78 research outputs found
Dust Abundance and Properties in the Nearby Dwarf Galaxies NGC 147 and NGC 185
We present new mid- to far-infrared images of the two dwarf compact elliptical galaxies that are satellites of M31, NGC 185, and NGC 147, obtained with the Spitzer Space Telescope. Spitzer's high sensitivity and spatial resolution enable us for the first time to look directly into the detailed spatial structure and properties of the dust in these systems. The images of NGC 185 at 8 and 24 μm display a mixed morphology characterized by a shell-like diffuse emission region surrounding a central concentration of more intense infrared emission. The lower resolution images at longer wavelengths show the same spatial distribution within the central 50" but beyond this radius, the 160 μm emission is more extended than that at 24 and 70 μm. On the other hand, the dwarf galaxy NGC 147, located only a small distance away from NGC 185, shows no significant infrared emission beyond 24 μm and therefore its diffuse infrared emission is mainly stellar in origin. For NGC 185, the derived dust mass based on the best fit to the spectral energy distribution is 1.9 × 10^3 M_⊙, implying a gas mass of 3.0 × 10^5 M_⊙. These values are in agreement with those previously estimated from infrared as well as CO and H I observations and are consistent with the predicted mass return from dying stars based on the last burst of star formation 1 × 10^9 yr ago. Based on the 70-160 μm flux density ratio, we estimate a temperature for the dust of ~17 K. For NGC 147, we obtain an upper limit for the dust mass of 4.5 × 10^2 M_⊙ at 160 μm (assuming a temperature of ~20 K), a value consistent with the previous upper limit derived using Infrared Space Observatory observations of this galaxy. In the case of NGC 185, we also present full 5-38 μm low-resolution (R ~ 100) spectra of the main emission regions. The Infrared Spectrograph spectra of NGC 185 show strong polycyclic aromatic hydrocarbons emission, deep silicate absorption features and H_2 pure rotational line ratios consistent with having the dust and molecular gas inside the dust cloud being impinged by the far-ultraviolet radiation field of a relatively young stellar population. Therefore, based on its infrared spectral properties, NGC 185 shows signatures of recent star formation (a few ×10^8 yr ago), although its current star formation rate is quite low
The Flux Ratio Method for Determining the Dust Attenuation of Starburst Galaxies
The presence of dust in starburst galaxies complicates the study of their
stellar populations as the dust's effects are similar to those associated with
changes in the galaxies' stellar age and metallicity. This degeneracy can be
overcome for starburst galaxies if UV/optical/near-infrared observations are
combined with far-infrared observations. We present the calibration of the flux
ratio method for calculating the dust attenuation at a particular wavelength,
Att(\lambda), based on the measurement of F(IR)/F(\lambda) flux ratio. Our
calibration is based on spectral energy distributions (SEDs) from the PEGASE
stellar evolutionary synthesis model and the effects of dust (absorption and
scattering) as calculated from our Monte Carlo radiative transfer model. We
tested the attenuations predicted from this method for the Balmer emission
lines of a sample starburst galaxies against those calculated using radio
observations and found good agreement. The UV attenuation curves for a handful
of starburst galaxies were calculated using the flux ratio method, and they
compare favorably with past work. The relationship between Att(\lambda) and
F(IR)/F(\lambda) is almost completely independent of the assumed dust
properties (grain type, distribution, and clumpiness). For the UV, the
relationship is also independent of the assumed stellar properties (age,
metallicity, etc) accept for the case of very old burst populations. However at
longer wavelengths, the relationship is dependent on the assumed stellar
properties.Comment: accepted by the ApJ, 18 pages, color figures, b/w version at
http://mips.as.arizona.edu/~kgordon/papers/fr_method.htm
Measuring Extinction Curves of Lensing Galaxies
We critique the method of constructing extinction curves of lensing galaxies
using multiply imaged QSOs. If one of the two QSO images is lightly reddened or
if the dust along both sightlines has the same properties then the method works
well and produces an extinction curve for the lensing galaxy. These cases are
likely rare and hard to confirm. However, if the dust along each sightline has
different properties then the resulting curve is no longer a measurement of
extinction. Instead, it is a measurement of the difference between two
extinction curves. This "lens difference curve'' does contain information about
the dust properties, but extracting a meaningful extinction curve is not
possible without additional, currently unknown information. As a quantitative
example, we show that the combination of two Cardelli, Clayton, & Mathis (CCM)
type extinction curves having different values of R(V) will produce a CCM
extinction curve with a value of R(V) which is dependent on the individual R(V)
values and the ratio of V band extinctions. The resulting lens difference curve
is not an average of the dust along the two sightlines. We find that lens
difference curves with any value of R(V), even negative values, can be produced
by a combination of two reddened sightlines with different CCM extinction
curves with R(V) values consistent with Milky Way dust (2.1 < R(V) < 5.6). This
may explain extreme values of R(V) inferred by this method in previous studies.
