Non-Axisymmetric Structure in the Satellite Dwarf Galaxy NGC2976: Implications for its Dark/Bright Mass Distribution and Evolution

Abstract

We present the result of an extensive search for non-axisymmetric structures in the dwarf satellite galaxy of M81: NGC 2976, using multiwavelength archival observations. The galaxy is known to present kinematic evidence for a bysimmetric distortion, anyhow the stellar bar presence is controversial, due to the possible interpretation of NGC 2976 presenting an elliptical disk triggered by a prolate dark matter halo. We applied diagnostics in order to detect stellar bars or spiral arms. The m=2 fourier phase has a jump around 60 arcsecs consistent with a central bar and bisymmetric arms. The CO, 3.6 $\mu$ surface brightness and the dust lanes are consistent with a gas rich central bar and possibly with gaseous spiral arms. The kinematic jumps related with the dust lanes suggest that the bar perturbation in the disk kinematics is non-negligible and the reported non-circular motions, the central gas excess and the nuclear X-ray source (AGN/Starburst) might be produced by the central bar. SPH simulations of disks inside triaxial dark halos suggest that the two symmetric spots at 130 arcsecs and the narrow arms may be produced by gas at turning points in an elliptical disk, alternatively the potential ellipticity can be originated by tidally induced strong stellar bar/arms. We conclude that, the gas rich bar and spiral arms triggered by tidal stirring, and primordial halo triaxiality, can explain most of the non-circular motions, mass redistribution and nuclear activity. Distinguishing between them requires detailed modeling of environmental effects. An analysis similar to this may reveal such kind of structures in other nearby dwarf satellite galaxies, and if confirmed, the same evolutionary scenario will be applicable to them. This implies biases constraining their dark matter distribution and also making comparison against theoretical predictions for isolated galaxies.

Accepted to appear in AJ, 11 pages, 6 figures

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