8266(  6) - 05/22/00 13:21  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      8266
                                                       Version: 6
                                                       Check-in Date: 22-May-2000
                                                                      13:19:39

1.Proposal Title:
The Coherence Length of Lyman-Alpha Absorbers at z ~ 1
------------------------------------------------------------------------------------
2. Proposal For  3. Cycle
GO               8
------------------------------------------------------------------------------------
4. Investigators
                                                                                     Contact?
    PI: Chris Impey                       University of Arizona
   CoI: Ms. Cathy Petry                   University of Arizona                         N

------------------------------------------------------------------------------------
5. Abstract

We propose to use STIS spectroscopy to measure the coherence length of Lyman-Alpha
absorbers in the redshift range 0.5 < z < 1.1, using a new pair of quasars with a
separation of 130 arcseconds. Quasars with this brightness (R = 18.1), redshift and
separation are very rare. Coincident absorbers along the line-of-sight to quasar
pairs can reveal the geometry of the diffuse but highly ionized gas that makes up
the Lyman-Alpha forest; this gas represents the majority of the baryons and it
traces the underlying dark matter distribution. The simple physical history of these
low density regions of the universe means that HST quasar spectra can be used to
derive the shape and amplitude of the power spectrum of mass fluctuations. We will
use automated line selection and fitting software to identically measure the HST
spectra and spectra extracted from SPH supercomputer simulations. The goals are to
(a) make an improved measurement of the coherence length of Lyman-Alpha absorbers at
z ~ 1, (b) determine whether or not the coherence length is a function of line
strength or column density, (c) look for evolution as a function of column density
across the range 0.5 < z < 1.1, and (d) make a direct comparison between the
absorber properties and the predictions of supercomputer simulations on scales of ~
0.5 h_100^-1 Mpc.
------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [  2]

