8689( 11) - 12/07/00 11:00 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8689 Version: 11 Check-in Date: 07-Dec-2000 10:59:44 1.Proposal Title: UV extinction in Dusty Ellipticals ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: James Rhoads Space Telescope Science Institute CoI: Dr. Sangeeta Malhotra Johns Hopkins University N CoI: Dr. Greg Kochanski Bell Laboratories, Lucent Technologies N CoI: Dr. Paul Goudfrooij Space Telescope Science Institute N CoI: Prof. Puragra Guhathakurta University of California at Santa Cruz N ------------------------------------------------------------------------------------ 5. Abstract We propose UV spectroscopy of bright inner parts of two dusty elliptical galaxies to study their ultraviolet extinction, in particular the 2175 Angstrom bump. These galaxies are selected from a sample of dusty ellipticals having very different, and in some cases highly chromatic extinction law in the visible (Goudfrooij et al. 1994). A chromatic extinction law requires either a deficiency of carbonaceous grains or a smaller typical grain size relative to Milky Way dust. The strength of the 2175 Angstrom\ feature, which is due to carbon grains, is the best way to distinguish between these two possibilities. A smaller typical grain size can come about due to grain erosion by x-ray gas, or due to lack of grain growth in dense molecular clouds. A deficiency of carbonaceous grains could come about due to chemical sputtering by hydrogen or due to reduced production of carbonaceous grains by aging stellar populations. These observations will distinguish between different physical explanations for anomalous optical extinction properties, and will thereby constrain the evolutionary history and physical conditions in the interstellar media of elliptical galaxies. ------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 2] Observations Description ------------------------ In order to accurately measure the 2175 Angstrom feature, we need (1) sufficient spectral coverage to include a good baseline on each side of the feature, which is ~ 300-500 Angstrom wide, (2) sufficient spatial coverage and resolution to compare spectra with and without substantial dust extinction, and (3) good signal to noise. These can be achieved using the G230L grating with the NUV MAMA detector and a wide (52x2) slit. In addition, we need NUV MAMA images with the quartz longpass filter to constrain the grey part of the reddening law by fitting a model to the surface brightness distribution in the galaxy. Planned total exposure times are 21000 seconds for the spectra and 3600 seconds for the images. We plan to coadd spectra from the entire sampled dust lane region (approximately 5'' along the slit) to obtain a high signal to noise spectrum with substantial dust extinction. To extract the reddening law, we will generate a similar coadded spectrum for nearby ``background'' regions with little or no dust (using information from ~ 15 -- 20'' along the slit). The wavelength dependent part of the extinction law can then be derived from the ratio of the two coadded spectra. The wavelength independent (grey) part of the extinction can also be extracted if we know the intrinsic surface brightness ratio between the dust lane and unobscured parts of the galaxy at some wavelength. This ratio can be derived from a model, fit to our image of the galaxy. Using the total magnitudes B_T=11.65 and V_T = 10.64 for NGC 1947 from the RC3, the effective radius r_e = 38'' from Goudfrooij et al (1994), and a De Vaucouleurs law for the spatial profile, we find surface brightnesses Mu_e(B) = 22.94 and Mu_e(V) = 21.93 at the effective radius. Burstein et al (1988) plot IUE spectra for a large sample of early type galaxies, and show that the 2200Angstrom flux is typically at least f_Lambda(2200Angstrom) >~ 10^-1.6 f_Lambda(V). Integrating over the central 5'' * 2'' region of the slit, we find <~ngle Sigma(2200Angstrom) rangle_ inner >~ 1.6* 10^-17 \erg/\cm^2/\sec/Angstrom/(")^2, while the mean surface brightness in the remainder of the slit is <~ngle Sigma(2200Angstrom) rangle_ outer >~ 2.9 * 10^- 18. The spectral slope for 2200 Angstrom <~ Lambda <~ 3000 Angstrom is reasonably approximated by f_Lambda \propto Lambda^3, while for Lambda < 2200 Angstrom the corresponding power law index varies between 3 and -3 for different early-type galaxies. Inserting these numbers in the STIS spectroscopic time estimator, we find that the final spectrum for the dust- free regions should have s/n ~ 25 at 2200 Angstrom, s/n ~ 15 in the blue baseline region at ~ 1900 Angstrom, and s/n > 50 in the red baseline region at ~ 2600. (These results are for the total spectrum, after coadding data from the entire dust- free region of the slit.) The signal to noise ratio for the dust lane will be comparable or greater, because the dust lane lies close to the center of the galaxy where the surface brightness rises sharply. We have discussed dark current subtraction with a STIS instrument scientist at STScI (S. Baum), and verified that by subtracting a deep dark frame (available from STScI) scaled to dark counts measured at the blue edge of the chip (Lambda < 1650 Angstrom, where object counts will be negligible), any residual dark subtraction errors will be below the photon noise level in our final spectra. The central optical depth of the dust lane is ~ 1 magnitude in V band, which is also the difference between extinction curves with and without a 2175Angstrom feature. Thus, our signal to noise should be sufficient to distinguish between the different reddening laws shown in figure 1 with high significance. In our final data analysis, we expect to replace the crude spatial binning into dust lane and dust free regions outlined above with a more sophisticated model of the spatial dust distribution based on STIS and ground based images. Near UV imaging observations with 3600 seconds total integration time will achieve a characteristic signal to noise ratio of ~ 30 on scales of 1", which is small compared to the width