transitions. Most 12CO emission is associated with small filaments and clumps
(<1pc, ~1000 Msun at the current resolution). Some clumps are associated with
protostars, including "pillars of creation" photoablated by intense radiation
from R136. Emission from molecular clouds is often analyzed by decomposition
into approximately beam-sized clumps. Such clumps in 30 Doradus follow similar
trends in size, linewidth, and surface density to Milky Way clumps. The 30
Doradus clumps have somewhat larger linewidths for a given size than predicted
by Larson's scaling relation, consistent with pressure confinement. They extend
to higher surface density at a given size and linewidth compared to clouds
studied at 10pc resolution. These trends are also true of clumps in Galactic
infrared-dark clouds; higher resolution observations of both environments are
required. Consistency of clump masses calculated from dust continuum, CO, and
the virial theorem reveals that the CO abundance in 30 Doradus clumps is not
significantly different from the LMC mean, but the dust abundance may be
reduced by ~2. There are no strong trends in clump properties with distance
from R136; dense clumps are not strongly affected by the external radiation
field, but there is a modest trend towards lower dense clump filling fraction
deeper in the cloud.Comment: accepted to Ap
The applicability of FIR fine-structure lines as Star Formation Rate tracers over wide ranges of metallicities and galaxy types
We analyze the applicability of far-infrared fine-structure lines [CII] 158
micron, [OI] 63 micron and [OIII] 88 micron to reliably trace the star
formation rate (SFR) in a sample of low-metallicity dwarf galaxies from the
Herschel Dwarf Galaxy Survey and compare with a broad sample of galaxies of
various types and metallicities in the literature. We study the trends and
scatter in the relation between the SFR (as traced by GALEX FUV and MIPS 24
micron) and far-infrared line emission, on spatially resolved and global galaxy
scales, in dwarf galaxies. We assemble far-infrared line measurements from the
literature and infer whether the far-infrared lines can probe the SFR (as
traced by the total-infrared luminosity) in a variety of galaxy populations. In
metal-poor dwarfs, the [OI] and [OIII] lines show the strongest correlation
with the SFR with an uncertainty on the SFR estimates better than a factor of
2, while the link between [CII] emission and the SFR is more dispersed
(uncertainty factor of 2.6). The increased scatter in the SFR-L([CII]) relation
towards low metal abundances, warm dust temperatures, large filling factors of
diffuse, highly ionized gas suggests that other cooling lines start to dominate
depending on the density and ionization state of the gas. For the literature
sample, we evaluate the correlations for a number of different galaxy
populations. The [CII] and [OI] lines are considered to be reliable SFR tracers
in starburst galaxies, recovering the star formation activity within an
uncertainty of factor 2. [Abridged]Comment: 35 pages, 13 figures, accepted for publication in A&A on May 7th 201
Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud
We present the detection and analysis of molecular hydrogen emission toward
ten interstellar regions in the Large Magellanic Cloud. We examined
low-resolution infrared spectral maps of twelve regions obtained with the
Spitzer infrared spectrograph (IRS). The pure rotational 0--0 transitions of
H at 28.2 and 17.1 are detected in the IRS spectra for ten
regions. The higher level transitions are mostly upper limit measurements
except for three regions, where a 3 detection threshold is achieved for
lines at 12.2 and 8.6. The excitation diagrams of the detected
H transitions are used to determine the warm H gas column density and
temperature. The single-temperature fits through the lower transition lines
give temperatures in the range . The bulk of the excited H
gas is found at these temperatures and contributes 5-17% to the total gas
mass. We find a tight correlation of the H surface brightness with
polycyclic aromatic hydrocarbon and total infrared emission, which is a clear
indication of photo-electric heating in photodissociation regions. We find the
excitation of H by this process is equally efficient in both atomic and
molecular dominated regions. We also present the correlation of the warm H
physical conditions with dust properties. The warm H mass fraction and
excitation temperature show positive correlations with the average starlight
intensity, again supporting H excitation in photodissociation regions.Comment: Accepted for publication in MNRA
The Spatial Distribution of Dust and Stellar Emission of the Magellanic Clouds
We study the emission by dust and stars in the Large and Small Magellanic
Clouds, a pair of low-metallicity nearby galaxies, as traced by their spatially
resolved spectral energy distributions (SEDs). This project combines Herschel
Space Observatory PACS and SPIRE far-infrared photometry with other data at
infrared and optical wavelengths. We build maps of dust and stellar luminosity
and mass of both Magellanic Clouds, and analyze the spatial distribution of
dust/stellar luminosity and mass ratios. These ratios vary considerably
throughout the galaxies, generally between the range and .
