8166( 3) - 03/07/00 15:56 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8166 Version: 3 Check-in Date: 17-Feb-1999 16:05:38 1.Proposal Title: WFPC2 Narrow-Band HAlpha Imaging of the Edge-on Galaxy NGC4631 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Q. Wang Northwestern University CoI: Dr. James Lauroesch Northwestern University N CoI: Prof. Rene Walterbos New Mexico State University N ------------------------------------------------------------------------------------ 5. Abstract Observations of nearby edge-on disk galaxies are essential to determine the role that the disk/halo interaction has in galaxy evolution. Located in a direction of exceptionally low Galactic extinction, NGC4631 is particularly suitable for a multi- wavelength investigation of extra-planar gas components. In fact, NGC4631 is one of the most well studied late-type disk galaxies. Broad-band optical, near-infrared, near-UV, and far-UV observations have been (or will be) taken with HST and FUSE. We have obtained AXAF observing time to conduct a spatially-resolved X-ray spectroscopic observation of the galaxy. Here we propose for HST WFPC2 narrow-band H -alpha imaging of the galaxy's active central region. This image will enable us to resolve various narrow filaments of warm ionized gas that extend more than 2 kpc away from the mid-plane of the galaxy. Based on the geometry, filling factor and line intensity of the filaments and their correlation with X-ray- emitting gas, we will stu dy their origins (chimney walls, b listers, condensations of cooled hot gas, shocks, etc). The results will be invaluable for our understanding of similar extra-planar gas observed in the Milky Way and other galaxies. ------------------------------------------------------------------------------------ 8166( 3) - 03/07/00 15:56 - [ 2] Observations Description ------------------------ We propose to observe one WFPC2 field centered at a position where the extra- planar filaments are most prominent (Fig. 2). The observation in H-alpha emission line and in the R band will reveal the filamentary structure of ionized gas and dust. The broad-band R image will also be used for continuum subtraction from the emission line image. An accurate sky level will be obtained by modeling the observed brightness distribution in comparison to our extensive ground-based imaging. The region with most prominent H-alpha filaments in the galaxy has a mean velocity of ~700 rm km s^-1 (Golla et al. 1996). Thus the H-alpha line is redshifted (to ~6580 Angstroms) well into the bandpass of the F658N filter (centered at 6590 Angstroms with an effective width of 29 Angstroms). We note that a considerable fraction of NII line emission will be in our bandpass; this will increase our sensitivity for the faint diffuse gas as N II/H-alpha ratios of up to 1 have been reported. Based on groun d-based observations, we see that the surface brightness of the most prominent H-alpha filaments is ~5-22*10^-17 ergs s^-1 cm^-2 arcsec^-2 (Rand et al. 1992). Since the filaments are mostly unresolved in these observations, the typical width of the filaments should be less than the resolution scale (~1") of the ground-based observations. The worst-case scenario is when the emission is actually distributed over the entire scale set by the ground based observations. In this scenario, the expected count rate per pixel is only ~4*10^-3 e- s^-1 pixel^-1; thus in each 2700 s exposure we would expect ~10 e- per pixel from the brightest H-alpha filaments. Given the dark current (~4*10^- 3 e- s^-1 pixel^-1), the read noise of 5.3 e-, and sky background (negligible in the F658N filter), we would thus expect to achieve a S/N of 1.5 per pixel per exposure, or a S/N of only about 4 per pixel in eight 2700 s exposures. We have confirmed these estimates using the on-line WFPC2 simulator, assuming CR- SPLIT=NO for the indiv idual exposures. Nevertheless, eve n in this case we will be able to determine the actual width and structure of the filaments, albeit not at the full instrument resolution. More likely, however, the emission is concentrated in small-scale clouds of cooled hot gas condensations, as predicted in the galactic fountain model, and/or along the walls of the chimneys, as expected in most popular models for the H-alpha filaments. If this is the scenario, we would expect the bulk of the observed emission to come from very narrow regions, most probably showing rim-brighten characteristics. Assuming that the emission regions are 0.1" in width, we expect to achieve S/N ratios for the prominent filaments of up to 7-18 per pixel in our H-alpha observations. The integrated counting statistics for an individual filament will certainly be much better. For emission of really low surface brightness, if still present in the HST images, we can resort to statistical analysis (auto -correlation) to measure characteristic scales and to constrain the filling factor o f filamentary structures. Only at the superb spatial resolution offered by the proposed HST observations does such a thorough spatial analysis of the extra-planar gas in a nearby galaxy become feasible. For the broad-band imaging we propose to take an one orbit F675W exposure (2500 s; CR- SPLIT=5). The contamination in the bandpass of the filter by some H-alpha should not matter since the total equivalent width of the line is low. This deep exposure will get down to R~27 mag for point sources and to a surface brightness (for extended sources) of ~23 mag arcsec^-2 (S/N~4). Such data are essential to subtract the continuum without significantly reducing the S/N ratio. Ground- based observations suggest that the surface brightness in this region is in the range 20.0-23.5 mag arcsec^-2, so we should be able to reliably identify and subtract any broad-band emission in our H-alpha image. Real Time Justification ----------------------- We will get supporting optical data from the ground using the ARC 3.5-m telescope, e.g. imaging in other filters and spectroscopy to determine exact velocity structure of the gas, which would help eliminate calibration uncertainties in the filter bandpass. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8166( 3) - 03/07/00 15:56 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8166( 3) - 03/07/00 15:56 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4631 GALAXY, HALO, ISM, RA=12H 42M 12S +/- 2", DEC=32D J2000 V = 10 FILAMENT 32' 18" +/- 2" SURF(V) = 22.0+/-1.0 SURF(6562)=14+/-10e-17 8166( 3) - 03/07/00 15:56 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 135D TO 225D 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 NGC4631 WFPC2 IMAGE PC1 F675W CR-SPLIT=DEF 1 1000 S ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4631 WFPC2 IMAGE PC1 F675W CR-SPLIT=DEF 1 1000 S MAX DUR 180% ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 8166( 3) - 03/07/00 15:56 - [ 6] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC4631 WFPC2 IMAGE PC1 F658N CR-SPLIT=NO 1 2700 S MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ 8166( 3) - 03/07/00 15:56 - [ 7] Summary Form for Proposal 8166 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=DEF CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 135D TO 225D MAX DUR 180% MAX DUR SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F675W F658N ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4631 ------------------------------------------------------------------------------------------------------------------------------------