The comparison of the luminosity profiles and rotation curves of disk galaxies provided the first evidence for dark matter in outer parts of galaxies (e.g., van Albada & Sancisi 1986). The luminous material, either in the form of stars or neutral gas, is not able to reproduce the approximate flatness of the rotation curves at large radii, implying that the Newtonian theory of gravitation needs to be modified or that the mass in the outer regions of most spirals is dominated by a dark component. The physical extent of such a "dark halo" cannot be inferred from the rotation curves, since these last are flat even where the luminous material ceases to be detected. Hence, the total mass in spiral galaxies is not yet well known.
The properties of dark matter (DM) and the structure of dark halos as derived from rotation curves have profound implications on cosmological issues such as galaxy formation. Recent N-body simulations of cold dark matter halos together with adiabatic infall models have shown that matching observed rotation curves requires a systematic increase of disk mass-to-light ratios with luminosity (Navarro et al. 1996). Such a trend is observed in large bodies of rotation curve data (Broeils 1992; Persic et al. 1996- hereafter PSS), and further comparisons between theoretical predictions and observations may help clarify the nature of dark matter in galaxies.
The presence of a dark halo makes global mass-to-light (M/L) ratios derived from rotation curves much higher than those expected from stellar populations, and produces radial gradients in the total M/L. The contribution of DM tends to be larger for low-luminosity systems, as the fraction of DM to luminous matter within the optical radius appears to decrease with increasing galaxy luminosity (Kormendy 1990; Salucci et al. 1991). In luminous systems, therefore, the mass distribution in the inner few disk scale lengths can be largely ascribed to the bulge and disk stars. Consequently, when stellar galaxy components can be accurately isolated with surface photometry, estimates of their M/L from rotation curves provide useful constraints for the age, abundance, and star formation history of their stellar populations. Recent population synthesis models predict trends of M/L with metallicity and with wavelength (Worthey 1994; Bruzual & Charlot 1993), and can be used to infer and compare properties of the stellar populations in bulges and disks.
We have reported in a previous paper (Moriondo et al. 1997 - hereafter Paper I) the results of two-dimensional near-infrared (NIR) surface brightness decompositions for a sample of early-type spirals. NIR wavelengths, especially when combined with those in the optical, are a powerful diagnostic tool since they trace more accurately the stellar mass content and minimize the complications of extinction. In this paper, we apply the bulge+disk decomposition results to the analysis of the rotation curves for our sample. In Sect. 2 we derive the radial mass distributions and evaluate the contribution of the bulge, disk, and dark halo to the observed rotation curves; as an alternative to dark matter halos, we also derive a value of the modified Newtonian dynamics critical acceleration parameter. The resulting NIR M/L ratios of the luminous components are analyzed in Sect. 3, and compared with those at optical wavelengths and with population synthesis models. Finally, we assess the importance of dark halos, and explore trends of M/L with luminosity in the context of the properties of the fundamental plane for gravitationally bound objects (e.g., Burstein et al. 1997).
© European Southern Observatory (ESO) 1998
Online publication: October 21, 1998