M63 is part a small grouping of galaxies which includes the grand design spiral M51. Unlike "Grand Design" spirals such as M51, M100, and M81, which all possess two symmetric large spiral arms, M63 is lacking in well defined spiral structure. Its fragmented and patchy spiral arms places it in the class of "flocculent" spiral galaxies which includes about one third of all spiral galaxies. Other well known galaxies of this type include NGC 2403, NGC 2841, NGC 3521, and M33. At present the best explanation for spiral arm formation is density wave theory (Lin and Shu 1964). This theory explains that density waves move through the disk at a different velocity than disk matter and stars. Stars move in and out of the density waves as they orbit the galaxy. As interstellar matter and gas encounters a density wave the matter tends to clump together. Giant molecular clouds form which then collapse, triggering star formation.
Although density waves are the likely driver for spiral arm formation, much remains uncertain about their role in spiral structure and star formation. If star formation occurs exclusively in the spiral arms of galaxies then what is the driving force for star formation in galaxies without significant spiral structure? Star formation in flocculent galaxies must have a different driving force. One hypothesis of how star formation occurs in flocculent spirals is the "stochastic" model (Seiden and Gerola 1982)which suggests that self-propagating star formation in random areas of the disk can occur in some galaxies driven by turbulence within the disk. The stellar winds of the more massive stars compress the adjacent molecular clouds causing them to collapse and form new stars. This process can trigger a cascade of star formation in neighboring regions. Computer simulations add support to this model of star formation within flocculent galaxies. In fact, self propagating star formation occurs commonly on a smaller scale within the spiral arms of grand design spirals.
Despite M63's prototypical flocculent design,
observations in near infrared (NIR) wavelengths show a two arm
spiral structure invisible at optical wavelengths. The weak spiral
arms extend out from the center some 20,000 light years. NIR studies
have revealed low level grand design spiral structure within several
optically flocculent galaxies suggesting that spiral density waves
remain an active driving force for star formation even in flocculent
galaxies. There is growing evidence that low level grand design
structure exists in many flocculent galaxies but is only evident
at NIR wavelengths. To make matters more confusing, grand design
spirals actually exhibit similar rates of star formation per unit
of gas compared to flocculent spirals which raises many questions
such as; do flocculent spirals represent weak counterparts to
grand design galaxies or are they dynamically entirely different?