  Observations Description
  ------------------------

     The proposed STIS spectroscopic observations are of a new pair of bright
     quasars, Q1649+305 A, B, which have emission redshifts of 1.345 and 1.125,
     respectively. This pair will be used to study Lyman-Alpha absorbers over
     transverse proper distances of 0.43-0.56 h_100^-1 Mpc for the range 0.5<z<1.1.
     Because neither quasar has yet been observed in the ultraviolet, there is a
     chance that the spectrum could be partially or completely absorbed by an
     intervening Lyman limit system (LLS). The longest wavelength that may be
     affected by a LLS is set by the emission redshift for the QSO. Using the higher
     of the two redshifts to preserve the common wavelength range between the
     spectra, a LLS at z=1.345 will absorb at wavelengths <= 2140 Angstrom, which
     corresponds to Lyman-Alpha absorption at z=0.76. Since the science goals of
     this project may be achieved using the wavelength range corresponding to 0.76 <
     z < 1.1, any absorption by LLSs is not an issue. The choice of instrumental co
     nf iguration is driven by the required wavelength coverage, resolution, and the
     signal-to-noise ratio (SNR) needed to detect absorbers to very low column
     densities (\log N_HI = 13.5) over the range 0.48 < z < 1.13 . The NUV-MAMA with
     the G230L grating centered at 2376 Angstrom\ provides spectral coverage over
     1570- 3180 Angstroms and includes the Lyman-Alpha emission feature of both
     quasars along with a significant portion of the Lyman-Alpha ``forest''. With
     the 52*0.1" slit, the resolution of this grating is moderate, 3.47 Angstroms or
     about 400 km s^-1, but both quasars are too faint to utilize any of the higher
     resolution configurations. An advantage of the G230L is that sufficient
     wavelength coverage is obtained to help reliably identify intervening metal-
     line systems. In using the 0.1"\ slit, a small decrease in transmission is
     traded for a well determined line spread function with minimal deviation from a
     Gaussian profile in the wings, which is essential for measuring an d deblending
     the absorption features. At a declination of 30 degrees the visibility is 53
     minutes. Overheads for the first orbit of a visit consists of a 7 min guide
     star acquisition, a 6 min target acquisition, a 6 min target ``peakup''
     required for the 0.1"\ slit, and 6 min for instrumental overhead --- a total of
     25 min. Overheads for subsequent orbits in a visit are 6 min for a guide star
     re- acquisition plus 1 min for instrumental overhead --- a total of 7 min. Time
     allocated to science exposures is 25 min for the first orbit and 46 min for
     subsequent orbits in a visit. The STIS ETC is used to estimate the SNR given an
     exposure time and a template spectral enery distribution. Given the wavelength
     range of interest and redshift of the quasars, the most accurate way estimate
     the UV flux of the Lyman-Alpha ``forest'' for each target is to use a composite
     spectrum made up of HST UV observations of quasars (Zheng et al. 1997, ApJ 475,
     469). The composite spectrum was reds hi fted to match each quasar and
     normalized to its O photographic magnitude (approximately Johnson B band)
     derived from POSS I plates, which have been corrected by subsequent APS
     measurements to reduce the rms error in magnitudes to about 0.1. Twenty orbits
     split in to 4 visits of 5 orbits each (to allow for the SAA), or 848 minutes of
     integration, is allotted to component A; 8 orbits split into 2 visits of 4
     orbits each, or 332 minutes of integration, is chosen for component B. The SNR
     per pixel is related to the inverse of the 1 sigma limiting equivalent width by
     the square root of the product of the dispersion and the instrumental
     resolution. The expected number of Lyman-Alpha lines is then calculated using
     the fitted coefficients derived for the low- redshift evolution of line density
     by Jannuzi, et al. 1998 (ApJS, 118, in press), and is 26 and 25 lines for
     components A and B, respectively, for 0.48 < z < 1.13. This is an improvement
     by a factor of ~ 2.5 on the statistics u sed in prior work on pairs at low
     redshift (Dinshaw et al. 1995, Petitjean et al. 1998). Sub-samples of lines may
     be formed, which are complete to various values of 4 sigma rest limiting
     equivalent width, W_thr, to look for coherence length dependence on line
     strength and evolution of the absorbers as a function of line strength and
     redshift. In particular , four samples may be formed, each with an upper bound
     of z_u = 1.13 with lower bounds of z_l=0.94, 0.73, 0.65, 0.56 and completeness
     limits of W_thr = 0.14 Angstrom, 0.18 Angstrom, 0.24 Angstrom, and 0.30
     Angstrom, respectively. The number of lines expected for each of these samples
     is about 10, 18, 17, and 15, respectively. The statistics are sufficient to
     test for the differential evolution as a function of line strength noted by
     Dobryzcki & Bechtold 1997 (Proc. 13^th IAP Astrophysics Colloquium) in QSO
     Absorption Line Key Project data, and to confirm the marginal detection by
     Petitjean et al. (1998) of a coherence le ngth that increases for weaker lines.
     As the completeness limit lowers, the line density increases, and line
     deblending will become more of an issue. The ratio of the line density to the
     instrumental resolution is one way of computing the probability that lines will
     be blended (Weymann et al. 1998, astro-ph/9806123). These values are 5.6 and
     7.1 for the two sub-samples with W_thr of 0.14 Angstrom\ and 0.18 Angstrom,
     respectively, and are much higher for the two remaining sub-samples. A value of
     less than ~ 5 is indicative that deblending the lines must be modelled
     effectively. We will utilize completely automated line finding software based
     on the QSO Absorption Line Key Project algorithms (Petry et al. 1998, ApJ 494,
     60) to measure and deblend the lines. The advantage of this procedure is that
     measurements of many realizations of simulated data will quantify how reliably
     lines may be deblended for each sub-sample of lines. We stress that the primary
     science goal of this pr oposal --- determination of the coherence length of
     Lyman-Alpha absorption at z ~ 1 --- is not compromised by issues of blending.

  Real Time Justification
  -----------------------

     Existing MMT spectroscopy is sufficient to identify strong metal line
     absorption systems (based on the MgII doublet) that might place high ionization
     metal lines in the Lyman-Alpha forest region. We will use the upgraded 6.5m MMT
     for full-coverage high-resolution optical spectroscopy of each quasar to more
     completely identify intervening metal- line systems which could contaminate the
     low-redshift Lyman- Alpha sample. The same data can be used to look for
     variations in metal lines systems across the transverse scale of 0.5 Mpc. Deep
     imaging with the Steward Observatory 2.3m is planned using a new thinned 2kx2k
     CCD to locate galaxies down to R ~ 24, which corresponds to L_* at z ~ 1.
     Galaxy redshifts will be measured from follow-up spectroscopic observations
     using the Hectospec multiobject spectrograph on the upgraded 6.5m MMT. The deep
     multi- color imaging will also result in a list of candidates for new quasars
     close in angular separation to Q1649+305A, B which can then be us ed to expand
     the grid of probes in this direction on the sky. Confirmation of the quasars
     and subsequent absorption line measurements will also use the Hectospec
     spectrograph.

