6579( 10) - 03/12/97 14:57 - [ 1]
PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only
ID: 6579
Version: 10
Check-in Date: 28-Mar-1996
19:26:22
1.Proposal Title:
The Cosmic Velocity of the Great Attractor
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2. Scientific Category 3. Proposal For 4. Cycle
COSMOLOGY GO 6
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5. Investigators
Contact?
PI: JOHN L. TONRY INSTITUTE FOR ASTRONOMY
CoI: Edward A. Ajhar National Optical Astronomy Observatories N
CoI: Jon A. Holtzman New Mexico State University N
CoI: Tod R. Lauer National Optical Astronomy Observatories N
CoI: Marc Postman Space Telescope Science Institute N
CoI: Alan Dressler Observatories of the Carnegie Institution N
of Washington
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6. Abstract
We request deep WFPC2 images of five galaxies in clusters near the Centaurus Cluster
to determine their distances and peculiar velocities using the Surface Brightness
Fluctuation (SBF) method. We expect to determine whether the Centaurus galaxies are
falling into the "Great Attractor", and distinguish between the claims that there is
convergence of the flow beyond Centaurus (return to CMB rest frame) instead of
continued ~600 km/s bulk flow. Confirmation of the latter would indicate that
coherent peculiar velocities are generated over an enormous scale. HST is required
as the ground-based SBF method becomes impossible beyond 3,000 km/s. The high
spatial resolution of HST permits precise measurement of the SBF signal at very
large distances, and allows easy recognition of globular clusters and background
galaxies in the galaxy images that must be removed prior to SBF detection. The
proposing team developed the SBF method, has worked extensively in the field of
large scale flows, played major roles in the calibration of WFPC and WFPC2, and are
conducting GTO observations which will help validate the zeropoint and application
of the SBF method for local galaxies.
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Observations Description
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We will measure distances to these galaxies by obtaining high signal-to-noise
images and observing their surface brightness fluctuations (SBF), the
irreduceable mottling of an otherwise smooth galaxy from the finite number of
stars. This has been described in detail in Tonry (1991) and Jacoby et al.
(1992). The fundamental idea is simply that the finite number of stars in a
galaxy gives rise to a measurable bumpiness in its surface brightness, even
though individual stars remain completely unresolved. The variance is the
product of the galaxy brightness per pixel and what we refer to as the "average
luminosity" of the stellar population, the ratio of the second moment of the
stellar luminosity function to the first, Lbar. Obviously there are many
sources of pixel-to-pixel noise (variance) in an image of a galaxy such as
white noise from detector noise and photon statistics, roughness from patchy
dust obscuration, spiral arms, and inhomogeneous stellar population, bumpiness
from globular clusters (GCs) or background galaxies, and finally the SBF
mottling from the Poisson statistics of the finite number of stars in a pixel.
Photon shot noise only grows as the square root of time, and the SBF signal
becomes detectable when a source of Lbar contributes more than one photon in an
image ( i.e., when stellar counting statistics dominate photon counting
statistics). We apply the SBF method primarily to E and S0 galaxies, which
avoids problems with dust and spiral arms. Unraveling the third source of
variance proceeds in two steps. First we identify as many "compact sources"
(stars, background galaxies, and primarily GCs) as possible and excise them
from the image. Second, we fit a luminosity function to the sources which we
found and integrate the extrapolation to fainter magnitudes to determine the
residual variance left in the image from the unseen sources. The variance from
the SBF of stars in the galaxy is then the difference between that which is
measured and the estimate of the residual. The fainter we can identify and
remove sources, the smaller the residual correction. In practice, if our
compact source completeness dies out two magnitudes brighter than the peak of
the GC luminosity function, the resulting SBF measurement is quite uncertain
because most of the variance in the picture results from the (uncertain)
residual variance. If we can identify sources all the way to the peak of the GC
luminosity function, the residual variance is typically only 20% of the net
variance and it contributes negligible uncertainty. Removal of compact sources
makes it critical that the images be as sharp as possible. Measurement of SBF
is accurate and reliable at the distance of Virgo (1300km/s) in 1" seeing, but
becomes difficult in 1.5" seeing. At the distance of the Great Attractor
environs (three or four times Virgo), even the best seeing from the ground
