8697( 4) - 05/10/00 14:02 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8697 Version: 4 Check-in Date: 10-May-2000 14:01:11 1.Proposal Title: Stellar Populations in the Disk-Halo Interface of NGC 55 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Annette Ferguson University of Cambridge CoI: Prof. Rosemary Wyse The Johns Hopkins University N CoI: Prof. John Gallagher III University of Wisconsin-Madison N ------------------------------------------------------------------------------------ 5. Abstract The disk-halo interface is where the global effects of stellar feedback on the ISM are expected to be most pronounced. Our deep ground-based emission--line images of the nearby, edge-- on SBm galaxy NGC 55 provide a particularly spectacular example of this phenomenon. In addition to numerous supergiant ionized filaments and shells, our images also reveal the surprising discovery of several candidate HII regions located at significant heights (1--2 kpc) above the disk plane. We have obtained high-resolution ground-based broad-band images for the brightest of these candidate HII regions, revealing several underlying blue continuum sources, supporting the idea that such regions might be the site of in situ star formation in the disk--halo interface. If recent star formation has occurred in these clumps, it would have extremely important implications for understanding the disk--halo connection in galaxies. Further, if this is not an isolated event, there should be a faint background thick disk/halo. We propose to obtain WFPC2 UVI images of these candidate HII regions to resolve the embedded stars that appear to be present, together with parallel STIS observations to investigate a possible background thick-disk/halo stellar population, about which there is little information in very late-type disk systems, and which will place our targetted WFPC2 HII region data in context. ------------------------------------------------------------------------------------ 8697( 4) - 05/10/00 14:02 - [ 2] Observations Description ------------------------ We propose to obtain F255W, F555W and F814W images of candidate extra--planar HII regions in the disk--halo interface of the nearest edge--on galaxy to the Milky Way, NGC~55, together with parallel STIS long-pass observations of their larger-scale environment. As discussed below, one orbit in each filter is required per HII region, resulting in a total request of 9 orbits. The large FOV of the WFPC2 provides the additional opportunity to study a significant fraction of the background halo/thick disk population. Using the WFPC2 exposure time estimator, we estimate that we can reach blue V=25 stars with a S/N $\sim$ 5 in F255W in one orbit ($\sim$ 2600 sec), after account is taken for overhead. At the distance modulus of NGC~55, $m-M=$25.85, this corresponds to a star with M$_V=-0.85$. Scaling from our past experience, we expect one orbit to be required in each of the F555W and F814W filters to reach moderately red stars of V=26.5 ($\approx$ I $\sim$ 24.5), corresponding to M$_V$=0.1 or M$_I=-1.3$, with a S/N $\sim$ 7 in both filters. Thus, we will be able to detect main sequence stars down to $\sim$ A0, red and blue supergiants, and AGB/RGB stars. Based on our measured H$\alpha$ luminosities for these regions, and assuming a normal IMF, we could expect to see as many as $\sim$ 10 young blue stars per candidate. We will reach several magnitudes below the tip of the red giant branch in an old population, M$_I=-4$. The Galactic foreground extinction towards NGC~55 is negligible (Burstein \& Heiles 1984). Internal extinction at a height of $\sim$ 1.5 kpc may be expected to be very small, but can be estimated by analysis of our UV and optical colors, and will be verified by ground--based long-slit spectroscopy (which will also provide a gas-phase abundance determination) which will be carried out at the ESO 3.6m in October 1999. The candidate extra--planar HII regions are small enough that they will fit entirely on the PC; a suitable roll angle will be chosen so that the WF chips probe the content and colours of the stellar populations in the thick disk/halo interface. The deepest probes of the background halo population in NGC~55 will be provided by STIS LP parallel observations, which will allow us to study the stellar luminosity function several magnitudes fainter than the WFPC2 observations. In 3 orbits per field, the STIS ETC suggests that we will reach stars with I$\sim$27 (M$_I$=1.2) with a S/N of $\sim$5. This will allow us to reach the region of the luminosity function that is densely populated with post-first-ascent red giant stars in the Helium-burning red clump or horizontal branch phase, which will provide excellent tracers of intermediate- age and old stellar populations, and allowing the construction of deep luminosity functions of evolved stars in regions far above the plane of the disk of NGC~55. