Exploring the Thermal State of the Low-Density Intergalactic Medium at z=3 with an Ultra-High Signal-to-Noise QSO Spectrum

Understand

At low densities the standard ionisation history of the intergalactic medium (IGM) predicts a decreasing temperature of the IGM with decreasing density once hydrogen (and helium) reionisation is complete.

• Heating the high-redshift, low-density IGM above the temperature expected from photo-heating is difficult, and previous claims of high/rising temperatures in low density regions of the Universe based on the probability density function (PDF) of the opacity in Lyman-$\alpha$ forest data at $2<z<4$ have been met with considerable scepticism, particularly since they appear to be in tension with other constraints on the temperature-density relation (TDR).
• We utilize here an ultra-high signal-to-noise spectrum of the QSO HE0940-1050 and a novel technique to study the low opacity part of the PDF.
• We show that there is indeed evidence (at 90% confidence level) that a significant volume fraction of the under-dense regions at $z \sim 3$ has temperatures as high or higher than those at densities comparable to the mean and above.