10703(  6) - 12/14/05 09:37  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 10703

Version: 6    Check-in Time: 14-Dec-2005 14:37:19

Title
Life in the fast lane: The dark-matter distribution in the most massive galaxy
clusters in the Universe at z>0.5
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Type             Cycle     Parallel Pointing Tolerance
GO/DD            13        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. Harald Ebeling                University of Hawaii
   CoI: Dr. Jean-Paul Kneib               California Institute of Technology            N
   CoI: Dr. Ian R. Smail                  University of Durham                          N
   CoI: Dr. Alastair C. Edge              University of Durham                          N
   CoI: Dr. Priyamvada Natarajan          Yale University                               N
   CoI: Dr. Graham Smith                  California Institute of Technology            N
   CoI: Dr. Nick Kaiser                   University of Hawaii                          N
   CoI: Mr. David Donovan                 University of Hawaii                          N
   CoI: Dr. Leon van Speybroeck           Smithsonian Institution Astrophysical         N
                                          Observatory
   CoI: Ms. Elizabeth Barrett             University of Hawaii                          N

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Abstract

We propose two-filter ACS observations of a complete sample of 12 very X-ray
luminous galaxy clusters at 0.5<z<0.7 as a cornerstone of a comprehensive multi-
wavelength study of the properties of the most massive clusters in the universe. Our
sample includes the famous systems Cl0016+16 and MS0451-03; all other clusters are
new discoveries from the MACS survey. Being the counterparts of the best-studied
systems at lower and higher redshift and comprising ALL massive clusters at
0.5<z<0.7 observable from Mauna Kea this sample will become the ultimate reference
for cluster studies at z>0.5. HST's unique capabilities will allow us to: 1) measure
accurately the clusters' dark matter distribution on scales from tens to more than
500/h_50 kpc from observations of strong and weak gravitational lensing, 2) use
galaxy-galaxy lensing to measure the shape, extent, and mass content of the dark-
matter halos of both cluster and field galaxies, and 3) study the color morphology
of mergers and the star formation history of galaxies in a high-density environment.
The proposed observations are complemented by Chandra observations of all our
targets (all 12 awarded, 11 executed to date) which provide independent constraints
on the dark matter and gas distribution in the cluster cores, as well a by extensive
groundbased observations of weak lensing on yet larger scales, galaxy dynamics, and
the SZ effect.
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  Observations Description
  ------------------------

