18225( 2) - 05-Feb-2026 14:11:01 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18225 Version: 2 Check-in Time: 05-Feb-2026 14:11:01 STScI Edit Number: 0 Title Do true Seyfert 2 galaxies truly exist ? The case of NGC 3660 ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Giovanni Miniutti Centro de Astrobiologia - CAB Y CoI: Dr. Marco Chiaberge Space Telescope Science Institute - ESA - Y JWST CoI: Prof. Stefano Bianchi Universita di Roma Tre N CoI: Dr. Alessandro Capetti Osservatorio Astronomico di Torino N CoI: Dr. Frederic Marin Universite de Strasbourg I N CoI: Dr. Ismael Garcia Bernete Centro de Astrobiologia - CAB N ------------------------------------------------------------------------------------ Abstract True Seyfert 2 galaxies are active nuclei with no optical/UV/IR broad emission lines that are simultaneously unabsorbed in X-rays and variable, demonstrating a direct view of the inner nucleus. The detection of optical and UV broad lines in the prototypical Seyfert 2 galaxy NGC 3147 in narrow-slit HST spectra has however questioned the very existence of the population as a whole. We propose here a joint XMM-Newton and HST program on one of the best remaining true Seyfert 2 candidates - NGC 3660 - with the goals of (I) searching for a broad, so far undetected, Halpha emission line using narrow-slit HST spectroscopy, and (II) deriveing a high quality intrinsic nuclear SED using quasi-simultaneous observations. The program has the potential of settling once and for all the debate on the existence of true Seyfert 2 galaxies in general. ------------------------------------------------------------------------------------ Observations Description ------------------------ In order to achieve our goals, we request narrow-slit optical spectra around H|*| using STIS/CCD and the G750M grating and optical/UV imaging with STIS/NUV-MAMA. We will use the 52x0.1 arcsec slit to reduce host galaxy contamination by a factor of |*| 10 with respect to ground based observations and to increase the contrast of any broad line component against both the stellar continuum and the superimposed narrow lines, which are mostly produced in the extended (kpc-scale) narrow line region (0.1|*| |*| 25 pc at z = 0.012). We replicate the highly successful strategy that led to the discovery of a broad H|*| in NGC 3147 (right panel of Fig. 1) by our team. The pointing will be aimed at the host galaxy photocenter. A target acquisition image with the F28X50LP will be taken to allow recentering on the brightest region, where we expect the AGN to be located, with exposure of 60 s, appropriate for the expected surface brightness of |*| 18225( 2) - 05-Feb-2026 14:11:01 - [ 2] 15 mag arcsec|*|2. To avoid missing the AGN at the center of the galaxy nucleus, we will obtain 5 spectra, as done for NGC 3147 (GO15225, PI: Bianchi). The first will be centered at the location derived following the acquisition image. The following will be taken with a POS-TARG of (+0.1, 0.0), (+0.2, 0.0), (-0.1, 0.0), (-0.2, 0.0). We performed accurate S/N simulations using the STIS ETC. The stellar continuum within the 0.1|*| slit is expected to be F|*| |*| 2 |*| 10|*|16 erg cm|*|2 s|*|1 |*|A|*|1, using the information derived from ground based spectra. The resulting S/N is 11 per resolution element. The broad emission line is expected to have a flux F(H|*|) |*| 1 |*| 10|*|13 erg cm|*|2 s|*|1 and FWHM|*|3000-6000 km s|*|1 (Bianchi et al 2012). Assuming a FWHM of 5000 km s|*|1, we expect a S/N of 37 at the peak. We use the 52x01E1 aperture to mitigate CTE losses and two CR-SPLIT images at each slit position to eliminate cosmic rays. As for the SED measurements, we will utilize the STIS/NUV-MAMA with the F25SFR2 filter, centered at 2320 |*|A to obtain imaging in the UV. Two dithered exposures will be obtained with a total exposure time of 300 sec, reaching a point source S/N of 17 for a flux of F|*| |*| 1 |*| 10|*|17 erg cm|*|2 s|*|1 |*|A|*|1. The acquisition image will instead provide a measurement of the AGN continuum at |*| 7000|*|A. Using the HST APT, our program can be completed within 4 HST orbits ------------------------------------------------------------------------------------ 18225( 2) - 05-Feb-2026 14:11:01 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-3660 GALAXY, NUCLEUS, RA=11H23M32.2781S, J2000 V = 14.45 SEYFERT DEC=-08D39'30.73" Reference Frame: ICRS Extended: YES Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 1.726341622286756E-5 -9.899997621687362E-5 3.23E-4 ------------------------------------------------------------------------------------------------------------------------------------ 18225( 2) - 05-Feb-2026 14:11:01 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-SPIRAL-DITH Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.567 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 26.6 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18225( 2) - 05-Feb-2026 14:11:01 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC-3660 STIS/NUV ACCUM F25SRF2 MIRROR 1 300 S (600 PATTERN 1 1 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC-3660 STIS/CCD ACQ F28X50LP MIRROR 1 60 S (60 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC-3660 STIS/CCD ACCUM 52X0.1E1 G750M 6581 1 1550 S (1550 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC-3660 STIS/CCD ACCUM 52X0.1E1 G750M 6581 1 1550 S POS TARG 0.1, +0.0 (1550 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC-3660 STIS/CCD ACCUM 52X0.1E1 G750M 6581 1 1550 S POS TARG 0.2, +0.0 (1550 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC-3660 STIS/CCD ACCUM 52X0.1E1 G750M 6581 1 1550 S POS TARG -0.1, -0.0 (1550 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC-3660 STIS/CCD ACCUM 52X0.1E1 G750M 6581 1 1550 S POS TARG -0.2, -0.0 (1550 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ NGC-3660 1 STIS/NUV ACCUM F25SRF2 MIRROR 1 none none none 1 N/A -MAMA NGC-3660 1 STIS/NUV ACCUM F25SRF2 MIRROR 2 none none none 1 N/A -MAMA NGC-3660 2 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A NGC-3660 3 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 1 1 N/A NGC-3660 3 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 2 2 N/A NGC-3660 4 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 1 2 N/A NGC-3660 4 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 2 2 N/A NGC-3660 5 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 1 3 N/A NGC-3660 5 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 2 3 N/A NGC-3660 6 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 1 3 N/A NGC-3660 6 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 2 4 N/A NGC-3660 7 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 1 4 N/A NGC-3660 7 STIS/CCD ACCUM 52X0.1E1 G750M 6581 none none none 2 4 N/A 18225( 2) - 05-Feb-2026 14:11:01 - [ 6] Summary Form for Proposal 18225 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.1E1, F25SRF2, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, POS TARG -0.1, -0.0, POS TARG -0.2, -0.0, POS TARG 0.1, +0.0, POS TARG 0.2, +0.0 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G750M, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC-3660 ------------------------------------------------------------------------------------------------------------------------------------