Proposal_Information Title: The Excitation Mechanism of Extended Emission-Line Regions in Active Galaxies Proposal_Category: GO Scientific_Category: AGN Cycle: 5 Investigators PI_name: Ian Evans PI_Institution: Space Telescope Science Institute CoI_Name: Mark Allen CoI_Institution: Mount Stromlo and Siding Spring Observatories CoI_Name: Michael Dopita CoI_Institution: Mount Stromlo and Siding Spring Observatories CoI_Name: Anuradha Koratkar CoI_Institution: Space Telescope Science Institute CoI_Name: Zlatan Tsvetanov CoI_Institution: Johns Hopkins University Abstract: Ionized gas in the form of extended emission-line regions extending for several kpc from the nucleus is commonly found associated with radio sources and active galactic nuclei. These emission-line regions form excellent laboratories for diagnosing the physical mechanisms operating in the host active galaxy, and for investigating the effects of the active nucleus on the surrounding galaxian environment. Although models invoking photoionization by the ionizing UV and soft X- ray nuclear continuum at first glance appear to do an excellent job of predicting the relative intensities of the strongest optical emission-lines, they all suffer from the serious inability to reproduce accurately the electron temperature in the gas, indicating that another source of heating is required. We propose to determine unambiguously whether ``radiative autoionizing shocks'' are an important contributor to the ionization balance in the extended emission -line regions. We will do this by observing the UV emission from the brightest knot in the extended emission-line regions of 3 highly preselected active galaxies with the Faint Object Spectrograph. Investigation of the ionization mechanism requires mandatory observations in the satellite UV because the UV emission-lines are the critical determinants of the emission mechanism. Questions Observing_Description: We will use the Faint Object Spectrograph (FOS) to obtain ultraviolet spectrophotometry of the brightest knot in the EELRs of three nearby active galaxies. The key discriminants between radiative autoionizing shocks and diluted nuclear photoionization are the fluxes and flux ratios observed in the emission-lines of C II Lambda1335, O IV Lambda1401, C IV Lambda1550, O III Lambda1666, C III Lambda1909, and C II Lambda2326. By using the FOS blue detector with the G160L low resolution grating, we can observe the wavelength region from LyAlpha to 2500Angstrom in one grating setting. Detailed line profile information is not required; gas velocity information can be obtained from our planned coordinated ground-based optical spectrophotometry. The FOS spectra will be obtained through the 1.0 arcsecond aperture to maximize the signal-to-noise ratio measurements of the weak diagnostic emission-lines in the UV and to determine the UV continuum. Using the 1.0 arcsecond aperture also simplifies intercomparison of the UV data with our planned coordinated ground-based optical spectrophotometry of the same knots that we will employ to constrain fully the models. The three objects in our sample have been preselected very carefully on the basis of the following criteria. (1) Observational evidence for the presence of an outflow or wind; (2) Observational evidence for radio emission from the EELR/nuclear region; (3) Observational evidence for soft X- ray emission from the EELR/nuclear region observed with the ROSAT PSPC; (4) Observational evidence for clumpiness in our high resolution ground-based (NTT) narrow-band imaging of the targets; (5) The presence of a clearly defined, bright, high excitation EELR that extends for at least several kpc from the position of the active nucleus; and (6) A hidden active nucleus. Only objects satisfying all six criteria were included in the sample. The presence of an outflow or wind, radio emission, and a highly clumpy EELR all suggest that interactions of the local ISM with the radio plasma/wind/outflow are occuring in the EELR (NGC 3281: Storchi-Bergmann et al. 1992; NGC 5643: Morris et al. 1985; Storchi-Bergmann et al. 1994; NGC 5728: Schommer et al. 1988; Wilson et al. 1993). These interactions are expected to generate shocks through the operation of the Kelvin- Helmholtz instability. Soft X-ray emission is a tracer of radiative, hot, shocked gas which may be present in the EELR (the spatial resolution of the existing PSPC data is insufficient to disentangle X-ray emission by the active nucleus from X-ray emission by the EELR gas). A hidden active nucleus and a bright, high excitation, clumpy, and relatively large scale EELR morphology suggests the existence of nuclear collimated radiation and a possible nuclear wind or outflow. We have in each case chosen to observe the brightest EELR knot that is at least 1 kpc from the nucleus, so as to tightly constrain the geometrical dilution of any nuclear ionizing radiation field. For the 3 objects in our sample, the O III Lambda5007 flux observed from the brightest EELR knots through a 1 arcsecond aperture is approximately the same, 2.0times10^-14 erg cm^-2 s^-1. The typical O III Lambda5007 / HBeta ratio in these objects is 12:1. Based on these fluxes, we can reach a 3 Sigma sensitivity limit in an emission-line of 5 percent flux at 1335Angstrom, in 3 orbits. This level of sensitivity is required to accurately measure the relative intensities of the faint UV diagnostic emission- lines. Additionally, for each object, one orbit is required to perform a 3-stage peakup acquisition to center the target in the 1.0 arcsecond aperture. Thus, 4 orbits are required per object. Real_Time_Justification: There are no special requirements. Coordinated optical spectrophotometry of the observed EELR knots will be obtained using ground-based facilities. Simultaneous optical and UV observations are not required, because the emission-line variability timescale for the EELRs is long. Since the FOS 1.0 arcsecond aperture encapsulates the bulk of the line emission from the knots, errors introduced in UV-optical line ratio measurements because of differences between the regions sampled by the HST and ground -based spectra will be negligible. The planned optical spectrophotometry will span the entire optical waveband, with a 3 Sigma sensitivity limit of a few percent of the HBeta flux. Fully self-consistent shock and photoionization models for the emission mechanism for these specific EELR knots will be developed using our existing EELR models as a staring point and employing the optical spectrophotometry to tightly constrain the models. The model predictions for the UV emission-lines will then be used to discriminate the mechanism or mechanisms responsible for ionizing the EELR. Finally, a global ionization model for the EELR will be developed using both the optical and UV emission-line diagnostics as constraints. Fixed_Targets Target_Number: 1 Target_Name: NGC5643-OFFSET Description: STAR Position: RA = 14H 32M 38.180S +/- 0.5", DEC = -44D 10' 35.30" +/- 0.5", PLATE-ID = 011M Equinox: J2000 Flux: V = 13.56 +/- 0.3 Comments: OFFSET STAR APPROX K2I Target_Number: 2 Target_Name: NGC5643-EELR Description: GALAXY, LINER, SPIRAL Position: RA-OFF = +2.517S +/- 0.014S, DEC-OFF = +6.57" +/- 0.15", FROM 1 Equinox: J2000 RV_or_Z: V = +1199 Comments: EXTENDED EMISSION LINE REGION Target_Number: 3 Target_Name: NGC5728-OFFSET Description: STAR Position: RA = 14H 42M 24.538S +/- 0.5", DEC = -17D 14' 54.65" +/- 0.5", PLATE-ID = 029T Equinox: J2000 Flux: V = 14.86 +/- 0.3 Comments: OFFSET STAR APPROX KOIII Target_Number: 4 Target_Name: NGC5728-EELR Description: GALAXY, LINER, SPIRAL Position: RA-OFF = -0.642S +/- 0.010S, DEC-OFF = -16.75" +/- 0.15", FROM 3 Equinox: J2000 RV_or_Z: V = +2788 Comments: EXTENDED EMISSION LINE REGION Visits Visit_Number: 1 Exposure_Number: 1 Target_Name: NGC5643-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 4.3 Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=1, SEARCH-SIZE-Y=3, SCAN-STEP-Y=1.23 Number_of_Iterations: 1 Time_Per_Exposure: 8.9S Special_Requirements: ONBOARD ACQ FOR 2 Exposure_Number: 2 Target_Name: NGC5643-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 1.0-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=6, SEARCH-SIZE-Y=2, SCAN-STEP-X=0.61, SCAN-STEP-Y=0.61, STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 9.8S Special_Requirements: ONBOARD ACQ FOR 3 Exposure_Number: 3 Target_Name: NGC5643-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 0.25-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=5, SEARCH-SIZE-Y=5, SCAN-STEP-X=0.17, SCAN-STEP-Y=0.17, STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 11.1S Special_Requirements: ONBOARD ACQ FOR 4-5 Exposure_Number: 4 Target_Name: NGC5643-EELR Config: FOS/BL Opmode: ACCUM Aperture: 1.0-PAIR-A Sp_Element: PRISM Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 1200S Exposure_Number: 5 Target_Name: NGC5643-EELR Config: FOS/BL Opmode: ACCUM Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 11300S Visit_Number: 2 Exposure_Number: 1 Target_Name: NGC5728-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 4.3 Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=1, SEARCH-SIZE-Y=3, SCAN-STEP-Y=1.23 Number_of_Iterations: 1 Time_Per_Exposure: 6.7S Special_Requirements: ONBOARD ACQ FOR 2 Exposure_Number: 2 Target_Name: NGC5728-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 1.0-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=6, SEARCH-SIZE-Y=2, SCAN-STEP-X=0.61, SCAN-STEP-Y=0.61, STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 7.3S Special_Requirements: ONBOARD ACQ FOR 3 Exposure_Number: 3 Target_Name: NGC5728-OFFSET Config: FOS/BL Opmode: ACQ/PEAK Aperture: 0.25-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=5, SEARCH-SIZE-Y=5, SCAN-STEP-X=0.17, SCAN-STEP-Y=0.17, STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 8.3S Special_Requirements: ONBOARD ACQ FOR 4-5 Exposure_Number: 4 Target_Name: NGC5728-EELR Config: FOS/BL Opmode: ACCUM Aperture: 1.0-PAIR-A Sp_Element: PRISM Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 1200S Exposure_Number: 5 Target_Name: NGC5728-EELR Config: FOS/BL Opmode: ACCUM Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 10700S Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS