124 research outputs found
Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: The KINGFISH Sample
Interstellar dust and starlight are modeled for the galaxies of the project "Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel." The galaxies were observed by the Infrared Array Camera and the Multiband Imaging Photometer for Spitzer on Spitzer Space Telescope, and the Photodetector Array Camera and Spectrometer and the Spectral and Photometric Imaging Receiver on Herschel Space Observatory. With data from 3.6 to 500 ÎŒm, dust models are strongly constrained. Using a physical dust model, for each pixel in each galaxy we estimate (1) dust surface density, (2) dust mass fraction in polycyclic aromatic hydrocarbons (PAHs), (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in subregions with high starlight intensity. The dust models successfully reproduce the observed global and resolved spectral energy distributions. With the angular resolution of Herschel, we obtain well-resolved maps (available online) for the dust properties. As in previous studies, we find the PAH fraction q_(PAH) to be an increasing function of metallicity, with a threshold oxygen abundance Z/Zâ â 0.1, but we find the data to be fitted best with q_(PAH) increasing linearly with log(O/H) above a threshold value of 0.15(O/H)â. We obtain total dust masses for each galaxy by summing the dust mass over the individual map pixels; these "resolved" dust masses are consistent with the masses inferred from a model fit to the global photometry. The global dust-to-gas ratios obtained from this study are found to correlate with galaxy metallicities. Systems with Z/Zâ âł 0.5 have most of their refractory elements locked up in dust, whereas in systems with Z/Zâ ⟠0.3 most of these elements tend to remain in the gas phase. Within galaxies, we find that q_(PAH) is suppressed in regions with unusually warm dust with vL_v(70 ÎŒm) âł 0.4L_(dust). With knowledge of one long-wavelength flux density ratio (e.g., fâââ/fâ
ââ), the minimum starlight intensity heating the dust (U_(min)) can be estimated to within ~50%, despite a variation in U_(min) of more than two orders of magnitude. For the adopted dust model, dust masses can be estimated to within ~0.2 dex accuracy using the fâââ/fâ
ââ flux ratio and the integrated dust luminosity, and to ~0.07 dex accuracy using the 500 ÎŒm luminosity vL_v(500 ”m) alone. There are additional systematic errors arising from the choice of dust model, but these are hard to estimate. These calibrated prescriptions for estimating starlight heating intensity and dust mass may be useful for studies of high-redshift galaxies
Common-Resolution Convolution Kernels for Space- and Ground-Based Telescopes
Multi-wavelength study of extended astronomical objects requires combining
images from instruments with differing point spread functions (PSFs). We
describe the construction of convolution kernels that allow one to generate
(multi-wavelength) images with a common PSF, thus preserving the colors of the
astronomical sources. We generate convolution kernels for the cameras of the
Spitzer Space Telescope, Herschel Space Observatory, Galaxy Evolution Explorer
(GALEX), Wide-field Infrared Survey Explorer (WISE), ground-based optical
telescopes (Moffat functions and sum of Gaussians), and Gaussian PSFs. These
kernels allow the study of the Spectral Energy Distribution (SED) of extended
objects, preserving the characteristic SED in each pixel. The convolution
kernels and the IDL packages used to construct and use them are made publicly
available
Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium
We compare the performance of several dust models in reproducing the dust
spectral energy distribution (SED) per unit extinction in the diffuse
interstellar medium (ISM). We use our results to constrain the variability of
the optical properties of big grains in the diffuse ISM, as published by the
Planck collaboration.
We use two different techniques to compare the predictions of dust models to
data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared
emission spectrum to recover the dust extinction and the intensity of the
interstellar radiation field (ISRF). Second, we infer the ISRF intensity from
the total power emitted by dust per unit extinction, and then predict the
emission spectrum. In both cases, we test the ability of the models to
reproduce dust emission and extinction at the same time.
We identify two issues. Not all models can reproduce the average dust
emission per unit extinction: there are differences of up to a factor
between models, and the best accord between model and observation is obtained
with the more emissive grains derived from recent laboratory data on silicates
and amorphous carbons. All models fail to reproduce the variations in the
emission per unit extinction if the only variable parameter is the ISRF
intensity: this confirms that the optical properties of dust are indeed
variable in the diffuse ISM.
Diffuse ISM observations are consistent with a scenario where both ISRF
intensity and dust optical properties vary. The ratio of the far-infrared
opacity to the band extinction cross-section presents variations of the
order of ( in extreme cases), while ISRF intensity varies
by ( in extreme cases). This must be accounted for in
future modelling.Comment: A&A, in pres
Dissecting the origin of the submillimeter emission in nearby galaxies with Herschel and LABOCA
We model the infrared to submillimeter spectral energy distribution of 11
nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and
compare model extrapolations at 870um (using different fitting techniques) with
LABOCA 870um observations. We investigate how the differences between
predictions and observations vary with model assumptions or environment. At
global scales, we find that modified blackbody models using realistic cold
emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed
emission within the uncertainties for most of the sample. Low values
(beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local
scales, we observe a systematic 870um excess when using beta_=2.0. The
beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with
observations in part of the disks. Some of the remaining excesses occur towards
the centres and can be partly or fully accounted for by non-dust contributions
such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In
three non-barred galaxies, the remaining excesses rather occur in the disk
outskirts. This could be a sign of a flattening of the submm slope (and
decrease of the effective emissivity index) with radius in these objects.Comment: 31 pages (including appendix), 7 figures, accepted for publication in
MNRA
The Star Formation in Radio Survey: GBT 33 GHz Observations of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions
We present 33 GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope as part of the Star Formation in Radio Survey. Among the sources without evidence for an active galactic nucleus, and also having lower frequency radio data, we find a median thermal fraction at 33 GHz of â76% with a dispersion of â24%. For all sources resolved on scales âŸ0.5 kpc, the thermal fraction is even larger, being âł90%. This suggests that the rest-frame 33 GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate (SFR) indicator. Taking the 33 GHz SFRs as a reference, we investigate other empirical calibrations relying on different combinations of warm 24 ÎŒm dust, total infrared (IR; 8-1000 ÎŒm), Hα line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33 GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33 GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of Ï_α NT âŸ0.13
A Detailed Study of the Radio--FIR Correlation in NGC6946 with Herschel-PACS/SPIRE from KINGFISH
We derive the distribution of the synchrotron spectral index across NGC6946
and investigate the correlation between the radio continuum (synchrotron) and
far-infrared (FIR) emission using the KINGFISH Herschel PACS and SPIRE data.
