The stellar mass to light ratio in the isolated spiral NGC 4414

Abstract

We present high resolution CO(1–0) interferometric observations and deep HST images of the flocculent isolated Sc type spiral NGC 4414. The goal is to determine the stellar mass-to-light (M/L) ratio in a galactic disk. NGC 4414 is an ideal object for this kind of study, as it is an unperturbed object at high galactic latitude with very extended atomic gas (Hi). Many Cepheid light curves were measured in NGC 4414 so its distance is known to be about 19.2 Mpc. NGC 4414 is quite axisymmetric, with no bar and poorly defined spiral structure, and the center is seen unobscured (no CO, HI, Hα, or thermal dust emission near the nucleus), as in many isolated spiral galaxies. Not only does this result in minimal non-circular velocities but also, and this is a key to our success, the central light profile traces the total mass. The stars are seen without a dust screen, the central gas mass is very low (undetected), and we show that the dark matter is negligible in the central regions. We have developed an axisymmetric analytical gravitational potential model to account for the central light (mass) profile, the dynamics of the molecular gas in the highly obscured molecular ring, and the stellar light profile outside the highly obscured region. A single dominant disk component reproduces the disk dynamics and outer stellar light profile such that even if other disk components were present they would not affect our results. The contribution of dark matter is constrained by the extremely extended HI rotation curve and is small, possibly negligible, at distances less than 5–7 kpc from the center. Furthermore, the 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'.