1999A&A...343..740G

The nucleus of the box-shaped galaxy NGC4013 has been observed with the IRAM interferometer in the J=1-0 and J=2-1 lines of ¹²CO. Our maps show the existence of a fast-rotating (130km/s) molecular gas disk of radius r∼2" (110pc). Several arguments support the existence of a bar potential in NGC 4013. The figure-of-eight pattern of the major axis p-v plot, the ring-like distribution of gas, and the existence of gas emission at non-circular velocities are best accounted by a bar. We have also detected gas at high z distances from the plane (z∼200-300pc=4"-5"). The latter component is related to a system of 4 Hα filaments of diffuse ionized gas that come out from the nucleus. The galactic fountain model seems the best to account for the Hα and CO filaments. Although the peanut distortion can be spontaneously formed by a stellar bar in the disk, gas at high z might have been ejected after a nuclear starburst. The Hα filaments start in the plane of the disk at r∼200pc(4"), and reach several Kpc height at r∼600pc(10"), coinciding with the maximum peanut distortion where the strength of the restoring forces of the plane have a minimum. We have critically examined other alternatives judged less probable: the existence of a CO warp (connected to the HI warping disk), the accretion of gas along stable inclined orbits and, finally, a vertical gas response near the resonances of the peanut (the latter is tested by numerical simulations). Although a link between the bar and the box-shaped bulge in NGC 4013 is suggested we find noticeable differences between the results of previous numerical simulations and the present observations. The discrepancy concerns the parameters of the bar generating the peanut. We see in NGC 4013 the existence of a strong ILR region. The inclusion of a dissipative component, which remains to be thoroughly studied, may change the evolution of the stellar peanut: although in simulations the peanut appears initially near a marginal ILR, the inflow of gas driven by the bar, can make two ILRs appear.