! Proposal 5946, submission 2
! PI: Chris Impey
! Received Fri Oct 27 17:00:28 EDT 1995
! From: pi16@stsci.edu
! Hubble Space Telescope Cycle 5 (1995) Phase II Proposal Template
! $Id: 5946,v 7.1 1996/04/24 10:36:30 pepsa Exp $
!
! Refer to the HST Phase II Proposal Instructions to fill this out
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! This partially completed template was generated from a Phase I proposal.
! Date generated: Sat Dec 17 11:08:26 EST 1994
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Proposal_Information
Title: The Stellar Populations of Low Surface
Brightness Galaxies
Proposal_Category: GO
Scientific_Category: Stellar Populations
Cycle: 5
Investigators
PI_name: Chris Impey
PI_Institution: Steward Observatory
CoI_Name: Greg Bothun
CoI_Institution: University of Oregon
Contact:
CoI_Name: James Schombert
CoI_Institution: Infrared Processing and Analysis Center
Contact:
CoI_Name: Stacey McGaugh
CoI_Institution: University of Cambridge
Contact:
Abstract:
We propose to take WFPC2 images of 10 extremely low surface
brightness (LSB) galaxies. Images in the U(F300W), V(F555W),
and I(F814W) bands with the WFC will dramatically improve on
the existing ground based constraints on the stellar
populations and ages of these enigmatic galaxies, as well as
their possible link to galaxies at higher redshift. These
galaxies offer a profound challenge to our understanding of
galaxy evolution in that: (a) the most extreme LSB galaxies
are 100 times more luminous than normal galaxies at a given
metal-abundance, (b) some are so so blue that either the
implied mean age is 1-2 Gyr, or there is a huge population of
blue horizontal branch stars, (c) their stellar volume mass
densities are at an unprecedentedly low level for normal star
formation to have occurred. These incongruent properties
coupled with the unexpectedly large co-moving space density of
faint blue galaxies suggest an evolutionary link only under
the extreme hypothesis that the galaxies remain blue while
fading. Such a hypothesis is possible in the context of non-
standard stellar populations. The spatial resolution of WFPC2
can be used to study stellar populations in LSB galaxies using
the technique of luminosity fluctuations. The fluctuations
signals are expected to be 20-40%, and they can be studied
with a sensitivity that cannot be achieved from the ground.
Questions
Observing_Description:
We propose to use the Wide Field Camera at f/12.9 to image 10
extremely low surface brightness galaxies. All the galaxies
will be observed in the I(F814W) bands, and all but one
will be observed in the U(F330W) band. The typical
integration times per galaxy are 20 minutes (F330W), 10 minutes
(F555W), and 10 minutes (F814W). The galaxies are selected to be
the bluest among 60 extreme LSB dwarfs with surface
photometry in the Virgo and Fornax clusters. The galaxies are
representative in surface brightness of over 500 dwarfs in
these two clusters in the range 23-26 V mag/square arcsec.
Analysis of ten objects is required to draw firm
conclusions about this ultra-faint galaxy population. The
targets are at known distances (i.e. cluster membership at
velocities less than 1500 km/s, so distances are well known
in the context of a model for large scale structure in the
local universe). The WFC filters are chosen for high
throughput, broad combined wavelength coverage, and
photometric characteristics that allow easy comparison with
ground-based data. A V(F555W) frame of a single target will
provide a reference
with existing surface photometry. Most of the galaxies are in
the fields of the Virgo and Fornax clusters, and have
diameters at the 27 V mag/square arcsec isophote of 1-2
arcminutes. Therefore, we will take full advantage of the 2.5
arcmin field-of-view of the camera. The exposure times are
chosen to match the instrument sensitivity and sky background
with the colors of LSB galaxies, and give an equivalent blue
surface brightness limit of 28.5 mag/square arcsec. This assumes
flat fielding to 1%. With this setup, sky noise will dominate
the errors in both bands, and the full
advantage of the space environment will be realized. For the
Wide Field Camera, total counts are calculated for a central
surface brightness of 25 V mag/square arcsec, or 30 V mag per
WFC pixel. The galaxies are chosen to have flat central
regions, so this surface brightness applies over a region of
5-20 arcseconds across. Using the sensitivities and filter
transmissions in the WFPC2 Instrument Handbook (Version 2.0,
May 1994), and assuming colors for the LSB galaxies of U-V =
0.5, and V-I = 1.0, the following number of electrons per
pixel will be collected for the surface brightness and
average integration times quoted: F330W - 5e (galaxy),
48e (sky); F555W - 4.4e (galaxy), 63e (sky); F814W - 14e
(galaxy), 135e (sky). A few of the LSB galaxies are at least
a factor of 2-3 brighter, so these are conservative
estimates. In each filter the observations are background
limited, so the signal to noise per pixel will be: F330W -
SNR = 0.59; F555W - SNR = 0.55; F814 - SNR = 1.20. This
gives us roughly equal sensitivity to fluctuations in each
filter. Sky surface brightness of 29.8 mag/pixel
(F330W), 27.3 mag/pixel (F555W), and 26.7 mag/pixel
(F814W) have been assumed. We note that the HST sky
brightness is 3 mag fainter in the U band and 2 mag fainter
in the I band than can be achieved from the ground. Assuming
flat-fielding to 1%, we can see that the flat field limit corresponds
to the following percentage of the expected galaxy signal per
pixel: F330W - 12%; F555W - 14%; F814W - 10%. Therefore
by averaging over as few as 4
pixels, we can detect a fluctuation signal of 10%, and the
sensitivity level over the large flat 5-20 arcsecond regions of the
galaxies is even greater. Basically, there will be many
independent samples with which to look for a small difference
between the variance of the sky signal and the variance of
the galaxy signal. The fluctuation signal in the U band may
well be dominated by a single star. The project was awarded
10 orbits. Integrations of more than 10 minutes will be split
to avoid excessive cosmic ray problems. In the viewing time
available in one orbit, there is time for a guide star
acquisition, three 10 minute exposures, and 2 associated
readouts (U and I band exposures).
