8642( 4) - 02/07/00 14:46 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8642 Version: 4 Check-in Date: 07-Feb-2000 14:46:03 1.Proposal Title: Deep Imaging of the Probable Einstein Ring 1517+656 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Claudia Urry Space Telescope Science Institute CoI: Dr. Riccardo Scarpa Space Telescope Science Institute N CoI: Dr. Rachel L. Webster University of Melbourne N CoI: Mr. Matthew O'Dowd Space Telescope Science Institute N CoI: Dr. Tom Broadhurst University of California, Berkeley N ------------------------------------------------------------------------------------ 5. Abstract A short WFPC2 F702W snapshot image of the BL Lac object 1517+656 has revealed three surrounding arcs, subsequently confirmed by a longer NOT observation, making this a probable new Einstein ring. Deeper high-resolution imaging is required to determine unambiguously the nature of this intriguing source. We propose to obtain a deep STIS/CCD + F28X50LP image to map the detailed structure of the arcs, and WFPC2 F555W and F814W images to measure their colors. The STIS data will have the depth and resolution required for iterative back-mapping of the arcs, providing a unique determination of the mass distribution of the lens. The extreme narrowness of the arcs makes ground-based observations useless for this purpose. Combined with the WFPC2 data, we will have sufficient color information to constrain the photometric redshift and stellar population of each arc (impossible with the ground-based data because of the instability of the required deconvolution), as well as of other galaxies detected in the field. ------------------------------------------------------------------------------------ 8642( 4) - 02/07/00 14:46 - [ 2] Observations Description ------------------------ Observing Plan We propose to observe 1517+656 with STIS CCD F28X52LP, WFPC2 F555W, and WFPC2 F814W, in order to obtain high S/N images of the arcs in three bands. The STIS F28X50LP, with its high throughput and fairly red response, will give the surface brightness sensitivity we need to accurately map the ring structure. Together with the existing WFPC2 F702W image and the proposed F555W and F814W images, we will have two independent colors. Lens models depend on the redshift of the background source but the low surface brightness of the arcs makes spectroscopy infeasible. Instead, we will use colors to constrain the redshift and type of the lensed galaxy (Fig. 2), thus (potentially) confirming that the arcs originate from a galaxy at redshift much greater than that of the BL Lac. Finally, the combination of images from these bands will provide us with sufficient signal-to-noise to detect the BL Lac host galaxy, even if fainter than expected. At z_lens ~ 0.7, the limiting absolute magnitude is M_R ~ -22.5 mag, more than one magnitude fainter than the average for nearby BL Lac hosts (Urry et al. 1999b); for z_lens~1, the limit is M_R ~ -23.5 mag. Given that host galaxies likely evolve at least passively, the actual host galaxy magnitudes should be well above these sensitivity limits. Note that because this object is in the CVZ (a total of 189 orbits in 7 opportunities, so it will likely schedule that way) it is possible to obtain nearly 5000 seconds in one orbit. STIS Observation Of the three proposed observations, this will be the deepest. Our goal is to detect both the arcs and the lensing galaxy at a very good S/N ratio, so the high throughput and broad red response of the F28X50LP are ideal. Such a deep observation will permit a complete characterization of the ring: its extent, radial width, variation of surface brightness, and total number of arcs (some may have been undetected in the F702W image). To have sufficiently accurate photometry of the arcs, we need to reach a surface brightness of Mu_V~23 mag/arcsec^2 at a S/N ratio of 5 per pixel in our STIS image. This corresponds to a S/N ratio ~3 times better, at ~1 magnitude fainter, than our short F702W image (which however still yields useful photometry). A S/N of 3 per pixel in the two WFPC2 images is sufficient to both gain accurate color information, and further increase the S/N (to about 7 per pixel) of the combined multi-color image. To estimate exposure times, we assume average conditions for sky background, and CRSPLIT=4 for a robust cosmic ray cleaning. To reach Mu_R=23.4 mag/arcsec^2 with S/N=5 using the F28X50LP filter requires ~3000 seconds (from the STIS exposure time calculator). With overhead for guide star acquisition (7 min), this can be accomodated easily within 1 CVZ orbit. Given that the lensing galaxy is very likely the BL Lac host, the required exposure time is easily sufficient to detect it. For an average host galaxy luminosity, M_V=-23.0 mag, and effective radius, r_e=10 kpc (Wurtz et al. 1996, Falomo et al. 1997, Urry et al. 1999a, b), at z=0.7, r_e=1 arcsec and the surface brightness at r_e is Mu_V=25.9 mag/arcsec^2. The BL Lac nucleus in our F702W image has Mu_V=22.8 mag/arcsec^2 at 1 arcsec from the center. Taking the latter as a source of noise, in 3000 s the galaxy light reaches S/N=0.9 per pixel at r_e=1 arcsec. Averaging azimuthally, the S/N ratio in an annulus at 1 arcsec is 10, easily sufficient to detect the host galaxy. WFPC2 Observations To obtain needed color information, we require images in two additional passbands: the F555W and F814W. Combined with the STIS F28X50LP image and our