(0.5") becomes inadequate. We believe that our attempts to observe SBF in
Centaurus (twice Virgo) from the ground have been successful, but we have
reservations about their reliability, since the seeing (0.75") was barely
adequate for the distance (2400km/s). However, HST has resolution to spare and
we should be able to identify GCs significantly fainter than the peak of the
luminosity function. Although the HST PSF is somewhat undersampled even with
the PC, we have enough experience with 0.48" pixels in 0.9" seeing at KPNO and
CTIO to know that this is not a problem with our power spectrum analysis. One
concern might be PSF spatial variability across the CCD fields; however, this
is expected to be small within WFPC2, and will be mapped by one of us
(Holtzman) as part of WFPC2 calibration. In any case, PSF variability can be
included in the SBF analysis and is not a limiting factor. We further note that
we will get useful information from both the WF and PC CCDs. Hot pixels had
been an initial concern with WFPC2, but they are much less important now that
the operating temperature of WFPC has been lowered, and Holtzman has extensive
experience with their detection and removal. Further, the observations will be
dithered to counter any residual hot pixels or PSF-scale flat-fielding errors
(We find the S/N and quality of the WFPC2 F814W flats as produced so far to be
more than sufficient for SBF work). A common worry about deriving distances
from SBF measurements is the extent to which we can predict the absolute
magnitudes Mbar which correspond to the mbar which we measure. We have found
empirically that in the I band stellar populations of differing age and
metallicity vary predictably with overall color, and this has been confirmed
theoretically (Worthey 1993). As a demonstration of this, Figure 2 shows the
SBF data we have obtained for the Fornax and Virgo clusters, plotting mbar_I
against the (V-I) color of the galaxy. It should be noted that these are all of
the galaxies we have observed in these clusters, without regard for E versus
S0, presence of dust, specific frequency of GCs (NGC4486, NGC1399, and NGC1316
appear here), etc. The rms scatter about the stellar population line is only
0.16 magnitude. In the case of Fornax we typically have 0.14 mag of
observational error (the median seeing was 1.54"), and Virgo has a significant
contribution from the depth of the cluster (the core of Virgo has a radius of
0.1 radian across the line of sight which presumably corresponds to a 10% depth
as well). These considerations, as well as the repeatability of multiple
measurements, suggests that with sufficient resolution we routinely achieve a
distance error of 5% per galaxy. A final concern is how well we understand the
SBF zero point itself. Ground based application of SBF out to 2,400 km/s has
yielded an important determination of H_0 (Tonry 1991), and there are vigorous
ongoing efforts to establish this zero point beyond any doubt. For example, at
present the SBF zero point is based on Cepheid distances to M31 and M81.
However the HST key project and SN1a long term project have produced
preliminary Cepheid distances to 6 other galaxies. The difference between SBF
and these new Cepheid distances is 0.06 magnitude with a scatter of 0.11
magnitude, indicating that SBF is indeed very accurate. As these Cepheid
distances are published our zero point will become firmly tied to the Cepheid
distance scale. On the theoretical front, the stellar models and calculations
of Worthey (1993) agree well with our empirical calibration of Mbar_I. We know
from nearby galaxies in Virgo that the apparent fluctuation magnitude for
galaxies of (V-I) = 1.20 is 29.64. Based on estimates of the Coma to Virgo
distance ratio (5.6), the distance to Virgo is 1286 km/s, so mbar_I will be
31.48 at 3000km/s. In order to achieve full 5% accuracy, it is essential that
the SBF signal significantly exceed the photon/read noise. Since we will have
extremely good compact source rejection, experience from ground based
observations indicates that we can get by with only 5 photons per mbar_I, but
less than this will seriously compromise the accuracy. WFPC2 with the F814W
filter detects 1 photon per second from a 23.78 magnitude source, so at a
distance D km/s the exposure time required to receive 5 photons per mbar_I is t
= 6000 sec (D/3000 km/s)^2 10^[1.6(V-I)-1.20]. The individual exposures will be
CR-split. We will request offsets of a few pixels between the exposures to
improve flat-fielding and rejection of any residual hot pixels or other CCD
defects.
Real Time Justification
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None. We have ground-based V and I images of many of the targets, and we will
obtain V and I photometry for the rest at LCO or CTIO. We have extensive
(>1500) SBF observations of nearby galaxies which we have used to verify the
method and understand systematics. Compared to ground based observations with
poor seeing, HST data will be much easier to work with.