of the dust lane (~ 5"). This calculation is based on the mean surface brightness in the annulus from 2.5" to 7.5"; the central signal to noise will be much higher, while that at the outer edge of the image will be somewhat lower. The images will allow a measurement of the spatial dust optical depth distribution averaged over the bandpass by measuring deviations from smooth model fits, as Goudfrooij et al did in the optical. This is required to extract the total extinction. Combining this with measurements of the selective extinction from the spectra yield the full extinction law as a function of wavelength. The spectra can also be used to help obtain a pure NUV image of the galaxy: The total light in the slit can be summed over wavelength and compared to the image to infer and correct for the fraction of total counts in the image coming from the optical tail of the NUV MAMA sensitivity. The observational parameters and analysis for NGC 1947 and NGC 4589 are identical, because the galaxies have very similar magnitudes, effective radii, dust lane geometries, and central optical depths. Real Time Justification ----------------------- We request CVZ observations for both galaxies to maximize signal to noise ratio using modest amounts of HST time. To attain the same depth without CVZ observations would require about twice as many orbits. Availability of CVZ orbits was included in our sample selection criteria, along with high surface brightness. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1947- CALIBRATION, RA=05H 26M 45.56S +/- 0.015S, J2000.0 V=+0 I = 16.15 OFFSET Astrometric DEC=-63D 45' 28.93" +/- 0.10", V = 16.15 PLATE-ID=02I7 Coordinate Source: IMAGE_TIED_TO_GSC_FRAME Comments: Offset star for NGC 1947. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0 0 0 2 NGC1947 GALAXY, Elliptical RA-OFF=2.02S+/-0.02S, DEC-OFF=- J2000.0 V=+1100 V = 10.64 7.27"+/-0.10", FROM 1, PLATE- B-V = 1.01 ID=02I7 Coordinate Source: IMAGE_TIED_TO_GSC_FRAME Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0 0 0 11 NGC4125- CALIBRATION, RA=12H 08M 12.6136S +/- 0.002S, J2000.0 V=+0 V = 17.1 OFFSET Astrometric DEC=65D 10' 16.995" +/- 0.01", V-I = 1.35 PLATE-ID=00MD Coordinate Source: HST_IMAGE Comments: Offset star for NGC 4125. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0 0 0 12 NGC4125 GALAXY, Elliptical RA-OFF=-6.811S +/- 0.002S, DEC- J2000.0 V=+0 V = 10.6 OFF=10.807" +/- 0.01", FROM 11, B-V = 1.0 PLATE-ID=00MD Coordinate Source: HST_IMAGE Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950 0 0 0 8689( 11) - 12/07/00 11:00 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-MAMA-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.23375 Coordinate_Frame POS-TARG Pattern_Orient 26.565 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.104536 Coordinate_Frame POS-TARG Pattern_Orient 90 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ ; ORIENT 18D TO 69D ; ORIENT 198D TO 249D On Hold Comments: Additional Comments: To minimize dark current and so get the best s/n ratio on this UV-faint target, we would prefer scheduling immediately after the end of an SAA passage, when the NUV-MAMA dark current is at its minimum level. This requirement is however less important than CVZ. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1947-OFF STIS/C ACQ F28X50L MIRROR 1 15.0 S SET CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC1947 STIS/N ACCUM F25QTZ MIRROR 3 302 S PATTERN 1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC1947 STIS/N ACCUM 52X2 G230L 2376 WAVECAL=NO 2 1400 S PATTERN 2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 5 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 6] Visit: 02 Visit Priority: Visit Requirements: CVZ ; ORIENT 200D TO 205D ; ORIENT 20D TO 25D On Hold Comments: Additional Comments: To minimize dark current and so get the best s/n ratio on this UV-faint target, we would prefer scheduling immediately after the end of an SAA passage, when the NUV-MAMA dark current is at its minimum level. This requirement is however less important than CVZ. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4125-OFF STIS/C ACQ F28X50L MIRROR 1 15.0 S SET CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4125 STIS/N ACCUM F25QTZ MIRROR 3 301 S PATTERN 1 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC4125 STIS/N ACCUM 52X2 G230L 2376 WAVECAL=NO 2 1386 S PATTERN 2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 5 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC4125 STIS/N ACCUM 52X2 G230L 2376 2 1386 S PATTERN 2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 7 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 7] Visit: 1A Visit Priority: Visit Requirements: CVZ ; SAME ORIENT AS 1 GROUP 01, 1A WITHIN 4D On Hold Comments: Additional Comments: To minimize dark current and so get the best s/n ratio on this UV-faint target, we would prefer scheduling immediately after the end of an SAA passage, when the NUV-MAMA dark current is at its minimum level. This requirement is however less important than CVZ. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1947-OFF STIS/C ACQ F28X50L MIRROR 1 15.0 S SET CD P ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC1947 STIS/N ACCUM 52X2 G230L 2376 WAVECAL=NO 2 1216 S PATTERN 2 UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 7 WAVE STIS/N ACCUM 52X0.1 G230L 2376 1 DEF UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8689( 11) - 12/07/00 11:00 - [ 8] Summary Form for Proposal 8689 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP F25QTZ 52X0.1 52X2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ; ORIENT 18D TO 69D ; ORIENT 198D TO 249D PATTERN 1 PATTERN 2 CVZ ; ORIENT 200D TO 205D ; ORIENT 20D TO 25D CVZ ; SAME ORIENT AS 1 GROUP 01, 1A WITHIN 4D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1947-OFFSET NGC1947 WAVE NGC4125-OFFSET NGC4125 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 ------------------------------------------------------------------------------------------------------------------------------------