A First Look
M74 / NGC 628 or The Phantom Galaxy is a face-on pinwheel nebula in Pisces, ~ 35 million light-years from Earth. It has the lowest surface brightness of any of the Messier objects, and is a difficult object to observe visually. But its large angular size and face-on orientation make it an archetypal example of a “Grand Design” Spiral Galaxy – and an ideal object for professional astronomers to study spiral structure and density waves.
Its remarkably symmetric appearance is caused by density waves sweeping around M 74’s disk, probably induced by gravitational interaction with neighboring galaxies. When gas clouds orbiting within the disk encounter such density waves, they are accelerated into the spiral-shaped wave crest, and converge into the spiral arm, enhancing the density wave.
Collisions and mergers of neighboring gas clouds occur along the density wave, which induce the observed starbirth activity along the spiral arms. They are traced with clusters of blue, young stars and coral-pink diffuse nebulae (H II regions) in color photos. The large number of H II regions – more than 193 – indicate that star formation currently occurs along the pronounced spiral pattern in M 74’s disk. These regions also show up as bright knots in the UV part of the spectrum. More to the point, HII regions arise from an ionization process following star formation in which the young blue stars generate high-energy photons from the gamma through the UV that ionizes all gases before them. Galaxies in local groups share both gases from HII and HI regions.
M74 is considered much to be like our own galaxy, The Milky Way, with a black hole at its nucleus. In the case of M74, Chandra X-Ray Telescope observed an ultra-luminous x-ray source which indicated an intermediate-size black hole.
M 74 was discovered by Pierre Mechain in September of 1780 and confirmed by Charles Messier a month later. M 74 was among the first 14 “Spiral Nebulae” listed by Lord Rosse in 1850.