9486( 4) - 03/20/02 09:51 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9486 Version: 4 Check-in Date: 20-Mar-2002 09:51:33 1.Proposal Title: What Excites LINERs: The Brilliant Case of NGC 3998 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Linda Dressel Space Telescope Science Institute CoI: Prof. Michael Dopita Mt.\ Stromlo and Siding Spring N Observatories ------------------------------------------------------------------------------------ 5. Abstract LINER emission is now known to occur in many galaxies at a variety of levels, with causes as diverse as old nuclear starbursts, cooling flows, accretion disks, and jets. The early prototypes, including NGC 3998, belong to a distinct class of AGN- like LINERs with the most luminous line emission and powerful nuclear radio emission. These LINERs have ionized gaseous disks on the scale of tens of parsecs, which have now been found to be in rotation about supermassive black holes. The disks are apparently the outer parts of energy- releasing accretion disks. Powerful new UV+optical emission line diagnostics have been developed to settle the long- standing debate over whether the gas is being photoionized or shock excited. With STIS, we will be able to test ionization models, to determine whether the UV continuum is produced by a nonthermal source or by hot stars, and to determine whether that continuum is sufficient to ionize the gas. We will also make a spatially resolved study of the nucleus to test ideas about the energy transport in the disk. NGC 3998 is an exceptionally good AGN-like LINER to apply the new modelling and the sensitivity and resolution of STIS to. It is very nearby, active, bright in emission lines, UV-bright, and free of obscuration. We propose to observe its nuclear spectrum from 1150Angstrom\ to 10000Angstrom\ to get what may be our best look yet at a LINER AGN. ------------------------------------------------------------------------------------ 9486( 4) - 03/20/02 09:51 - [ 2] Observations Description ------------------------ We will cover the spectrum of the nucleus of NGC 3998 from 1150Angstrom\ to 10266Angstrom\ using each of the gratings G140L, G230L, G430L, G750L at one tilt setting with the 0.2" slit. To maximize the amount of emission line flux entering the slit, and to get the best linear resolution of the emission line disk, we will place the slit along the major axis of the disk. The position angle of the major axis is well known from our previous HST observations. Additionally, we will make an observation of the profile of OILambda6300 with high spectral and spatial resolution using the 0.1" slit with the G750M grating. We are requesting one orbit for G750M with the 0.1"\ slit and G430L and G750L with the 0.2" slit. Allowing 7 minutes for guide star acquisiton, 4 for target aquisition, 6 for an acq/peak, 5 for the initial overhead with each grating, 1 for a CR split overhead for each grating, 1 for a slit change, (with 2 for a fringe flat free of charge at the end), we will divide the remaining 21 minutes between observations with each grating, giving 420 sec to each. This will be followed by one orbit for G140L and one for G230L. Each of these will include 6 min for a guide star reacquisition, 8 minutes for the initial overhead for the grating, and 0.5 minutes for a CR split, leaving 2550 sec for observing with each grating. Since we have already observed this target with STIS, the acquisition and CCD overhead times are based on experience. Our HST imaging has shown that the optical emission line region is very compact, and the nuclear continuum light somewhat less so. To optimize signal to noise in the emission lines, and to allow for the possibility of a spatially resolved study of the emission lines and continuum, we will use the 0.2"\ slit. Measurements of our PC H Alpha+NII and continuum images indicate that integration over 0.5"\ along the 0.2"\ slit gives 75\ and 55\ 0.5"\ slit. By using the narrower slit, we can thus still get about the same signal to noise on weak optical lines as with the wider slit. For the stronger lines, it will be possible to integrate over much shorter distances, and make separate measurements at the nucleus and at positions at different distances from the nucleus. For S/N calculations, below, we will assume integration over 0.2"\ x 0.2". FOC observations have shown that a nuclear continuum point source becomes more prominent in the UV. In going from the 0.5"\ slit to the 0.2"\ slit, the slit loss is relatively greater for this point source than for the more extended emission line disk, helping to keep up the emission line S/N in the 0.2"\ slit. The 0.2"\ slit, which allows the possibility of a spatially resolved study of the stronger emission lines, is thus a good choice in the UV as well as in the optical. CIILambda2326 was strongly detected in IUE observations of the nucleus of NGC 3998 (Reichert et al. 1992). Since this line is strong in the circumnuclear LINER disk of M87, we are using the spectrum of M87 to predict line strengths in the disk in NGC 3998. The line strengths are scaled to the observed flux of the SIILambda6717, 6731 lines in the central 0.2"\ x 0.2"\ of our STIS spectra of NGC 3998, correcting for slit losses and extraction length as a function of wavelength. We expect that the lines characteristic of shock excitation