9414( 2) - 01/28/02 06:54 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9414 Version: 2 Check-in Date: 28-Jan-2002 06:53:36 1.Proposal Title: Resolved halo stellar populations in the Milky Way analogue edge-on galaxy NGC 891 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Richard De Grijs University of Cambridge CoI: Dr. Reynier Peletier University of Nottingham N CoI: Prof. Gerard Gilmore University of Cambridge N CoI: Mr. Colin Frayn University of Cambridge N CoI: Dr. Chisato Ikuta University of Nottingham N ------------------------------------------------------------------------------------ 5. Abstract The stellar halo is one of the fundamental building blocks of galaxies. Although ground-based surface photometry has shown that stellar halos exist in some spiral galaxies, with colours similar to those of the main disk, little else is known about their stellar populations. We propose to carry out a systematic study of the halo regions of the edge-on galaxy NGC 891. The resulting deep colour-magnitude diagrams will be used to infer the mean metallicity of the bright halo giants at the tip of the RGB and below, and its metallicity spread. Recently, Zepf et al. (2000) inferred that the halo metallicity of NGC 5907 has to be low (Fe/H <= -1.7), with a large stellar M/L ratio. We will be able to test this hypothesis with much better and deeper data. If we can confirm a similar halo composition in NGC 891, this would imply that the Milky Way halo might be very different from external halos. Since galactic halos trace the history of galaxy formation, this will have important implications for our understanding of galactic evolution on cosmological time scales. Using these survey data, we will be able to conclusively distinguish between genuine halo objects and merger remnants, without having to assume a scenario in which the halo field population is fully mixed. Surprisingly, such studies have not yet been undertaken for NGC 891, although the galaxy is a prime Milky Way analogue. ------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 2] Observations Description ------------------------ 1. Field choice: We will use ACS/WFC to pursue an imaging survey of the halo regions of NGC 891. The field centres will be positioned at (r=0, z=120''), (r=190'', z=120'') and (r=380'', z=120''), so that the individual pointings will overlap slightly. By selecting these fields, we will cover all vertical distances 20'' <= z <= 220''. For a vertical scale height h_z = 11'' (van der Kruit & Searle 1981; KS) and a radial scale length h_R = 106'' (KS) this corresponds to vertical distances between 2 and 20 h_z. The choice of these fields will ensure minimal contamination by dust and stars belonging to the dominant galactic disk component: dust contamination, as traced by the most recent submm SCUBA maps (Alton et al. 1998, 1999) becomes negligible at z ~ 4 h_z, even along the minor axis. At z >= 7 h_z = 77'', the halo of NGC 891 is more than 2 mag brighter than the disk, using surface photometry of KS (the bulge component is very small in this galaxy). At 77'' the surface brightness is F=23.7 mag arcsec^-2 (``masq'') (KS), or V=24.1 mag/square arcsec (using KS' conversion, assuming B-V=0.8) on the minor axis. In field 2, at (r, z) = (2h_r, 7h_z), V=24.7 mag/square arcsec and in field 3, at (r, z) = (4h_r, 7h_z), V=26.0 mag/square arcsec (KS, their Fig. 5). As a comparison, at 18.3' from the centre of NGC 5128 (Harris et al. 1999), extrapolated surface photometry of van den Bergh (1976) gives V=24.9 mag/square arcsec. To estimate the number of RGB stars that we will find, we scale the data of Harris et al. (1999) by the distance ratio, resulting in a halo stellar density in NGC 891 at (0, 7h_z) of 12.4x that in the field of Harris et al. (1999) and 7.1x in field 2. In field 3, the density is still a factor 2.2 higher. In their NGC 5128 field Harris et al. (1999) found 75 TRGB stars and 330 stars of ~ 1 mag fainter. We therefore expect to find thousands of stars, so that it will be possible to derive the metallicity and metallicity width of the RGB. We will be able to use most of the ACS/WFC field, which has 2x the collecting area of WFPC2, for our halo study. In addition, our large radial coverage of the halo regions (Fig. 1) ensures that a robust analysis can be carried out to avoid contamination by possible low-luminosity merger remnants in the halo of NGC 891. We will be able to determine radial metallicity gradients of the halo population, and also investigate its shape, whether it is spherical, as in our Galaxy, or more similar to an exponential thick