Effects Of Dust Absorption Research Articles

Dust and gas absorption in the high mass X-ray binary IGR J16318−4848

Context. With an absorption column density on the order of 10²⁴ cm⁻², IGR J16318−4848 is one of the most extreme cases of a highly obscured high mass X-ray binary. In addition to the overall continuum absorption, the source spectrum exhibits a strong iron and nickel fluorescence line complex at 6.4 keV. Previous empirical modeling of these features and comparison with radiative transfer simulations raised questions about the structure and covering fraction of the absorber and the profile of the fluorescence lines. Aims. We aim at a self-consistent description of the continuum absorption, the absorption edges, and the fluorescence lines to constrain the properties of the absorbing material, such as ionization structure and geometry. We further investigate the effects of dust absorption on the observed spectra and the possibility of fluorescence emission from dust grains. Methods. We used XMM-Newton and NuSTAR spectra to first empirically constrain the incident continuum and fluorescence lines. Next we used XSTAR to construct a customized photoionization model where we vary the ionization parameter, column density, and covering fraction. In the third step we modeled the absorption and fluorescence in a dusty olivine absorber and employed both a simple analytical model for the fluorescence line emission and a Monte Carlo simulation of radiative transfer that generates line fluxes, which are very close to the observational data. Results. Our empirical spectral modeling is in agreement with previous works. Our second model, the single gas absorber does not describe the observational data. In particular, irrespective of the ionization state or column density of the absorber, a much higher covering fraction than previously estimated is needed to produce the strong fluorescence lines and the large continuum absorption. A dusty, spherical absorber (modeled as consisting of olivine dust, although the nature of dust cannot be constrained) is able to produce the observed continuum absorption and edges. Conclusions. A dense, dusty absorber in the direct vicinity of the source consisting of dust offers a consistent description of both the strong continuum absorption and the strong emission features in the X-ray spectrum of IGR J16318−4848. In particular, for low optical depth of individual grains, which is the case for typical volume densities and grain size distribution models, the dust will contribute significantly to the fluorescence emission.

Effect of dust absorption on the electron avalanche process occurring within Martian dust storms

Martian dust devils and storms are believed to generate and maintain large‐scale E‐fields which may be strong enough to create a weakly ionized plasma in the low pressure CO2 atmosphere. Previous studies of the electron development included a determination of the avalanching electron growth via electron impact ionization with CO2 at electron energy levels above 14 eV. Townsend's first coefficient defining the path length for impact ionization was calculated as a function of driving electric field. Such calculations demonstrated the growth of the electron avalanche but did not include any electron loss processes. Herein, we consider the effect of such a loss process: electron absorption by dust grains. The inclusion of electron‐dust absorption has a moderating effect, controlling electron avalanche growth. A critical E‐field to initiate the avalanche is derivable, with fields below ∼7 kV/m quenching the process but above 7 kV/m fully capable of stimulating electron growth in the storm.

Dust Absorption Along the Line of Sight for High‐Redshift Objects

We estimate the optical depth distribution of dust present in absorption systems along the line of sight of high-redshift galaxies and the resulting reddening. We characterize the probability distribution of the transmission to a given redshift and the shape of the effective mean extinction law by means of analytical estimates and Monte Carlo simulations. We present our results in a format useful for applications to present samples of high-redshift galaxies and discuss the implications for observations with the James Webb Space Telescope. Our most realistic model takes into account the metallicity evolution of damped Lyα absorbers and predicts that the effects of dust absorption are modest: at redshift z 5 the transmission is above 0.8 at an emitted wavelength λe = 0.14 μm with probability 90%. Therefore, dust obscuration along the line of sight will affect only marginally observations at very high redshift.

Optical Identification of the ASCA Medium Sensitivity Survey in the Northern Sky: Nature of Hard X‐Ray–Selected Luminous Active Galactic Nuclei

We present the results of optical spectroscopic identifications of a bright subsample of 2-10 keV hard X-ray-selected sources from the ASCA Medium Sensitivity Survey in the northern sky (AMSSn). The flux limit of the subsample is 3 × 10-13 erg s-1 cm-2 in the 2-10 keV band. All but one of the 87 hard X-ray-selected sources are optically identified, with AGNs (including broad-line AGNs, narrow-line AGNs, and three BL Lac objects), seven clusters of galaxies, and one galactic star. It is the largest complete sample of hard X-ray-selected AGNs at the bright flux limit. Amounts of absorption to their nuclei are estimated to be hydrogen column densities (NH) of up to ~ 3 × 1023 cm-2 from their X-ray spectra. Optical properties of X-ray absorbed AGNs with NH > 1 × 1022 cm-2 indicate the effects of dust absorption: at redshifts z 0.6, the X-ray absorbed AGNs have a large hard X-ray to optical flux ratio (log f2-10 keV/fR > +1). However, three X-ray absorbed z > 0.6 AGNs show strong broad lines. In combination with hard X-ray-selected AGN samples from the ASCA Large Sky Survey, the ASCA Deep Survey in the Lockman Hole, and Chandra Deep Field North, the luminosity distributions of absorbed (NH > 1 × 1022 cm-2) and less-absorbed (NH < 1 × 1022 cm-2) AGNs are compared.

