The Program of Last Resort (An Unusual ACS SNAP Gap Filler)

Andrea Bellini, bellini[at]stsci.edu, and Norman Grogin, nagrogin[at]stsci.edu

Every year, the Institute allocates over 3000 orbits of Hubble time to approved General Observer, Snapshot (SNAP), and Director's Discretionary programs. The many targets among all these programs are not distributed uniformly around the celestial sphere, and most targets have observational constraints that limit their schedulability to something less than the entire year. Despite the best efforts of the Hubble schedulers to allocate every last orbit, a small but persistent fraction (∼2–3%) of the orbits go unused. Salvaging this unused observing time presents an opportunity for the Institute to benefit the astronomy community.

The Institute's Hubble Mission Office has initiated a pilot, ultra-low priority, SNAP program (14840, PI: Bellini) in Cycle 24, with the goal of taking useful data in Hubble orbits that absolutely no other program is able to use. The current target list, subject to change, comprises Hubble-unobserved, moderately large, and bright NGC/IC galaxies. All targets are identically observed with two dithered 337‑second exposures, using the F606W (wide‑V) filter of the Advanced Camera for Surveys (ACS)/Wide Field Channel (WFC). This shortest possible exposure time allows a parallel buffer dump for ACS/WFC, thereby maximizing the efficiency of scheduling. There are no configuration constraints placed on the observations, again allowing for maximal Hubble schedulability.

The initial target list for this program is drawn from the revised NGC/IC catalog maintained by W. Steinicke. After excluding non-galaxies and the already-Hubble-observed galaxies, the catalog is then culled based on galaxy size. Galaxies are selected with diameters in the range of 0.9–1.6 arcminutes, which largely spans one of the two ACS/WFC charge-coupled devices (CCDs). To obtain a suitably large number of galaxies (≈500) for a full Hubble cycle of observations, the sample is further restricted to the magnitude range 11.4 ≤ mV ≤ 12.6.

Milky Way obscuration naturally results in an uneven distribution in Right Ascension (R.A.) of magnitude-limited NGC/IC galaxies. It is desirable for a snapshot target list to be reasonably homogeneous across the sky, so that Hubble can always slew to an available target regardless of the prior orbit's pointing. This program's R.A. distribution is therefore leveled, in R.A. ranges impacted by heavier Milky Way obscuration, by adding NGC/IC galaxies up to 0.7 mag fainter (mV ≤ 13.3). Figure 1 shows the locations of the final list of 491 NGC/IC targets on the sky (in blue).

Figure 2 shows a snapshot of one already-observed galaxy.

Figure 1: The locations of the final list of 491 NGC/IC targets (blue symbols) on the sky. At the time of writing, 13 galaxies have already been observed (green symbols). Targets scheduled for observation in the coming weeks are denoted by red symbols. The histograms at the bottom show the distribution of targets in R.A. and Declination (Dec.). The red line in the histograms indicates perfectly uniform distribution on the celestial sphere.

Figure 2: A snapshot of one already-observed galaxy.

This program is ordained to have the very lowest priority of any Hubble program currently scheduled. Its sole purpose is to fill schedule gaps remaining after all other programs have populated the observing calendar. In the summer of 2017, the Institute will solicit additional target lists from the astronomical community, likely involving a short justification outside of the Astronomer's Proposal Tool. The limitations of this call will be: (1) two fixed-length (337 second), dithered, single-filter ACS/WFC exposures per target; (2) single guide-star suitability; (3) no proprietary period; and (4) no associated funding. A uniform distribution of targets across the celestial sphere is desirable.