14886( 3) - 04-Jan-2017 12:07:26 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14886 Version: 3 Check-in Time: 04-Jan-2017 12:07:26 STScI Edit Number: 0 Title The unusual lensing BCG in Abell 1201: A steep IMF gradient, or an ultra-massive central black hole? ------------------------------------------------------------------------------------ Type Cycle GO 24 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Russell Julian Smith Durham Univ. Y CoI: Dr. John Lucey Durham Univ. N CoI: Dr. Alastair C. Edge Durham Univ. N CoI: Dr. Mark Swinbank Durham Univ. N CoI: Dr. James William Nightingale Durham Univ. N ------------------------------------------------------------------------------------ Abstract Abell 1201 is a cluster of galaxies at z=0.17, with a brightest cluster galaxy (BCG) which acts as a gravitational lens to a background surce at z=0.45. The lensing configuration is unusual, with a single bright arc formed at small radius (~5kpc), where the stars are expected to contribute substantially to the total lensing mass. Using VLT/MUSE data, we have recently discovered a faint counter-image to the main arc, located just 0.6" from the lens centre. This image is not predicted at all in lensing models which incorporate the mass contributions from stars (following the observed light profile) and dark matter. Modelling shows that the formation of the counter-image requires additional mass at small radius. Possible solutions include (a) imposing a gradient of the stellar-mass-to-light ratio, reflecting a heavier initial mass function (IMF) in the galaxy centre, or (b) adding a central black hole with mass above ten billion solar masses, comparable to the largest dynamical measurements. The first scenario would be relevant to ongoing controversies concering possible non-standard IMFs in massive elliptical galaxies. The second explanation would be the first lensing-based detection of a central black hole, with implications for the still sparsely-populated upper end of the black hole scaling relations. The two alternative models yield different predictions for the morphology of the counter-image. Our proposed WFC3/UVIS observations will establish whether the image is (a) a radial arc, supporting the IMF gradient interpretation, or (b) a compact counter-image, confirming the ultra-massive black hole scenario. HST resolution is essential for this measurement. ------------------------------------------------------------------------------------ Observations Description ------------------------ The target is a z=0.17 cluster-dominant elliptical galaxy, with an unusual gravitational-lensing arc system. The particular aim of these observations is to get improved information on a very faint inner counter-image to the main arc, buried behind the bright elliptical. 14886( 3) - 04-Jan-2017 12:07:26 - [ 2] We observe in a blue bandpass (F390W), to improve contrast between the star-forming background galaxy and the red foreground galaxy. We also take imaging in a red bandpass (F814W), to help model the stellar mass distribution in the lens. The observation should be fairly straightforward, with a simple five-point small-dither pattern in each band for cosmic ray and detector defect rejection. The dither is a typical N+1/2 pixel "box", with an additional "central" position. But since the observations get split over three orbits, pointing offset errors probably introduce their own significant random dither at similar level. We elected to use post-flash to mitigate CTE losses in the F814W where the exposures are short; we also use the C1K1C-CTE aperture to keep target close to the amplifier. ------------------------------------------------------------------------------------ 14886( 3) - 04-Jan-2017 12:07:26 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ABELL1201-BC CLUSTER OF GALAXIES, RA=11H12M54.5022S, J2000 V = 16.5 +/- 0.1 G GRAVITATIONAL LENS, DEC=+13D26'8.98" RICH CLUSTER Reference Frame: Other: SIMBAD Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 14886( 3) - 04-Jan-2017 12:07:26 - [ 4] Visit: 02 (A1201BCG) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F814W FLASH=4 1 336 S (336 POS TARG +0.185, +0.197 CG S 1C-CTE S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F814W FLASH=4 1 336 S (336 POS TARG 0, 0 CG S 1C-CTE S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F390W 1 1430 S POS TARG 0, 0 CG S 1C-CTE (1430 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F814W FLASH=4 1 337 S (337 POS TARG +0.092, 0.098 CG S 1C-CTE S) ------------------------------------------------------------------------------------------------------------------------------------ 5 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F390W 1 1430 S POS TARG -0.09, -0.09 CG S 1C-CTE (1430 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F390W 1 1430 S POS TARG -0.09, +0.09 CG S 1C-CTE (1430 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F390W 1 1430 S POS TARG +0.09, -0.09 CG S 1C-CTE (1430 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 ABELL1201-B WFC3/UVI ACCUM UVIS2-C1K F390W 1 1430 S POS TARG +0.09, +0.09 CG S 1C-CTE (1430 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ ABELL1201-BCG 1 WFC3/UVI ACCUM UVIS2-C1 F814W none none none none 1 N/A S K1C-CTE ABELL1201-BCG 2 WFC3/UVI ACCUM UVIS2-C1 F814W none none none none 1 N/A S K1C-CTE ABELL1201-BCG 3 WFC3/UVI ACCUM UVIS2-C1 F390W none none none none 1 N/A S K1C-CTE ABELL1201-BCG 4 WFC3/UVI ACCUM UVIS2-C1 F814W none none none none 1 N/A S K1C-CTE ABELL1201-BCG 5 WFC3/UVI ACCUM UVIS2-C1 F390W none none none none 2 N/A S K1C-CTE ABELL1201-BCG 6 WFC3/UVI ACCUM UVIS2-C1 F390W none none none none 2 N/A S K1C-CTE 14886( 3) - 04-Jan-2017 12:07:26 - [ 5] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ ABELL1201-BCG 7 WFC3/UVI ACCUM UVIS2-C1 F390W none none none none 3 N/A S K1C-CTE ABELL1201-BCG 8 WFC3/UVI ACCUM UVIS2-C1 F390W none none none none 3 N/A S K1C-CTE 14886( 3) - 04-Jan-2017 12:07:26 - [ 6] Summary Form for Proposal 14886 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-CTE ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG +0.09, +0.09, POS TARG +0.09, -0.09, POS TARG +0.092, 0.098, POS TARG +0.185, +0.197, POS TARG -0.09, +0.09, POS TARG -0.09, -0.09, POS TARG 0, 0 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F390W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ABELL1201-BCG ------------------------------------------------------------------------------------------------------------------------------------