A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. II. Vertical Distribution of the Resolved Stellar Population

We analyze the vertical distribution of the resolved stellar populations in six low-mass (Vmax = 67-131 km s-1), edge-on, spiral galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys. In each galaxy we find evidence for an extraplanar stellar component extending up to 15 scale heights (3.5 kpc) above the plane, with a scale height typically twice that of two-dimensional fits to Ks-band Two Micron All Sky Survey images. We analyze the vertical distribution as a function of stellar age by tracking changes in the color-magnitude diagram. The young stellar component (108 yr) is found to have a scale height larger than the young component in the Milky Way, suggesting that stars in these low-mass galaxies form in a thicker disk. We also find that the scale height of a stellar population increases with age, with young main-sequence stars, intermediate-age asymptotic giant branch stars, and old red giant branch (RGB) stars having successively larger scale heights in each galaxy. This systematic trend indicates that disk heating must play some role in producing the extraplanar stars. We constrain the rate of disk heating using the observed trend between scale height and stellar age and find that the observed heating rates are dramatically smaller than in the Milky Way. The color distributions of the RGB stars well above the midplane indicate that the extended stellar components we see are moderately metal-poor, with peak metallicities around [Fe/H] = -1 and with little or no metallicity gradient with height. The lack of metallicity gradient can be explained if a majority of extraplanar RGB stars were formed at early times and are not dominated by a younger heated population. Our observations suggest that, like the Milky Way, low-mass disk galaxies also have multiple stellar components. In its structure, mean metallicity, and old age, the RGB component in these galaxies seems analogous to the Milky Way thick disk. However, without additional kinematic and abundance measurements, this association is only circumstantial, particularly in light of the clear existence of some disk heating at intermediate ages. Finally, we find that the vertical dust distribution has a scale height somewhat larger than that of the main-sequence stars.