Enabling HST UV Exploration of the Low Surface Brightness Universe: A Pilot Study with the WFC3 X Filter Set

final 97436022 Assigned ID: 558 ----- Attempting Submission 1 (Fri Apr 07 15:07:23 GMT 2017) ----- HST Phase I Proposal 558 (HST25_ULTRApilot.apt) successfully submitted. Receipt: # 558-1 ----- Attempting Submission 2 (Fri Apr 07 22:02:20 GMT 2017) ----- HST Phase I Proposal 558 (HST25_ULTRApilot.apt) successfully submitted. Receipt: # 558-2 ----- Attempting Submission 3 (Fri Apr 07 23:26:21 GMT 2017) ----- HST Phase I Proposal 558 (HST25_ULTRApilot.apt) successfully submitted. Receipt: # 558-3 Revised version in order to obtain three F600LP / F814W exposures (rather than only two) for the targets that have not yet been observed as of April 2018. -316095318 ----- Attempting Submission 1 (Tue Jul 18 07:53:18 GMT 2017) ----- ----- Attempting Submission 2 (Tue Apr 10 21:18:40 GMT 2018) ----- Enabling HST UV Exploration of the Low Surface Brightness Universe: A Pilot Study with the WFC3 X Filter Set We request 17 orbits to conduct a pilot study to examine the effectiveness of the WFC3/UVIS F300X filter for studying fundamental problems in star formation in the low density regime. In principle, the broader bandpass and higher throughput of F300X can halve the required observing time relative to F275W, the filter of choice for studying young stellar populations in nearby galaxies. Together with F475W and F600LP, this X filter set may be as effective as standard UVIS broadband filters for characterizing the physical properties of such populations. We will observe 5 low surface brightness targets with a range of properties to test potential issues with F300X: the red tail to 4000A and a red leak beyond, ghosts, and the wider bandpass. Masses and ages of massive stars, young star clusters, and clumps derived from photometry from the X filter set will be compared with corresponding measurements from standard filters. Beyond testing, our program will provide the first sample spanning a range of LSB galaxy properties for which HST UV imaging will be obtained, and a glimpse into the ensemble properties of the quanta of star formation in these strange environments. The increased observing efficiency would make more tractable programs which require several tens to hundreds of orbits to aggregate sufficient numbers of massive stars, young star clusters, and clumps to build statistical samples. We are hopeful that our pilot observations will broadly enable high-resolution UV imaging exploration of the low density frontier of star formation while HST is still in good health. To demonstrate the utility of imaging with the HST X filter set for exploring star formation in low surface brightness (LSB) environments, we select 5 archetypal LSB targets. Our sample spans a range of distances to test the technique for studying young stars and star clusters in the outskirts of the nearest galaxies, as well as for studying clumps in massive LSB disk galaxies such as Malin 1 which can only be found at larger distances. The resolved star formation "unit" will thus span a range of physical scales. In order of increasing distance, our targets are: Holmberg I (UGC 5139) - dwarf irregular galaxy at 4 Mpc; NGC 628 - outer disk at 9.5 Mpc; UGC 9024 - archetypal LSB disk at 24 Mpc; UGC 1382 - ETG-XUV at 80 Mpc; and Malin 2 - giant LSB disk galaxy at 192 Mpc. Coverage of the SED at three points across the NUV, blue and red bands is needed to constrain ages and masses of clusters and clumps. Filters F300X and F475W straddle the 4000A break (for age) and stellar mass is probed with F600LP. Age/extinction degeneracy is a concern but extinction is generally low in LSB environments, and any strong dust features should be readily apparent in multi-color images from our observations, helping us to recognize which sources may be of increased concern regarding dust. All but one of our targets will be observed with a visit of 3 or 4 orbits duration, devoted to WFC3/UVIS (F300X, F475X, F600LP) X-filter imaging during two orbits and standard filter imaging during either one or two orbits. Holmberg I already has enough archival (standard band) data for our purposes so we request only 2 orbits. For the NGC628 (XUV-disk) field we do not need F438W data (as data is already from LEGUS parallel imaging). We do require F275W imaging for testing against F300X in NGC 628 for CMD analyses. In this case, we will expose F275W for the same duration as F300X (full orbit). In the remaining 3 targets (UGC9024, UGC1382 and Malin 2) we will acquire the X-filter imaging as for the 2 other targets, and then devote the third and fourth orbits for F275W, F438W, and F814W.This clearly implies standard W bands will not be as deep as our X filter data, but we will focus on the brighter end of the detected cluster and clump population for testing. We plan three dithered exposures per target in F300X and F475X. Two exposures will be taken per target with F600LP. Post-flash of a minimal number of electrons will be used for F300X and F275W, respectively, and for other bands as needed. We will employ LA-cosmic to flag and repair cosmic rays in the two exposure datasets before drizzling. We plan 2600s, 1600s and 850s total exposure respectively for F300X, F475X, F600LP (allowing for overheads of GS acquisition, dithering, filter wheel, and readout). Standard filter imaging time will split within orbits by filter to mimic our relative depth between ultra-wide filters. Galaxies and the IGM 0 false true false false false false SUPPORTED GALAXY LSB This object was generated by the targetselector and retrieved from the NED database. 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POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 2

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