0 Assigned ID: 6617 ----- Attempting Submission 1 (Thu Apr 10 19:13:59 GMT 2025) ----- 0 ----- Attempting Submission 1 (Wed Aug 13 22:15:31 GMT 2025) ----- In a sample of 15,900 SDSS quasars, Eracleous et al. (2012) found 88 objects exhibiting broad emission lines with >1000 km/s velocity offsets with respect to the host galaxy rest frame. One leading hypothesis for the origin of these velocity-offset broad lines is the dynamics of a binary supermassive black hole (SMBH), with competing hypotheses of recoiling SMBH and a broad line region (BLR) gas outflow due to entrainment by a relativistic jet. Three of these quasars were also shown to have signficiant VLBI/Gaia offsets in follow-up VLBA observations, where recoiling SMBHs, binary SMBHs, and optical synchrotron jets are all potential causes of these offsets. We propose HST observations to test the various competing physical models for both the observed spectroscopic and astrometric offsets, requiring a total of 3 ACS/WFC + F606W orbits to achieve our science goals. These observations are focused on three quasars (all z~0.2-0.3), where we allocate one orbit per target with ACS/WFC + F606W. The F606W filter is required for our science goals as it matches the Gaia filter used to measure the quasar radio-optical astrometric offsets. These observations require long exposures in order to maximize sensitivity to low surface brightness indicators of a previous major galaxy merger in the host galaxy of our quasar targets (this is essential to our science goals). We will use 4x438s science exposures in order to achieve 1752s of total integration time on each target. According to the online ACS Exposure Time Calculator, this will allow for 5-sigma surface brightness sensitivity limits of ~ 26 AB magnitudes per square arcsec (which should be sufficient for faint host galaxy structural analysis). The WFC1-FIX aperture and POS TARG offset of X=40 arcsec and Y=5 arcsec (moving towards the B amplifier of the WFC1 chip) was chosen to improve charge transfer efficiency performance and lower amplifier readnoise -- allowing us the best sensitivity to faint host galaxy componenets which are relevant to our science goals (e.g., imaging faint tidal structures or possible faint jet structures). This strategy will also keep our quasar position far away from the chip gap during the dithers. We will use the standard ACS/WFC 4-point box dither pattern (4x438s exposures) in order to allow for a good sampling of the PSF for PSF modeling and subtraction. These exposure times may lead to saturation of a few central core PSF pixels for our quasars (note that the AB V-band magnitudes for our quasars range from ~ 18.3-19.2). Therefore, we also employ a short 30s exposure at the start of each orbit, allowing us to obtain a good model of the central core and inner wings of the PSF. We can use the unsaturated core pixels from the short 30s exposures to reconstruct the saturated pixels in the PSF cores of the longer dithered exposures. The short 30s exposure and the longer dithered 4x438s exposures are in "non-interruptible sequences" with the same WFC1-FIX position offsets. This will ensure that the short exposures occur at the same chip position as the first dither point (and without much time separation), allowing us to construct a PSF model for the central pixels which is very close to the PSF core sampled by the subsequent dithered exposures. Dr. Peter Breiding is an expert in radio-loud AGNs. He is a member of the NANOGrav collaboration (and active member of the astrophysics working group), SDSS collaboration, 3CR collaboration, and has a great deal of multi-wavelength observational expertise in AGN science. He is PI of a successful cycle 28 HST (GO-16483) proposal concerning 4 quasars who are also binary SMBH candidates and co-I of other successful HST proposals. His recent publications also concern the identification and characterization of pc-to-sub pc-scale binary SMBH candidates, gravitational-wave recoiling SMBH and AGN candidates, and quasars with powerful radio jets which reside in major galaxy mergers. Supermassive Black Holes and Active Galaxies 6 false false false false false false SUPPORTED GALAXY QSO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY QSO YES This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 GALAXY QSO YES This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 The scheduling data was generated by CASM 31.0. CASM was run in Phase I mode. ACS/WFC Imaging No No F606W The scheduling data was generated by CASM 31.0. CASM was run in Phase I mode. ACS/WFC Imaging No No F606W The scheduling data was generated by CASM 31.0. CASM was run in Phase I mode. ACS/WFC Imaging No No F606W POS-TARG DITHER 0.2637 LINE 0.1856 false false 69.02 4 20.7 ACS-WFC-DITHER-BOX 1 false 1755116517020 /Users/peter/Desktop/OP-cache/pdb_hst_binaries_cycle_33_phase_2_18047-v01.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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