near-IR Extinction Research Articles

Nanoparticle Spectroscopy: Plasmon Coupling in Finite-Sized Two-Dimensional Arrays of Cylindrical Silver Nanoparticles

This study provides new data for interpreting plasmon coupling in finite two-dimensional arrays, which are important in plasmonic devices and sensing applications. Finite square arrays of cylindrical silver nanoparticles with two different nanoparticle diameters (339 and 163 nm) and two different interparticle spacings (400 and 450 nm) were fabricated by e-beam lithography. The plasmonic properties of these arrays were obtained by UV−visible−near-IR extinction spectroscopy and compared with isolated particle spectra. In the case of the large-diameter particle arrays, the semi-infinite array resonance is quite blue-shifted from the isolated particle resonance; the resonant wavelength of the 2 × 2 particles and successively larger blocks keeps blue-shifting relative to the isolated particle resonance and converges to that of a semi-infinite array. For the small-diameter particle arrays, the semi-infinite resonance is red-shifted from the isolated particle resonance; the resonant wavelength of the 2 × 2 particles and successively larger blocks keeps red-shifting and converges to the semi-infinite array. The experimental resonances for progressively increasing array sizes show systematic resonance shifts consistent with increasing numbers of particles, and the bandwidth trends require models and more experiments to obtain better insight into trends.

The Nature of the Dense Core Population in the Pipe Nebula: Core and Cloud Kinematics from C18O Observations

We present molecular line observations of 94 dark cloud cores identified in the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio Observatory 12 m telescope, we obtained spectra of these cores in the J = 1–0 transition of C18O. We used the measured core parameters, TR*, Δ v, vlsr, radius, and mass, to explore the internal kinematics of the cores, as well as their radial motions through the larger molecular cloud. We find that the vast majority of the dark extinction cores are true cloud cores, rather than the superposition of unrelated filaments. While we identify no significant correlations between the cores' internal gas motions and their other physical parameters, we identify spatially correlated radial velocity variations that outline two main kinematic components of the cloud. The largest is a 15 pc long filament that is surprisingly narrow both in spatial dimensions and in radial velocity. Beginning in the Stem of the Pipe, this filament displays uniformly small C18O line widths (Δ v ~ 0.4 km s−1), as well as core-to-core motions only slightly in excess of the gas sound speed. The second component outlines what appears to be part of a large (2 pc; 103 M☉) ringlike structure. Cores associated with this component display both larger line widths and core-to-core motions than cores in the main cloud. The Pipe molecular ring may represent a primordial structure related to the formation of this cloud.

Pixie Dust: The Silicate Features in the Diffuse Interstellar Medium

We have analyzed the 9.7 and ``18'' micron interstellar silicate absorption features along the line of sight toward four heavily extincted galactic WC-type Wolf-Rayet (WR) stars. We construct two interstellar extinction curves from 1.25 to 25 micron using near-IR extinction measurements from the literature along with the silicate profiles of WR 98a (representing the local ISM) and GCS 3 (representing the Galactic Center). We have investigated the mineralogy of the interstellar silicates by comparing extinction profiles for amorphous silicates with olivine and pyroxene stochiometry to the 9.7 and ``18'' micron absorption features in the WR 98a spectrum. In this analysis, we have considered solid and porous spheres and a continuous distribution of ellipsoids. While it is not possible to simultaneously provide a perfect match to both profiles, we find the best match requires a mixture of these two types of compounds. We also consider iron oxides, aluminosilicates and silicate carbide (SiC) as grain components. Iron oxides cannot be accommodated in the observed spectrum, while the amount of Si in SiC is limited to <4%. Finally, we discuss the cosmic elemental abundance constraints on the silicate mineralogy, grain shape and porosity.

A model for the infrared dust emission from forming galaxies

Dust plays various important roles in galaxy formation and evolution. In the early epoch of galaxy evolution, dust is only supplied by supernovae (SNe). With the aid of a new physical model of dust production by SNe developed by Nozawa et al. (N03), we constructed a model of dust emission from forming galaxies on the basis of the theoretical framework of Takeuchi et al. (T03). N03 showed that the produced dust species depends strongly on the mixing within SNe. We treated both unmixed and mixed cases and calculated the infrared (IR) spectral energy distribution (SED) of forming galaxies for both cases. Our model SED is less luminous than the SED of T03 model by a factor of 2–3. The difference is due to our improved treatment of the ultraviolet (UV) photon absorption cross-section, as well as the different grain size and species newly adopted in this work. The SED for the unmixed case is found to have an enhanced near to mid-IR (N-MIR) continuum radiation in its early phase of the evolution (age ≲ 107.25 yr) compared with that for the mixed case. The strong N-MIR continuum is due to the emission from silicon grains, which only exist in the species of the unmixed dust production. We also calculated the IR extinction curves for forming galaxies. N03 dust was found to yield a smaller extinction than that of T03 model. For the unmixed case, near-IR (NIR) extinction is dominated by large grains of silicon and amorphous carbon, and silicate features are less prominent compared to the curve given by T03. To the contrary, the extinction curve of the mixed case has a similar shape to that of T03. Then we calculated the SED of a local starbursting dwarf galaxy SBS 0335−052. Our present model SED naturally reproduced the strong N-MIR continuum and the lack of cold far-IR emission of SBS 0335−052. We found that only the SED of unmixed case can reproduce the NIR continuum of this galaxy. We then made a prediction for the SED of another typical star-forming dwarf, I Zw 18. The MIR continuum of I Zw 18 is expected to be much weaker than that of T03 SED. We also presented the evolution of the SED of Lyman-break galaxies. Finally, we discussed the possibility of observing forming galaxies at z≳ 5.

