Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Exploring the Thermal State of the Low-Density Intergalactic Medium at
  z=3 with an Ultra-High Signal-to-Noise QSO Spectrum   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. We further demonstrate
that this conclusion is nevertheless consistent with measurements of a slope of
the TDR in over-dense regions that imply a decreasing temperature with
decreasing density, as expected if photo-heating of ionised hydrogen is the
dominant heating process. We briefly discuss implications of our findings for
the need to invoke either spatial temperature fluctuations, as expected during
helium reionization, or additional processes that heat a significant volume
fraction of the low-density IGM.

8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
26
The categories of this paper are: bayes-an
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph
7
The categories of this paper are: hep-th
10
The categories of this paper are: q-bio
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
9
The categories of this paper are: nlin