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In the Atlas of Peculiar Galaxies
(Arp 1966)
this Category was called
``companions attached to spiral arms''. It was one of the best-defined
and important categories and was ordered according to the relative
size of the companion, going from the smallest to largest. In the
present Catalogue it is clear that there are four parameters which
could characterize an M51-type: (1) relative size of companion, (2)
openness of spiral arm on which the companion appears, (3) type of
central spiral, and (4) type of companion galaxy.
Any of these parameters, or others, might be deemed the most
significant by which to order the Category. In the illustrations we
have arranged the objects very approximately by the degree of openness
of the arm with which the companion seems to be associated.
The first object illustrated, AM 0021-623, is considered to be a
prototype object of this Category. The picture shown is from a 4m CTIO
plate (1413A). On page 9.4, object AM 1823-513 is an M51-type in a
chain of galaxies and is a reminder that there are some additional
examples of M51-types in Category 17. On the
same page, object AM
2025-224 appears to be of a type discussed by Vorontsov-Velyaminov
(1976, 1977) where there is a companion on the end of each arm. The
object is small, however, and requires better resolution in order to
check this conclusion. On page 9.8, AM 1950-585 exhibits an apparent
companion which is an M51-type and is also illustrated under
Category 8. On the same page, object AM
0324-524 has two galaxies south of the
obvious spiral, connected by a very faint filament to the other. On
page 9.9 the picture shown is from a plate taken with the 2.5m Las
Campanas telescope (CD 591) in about one arc second
seeing. Immediately following, page 9.10 contains another picture
taken with the same telescope (CD 587). On page 9.11 a 4m CTIO
photograph is shown (1377A).
CATEGORY 9:
M51-TYPES