The ISOGAL field FC–01863+00035: Mid-IR interstellar extinction and stellar populations

A 0.35° 0.29°field centered at °, ° was observed during the ISOGAL survey by ISOCAM imaging at 7 μm and 15 μm. 648 objects were detected and their brightness are measured. By combining with the DENIS data in the near-infrared J and KS bands, one derives the extinction at 7 μm through which yields from the near-IR extinction values of van de Hulst–Glass (Glass 1999). The extinction structure along the line of sight is then determined from the values of or KS–[7] of the ISOGAL sources identified as RGB or early AGB stars with mild mass-loss. The distribution of AV ranges from 0 to ~45 and it reflects the concentration of the extinction in the spiral arms. Based on their locations in color-magnitude diagrams and a few cross-identifications with IRAS and MSX sources, the nature of objects is discussed in comparison with the case of a low extinction field in Baade's Window. Most of the objects are either AGB stars with moderate mass loss rate or luminous RGB stars. Some of them may be AGB stars with high mass loss rate. In addition, a few young stellar objects (YSOs) are present.

The Extinction Distribution in the Galaxy UGC 5041

We probe the dust extinction through the foreground disk of the overlapping galaxy pair UGC 5041 by analyzing B-, I-, and H-band images. The inclined foreground disk of this infrared-selected pair is almost opaque in B at a projected distance of ~8 kpc. From the images, we directly estimate the area-weighted distribution of differential near-IR extinction: it is nearly Gaussian, with τI - τH = 0.6 and σ = 0.27. For a homogenous dust distribution and a Milky Way extinction curve, this corresponds to a face-on distribution p(τ) with a mean of τV = 0.34 and σV = 0.15. For a clumpy dust model, the optical depth estimate increases to τV = 0.41 and σV = 0.19. Even though the galaxy pair is subject to different selection biases and our analysis is subject to different systematics, the result is consistent with existing case studies, indicating that τV ~ 0.3 is generic for late-type spirals near their half-light radii. We outline how to estimate from p(τ) by how much background quasars are underrepresented, where projected within ~10 kpc of nearby spirals, such as damped Lyα absorbers or gravitational lenses; from our data we derive a factor of 2 deficit for flux-limited, optical surveys.

The Near-Infrared Extinction Law and Limits on the Pre–Main-Sequence Population of the ρ Ophiuchi Dark Cloud

We describe new techniques to measure the near-infrared extinction law and to place limits on the pre–main-sequence stellar population of a dark cloud. We analyze JHK imaging data for the central 1 deg2 of the ρ Ophiuchi cloud core and show that nearly all stars projected onto regions of low CS intensity, ICS ≤ 10 K km s-1, are background stars. Most sources at larger CS intensities lie within cloud material. We use the background stars to derive the slope of the near-IR extinction law, EJ - H/EH - K = 1.57 ± 0.03. This result is consistent with previous extinction laws but has a factor of 2–3 smaller uncertainty. The new ρ Oph extinction law yields strong constraints on the number of previously undiscovered pre–main-sequence stars in the cloud, 46 ± 11, and the number of previously undiscovered young stars with near-IR excesses, 15 ± 4. Neither limit exceeds the number of known pre–main-sequence stars in the cloud, ~100. Thus, current samples of pre–main-sequence stars are reasonably complete for K ≤ 14.

K'-band observations of the evil eye galaxy: Are the optical and near-infrared dust albedos identical?

New measurements of the reduction of the V-band surface brightness across the prominent dust feature in the galaxy NGC 4826 are compared with corresponding increases in the V-K' color within the context of radiative transfer models invoking both absorption and scattering. The K'-band surface brightness is found to be higher than expected from standard dust models. We interpret the difference as resulting from a high effective dust albedo at K', with a likely value in excess of 0.8, provided the near-IR extinction curve in NGC 4826 is identical to the Galactic one. The high effective albedo may result from scattering by dust with a maximum grain size at least twice as large as assumed by standard models, a conclusion already indirectly hinted at by recent studies of dust star-forming regions and reflection nebulae. At least part of the high effective albedo at K' may result from near-IR nonequilibrium continuum emission attributable to very small grains.

Wavelength scaling of atmospheric aerosol scattering and extinction

Several scaling laws have been developed relating hypothetical measurements of the scattering or extinction of the atmospheric aerosols to the scattering or extinction properties of the aerosols at other wavelengths. These scaling laws were tested in a series of numerical simulations using a number of representatives models of the atmospheric aerosols and calculations of their expected scattering and extinction properties. These simulations showed that forward scattering at fixed angles for an ultraviolet wavelength could provide useful predictions of near-IR extinction. A good correlation is also presented between forward scattering for a wavelength of 0.25 microm and backscattering at a wavelength of 1.06 microm. A comparison of 10.6microm extinction with backscattering at 10.6 microm and extinction at a wavelength of 1.06 microm showed too much scatter to develop useful scaling laws in those case.