8133( 4) - 07/22/99 16:41 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8133 Version: 4 Check-in Date: 22-Jul-1999 16:40:25 1.Proposal Title: The Starburst-ISM Interaction in NGC 1569 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Patrick Shopbell University of Maryland CoI: Dr. Reginald Dufour Rice University N CoI: Dr. Donald Walter South Carolina State University N ------------------------------------------------------------------------------------ 5. Abstract We propose to use the WFPC2 camera to obtain narrowband images of the complex ionized gas component of the dwarf starburst galaxy NGC 1569. We will image the central regions of the galaxy in the emission lines of H-alpha, H-beta, [S II] 6717A+6731A, [O III] 5007A, and He II 4686A. The H-alpha imagery will provide a map of the optical emission associated with the filamentary ionized gas, at much higher resolution than ever before studied. From recent ground-based imagery and Fabry- Perot observations with 1"-2" resolution, we expect the increased resolution of HST to reveal immense detail in the filament system, allowing for accurate isolation of the filamentary structure from the background, as well as detailed comparison of the ionized gas and stellar components. (The latter has already been imaged in detail by HST/WFPC2.) The [SII] and [OIII] imagery will provide high-resolution maps of the filament excitation, leading to improved models of the outflow emission mechanis ms and energetics. The H-beta image will be used to create a map of the extinction, an important issue in the inner regions of starburst systems, and in NGC 1569 particularly. Finally, the He II image will be used to separate regions under the influence of Wolf-Rayet stars from those interacting with O stars, again allowing for more accurate models. ------------------------------------------------------------------------------------ 8133( 4) - 07/22/99 16:41 - [ 2] Observations Description ------------------------ Observations in the emission line of H-alpha (filter F656N) have been chosen to provide a high-resolution map of the ionized gas in NGC 1569. The required exposure times have been estimated using published ground-based narrowband observations and the on-line exposure time calculator (ETC). Detailed H-alpha imaging observations of NGC 1569 have been made by Waller (1991), in which the surface brightness in the brightest knot (his knot #2) is measured to be 3.7 * 10^-14 erg cm^-2 sec^-1 arcsec^-2. The ETC shows that this flux level can be reached at a signal-to-noise ratio of 3 in a mere 33 seconds. If we instead compute an exposure time using a contour of the extended H-alpha component in NGC 1569 encompassing the majority of the optical body, which Figure 3a of Waller (1991) shows to be at a level of 10^-15 erg cm^-2 sec^-1 arcsec^-2, we derive an exposure time of 1352 seconds, or t~900 seconds each for a CR-SPLIT pair of images. Since this image will be used to paramete rize the ionized gas distribution throughout the galaxy, it has been increased to a total of 2400 seconds. This image should then achieve a surface brightness of 10^-15.5 erg cm^-2 sec^-1 arcsec^-2 with a signal-to-noise ratio in excess of 1.5, allowing detection of the entire outer envelope of the galaxy and the western ``H-alpha arm''. The brightest regions will not exceed ~5% of saturation. Exposure times for the other filters have been estimated by combining the above numbers with published line ratios (Heckman et al. 1995; Kobulnicky & Skillman 1997). The [O III] emission is stronger than H-alpha by a factor of ~20%; the ETC derives an exposure time of 2100 seconds, for a CR-SPLIT pair of images with S/N~3. Similarly, the [S II] surface brightness in NGC 1569 is greater than 10% of H- alpha along most of Heckman et al.'s slit. For these fainter lines (i.e., all except [O III]), the goal is to limit the exposure time requirement by imaging only the main body of the ga laxy, particularly the ionized gas surrounding the starburst clusters. We therefore compute 10% of an intermiediate contour in Waller (1991) and use the ETC to compute an exposure time of ~3000 seconds. In a similar manner, the [NII] flux is observed to be ~6% of H-alpha (dwarf starbursts are often metal-poor); the computed exposure time of 5700 seconds is rounded to 5400, i.e., one CVZ orbit. Due to the long exposure time for this image, the TAC has required that the [NII] observation be removed from the proposal; we have done so. Due to its low