Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: The cosmic atomic hydrogen mass density as a function of mass and galaxy
  hierarchy from spectral stacking
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
1
The categories of this paper are: cs
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
3
The categories of this paper are: physics
34
The categories of this paper are: chem-ph
2
The categories of this paper are: math
17
The categories of this paper are: q-fin
21
The categories of this paper are: adap-org