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Dust and Stellar Emission of Nearby Galaxies
in the KINGFISH Herschel Survey
Ramin A. Skibba ([email protected]), Charles W. Engelbracht, et al.
I. Introduction
We exploit data from the UV to submillimeter wavelengths of a
heterogeneous sample of 62 galaxies from the KINGFISH project (Key
Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel), to
empirically study the emission from stars and dust in these galaxies.
We use the spectral energy distributions computed by Dale et al.,
using data from GALEX, SDSS (and other optical measurements),
2MASS, Spitzer, SCUBA (when available), as well as some new data
from Herschel. We estimate the stellar and dust emission of these
galaxies, in a way that is as empirical and model-independent as
possible, and we use these to estimate the ratio of stellar-to-dust
emission. The stellar-to-dust ratios can be compared to gas-to-dust
ratios determined from SED models, and will yield important
information about the properties of the ISM in these galaxies.
This is an ongoing project. We expect that Herschel
observations will allow us to trace cold dust components invisible to
Spitzer and will have greatly reduced systematic uncertainties relative
to ground-based submillimeter measurements. We examine how our
estimated stellar-to-dust ratios correlate with various galaxy
properties: total infrared luminosity, morphology, stellar mass, and
metallicity. Finally, for a few well-resolved galaxies, such as M101, we
plan to use Herschel observations to study the spatial variations of
the stellar-to-dust ratio within the galaxies.
II. Data
For the 58 SINGS galaxies included in KINGFISH, we use the global
flux densities estimated in Dale et al. (2007). For the 17 LVL galaxies
included in KINGFISH, two of which are not in SINGS (M101, NGC
3077), we use the global flux densities estimated in Dale et al. (2009).
Data for the remaining two galaxies, IC 342 and NGC 2146, were
obtained separately. UV data from GALEX (1528, 2271 Å); optical
data from either SDSS (ugriz) or Kitt Peak (BVRI); near-infrared data
from 2MASS (JHK); mid-infrared data from Spitzer (3.6, 4.5, 5.8, 8, 24,
70, 160 m; submm data for 1/3 of the galaxies from SCUBA (450,
850 m).
III. Stellar-to-Dust Ratio
In order to demarcate “stellar” and “dust” emission in the SEDs,
(f)stars and (f)dust, we choose a strict wavelength cut at 5m and
simply integrate over the SED at <5m for the former and >5m for
the latter. The area under the SED is computed directly, without
applying any model, using Jy and log(m) as our units. We choose
not to extrapolate the SEDs for the galaxies missing UV data (4/60
galaxies) or submm data (38/60). Finally, we take the ratio (f)stars /
(f)dust for the stellar/dust
emission. To estimate
the uncertainties, we
simply assume that the
errors of the flux
densities have a Gaussian
distribution; we sample
from these distributions
100 times, and compute
the variance around the
mean stellar/dust ratio.
The figure shows two
example SEDs:
IV. Current Results
Distribution of stellar-todust ratios for 60 nearby
galaxies:
Many of the galaxies have
stellar/dust emission near
unity, with very few
extremely dusty passive
galaxies. NGC 1404 and
DDO 165 have the largest
ratios, and NGC 1482 and
1266 have the lowest.
Correlation between total infrared
(TIR) luminosity (estimated from
8, 24, 70, 160µm; Draine & Li
2007) vs. stellar/dust emission:
There appear to be two distinct
populations: some of the galaxies
with large stellar/dust emission
and large LTIR are earlier types,
while some of them with large
stellar/dust and small LTIR are
irregulars. Many of the galaxies
with stellar/dust~1 and high LTIR
are metal-rich late-type spirals,
suggesting an evolutionary
sequence (such that they’re
between the two populations
with higher stellar/dust).
Specific SFR vs. stellar-to-dust
ratio of the 32 KINGFISH
galaxies studied by Noll et al.
(2009; their sample excludes
the dwarf galaxies).
This dramatically strong
correlation could simply be
explained by mostly warm
dust heated by stars in
galaxies with higher
formation rates.
V. Plans for Herschel
This is a work in progress.
By adding new data from Herschel (especially SPIRE), we will detect
more cold dust in these galaxies, reducing systematic uncertainties in
our estimates of the amount of dust (vis-à-vis stellar) emission.
We plan to estimate the mass in dust relative to mass in stars (with
few assumptions), rather than just the luminosity ratio. The mass
ratio Mstellar/Mdust is a more physical quantity that could distinguish
between two possibilities: the trends may be due to galaxies with
different dust populations, or to some galaxies with stronger
interstellar radiation fields.
We also plan to analyze the spatial dependence of stellar/dust
emission for a couple galaxies, such as M101, IC 342, or M33.