804 research outputs found
Environmental Effects in Clusters: Modified Far-Infrared--Radio Relations within Virgo Cluster Galaxies
(abridged) We present a study on the effects of the intracluster medium (ICM)
on the interstellar medium (ISM) of 10 Virgo cluster spiral galaxies using {\it
Spitzer} far-infrared (FIR) and VLA radio continuum imaging. Relying on the
FIR-radio correlation within normal galaxies, we use our infrared data to
create model radio maps which we compare to the observed radio images. For 6 of
our sample galaxies we find regions along their outer edges that are highly
deficient in the radio compared with our models. We believe these observations
are the signatures of ICM ram pressure. For NGC 4522 we find the radio deficit
region to lie just exterior to a region of high radio polarization and flat
radio spectral index, although the total 20 cm radio continuum in this region
does not appear strongly enhanced. These characteristics seem consistent for
other galaxies with radio polarization data in the literature. The strength of
the radio deficit is inversely correlated with the time since peak pressure as
inferred from stellar population studies and gas stripping simulations,
suggesting the strength of the radio deficit is good indicator of the strength
of the current ram pressure. We also find that galaxies having {\it local}
radio {\it deficits} appear to have {\it enhanced global} radio fluxes. Our
preferred physical picture is that the observed radio deficit regions arise
from the ICM wind sweeping away cosmic-ray (CR) electrons and the associated
magnetic field, thereby creating synchrotron tails as observed for some of our
galaxies. We propose that CR particles are also re-accelerated by ICM-driven
shocklets behind the observed radio deficit regions which in turn enhances the
remaining radio disk brightness.Comment: 19 pages, 10 figures; Astrophysical Journa
APM 08279+5255: Keck Near- and Mid-IR High-Resolution Imaging
We present Keck high-resolution near-IR (2.2 microns; FWHM~0.15") and mid-IR
(12.5 microns; FWHM~0.4") images of APM08279+5255, a z=3.91 IR-luminous BALQSO
with a prodigious apparent bolometric luminosity of 5x10^{15} Lsun, the largest
known in the universe. The K-band image shows that this system consists of
three components, all of which are likely to be the gravitationally lensed
images of the same background object, and the 12.5 micron image shows a
morphology consistent with such an image configuration. Our lens model suggests
that the magnification factor is ~100 from the restframe UV to mid-IR, where
most of the luminosity is released. The intrinsic bolometric luminosity and IR
luminosity of APM08279+5255 are estimated to be 5x10^{13} Lsun and 1x10^{13}
Lsun, respectively. This indicates that APM 08279+5255 is intriniscally
luminous, but it is not the most luminous object known. As for its dust
contents, little can be determined with the currently available data due to the
uncertainties associated with the dust emissivity and the possible effects of
differential magnification. We also suggest that the lensing galaxy is likely
to be a massive galaxy at z~3.Comment: 32 pages, 4 tables, 11 figures; Accepted for publication in Ap
An Accounting of the Dust-Obscured Star Formation and Accretion Histories Over the Last ~11~Billion Years
(Abridged) We report on an accounting of the star formation and accretion
driven energetics of 24um detected sources in GOODS North. For sources having
infrared (IR; 8-1000um) luminosities >3x10^12 L_sun when derived by fitting
local SEDs to 24um photometry alone, we find these IR luminosity estimates to
be a factor of ~4 times larger than those estimated when the SED fitting
includes additional 16 and 70um data (and in some cases mid-infrared
spectroscopy and 850um data). This discrepancy arises from the fact that high
luminosity sources at z>>0 appear to have far- to mid-infrared ratios, as well
as aromatic feature equivalent widths, typical of lower luminosity galaxies in
the local Universe. Using our improved estimates for IR luminosity and AGN
contributions, we investigate the evolution of the IR luminosity density versus
redshift arising from star formation and AGN processes alone. We find that,
within the uncertainties, the total star formation driven IR luminosity density
is constant between 1.15 < z < 2.35, although our results suggest a slightly
larger value at z>2. AGN appear to account for <18% of the total IR luminosity
density integrated between 0< z < 2.35, contributing <25% at each epoch. LIRG
appear to dominate the star formation rate (SFR) density along with normal
star-forming galaxies (L_IR < 10^11 L_sun) between 0.6 < z < 1.15. Once beyond
z >2, the contribution from ultraluminous infrared galaxies ULIRGs becomes
comparable with that of LIRGs. Using our improved IR luminosity estimates, we
find existing calibrations for UV extinction corrections based on measurements
of the UV spectral slope typically overcorrect UV luminosities by a factor of
~2, on average, for our sample of 24um-selected sources; accordingly we have
derived a new UV extinction correction more appropriate for our sample.Comment: Accepted for publication in Ap
Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946
Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10
star-forming regions in the nearby galaxy NGC6946, including its starbursting
nucleus, we compare a number of SFR diagnostics that are typically considered
to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e.,
1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission,
along with the hybrid (obscured + unobscured) indicators of H\alpha+24um and
UV+IR. The 33GHz free-free emission is assumed to provide the most accurate
measure of the current SFR. Among the extranuclear star-forming regions, the
24um, H\alpha+24um and UV+IR SFR calibrations are in good agreement with the
33GHz free-free SFRs. However, each of the SFR calibrations relying on some
form of dust emission overestimate the nuclear SFR by a factor of ~2. This is
more likely the result of excess dust heating through an accumulation of
non-ionizing stars associated with an extended episode of star formation in the
nucleus rather than increased competition for ionizing photons by dust. SFR
calibrations using the non-thermal radio continuum yield values which only
agree with the free-free SFRs for the nucleus, and underestimate the SFRs from
the extranuclear star-forming regions by a factor of ~2. This result likely
arises from the CR electrons decaying within the starburst region with
negligible escape compared to the young extranuclear star-forming regions.
Finally, we find that the SFRs estimated using the total 33GHz emission agree
well with the free-free SFRs due to the large thermal fractions present at
these frequencies even when local diffuse backgrounds are not removed. Thus,
rest-frame 33GHz observations may act as a reliable method to measure the SFRs
of galaxies at increasingly high redshift without the need of ancillary radio
data to account for the non-thermal emission.Comment: 18 pages, 7 Figures, Accepted for publication in Ap
van der Kruit to Spitzer: A New Look at the FIR-Radio Correlation
We present an initial look at the far infrared-radio correlation within the
star-forming disks of four nearby, nearly face-on galaxies (NGC~2403, NGC~3031,
NGC~5194, and NGC~6946). Using {\it Spitzer} MIPS imaging and WSRT radio
continuum data, we are able to probe variations in the logarithmic
70~m/22~cm () flux density ratios across each disk at sub-kpc
scales. We find general trends of decreasing with declining surface
brightness and with increasing radius. We also find that the dispersion in
within galaxies is comparable to what is measured {\it globally} among
galaxies at around 0.2 dex. We have also performed preliminary phenomenological
modeling of cosmic ray electron (CR) diffusion using an image-smearing
technique, and find that smoothing the infrared maps improves their correlation
with the radio maps. The best fit smoothing kernels for the two less active
star-forming galaxies (NGC~2403 and NGC~3031) have much larger scale-lengths
than that of the more active star-forming galaxies (NGC~5194 and NGC~6946).
This difference may be due to the relative deficit of recent CR
injection into the interstellar medium (ISM) for the galaxies having largely
quiescent disks.Comment: 6 pages, 3 figures, To appear in the proceedings of the "Island
Universes: Structure and Evolution of Disk Galaxies" conference held in
Terschelling, Netherlands, July 2005, ed. R. de Jong (Springer: Dordrecht
Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: NGC 628 and NGC 6946
We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in
the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly
constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and
carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass
surface density, (2) dust mass fraction contributed by polycyclic aromatic
hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the
dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR
luminosity originating in regions with high starlight intensity. We obtain maps
for the dust properties, which trace the spiral structure of the galaxies. The
dust models successfully reproduce the observed global and resolved spectral
energy distributions (SEDs). The overall dust/H mass ratio is estimated to be
0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with
what is expected for galaxies of near-solar metallicity. Our derived dust
masses are larger (by up to a factor 3) than estimates based on
single-temperature modified blackbody fits. We show that the SED fits are
significantly improved if the starlight intensity distribution includes a
(single intensity) "delta function" component. We find no evidence for
significant masses of cold dust T<12K. Discrepancies between PACS and MIPS
photometry in both low and high surface brightness areas result in large
uncertainties when the modeling is done at PACS resolutions, in which case
SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting
to model dust at the angular resolution of PACS.Comment: To be published in Apj, September 2012. See the full version at
http://www.astro.princeton.edu/~ganiano/Papers
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
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Far-infrared line imaging of the starburst ring in NGC 1097 with the Herschel/PACS spectrometer
NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Observatory. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 μm, [OIII] 88 μm, [NII] 122 μm, [CII] 158 μm and [NII] 205 μm. We observe that in the [OI] 63 μm, [OIII] 88 μm, and [NII] 122 μm line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220 km s-1 (inclination uncorrected) measured in all lines. The [OI] 63 μm/[CII] 158 μm ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 μm and [OIII] 88 μm maps. Variations of the [NII] 122 μm/[NII] 205 μm ratio correspond to a range in the ionized gas density between 150 and 400 cm-3
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
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