6635( 7) - 11/14/96 15:37 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6635 Version: 7 Check-in Date: 14-Nov-1996 15:34:30 1.Proposal Title: Unveiling the Nature of the Ultraviolet Spike at the Center of the Sa Galaxy NGC 2681 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Francesco Bertola University of Padova CoI: David Burstein Arizona State University N CoI: Lucio Buson Padova Observatory N CoI: Michele Cappellari University of Padova Y CoI: Jose Funes University of Padova N CoI: Laura Greggio University of Bologna N CoI: Alvio Renzini ESO N ------------------------------------------------------------------------------------ 6. Abstract We propose to investigate the nature of the central UV-bright spike in the bulge- dominated Sa galaxy NGC 2681, first detected in the course of our previous HST/FOC observations of this galaxy. The feature appears point-like in our Pre-Costar images and is located within ~ 2 pc from the galaxy center. New combined FOS and FOC/96 observations will provide its UV/optical SED and higher resolution imaging. These data will allow to determine the physical nature of the source (a compact cluster of hot stars, or a non-thermal, perhaps transient nuclear source). In particular, these data should allow to establish whether the UV spike in NGC 2681 is the same kind of transient phenomenon we have detected in NGC 4552, an otherwise normal elliptical galaxy in which a similar UV-bright central spike underwent a flare, brightening by over a factor of 7 between 1991 and 1993. ------------------------------------------------------------------------------------ Observations Description ------------------------ The proposed observations are almost identical to those requested in our DD Cycle 5 proposal (ID 6309) aimed at imaging the central spike of NGC 4552. As a consequence, we expect to be able to reduce the data in a homogeneous way, though taking advantage of a higher S/N ratio for NGC 2681., Using as reference our own previous (epoch 1993) FOC/96 images of the same galaxy, the expected background subtracted flux from the spike in NGC 2681 is ~ 3*10^-16 erg cm^-2 s^-1 AA^-1 at Lambda ~ 2700 Angstrom. Its SED appears consistent -- in the 1700 -2800 Angstrom range -- with that of a T ~ 10000 K Black Body., As in the case of NGC 4552, we plan to spend about two orbits for the acquisition of the target using the FOS ACQ/PEAK mode. The remaining two orbits will be devoted to the science exposures: we will acquire the spectra using the gratings G270H (3rd orbit) as well as G780H and G650L (4th orbit). Given the flux, the expected S/N ratio over 6 Angstrom intervals (3 diodes) for the G270H spectrum is ~20 with the spike contributing about 4/5 of the total count rate in one of the 0.25-PAIR aperture. The contaminating galaxy light will be removed using the existing IUE spectrum. The G780H grating will possibly allow us to detect HAlpha emission, as already detected by Eracleous et al. (1995, ApJ 438, 610) at the 7*10^-16 erg cm^-2 s^-1 ang^-1 level in the nucleus of NGC 1097, and attributed to accretion around a massive black hole in that galaxy. At the distance of NGC 2681 this emission would appear at the 1.1*10^-15 erg cm^-2 s^- 1 ang^-1 level and would be easily detectable with good S/N over the galaxy continuum. Finally, the G650L grating will permit us to complete the full SED of the spike with little additional overhead. Our FOS observations will be supplemented by contemporary FOC imaging, primarily to try to determine the spatial extent of the spike in NGC 2681. The F175W image permits extension of the SED of the spike from 2200 down to 1600 Angstrom, using known sensitivity curves and our FOS spectra (to account for the red--leak). Together with the F275W image it will be used to check for variability of the spike. The F342W image will be used, together with the other images, to determine the UV color of any additional structure that may exist around the spike., Using our previous FOC observations and owing to the improved performance of the Post- COSTAR HST, we estimate that we will be able to get a better S/N in less than half the exposure time, so we can efficiently pack all FOC observations within one orbit. Using the formulas for nonlinearity correction of point sources over a bright background (Greenfield, P., ISR FOC-074, 1994), we estimate that the Post-COSTAR count rate will remain in the linear range of the FOC 512*512 mode both in the F175W and F275W filter, whereas we will need an additional neutral F1ND filter for the F342W exposure. