The current state of the Lin--Shu density wave theory is discussed in the light of modern observational data. Much attention is paid to the problem of wave excitation and to the response of the interstellar gas to the wave gravitational potential. It is noted that the major predictions of the density wave theory--the galactic shock waves, the spiral velocity field of stars, and the age gradient across the spiral arms--have become fundamental observational facts at present, so that the density wave theory now has no competition from alternative theories. The nature of flocculent spirals is also discussed since, unlike regular spirals, they are probably not connected with density waves but with the effects of induced star formation in differentially rotating galactic disks.

PT Journal Article
TI The modern view of the nature of the spiral structure of galaxies
AU Efremov Yu N
FAU Efremov YN
AU Korchagin V I
FAU Korchagin VI
AU Marochnik L S
FAU Marochnik LS
AU Suchkov A A
FAU Suchkov AA
DP 10 Apr, 1989
TA Phys. Usp.
VI 32
IP 4
PG 310-327
RX 10.1070/PU1989v032n04ABEH002699
URL https://ufn.ru/en/articles/1989/4/b/
SO Phys. Usp. 1989 Apr 10;32(4):310-327