M64’s unusual dust feature

We had a really clear night June 3rd, following a cold front, so I set up an imaging run on M64, the descriptively named “Black Eye” galaxy.  I got a good hour of luminance data, but only about 15 minutes each of RGB.  I haven’t made anything acceptable with the color data yet, but the luminance is ok.  M64 (aka NGC 4826) is a crowd-pleaser  for amateurs because it shows visible structure in modest scopes due to its startlingly obvious dust lane.

M64 (NGC 4826), exposed 6-3-2013; luminance 12 x 300 seconds. 12″ LX-200 at f/10 with SBIG ST-10XME, resolution .47 arc-seconds/pixel. Click image for larger size.

But the dust lane wants an explanation.  It’s located at the inner third of of a broad disk, and is obvious even at the fairly broad angle at which the galaxy presents itself.  The outer disk has suggestions of spiral structure, but the details have apparently softened over time.  The explanation is probably related to the observation that the outer part of the galaxy includes a gas disk rotating in the opposite direction of the stars and gas in the inner disk, presumably from accretion or collision at some time in the distant past.  From  a paper by Corsini, E. M.; Bertola, F,  “The Phenomenon of Counterrotation in Galaxies” (http://adsabs.harvard.edu/abs/1998JKPS…33S.574C):

This galaxy contains two nested counter-rotating gaseous disks.  Radio and optical observations revealed an inner disk of about 1 kpc radius containing ~107 solar masses in HI and  ~108 solar masses in H2 and a counter-rotating outer gas disk extending from 1.5 to 11 kpc and containing ~108 solar masses in HI.


They are coplanar to the stellar  disk. Stars co-rotate with the inner gas  but beyond the dust lane less than 5% of them (~108M ) co-rotate with the outer gas.  The kinematical features of NGC4826 are interpreted considering an original gas-poor galaxy with prograde gas which slowly acquires a comparable mass of external retrograde gas.  The new counterrotating gas settles in the outer parts of the stellar disk,leaving undisturbed the galaxy morphology.

The galaxy is now relatively isolated, so there is no obvious smoking gun.  The culprit may have been a counter-rotating dwarf galaxy in orbit around M64, which has now completely lost its identity, and is suggested only by the Black Eye.



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