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_lambda/A_Ks, the exti
nction in the four IRAC bands (i.e., [3.6], [4.5], [5.8] and [8.0]um) relative to the 2MASS Ks band at 2.16um. We obtain the near-infrared (NIR) extinction coefficient to be E(J-H)/E(H-Ks)=1.29pm0.04 and E(J-Ks)/E(H-Ks)=1.94pm0.04. The wavelength dependence of the MIR extinction A_lambda/A_Ks in the LMC varies from one sightline to another. The overall mean MIR extinction is A_[3.6]/A_Ks=0.72pm0.03, A_[4.5]/A_Ks=0.94pm0.03, A_[5.8]/A_Ks=0.58pm0.04, and A_[8.0]/A_Ks=0.62pm0.05. Except for the extinction in the IRAC [4.5] band which may be contaminated by the 4.6um 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 Rv = 5.5 model extinction curve (where Rv is the optical total-to-selective extinction ratio). The possible systematic bias caused by the correlated uncertainties of Ks-lambda and J-Ks is explored in terms of Monte-Carlo simulations. It is found that this could lead to an overestimation of A_lambda/A_Ks in the MIR.
We report the detection and successful modeling of the unusual 9.7mum Si--O stretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature
in the central 230 pc of M81 to be in strong emission, with a peak at ~10.5mum. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and graphite. In addition to silicate dust, there is weak PAH emission present (particularly at 11.3mum, arising from the C--H out-of-plane bending vibration of relatively large PAHs of ~500--1000 C atoms) whose character reflects the low-excitation AGN environment, with some evidence that small PAHs of ~100--200 C atoms (responsible for the 7.7mum C--C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. (abstract continues)
The dust extinction of gamma-ray bursts (GRBs) host galaxies, containing important clues to the nature of GRB progenitors and crucial for dereddening, is still poorly known. Here we propose a straightforward method to determine the extinction of GRB
host galaxies by comparing the observed optical spectra to the intrinsic ones extrapolated from the X-ray spectra. The rationale for this method is from the standard fireball model: if the optical flux decay index equals to that of the X-ray flux, then there is no break frequency between the optical and X-ray bands, therefore we can derive the intrinsic optical flux from the X-ray spectra. We apply this method to three GRBs of which the optical and X-ray fluxes have the same decay indices and another one with inferred cooling break frequency, and obtain the rest-frame extinction curves of their host galaxies. The derived extinction curves are gray and do not resemble any extinction curves of local galaxies (e.g. the Milk Way, the Small/Large Magellanic Clouds, or nearby starburst galaxies). The amount of extinction is rather large (with visual extinction $A_V$ $sim$ 1.6--3.4$magni$). We model the derived extinction curves in terms of the silicate-graphite interstellar grain model. As expected from the ``gray nature of the derived extinction curve, the dust size distribution is skewed to large grains. We determine, for the first time, the local dust-to-gas ratios of GRB host galaxies using the model-derived dust parameters and the hydrogen column densities determined from X-ray absorptions.