disk. ACS/WFC, with its angular resolution of 0.05'', is the ideal instrument to obtain deep photometric images with the significantly subarcsecond resolution needed to resolve and distinguish between the individual red giants and halo globular clusters. At the distance of NGC 891 (~ 9.8 Mpc Ferrarese et al. 2000a, b), its resolution is ~ 4.6 pc, more than enough to resolve globular and young star clusters (e.g., de Grijs et al. 2001 and references therein). For our NICMOS parallel programme, we opt to use NIC3 because of its large field of view; in addition, the focus problems in past cycles have now been reduced to a minimum, so that we expect to obtain good quality, sensitive observations of reasonably large fields, i.e., NIC3 is the optimal choice for our proposed programme. 2. Filters: The standard ACS/WFC broad-band V (F606W, broad V) and I (F814W) filters are best suited for the study of stellar populations. We will use the F606W filter instead of F555W because of its better sensitivity. 3. Exposure times and programme feasibility: Exposure times in this proposal are based on (i) the observations by Harris et al. (1999) of TRGB stars in NGC 5128; (ii) the results of Sakai & Madore (1999) for the halo of M82, corrected for the distance differential (Sakai & Madore [1999] easily resolve the halo red giant population in M82 with a total of 1000s per position per filter in F555W and F814W, using WFPC2); and (iii) the ACS exposure time calculator (ETC), assuming a S/N = 5 and gain=1 per 3x3 pixel aperture for stars ~ 1 mag fainter than the TRGB, for metallicities ranging from Fe/H = -1.7 to-0.5. For NGC 5128, Harris et al. (1999) found I(TRGB) ~ 24.1 (their Fig. 7) and (V-I)_TRGB ~ 2 (their Fig. 6), so that V(TRGB) ~ 26.1 for this galaxy (metallicity Fe/H = -0.4). Taking into account the distance differential between NGC 5128 and NGC 891, V(TRGB) would be 28.1 at the distance of NGC 891. So, for such a population of Fe/H =-0.5 observing stars 1 mag below the TRGB would correspond to V ~ 29.1. For Milky Way type halo stars V=29.1 enables us to go 2 V-magnitudes below the TRGB. The (V-I) colours of (T)RGB stars at a metallicity of Fe/H = -2 correspond to K0III stars; for Fe/H =-0.4 (Harris et al. 1999), colours of early-M type stars (M1III) are the best approximation. Exposure times for late-G (G8 template) to late-K/early-M (K7/M2 template) type stars are <= 9500s for F606W and <= 8800s for F814W (decreasing towards later spectral types). Including overhead, the observing time required to successfully complete this programme will therefore be 18 orbits (6 orbits per field). This ensures that we are sensitive to Fe/H ~ -0.30 stars, as in NGC 5128 (Harris et al. 1999), and to more metal-poor populations, as suggested for NGC 5907 (Zepf et al. 2000; cf. Fig. 1). With the proposed exposure times, 3 orbits per filter per position, our parallel NICMOS observations will be able to detect J=26.5 mag stars at the 4.2 Sigma level in F110W and H = 26.0 mag stars at S/N=3.6 in F160W. We will split up the observations in individual exposures of about 1000s each. We will further CR-split the individual exposures in 3 splits to reduce the effects of cosmic ray contamination. The individual exposures will also be dithered by small amounts to control for hot pixels and improve resolution. 4. Our group's expertise; the method to be used: This project builds on, complements and extends de Grijs & Peletier's ongoing NIR programme of highly inclined spiral galaxies. Our main aim is to analyze and disentangle the physical characteristics of galactic (stellar) disk components in disk-dominated spiral galaxies. Both de Grijs (e.g., de Grijs et al. 2001) and Peletier have the necessary expertise to successfully model and analyze the physical characteristics of the resolved stellar populations that will be observed for this programme. This work also extends Gilmore's long- standing study of the stellar populations and disk components in both our own and external spiral galaxies. This proposal is highly relevant for Frayn's PhD thesis, which deals partially with halo stellar populations in disk galaxies. Ikuta has recently completed her PhD thesis on ``Star Formation Histories of Local Group Dwarfs'', and has developed a code to simulate the properties of various mixtures of stellar populations. She has gained significant expertise in the analysis of colour-magnitude data of stellar populations, which complements the expertise available in Cambridge (de Grijs, Gilmore, Frayn). We expect to have the photometry database for our halo fields prepared within 2 months of the data acquisition; the main challenge will then be the modelling of the CMD data using maximum likelihood analysis. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC891-HALO1 GALAXY, HALO, GALAXY, RA=02H 22M 42.7S +/- 1.5', J2000 V = 26.9 SPIRAL DEC=42D 19' 42.0" +/- 1.5', REGION Coordinate Source: OTHER_SOURCE 2 NGC891-HALO1 GALAXY, HALO, GALAXY, XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF1 SPIRAL 0.01", ETA-OFF=0.0"+/- 0.01", FROM 1 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 3 NGC891-HALO1 GALAXY, HALO, GALAXY, XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF2 SPIRAL 0.1", ETA- OFF=0.40000000000000002"+/- 0.1", FROM 1 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 4 NGC891-HALO2 GALAXY, HALO, GALAXY RA=02H 22M 49.7S +/- 1.5', J2000 V = 26.9 DEC=42D 22' 49.2" +/- 1.5', REGION Coordinate Source: OTHER_SOURCE 5 NGC891-HALO2 GALAXY, HALO, GALAXY XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF1 0.1", ETA-OFF=0.0"+/- 0.01", FROM 4 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 6 NGC891-HALO2 GALAXY, HALO, GALAXY XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF2 0.1", ETA- OFF=0.40000000000000002"+/- 0.1", FROM 4 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 7 NGC891-HALO3 GALAXY, HALO, GALAXY, RA=02H 22M 56.6S +/- 1.5', J2000 V = 26.9 SPIRAL DEC=42D 25' 54.7" +/- 1.5', REGION Coordinate Source: OTHER_SOURCE 8 NGC891-HALO3 GALAXY, HALO, GALAXY, XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF1 SPIRAL 0.1", ETA-OFF=0.0"+/- 0.01", FROM 7 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 9 NGC891-HALO3 GALAXY, HALO, GALAXY, XI-OFF=0.40000000000000002"+/- J2000 V = 26.9 -OFF2 SPIRAL 0.1", ETA- OFF=0.40000000000000002"+/- 0.1", FROM 7 Coordinate Source: OTHER_SOURCE Comments: Offset between orbits to mimick dithering 9414( 2) - 01/28/02 06:54 - [ 4] Visit: 01 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 1 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 1-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 1-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 5] Visit: 02 Visit Priority: Visit Requirements: SAME ORIENT AS 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 1 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 1-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 1-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 6] Visit: 03 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 2 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 2-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 2-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 7] Visit: 04 Visit Priority: Visit Requirements: SAME ORIENT AS 03 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 2 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 2-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 2-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 8] Visit: 05 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 3 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 3-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F606W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 3-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F160W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 9] Visit: 06 Visit Priority: Visit Requirements: SAME ORIENT AS 05 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2472 S PAR 20 WITH 10 3 C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 40 WITH 30 3-OFF1 C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC891-HALO ACS/WF ACCUM WFC F814W CR-SPLIT=3 1 2620 S PAR 60 WITH 50 3-OFF2 C ------------------------------------------------------------------------------------------------------------------------------------ 60 ANY NIC3 MULTIAC NIC3 F110W NSAMP=21,SAMP-SEQ=STEP256 1 DEF CUM ------------------------------------------------------------------------------------------------------------------------------------ 9414( 2) - 01/28/02 06:54 - [ 10] Summary Form for Proposal 9414 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=3 SAMP-SEQ=STEP256 NSAMP=21 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PAR 20 WITH 10 PAR 40 WITH 30 PAR 60 WITH 50 SAME ORIENT AS 01 SAME ORIENT AS 03 SAME ORIENT AS 05 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F160W F814W F110W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC891-HALO1 ANY NGC891-HALO1-OFF1 NGC891-HALO1-OFF2 NGC891-HALO2 NGC891-HALO2-OFF1 NGC891-HALO2-OFF2 NGC891-HALO3 NGC891-HALO3-OFF1 NGC891-HALO3-OFF2 ------------------------------------------------------------------------------------------------------------------------------------