0Assigned ID: 494
----- Attempting Submission 1 (Fri Apr 05 16:01:45 GMT 2019) -----(1) Removed the group scheduling constraints.
(2) Increased the dithering spacing to avoid persistence.
(3) Increased NSAMP to be greater or equal to 8.-339502399----- Attempting Submission 1 (Thu Jul 25 02:39:53 GMT 2019) -----
HST Phase II Proposal 15975 (15975_phase2.apt) successfully submitted.
Receipt: # 15975-1
----- Attempting Submission 2 (Fri Jul 26 03:29:57 GMT 2019) -----
----- Attempting Submission 3 (Sat Jul 27 15:43:11 GMT 2019) -----
----- Attempting Submission 4 (Mon Jul 29 14:01:12 GMT 2019) -----
----- Attempting Submission 5 (Mon Sep 02 18:23:52 GMT 2019) -----Host Galaxy Morphologies of Candidate Sub-parsec Binary Supermassive Black Holes: Testing the Merger HypothesisSub-parsec (sub-pc) binary supermassive black holes (BSBHs) should be common from galaxy mergers, yet direct evidence has been elusive. We propose WFC3/IR imaging for a sample of 8 candidate sub-pc BSBHs at redshifts z~0.1-0.5. The targets were identified from ordinary quasars and kinematically offset quasars whose broad H-beta lines are significantly offset (by >~ a few hundred km/s) from the systemic redshifts. The offsets may be signposts for BSBHs, but they could also originate in broad-line region (BLR) kinematics or gravitational wave recoils. Using long-term spectroscopic monitoring, 8 strong BSBH candidates have been selected whose broad H-beta radial velocity (RV) shifts are consistent with binary orbital motion without line profile changes. However, complementary tests are necessary, because the time baselines required to cover a full binary cycle are typically much longer than human lifetimes. The proposed observations will directly test the prediction that the host galaxies of BSBHs should have a higher fraction of disturbed morphologies and younger stellar bulges (with low Sersic indices) from recent interactions than those of control quasars. The results will test the merger hypothesis, potentially providing supporting evidence for these sub-pc BSBH candidates, which are important as the progenitors of low-frequency gravitational wave sources. If confirmed, the results will help validate and improve the spectroscopic RV technique for selecting sub-pc BSBHs.Our program includes 10 orbits, of which 8 orbits are for 8 science targets, and 2 orbits are for PSF stars.
Our science targets are a sample of 8 SDSS quasars at redshifts 0.1<z<0.5. They are currently the strongest candidates known for hosting sub-pc BSBHs. They were selected from long-term, spectroscopic monitoring studies using the largest sample of quasars with multi-epoch spectra. To ensure robust measurements of host properties of our targets at 0.1<z<0.5, we need both deep exposures and high imaging resolution. HST WFC3 is the ideal instrument for our scientific goals, given its superb sensitivity and diraction-limited imaging quality.
We choose the WFC3/IR channel with the F160W (H) filter, which samples the rest-frame ~1 um and probes the old stellar population. Working at 1 um has the advantage over shorter wavebands of reducing the impact of dust extinction and massive star forming regions on morphological measurements. The host-quasar contrast is also close to maximum at this wavelength and no strong quasar emission lines are included, optimal for the measurements of host morphologies. To detect the host and to obtain robust morphological measurements of its surface brightness profile, deep exposures are required. Studies have shown that shallow imaging data (HST SNAP or ground-based) make light profile fitting ambiguous, and are unable to detect low surface brightness features (such as extended disks, tidal features and faint companions), especially in the presence of a bright quasar point source.
We use the 512x512 subarray to reduce readout and buffer dump. This provides a field-of-view (FOV) of 60''x60'', or 400 kpc at z~0.5, enough to cover the host and close companions. The control quasar sample has a median redshift z~0.2 observed to a surface brightness limit of H~22 mag arcsec$^{-2}$ (Vega) with NICMOS. Accounting for surface brightness dimming for the targets at higher redshifts, H~23.5 mag arcsec$^{-2}$ is needed for a fair comparison. For the faintest target with H~16.3 mag, we will take 12 exposures each using STEP25 with NSAMP=10 (total exposure time ~30 min). We will use the 4-step BOX dither pattern for cosmic-ray and hot-pixel rejection, and to better sample the PSF. We will reach H~24 mag arcsec$^{-2}$ (Vega, for a 3\sigma detection within a 0.65"x0.65" aperture, assuming an elliptical galaxy SED at z~0.5), needed for detecting extended disks and tidal features. We need a full orbit for each target accounting for overhead.
Accurate PSFs are crucial for our science goals. Studies have shown that the PSFs vary from weeks to months, and the rms difference between PSFs taken 6 months apart can be as large as 20%. Thus ideally we want simultaneously observing PSF stars and science targets. In practice, we will divide the targets into two groups by RA, and request block visits for each. In each block we will observe one additional orbit for a PSF star, with the same filters and dither patterns as our targets, to create high dynamic range PSFs. As a backup, we will utilize archival PSFs and stars within the FOVs of our targets to monitor the spatial, temporal and color variations of the PSFs.
The impact of reduced-gyro operations on our program is likely to be significant. Given the rarity of our targets as the best 8 BSBH candidates, we need to observe all of them, if not more, to put the comparative study on firm statistical ground. Completing only part of our program will make the interpretation of the results more difficult.Dr. Xin Liu and Dr. Yue Shen have expertise in HST data reduction for WFC3, the prime instrument used in the proposed observations. Dr. Liu is an expert in binary and dual AGNs, and has more than a dozen papers published in the past decade on the general topic of merging supermassive black holes; she has led three past HST observing programs with 3 papers published based on the HST observations. Dr. Shen is an expert in quasars and supermassive black holes; he has led three past HST observing programs, and has extensive experiences in the analysis of quasar host galaxies using HST images that will be required to analyze and interpret the data.Massive Black Holes And Their Host Galaxies6falsetruefalsefalsefalsefalsefalseSUPPORTEDGALAXYQUASARYESJ2000J2000H = 16.00GALAXYQUASARYESJ2000J2000H = 16.07GALAXYQUASARYESJ2000J2000H = 14.73GALAXYQUASARYESJ2000J2000H = 15.39GALAXYQUASARYESJ2000J2000H = 16.32GALAXYQUASARYESJ2000J2000H (AB) = 15.85GALAXYQUASARYESJ2000J2000H = 16.02GALAXYQUASARYESJ2000J2000H = 14.25STARDAThis is a PSF star which was approved as two, generic PSF star targets in phase 1.
It is NOT a new target different from what was already approved.J2000J2000STARDAThis is a PSF star which was approved as two, generic PSF star targets in phase 1.
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