We observe that the dust/stellar ratios depend on the interstellar medium (ISM)
environment, such as the distance from currently or previously star-forming
regions, and on the intensity of the interstellar radiation field (ISRF). In
addition, we construct star formation rate (SFR) maps, and find that the SFR is
correlated with the dust/stellar luminosity and dust temperature in both
galaxies, demonstrating the relation between star formation, dust emission and
heating, though these correlations exhibit substantial scatter.Comment: 15 pages, 18 figures; ApJ, in press; version published in the journal
will have higher-resolution figure
Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud. I. N159W
We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations
of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In
our observations, a number of far-infrared cooling lines including CO(4-3) to
CO(12-11), [CI] 609 and 370 micron, and [NII] 205 micron are clearly detected.
With an aim of investigating the physical conditions and excitation processes
of molecular gas, we first construct CO spectral line energy distributions
(SLEDs) on 10 pc scales by combining the FTS CO transitions with ground-based
low-J CO data and analyze the observed CO SLEDs using non-LTE radiative
transfer models. We find that the CO-traced molecular gas in N159W is warm
(kinetic temperature of 153-754 K) and moderately dense (H2 number density of
(1.1-4.5)e3 cm-3). To assess the impact of the energetic processes in the
interstellar medium on the physical conditions of the CO-emitting gas, we then
compare the observed CO line intensities with the models of photodissociation
regions (PDRs) and shocks. We first constrain the properties of PDRs by
modelling Herschel observations of [OI] 145, [CII] 158, and [CI] 370 micron
fine-structure lines and find that the constrained PDR components emit very
weak CO emission. X-rays and cosmic-rays are also found to provide a negligible
contribution to the CO emission, essentially ruling out ionizing sources
(ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source
for CO in N159W. On the other hand, mechanical heating by low-velocity C-type
shocks with ~10 km/s appears sufficient enough to reproduce the observed warm
CO.Comment: accepted for publication in A&
Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-Dust Ratio Variations across ISM Phases
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on
the chemical evolution and lifecycle of dust in galaxies. We examine the
relation between dust and gas at 10-50 pc resolution in the Large and Small
Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21
cm, CO, and Halpha observations. In the diffuse atomic ISM, we derive the
gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust
ratios of 380+250-130 in the LMC, and 1200+1600-420 in the SMC, not including
helium. The atomic-to-molecular transition is located at dust surface densities
of 0.05 Mo pc-2 in the LMC and 0.03 Mo pc-2 in the SMC, corresponding to AV ~
0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion
factor to best account for all the molecular gas in the beam of the
observations, and find upper limits on XCO to be 6x1020 cm-2 K-1 km-1 s in the
LMC (Z=0.5Zo) at 15 pc resolution, and 4x 1021 cm-2 K-1 km-1 s in the SMC
(Z=0.2Zo) at 45 pc resolution. In the LMC, the slope of the dust-gas relation
in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after
accounting for the effects of CO-dark H2 in the translucent envelopes of
molecular clouds. Coagulation of dust grains and the subsequent dust emissivity
increase in molecular clouds, and/or accretion of gas-phase metals onto dust
grains, and the subsequent dust abundance (dust-to-gas ratio) increase in
molecular clouds could explain the observations. In the SMC, variations in the
dust-gas slope caused by coagulation or accretion are degenerate with the
effects of CO-dark H2. Within the expected 5--20 times Galactic XCO range, the
dust-gas slope can be either constant or decrease by a factor of several across
ISM phases. Further modeling and observations are required to break the
