A precise measure of the mid-infrared interstellar extinction law is crucial to the investigation of the properties of interstellar dust, especially of the grains in the large size end. Based on the stellar parameters derived from the SDSS-III/APOGEE spectroscopic survey, we select a large sample of G-and K-type giants as the tracers of the Galactic mid-infrared extinction. We calculate the intrinsic stellar color excesses from the stellar effective temperatures and use them to determine the mid-infrared extinction for a given line of sight. For the entire sky of the Milky Way surveyed by APOGEE, we derive the extinction (relative to A K S , the K S band extinction at wavelength λ = 2.16 µm) for the four WISE bands at 3.4, 4.6, 12 and 22 µm, the four Spitzer /IRAC bands at 3.6, 4.5, 5.8 and 8 µm, the Spitzer /MIPS24 band at 23.7 µm and for the first time, the AKARI /S9W band at 8.23 µm. Our results agree with previous works in that the extinction curve is flat in the ∼3-8 µm wavelength range and is generally consistent with the R V = 5.5 model curve except our determination exceeds the model prediction in the WISE /W4 band. Although some previous works found that the mid-IR extinction law appears to vary with the extinction depth A K S , no noticeable variation has been found in this work. The uncertainties are analyzed in terms of the bootstrap resampling method and Monte-Carlo simulation and are found to be rather small.

We present a new method to decompose the emission features of polycyclic aromatic hydrocarbons (PAHs) from mid-infrared spectra using theoretical PAH templates in conjunction with modified blackbody components for the dust continuum and an extinction term. The primary goal is to obtain robust measurements of the PAH features, which are sensitive to the star formation rate, in a variety of extragalactic environments. We demonstrate the effectiveness of our technique, starting with the simplest Galactic high-latitude clouds to extragalactic systems of ever-increasing complexity, from normal star-forming galaxies to low-luminosity active galaxies, quasars, and heavily obscured infrared-luminous galaxies. In addition to providing accurate measurements of the PAH emission, including the upper limits thereof, our fits can reproduce reasonably well the overall continuum shape and constrain the line-of-sight extinction. Our new PAH line flux measurements differ systematically and significantly from those of previous methods by ∼ 15% to as much as a factor of ∼ 6. The decomposed PAH spectra show remarkable similarity among different systems, suggesting a uniform set of conditions responsible for their excitation.

Type Ia supernovae (SNe Ia) are powerful cosmological "standardizable candles" and the most precise distance indicators. However, a limiting factor in their use for precision cosmology rests on our ability to correct for the dust extinction toward them. SN 2014J in the starburst galaxy M82, the closest detected SN Ia in three decades, provides unparalleled opportunities to study the dust extinction toward an SN Ia. In order to derive the extinction as a function of wavelength, we model the color excesses toward SN 2014J, which are observationally derived over a wide wavelength range in terms of dust models consisting of a mixture of silicate and graphite. The resulting extinction laws steeply rise toward the far ultraviolet, even steeper than that of the Small Magellanic Cloud (SMC). We infer a visual extinction of A V ≈ 1.9 mag, a reddening of E(B − V ) ≈ 1.1 mag, and a totalto-selective extinction ratio of R V ≈ 1.7, consistent with that previously derived from photometric, spectroscopic, and polarimetric observations. The size distributions of the dust in the interstellar medium toward SN 2014J are skewed toward substantially smaller grains than that of the Milky Way and the SMC.

Based on the photometric data from the Spitzer /SAGE survey and with red giants as the extinction tracers, the mid-infrared (MIR) extinction laws in the Large Magellanic Cloud (LMC) are derived for the first time in the form of A λ /A K S , the extinction in the four IRAC bands (i.e., [3.6], [4.5], [5.8] and [8.0] µm) relative to the 2MASS K S band at 2.16 µm. We obtain the near-infrared (NIR) extinction coefficient to beExcept for the extinction in the IRAC [4.5] band which may be contaminated by the 4.6 µm CO gas absorption of red giants (which are used to trace the LMC extinction), the extinction in the other three IRAC bands show a flat curve, close to the Milky Way R V = 5.5 model extinction curve (where R V is the optical total-to-selective extinction ratio). The possible systematic bias caused by the correlated uncertainties of K S − λ and J − K S is explored in terms of Monte-Carlo simulations. It is found that this could lead to an overestimation of A λ /A K S in the MIR.

The Spitzer /Infrared Spectrograph spectra of three spectroscopically anomalous galaxies (IRAS F10398+1455, IRAS F21013-0739 and SDSS J0808+3948) are modeled in terms of a mixture of warm and cold silicate dust, and warm and cold carbon dust. Their unique infrared (IR) emission spectra are characterized by a steep ∼ 5-8 µm emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent silicate emission. The steep ∼ 5-8 µm emission continuum and strong PAH emission features suggest the dominance of starbursts, while the silicate emission is indicative of significant heating from active galactic nuclei (AGNs). With warm and cold silicate dust of various compositions ("astronomical silicate," amorphous olivine, or amorphous pyroxene) combined with warm and cold carbon dust (amorphous carbon, or graphite), we are able to closely reproduce the observed IR emission of these galaxies. We find that the dust temperature is the primary cause in regulating the steep ∼5-8 µm continuum and silicate emission, insensitive to the exact silicate or carbon dust mineralogy and grain size a as long as a 1 µm. More specifically, the temperature of the ∼ 5-8 µm continuum emitter (which is essentially carbon dust) of these galaxies is ∼250-400 K, much lower than that of typical quasars which is ∼640 K. Moreover, it appears that larger dust grains are preferred in quasars. The lower dust temperature and smaller grain sizes inferred for these three galaxies compared with that of quasars could be due to the fact that they may harbor a young/weak AGN which is not maturely developed yet.