The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. II. Fluid-Dynamical Models

Details The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. II. Fluid-Dynamical Models The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. II. Fluid-Dynamical Models

2000-08-14

arxiv.org/abs/astro-ph/0008205v1

Astrophys.J.546:931-951,2001

Astronomy and Astrophysics

Astrophysics

accepted by ApJ, 20 pages, 11 figures (2 color), uses emulateapj.sty, onecolfloat.sty

Scientific paper

10.1086/318289

We report a dynamical determination of the separate contributions of disk and dark halo masses to the rotation curve of a spiral galaxy. We use fluid-dynamical models of gas flow in the barred galaxy NGC 4123 to constrain the dynamical properties of the galaxy: disk M/L, bar pattern speed, and the central density and scale radius of the dark halo. We derive a realistic barred potential directly from the light distribution. For each model we assume a value of the stellar M/L and a bar pattern speed Omega_p and add a dark halo to fit the rotation curve. We then compute the gas flow velocities with a 2-D gas dynamical code, and compare the model flow patterns to a 2-D velocity field derived from Fabry-Perot observations. The strong shocks and non-circular motions in the observed gas flow require a high stellar M/L and a fast-rotating bar. Models with I-band disk M/L of 2.0 -- 2.5 h_75, or 80 -- 100% of the maximum disk value, are highly favored. The corotation radius of the bar must be <= 1.5 times the bar semi-major axis. These results contradict some recent claimed ``universal'' galaxy disk/halo relations, since NGC 4123 is of modest size (rotation curve maximum 145 km/sec, and V_flat = 130 km/sec) yet is quite disk-dominated. The dark halo of NGC 4123 is less concentrated than favored by current models of dark halos based on cosmological simulations. Since some 30% of bright disk galaxies are strongly barred and have dust lanes indicating shock morphology similar to that of NGC 4123, it is likely that they also have high stellar M/L and low density halos. We suggest that luminous matter dominates inside the optical radius R_25 of high surface brightness disk galaxies.

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