But lens difference curves with more normal values of R(V) are just as likely
to be composed of two dust extinction curves with R(V) values different than
that of the lens difference curve. While it is not possible to determine the
individual extinction curves making up a lens difference curve, there is
information about a galaxy's dust contained in the lens difference curves.Comment: 15 pages, 4 figues, ApJ in pres
One Relation for All Wavelengths: The Far-Ultraviolet to Mid-Infrared Milky Way Spectroscopic R(V) Dependent Dust Extinction Relationship
Dust extinction is one of the fundamental measurements of dust grain sizes,
compositions, and shapes. Most of the wavelength dependent variations seen in
Milky Way extinction are strongly correlated with the single parameter
R(V)=A(V)/E(B-V). Existing R(V) dependent extinction relationships use a
mixture of spectroscopic and photometry observations, hence do not fully
capture all the important dust features nor continuum variations. Using four
existing samples of spectroscopically measured dust extinction curves, we
consistently measure the R(V) dependent extinction relationship
spectroscopically from the far-ultraviolet to mid-infrared for the first time.
Linear fits of A(lambda)/A(V) dependent on R(V) are done using a method that
fully accounts for their significant and correlated uncertainties. These linear
parameters are fit with analytic wavelength dependent functions to determine
the smooth R(V) (2.3-5.6) and wavelength (912 A-32 micron) dependent extinction
relationship. This relationship shows that the far-UV rise, 2175 A bump, and
the three broad optical features are dependent on R(V), but the 10 and 20
micron features are not. Existing literature relationships show significant
deviations compared to this relationship especially in the far-ultraviolet and
infrared. Extinction curves that clearly deviate from this relationship
illustrate that this relationship only describes the average behavior versus
R(V). We find tentative evidence that the relationship may not be linear with
1/R(V) especially in the ultraviolet. For the first time, this relationship
provides measurements of dust extinction that spectroscopically resolve the
continuum and features in the ultraviolet, optical, and infrared as a function
of R(V) enabling detailed studies of dust grains properties and full
spectroscopic accounting for the effects of dust extinction on astrophysical
objects.Comment: 16 pages, 9 figures, ApJ, in pres
The Dusty Starburst Nucleus of M33
We have thoroughly characterized the ultraviolet to near-infrared (0.15 - 2.2
micron) spectral energy distribution (SED) of the central parsec of the M33
nucleus through new infrared photometry and optical/near-infrared spectroscopy,
combined with ultraviolet/optical observations from the literature and the HST
archive. The SED shows evidence for a significant level of attenuation, which
we model through a Monte Carlo radiative transfer code as a shell of clumpy
Milky Way-type dust (tau_V ~ 2 +/- 1). The discovery of Milky Way-type dust
(with a strong 2175 A bump) internal to the M33 nucleus is different from
previous work which has found SMC-like dust (no bump) near starburst regions.
The amount by which dust can be processed may be related to the mass and age of
the starburst as well as the extent to which the dust can shield itself. Our
starburst models include the effects of this dust and can fit the SED if the
nucleus was the site of a moderate (~10^8 L_sun at 10 Myrs) episode of coeval
star formation about 70 Myrs ago. This result is quite different from previous
studies which resorted to multiple stellar populations (between 2 and 7)
attenuated by either no or very little internal dust. The M33 nuclear starburst
is remarkably similar to an older version (70 Myr versus 10 Myr) of the
ultra-compact starburst in the center of the Milky Way.Comment: 29 pages, 9 embedded figures, ApJ, in pres
The DIRTY Model. I. Monte Carlo Radiative Transfer Through Dust
We present the DIRTY radiative transfer model in this paper and a companion
paper. This model computes the polarized radiative transfer of photons from
arbitrary distributions of stars through arbitrary distributions of dust using
Monte Carlo techniques. The dust re-emission is done self-consistently with the
dust absorption and scattering and includes all three important emission paths:
equilibrium thermal emission, non-equilibrium thermal emission, and the
aromatic features emission. The algorithm used for the radiative transfer
allows for the efficient computation of the appearance of a model system as
seen from any viewing direction. We present a simple method for computing an
upper limit on the output quantity uncertainties for Monte Carlo radiative
transfer models which use the weighted photon approach.Comment: 8 pages, 3 figures, accepted to the Ap
A Reanalysis of theUltraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)
extinction using data from the IUE final archive. Our new analysis takes
advantage of the improved signal--to--noise of the IUE NEWSIPS reduction, the
exclusion of stars with very low reddening, the careful selection of well
matched comparison stars, and an analysis of the effects of Galactic foreground
dust. Differences between the average extinction curves of the 30 Dor region
and the rest of the LMC are reduced compared to previous studies. We find that
there is a group of stars with very weak 2175 Ang. bumps that lie in or near
the region occupied by the supergiant shell, LMC 2, on the southeast side of 30
Dor. The average extinction curves inside and outside LMC 2 show a very
significant difference in 2175 Ang. bump strength, but their far--UV
extinctions are similar. While it is unclear whether or not the extinction
outside the LMC 2 region can be fit with the relation of Cardelli, Clayton and
Mathis (CCM), sightlines near LMC 2 cannot be fit with CCM due to their weak
2175 Ang. bumps. While the extinction properties seen in the LMC lie within the
range of properties seen in the Galaxy, the correlations of UV extinction
properties with environment seen in the Galaxy do not appear to hold in the
LMC.Comment: 29 pages, 10 figures, to be published in Ap
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