9115( 4) - 05/24/01 13:05 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9115 Version: 4 Check-in Date: 24-May-2001 13:04:25 1.Proposal Title: The Evolution of Extreme Gas Rich Galaxies ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Patricia Knezek Space Telescope Science Institute CoI: Dr. Michael Disney University of Wales, Cardiff N CoI: Dr. Jonathan Davies University of Wales, Cardiff N CoI: Dr. Kenneth Freeman Research School of Astronomy and N Astrophysics/MSSSO CoI: Dr. John Gallagher III University of Wisconsin-Madison N CoI: Mr. Robert Minchin University of Wales, Cardiff N ------------------------------------------------------------------------------------ 5. Abstract We propose to observe a recently discovered sample of the closest known low surface brightness (LSB), very gas-rich dwarf galaxies outside of the Local Group. These galaxies represent an extreme star formation environment, where the fuel for star formation appears plentiful, the local environment (the Centaurus A group of galaxies) provides opportunities for tidally-induced star formation, and yet star formation appears to be surpressed (by two orders of magnitude) compared to other `typical' galaxies. Our goal is to understand the star formation history in this puzzling environment of low stellar density and high gas content. By resolving the underlying stellar component with WFPC2, we will obtain critical information on the mean metallicity and star formation history of these galaxies. The HST data will be combined with a vigorous ground-based program to study the ongoing massive star formation rate and the mean age of the stellar population, through a combination of broadband (UBVRI) and narrowband (H-alpha and [S II]) imaging. The combined data sets will help us understand the star formation process in these extreme gas-rich LSB galaxies, and in gaseous galaxies in general. Such knowledge is key for our comprehension of the present day role of the most common type of potentially star forming galaxies in the Universe, and may aid us in interpreting results at high redshifts on star formation in the early universe. ------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 2] Observations Description ------------------------ Deep WFPC2 images through filters F555W and F814W will be used to study the stellar populations and star formation history of the four gas-rich Cen A Group dwarfs. The F555W and F814W bandpasses are chosen since they have good transformations to the Johnson/Cousins V and I system, which is the standard for observing old stellar populations. The Cycle 4 TAC strongly recommended these filters for galaxy population studies. The galaxies will be centered in the WF3 to get the largest spatial coverage possible. Given the relative nearness of the Cen A Group and the diffuse nature of the proposed target galaxies, the WF detectors should provide adequate resolution (1 WF pixel~3 pc^2 at the distance of the Cen A Group. Note: we expect 1-5 RGB stars per square arcsec, i.e. per ~300 pc^2). Furthermore, the increased sensitivity of the WF chips is very desirable for these low surface brightness galaxies. The four Cen A dwarfs proposed here fit nicely on to a WFPC2 frame, as each has an optical diameter of ~1-2 arcmin. Their surface brightnesses are low (mu_B ~ 24 mag/arcsec^2), and their distance modulus is (m-M)_0=27.8 (Saha et al. 1995, Banks et al. 1999). Earlier work on Local Group dwarfs with HST (Tolstoy et al. 1998, Gallagher 1996) use 4 orbits per galaxy to reach about 26 in V and corresponding figures in B and I with enough accuracy to get good color magnitude diagrams (CMDs). These CMDs are then used to deduce star formation history (SFH) using red giant branch, red clumps, blue loops, AGB fitting, etc.(Mateo 1998). In the Cen A group, many of these features would be out of reach (HB~28.8, RC~27 in V) but we would see the upper RGB (24-26.5 in V), some of the AGB and any blue supergiants and HII regions present. Our targets are 1- 2 kpc across, have internal velocity dispersions of order 50 km/s, and therefore dynamical timescales of 3x10^7 years. They are low in the galactic plane (~15 deg) but presumably their stars will be much fainter than galactic stars and therefore confusion will be minimal. To address this issue and make efficient use of HST we will also take images in parallel with STIS. By imaging through both the clear aperture and the longpass filter, we can measure colors similar to (B-I) for the detected objects (Gregg & Minniti 1997, PASP, 109, 1062). Because of the wide filters these images will be very deep, V~28.5 in 4800 sec. The offset field of STIS will give us a better handle on background/foreground contamination in the WFPC2 images. We will determine accurate foreground extinctions for these galaxies from a combination of IRAS/COBE data, checked using colors of background ellipticals (e.g. Banks et al. 1999). One of the primary goals of this project is to go two magnitudes below the tip of the red giant branch (TRGB) in order to sample the red giant branch well. The TRGB for old stellar populations is at M_I=-4, (V-I)~1.5, in the color-magnitude diagram. Therefore, for an assumed distance to the Cen A Group of (m-M)_0=27.8, the TRGB will have m_I~23.8 and our detection limit (S/N~3) will need to be m_I~26. The WFPC2 exposure time calculator (ETC) suggests that we can reach this limit in ~3100 sec in F555W, or <=2 orbits, and ~1100 sec in F814W, or <=1 orbit. We have assumed that objects are centered in the corner of a PC pixel and we have accounted for loss of S/N due to multiple readouts. However, experience with HST imaging of extragalactic globular clusters (Miller et al. 1997, AJ, 114, 2381) suggests that the ETC tends to underestimate actual exposure times. Furthermore, prior stellar population work at similar distances indicates that longer exposures will be necessary for the accuracy we need to achieve. For instance, Soria et al.