The radio--FIR correlation is studied as a function of star formation rate,
magnetic field strength, radiation field strength, and the total gas surface
brightness. The synchrotron emission follows both star-forming regions and the
so-called magnetic arms present in the inter-arm regions. The synchrotron
spectral index is steepest along the magnetic arms (), while
it is flat in places of giant H{\sc ii} regions and in the center of the galaxy
(). The map of provides an observational
evidence for aging and energy loss of cosmic ray electrons propagating in the
disk of the galaxy. Variations in the synchrotron--FIR correlation across the
galaxy are shown to be a function of both star formation and magnetic fields.
We find that the synchrotron emission correlates better with cold rather than
with warm dust emission, when the interstellar radiation field is the main
heating source of dust. The synchrotron--FIR correlation suggests a coupling
between the magnetic field and the gas density. NGC6946 shows a power-law
behavior between the total (turbulent) magnetic field strength B and the star
formation rate surface density with an index of
0.14\,(0.16)0.01. This indicates an efficient production of the turbulent
magnetic field with the increasing gas turbulence expected in actively star
forming regions. The scale-by-scale analysis of the synchrotron--FIR
correlation indicates that the ISM affects the propagation of old/diffused
cosmic ray electrons, resulting in a diffusion coefficient of \,cm\,s for 2.2\,GeV CREs.Comment: 23 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics Journa
Mapping the cold dust temperatures and masses of nearby Kingfish galaxies with Herschel
Taking advantage of the sensitivity and angular resolution of the Herschel
Space Observatory at far-infrared and submm wavelengths, we aim to characterize
the physical properties of cold dust within nearby galaxies and study the
robustness of the parameters we derive using different modified blackbody
models. For a pilot subsample of the KINGFISH program, we perform 2 temperature
fits of the Spitzer and Herschel photometric data (24 to 500um), with a warm
and a cold component, globally and in each resolution element.At global scales,
we observe ranges of values for beta_c(0.8 to 2.5) and Tc(19.1 to 25.1K).We
compute maps of our parameters with beta fixed or free to test the robustness
of the temperature and dust surface density maps we deduce. When the emissivity
is fixed, we observe temperature gradients as a function of radius.When the
emissivity is fitted as a free parameter, barred galaxies tend to have uniform
fitted emissivities.Gathering resolved elements in a Tc-beta_c diagram
underlines an anti-correlation between the two parameters.It remains difficult
to assess whether the dominant effect is the physics of dust grains, noise, or
mixing along the line of sight and in the beam. We finally observe in both
cases that the dust column density peaks in central regions of galaxies and bar
ends (coinciding with molecular gas density enhancements usually found in these
locations).We also quantify how the total dust mass varies with our assumptions
about the emissivity index as well as the influence of the wavelength coverage
used in the fits. We show that modified blackbody fits using a shallow
emissivity (beta_c < 2.0) lead to significantly lower dust masses compared to
the beta_c < 2.0 case, with dust masses lower by up to 50% if beta_c=1.5 for
instance.The working resolution affects our total dust mass estimates: masses
increase from global fits to spatially-resolved fits.Comment: 26 pages, 12 figures, 4 tables, accepted for publication in MNRAS,
2012 June 2
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Cool dust heating and temperature mixing in nearby star-forming galaxies
Physical conditions of the interstellar medium in galaxies are closely linked
to the ambient radiation field and the heating of dust grains. In order to
characterize dust properties in galaxies over a wide range of physical
conditions, we present here the radial surface brightness profiles of the
entire sample of 61 galaxies from Key Insights into Nearby Galaxies:
Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the
characterization of the grain emissivities, dust temperatures, and interstellar
radiation fields responsible for heating the dust. After fitting the dust and
stellar radial profiles with exponential functions, we fit the far-infrared
spectral energy distribution (SED) in each annular region with
single-temperature modified black bodies using both variable (MBBV) and fixed
(MBBF) emissivity indices beta, as well as with physically motivated dust
models. Results show that while most SED parameters decrease with radius, the
emissivity index beta also decreases with radius in some galaxies, but in
others is increasing, or rising in the inner regions and falling in the outer
ones. Despite the fixed grain emissivity (average beta~ 2.1) of the
physically-motivated models, they are well able to accommodate flat spectral
slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated
with cooler temperatures, contrary to what would be expected from the usual
Tdust-beta degeneracy. This trend is related to variations in Umin since beta
and Umin are very closely linked over the entire range in Umin sampled by the
KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results
strongly suggest that the low apparent \beta values (flat slopes) in MBBV fits
are caused by temperature mixing along the line-of-sight, rather than by
intrinsic variations in grain properties. Abstract truncated for arXiv.Comment: 28 pages, 20 figures, accepted for publication in A&
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