381 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
3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium
We use 3-D radiative transfer models to show the effects of clumpy
circumstellar material on the observed infrared colors of high mass stars
embedded in molecular clouds. We highlight differences between 3-D clumpy and
1-D smooth models which can affect the interpretation of data. We discuss
several important properties of the emergent spectral energy distribution
(SED): More near-infrared light (scattered and direct from the central source)
can escape than in smooth 1-D models. The near- and mid-infrared SED of the
same object can vary significantly with viewing angle, depending on the clump
geometry along the sightline. Even the wavelength-integrated flux can vary with
angle by more than a factor of two. Objects with the same average circumstellar
dust distribution can have very different near-and mid-IR SEDs depending on the
clump geometry and the proximity of the most massive clump to the central
source.
Although clumpiness can cause similar objects to have very different SEDs,
there are some observable trends. Near- and mid-infrared colors are sensitive
to the weighted average distance of clumps from the central source and to the
magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared
emission remains a robust measure of the total dust mass. We present simulated
SEDs, colors, and images for 2MASS and Spitzer filters. We compare to
observations of some UCHII regions and find that 3-D clumpy models fit better
than smooth models. In particular, clumpy models with fractal dimensions in the
range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with
shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures:
http://www.astro.virginia.edu/~ri3e/clumpy3d.pd
Infrared Dust Emission in Starburst Galaxies: Self-Consistent Modeling From the UV to Far Infrared.
In the first part of this thesis, the ultraviolet extinction from dust in the Large Magellanic Cloud is re-analyzed. Differences in the extinction properties between different lines of sight, as well as between the LMC, SMC, and Milky Way are discussed in the context of their implications for the constitution and size distributions of the dust grains. The UV extinction curves derived in this work, along with those from other studies, have been used to constrain dust models used as inputs to dust heating models. Next, the DIRTY model, a Monte Carlo radiative transfer code from the ultraviolet to the far IR, is extended to self-consistently include dust heating and emission, fully accounting for the effects of the transient heating of small grains. The code is completely general; the density structure of the dust, heating sources, and their geometric configurations can be specified arbitrarily. The dust scattering, absorbing, and emitting properties are calculated from realistic dust models derived by fitting observed extinction curves. Dust self-absorption is also accounted for by treating photons emitted by the dust as an additional heating source and adopting an iterative radiative transfer scheme. The dependence of the UV-FIR SED, dust temperatures, and dust masses predicted by DIRTY on variations of the input parameters is examined. Finally, the DIRTY model is applied directly to a sample of nearby starburst galaxies. The UV to far IR SEDs of the galaxies are well reproduced using DIRTY, and quantitative information including star formation rates, dust masses, and dust temperatures are derived. The ability to accurately reproduce the full SED of starburst galaxies is discussed in the context of modeling galaxies at high redshift. The model developed in this thesis is well suited to simulate galaxies at different evolutionary stages and hence has promise for investigating the star formation history of the universe. However, it must be emphasized that its range of applicability is not limited to galaxies. It should prove a useful tool in future investigations of a wide range of astrophysical systems in which dust plays an important role
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
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