8565( 2) - 08/03/00 17:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8565 Version: 2 Check-in Date: 01-Mar-2000 12:11:22 1.Proposal Title: Where Does Lyman Alpha Escape from Galaxy Disks? ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: William Keel University of Alabama ------------------------------------------------------------------------------------ 5. Abstract The frequent detection of Lyman Alpha emission at high redshifts underscores the need to understand how it arises in nearby galaxies. Despite the obstacles posed by radiative transfer, we find empirically that significant Lyman Alpha emerges from some nearby galaxies. Data on M33, in particular, suggest that much of this escape could be from the diffuse ISM rather than from H II regions specifically, an idea with some theoretical attraction as well. This proposal examines the structure of known Lyman Alpha emission from the starburst disk of Markarian 357 (PG 0119+229), to separate the contributions of discrete and diffuse sources of this line. ------------------------------------------------------------------------------------ 8565( 2) - 08/03/00 17:13 - [ 2] Observations Description ------------------------ Selecting a target for this measurement requires: z large enough to get the line well free of geocoronal emission and Galactic absorption, and small enough to resolve discrete from diffuse sources; line flux low enough to avoid issues of MAMA count rate; and, ideally, data showing that Lyman Alpha emission exists at a useful level and that there may be extended emission in at least some line. These are satisfied by the starburst system PG 0119+229 (Mkn 357) at z=0.0532, shown from IUE to have Lyman Alpha emission and from HAlpha imaging to have extended structure out to 2-3" scales. This is a luminous interacting system, so its ISM might be more like those of bright spirals than found in the lower- luminosity and metal-poor H II systems that have been more frequently found to show Lyman Alpha. The integrated HAlpha luminosity of 2.5 * 10^42 erg/s places it with bright star- forming spirals, such as NGC 2276. The strategy includes a far-UV image, for the distribution of UV starlight, a wide- slit spectrum across Lyman Alpha, and a narrowband HAlpha observation with accompanying continuum image. The key measurements will be in comparing the Lyman Alpha distribution to the UV continuum and HAlpha images; if the diffuse ISM is a preferential source of Lyman Alpha, this will appear in the comparison once PSF differences with wavelength are accounted for. If Lyman Alpha and HAlpha both show similar extended components compared to the UV continuum, this implies that the escape of UV photons from H II regions is complex, but not preferential behavior for Lyman Alpha. The total Lyman Alpha flux (observed Lambda=1283 Angstroms) from IUE is 1.7 * 10^-13 erg cm^-2 s^-1 (global equivalent width 12.5 Angstroms). The HAlpha scale length is close to 1", from narrowband KPNO imagery. As a rough model, I consider the case of the line flux evenly divided between 10 unresolved H II regions and a uniform 1" disk. The G140M grating rejects the continuum well, compared to G140L, while only barely resolving typical H II region linewidths; this way, a wide-slit spectrum is nearly a monochromatic image in the line. A 2" slit would be used to reduce the detector area affected by geocoronal emission (oriented, if feasible, to include one of two nearby companions within 12 arcseconds). Giant H II regions have linewidths only slightly greater than the spectral pixel scale of the FUV- MAMA (12 km/s per pixel) with the G140M grating, so it will give significant continuum reduction compared to G140L. Each of the point sources then has a line flux 3.5 * 10^-15 erg/cm^2 s, within a line width of 30 km/s (0.13 Angstrom =2.5 pixels) or less, with an additional 7 * 10^-14 ergs cm^2 s smoothly spread over 1". The STIS exposure-time calculator is confusing enough for sources with strong, narrow emission lines, that I checked the estimates by starting with the raw photon flux and system throughput, and partitioning the flux among possible spatial components in Lyman Alpha. This exercise gives each model H II region a total count rate of 0.04 per second in the emission line, about three times higher than the rate from the ETC using the scaled IUE spectrum as input. While the difference may reflect the low spectral resolution of the IUE spectrum compared to the actual linewidth, the time request is based on the ETC results as a more conservative estimate. A benchmark S/N of 10 for these bright knots is desirable, so that one can sensibly ask how much of the total comes from more diffuse material (``diffuse" being defined with reference both to the UV continuum and the HAlpha structure). To compare the distribution of line emission and UV starlight, part of an orbit would be devoted to a broad -band FUV direct image. An additional orbit would use WFPC2 for narrow-band HAlpha and broadband continuum imaging. If one cheats by only 14 Angstroms in setting the center wavelength, a small fraction of the filter width, the WFPC2 ramp filter FR680N puts HAlpha at z=0.0532 (6912 Angstroms) on the PC chip, giving the best available sampling for a narrow band at this wavelength. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8565( 2) - 08/03/00 17:13 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 MRK357 GALAXY, Interacting RA=01H 22M 40.64S +/- 0.1S, 2000.0 Z=0.053 V = 15.3 galaxy DEC=+23D 10' 14.7" +/- 1.0", B-V = 0.25 PLATE-ID=02BF Coordinate Source: GSC_SURVEY_PLATE 8565( 2) - 08/03/00 17:13 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 0D TO 48D ORIENT 174D TO 228D ORIENT 352D TO 360D 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 MRK357 STIS/C ACQ F28X50L MIRROR ACQTYPE=DIFFUSE, 1 100 S CD P DIFFUSE-CENTER=FLUX-CENTROID, CHECKBOX=15 ------------------------------------------------------------------------------------------------------------------------------------ 2 MRK357 STIS/F ACCUM 52X2 G140M 1272 3 1300 S UV-MAM A Comments: Wide-slit spectrum with STIS is essentially slitless, using the 2-second slit to block geocoronal O I emission and keep both background and overall count rate low. The optimal orientation has the slit along celestial PA 151 or -29, parallel to the major axis of the galaxy and encompassing its companion 12 arcseconds to the southeast (which would be close to the end of the slit). The allowed ORIENT ranges are centered near this value, which translates to U3 position angle 16 or 196 degrees (STIS slit 45 degrees clockwise from U3). ------------------------------------------------------------------------------------------------------------------------------------ 3 MRK357 STIS/F ACCUM F25SRF2 MIRROR 1 1800 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 MRK357 WFPC2 IMAGE PC1 F702W 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 5 MRK357 WFPC2 IMAGE LRF LRF 6922 1 1600 S Comments: Wavelength set slightly to the red to get target on PC ------------------------------------------------------------------------------------------------------------------------------------ 8565( 2) - 08/03/00 17:13 - [ 5] Summary Form for Proposal 8565 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X2 F25SRF2 PC1 LRF ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=DIFFUSE DIFFUSE-CENTER=FLUX-CENTROID CHECKBOX=15 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 0D TO 48D ORIENT 174D TO 228D ORIENT 352D TO 360D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140M F702W LRF ------------------------------------------------------------------------------------------------------------------------------------ Target Names MRK357 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1272 6922 ------------------------------------------------------------------------------------------------------------------------------------