NGC 4826 – The “Black Eye” Galaxy
Sometimes familiar objects have strange things going on with them. This is the case with the interesting spiral galaxy NGC 4826 (M64) in Coma Berenices, more commonly known as the “Sleeping Beauty” or the “Blackeye Galaxy”. First discovered by J. E. Bode on April 4, 1779 it was catalogued by Charles Messier in 1780 as a “nebulous” object. Much attention has been drawn to M64 due to the great dust lane in the center of the galaxy which gives it that “black eye“ appearance.
M64 is located at RA = 12h 54m 18 sec, DEC = +21d 57m. It is well placed for observation in the spring and early summer. It has a total visual magnitude of about 8.8. M64 is classified as a Sb galaxy by Allan Sandage, appearing as an elliptical galaxy about 7 x 4 arc minutes visually with soft, smooth circular arms. Photographs have shown few if any details in the spiral arms such as HII regions. Yet the dust lane shown numerous knots and features. At the Texas Star Party in Ft. Davis, Texas, in a dark sky, the dark dust lane near the center was easily seen in a 10” scope. M64’s redshift is measured at 474 km/sec giving a distance of 6.3 Megaparsecs (21 million light years, assuming a Hubble Constant of 75 km/sec/Mpc). No supernovae have been seen in M64 hence the estimated distance is only good to no better than 30% (or more).
Early photographs of M64 taken by George Ritchey with the newly installed 60” reflector in 1910 showed the dust lane clearly along with the smooth spiral arms. Long exposures showed an elliptical halo drowning out the dust lane and the dark compact clouds giving the appearance of a totally different galaxy double in size than the shorter exposures. Not much more than the usual research has been carried out with M64 until recently. In 1992 Robert Braun and his associates reported in the Astrophysical Journal that they had found evidence that the area within the dust lane of M64 was counter-rotating from the outer spiral arms! In 1994 they published details of the counter rotation. They analyzed the velocities of neutral hydrogen clouds using the Very Large Array (VLA) in New Mexico and the Westerbork Synthesis Radio Telescope (WRST). Braun found that the inner gas disk near the core had a high density greater than 50 M> /parsec and a high star formation rate. Between 1 and 2.8 kpc radius from the core there was a low gas density. The outer gas disk beyond 5.5 kpc to 11 kpc radius was rotating in the opposite direction (with same position angle) as the inner disk. The friction caused by the two disks counter rotating may be responsible for the dark dust lane (the “black eye”).
Braun has theorized that the cause of the counter-rotation may be the merger of a star poor dwarf galaxy with a gas poor spiral galaxy with each galaxy having gas clouds weighing in at a few hundred million solar masses. In this case one galaxy just happens to be rotating in the opposite direction as the other. If this is so, why don’t we see any tidal plumes (“tails”) as in NGC 4038 and 4039? Another explanation would be the continuous accretion of anti-parallel spin gas, weighing in at a few hundred million solar masses. The counter-rotating disks may be a short lived phenomena in galaxy evolution since we do not see this very often in other galaxies. It may be the case that galaxy mergers were very common in the early universe when galaxies were much closer together. Such mergers triggered star formation when neutral hydrogen clouds were perturbed.
NGC 4826, the “Blackeye” galaxy is shown as a negative print to bring out the details of the dust lane seen here as the whitish area above the core. From the center outward through the dusty region, the stars and gas rotate in the same sense. Beyond the dust lane, the gas reverses direction and counter rotates.
M64 can be a nice object to show visitors at star parties this spring and summer. Many people have seen photos of galaxies in books and sense they are rotating by the shape of the spiral arms (like the classic M51, the “Whirlpool” galaxy”). When showing these visitors M64, make mention of the mysterious counter-rotation just recently discovered outside the radius of the dust lane. This situation just goes to show that galaxy evolution and kinematics is not an exact science. And investigators studying the evolution of galaxies have a lot of explaining to do with M64.
Finder chart for M64, The Black Eye galaxy.
REFERENCES
Braun, R., Walterbos, A.,M.R., Kennicutt, R.C., Tacconi, L., J., 1994, Counter-Rotating Gaseous Disks in NGC 4826, Astrophysical Journal, 420, p. 558-569.