8119( 6) - 10/13/99 10:11 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8119 Version: 6 Check-in Date: 13-Oct-1999 10:10:44 1.Proposal Title: Measuring the black hole mass in Centaurus A, the nearest active galaxy ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Ethan Schreier Space Telescope Science Institute CoI: Dr. Dave Axon Space Telescope Science Institute N CoI: Dr. Alessandro Capetti Osservatorio Astronomico di Torino N CoI: Dr. Anton Koekemoer Space Telescope Science Institute N CoI: Dr. Duccio Macchetto Space Telescope Science Institute N CoI: Dr. Alessandro Marconi Osservatorio Astrofisico di Arcetri N ------------------------------------------------------------------------------------ 5. Abstract We will use STIS to measure the mass of the black hole at the center of NGC5128 (Cen A), the nearest active galaxy. The presence of a super-massive black hole, comparable to that of M87, is suggested by the jet, the extent of the radio lobes, and the presence of strong nuclear radiation extending to hard X-rays and Gamma- rays. A 1" (~ 17pc) radius ionized accretion disk at the nucleus of Cen A was discovered in PaAlpha using HST NICMOS (Schreier et al. 1998); we have now (1998 August) also seen the disk in Fe IILambda1.643Mu m. Recent near-IR ground based spectra (Blum, 1998 private communication) reveal broadened lines near the nucleus, consistent with a central mass ~eq 10^9 M_\odot. These and other observations demonstrate that notwithstanding the high dust extinction at the center of NGC5128, a number of lines in the ~eq0.9-2.2Mu m range can be used to study the mass distribution. HST's spatial resolution is required to yield a good mass determination. S IIILambda 9532Angstrom\ is the shortest wavelength line which is seen to be present and is readily accessible to STIS. Observing with the G750L low- dispersion grating of STIS through the 0arcs1 wide slit, we will thus use the S III emission line as a gas tracer to measure the rotation curve of the disk, measure the mass distribution in the central few parsecs of the galaxy, and determine the mass of the black hole. ------------------------------------------------------------------------------------ 8119( 6) - 10/13/99 10:11 - [ 2] Observations Description ------------------------ The circumnuclear disk of Centaurus A will be observed using the low dispersion grating G750L of STIS through the 0arcs1 wide slit. The main issue to be addressed is extinction from the dust lane which, in the innermost regions, is as high as A_V ~ 11 magnitudes. While at the wavelength of the emission lines traditionally used for similar studies, such as HAlpha, the disk emission would be reduced by a factor of ~ 4000, at the wavelength of the S IIILambda9532 line this factor is less than 100. The ground based spectra of Simpson & Meadows (1998) show that S III is indeed quite bright, with a total flux of 5.4times10^ -14 erg cm^-2 s^-1 (see Fig. 2). To estimate the exposure time required for our observations, we derive a lower limit to the surface brightness of SIII from the Simpson & Meadows data of 2.4times10^-14 erg cm^-2 s^-1 arcsec^-2 (1arcs4*1arcs6 aperture). Assuming first that the flux is uniformly distributed over the aperture, we find that with a 52"*0arcs1 slit and the G750L grating, an exposure time of 3000s will provide a S/N of ~ 25 in the integrated line profile at each pixel position along the slit. We note that if, as we would expect, the emission is distributed approximately in the disk-like morphology we observe in Paschen-Alpha, then the S/N for the integrated line profile at each pixel position along the slit ranges from a minimum of ~ 10 in the fainter outer part of the disk to >~ 40 near the nucleus. This S/N is sufficient to permit a very accurate determination of the rotation curve. The proximity of Centaurus A makes it possible to use the low resolution G750L, with its maximal throughput. Even with its 4.92Angstrom/pix dispersion at the S III wavelength, corresponding to 150 km/s per pixel, we will be sensitive to a central mass concentration of the order of 10^6 solar masses within ~ 2 pc. Given the good signal to noise achievable, we expect to be able to measure the velocity difference with an uncertainty of less than 50 km/s. The difference in velocity across a Keplerian disk around a black hole is Delta v ~ 130 (M_6/r)^1/2 km/s where M_6 is the black hole mass in units of 10^6 Msun and r is the distance from the center in pc. At the distance of Centaurus A, 1"\ is 17 pc and the spatial resolution of STIS allows us