Cannot find file /Users/bholwerd/Desktop/Proposals/HST/C25/OCG-JWST/OCG-VV191-HST-c25-v3.pdfmildly updated v3. One Fig enlarged-1190228197Assigned ID: 150
----- Attempting Submission 1 (Wed Apr 05 16:42:40 GMT 2017) -----
HST Phase I Proposal 150 (OCG-VV191-HST-c25-v1.apt) successfully submitted.
Receipt: # 150-1
----- Attempting Submission 2 (Thu Apr 06 01:57:52 GMT 2017) -----
HST Phase I Proposal 150 (OCG-VV191-HST-c25-v1.apt) successfully submitted.
Receipt: # 150-2
----- Attempting Submission 3 (Thu Apr 06 19:33:56 GMT 2017) -----slightly dithered UV observations of VV 191. This may well be considered for a joint HST/JWST image release, please check if a good OPO image can be made with these settings.2015174467----- Attempting Submission 1 (Mon Jul 17 14:45:50 GMT 2017) -----
HST Phase II Proposal 15106 (vv191_Phase2_v2.apt) successfully submitted.
Receipt: # 15106-1
----- Attempting Submission 2 (Mon Jul 17 14:50:49 GMT 2017) -----The UV attenuation in JWST target VV 191We aim to map the UV--near--IR attenuation curve along many sightlines within nearby disk galaxies to resolve a large fundamental uncertainty in galaxy evolution studies: the variance in the attenuation curve within an indivual galaxy disk on linear scales <50pc, and the prevalence of the 2175 Angstrom "bump" specifically. We developed a technique to obtain spatially-resolved attenuation measurements using overlapping ("occulting") galaxy pairs. The GalaxyZoo citizen science project and our STARSMOG HST/ACS F606W snapshot survey have only recently identified and validated overlapping pairs with a geometry that lends itself particularly well for such analysis.
The most promising pair, VV 191, will be observed as JWST/NIRCam GTO targets to cover the near-IR part of the attenuation curves. To complement the JWST NIR observations with UV, we request WFC3/UVIS observations in the F225W and F336W filters. These cover (together with the existing F606W ACS images) the UV--visible portion of the UV--near-IR attenuation curves for each <50pc resolution element in the region of overlap in the non-interacting galaxy pair VV191 (MCG+04-33-005), in which a relatively blue elliptical beautifully backlights the outer disk of a foreground face-on spiral galaxy.
Dither strategy:
We opt for a 2-point dither in the case of the F336W observations (1 orbit) and a 3pt dither strategy for the F225W observations.
The 9 orbits for the F225W observations are broken into three groupings of 3 orbits in the 3 dither pattern. This is to ensure correction of cosmics and detector artifacts.
Our secondary aim is an HST/JWST image with good public outreach potential and our aim is to maximize image quality for this reason as well.HST/WFC3 Observations
We ask for HST/WFC3 UVIS observations for ten orbits, distributed over the F225W and F336W
filters to ensure coverage of the attenuation "bump" observed by Calzetti & Heckman (1999).
Exposures are to be dithered to remove cosmics and mitigate detector effects. Roll angle is not
particularly constrained as the pair fits comfortably in the WFC3 field-of-view. The STARSMOG
program already obtained F606W observations and we ask only for the UV filters. The planned
JWST/NIRcam GTO observations cover the near-infrared through K-band.
Filter Choices
Our choice of filters reflects the need to sample the attenuation curve in
the ultra-violet (see Fig. 4). The F225W maximizes leverage on the attenuation in the ultraviolet
combined with excellent throughput and sensitivity for short exposures. Combined with the F336W
filter, it traces the strength of the 2175 Angstrom "bump" in the attenuation curve. We will only
use one side of WRC3/UVIS to ensure homogeneity among the observations and include a 10-14e-
of post-flash for each exposure. With seven filters, the attenuation curve is sufficiently sampled
to distinguish between the different ones observed in local galaxies, e.g., Milky Way
(Cardelli et al., 1989), Magellanic Clouds (Gordon & Clayton, 1998; Misselt et al., 1999)
or star-forming (Calzetti et al., 1994).
Exposure time
A single orbit exposure with F606W results in s/n > 7 in the outer parts of
both galaxies, evident by the image in Figure 5, ensuring a wide overlap region for attenuation
measurements. In the UV, the observation is more challenging because the background elliptical
lower surface brightness in the UV. The central region of the elliptical has the highest surface
brightness, enough for UV comparisons. To ensure enough overlap, we require a s=n 1 at the
effective radius of the elliptical in both filters. Starting from our F606W image and according to
the WFC3 ETC this can be achieved with 9 orbits (23000s, including post-flash) in F225W and 1
orbit (2700s) in F336W, resulting in a total of 10 orbits. The 10 orbits requested can be broken
into two visits (0.5+4.5 orbits F336W+F225W).
Flash
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