Optical and Near‐Infrared Structural Properties of Cluster Galaxies atz∼ 0.3

Structural parameters (half-light radius re, mean effective surface brightness μe, and Sersic index n, parameterizing the light profile shape) are derived for a sample of galaxies in the rich cluster AC 118 at z = 0.31, so far the largest (N = 93) sample of galaxies at intermediate redshift with structural parameters measured in the near-infrared. The parameters are obtained in two optical wavebands (V and R) and in the K band, corresponding approximately to the B, V, and H rest frame. The distributions of re at z = 0.31 match those for the Coma Cluster (i.e., for the local universe) both in the optical and in the NIR. The K-band distribution is of particular interest, since the NIR light mimics the mass distribution of galaxies. The similarity of the distributions for the two clusters (AC 118 and Coma) proves that the galaxies at the bright end of the luminosity function did not significantly change their sizes since z ~ 0.3 to the present epoch. The ratio of the optical to the NIR half-light radius shows a marked trend with the shape of the light profile (Sersic index n). In galaxies with n 4 (typical bright ellipticals) re,NIR ~ 0.6re,opt, while the average ratio is 0.8 for galaxies with lower n (typical disk systems). Moreover, the NIR Sersic index is systematically larger than in the optical for n 4. These results, translated into optical and optical-NIR color gradients, imply that the optical color gradients at z ~ 0.3 are similar to those of nearby galaxies. The optical-NIR color gradients are in the average larger, ranging from -0.73 mag dex-1 for n 4 to -0.35 mag dex-1 for n 4. Models with pure or pure gradients are unable to reconcile our color gradients estimates with observations at z ~ 0, but we argue that the combined effects of age and metallicity might explain consistently the observed data: passive evolution (plus the possible effect of dust absorption) may account for the differences between the optical and NIR structural properties. The lack of any major change in re,NIR since z ~ 0.3 suggests that merging involving bright galaxies did not play a significant role in the last ~4.4 Gyr (ΩM = 0.3, ΩΛ = 0, H0 = 50 km s-1 Mpc-1). The results of the present paper will be applied to the study of the scaling laws in subsequent works.

A Very Deep Spectrum of the Diffuse Ionized Gas in NGC 891

A deep long-slit spectrum of the diffuse ionized gas (DIG) in the edge-on spiral galaxy NGC 891 has been obtained. The slit crosses the plane at a galactocentric radius of 5 kpc and covers DIG on both the east and west sides of the galaxy. The primary motivation was to attempt to detect the recombination line He I λ5876, the strength of which relative to Hα provides a direct constraint on the hardness of the ionizing spectrum. In the DIG of the Milky Way, this line ratio has turned out to be surprisingly low (Reynolds & Tufte; Heiles et al.), implying an ionizing spectrum much softer than had been inferred from the more readily observable forbidden lines, such as [N II] λ6583 and [S II] λ6717. The He I line has been detected in NGC 891 to a height of about 1.5 kpc from the plane—well into the diffuse gas layer. In the DIG, He I/Hα ≈ 0.034, implying that helium is about 70% ionized and that the ionizing spectrum is significantly harder than in the Reynolds layer. The spectrum also allows very detailed mapping of the [N II] λ6583/Hα ratio as a function of height off the plane. This ratio reaches peak values of about 1.4 at heights of z = 2 kpc on the east side of the plane and z = 4 kpc on the west side. Beyond this height on the east side, the ratio clearly declines again, falling to about 1.0 at z = 4 kpc. Previous modeling indicates that such high values of [N II] λ6583/Hα require an ionizing spectrum significantly harder than that indicated by the He I/Hα ratio. Hence, despite the higher values of He I/Hα, the same dilemma exists as in the Reynolds layer case. These results suggest that we do not understand the heating and ionization of the diffuse gas well enough. A determination of the gas temperature would help, but the nondetection of the line [N II] λ5755 allows only upper limits to be set: 13,000 K on the east side and 10,000 K on the west side. Both the [N II] and Hα lines are detected beyond z = 5 kpc; thus, the DIG layer extends much further than indicated by previous narrowband images. A model of the electron density distribution consisting of two components with scale heights of about 1 kpc and 5-6 kpc provides a good fit to the Hα emission profile. The very extended component was also seen in the image of Rand, Kulkarni, & Hester, but the inferred scale height is now larger. Apart from the known effects of dust absorption on the velocity centroids at low z, there is a smooth gradient in velocity centroids with increasing z above the dust lane in the sense that they become closer to the systemic velocity, changing by about 30 km s-1 from z = 1 kpc to z = 4.5 kpc. We postulate that the effect is due in part to a decreasing rotation speed and an outward radial migration of gas with z. An illustrative model suggests that the rotation speed at z = 4.5 kpc may be about 20 km s-1 slower than in the disk, although other effects could change this estimate. Such a change in rotation velocity is expected in galactic fountain models, but some simple experiments with stellar orbits show that the flow in a fountain may be more complicated than previously thought.