     PROPOSED OBSERVATIONS We propose moderately deep ACS observations in two
     passbands (two orbit in each of F555W and F814W) of a complete subsample of the
     12 most distant MACS clusters at 0.5<z<0.7 as a cornerstone of our
     comprehensive, quantitative, multi-waveband study of the properties of massive
     distant clusters. The depth of the proposed observations is dictated by the
     requirement for an accurate measurement of the gravitational shear field;
     resolved color information is critical for secure identifications of all faint,
     multiply imaged background objects and thus an unambiguously determined lens
     model, as well as for the proposed detailed study of the cluster galaxy
     population and its evolution. We expect MACS to become the sample of choice for
     future studies of massive clusters at intermediate redshift 0.3<z<0.7 and
     stress the heritage character of the observations proposed here which will
     prove useful to a wider community for cluster-related studies. SUPPORTING
     OBSERVATIONS X-ray Thanks to an investment of several hundreds of ks of Chandra
     GTO time by Leon Van Speybroeck all 12 proposed clusters have been or will be
     observed with ACIS-I, yielding superb X-ray data for this best-defined sample
     of massive distant clusters ever (see Fig.~5). Groundbased optical The proposed
     HST observations are supported and complemented by observations with Subaru,
     Gemini, and Keck. We are obtaining multi-object spectroscopy (MOS) of the
     galaxies in MACS clusters to assess the prevalence of velocity substructure.
     MOS data for 9 of the 12 MACS clusters at z>0.5 are in hand, and yield velocity
     dispersions ranging from 820 to 1700 km/s, time to complete the sample has been
     awarded on Gemini/GMOS. In addition, we are conducting a wide-field VRIz'
     imaging survey of MACS clusters with Subaru/SuprimeCam to constrain the large-
     scale mass distribution beyond the ACS field of view via weak lensing, and to
     obtain global colours for galaxies in these regions. SuprimeCam data for eight
     of our 12 most distant MACS clusters are in hand (e.g. Fig.~4); the full sample
     will be completed using three additional SuprimeCam nights in the coming
     semester. Sub-mm and radio We are collaborating with John Carlstrom's group to
     secure detections of the Sunyaev-Zel`dovich effect in MACS clusters with the
     OVRO and BIMA mm arrays. Strong detections have been obtained for all 8
     clusters at z>0.5 observed so far (LaRoque et al. 2003). TECHNICAL
     JUSTIFICATION Our primary goal is to map in detail the dark matter distribution
     in these clusters. The ACS field extends to a median radius of 120'' or
     ~0.9/h50 Mpc at z=0.5-0.7. The high masses expected for these X-ray luminous
     systems predict typical shears of 20% at this radius. Our analysis of our
     Subaru data imaging (see Fig.~4) confirm that ground-based data can measure the
     radially averaged shear on these scales, but can not provide the high-
     resolution mass maps essential for comparing the dark matter and intracluster
     gas on ~100 kpc scales in the cluster cores. The limiting shear necessary per
     resolution element in our mass map determines the required depth. The rms error
     of the shear averaged over the shapes of galaxies in a patch of sky is
     sigma_gamma ~ \sigma_epsilon/sqrt(N), where sigma_epsilon ~ 0.25 is the rms
     intrinsic ellipticity of field galaxies and N is the number per patch. A 3sigma
     detection on 20'' (~150 kpc) scales of the shear at the edge of our field thus
     requires a sensitivity of sigma_gamma ~ 7% (the systematic errors due to the
     HST optics can be removed to a negligible level compared to this, Rhodes et al.
     2000; Hoekstra et al. 1998). This equates to a galaxy surface density of
     150/arcmin^2, or a limiting magnitude of I=25.5, for clusters at z~0.5-0.7,
     after accounting for 30% of field galaxies lying in front of the clusters
                                                10703(  6) - 12/14/05 09:37  - [  3]

     (Kneib et al. 1996). Based on our previous lensing work with HST we estimate
     that we need to measure the shapes of individual galaxies to better than 20%
     accuracy to be dominated by their intrinsic ellipticity distribution. This
     requires that 10 pixels across the galaxy are detected above a S/N=1 per pixel
     threshold. With 4 WFC pixels averaged to produce a 0.1''x 0.1'' superpixel, and
     for galaxies at I=25.5, with scale sizes of r_hl ~ 0.25'', this corresponds to
     mu_F814W=25.7 mag/arcsec^2. For a 1sigma detection (per 2x2 WFC pixel) of this
     isophote using F814W, we calculate an exposure time of 5000 s using the WWW
     exposure tool. This exposure time translate into 2 orbits using LOW-SKY (to
     reduce Earth-shine) and hence a visibility of 2800 sec for a typical target. We
     will dither between exposures to cover the gaps between the WFC chips and also
     to reduce the effects of flatfield and hot pixel errors. Customized software
     (ImCat) designed to take into account the HST PSF (Kaiser 2000) is in place for
     a comprehensive analysis of the observed shear field. The F814W observations
     will probe the restframe morphology of the cluster members longward of 4000A.
     Shortwards of 4000A we complement these with observations in F555W to study the
     internal color structure of the cluster members and most importantly to use
     colors to confirm the identity of multiply imaged background sources in the
     crowded cluster cores. To achieve the latter we require photometry to 20%
     precision at the limit of our arclet catalog, I=25.5, or V=26.5 using the
     typical color at this depth. For a 5-sigma detection of V=26.5 in F555W within
     a 1'' aperture, this requires 5600 s and hence 2 orbits per cluster (again
     based on LOW-SKY).