to probe the nuclear disk down to a distance of ~ 2 pc. We are therefore sensitive to a central mass concentration of the order of 10^6 solar masses within this radius, while by comparison with M87, and from the estimated 800 km/s line width of the ground based BrGamma observation, we expect a mass of severaltimes10^8 to 10^9Msun. The major uncertainty in the estimate of the black hole mass is therefore likely to arise from the unknown geometry of the disk -- the exact position of the line of nodes and its inclination -- rather than from the actual velocity measurements. The degeneracy is eliminated by taking spectra along the disk axis and along two parallel lines, offset from the nucleus by +/-0arcs2, as shown in Fig. 3. The right panel in the figure shows the expected rotation curves at the three positions for two different models. The solid lines are simulated results for a 10^8 Msun\ black hole with a disk inclination of 60^degrees and the slit aligned along the line of nodes (i.e. the disk geometry expected from the analysis of the PaAlpha image). The dashed lines are for a 2.5* 10^8 Msun\ black hole, but with a disk inclination of 40^degrees and a 25^degrees angle between the slit and the line of nodes. Note that while the curves for the two models are completely indistinguishable for the central slit alone, they vary significantly in the off-nuclear positions. The proposed observations follow the same strategy we successfully adopted to measure the mass of the central black hole in M87 (Macchetto et al. 1997). We will use the software already developed for that paper to simultaneously fit the three rotation curves. This technique strongly constrains the geometry of the disk and significantly improves the black hole mass determination. We require a total of four orbits, in order to obtain >3000 seconds of exposure time at each of the three parallel slit positions. beginfigure centerline beginminipagec.4\linewidth psfigfigure=fig3.ps, width=0.9\linewidth, clip=yes endminipage \skip 0.1\linewidth beginminipagec.55\linewidth psfigfigure=fig4.ps, width=0.9\linewidth, clip=yes endminipage caption Left Panel: PaAlpha NICMOS image of Cen A's circumnuclear disk with the three proposed slit positions indicated. Slits are oriented along the major axis of the disk (PA 33^degrees) and are separated by 0arcs2. Right Panel: Expected rotation curves in the three slits, for two different black hole masses and geometries (solid and dashed lines; see text for details). endfigure The slit will be positioned on the nucleus of Centaurus A and the flanking fields by performing a target acquisition (STIS ACQ + ACQ/PEAK) on an isolated foreground star (V~20) located 9" SW of the nucleus. The relative offset has been measured from our WFPC2 images with an accuracy of ~ 0.02 "--both the star and the nucleus lie in a single PC field. This will ensure a successful acquisition of the nucleus in the slit(s) with a small angle maneuver. At Cen A's declination, the available time/orbit is ~55min. In the first orbit, GS ACQ + STIS ACQ + ACQ/PEAK +OH +SAM = 25min, leaving 24min (+6min OH) for the first science exposure. In subsequent orbits, GS REACQ + OHs +SAMs will allow a total of ~45min/orbit exposure time. Note that the exposures at each slit position are split across two orbits and thus enable cosmic ray rejection. In addition, note that our observations are in a region where fringing is significant; we will take a fringe flat every orbit for 2min, including overheads. 4 orbits will provide the required exposure time. For the parallel observations we require the same filter complement adopted for all the Cen A fields we previously imaged with WFPC2 both as primary and as parallel observations, i.e. the standard broad band filters F336W, F555W and F814W. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8119( 6) - 10/13/99 10:11 - [ 3] Data Distribution Paper Products: YES Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8119( 6) - 10/13/99 10:11 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CENA-STAR STAR RA=13H 25M 26.537S +/- 0.002S, J2000.0 Z=0.0 V = 20.0 DEC=-43D 1' 13.56" +/- 0.02", E(B-V) = 0.0 PLATE-ID=00AK 2 CENA-NUC GALAXY, Elliptical, RA=13H 25M 27.390S +/- 0.002S, J2000.0 Z=0.0018 V = 7.8 Radio Galaxy DEC=-43D 1' 10.22" +/- 0.02", 25 E(B-V) = 11.0 PLATE-ID=00AK 8119( 6) - 10/13/99 10:11 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 20D TO 40D ORIENT 200D TO 220D On Hold Comments: Additional Comments: VISIT 01 REQUIRES CONTIGUOUS ORBITS IN ORDER TO AVOID COMPROMISING THE SCIENCE OBJECTIVES. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CENA-STAR STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 3 S SEQ 1-14 NON-INT CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 CENA-STAR STIS/C ACQ/PEA 52X0.1 MIRROR 1 10 S CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 CENA-NUC STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 600 S PAR 4 WITH 3 CD P ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 11 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1250 S PAR 12-13 WITH 11 CD Comments: CENA-NUC - Dither 1 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 13 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 14 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 21 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1370 S POS TARG 0.00, 0.50 PAR CD 22-23 WITH 21 SEQ 21-34 NON-INT Comments: CENA-NUC - Dither 2 ------------------------------------------------------------------------------------------------------------------------------------ 22 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 23 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 31 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1370 S POS TARG 0.00, 1.00 PAR CD 32-33 WITH 31 Comments: CENA-NUC - Dither 3 ------------------------------------------------------------------------------------------------------------------------------------ 32 ANY WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 33 ANY WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 34 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 8119( 6) - 10/13/99 10:11 - [ 6] Visit: 02 Visit Priority: Visit Requirements: ORIENT 68D TO 88D ORIENT 248D TO 268D On Hold Comments: Additional Comments: VISIT 02 REQUIRES CONTIGUOUS ORBITS IN ORDER TO AVOID COMPROMISING THE SCIENCE OBJECTIVES. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CENA-STAR STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 3 S SEQ 1-14 NON-INT CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 CENA-STAR STIS/C ACQ/PEA 52X0.1 MIRROR 1 10 S CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 CENA-NUC STIS/C ACCUM F28X50L MIRROR CR-SPLIT=2 1 600 S PAR 4 WITH 3 CD P ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 11 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1250 S PAR 12-13 WITH 11 CD Comments: CENA-NUC - Dither 1 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 13 ANY WFPC2 IMAGE WFALL F336W CR-SPLIT=NO 1 500 S ------------------------------------------------------------------------------------------------------------------------------------ 14 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 21 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1370 S POS TARG 0.00, 0.50 PAR CD 22-23 WITH 21 SEQ 21-34 NON-INT Comments: CENA-NUC - Dither 2 ------------------------------------------------------------------------------------------------------------------------------------ 22 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 23 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 31 CENA-NUC STIS/C ACCUM 52X0.1 G750L 7751 CR-SPLIT=2 1 1370 S POS TARG 0.00, 1.00 PAR CD 32-33 WITH 31 Comments: CENA-NUC - Dither 3 ------------------------------------------------------------------------------------------------------------------------------------ 32 ANY WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 33 ANY WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 510 S ------------------------------------------------------------------------------------------------------------------------------------ 34 CCDFLAT STIS/C ACCUM 52X0.1 G750L 7751 2 DEF CD ------------------------------------------------------------------------------------------------------------------------------------ 8119( 6) - 10/13/99 10:11 - [ 7] Summary Form for Proposal 8119 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1 WFALL ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT CR-SPLIT=2 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 20D TO 40D ORIENT 200D TO 220D SEQ 1-14 NON-INT PAR 4 WITH 3 PAR 12-13 WITH 11 POS TARG 0.00, 0.50 PAR 22-23 WITH 21 SEQ 21-34 NON-INT POS TARG 0.00, 1.00 PAR 32-33 WITH 31 ORIENT 68D TO 88D ORIENT 248D TO 268D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR F336W G750L F555W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names CENA-STAR CENA-NUC ANY CCDFLAT ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 7751 ------------------------------------------------------------------------------------------------------------------------------------