Traditionally, the radial surface-brightness profile of a galaxy disk has been modelled by a single exponential profile. However, the vast majority of disk galaxies actually show signs of a disk truncation, where the profile suddenly breaks to a steeper second exponential profile. The cause of this break remains poorly understood. In Lambda-CDM cosmology, the radii at which disk truncations are found are also the most recently formed. Disk truncations may therefore be a direct result of the disk-galaxy assembly process. Many theories have thus been proposed to connect the truncations to theories of disk galaxy formation. Generally, break formation models either form the stars in the observed distribution (e.g., a star formation threshold) or employ dynamical effects to redistribute them into the current configuration (e.g., tidal stripping). These models need to be constrained observationally to probe the underlying mechanism of outer-disk formation responsible for creating the truncations. Using our previous experience with deep CMDs from HST observations of resolved extragalactic stellar populations, we propose a comprehensive survey of disk truncations across a variety of disk morphologies and sizes. With resolved stellar populations we are able to map the truncations as a function of age to a very low equivalent surface brightness. Additionally, we will carry out simulations of different truncation formation scenarios for each galaxy (based on disk type and mass) to compare directly with the observations. Combining the two will prove crucial to understanding the mechanism for creating disk truncations and the ongoing processes in disk formation. HST is the only observatory capable of discriminating the formation models due the unique resolution and depth of the ACS and WFC3 instruments.

We will obtain WFC3/UVIS and ACS/WFC coordinated parallel observations to map disk truncations across five nearby disk galaxies. These observations will help reveal the cause of the phenomenon. The work will build upon the GHOSTS survey, which has in place extensive tools to analyze resolved stellar populations from HST observations. Simulations of the various formation scenarios will be carried out in coordination with the observations. To differentiate the various models, we will measure the radial stellar profiles as a function of stellar population. GHOSTS have demonstrated that the various populations can be easily extracted from a color-magnitude diagram using the F606W (~ V -band) and F814W (~ I-band) 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 and outer profiles we will continue to observe in F606W and F814W. HST is the only observatory capable of resolving extragalactic stellar populations and removing contaminating background galaxies. One field will be placed on the disk of each galaxy where crowding can be high. However, for a galaxy surface brightness of \mu_V = 25.5 at a distance modulus of m - M ~ 28, we expect ~ 1200 stars/arcmin2 brighter than V = 26.8 mag. For WFC3/UVIS this is about 450 resolution elements per star. This is more than sufficient for reliable photometry. At the lower surface brightnesses of the non-disk fields, we are only limited by unresolved background galaxies (~ 2/arcmin^2 from GOODS) and foreground Milky Way stars (~ 2/arcmin^2 from Besancon models, Robin et al. 2003). To resolve enough stars for each stellar population requires us to reach about 1 mag below the tip of the Red Giant Branch. This occurs almost independent of metallicity at M_I = -4 mag, with V - I ~ 1. In one orbit, ACS and WFC3 in parallel can both observe two filters, each filter taking 1200s (400+800s). For NGC7793 and NGC4244 at an average m - M ~ 28 and low extinction (A_V = 0.02 - 0.07 mag), the WFC3 ETC calculates an average S/N=10 for one orbit. We thus request one orbit per field with two dithered exposures per passband for cosmic ray and bad pixel removal. Similar calculations show IC 5052 (m - M = 28.8, A_V ~ 0.12) requires two orbits, NGC891 (m - M = 29.6, A_V ~ 0.24) requires 3 orbits for both fields and NGC4565 (m - M = 30.5, A_V = 0.02) requires 6 orbits. These results agree with the observed S/N of the existing GHOSTS ACS fields in the respective galaxies. Where appropriate, WFC3 fields are primarily placed on the disk truncation (as the higher resolution is of benefit in crowded regions), whilst the ACS parallel is situated on the outer profile (where the greater sensitivity of ACS is more important). Orientations have been chosen to maximize schedulability. POS-TARGs for visits 3, 4, 7 and 8 are placed between the ACS and WFC3 apertures to maximise the range of orientations available. COSMOLOGY 12 false 0.0 0.0 false false false false false false SUPPORTED GALAXY DISK HALO This object was generated by the targetselector and retrieved from the SIMBAD database. 26 I-mag J2000 02 22 46.9621 +42 27 11.97 J2000 GALAXY DISK HALO This object was generated by the targetselector and retrieved from the SIMBAD database. 26 I-mag J2000 02 22 46.9621 +42 27 11.97 J2000 GALAXY DISK HALO J2000 12 17 29.6000 +37 48 39.60 J2000 GALAXY DISK HALO J2000 12 17 29.6000 +37 48 39.60 J2000 GALAXY DISK HALO J2000 12 36 41.7937 +25 54 25.41 J2000 GALAXY DISK HALO J2000 20 51 45.3403 -69 09 36.15 J2000 GALAXY DISK HALO J2000 23 58 9.7752 -32 35 38.34 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. POS-TARGs outside aperture to increase range of possible orientations. (Tic-Visit :proposal-id 12196 :visit-id "03" :proposal-version -1 :trans-version "20100505" :date "09-Jul-2010 13:41:15.0" :trans-errors '(:WARNINGS 2 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291888 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12196 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12196 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2968 :al-overhead-at-end 116 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3301 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. POS-TARGs outside aperture to increase range of possible orientations. 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