degeneracy between dust grain coagulation, accretion, and CO-dark H2
The elusive ISM of dwarf galaxies: excess submillimetre emission & CO-dark molecular gas
The Herschel Dwarf Galaxy Survey investigates the interplay of star formation activity and the the metal-poor gas and dust of dwarf galaxies using FIR and submillimetre imaging spectroscopic and photometric observations in the 50 to 550mu window of the Herschel Space Observatory. The dust SEDs are well constrained with the new Herschel and MIR Spitzer data. A submillimetre excess is often found in low metallicity galaxies, which,if tracing very cold dust, would highlight large dust masses not easily reconciled in some cases, given the low metallicities and expected gas-to-dust mass ratios. The galaxies are also mapped in the FIR fine-structure lines (63 and 145mu OI, 158mu CII, 122 and 205mu NII, 88mu OIII) probing the low density ionised gas, the HII regions and photodissociation regions. While still early in the Herschel mission we can already see, along with earlier studies, that line ratios in the metal-poor ISM differ remarkably from those in the metal-rich starburst environments. In dwarf galaxies, L[CII]/L(CO) (>10^4) is at least an order of magnitude greater than in the most metal-rich starburst galaxies. The enhanced [CII] arises from the larger photodissociation region where H2, not traced by the CO, can exist. The 88mu [OIII] line usually dominates the FIR line emission over galaxy-wide scales in dwarf galaxies, not the 158mu [CII] line which is the dominant FIR cooling line in metal-rich galaxies. All of the FIR lines together can contribute 1% to 2% of the L(TIR). The Herschel Dwarf Galaxy survey will provide statistical information on the nature of the dust and gas in low metallicity galaxies, elucidating the origin of the submm excess in dwarf galaxies, and help determine a ([CII] +CO) to H2 conversion factor, thus providing observational constraints on chemical evolution models of galaxies
The resolved scaling relations in DustPedia: Zooming in on the local Universe
We perform a homogeneous analysis of an unprecedented set of spatially
resolved scaling relations (SRs) between ISM components and other properties in
the range of scales 0.3-3.4 kpc. We also study some ratios: dust-to-stellar,
dust-to-gas, and dust-to-metal. We use a sample of 18 large, spiral, face-on
DustPedia galaxies. All the SRs are moderate/strong correlations except the
dust-HI SR that does not exist or is weak for most galaxies. The SRs do not
have a universal form but each galaxy is characterized by distinct
correlations, affected by local processes and galaxy peculiarities. The SRs
hold starting from 0.3 kpc, and if a breaking down scale exists it is < 0.3
kpc. By evaluating all galaxies at 3.4 kpc, differences due to peculiarities of
individual galaxies are cancelled out and the corresponding SRs are consistent
with those of whole galaxies. By comparing subgalactic and global scales, the
most striking result emerges from the SRs involving ISM components: the
dust-total gas SR is a good correlation at all scales, while the dust-H2 and
dust-HI SRs are good correlations at subkpc/kpc and total scales, respectively.
For the other explored SRs, there is a good agreement between small and global
scales and this may support the picture where the main physical processes
regulating the properties and evolution of galaxies occur locally. Our results
are consistent with the hypothesis of self-regulation of the SF process. The
analysis of subgalactic ratios shows that they are consistent with those
derived for whole galaxies, from low to high z, supporting the idea that also
these ratios could be set by local processes. Our results highlight the
heterogeneity of galaxy properties and the importance of resolved studies on
local galaxies in the context of galaxy evolution. They also provide
observational constraints to theoretical models and updated references for
high-z studies.Comment: 42 pages, 11 figures and 5 tables in the main text, 2 figures and 1
table in Appendix. Accepted for publication in A&
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