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The Extragalactic Interstellar Medium - Measuring Dust Extinction using Gravitational Lensing

Final Report Summary - DUST-ISM/ (The Extragalactic Interstellar Medium - Measuring Dust Extinction using Gravitational Lensing)

The goals of the project were to study dust extinction in galaxies outside the local group. Traditional methods of comparing two stars of the same spectral type are not applicable, as individual stars can not be resolved. Therefore, this project used an alternative method which consists of comparing the multiple images of the same background source shining through a strong lensing galaxy. As the lensing effect which splits the background image into the multiple images is in its nature an achromatic effect, the colour difference between the images is a measure of the dust extinction.

Project objectives:

- Quantify the effects of systematics and the scatter in the extinction of individual galaxies on mean extinction properties
- Develop the technique to be applicable to finite source sizes, i.e. galaxy-galaxy lenses in addition to quasar-galaxy lenses
- Measure the extinction properties in recent pre-existing lens surveys
- Determine the prevalence of Milky Way (MW) type of dust.

Work performed since beginning of the project:

The first objective was addressed with simulations. The results strongly support that an average extinction law can be deduced from the data despite extinction along both lines of sight. For the microlensing, more limited simulations were performed as discussions with a visiting researcher at Princeton (Prof. Wambsganss) led to the conclusion that microlensing would prove a much more limited problem.

The second objective was the most risky part of the project and it did run into some stumbling blocks. It was concluded that this method is more suitable for a detailed study of a single system but less useful for finding a statistical extinction curve to be of use in a cosmological context.

The third objective was put on hold as it relied on the second objective being carried out. Instead the researcher broadened the method to apply it to quasars in pre-existing surveys (SDSS) and gamma ray bursts (GRBs) to address the fourth objective. It was found that around 30 % of the SDSS quasars showed MW type of dust, although both the quasar and the GRB sample show that the characteristic bump of the MW extinction curve is weaker in the high redshift systems.

Main results:

The effect of dust extinction along both sightlines was quantified. The effect of chromatic microlensing was found not to be a concern for the goals of this project. A paper on the highest redshift detection of a Galactic type of reddening was published (in a GRB host galaxy, Elíasdóttir et al., 2009). The study of the SDSS quasars indicates that 30 % of high redshift galaxies show signs of MW type of dust. Preliminary results indicate that this ratio depends somewhat on redshift although not very strongly. The extinction curve of GRB 071025 is not compatible with MW type of dust. A study of the properties of the 2175 Angstrom bump in GRBs (Zafar, Watson, Elíasdóttir et al., 2011, submitted), shows that the bump is less prominent than for the MW. A code was written to fit such extinction curves to the dust models of Draine et al. to compare the dust composition of the extragalactic galaxies to that of the MW.

Final results and their potential impact and use:

The final results were expected to be statistically significant average extinction curves for high redshift galaxies. As the use of galaxy-galaxy lenses was found to be limited, a greater focus was put on the simulated curves, setting them up for dealing with the huge datasets of missions like LSST which will in the next years provide thousands of quasar-galaxy systems. In addition, the study was expanded to include quasars and GRBs. The results showed that high redshift galaxies have dust properties which differ in significant way from that of the Milky Way. This has impact on studies of evolution of galaxies, where dust plays a significant role, and on cosmological studies, such as the supernova dark energy surveys, where dust is a significant source of a systematic error.