current F702W image, these observations will allow us to constrain the redshift of the arcs and any observed lensing galaxy, as well as constraining the star-formation level (see Figure 2; we explored color-color space extensively to find the best combination of two new sensitive filters with the existing F702W and proposed F28X50LP, best meaning the clearest separation of redshift and galaxy type.) Here a S/N of 3 per pixel is sufficient for accurate photometry --- the important point is that the arcs be well and accurately resolved. The F555W filter samples the rest-UV band, which is the band that most efficiently breaks the redshift degeneracy in combination with our current F702W image. Assuming V-R=0.6 mag for the arcs, appropriate for a galaxy at z=1.5 with a low star formation rate, Mu_V=23 mag/arcsec^2, and reaching S/N=3 per pixel requires 4400 s (1 orbit for this CVZ object). An image in the F814W filter is needed to distinguish between a very blue stellar population at z < 1.0 and a higher redshift moderately star-forming population, as it is very sensitive to the 4000Angstrom break in the latter. Given R-I=0.7 mag, we expect Mu_I=21.8 mag/arcsec^2, making a S/N of 3 achievable in 3800 s (i.e., 1 orbit). Total Requested Time A total of 3 orbits are therefore required to complete the project, 1 for STIS CCD F28X50LP, 1 for WFPC2 F555W, and 1 for WFPC2 F814W. Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8642( 4) - 02/07/00 14:46 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 1517+656 GALAXY, GRAVITATIONAL RA=15H 17M 47.6S +/- 0.5", J2000 V = 16.6+/-1 LENS, GALAXY, BL LAC DEC=65D 25' 23.9" +/- 0.5", PLATE-ID=01FX 2 1517+656- GALAXY, GRAVITATIONAL XI-OFF=0.1265"+/- 0.01", ETA- J2000 V = 16.6+/-1 POS1 LENS, GALAXY, BL LAC OFF=0.1265"+/- 0.01", FROM 1, PLATE-ID=01FX 3 1517+656- GALAXY, GRAVITATIONAL XI-OFF=0.253"+/- 0.01", ETA- J2000 V = 16.6+/-1 POS2 LENS, GALAXY, BL LAC OFF=0"+/- 0.01", FROM 1, PLATE- ID=01FX 4 1517+656- GALAXY, GRAVITATIONAL XI-OFF=0.1265"+/- 0.01", ETA- J2000 V = 16.6+/-1 POS3 LENS, GALAXY, BL LAC OFF=-0.1265"+/- 0.01", FROM 1, PLATE-ID=01FX 8642( 4) - 02/07/00 14:46 - [ 4] Visit: 01 Visit Priority: Visit Requirements: CVZ On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 1517+656 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=NO 1 30 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 1517+656 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 750 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 30 1517+656-PO STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 750 S S1 CD P ------------------------------------------------------------------------------------------------------------------------------------ 40 1517+656-PO STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 750 S S2 CD P ------------------------------------------------------------------------------------------------------------------------------------ 50 1517+656-PO STIS/C ACCUM F28X50L MIRROR CR-SPLIT=NO 1 800 S S3 CD P ------------------------------------------------------------------------------------------------------------------------------------ 60 1517+656-PO WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 50 S S3 ------------------------------------------------------------------------------------------------------------------------------------ 80 1517+656-PO WFPC2 IMAGE PC1 F814W CR-SPLIT=0.5 1 1000 S S3 ------------------------------------------------------------------------------------------------------------------------------------ 90 1517+656-PO WFPC2 IMAGE PC1 F814W CR-SPLIT=0.5 1 1000 S S2 ------------------------------------------------------------------------------------------------------------------------------------ 100 1517+656-PO WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1000 S S1 ------------------------------------------------------------------------------------------------------------------------------------ 110 1517+656 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1000 S ------------------------------------------------------------------------------------------------------------------------------------ 120 1517+656 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 1 60 S ------------------------------------------------------------------------------------------------------------------------------------ 130 1517+656 WFPC2 IMAGE PC1 F555W CR-SPLIT=0.5 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 140 1517+656-PO WFPC2 IMAGE PC1 F555W CR-SPLIT=0.5 1 1200 S S1 ------------------------------------------------------------------------------------------------------------------------------------ 150 1517+656-PO WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 1 1100 S S2 ------------------------------------------------------------------------------------------------------------------------------------ 160 1517+656-PO WFPC2 IMAGE PC1 F555W CR-SPLIT=NO 1 1100 S S3 ------------------------------------------------------------------------------------------------------------------------------------ 8642( 4) - 02/07/00 14:46 - [ 5] Summary Form for Proposal 8642 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO CR-SPLIT=2 CR-SPLIT=0.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names 1517+656 1517+656-POS1 1517+656-POS2 1517+656-POS3 ------------------------------------------------------------------------------------------------------------------------------------