8122(  4) - 07/24/00 10:40  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      8122
                                                       Version: 4
                                                       Check-in Date: 07-Jul-1999
                                                                      16:52:16

1.Proposal Title:
The evolution of galaxies -- mining the stellar content of the two most local Blue
Compact Dwarf Galaxies
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2. Proposal For  3. Cycle
GO               8
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4. Investigators
                                                                                     Contact?
    PI: Regina Schulte-Ladbeck            University of Pittsburgh
   CoI: Dr. Ulrich Hopp                   Observatory of the University of Munich       N
   CoI: Prof. Mary Crone                  Skidmore College                              N
   CoI: Dr. Laura Greggio                 University of Bologna                         N

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5. Abstract

We propose to exploit the unique, high spatial resolution of the WFPC2 to obtain
deep color-magnitude diagrams (CMDs) of two newly recognized very nearby (D<3 Mpc)
blue compact dwarf galaxies (BCDs). The CMDs will be modeled to extract the star-
formation histories; in particular, we will use red-giant stars to determine WHEN
these galaxies first started to make the bulk of their stellar population. This will
provide an important alternative to galaxy counts for testing the predictions of
dwarf galaxy evolution. BCDs are very small, nearby, starbursting systems which (at
least in some cases) also contain an older stellar population. The star-formation
history of BCDs is a key to understanding the enigmatic evolution of dwarf galaxy
populations -- for example, the faint blue excess, thought to be due to low-mass
galaxies experiencing a short epoch of star formation at redshifts 0.5<z<1. Possibly
the most important processes driving counts at very faint magnitudes are 1) the
delayed formation of dwarfs, where the collapse time-scales of the interstellar gas
is governed by photoionization by the metagalactic UV background; and 2) the ensuing
superwinds, which could allow loose gas to escape these low-mass systems. Besides
being candidates for the remnants of the faint blue excess, BCDs provide a unique
laboratory for understanding the effects of starburst feedback on the interstellar
meduim.
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  Observations Description
  ------------------------

     While our main goal is to obtain deep HST/WFPC2 images in V and I, we also need
     to obtain images in the UV, and in HAlpha. The UV color is required to
     determine the internal extinction in the program galaxies. As was recently
     demonstrated with HST imaging by Calzetti et al. (1997), there can be
     significant extinction by dust in a very patchy distribution even in a dwarf
     irregular type galaxy. UV-based CMDs will also contribute information about the
     massive, young and blue stars formed in the last 50 Myrs. The HAlpha images are
     required to subtract nebular emission from the broad-band images and thus
     remove spurious point sources from the CMDs and obtain accurate colors within
     HII regions (see Schulte-Ladbeck et al. 1998). In addition, high resolution
     HAlpha images are needed to understand the effects of star formation and
     feedback from superwinds in these BCDs. Because they are so small and have so
     much star formation, BCDs are an excellent laboratory for observing the effects
     of supernova feedback on the interstellar medium --- the very effects we need
     to understand in order to explain dwarf galaxy evolution. In particular, high-
     resolution HAlpha images will reveal the luminosities and spatial distribution
     of HII regions, as well as large-scale low-surface brightness bubbles caused by
     superwinds. Finally, the integrated HAlpha flux will yield a measurement of the
     present-day star- formation rate of the program galaxies. To realize the
     observations, we selected broad-band filters that transform well into the
     Johnson-Cousins system, F336W, F555W, F814W, and F656N for the HAlpha images.
     The integration times were patterned after proposal 6276 which obtained the UGC
     6456 data shown in Fig. 1. For UGC 6456, the integration times realized were
     F555W=4200s, F814W=4200s, F336W=5600s, F656N=2400s. Our single-star photometry
     showed that errors reached 0.1 mag for F555W and F814W at about 25.5 mag, and
     in F336W at about 24.0 mag. Incompleteness set in in F555W at 26 mag and in
     F814W and F336W at 25 mag. The data for the two program galaxies will be
     acquired using canned dithering routines. CR-splits will allow for good cosmic
     ray rejection; dithering, for a better sampling of the PSF required for the
     point-source photometry. In order to obtain data of the same quality as for UGC
     6456 for the two program galaxies, taking into account the observing overheads,
     we request for each object F555W=2 orbits, F814W=3 orbits, F336W=2 orbits,
     F656N=1 orbit. We have added exposure time to F814W at the expense of F336W, in
     order to try to go deeper in F814W. F814W does not achieve the same S/N as
     F555W for the same exposure time, and we need to be able to detect a similar
     number of point-sources in both to get the ``red tangle" well populated. This
     results in our request of 16 orbits. The main work of this proposal is in the
     single-star photometry; each of the WPFC2 chips will contain thousands of point
     sources, with different PSFs for the 4 chips. Our goal is to obtain single star
     photometry, and to construct color-magnitude diagrams and color-color diagrams;
     the photometry will be carried out with IRAF/DAOPHOT. Then, we need to carry
     out false-star simulations to investigate the completeness of the data. This is
     done by adding test stars to the images, and re-doing the photometry. After
     photometry is completed in the different filters, we need to transform to the
     Johnson- Cousins system, correct for galactic extinction, and investigate
     internal extinction. Reddening-free CMDs will be constructed and the analysis
     will proceed to measure the TRGB and so, derive an absolute scale for the CMDs.
     Synthetic CMDs will be constructed using as input a variety of stellar
     evolution tracks, IMFs, SFHs, and CELs. The data in these synthetic CMDs will
     then be convolved with the photometric errors of the HST data, and their
     incompleteness. This results in model CMDs for our galaxies. We will compare
     the model CMDs with a number of empirical measurements of the ``red tangle", to
     determine the best-fitting model and from that, to derive the SFHs.

