The vertical disk structure of spiral galaxies and the origin of their thick disks
Our knowledge of the formation and evolution of the main galactic components (bulge, disks, halo) has vastly increased in recent years, but especially the formation of thin and thick disks in spiral galaxies are still poorly understood. Despite the large of number of galaxies for which the structural parameters of thin and thick disks have been measured, and the increasing amount of data about the Milky Way's disks, there is no consensus on the mechanism that drive the formation and evolution of thick disk components.
Thick disk formation models can be classified in three categories: 1) the thick disk is created in situ as a hot component from merging turbulent gas clouds at high redshift, 2) an original thin disk created from cold gas is heated by one or more dynamical processes (scattering off spiral arms, bars, molecular clouds in both vertical and radial direction, merger events, bombardment by dark matter subhalos), and 3) the thick disk consists of the remnants of accreted satellite galaxies. Each of these models is expected to leave distinct signatures in the radial and vertical distribution of stars of different ages.
Here we propose to use the unique HST capabilities to resolve the stellar disk contents of three nearby massive edge-on disk galaxies using ACS and WFC3 imaging. Resolving stellar populations in both vertical and radial direction allows us to disentangle stars of different ages and to measure their disk parameters separately. We can thus quantify the temporal evolution of scaleheights and scalelengths, and the amount of flaring in these galaxies, which will enable us to give definitive verdicts on the various thick disk formation scenarios.
We propose to map the stellar content in the disks of three galaxies. We will use ACS and WFC3 coordinated parallel observations in F606W and F814W to resolve stellar populations. This work will build upon the GHOSTS survey, which has in place extensive tools to analyze resolved stellar populations from HST observations.
To differentiate the various models, the vertical and radial stellar profiles need to be measured as a function of stellar population. The GHOSTS survey results show that the various populations can be easily extracted from a color-magnitude diagram using the F606W and F814W filters. These two filters sufficiently separate the various populations, have good throughput and relatively little extinction compared to shorter wavelength filters. As we will use the existing GHOSTS fields to match the inner profiles we will continue to observe in F606W and F814W.
To resolve enough stars for each stellar population requires us to reach S/N=6 at least 1 mag below the Tip of the Red Giant Branch (TRGB). This TRGB occurs almost independent of metallicity at M_I=4mag, with V-I=1. We have used the ETC and our GHOSTS experience to calculate the required exposure times for optimal PSF fit photometry based on the best available distances. Per orbit we will typically expose 1000s and 1400s in F606W and F814W with WFC3, and 1000s and 1200s with ACS, using a dither in each exposure for removal of cosmic rays and bad pixels.
We use POSTARGS to do the dithering, as this gives us the largest flexibility to determine offsets and to pack the orbits efficiently with the ACS & WFC3 parallels. The POSTARGS also put the apertures outside the target position, but this is done on purpose such that the rotation point where the ACS and WFC3 parallel rotate around is in between the two cameras and the positional shift remains small for both cameras independent of the final ORIENT chosen.
RESOLVED STELLAR POPULATIONS
Default
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This object was generated by the targetselector and retrieved from the NED database.
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This object was generated by the targetselector and retrieved from the NED database.
J2000
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