  Calibration Justification
  -------------------------


  Additional Comments
  -------------------

------------------------------------------------------------------------------------

                                                8266(  6) - 05/22/00 13:21  - [  3]

  Data Distribution

  Paper Products:  NO
  Media:           ELECTRONIC
  Blocking Factor: 10
  Ship To:         PI_Address
  Ship Via:
  Email:
------------------------------------------------------------------------------------

                                                8266(  6) - 05/22/00 13:21  - [  4]

TARGET LIST    Fixed Targets
------------------------------------------------------------------------------------------------------------------------------------
Tar|   Target   |         Target        |            Target              |Coord   | Radial |             Flux data
No |    Name    |       Description     |           Position             |Eqnx    |  Vel.  |
------------------------------------------------------------------------------------------------------------------------------------
1   FIRST-       GALAXY, QSO, LYMAN      RA=16H 49M 19.01S +/- 1",        J2000    Z =      B = 18.7+/-0.1
    J164919.0+30 ALPHA CLOUD             DEC=30D 47' 18.0" +/- 1"                  1.345    R = 18.1+/-0.1
    4718                                                                                    V = 18.3+/-0.3

2   FIRST-       GALAXY, QUASAR, LYMAN   RA=16H 49M 28.87S +/- 1",        J2000    Z =      B = 18.0+/-0.1
    J164928.9+30 ALPHA CLOUD             DEC=30D 46' 52.4" +/- 1"                  1.125    R = 18.1+/-0.1
    4652                                                                                    V = 18.2+/-0.3

                                                8266(  6) - 05/22/00 13:21  - [  5]

Visit: 01
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   13 S
         19.0+304718 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   19 S
         19.0+304718 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      4   45 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [  6]

Visit: 02
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   10 S
         19.0+304718 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   15 S
         19.0+304718 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      4   45 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [  7]

Visit: 03
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   10 S
         19.0+304718 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   15 S
         19.0+304718 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      4   45 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [  8]

Visit: 04
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   10 S
         19.0+304718 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   15 S
         19.0+304718 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      2   45 M    EXPAND
         19.0+304718 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [  9]

Visit: 05
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   11 S
         28.9+304652 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   16 S
         28.9+304652 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         28.9+304652 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      4   45 M    EXPAND
         28.9+304652 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [ 10]

Visit: 06
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
10       FIRST-J1649 STIS/C ACQ     F28X50L MIRROR           ACQTYPE=POINT                     1   10 S
         28.9+304652 CD             P
------------------------------------------------------------------------------------------------------------------------------------
20       FIRST-J1649 STIS/C ACQ/PEA 52X0.1  MIRROR                                             1   15 S
         28.9+304652 CD     K
------------------------------------------------------------------------------------------------------------------------------------
30       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      1   30 M    EXPAND
         28.9+304652 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
40       FIRST-J1649 STIS/N ACCUM   52X0.1  G230L    2376                                      4   45 M    EXPAND
         28.9+304652 UV-MAM
                     A
------------------------------------------------------------------------------------------------------------------------------------
                                                8266(  6) - 05/22/00 13:21  - [ 11]

            Summary Form for Proposal  8266

  Item                         Used in this proposal
------------------------------------------------------------------------------------------------------------------------------------
  Apertures                    F28X50LP 52X0.1
------------------------------------------------------------------------------------------------------------------------------------
  Configurations               STIS/CCD STIS/NUV-MAMA
------------------------------------------------------------------------------------------------------------------------------------
  Opmodes                      ACQ ACQ/PEAK ACCUM
------------------------------------------------------------------------------------------------------------------------------------
  Optional Parameters          ACQTYPE=POINT
------------------------------------------------------------------------------------------------------------------------------------
  Proposal Category            GO
------------------------------------------------------------------------------------------------------------------------------------
  Special Requirements         EXPAND
------------------------------------------------------------------------------------------------------------------------------------
  Spectral Elements            MIRROR G230L
------------------------------------------------------------------------------------------------------------------------------------
  Target Names                 FIRST-J164919.0+304718 FIRST-J164928.9+304652
------------------------------------------------------------------------------------------------------------------------------------
  Wavelengths                  2376
------------------------------------------------------------------------------------------------------------------------------------