Real_Time_Justification:
None.
We have an approved UIT Astro-2 program to measure fluxes at
2000 Angstroms for a large sample of these LSB objects.
Pushing the observation of luminosity fluctuations with WFPC2
as far into the blue as possible will be an important tie-in
to the UIT data and will help isolate the stellar types which
dominate the UV light.
Calibration_Justification:
None.
Additional_Comments:
Fixed_Targets
Target_Number: 01
Target_Name: V1L4
Description: Galaxy, LSB
Position: RA = 12H 32M 11.3S +/- .7S,
DEC = +14D 29' 56" +/- 5"
Equinox: 1950
RV_or_Z:
Flux: V = 16.1 +/- 0.5,
B-V = 0.37 +/- 0.2
Comments:
Target_Number: 02
Target_Name: V2L8
Description: Galaxy, LSB
Position: RA = 12H 28M 44.7S +/- .7S,
DEC = +14D 54' 59" +/- 5"
Equinox: 1950
RV_or_Z:
Flux: V = 18.4 +/- 0.5,
B-V = 0.30 +/- 0.2
Comments:
Target_Number: 03
Target_Name: V7L3
Description: Galaxy, LSB
Position: RA = 12H 26M 24.3S +/- .7S,
DEC = +13D 11' 15" +/- 5"
Equinox: 1950
RV_or_Z:
Flux: V = 17.2 +/- 0.5,
B-V = 0.52 +/- 0.2
Comments:
Target_Number: 04
Target_Name: UGC12695
Description: Galaxy, LSB
Position: RA = 23H 36M 2.2S +/- 1S,
DEC = +12D 52' 32.6" +/- 1"
Equinox: 2000
RV_or_Z:
Flux: V = 15.2 +/- 0.5,
B-V = 0.37 +/- 0.2
Comments:
Target_Number: 05
Target_Name: MALIN1
Description: Galaxy, LSB
Position: RA = 12H 34M 28.1S +/- .7S,
DEC = +14D 36' 19" +/- 5"
Equinox: 1950
RV_or_Z:
Flux: V = 14.2 +/- 0.5,
B-V = 0.50 +/- 0.2
Comments:
Visits
Visit_Number: 1
Visit_Requirements:
Visit_Comments:
Exposure_Number: 1
Target_Name: V1L4
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F300W
Wavelength: 2911
Optional_Parameters: SUM=2x2
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: U Filter
Exposure_Number: 2
Target_Name: V1L4
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F814W
Wavelength: 7940
Optional_Parameters:
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: I Filter
Visit_Number: 2
Visit_Requirements:
Visit_Comments:
Exposure_Number: 1
Target_Name: V2L8
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F300W
Wavelength: 2911
Optional_Parameters: SUM=2x2
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: U Filter
Exposure_Number: 2
Target_Name: V2L8
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F814W
Wavelength: 7940
Optional_Parameters:
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: I Filter
Visit_Number: 3
Visit_Requirements:
Visit_Comments:
Exposure_Number: 1
Target_Name: V7L3
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F300W
Wavelength: 2911
Optional_Parameters: SUM=2x2
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: U Filter
Exposure_Number: 2
Target_Name: V7L3
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F814W
Wavelength: 7940
Optional_Parameters:
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: I Filter
Visit_Number: 4
Visit_Requirements:
Visit_Comments:
Exposure_Number: 1
Target_Name: UGC12695
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F300W
Wavelength: 2911
Optional_Parameters: SUM=2x2
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: U Filter
Exposure_Number: 2
Target_Name: UGC12695
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F814W
Wavelength: 7940
Optional_Parameters:
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: I Filter
Visit_Number: 5
Visit_Requirements:
Visit_Comments:
Exposure_Number: 1
Target_Name: MALIN1
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F300W
Wavelength: 2911
Optional_Parameters: SUM=2x2
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: U Filter
Exposure_Number: 2
Target_Name: MALIN1
Config: WFPC2
Opmode: IMAGE
Aperture: WFALL
Sp_Element: F814W
Wavelength: 7940
Optional_Parameters:
Number_of_Iterations: 4
Time_Per_Exposure: 500S
Special_Requirements: EXPAND
Comments: I Filter
Data_Distribution ! Defaults indicated; change if desired
Medium: 8MM ! 8MM or 6250BPI or 1600BPI
Blocking_Factor: 10 ! 10 or 1
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Recipient_Email: ! Needed if Ship_To: is not PI_Address
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