Calibration Justification
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Additional Comments
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6579( 10) - 03/12/97 14:57 - [ 3]
Data Distribution
Media: 8MM
Blocking Factor: 10
Ship To: PI_Address
Ship Via:
Email:
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TARGET LIST a) 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 NGC4767 GALAXY,ELLIPTICAL RA=12H 53M 52.84S +/- 1",DEC=- J2000 V = 2997 SURF(V) = 21.4+/-0.3
39D 42' 49.7" +/- 1",PLATE-
ID=0162
2 NGC4709 GALAXY,ELLIPTICAL RA=12H 50M 03.96S +/- 1",DEC=- J2000 V = 4683 SURF(V) = 21.4+/-0.3
41D 22' 53.7" +/- 1",PLATE-
ID=0162
3 NGC5516 GALAXY,ELLIPTICAL RA=14H 15M 54.69S +/- 1",DEC=- J2000 V = 4460 SURF(V) = 21.4+/-0.3
48D 06' 53.7" +/- 1",PLATE-
ID=01F7
4 ESO507-G045 GALAXY,ELLIPTICAL RA=12H 55M 35.60S +/- 1",DEC=- J2000 V = 4825 SURF(V) = 21.4+/-0.3
26D 49' 30.2" +/- 1",PLATE-
ID=04LL
5 NGC6876 GALAXY,ELLIPTICAL RA=20H 18M 19.04S +/- 1",DEC=- J2000 V = 3836 SURF(V) = 21.4+/-0.3
70D 51' 30.1" +/- 1",PLATE-
ID=01CB
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Visit: 01
Visit Requirements: <none>
On Hold Comments: <none>
Additional Comments: <none>
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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101 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 0.00,0.00
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102 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 0.8969,0.0946
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103 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 1.0010,0.8908
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104 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.1052,0.9954
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105 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG -0.8903,1.1006
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106 NGC4767 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.9965,-0.0940
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6579( 10) - 03/12/97 14:57 - [ 6]
Visit: 02
Visit Requirements: ORIENT 240D TO 40D
On Hold Comments: <none>
Additional Comments: <none>
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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201 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 0.00,0.00
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202 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.8969,0.0946
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203 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 1.0010,0.8908
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204 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.1052,0.9954
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205 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG -0.8903,1.1006
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206 NGC4709 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.9965,-0.0940
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6579( 10) - 03/12/97 14:57 - [ 7]
Visit: 03
Visit Requirements: <none>
On Hold Comments: <none>
Additional Comments: <none>
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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301 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 0.00,0.00
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302 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.8969,0.0946
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303 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 1.0010,0.8908
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304 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.1052,0.9954
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305 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG -0.8903,1.1006
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306 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.9965,-0.0940
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307 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG -1.1006,-0.8903
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308 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.1052,-0.9954
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309 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 1.0895,-1.1017
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310 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 1.8857,-1.2058
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311 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 1.9931,0.1881
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312 NGC5516 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 2.0982,1.1835
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Visit: 04
Visit Requirements: <none>
On Hold Comments: <none>
Additional Comments: <none>
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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401 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1000 S POS TARG 0.00,0.00
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402 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.8969,0.0946
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403 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 1.0010,0.8908
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404 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 0.1052,0.9954
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405 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG -0.8903,1.1006
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406 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.9965,-0.0940
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407 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG -1.1006,-0.8903
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408 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.1052,-0.9954
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409 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 1.0895,-1.1017
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410 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 1.8857,-1.2058
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411 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1100 S POS TARG 1.9931,0.1881
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412 ESO507-G045 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG 2.0982,1.1835
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6579( 10) - 03/12/97 14:57 - [ 9]
Visit: 05
Visit Requirements: CVZ ORIENT 180D TO 335D
On Hold Comments: <none>
Additional Comments: <none>
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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501 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 0.00,0.00
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502 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 0.8969,0.0946
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503 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 1.0010,0.8908
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504 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG 0.1052,0.9954
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505 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG -0.8903,1.1006
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506 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1300 S POS TARG -0.9965,-0.0940
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507 NGC6876 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO 1 1200 S POS TARG -1.1006,-0.8903
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6579( 10) - 03/12/97 14:57 - [ 10]
Summary Form for Proposal 6579
Item Used in this proposal
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Configurations WFPC2
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Opmodes IMAGE
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Optional Parameters CR-SPLIT=NO
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Proposal Category GO
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Scientific Category COSMOLOGY
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Special Requirements POS TARG 0.00,0.00 POS TARG 0.8969,0.0946 POS TARG 1.0010,0.8908 POS TARG 0.1052,0.9954 POS TARG
-0.8903,1.1006 POS TARG -0.9965,-0.0940 ORIENT 240D TO 40D POS TARG -1.1006,-0.8903 POS TARG
-0.1052,-0.9954 POS TARG 1.0895,-1.1017 POS TARG 1.8857,-1.2058 POS TARG 1.9931,0.1881 POS TARG
2.0982,1.1835 CVZ ORIENT 180D TO 335D
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Spectral Elements F814W
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Target Names NGC4767 NGC4709 NGC5516 ESO507-G045 NGC6876
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