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: , ------------------------------------------------------------------------------------ 6635( 7) - 11/14/96 15:37 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC2681-NUC GALAXY,SPIRAL, RA=8H 53M 32.80S +/- J2000 F(1750)=1.5+/-0.5e-16 0.1S,DEC=51D 18' 49.4" +/- 1" F(2750)=3.0+/-0.5e-16 F(3400)=3.0+/-0.5e-16 W-LINE(6600)=200+/-100 F-LINE(6600)=1+/-0.5e-15 Comments: The target luminosity might be variable. The fluxes at specified wavelength refer to the last values measured with FOC in october 1993. The line flux is a very uncertain extrapolation based on theoretical considerations. 6635( 7) - 11/14/96 15:37 - [ 2] Visit: 01 Visit Requirements: SEQ 1-2 WITHIN 24H On Hold Comments: Additional Comments: FOC photometry will be used in conjunction with FOS spectra to determine the Spectral Energy Distribution of the flare. This proposal includes FOS observations which have to be done before the 2nd Servicing Mission in Feb 1997. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC2681-NUC FOC/96 IMAGE 512X512 F342W, 3410 1 400 S F2ND ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC2681-NUC FOC/96 IMAGE 512X512 F275W, 2770 1 550 S SAME POS AS 1 F1ND ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC2681-NUC FOC/96 IMAGE 512X512 F175W 1730 1 1025 S END ORBIT EXPAND SAME POS AS 1 ------------------------------------------------------------------------------------------------------------------------------------ 6635( 7) - 11/14/96 15:37 - [ 3] Visit: 02 Visit Requirements: 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 NGC2681-NUC FOS/RD ACQ/PEA 4.3 G270H SCAN-STEP-Y=1.23,SEARCH-SIZE-X=1, 1 12 S ONBOARD ACQ FOR 2 K SEARCH-SIZE-Y=3 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC2681-NUC FOS/RD ACQ/PEA 1.0-PAI G270H SCAN-STEP-X=.61,SCAN-STEP-Y=.61, 1 12 S ONBOARD ACQ FOR 3 K R-A SEARCH-SIZE-X=6,SEARCH-SIZE-Y=2 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC2681-NUC FOS/RD ACQ/PEA 0.25-PA G270H SCAN-STEP-X=.17,SCAN-STEP-Y=.17, 1 12 S ONBOARD ACQ FOR 4 K IR-A SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC2681-NUC FOS/RD ACQ/PEA 0.25-PA G270H SCAN-STEP-X=.052,SCAN-STEP-Y=.052 1 68 S ONBOARD ACQ FOR 5 K IR-A ,SEARCH-SIZE-X=5,SEARCH-SIZE-Y=5 Comments: Critical ACQ/PEAK stage ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC2681-NUC FOS/RD ACCUM 0.25-PA G270H STEP-PATT=SINGLE 1 2590 S END ORBIT EXPAND IR-A ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC2681-NUC FOS/RD ACCUM 0.25-PA G650L STEP-PATT=SINGLE 1 600 S SAME POS AS 5 IR-A ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC2681-NUC FOS/RD ACCUM 0.25-PA G780H STEP-PATT=SINGLE 1 1720 S END ORBIT EXPAND SAME POS IR-A AS 5 ------------------------------------------------------------------------------------------------------------------------------------ 6635( 7) - 11/14/96 15:37 - [ 4] Summary Form for Proposal 6635 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOC/96 FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-Y=1.23 SEARCH-SIZE-X=1 SEARCH-SIZE-Y=3 SEARCH-SIZE-Y=2 SEARCH-SIZE-Y=5 SEARCH-SIZE-X=5 STEP-PATT=SINGLE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 1-2 WITHIN 24H SAME POS AS 1 END ORBIT EXPAND SAME POS AS 1 ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3 ONBOARD ACQ FOR 4 ONBOARD ACQ FOR 5 END ORBIT EXPAND SAME POS AS 5 END ORBIT EXPAND SAME POS AS 5 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F342W,F2ND F275W,F1ND F175W G270H G650L G780H ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC2681-NUC ------------------------------------------------------------------------------------------------------------------------------------