Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyman
  Alpha Forest Absorbers   Probing the size and shape of the absorbers that lie in front of the QSOs, in
particular the Ly alpha forest, we analyze 785 absorption lines in spectra of
five QSOs in close groupings: a pair and a triplet. Both QSO groups have been
observed before, but these data are a drastic improvement over earlier data,
and provide a qualitatively different view of the nature of the absorbers. The
pair samples a scale critical in determining the size upper bound of Ly alpha
absorbers, with significant leverage in redshift compared to previous studies.
The triplet data represents the spatially densest sample of Ly alpha forest
ever studied, and an almost ideally-suited probe of the shape of absorbers. We
observe a significant number of Ly alpha lines in common between the triplet
sightlines, for lines stronger than rest equivalent width W > 0.4A (and no
detected metal lines) and velocity differences up to 200 km/s, corresponding to
a two-point correlation function xi = 1.88 +0.78/-0.50 on scales 0.5 to 0.8
Mpc/h with < z > = 2.14, and inconsistent at the 99.999% level with the absence
of any clustering. These data also show that a significant fraction of the W >
0.4A Ly alpha forest absorbers span all three sightlines to the triplet,
indicating that the strong-lined absorbers are consistent with nearly round
shapes, chosen from a range of possible cylinders of different elongations.
This may be inconsistent with results from hydrodynamic/gravitational
simulations of H I in the early Universe indicating that the theoretical
counterparts of Ly alpha forest clouds are long and filamentary. We conclude,
tentatively, that the W > 0.4A Ly alpha forest objects are sheetlike. This is
supported by the uniformity of linestrengths between the three sightlines, for
W > 0.4A. (Abstract continues.)

34
The categories of this paper are: chem-ph
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
21
The categories of this paper are: adap-org
32
The categories of this paper are: atom-ph
13
The categories of this paper are: quant-ph
20
The categories of this paper are: acc-phys
17
The categories of this paper are: q-fin