9477( 3) - 03/26/03 11:45 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9477 Version: 3 Check-in Date: 26-Mar-2003 11:43:59 1.Proposal Title: TRGB Distance to the Maser Galaxy NGC4258 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Barry Madore Carnegie Institition of Washington CoI: Dr. Wendy L. Freedman Carnegie Institution of Washington N CoI: Ms. Olga Pevunova Infrared Processing and Analysis Center N ------------------------------------------------------------------------------------ 5. Abstract Masers surrounding the nucleus of the galaxy NGC 4258 have been used to derive a geometric distance to this galaxy from proper motions and accelerations of individual sources assumed to be in Keplerian orbits. This provides us with a unique opportunity to test the zero point of the Cepheid Period- Luminosity relation in specific, and the extragalactic distance scale in general. A Cepheid distance has also been determined to NGC 4258 by us following the exact methodology adopted by the HST Key Project on the Extragalactic Distance Scale. The two answers differ, with the maser distance being smaller by 12\ We are proposing to use a third and totally independent means to assess the sense and significance of the difference that may need to be applied to the extragalactic distance scale. By using the tip of the red giant branch (TRGB) method we will obtain a Population II distance measurement to the maser galaxy NGC 4258 good to a precision of better than 10\ Resolving this issue is important because if the maser distance is adopted and used to recalibrate the HST Cepheid Key Project distances, then the Hubble constant increases from H_o = 70 to 80 km/sec/Mpc. Both distance scales must be scrutinized and tested very closely before any recalibration takes place. ------------------------------------------------------------------------------------ 9477( 3) - 03/26/03 11:45 - [ 2] Observations Description ------------------------ NGC 4258 is an extremely large galaxy, both intrinsically and as projected on the sky. Indeed its angular size is almost as large as M81, yet at its presently estimated distance of 7-8 Mpc it is 2-3 times further away than M81. Its angular size poses both a challenge to this program and an opportunity. Because NGC 4258 is so large on the sky (19 by 7 arcmin) it is simply impractical to overlap a minor-axis TRBG field with the existing major-axis Cepheid field; so no direct photometric tie-in between the fields will be possible. The advantage of its angular size is that there is the opportunity for large areal coverage of the pure halo so as to image TRGB stars in abundance without having to press too close to the contaminating disk or into the potentially crowded inner halo. \medskip TRGB- We will image the outer halo of NGC 4258 in both V and I bands, in order to detect and then unambiguously measure the luminosities of the brightest low- metallicity, red-giant branch population of this galaxy's old Population II. It is expected that these stars will be resolved at I < 25.5 mag. Recent direct experience in using HST to resolve the same population of stars in the M81 Group dwarf galaxy BK5N (at approximately half the distance of NGC 4258, 3.6 Mpc or 1.5 mag closer) reveals hundreds of uncrowded red-giant branch stars, probing two full magnitudes down into the RGB luminosity function. That study by Caldwell, Armandroff, DaCosta & Seitzer (1998, AJ, 115, 535) used 22 orbits: fifteen (15) orbits in the I-band and seven (7) orbits in the V-band, defining the TRGB with random and systematic errors quoted to be less than 5\ With ACS we can cut those excessive exposure times down to only three orbits for NGC 4258. For the TRGB analysis the complementary V-band data are essential for obtaining a color-magnitude diagram, in addition to the I-band luminosity function. This allows one to confirm the metallicity range of the TRGB stars being measured (by the color and shape of the red giant branch) and to identify, quantify and eliminate residual asymptotic giant branch stars at higher luminosities (and distinctly redder colors) than the TRGB stars needed for a distance determination. Any diluting effects of extremely high-metallicity RGB stars are also eliminated by a color cut because of their significantly redder (line-blanketed) V-I colors (and their fortuitously lower intrinsic luminosities). All frames will be CR split. Regrettably, the VI data already obtained for NGC 4258 by the Key Project team