Galactic latitude (l ~ 11 deg) and the resulting large extinction towards NGC 1569 (A_v~1.58; Kobulnicky & Skillman 1997), as well as the complex, dusty morphology of typical starburst regions, accurate modeling of the emission line imagery requires a reddening correction. We propose to do this using the customary Balmer decrement calculation between the H-alpha and H-beta emission lines. In the same manner as above, we find an exposure time of ~1 orbit for the H-beta exposure. The long length of this exposure is the result of both high extinction and decreasing telescope-instrument efficiency at bluer wavelengths. In order to enable continuum subtraction, a short off-band (filter F547M) image will be taken. A comparison of the dimensionless efficiencies of the F656N and F547M filters (Table 68 in the WFPC2 Instrument Handbook) suggests an exposure time of approximately one minute to obtain a comparable continuum to that seen in the H-alpha image. Finally, in order to help integrate this emission line imagery with the HST archival stellar broadband imagery, we propose to obtain an image spanning the Wolf-Rayet-sensitive He II line. An estimate of the He II surface brightness is given by Gonzalez Delgado et al. (1997) as ~1.3 * 10^-14 erg cm^-2 sec^-1 arcsec^-2. After correction for extinction, the ETC has been used to compute an exposure time of ~1500 seconds. This number has been increased to 2000, in order to fill the orbit. All of the exposure times have bee n computed using the published radial velocity for NGC 1569 of v_r=-10 4 kms. Published radial velocity data (e.g., Heckman et al. 1995), as well as our Fabry-Perot observations, indicate that the intrinsic velocities of the ionized gas do not vary from this value by more than +/-100 kms. The resulting shift of 3-4 Angstroms is sufficiently small that the flux should remain within the narrow filter bandpasses. To summarize, we propose a single visit of 3 CVZ orbits (5760 seconds each): orbit observation: exposure t_exp t_ohead t_total-- ------------------------------------------------------------------------------- ---------------- 1 H-alpha/F656N: 2400 He II/F469N: 2000 y/F547M: 60 4460 1260 5720 2 [S II]/F673N: 3000 [O III]/F502N: 2100 5100 600 5700 3 H-beta/F487N: 5400 5400 300 5700------------------------------------------------------------- ------------------------------------ All exposures will use the CR-SPLIT option; the 1-minute WFPC2 overhead for filter changes has been ignored. The se exposures will image the entire central starburst region of NGC 1569. The detector will be positioned such that the starburst region will appear roughly centered on chip WF3, with extended emission possibly apparent on the other chips (especially in the H-alpha image). Use of the WF3-FIX aperture will allow us to image the entire main body of the galaxy, while avoiding the need for a specific ORIENT and preventing the bright star directly north of NGC 1569 from falling on the same chip (Figure 2). The bright star will be either off of the field, or imaged on one of the other chips. In order to avoid PC1 stray light patterns, the star will be positioned close enough to WF3 that it cannot appear in the portion of the PC pyramid facet not imaged by the PC chip. There will be a small possibility of a ``Dragon's Breath'' reflection, should a roll angle place the bright star near a chip edge, but it will be on either WF2 or WF4, not WF3. Real Time Justification ----------------------- NGC 1569 is well-positioned in the heart of the continuous viewing zone (CVZ). As determined from the on-line tables, the position 04:30, +64:50.75 appears in 7 CVZ windows during Cycle 8, which include a total of 216 orbits. This should provide ample scheduling opportunities for the small number of orbits requested in this proposal. In the past two years, our collaboration has used ground- based facilities to investigate NGC 1569 and a number of similar dwarf starburst galaxies. Of particular relevance to this proposal are ground-based narrowband imagery, longslit spectra, and Fabry-Perot observations of NGC 1569. Our preliminary reduction of the latter dataset, obtained recently at the Palomar 60-inch telescope, reveals a wealth of morphological and kinematic information. Although the seeing was quite good for that run, the ionized gas in NGC 1569 is extremely complex, exhibiting numerous filaments and bubbles that are unresolved from the ground. Our 23 km/s resolutio n is able to adequately sample the