Stellar Populations in Edge-On Galaxies From Deep CCD Surface Photometry.II. One-Dimensional FITS of NGC 891
We present deep R-band CCD surface photometry of the nearby edge-on spiral NGC 891, which bears many similarities to the Milky Way. Our data confirm van der Kruit and Searle's (1981b) finding that NGC 891 has a thick disk. We use one-dimensional fits of exponential disk models to averaged vertical profiles to show that this popular technique has a major drawback: it does not produce unique solutions when two or more stellar components are present, and no component dominates in the regions studied. For this galaxy, adequate fits are possible for thin disk scale heights ranging from 400-650 pc and thick disk scale heights from 1.5 - 2.5 kpc. Our next paper will use full two-dimensional modelling of the image to improve the fits. Although NGC 891 and the Milky Way have similar rotation curves and presumably similar total masses, and their disks have similar radial extent, NGC 891's disk and thick disk are both significantly more extended in the vertical direction. We suggest that the more extended thin disk may be caused by more efficient heating by NGC 891's more massive and thicker molecular cloud layer. The large thick disk scale height is harder to explain in the context of recent n-body simulations of thick disk formation via minor mergers, and shows the need for a more extensive exploration of parameter space in these models.