The Near Edge of Infall into the Virgo Cluster
Infall upon a gravitationally collapsed core displays a characteristic S-wave pattern in velocity as a function of distance to an external observer. Foreground galaxies have motions larger than Hubble flow as they fall away; background galaxies have motions smaller than Hubble flow as they fall forward. In the rest frame of the collapsed core there is a zero-velocity surface separating the domains of Hubble expansion and infall. The radius of this surface depends on the mass internal to the surface and on Omega_Lambda. The infall of galaxies into the Virgo Cluster can provide an interesting test: specifically, galaxies located near the line-of-sight to the cluster at the intersection with the foreground part of the zero-velocity surface. Here, the surface is anticipated to lie about 7 Mpc in front of the Virgo Cluster or about 10 Mpc away from us. Accurate distances can be determined to galaxies near this surface through observations of the Tip of the Red Giant Branch in the resolved stellar populations. An accurate mass can be determined for the region centered on the cluster interior to the
targets. If there is an unexpectedly large amout of mass in an extended halo around the cluster then the zero velocity surface is expanded and the test galaxies will be closer to us than anticipated.
We intent to measure distances to 9 galaxies in front of Virgo situated at expected distances around 8-10Mpc from us. The TRGB method is the best way
to determine the distances to nearby galaxies with an accuracy of about 5%
independent of their morphology. In an old stellar population of a galaxy
the Tip of the Red Giant Branch will be at MI = -4.0, MV = -2.5. For a galaxy
at 10 Mpc, the TRGB will occur at I = 26.0 and V = 27.5. The ACS Exposure Time
Calculator predicts that exposures of 1600 and 1200 seconds will give S/N
of 11 and 12 in V and I bands respectively. Hence adequate data for the
determination of distances can be obtained within two orbits. The exposure
time will be divided between F814W and F606W filters with 4 frame dithers
in each filter.
In most cases the visible galaxy is small enough to be contained within
the WFC1 chip, yet large enough that it extends widely across the chip
and is centered with the WFC1-FIX aperture. Larger galaxies require the
entire ACS/WFC field and are centered with the the WFC-FIX aperture.
A complete visit requires slightly less than 50 minutes including overheads.
Galaxy targets are observed for 1200 sec in each of the F814W and F606W
filters. The observations in each filter are split into two exposures with a
small dither that optimizes the PSF reconstruction. The TRGB is determined
from the luminosities of stars in the F814W filter. The F606W filter
provides discrimination against young blue stars while retaining
coverage of the full width of the RGB.
RESOLVED STELLAR POPULATIONS
12
false
0.0
0.0
false
false
false
false
SUPPORTED
GALAXY
MAGELLANIC IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
GALAXY
AMORPHOUS IRREGULAR
J2000
J2000
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
ACS/WFC
Imaging
No
No
F814W
ACS/WFC
Imaging
No
No
F606W
POS-TARG
DITHER
3.011
LINE
false
false
2
85.28
ACS-WFC-DITHER-LINE
1
false
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(Tic-Orbit
:proposal-id 12878
:visit-id "18"
:orbit-number 1
:pcs-time 333
:si-vis-utilization 2681
:al-overhead-at-end 0
:internal-dur-at-end 347
:dark-offset NIL
:dark-utilization 0
:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 3015
:pcs-gap 0
)
false
1
0