will either be about this strong, if that is the relevant mechanism, or much weaker (generally undetectable in the UV) if it is not. To estimate the continuum flux in 0.2"\ x 0.2" we have used the STIS data in combination with observations by Blackman, Wilson, and Ward (1983) and Reichert et al. (1992), assuming the unresolved IUE component to be a point source. We have used the above described fluxes and observing times to predict the S/N for the line emission in an 0.2"\ x 0.2"\ area. Based on our existing STIS data, we have assumed integration over a full width of 1300 km/sec per line. For the lines that we are considering, S is determined by the line flux and N by the line and continuum fluxes (taking slit losses and extraction length into account). Sky brightness, dark counts, and read noise are negligible. The S/N are as follows for all of the lines which are bright in the M87 shock model and less bright in other modelsvspace5mm: CIV Lambda1550: 36 HeII Lambda1640: 10 in G140L, 6 in G230L N IIILambda1750: 5 C IIILambda1909: 19 C IILambda2326: 25 NeIVLambda2424: 5 OIILambda3727: 79 O1Lambda6300: 66 in G750L, 50 in G750M NIILambda6584: 114 SIIvspace5mmLambda6717, 6731: 98 These figures are high enough to allow for good flux measurements in most of the lines even if there is some error in the estimates of the nuclear UV line and continuum fluxes, and to allow for separate nuclear and off-nuclear measurements of most lines. (i.e., the 0.2"\ x 0.2"\ area will be adequate for the nucleus, and the off-nuclear S/N will be lower but still adequate for most lines.) Note that 420 sec is a lot of integration time for optical emission lines this bright. In program 7354, integrations of 328 sec per slit position with the 0.1"\ slit on and near the nucleus of NGC 3998 gave more than enough S/N for emission line measurements. The continuum S/N per Angstrom\ is 10 to 20 below 3000 Angstrom and higher above 3000Angstrom so we should get a good measurement of the UV continuum slope or shape. The throughput of STIS+G140L is 10 times that of FOS+G130H at 1400Angstrom. (FOS+G160L was somewhat worse.) From 1200Angstrom\ to 1550Angstrom, it is 30 to 4 times as high. We should therefore get better observations at the shortest wavelengths than were ever obtained for LINERs with FOS, considering both the greater throughput of STIS and the brightness of this galaxy. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9486( 4) - 03/20/02 09:51 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3998 GALAXY, LINER, RA=11H 57M 56.12S +/- 0.5", J2000 V = V = 17.0+/-0.1 LENTICULAR, NUCLEUS DEC=55D 27' 12.5" +/- 0.5", +1045 PLATE-ID=00LK Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 9486( 4) - 03/20/02 09:51 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.3551 Coordinate_Frame POS-TARG Pattern_Orient 90 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.1776 Coordinate_Frame POS-TARG Pattern_Orient 90 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9486( 4) - 03/20/02 09:51 - [ 5] Visit: 01 Visit Priority: Visit Requirements: GROUP 1-2 WITHIN 30D ORIENT 342.0D TO 345.0D ORIENT 162.0D TO 165.0D 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC3998 STIS/C ACQ F28X50L MIRROR 1 10 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC3998 STIS/F ACCUM 52X0.2 G140L 1425 1 2500 S EXPAND UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC3998 STIS/N ACCUM 52X0.2 G230L 2376 1 3000 S EXPAND UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 9486( 4) - 03/20/02 09:51 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC3998 STIS/C ACQ F28X50L MIRROR 1 10 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC3998 STIS/C ACQ/PEA 52X0.1E MIRROR SIZEAXIS2=16 1 10 S CD K 1 ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC3998 STIS/C ACCUM 52X0.1E G750M 6581 CR-SPLIT=NO,BINAXIS1=2 1 130 S PATTERN 2 30 MAX DUR 130 CD 1 S ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC3998 STIS/C ACCUM 52X0.2E G430L 4300 CR-SPLIT=NO 1 200 S PATTERN 1 40 MAX DUR 200 CD 1 S ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC3998 STIS/C ACCUM 52X0.2E G750L 7751 CR-SPLIT=NO 1 200 S EXPAND PATTERN 1 50 MAX CD 1 DUR 200 S ------------------------------------------------------------------------------------------------------------------------------------ 60 CCDFLAT STIS/C ACCUM 52X0.2E G750L 7751 2 DEF CD 1 ------------------------------------------------------------------------------------------------------------------------------------ 9486( 4) - 03/20/02 09:51 - [ 7] Summary Form for Proposal 9486 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.2 52X0.1E1 52X0.2E1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ACQ/PEAK ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SIZEAXIS2=16 BINAXIS1=2 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 1-2 WITHIN 30D ORIENT 342.0D TO 345.0D ORIENT 162.0D TO 165.0D EXPAND SAME ORIENT AS 01 PATTERN 2 30 MAX DUR 130 S PATTERN 1 40 MAX DUR 200 S EXPAND PATTERN 1 50 MAX DUR 200 S ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L G230L G750M G430L G750L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC3998 CCDFLAT ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 2376 6581 4300 7751 ------------------------------------------------------------------------------------------------------------------------------------