8125( 7) - 08/24/00 14:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8125 Version: 7 Check-in Date: 24-Aug-2000 14:13:25 1.Proposal Title: Primordial Low-z LyAlpha Clouds towards PKS 2155-304? ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: J. Shull University of Colorado at Boulder CoI: Dr. John Stocke University of Colorado, Boulder N CoI: Dr. Mark Giroux University of Colorado, Boulder N ------------------------------------------------------------------------------------ 5. Abstract The origin of the low-redshift LyAlpha clouds remains a mystery, made more tantalizing by the possibility that they contain a significant fraction of baryons. Our GHRS/G160M study of PKS 2155-304 (Shull et al. 1998) identified a cluster of strong LyAlpha absorbers between 16, 000 and 18, 500 km s^-1 with N_ HI ~ 2 * 10^16 cm^-2 (Ly-edge estimate from ORFEUS). From expected positions of Si III and C IV absorption, we set 4Sigma limits Si/C < 0.003 and C/H < 0.005 solar abundance. Using the VLA, we identify these absorbers with the extended halos or intragroup gas surrounding four H I galaxies offset by (400-800)h_75^-1 kpc. PKS 2155-304 is a FUSE Key Science target for studies of extragalactic D/H and will produce accurate N_ HI from higher Lyman series and Lyman limits. Here, we propose to obtain a STIS/E140M spectrum (1150-1740 Angstrom) at 10 km s^-1 resolution to detect or limit metal absorption lines (C IV, Si III, Si IV, N V, C II, Si II, S II) down to below 10^-3 solar abundance (4 mAngstrom\ EW limit at S/N ~30) as well as D I LyAlpha. These data will ascertain whether the IGM in these absorbers is primordial and thus will probe the origin and chemical evolution of gas at large distances from galaxies. The combination of a bright target (V=13.1) and high-N_HI absorbers make this an ideal situation for studying metals in LyAlpha clouds. These clouds have the potential for being the most metal-free gas ever studied. ------------------------------------------------------------------------------------ 8125( 7) - 08/24/00 14:13 - [ 2] Observations Description ------------------------ We propose to obtain a 10-orbit exposure (totaling 475 minutes) of PKS 2155-304 with the STIS/E140M echelle (1150- 1740 Angstrom) taken through the 0.2'' * 0.2'' slit. This exposure should provide S/N ~ 30 per 0.0145 Angstrom resolution element, adequate to detect weak features due to metal-ion absorption at 4Sigma significance down to 4 mAngstrom\ equivalent width. The S/N estimates are based on the STIS Phase II simulator, using the median flux level, F_Lambda = 1.1 * 10^-13 ergs cm^-2 s^-1 AA^-1 at 1300 Angstrom from the Penton & Shull (1997) IUE-AGN database. Real Time Justification ----------------------- None. We have imaged the PKS 2155-304 field (VLA) in the 21-cm line of H I. At the velocity of the LyAlpha absorbers (16, 000-- 17, 000 km s^-1) we find four large H I galaxies and one dwarf (van Gorkom et al. 1996; Shull et al. 1998), offset from the sightline by projected distances of (400-800)h_75^-1 kpc (Fig. 2). In September 1998, Stocke and Weymann will use the multi-object spectrograph on the Dupont (Las Campanas) telescope to search for the locations and redshifts of possible absorbers near the 2155-304 sightline. Approximately 60 galaxy redshifts will be obtained within a 20' radius of 2155-304, down to m_B ~ 20, corresponding to M_B = -17 at the redshift distance, (225 Mpc)h_75^- 1, of the 17, 000 km s^-1 absorbers. With van Gorkom, we are planning a much deeper VLA 21-cm image of the 2155-304 field. Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8125( 7) - 08/24/00 14:13 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 PKS2155-304 GALAXY, BL Lac RA=21H 58M 51.99S +/- 0.3S, DEC= J2000 Z=0.117 V = 13.6+/-0.1 -30D 13' 32.2" +/- 3.0", PLATE- B-V = 0.35+/-0.1 ID = 02J3 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) B1979.0 0.0 0.0 0.0 8125( 7) - 08/24/00 14:13 - [ 4] Visit: 01 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 PKS2155-304 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.5 S ONBOARD ACQ FOR 2-3 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 PKS2155-304 STIS/F ACCUM 0.2X0.2 E140M 1425 1 2347.0 EXPAND UV-MAM A Comments: First orbit exposure short due to overheads ------------------------------------------------------------------------------------------------------------------------------------ 3 PKS2155-304 STIS/F ACCUM 0.2X0.2 E140M 1425 4 2945.0 EXPAND UV-MAM A Comments: Four orbit exposure ------------------------------------------------------------------------------------------------------------------------------------ 8125( 7) - 08/24/00 14:13 - [ 5] Visit: 02 Visit Priority: Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 PKS2155-304 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 0.5 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 PKS2155-304 STIS/F ACCUM 0.2X0.2 E140M 1425 1 2347.0 EXPAND UV-MAM A Comments: First orbit exposure short due to overheads ------------------------------------------------------------------------------------------------------------------------------------ 3 PKS2155-304 STIS/F ACCUM 0.2X0.2 E140M 1425 4 2945.0 EXPAND UV-MAM A Comments: Four orbit exposure ------------------------------------------------------------------------------------------------------------------------------------ 8125( 7) - 08/24/00 14:13 - [ 6] Summary Form for Proposal 8125 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 0.2X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-3 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names PKS2155-304 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------