Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cosmic ray propagation and star formation history of NGC 1961   We present new radio continuum data at 4 frequencies on the supermassive,
peculiar galaxy NGC 1961. These observations allow us to separate the thermal
and the nonthermal radio emission and to determine the nonthermal spectral
index distribution. This spectral index distribution in the galactic disk is
unusual: at the maxima of the radio emission the synchrotron spectrum is very
steep, indicating aged cosmic ray electrons. Away from the maxima the spectrum
is much flatter. The steep spectrum of the synchrotron emission at the maxima
indicates that a strong decline of the star formation rate has taken place at
these sites. The extended radio emission is a sign of recent cosmic ray
acceleration, probably by recent star formation. We suggest that a violent
event in the past, most likely a merger or a collision with an intergalactic
gas cloud, has caused the various unusual features of the galaxy.

10
The categories of this paper are: q-bio
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
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The categories of this paper are: eess
4
The categories of this paper are: cond-mat
3
The categories of this paper are: physics
23
The categories of this paper are: solv-int
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The categories of this paper are: nlin
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
18
The categories of this paper are: econ