Dr Sarah E. I.
Bosman
Bosman
Published papers explained simply
New constraints on Lyman-α opacity with a sample of 62 quasars at z > 5.7
Bosman et al. 2018
We measured how in-homogeneous the last stages of reionisation were, and the answer was "definitely too much". None of the theoretical models we tested could match it. This changed in the following years!
A deep search for metals near redshift 7: the line of sight towards ULAS J1120+0641
Bosman et al. 2017
Quasars are very bright, so we can use them as flashlights - shining through the Universe between us and them. Their light gathers imprints along the way every time it passes through clouds of gas with metals in them (which, to astronomers, means any elements other than H, He and Li). Metals absorb specific frequencies of light and create metal absorbers whose exact shape depends on the amount of metal, its temperature, and its ionisation state.
This means we can use quasars to find out how the abundance of elements over the timescale of the Universe. The quasar J1120+0641 was the most distance quasar ever known for a long time at a redshift of z=7.085. The light from J1120 has traversed 94% of the observable Universe to reach us! This obviously makes it a great way to trace how the abundances of elements have changed over time.
Using a very high-quality, high-resolution spectrum of J1120 taken by the X-Shooter instrument on the Very Large Telescope, we measured the global densities of carbon and magnesium (or more specifically, their CII, CIV and MgII ions) for the first time at such great distances. The light of J1120 only passed through 9 clouds of enriched gas on the way to us as far as we could detect, so the measurements are not very precise - but you got to start somewhere!
Bosman & Becker 2015
Active supermassive black holes, known as quasars, are very useful to study the Universe in its young age because they are extremely bright. This means very detailed information on quasars, like their emission properties, metalicity, temperature, and so on, can be measured accurately even at redshift z>7, or more than 28 billion light-years away. Quasars are also very similar to each other and don't change much over the history of the Universe.
One thing we can use quasars for is to measure the fraction of neutral gas in their surroundings at the time they are observed. This is very important to find out about cosmic reionisation. Specifically, if the inter-galactic gas is more than 10% neutral, a feature called the Lyman-alpha emission line of quasars gets distorted and weakened. The quasar J1120+0641 was the first quasar ever found at z>7, and its Lyman-alpha line looked kind of weak - which gave rise to strong claims that reionisation had been observed directly.
However, knowing for sure whether the Lyman-alpha line is too weak requires assuming what it is intrinsically - quasars are very similar to each other, but not identical. We noticed that J1120+0641 had a weird feature: a very blueshifted CIV emission line. No-one knows exactly what this feature means, but it's rare enough that it was hard to find local quasars with the same feature. We dug in the SDSS catalogue, and found a few - and showed that these have intrinsically weaker Lyman-alpha lines than average. The difference was large enough that, compared to those quasars, the weakness of the Lyman-alpha line in J1120+0641 was not statistically significant anymore.
Some more clear-cut cases have emerged since this work, but we showed that the careful modelling of the intrinsic emission of quasars would be crucial to measure reionisation with this technique in the future.
More coming soon