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


  Calibration Justification
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  Additional Comments
  -------------------

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TARGET LIST    Fixed Targets
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Tar|   Target   |         Target        |            Target              |Coord   | Radial |             Flux data
No |    Name    |       Description     |           Position             |Eqnx    |  Vel.  |
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1   MACSJ0025-   CLUSTER OF GALAXIES,    RA=00H25M30.2300S +/- 1", DEC=-  J2000             V = 18.0
    1222         HIGH REDSHIFT CLUSTER,  12D22'43.00" +/- 1"
                 RICH CLUSTER
    Coordinate Source: Other_Source: Chandra

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Patterns
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Pattern_Number: 1

                    Primary_Pattern                   Secondary_Pattern
Pattern_Type        LINE                              ACS-WFC-DITHER-LINE
Pattern_Purpose     DITHER                            DITHER
Number_Of_Points    2                                 3
Point_Spacing       3.011                             0.138
Line_Spacing        <none>                            <none>
Coordinate_Frame    POS-TARG                          POS-TARG
Pattern_Orient      85.28                             32.99
Angle_Between_Sides <none>                            <none>
Center_Pattern      YES                               YES

Pattern_Comments:    6-position dither
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                                                10703(  6) - 12/14/05 09:37  - [  6]

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


Exposures
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Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MACSJ0025-1 ACS/WFC  ACCUM   WFC       F555W            CR-SPLIT=NO                    1   690 S      LOW-SKY; PATTERN 1 1
     222
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2    MACSJ0025-1 ACS/WFC  ACCUM   WFC       F814W            CR-SPLIT=NO                    1   700 S      LOW-SKY; PATTERN 1 2
     222
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Sub Exposures
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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 |
------------------------------------------------------------------------------------------------------------------------------------
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            1            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            2            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            3            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            1            none       none       2       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            2            none       none       2       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            3            none       none       2       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            1            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            2            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            3            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            1            none       none       4       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            2            none       none       4       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            3            none       none       4       N/A
                                                10703(  6) - 12/14/05 09:37  - [  7]

Visit: 51
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
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Exp |   Target  | Instr  | Oper. | Aper    |Spectral|Central|           Optional           |Num|   Time   |       Special
Num |    Name   | Config | Mode  | or FOV  |Element |Waveln.|          Parameters          |Exp|          |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1    MACSJ0025-1 ACS/WFC  ACCUM   WFC       F555W            CR-SPLIT=NO                    1   690 S      LOW-SKY; PATTERN 1 1
     222
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2    MACSJ0025-1 ACS/WFC  ACCUM   WFC       F814W            CR-SPLIT=NO                    1   700 S      LOW-SKY; PATTERN 1 2
     222
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Sub Exposures
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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 |
------------------------------------------------------------------------------------------------------------------------------------
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            1            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            2            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          1            3            none       none       1       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            1            none       none       2       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            2            none       none       2       N/A
MACSJ0025-1222      1     ACS/WFC  ACCUM   WFC      F555W          2            3            none       none       2       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            1            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            2            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          1            3            none       none       3       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            1            none       none       4       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            2            none       none       4       N/A
MACSJ0025-1222      2     ACS/WFC  ACCUM   WFC      F814W          2            3            none       none       4       N/A
                                                10703(  6) - 12/14/05 09:37  - [  8]

            Summary Form for Proposal  10703

  Item                         Used in this proposal
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  Apertures                    WFC
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  Configurations               ACS/WFC
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  Opmodes                      ACCUM
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  Optional Parameters          CR-SPLIT=NO
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  Special Requirements         LOW-SKY; PATTERN 1 1; PATTERN 1 2;
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  Spectral Elements            F555W F814W
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  Target Names                 MACSJ0025-1222
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