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Colors and Mass-to-Light Ratios of Bulges and Disks of Nearby Spiral Galaxies

We investigate colors and mass-to-light ratios ($M/L$s) of the bulges and disks for 28 nearby spiral galaxies with various morphological types of Sab to Scd, using images in optical and near-infrared ($V$, $I$, and $J$) bands and published rotation curves. It is shown that the observed colors and $M/L$s generally agree with the galaxy formation model with an exponentially declining star formation rate and shallow slope (ex. Scalo) initial mass function (IMF) for both the bulges and the disks. We find that the bulge $M/L$ is generally higher than the disk $M/L$ and that the galaxies with larger bulge-to-total luminosity ratio tend to have a smaller bulge $M/L$. The fact indicates that the luminosity-weighted average age of bulges for early-type spirals is younger than that of later-type spirals. These results support a formation scenario that produces young stars for the bulges of middle-type and early-type spirals.