95 research outputs found
Spectral Mapping Reconstruction of Extended Sources
Three dimensional spectroscopy of extended sources is typically performed
with dedicated integral field spectrographs. We describe a method of
reconstructing full spectral cubes, with two spatial and one spectral
dimension, from rastered spectral mapping observations employing a single slit
in a traditional slit spectrograph. When the background and image
characteristics are stable, as is often achieved in space, the use of
traditional long slits for integral field spectroscopy can substantially reduce
instrument complexity over dedicated integral field designs, without loss of
mapping efficiency -- particularly compelling when a long slit mode for single
unresolved source followup is separately required. We detail a custom
flux-conserving cube reconstruction algorithm, discuss issues of extended
source flux calibration, and describe CUBISM, a tool which implements these
methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared
Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS
The Opacity of Spiral Galaxy Disks VIII: Structure of the Cold ISM
The quantity of dust in a spiral disk can be estimated using the dust's
typical emission or the extinction of a known source. In this paper, we compare
two techniques, one based on emission and one on absorption, applied on
sections of fourteen disk galaxies. The two measurements reflect, respectively
the average and apparent optical depth of a disk section. Hence, they depend
differently on the average number and optical depth of ISM structures in the
disk. The small scale geometry of the cold ISM is critical for accurate models
of the overall energy budget of spiral disks. ISM geometry, relative
contributions of different stellar populations and dust emissivity are all free
parameters in galaxy Spectral Energy Distribution (SED) models; they are also
sometimes degenerate, depending on wavelength coverage. Our aim is to constrain
typical ISM geometry. The apparent optical depth measurement comes from the
number of distant galaxies seen in HST images through the foreground disk. We
measure the IR flux in images from the {\it Spitzer} Infrared Nearby Galaxy
Survey in the same section of the disk that was covered by HST. A physical
model of the dust is fit to the SED to estimate the dust surface density, mean
temperature, and brightness in these disk sections. The surface density is
subsequently converted into the average optical depth estimate. The two
measurements generally agree. The ratios between the measured average and
apparent optical depths of the disk sections imply optically thin clouds in
these disks. Optically thick disks, are likely to have more than a single cloud
along the line-of-sight.Comment: 31 pages, 5 figures, 4 tables, accepted for publication in A
The Mid-Infrared Spectrum of Star-Forming Galaxies: Global Properties of PAH Emission
We present a sample of low-resolution 5-38um Spitzer IRS spectra of the inner
few square kiloparsecs of 59 nearby galaxies spanning a large range of star
formation properties. A robust method for decomposing mid-infrared galaxy
spectra is described, and used to explore the behavior of PAH emission and the
prevalence of silicate dust extinction. Evidence for silicate extinction is
found in ~1/8 of the sample, at strengths which indicate most normal galaxies
undergo A_V < ~3 magnitudes averaged over their centers. The contribution of
PAH emission to the total infrared power is found to peak near 10% and extend
up to ~20%, and is suppressed at metallicities Z < ~Z_sun/4, as well as in
low-luminosity AGN environments. Strong inter-band PAH feature strength
variations (2-5x) are observed, with the presence of a weak AGN and, to a
lesser degree, increasing metallicity shifting power to the longer wavelength
bands. A peculiar PAH emission spectrum with markedly diminished 5-8um features
arises among the sample solely in systems with relatively hard radiation fields
harboring low-luminosity AGN. The AGN may modify the emitting grain
distribution and provide the direct excitation source of the unusual PAH
emission, which cautions against using absolute PAH strength to estimate star
formation rates in systems harboring active nuclei. Alternatively, the low star
formation intensity often associated with weak AGN may affect the spectrum. The
effect of variations in the mid-infrared spectrum on broadband infrared surveys
is modeled, and points to more than a factor of two uncertainty in results
which assume a fixed PAH emission spectrum, for redshifts z=0-2.5.Comment: Accepted for publication in ApJ, 24 pages (abstract typo fixed,
reference added
The Calibration of Mid-Infrared Star Formation Rate Indicators
With the goal of investigating the degree to which the mid-infrared emission
traces the star formation rate (SFR), we analyze Spitzer 8 um and 24 um data of
star-forming regions in a sample of 33 nearby galaxies with available
HST/NICMOS images in the Paschen-alpha (1.8756 um) emission line. The galaxies
are drawn from the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample, and
cover a range of morphologies and a factor ~10 in oxygen abundance. Published
data on local low-metallicity starburst galaxies and Luminous Infrared Galaxies
are also included in the analysis. Both the stellar-continuum-subtracted 8 um
emission and the 24 um emission correlate with the extinction-corrected
Pa-alpha line emission, although neither relationship is linear. Simple models
of stellar populations and dust extinction and emission are able to reproduce
the observed non-linear trend of the 24 um emission versus number of ionizing