line structure revealed by previous longslit observations (e.g., Tomita et al. 1994; Heckman et al. 1995), but across the entire object, rather than at arbitrary slit positions. Detailed modeling of the kinematic information has been hampered, primarily by the complexity and small scale of the structure in the ionized gas. Our ground-based observations are unable to clearly delineate the filamentary structure, revealing instead emission from a combination of background sources and overlapping filaments in each pixel. HST observations are necessary to help deconvolve the various components of NGC 1569. We note that we also have Fabry-Perot (Palomar 60-inch and 200-inch), longslit spectra (Steward 90-inch, HST), and narrowband imagery (San Pedro Matir, HST) of several similar dwarf starburst galaxies, including NGC 4449 and I Zw 18. These data will be used for direct comparison with the proposed HST observations of NGC 1569, to better sep arate more general astrophysical issues (e.g., expanding superbubbles) from details specific to NGC 1569. Data from the literature will be used for auxiliary purposes, including particularly longslit spectral observations of specific emission regions (Kobulnicky & Skillman 1997), broadband HST observations of the stellar population, and ROSAT X-ray imaging observations of the hot gas (Heckman et al. 1995). Martin's scheduled deep AXAF imaging observation will be extremely useful for comparison as well. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8133( 4) - 07/22/99 16:41 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8133( 4) - 07/22/99 16:41 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1569 GALAXY, DWARF COMPACT, RA=04H 30M 55.45S +/- 0.25", J2000 V = -104 B = 11.86+/-0.09 STAR FORMING REGION, DEC=64D 50' 26" +/- 0.25" V = 11.51+/-0.06 STARBURST, FILAMENT, SURF-LINE(6563)=1e-15 WIND SURF-LINE(5007)=1.2e-15 SIZE=100+/-10 8133( 4) - 07/22/99 16:41 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ ORIENT 90D TO 270D On Hold Comments: Additional Comments: The specified ORIENT value places a nearby bright star off of the field of view. As it only restricts the schedulability from 27 days (over the 2-year RPS2 window) to 22 days, we feel this is a reasonable request. The optimal ORIENT for this program is 116 degrees or 206 degrees. This will place the major axis of the galaxy along the diagonal of the WF3 chip. We request scheduling as close to one of these two ORIENTs as possible. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC1569 WFPC2 IMAGE WF3-FIX F656N CR-SPLIT=0.5 1 800 S POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ 15 NGC1569 WFPC2 IMAGE WF3-FIX F656N CR-SPLIT=0.5 1 800 S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC1569 WFPC2 IMAGE WF3-FIX F469N CR-SPLIT=0.5 1 1400 S ------------------------------------------------------------------------------------------------------------------------------------ 25 NGC1569 WFPC2 IMAGE WF3-FIX F469N CR-SPLIT=0.5 1 1200 S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC1569 WFPC2 IMAGE WF3-FIX F547M CR-SPLIT=0.5 1 60 S ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC1569 WFPC2 IMAGE WF3-FIX F673N CR-SPLIT=0.533 1 1502 S ------------------------------------------------------------------------------------------------------------------------------------ 45 NGC1569 WFPC2 IMAGE WF3-FIX F673N CR-SPLIT=0.533 1 1502 S ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC1569 WFPC2 IMAGE WF3-FIX F502N CR-SPLIT=0.5 1 1000 S ------------------------------------------------------------------------------------------------------------------------------------ 55 NGC1569 WFPC2 IMAGE WF3-FIX F502N CR-SPLIT=NO 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC1569 WFPC2 IMAGE WF3-FIX F487N CR-SPLIT=0.5 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 70 NGC1569 WFPC2 IMAGE WF3-FIX F487N CR-SPLIT=0.5 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 8133( 4) - 07/22/99 16:41 - [ 6] Summary Form for Proposal 8133 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WF3-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=0.5 CR-SPLIT=0.533 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ORIENT 90D TO 270D POS TARG 0.25, 0.25 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F656N F469N F547M F673N F502N F487N ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1569 ------------------------------------------------------------------------------------------------------------------------------------