The Red Sequence at 1.3 < z < 1.4 in Galaxy ClustersWe propose to obtain NIC3/F160W imaging of three new IRAC-selected galaxy clusters at 1.3 < z < 1.5. In combination with deep ACS/F850LP images being obtained in Cycle 14, the resulting precision photometry in a rest ~U - R color will allow us to construct color-magnitude diagrams
which can be used to measure the slope and scatter in the red sequence galaxies, thereby constraining the history of star formation in the early-type galaxies. The number of morphologically-selected early-type galaxies more luminous than L* will allow us to test the predictions of the hierarchical merging scenario for galaxy formation in clusters at the highest available redshifts in galaxy clusters.We require high precision color measurements so that we can determine
the average scatter in a color spanning the 4000 \AA\ break down to
levels of $\sim$0.02 mag to constrain the evolutionary history of the
stellar populations in the galaxies selected by their morphologies in
the ACS data to be early-types (Blakeslee et al.\ 2003; Mei et al.\
2006). We are already obtaining eep imaging with ACS/WFC in the
F850LP band to sample rest frame $U$ in Cycle 14. For the red band
above the 4000 \AA\ break we must turn to NICMOS. Although the F110W
filter samples close to rest frame $B$, we chose to use F160W because
of its greater sensitivity and because its passband is cleaner---the
F110W filter overlaps with the ACS F850LP bandpass. We have chosen to
ask for NIC3 observations even though its PSF is much larger than that
of ACS/WFC, in order to be able to cover most of the ACS FOV in a
reasonable number of orbits. The ACS data will yield better
morphological information, despite sampling a relatively blue rest
frame portion of the spectrum. The morphologies that are determined
from the NIC3 data will be used to see if the bluer rest
frame wavelengths bias the ACS morphologies, particularly for the
larger, more luminous early-type galaxies. The primary use of the
NIC3 data is in enabling the construction of the color-magnitude
diagram with very high precision. It is very difficult to combine
ground-based near-IR photometry with ACS photometry in making a
color-mag diagram for a high-$z$ galaxy cluster due to blending issues
in the dense cluster core, and also due to the required very high S/N
at relatively faint limits. An excellent example of the superior results
that can be obtained using HST for both bands in constructing a color-magnitude
diagram of a high redshift galaxy cluster is shown in Figure 5. The
ground-based attempt used the VLT and obtained spent 10 hours of integration
in the near-IR band. Yet clearly the intrinsic scatter and slope of the
red sequence can be measured far more accurately using the HST-only photometry.
At $z=1.35$, an $L^*$ early-type galaxy will have $H_{160}$ = 19.8.
At $H_{160} = 21.8$, 2 magnitudes below $L^*$, a single 900~s exposure
with NIC3 yields a S/N = 26 as measured in an aperture containing 80\% of the
total light. For our targets there is enough visibility time, after
allowing for guide star and instrument overheads, to obtain three
900~s exposures per orbit. Thus in two orbits containing six 900~s
exposures, the achieved S/N will be 63, slightly higher than
desired---but it would be impractical and inefficient to reduce the
exposure time per pointing below 2 orbits. With 6 dither positions
per pointing we will have enough to adequately reduce the data.
We can cover nearly all of the ACS field with a $3 \times 3$ mosaic of 9 NIC3
pointings, allowing for 3 arcsec of overlap between the pointings. The
relative areas of the proposed NIC3 mosaic and a single ACS FOV are illustrated
in Figures 1-3 for each of the three target clusters.
Thus the total request for each cluster is 18 orbits, and 54 orbits for the
whole proposal. For each cluster the NIC3 orbits will need to be divided into 4
visits, with some restriction on the allowable roll angle so as to
keep the tiling of the area relatively efficient and keep gaps from
appearing between pointings in different visit.UNRESOLVED STELLAR POPULATIONS12false00.00falsefalsefalsefalsefalsefalseSUPPORTEDCLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 33 50.6470+33 25 35.73J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 38 7.8940+34 13 45.46J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 34 46.3300+35 19 45.00J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 33 50.7230+33 26 25.18J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 33 51.9190+33 27 31.57J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 33 54.6300+33 26 20.11J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 33 55.9160+33 24 32.82J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 38 6.5470+34 14 36.84J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 38 11.8220+34 12 58.38J2000CLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERJ200014 38 10.6420+34 14 24.18J2000NIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WNIC3ImagingNoNoF160WPOS-TARGDITHERDITHER1.1NIC-XSTRIP-DITHfalsetrue80POS-TARGNIC-SPIRAL-DITH01false
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