photons, including the modest deficiency of 24 um emission in the low
metallicity regions, which results from a combination of decreasing dust
opacity and dust temperature at low luminosities. Conversely, the trend of the
8 um emission as a function of the number of ionizing photons is not well
reproduced by the same models. The 8 um emission is contributed, in larger
measure than the 24 um emission, by dust heated by non-ionizing stellar
populations, in agreement with previous findings. Two SFR calibrations, one
using the 24 um emission and the other using a combination of the 24 um and
H-alpha luminosities (Kennicutt et al. 2007), are presented. No calibration is
presented for the 8 um emission, because of its significant dependence on both
metallicity and environment. The calibrations presented here should be directly
applicable to systems dominated by on-going star formation.Comment: 67 pages, 15 figures, accepted for publication on the Astrophysical
Journal; replacement contains: correction to equation 8; important tweaks to
equation 9; various typos correcte
An Ultraviolet-to-Radio Broadband Spectral Atlas of Nearby Galaxies
The ultraviolet-to-radio continuum spectral energy distributions are
presented for all 75 galaxies in the Spitzer Infrared Nearby Galaxies Survey
(SINGS). A principal component analysis of the sample shows that most of the
sample's spectral variations stem from two underlying components, one
representative of a galaxy with a low infrared-to-ultraviolet ratio and one
representative of a galaxy with a high infrared-to-ultraviolet ratio. The
influence of several parameters on the infrared-to-ultraviolet ratio is studied
(e.g., optical morphology, disk inclination, far-infrared color, ultraviolet
spectral slope, and star formation history). Consistent with our understanding
of normal star-forming galaxies, the SINGS sample of galaxies in comparison to
more actively star-forming galaxies exhibits a larger dispersion in the
infrared-to-ultraviolet versus ultraviolet spectral slope correlation. Early
type galaxies, exhibiting low star formation rates and high optical surface
brightnesses, have the most discrepant infrared-to-ultraviolet correlation.
These results suggest that the star formation history may be the dominant
regulator of the broadband spectral variations between galaxies. Finally, a new
discovery shows that the 24 micron morphology can be a useful tool for
parametrizing the global dust temperature and ultraviolet extinction in nearby
galaxies. The dust emission in dwarf/irregular galaxies is clumpy and warm
accompanied by low ultraviolet extinction, while in spiral galaxies there is
typically a much larger diffuse component of cooler dust and average
ultraviolet extinction. For galaxies with nuclear 24 micron emission, the dust
temperature and ultraviolet extinction are relatively high compared to disk
galaxies.Comment: Accepted for publication in the Astrophysical Journal; Fixed radio
flux density units (mJy
The Spitzer Infrared Nearby Galaxies Survey: A High-Resolution Spectroscopy Anthology
High resolution mid-infrared spectra are presented for 155 nuclear and
extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS).
The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared
emission lines are also provided, along with upper limits in key lines for
infrared-faint targets. The SINGS sample shows a wide range in the ratio of
[SIII]18.71um/[SIII]33.48um, but the average ratio of the ensemble indicates a
typical interstellar electron density of 300-400 cm^{-3} on ~23"x15" scales and
500-600 cm^{-3} using ~11"x9" apertures, independent of whether the region
probed is a star-forming nuclear, a star-forming extranuclear, or an AGN
environment. Evidence is provided that variations in gas-phase metallicity play
an important role in driving variations in radiation field hardness, as
indicated by [NeIII]15.56um/[NeII]12.81um, for regions powered by star
formation. Conversely, the radiation hardness for galaxy nuclei powered by
accretion around a massive black hole is independent of metal abundance.
Furthermore, for metal-rich environments AGN are distinguishable from
star-forming regions by significantly larger [NeIII]15.56um/[NeII]12.81um
ratios. Finally, [FeII]25.99um/[NeII]12.81um versus [SiII]34.82um/[SIII]33.48um
also provides an empirical method for discerning AGN from normal star-forming
sources. However, similar to [NeIII]15.56um/[NeII]12.81um, these mid-infrared
line ratios lose their AGN/star-formation diagnostic powers for very low
metallicity star-forming systems with hard radiation fields.Comment: Accepted for publication in Ap
The Incidence of Highly-Obscured Star-Forming Regions in SINGS Galaxies
Using the new capabilities of the Spitzer Space Telescope and extensive
multiwavelength data from the Spitzer Infrared Nearby Galaxies Survey (SINGS),
it is now possible to study the infrared properties of star formation in nearby
galaxies down to scales equivalent to large HII regions. We are therefore able
to determine what fraction of large, infrared-selected star-forming regions in
normal galaxies are highly obscured and address how much of the star formation
we miss by relying solely on the optical portion of the spectrum. Employing a
new empirical method for deriving attenuations of infrared-selected
star-forming regions we investigate the statistics of obscured star formation
on 500pc scales in a sample of 38 nearby galaxies. We find that the median
attenuation is 1.4 magnitudes in H-alpha and that there is no evidence for a
substantial sub-population of uniformly highly-obscured star-forming regions.
