! Proposal 6430, submission 1 ! PI: Roeland P. van der Marel ! Received Wed Feb 14 12:15:34 EST 1996 ! From: marel@sns.ias.edu Proposal_Information Title: Nuclear structure & merger-starburst relation in the ultraluminous IRAS galaxy NGC 6240 Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_Name: Roeland P. van der Marel PI_Institution: Institute for Advanced Study, Princeton CoI_Name: Chris Mihos CoI_Institution: Johns Hopkins University, Baltimore Contact: N CoI_Name: Josh Barnes CoI_Institution: Institute for Astronomy, University of Hawaii Contact: N CoI_Name: Lars Hernquist CoI_Institution: Lick Observatory, Santa Cruz Contact: N Abstract: The ultraluminous IRAS galaxy NGC 6240 has been studied extensively from the ground at all accessible wavelengths. Loops, tidal tails and a double nucleus (separation approximately 1.5 arcsec = 0.7 kpc) suggest an ongoing merger. The far-IR luminosity is generally attributed to dust-heating by a nuclear starburst. This is supported by most, but not all data. In particular, the nuclear stellar velocity dispersion (approximately 350 kms) indicates too high an M/L for a starburst and the UV field strength appears to be too low. Alternative models for the dust heating, e.g. invoking an AGN, therefore remain viable. Of equal interest has been a claim for the presence of a 10^11 solar mass black hole (BH) at 12 arcsec from the double nucleus, from a decomposition of the ionized gas velocity field into that of two disks, presumed to be the merging progenitors. We will obtain broad- and narrow-band WFPC2 images of NGC 6240 to study the nuclear distributions of stars, dust and ionized gas. We will search for an AGN component. We will construct dissipational (tree+SPH) N-body models, to interpret the data, test the starburst hypothesis, and study whether equilibrium kinematics can be used to infer M/L values and BH masses for a merging galaxy like NGC 6240. Questions Observing_Description: WFPC2 exposures of NGC 6240 will be taken with five different filters. The entire program requires a total of 4 HST orbits, to be executed in one visit. The total exposure time for each filter is split between two or three separate exposures. This allows an accurate cosmic ray subtraction, and identification of CCD defects. It also allows for efficient packing of the exposures into the available orbits. Most of the exposures will be taken with the target positioned on the PC CCD (yielding 0.046'' * 0.046'' pixels) to maximize the spatial resolution. We will take broad-band images of NGC 6240 with the F450W (B-band), F547M (V-band) and F814W (I-band) filters. For F450W and F814W three exposures are taken. For F547M two exposures are taken. In addition to the broad-band exposures with the galaxy on the PC CCD, we will take one CR-SPLIT exposure with the F814W filter and the galaxy on the WF3 CCD. This will be used to study the fainter surface brightness regions in the outer parts of the galaxy. To reduce the effective read-noise, this CR-SPLIT exposure will be obtained with the `SUM=2x2' on-chip-binning special requirement. To construct an H_alpha+[NII] image we obtain three narrow-band exposures with the F673N filter and the target on the PC CCD. This is not the conventional use of this filter. It was designed to observe [SII] emission lines at zero redshift. However, at the redshift of NGC 6240 (z=0.02427, corresponding to approximately 161 Angstrom at the wavelength of these lines), the H_alpha+[NII] lines are shifted exactly into the F673N filter. We will also obtain two shorter exposures with the narrow F658N filter and the galaxy on the PC CCD. This filter is emission-free at the redshift of NGC 6240. It will be used in combination with the F814W and F547M images to construct an off-band image for the on-minus-off-band subtraction. All the PC exposures will be obtained with the target at one of two different positions on the CCD, which are offset by a `POS TARG 0.5010,0.5010' special requirement. This corresponds to a shift of (11,11) PC pixels, and (5,5) WFC pixels. This strategy allows accurate identification of CCD defects and hot pixels. In cases where three exposures are taken for the same filter, we do not use three different positions, to save overhead time. We do not use sub-pixel dithering. The central surface brightness of the (brighter) nucleus of NGC 6240 in B, V and I is listed in the target definition, together with the total H_alpha+[NII] emission line flux in the central arcsec^2. The expected continuum and emission- line count rates per pixel calculated using the STSDAS/SYNPNOT package are listed in the comments to each exposure. These are averages over the central arcsec^2. Based on our experience with HST images of other galaxies (project GO-5848) we are confident that the resulting S/N allows a detailed study of the (stellar) light distribution, dust absorption, and ionized gas morphology near the nucleus. All exposures are taken with the low gain setting. The count rates are low enough that there is no danger of saturating the galaxy nucleus. There is a star with visual magnitude 13.16 at 36 arcsec from the target. It is important that this star does not fall in the unimaged region adjacent to the PC CCD, since that can lead to stray light being imaged onto the PC (see the recent WFPC2 ISR 95/06). Hence we require an ORIENT special requirement for the observations. The orientation requested below also ensures that the tidal-arm to the north-east of the galaxy nucleus is imaged on one of the WF CCDs, while the PC CCD images the galaxy nucleus. The most preferred ORIENT is 215D TO 235D. However, we have entered a larger range below, to allow for more flexible scheduling. For