(1996) studied the TRGB in Centaurus A itself. They obtained total exposure times of 3200 sec in F555W and 3800 sec in F814W. Formally, their 50% completeness limits at (V-I)~1.5 are m_I~25 and m_V~26.5. In addition, the errors at m_I~25 are ~0.4-0.5 mag. Cole et al.(1999) find formal limiting magnitudes of m_I~25.5 and m_V~26.5 in 10700 sec exposures for each filter in their study of the Local Group galaxy IC 1613. Other consistent studies include Mould et al.(2000) for Centaurus A itself; Caldwell et al.(1998) for members of the M81 group; and e.g. Da Costa et al.(1999), and Tolstoy et al.(1998) in the Local Group. Therefore, we feel that a minimum of 2 full orbits in F555W and 2 full orbits in F814W are needed to achieve the desired sensitivity and photometric accuracy for each of the four target galaxies. Therefore, our project requires 16 total orbits. The long exposures will be CR-SPLIT for removal of cosmic rays. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HIPASS1321- GALAXY, LSB RA=13H 21M 8.5S +/- 2", DEC=-31D J2000 V = 570 B = 17.1+/-0.2 31 31' 51" +/- 2" E(B-V) = 0.062 Coordinate Source: OTHER_SOURCE 2 HIPASS1328- GALAXY, LSB RA=13H 28M 55.6S +/- 2", DEC=- J2000 V = 200 B = 17.0+/-0.2 30 30D 29' 34" +/- 2" E(B-V) = 0.055 Coordinate Source: OTHER_SOURCE 3 HIPASS1337- GALAXY, LSB RA=13H 37M 25.5S +/- 2", DEC=- J2000 V = 490 B = 16.5+/-0.3 39 39D 53' 51" +/- 2" E(B-V) = 0.074 Coordinate Source: OTHER_SOURCE 4 DEEP1337-33 GALAXY, LSB RA=13H 37M 0.6S +/- 2", DEC=-33D J2000 V = 591 B = 17.3+/-0.3 21' 47" +/- 2" E(B-V) = 0.044 Coordinate Source: OTHER_SOURCE 9115( 4) - 05/24/01 13:05 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HIPASS1321- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2400 S 31 ------------------------------------------------------------------------------------------------------------------------------------ 3 HIPASS1321- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 31 ------------------------------------------------------------------------------------------------------------------------------------ 5 HIPASS1321- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 31 ------------------------------------------------------------------------------------------------------------------------------------ 7 HIPASS1321- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 31 ------------------------------------------------------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 5] Visit: 02 Visit Priority: Visit Requirements: PCS MODE FINE On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HIPASS1328- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2400 S 30 ------------------------------------------------------------------------------------------------------------------------------------ 3 HIPASS1328- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 30 ------------------------------------------------------------------------------------------------------------------------------------ 5 HIPASS1328- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 30 ------------------------------------------------------------------------------------------------------------------------------------ 7 HIPASS1328- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 30 ------------------------------------------------------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 6] Visit: 03 Visit Priority: Visit Requirements: PCS MODE FINE On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HIPASS1337- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2400 S 39 ------------------------------------------------------------------------------------------------------------------------------------ 3 HIPASS1337- WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 39 ------------------------------------------------------------------------------------------------------------------------------------ 5 HIPASS1337- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 39 ------------------------------------------------------------------------------------------------------------------------------------ 7 HIPASS1337- WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S 39 ------------------------------------------------------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 7] Visit: 04 Visit Priority: Visit Requirements: PCS MODE FINE On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DEEP1337-33 WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2400 S ------------------------------------------------------------------------------------------------------------------------------------ 3 DEEP1337-33 WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S ------------------------------------------------------------------------------------------------------------------------------------ 5 DEEP1337-33 WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S ------------------------------------------------------------------------------------------------------------------------------------ 7 DEEP1337-33 WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 2600 S ------------------------------------------------------------------------------------------------------------------------------------ 9115( 4) - 05/24/01 13:05 - [ 8] Summary Form for Proposal 9115 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WF3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN=7 CR-SPLIT=0.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names HIPASS1321-31 HIPASS1328-30 HIPASS1337-39 DEEP1337-33 ------------------------------------------------------------------------------------------------------------------------------------