  Real Time Justification
  -----------------------


  Calibration Justification
  -------------------------


  Additional Comments
  -------------------

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                                                8122(  4) - 07/24/00 10:40  - [  3]

  Data Distribution

  Paper Products:  YES
  Media:           ELECTRONIC
  Blocking Factor: 10
  Ship To:         PI_Address
  Ship Via:
  Email:
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                                                8122(  4) - 07/24/00 10:40  - [  4]

TARGET LIST    Fixed Targets
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Tar|   Target   |         Target        |            Target              |Coord   | Radial |             Flux data
No |    Name    |       Description     |           Position             |Eqnx    |  Vel.  |
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1   UGCA290      GALAXY, DWARF COMPACT   RA=12H 37M 21.75S+/-0.1S,        J2000             B = 15.76
                                         DEC=38D 44' 37.5"+/-0.1"

2   NGC6789      GALAXY, DWARF COMPACT   RA=19H 16M 41.79S+/-0.1S,        J2000             B = 13.83
                                         DEC=63D 58' 22.3"+/-0.1"

                                                8122(  4) - 07/24/00 10:40  - [  5]

Visit: 01
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
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Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
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1        UGCA290     WFPC2  IMAGE   PC1-FIX F555W                                              1   2600 S
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2        UGCA290     WFPC2  IMAGE   PC1-FIX F555W                                              1   2700 S  POS TARG 0.25, 0.25
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3        UGCA290     WFPC2  IMAGE   PC1-FIX F555W                                              1   2700 S  POS TARG 0.5, 0.5
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4        UGCA290     WFPC2  IMAGE   PC1-FIX F336W                                              1   2700 S
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                                                8122(  4) - 07/24/00 10:40  - [  6]

Visit: 02
     Visit Priority:      <none>
     Visit Requirements:  GROUP 1-2 WITHIN 7D SAME ORIENT AS 01
     On Hold Comments:    <none>
     Additional Comments: We need F555W and F814W at the same time, and at the same orient. We only
                          split this into 2 visits because of the new "5 orbit" rule.


Exposures
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Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1        UGCA290     WFPC2  IMAGE   PC1-FIX F814W                                              1   2600 S
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2        UGCA290     WFPC2  IMAGE   PC1-FIX F814W                                              1   2700 S  POS TARG 0.25, 0.25
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3        UGCA290     WFPC2  IMAGE   PC1-FIX F814W                                              1   2700 S  POS TARG 0.5, 0.5
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4        UGCA290     WFPC2  IMAGE   PC1-FIX F656N                                              1   2700 S
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                                                8122(  4) - 07/24/00 10:40  - [  7]

Visit: 03
     Visit Priority:      <none>
     Visit Requirements:  <none>
     On Hold Comments:    <none>
     Additional Comments: <none>


Exposures
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Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
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1        NGC6789     WFPC2  IMAGE   PC1-FIX F555W                                              1   2700 S
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2        NGC6789     WFPC2  IMAGE   PC1-FIX F555W                                              1   2800 S  POS TARG 0.25, 0.25
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3        NGC6789     WFPC2  IMAGE   PC1-FIX F555W                                              1   2800 S  POS TARG 0.5, 0.5
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4        NGC6789     WFPC2  IMAGE   PC1-FIX F336W                                              1   2800 S
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                                                8122(  4) - 07/24/00 10:40  - [  8]

Visit: 04
     Visit Priority:      <none>
     Visit Requirements:  GROUP 3-4 WITHIN 7D SAME ORIENT AS 03
     On Hold Comments:    <none>
     Additional Comments: We need F555W and F814W at the same time, and at the same orient. We only
                          split this into 2 visits because of the new "5 orbit" rule.


Exposures
------------------------------------------------------------------------------------------------------------------------------------
Exposure|   Target  |Instr | Oper. | Aper  |Spectral|Central|             Optional            |Num|  Time |       Special
 Number |    Name   |Config| Mode  |or FOV |Element |Waveln.|            Parameters           |Exp|       |     Requirements
------------------------------------------------------------------------------------------------------------------------------------
1        NGC6789     WFPC2  IMAGE   PC1-FIX F814W                                              1   2700 S
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2        NGC6789     WFPC2  IMAGE   PC1-FIX F814W                                              1   2800 S  POS TARG 0.25, 0.25
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3        NGC6789     WFPC2  IMAGE   PC1-FIX F814W                                              1   2800 S  POS TARG 0.5, 0.5
------------------------------------------------------------------------------------------------------------------------------------
4        NGC6789     WFPC2  IMAGE   PC1-FIX F656N                                              1   2800 S
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                                                8122(  4) - 07/24/00 10:40  - [  9]

            Summary Form for Proposal  8122

  Item                         Used in this proposal
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  Apertures                    PC1-FIX
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  Configurations               WFPC2
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  Opmodes                      IMAGE
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  Proposal Category            GO
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  Special Requirements         POS TARG 0.25, 0.25 POS TARG 0.5, 0.5 GROUP 1-2 WITHIN 7D SAME ORIENT AS 01 GROUP 3-4 WITHIN 7D
                               SAME ORIENT AS 03
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  Spectral Elements            F555W F336W F814W F656N
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  Target Names                 UGCA290 NGC6789
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