! 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 ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Debra Wallace ! Phone: 410 338-4506 , E-mail: wallace@stsci.edu ! ! This partially completed template was generated from a Phase I proposal. ! Date generated: Sat Dec 17 11:08:26 EST 1994 ! 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 ! 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