for the search and discovery of Cepheids are not at all suitable for this test. As can be seen in Figure 1 the I vs (V-I) CMD for the Cepheid fields in NGC 4258 are dominated by Pop I supergiants to the blue, and by red Pop I supergiants and intermediate-age (AGB) stars to the red. Even more damaging is the I-band cut-off in the red (where the TRGB would be) imposed not by exposure-time or signal-to-noise considerations but by severe disk crowding and confusion at those high-surface- brightness levels in the main disk of the galaxy. An approved archival proposal by Sakai & Gibson (Archival Proposal 8747) to extract a TRGB distance out of these frames has not been successful. The TRGB in NGC 4258 is expected to show up at I ~ 25.5 mag. In order to estimate the amount of integration time needed in the V and I filters for this calibration experiment we have used the ACS Exposure Time Calculator. A single- orbit CR split observation of 2900 sec gives S/N = 20 at I ~ 25.5 mag. For a comparable S/N in V the required integration time is 5, 500 sec (or somewhat less than two orbits). In other words the TRGB and a significant proportion of the RGB below the tip can be detected, unambigously identified, and measured to +/- 0.05 mag in only three orbits, as requested. This enormous increase in 9477( 3) - 03/26/03 11:45 - [ 3] efficiency should be compared with previously approved projects using HST and the TRGB method on substantially closer galaxies, having expended between 12 and 22 orbits each: \medskip o One such study is that by Caldwell et al. (1998), as cited above. They used approximately 22 HST orbits to observe a dwarf galaxy BK5N in the M81 Group (2-3 times closer than NGC 4258). M81 Group. They accumulated 36, 000 sec (15 orbits) of I-band integration and 18, 000 sec (7 orbits) of V-band data and measured a TRGB distance of 3 Mpc. They penetrated (at excellent signal-to-noise) a full two magnitudes below the tip of the red giant branch, truncating their published photometry at I ~ 26 mag ( i.e., 0.5 mag fainter than the tip magnitude expected for NGC 4258). o Elson (MNRAS, 286, 771, 1997) used 22, 000 sec (10 orbits) in I and 17, 000 sec (6 orbits) in V to obtain a VI CMD in the halo of the S0 galaxy NGC 3115 at 11 Mpc, detecting the tip at I ~ 26 mag. o For the galaxy NGC 3379 in the Leo Group, at 12 Mpc, Sakai et al. ApJ, 478, 4957, 1997) detected the TRGB in at I ~ 26.3 mag in a combined I-band exposure totaling 29, 000 sec (12 orbits). No V-band data were obtained and severe incompleteness occurred only 0.5 mag below the tip. o Finally, Harris et al., Nature, 395, 46, 1998) used 32, 000 sec (12 orbits) to obtain a TRGB distance of 16 Mpc to a Virgo dwarf galaxy, having detected the tip at I ~ 26.7 mag. No V-band exposures were obtained. Completeness tests gave a 50\ I ~ 27.5 mag \medskip It needs to be emphasized that many of the errors in the distances derived from the Cepheids and the TRGB are largely uncorrelated: (a) errors in the photometric calibration are reduced for the TRGB because they are not propagated through the dereddening procedure necessarily applied to the Cepheids, (b) there is no metallicity dependence on the brightest measured TRGB magnitude, and (c) crowding is significantly less of an issue in the low-surface-brightness halo fields (for the very brightest population of TRGB stars) when compared to to the dusty, complex and mottled disk field containing the Cepheids. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9477( 3) - 03/26/03 11:45 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC4258-TRGB GALAXY RA=12H 19M 19.0S +/- 0S, DEC=47D J2000 V = 26.5 20' .0" +/- 0" I = 25.5 Coordinate Source: Other_Source: NED 9477( 3) - 03/26/03 11:45 - [ 5] Visit: 01 Visit Priority: Visit Requirements: BEFORE 09-May-2003 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 NGC4258-TRG ACS/WF ACCUM WFC F814W CR-SPLIT=2 1 2600 S B C ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC4258-TRG ACS/WF ACCUM WFC F555W CR-SPLIT=2 1 5700 S B C ------------------------------------------------------------------------------------------------------------------------------------ 9477( 3) - 03/26/03 11:45 - [ 6] Summary Form for Proposal 9477 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BEFORE 09-May-2003 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC4258-TRGB ------------------------------------------------------------------------------------------------------------------------------------