! Hubble Space Telescope Proposal 6588 ! PI: Ralf J. Dettmar ! From: dettmar@astro.ruhr-uni-bochum.de ! +====================+ ! | PED OUTPUT PRODUCT | ! +====================+ Proposal_Information Title: High Resolution Imaging of Ionized Gas in the Disk-Halo Interface of Spiral Galaxies Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_Name: Ralf J. Dettmar PI_Institution: Astronomisches Institut der Ruhr-Universitaet Bochum CoI_Name: Rene Walterbos CoI_Institution: New Mexico State University Contact: CoI_Name: Colin Norman CoI_Institution: Johns Hopkins University Contact: Abstract: Observations of the interstellar medium at the disk-halo interface of spiral galaxies provide us with very significant information on the physical state and structure of the interstellar medium (ISM) in a global sense. The nature of disk-halo interaction is particularly important for our understanding of the evolution of galaxies. We propose to study the morphology of diffuse ionized gas and its correlation with dust and other ISM components in the disk- halo interface of several edge-on galaxies. This gas is the extra-galactic analog of the Reynolds layer in the Milky Way. The morphology and vertical extent of this gas seems to vary from object to object and to change even within individual galaxies. Correlations with tracers of star formation strongly indicate that extraplanar gas is indeed correlated with star formation activity in the underlying disk, in agreement with galactic fountain and chimney models of the disk-halo interface. However, while ground-based data begin to resolve the largest filamentary structures and bubbles, the resolution is completely inadequate to characterize the structure of the ionized medium well enough to test specific predictions of these models. In view of the important role that transport of energy and matter from disk to halo and vice versa has in models of galactic evolution, it is crucial to accurately image the diffuse ionized gas layer in different galaxies to better test the theoretical predictions. Questions Observing_Description: Following the recommendation by TAC we will observe one WFPC2 field in NGC 891 centered at a position where star formation in the disk and the associated extraplanar gas are most prominent. The observations in H Alpha +NII emission line filters and in the R broad band filter will reveal the filamentary distribution of ionized gas and associated structures in the dust. The broad band R image is also used to correct for the continuum contribution in the emission line filter. An accurate sky level will be obtained by modeling the observed brightness distribution in comparison to our extensive ground based imaging of all objects. Due to the radial velocity of the objects and their internal velocity field the H Alpha + NII filter has to be chosen carefully. The complete velocity range can not be covered with only one of the F656N or F658N filters. For NGC 891 we therefore have chosen a field centered 2' NE of the nucleus (Rupen 1990). All HI velocities observed in this area fall into the wavelength range 3Sigma detection can be expected even if the continuum subtraction (introducing the same noise level) is taken into account. This is a worst case scenario; at high resolution the filaments are likely much brighter, giving a correspondingly large S/N ratio. The target is in the DEC range of 35degrees -- 45degrees allowing us ~42 minutes of integration from the second orbit onwards. 9 orbits will be used for the narrow band filter one additional orbit is required for the broad band image. Real_Time_Justification: None The target has been been well studied from the ground and the applicants have been involved over the last couple of years in several optical, radio, and X-ray studies addressing in particular the disk-halo interaction of the ISM. Therefore X-ray, radio, and ground based optical data are already to our disposal (for NGC 891: optical imaging and spectra: Dettmar 1990, Keppel, Dettmar, Gallagher & Roberts 1991, Dettmar & Schulz 1992, Dettmar and Allen have obtained Fabry- Perot observations in H Alpha, radio: Dahlem, Dettmar, Hummel 1994) Walterbos will have access to 3.5m telescopes (ARC) for supporting observations if needed. Calibration_Justification: Additional_Comments: Fixed_Targets Target_Number: 1 Target_Name: NGC891 Alternate_Names: UGC1831 Description: EXT-MEDIUM,HALO,DUST LANE,IONIZATION FRONT,GALAXY,SPIRAL Position: RA=02H 22M 35.5S +/- 5", DEC=42D 21' 43" +/- 5", PLATE-ID=01WN Equinox: J2000 RV_or_Z: V = 528 Epoch: Flux: SURF(V) = 20.5+/-0.2 B-V = 0.9 Comments: Visits Visit_Number: 01 Visit_Requirements: ORIENT 240D TO 240D Visit_Comments: If scheduling is not possible within the given range for ORIENT, other solutions are possible. However, this would require fine tuning of position and ORIENT together. A second visit is used only to account for a Feasibility Diagnostic telling that only 8 consecutive orbits can be used in one visit. Specific orientation set to best avoid stray light from foreground stars. Exposure_Number: 1 Target_Name: NGC891 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F675W Wavelength: Optional_Parameters: DITHER-TYPE=BOX Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: Comments: Exposure_Number: 2 Target_Name: NGC891 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F656N Wavelength: Optional_Parameters: CR-SPLIT=NO,DITHER-TYPE=BOX Number_of_Iterations: 1 Time_Per_Exposure: 2800S Special_Requirements: Comments: Exposure times are chosen under the assumption that read-out into occultation is possible Visit_Number: 02 Visit_Requirements: ORIENT 240D TO 240D; SCHED 30% On_Hold_Comments: Visit_Comments: Specific orientation set to best avoid stray light from foreground stars. Exposure_Number: 3 Target_Name: NGC891 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F656N Wavelength: Optional_Parameters: CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 2700S Special_Requirements: POS TARG -0.25,0.25 Comments: Exposure_Number: 4 Target_Name: NGC891 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F656N Wavelength: Optional_Parameters: DITHER-TYPE=BOX,CR-SPLIT=NO Number_of_Iterations: 1 Time_Per_Exposure: 2800S Special_Requirements: POS TARG 0.25,0.25 Comments: Data_Distribution ! Defaults indicated; change if desired Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS Recipient_Email: