The optical morphology of the kinematically peculiar galaxy NGC 4826

Details The optical morphology of the kinematically peculiar galaxy NGC 4826 The optical morphology of the kinematically peculiar galaxy NGC 4826

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aj....107..184w&link_type=abstract

The Astronomical Journal, vol. 107, no. 1, p. 184-194

Astronomy and Astrophysics

Astronomy

26

Disk Galaxies, Galactic Bulge, Galactic Rotation, Galactic Structure, Interstellar Extinction, Peculiar Galaxies, Spiral Galaxies, Absorption Spectra, Astronomical Models, Astronomical Photometry, Charge Coupled Devices, Cosmic Dust, Kinematics, Morphology, Stellar Mass

Scientific paper

We present charge coupled device (CCD) BVI photometry of the galaxy NGC 4826, the Evil- or Black-Eye galaxy, which was recently found to have two counter-rotating gas disks. We study the extinction in the inner gas disk, which gives NGC 4826 its nickname, and find that this disk can be coplanar or close to coplanar with the stellar disk and still cause the strong absorption that is seen on one side of the galaxy. We try to constrain the orientation of the outer gas disk by looking for a small overall asymmetry in the light distribution which would be present if there is dust in this disk, and if it is significantly tilted with respect to the main body of the galaxy. The test shows that the light distribution does not preclude the outer gas disk from being coplanar with the stellar disk as well. NGC 4826 has a small bulge, with a bulge to total light ratio of 0.17 in B. We confirm that this galaxy is indeed a spiral, with a perfect exponential disk down to 27 mag/sq arcsec in B. The close to coplanar orientation of the gas disks is one aspect which is in good agreement with what is expected on the basis of a merger model for the counter-rotating gas. The rotation direction of the inner gas disk with respect to the stars, however, is not. In addition, the existence of a well defined exponential disk probably implies that if a merger did occur it must have been between a gas-rich dwarf and a spiral, not between two equal mass spirals. The stellar spiral arms of NGC 4826 are trailing over part of the disk and leading in the outer disk. Recent numerical calculations by Byrd et al. for NGC 4622 suggest that long lasting leading arms could be formed by a close retrograde passage of a small companion. In this scenario, the outer counter-rotating gas disk in NGC 4826 might be the tidally stripped gas from the dwarf. However, in NGC 4826 the outer arms are leading, while it appears that in NGC 4622 the inner arms are leading. A realistic N-body/hydro simulation of a dwarf-spiral encounter is clearly needed. It may also be possible that the counter-rotating outer gas disk is due to gradual infall of gas from the halo, rather than from a discrete merger event.

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