the same reason we have added a special requirement `SCHED 50%', which allows the observations to be scheduled also in orbits with relatively low target visibility time. Real_Time_Justification: No real time requirements. Calibration_Justification: No special calibration requirements. Additional_Comments: The target coordinates listed below were obtained by combining the results of two measurements: (i) a measurement from pre-COSTAR FOC images of this galaxy in the HST Archive; (ii) a measurements from DSS plate 00JO. These measurements agree to within approximately 1.5 arcsec. We are confident that this is sufficient for the purposes of our program (which only involves imaging of an extended source). There were problems with the DSS jukebox for this field while this Phase II proposal was being prepared. Hence, we might not have used the correct DSS plate for our measurements. This should be verified after the DSS jukebox problems are fixed. Fixed_Targets Target_Number: 10 Target_Name: NGC6240 Alternate_Names: IRAS16504+0228, UGC10592 Description: GALAXY, ULTRALUMINOUS IR GAL, INTERACTING GALAXY, STARBURST, MULTIPLE NUCLEI Position: RA=16H 52M 58.91S +/- 1.0", DEC=02D 24' 04.0" +/- 1.0", PLATE-ID=00JO Equinox: J2000.0 RV_or_Z: V = 7339 RA_PM: 0.0 Dec_PM: 0.0 Epoch: Annual_Parallax: 0.0 Flux: SURF(I) = 15.8 +/- 0.5 SURF(R) = 17.5 +/- 0.5 SURF(V) = 18.9 +/- 0.5 SURF(B) = 20.5 +/- 0.5 F-LINE(6565) = 2 +/- 1 e-14 Comments: Listed Surface brightness values are averages over the central arcsec^2, determined from ground-based observations in the literature. Listed line flux is total H_alpha+[NII] flux in the central arcsec^2. Visits Visit_Number: 01 Visit_Requirements: ORIENT 143D TO 295D SCHED 50% On_Hold_Comments: Visit_Comments: The most preferred ORIENTation is 215D TO 235D, but the whole range 170D TO 280D is allowed. See the `Observational description'. Exposure_Number: 110 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SEQ 110-140 NON-INT Comments: Expected average count rate in central arcsec^2 of galaxy: 3.0 electrons/sec/pix. Sky: 0.016 electrons/sec/pix. [pix = 1 PC pixel] Exposure_Number: 120 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SAME POS AS 110 Comments: Expected count rates as for exp.110. Exposure_Number: 130 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: POS TARG 0.5010,0.5010 Comments: Expected count rates as for exp.110. The `POS TARG' shifts the target by (11,11) PC pixels = (5,5) WFC pixels w.r.t. its position in exp.110. Exposure_Number: 140 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SAME POS AS 130 EXPAND MIN DUR 80% MAX DUR 300% END ORBIT Comments: Last exposure in orbit #1. Expected average count rate in central arcsec^2 of galaxy: 0.17 electrons/sec/pix. Sky: 0.005 electrons/sec/pix. Exposure_Number: 210 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F450W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 700S Special_Requirements: SEQ 210-230 NON-INT SAME POS AS 110 Comments: Expected average count rate in central arcsec^2 of galaxy: 0.050 electrons/sec/pix. Sky: 0.005 electrons/sec/pix. Exposure_Number: 220 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F450W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 700S Special_Requirements: SAME POS AS 110 Comments: Expected count rates as for exp.210. Exposure_Number: 230 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F450W Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 800S Special_Requirements: SAME POS AS 130 END ORBIT EXPAND MIN DUR 80% MAX DUR 300% Comments: Last exposure in orbit #2. Expected count rates as for exp.210. Exposure_Number: 310 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F673N Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 700S Special_Requirements: SEQ 310-330 NON-INT SAME POS AS 110 Comments: Expected average emission line count rate in central arcsec^2 of galaxy: 0.063 electrons/sec/pix. Continuum: 0.044 electrons/sec/pix. Sky: 0.001 electrons/sec/pix. Exposure_Number: 320 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F673N Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 700S Special_Requirements: SAME POS AS 110 Comments: Expected count rates as for exp.310. Exposure_Number: 330 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F673N Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 800S Special_Requirements: SAME POS AS 130 END ORBIT EXPAND MIN DUR 80% MAX DUR 300% Comments: Last exposure in orbit #3. Expected count rates as for exp.310. Exposure_Number: 410 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F658N Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SAME POS AS 130 Comments: Expected continuum count rate in central arcsec^2 of galaxy: 0.029 electrons/sec/pix. Sky: 0.001 electrons/sec/pix. Exposure_Number: 420 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F658N Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SAME POS AS 110 Comments: Expected count rates as for exp.410. Exposure_Number: 430 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: PC1-FIX Sp_Element: F547M Wavelength: Optional_Parameters: CR-SPLIT=NO ATD-GAIN=7 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: SAME POS AS 110 EXPAND MIN DUR 50% MAX DUR 300% Comments: Expected count rates as for exp.140. Exposure_Number: 440 Target_Name: NGC6240 Config: WFPC2 Opmode: IMAGE Aperture: WF3-FIX Sp_Element: F814W Wavelength: Optional_Parameters: CR-SPLIT=0.5 CR-TOLERANCE=0.0 ATD-GAIN=7 SUM=2x2 Number_of_Iterations: 1 Time_Per_Exposure: 400S Special_Requirements: END ORBIT Comments: Last exposure in orbit #4. Expected count rates (per arcsec^2) as for exp.110. Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS Recipient_Email: