Presentation 1 : Oral
The mass to light ratio in NGC 4414 and constraints on the dark matter distribution
Vallejo O., Braine J., Baudry A.
The purpose of our work is to strongly constrain the mass to light ratio in the disk of the flocculent isolated Sc type spiral NGC 4414
using a combined high resolution interferometric CO(1-0)and HI rotation curve, and deep HST B-V-I images of the galaxy. To
identify the mass contributions of the visible - stellar and gaseous - components, we observed the rotation curve with high resolution
in the optical disk. The M/L ratios we derive are low, about 1.5 in I band and 0.5 in K\' band. The B and V band M/L ratios vary greatly
due to absorption by dust, reaching 4 in the molecular ring and decreasing to about 1.6 -- 1.8 at larger radii. This unequivocally
shows that models, like most maximum disk models, assuming constant M/L ratios in an optical waveband, simply are not appropriate.
We illustrate this by making mock maximum disk models with a constant V band M/L ratio. The key is having the central light
distribution unobscured such that it can be used to trace the mass. The K\' band M/L ratio is virtually constant over the disk,
suggesting that the intrinsic (unobscured) stellar M/L ratio is roughly constant. A primitive attempt to determine the intrinsic M/L ratio
yields values close to unity in the B,V, and I bands and slightly below 0.5 in K\'. We use our knowledge of the visible mass distribution
to test disk and halo dark matter models. Fitting an NFW (Navarro et al 1996) halo to the rotation curve suggests that NGC 4414 is in
a low mass concentrated halo typical of small galaxies and early halo formation times.