GRB Afterglow Dust Extinction

Dust Extinction Curves
A powerful way of studying the physical dust properties within an environment is through the extinction of the dust along the observed line-of-sight as a function of wavelength. This is known as the environmental dust extinction curve, and is a function of both the dust grain size distribution as well as its composition along the line-of-sight. This is illustrated in Fig.1, where the left and right panels show the extinction curve produced from a population of purely silicate and graphite grains, respectively, with a 'normal' power-law grain size distribution (black), and a grain distribution skewed to large dust grains (gray). This latter distribution is also referred to as a 'gray' distribution due to the relatively weak dependence that the extinction law has on wavelength.

Figure 1: Left: Extinction curves for a population of silicate dust grains with a normal particle size distribution (black) with grain diameter ranging from a=0.005-0.25μm, and a gray distribution (or distribution skewed to large dust grains), with diameter ranging from a=0.005-10.0μm. The particle size distribution is of the form (a/a0)-3.5 (Mathis et al., 1977). Right: Same as left figure, but for a population of graphite dust grains. In both panels, the y-axis is the optical depth, τ, divided by the column density of dust, Σ. (Credit: A.C.Updike)

Dust extinction within the local Universe
The extinction curve along hundreds of lines of sight within the Milky Way have been studied, revealing a persistent dust extinction feature centred at ~2175Å along almost all Milky Way sight lines. The prominence of this feature and the slope of the underlying extinction curve is known to be a strong function of the total-to-selective extinction, RV=AV/E(B-V) for a total, visual extinction AV, and a Galactic reddening E(B-V) (the CCM relation). RV can also be thought of as a measure of the overall dust grain size, whereby larger RV signifies a flatter dust grain size distribution. There are, nevertheless, significant deviations from the CCM relation in different Galactic environments and the reason for these deviations, and thus the global dependencies on the physical dust properties within an environment remain elusive.

A greater variety of environmental conditions is probed by sight lines through the nearby Small and Large Magellanic Clouds (SMC and LMC respectively), where variations are evident, although the origin of the differences is not clear. The average Milky Way, SMC and LMC extinction curves are plotted in Fig.2 along with a number of other GRB-derived extinction laws and a theoretical supernova-dust extinction law. The variation in extinction curves in general between galaxies as well as within the same galaxy imply the physical properties of dust to be dependent on a number of environmental parameters, such as metallicity and star formation activity. However, a detailed investigating into the relation between dust properties and environmental factors has been hindered by the relatively narrow range of galaxy conditions within which the physics of dust has been investigated. To gain a solid understanding of the multi-phase ISM, it is important to study the dust and gas in the ISM of a larger and more diverse population of galaxies than are available within the Local Group of galaxies. GRBs have already proven to provide deep and accurate probes to the conditions of distant, star-forming galaxies, and much of the recent investigation into the physical dust properties of GRB host galaxies has been led by myself and members of the GRB group at MPE.

Figure 2: Average extinction curves from numerous lines of sight to the Small Magellanic Cloud (SMC; large-dash, green), the Large Magellanic Clouds (LMC; small-dash, blue), to the Milky Way (dots, pink), and to a sample of GRBs dot-dash, yellow; Schady et al. 2011). The figure also shows the extinction curves best-fit to the SED of GRB070802 (solid red; Eliasdottir et al. 2009), GRB080607 (blank-dash black; Perley et al. 2011) and the theoretical extinction law from supernova (SN) produced dust (dot-dot-dash orange; Todini & Ferrara, 2011.)

Dust extinction within the distant Universe
Direct measurements of the interstellar dust and other properties of the ISM in high redshift galaxies have focused on the study of either the whole galaxy, or within absorption systems along a single line-of-sight to distant QSOs, that hold little knowledge of the global properties of the intervening galaxy. The dust extinction imprint left on composite spectra of high-redshift galaxies is challenging to measure due to the difficulty in modelling the complex radiative transfer of stellar light that goes through numerous episodes of gas and dust absorption, emission and scattering. The resultant observed stellar attenuation is thus the galaxy's attenuation curve, which is distinct from its dust extinction curve, and is highly dependant on the geometric distribution of the dust, gas and stars. A similar problem arises when studying the emission properties of dust, where assumptions also have to be made on the distribution and composition of the dust and the galaxy stellar population.

Figure 3: The distribution of measured AV from the GROND afterglow sample with redshift as compared to that of an optically-bright biased sample. The dashed line is the theoretical distribution (normalised to the same number of objects in the sample) of a Monte Carlo simulation of random sightlines through an evolving galaxy model. (Credit: A.C.Updike)

Extracting information on the physical dust properties of intervening systems in the point-like line-of-sight to QSOs on the other hand, suffers from the relatively complex spectral shape of QSOs, which makes distinguishing dust from intrinsic colour variation hugely challenging, and from the typically small amounts of dust that exist in the QSO-DLAs. As with high-z interstellar gas, the deep and point-like line-of-sight to the GRB afterglow, their intrinsically simple spectra, and transient nature, provide an excellent and unique opportunity to study the extinction properties of interstellar dust for a statistically meaningful sample of sightlines on a cosmological scale. Around 50% of afterglows show negligible evidence for dust extinction. However, the distribution has a long tail that extends to significant levels of dust extinction (see Fig.3), and which is becoming increasingly populated with the continual observations of GRB afterglows at less attenuated near-IR wavelengths. I am interested in continuing the investigation in this area, using a large multi-wavelength dataset of GRB afterglows, in particular GROND data, to study the largest sample of GRB host galaxy extinction curves in unprecedented detail.