The regions in the highly-obscured tail of the attenuation distribution
(A_H-alpha > 3) make up only ~4% of the sample of nearly 1800 regions, though
very embedded infrared sources on the much smaller scales and lower
luminosities of compact and ultracompact HII regions are almost certainly
present in greater numbers. The highly-obscured cases in our sample are
generally the bright, central regions of galaxies with high overall attenuation
but are not otherwise remarkable. We also find that a majority of the galaxies
show decreasing radial trends in H-alpha attenuation. The small fraction of
highly-obscured regions seen in this sample of normal, star-forming galaxies
suggests that on 500pc scales the timescale for significant dispersal or break
up of nearby, optically-thick dust clouds is short relative to the lifetime of
a typical star-forming region.Comment: Accepted for publication in ApJ; emulateapj style, 30 pages, 18
figures (compressed versions), 3 table
Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations
We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and
radio images of 14 radio-selected objects in M 33. These objects are thought to
represent the youngest phase of star cluster formation. We have detected the
majority of cluster candidates in M 33 at all wavelengths. From the near-IR
images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and
stellar masses of 10^3-10^4 M_solar. We have generated spectral energy
distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these
SEDs, we have modeled the dust emission around these star clusters to determine
the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters'
local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV
images are similar to within a factor of 2 or 3. These results suggest that
eleven of the fourteen radio-selected objects are optically-visible young star
clusters with a surrounding H II region, that two are background objects,
possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.Comment: 57 pages total; 20 figures; 3 tables under review by ApJS; first
review complet
An Aromatic Inventory of the Local Volume
Using infrared photometry from the Spitzer Space Telescope, we perform the
first inventory of aromatic feature emission (AFE, but also commonly referred
to as PAH emission) for a statistically complete sample of star-forming
galaxies in the local volume. The photometric methodology involved is
calibrated and demonstrated to recover the aromatic fraction of the IRAC 8
micron flux with a standard deviation of 6% for a training set of 40 SINGS
galaxies (ranging from stellar to dust dominated) with both suitable
mid-infrared Spitzer IRS spectra and equivalent photometry. A potential factor
of two improvement could be realized with suitable 5.5 and 10 micron
photometry, such as what may be provided in the future by JWST. The resulting
technique is then applied to mid-infrared photometry for the 258 galaxies from
the Local Volume Legacy (LVL) survey, a large sample dominated in number by
low-luminosity dwarf galaxies for which obtaining comparable mid-infrared
spectroscopy is not feasible. We find the total LVL luminosity due to five
strong aromatic features in the 8 micron complex to be 2.47E10 solar
luminosities with a mean volume density of 8.8E6 solar luminosities per cubic
Megaparsec. Twenty-four of the LVL galaxies, corresponding to a luminosity cut
at M = -18.22 in the B band, account for 90% of the aromatic luminosity. Using
oxygen abundances compiled from the literature for 129 of the 258 LVL galaxies,
we find a correlation between metallicity and the aromatic to total infrared
emission ratio but not the aromatic to total 8 micron dust emission ratio. A
possible explanation is that metallicity plays a role in the abundance of
aromatic molecules relative to the total dust content, but other factors such
as star formation and/or the local radiation field affect the excitation of
those molecules.Comment: ApJ in press; 29 pages, 14 figures, 3 tables; emulateapj forma
The Effect of Star Formation on the Far-Infrared--Radio Correlation within Galaxies
Using data obtained for twelve galaxies as part of the {\it Spitzer} Infrared
Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope
(WSRT)-SINGS radio continuum survey, we study how star formation activity
affects the far-infrared (FIR)--radio correlation {\it within} galaxies by
testing a phenomenological model, which describes the radio image as a smeared
version of the FIR image. The physical basis of this description is that
cosmic-ray (CR) electrons will diffuse measurably farther than the mean free
path of dust-heating photons before decaying by synchrotron radiation. This
description works well in general. Galaxies with higher infrared surface
brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs,
substantially shorter than those for lower surface brightness galaxies. We
interpret this result to suggest that galaxies with higher disk averaged star
formation rates have had a recent episode of enhanced star formation and are
characterized by a higher fraction of young CR electrons that have traveled
only a few hundred parsecs from their acceleration sites in supernova remnants
compared to galaxies with lower star formation activity.Comment: 5 pages, 3 figures; Accepted for publication in ApJ Letter
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