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8697( 4) - 05/10/00 14:02 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC55-HII1 EXT-CLUSTER, OB RA=00H 14M 48.80S +/- 0.5", DEC= J2000 F(6563)=1+/-1e-14 ASSOCIATION, EXT- -39D 15' 0.1" +/- 0.5", PLATE- MEDIUM, HII REGION ID=011S Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 2 NGC55-HII2 EXT-CLUSTER, OB RA=00H 15M 0.37S +/- 0.5", DEC=- J2000 F(6563)=1+/-1e-14 ASSOCIATION, EXT- 39D 15' 37.4" +/- 0.5", PLATE- MEDIUM, HII REGION ID=011S Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 3 NGC55-HII3 EXT-CLUSTER, OB RA=00H 15M 46.12S +/- 0.5", DEC= J2000 F(6563)=1+/-1e-14 ASSOCIATION, EXT- -39D 18' 5.3" +/- 0.5", PLATE- MEDIUM, HII REGION ID=011S Coordinate Source: IMAGE_TIED_TO_GSC_FRAME 8697( 4) - 05/10/00 14:02 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 70D TO 215D On Hold Comments: Additional Comments: The range in allowable ORIENT has been calculated to place the STIS parallel pointing above the bright disk of the galaxy, thus probing the lower halo, and to avoid having bright stars in the STIS aperture. A bright (12th mag) guide star will unavoidably land on one of the 3 WF chips but should not be a problem for our main target, which is centered on the PC. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC55-HII1 WFPC2 IMAGE PC1-FIX F555W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 21 WITH 10-20 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC55-HII1 WFPC2 IMAGE PC1-FIX F555W ATD-GAIN=7,CR-SPLIT=NO 1 800 S ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC55-HII1 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=2,GAIN=1 1 2765 S CD ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC55-HII1 WFPC2 IMAGE PC1-FIX F255W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 41 WITH 30-40 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC55-HII1 WFPC2 IMAGE PC1-FIX F255W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC55-HII1 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=2,GAIN=1 1 2850 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC55-HII1 WFPC2 IMAGE PC1-FIX F814W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 61 WITH 50-60 ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC55-HII1 WFPC2 IMAGE PC1-FIX F814W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 61 NGC55-HII1 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=3,GAIN=1 1 2787 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 8697( 4) - 05/10/00 14:02 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENT 75D TO 115D ORIENT 155D TO 245D On Hold Comments: Additional Comments: The range in allowable ORIENT has been calculated to place the STIS parallel pointing above the bright disk of the galaxy, thus probing the lower halo, and to avoid having bright stars in the STIS aperture. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC55-HII2 WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 21 WITH 10-20 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC55-HII2 WFPC2 IMAGE WF3 F555W ATD-GAIN=7,CR-SPLIT=NO 1 800 S ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC55-HII2 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=2,GAIN=1 1 2768 S CD ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC55-HII2 WFPC2 IMAGE WF3 F255W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 41 WITH 30-40 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC55-HII2 WFPC2 IMAGE WF3 F255W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC55-HII2 STIS/C ACCUM 50CCD MIRROR CR-SPLIT=2,GAIN=1 1 2850 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC55-HII2 WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 61 WITH 50-60 ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC55-HII2 WFPC2 IMAGE WF3 F814W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 61 NGC55-HII2 STIS/C ACCUM F28X50L MIRROR CR-SPLIT=3,GAIN=1 1 2787 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 8697( 4) - 05/10/00 14:02 - [ 6] Visit: 03 Visit Priority: Visit Requirements: ORIENT 150D TO 160D ORIENT 175D TO 240D On Hold Comments: Additional Comments: The range in allowable ORIENT has been calculated to place the STIS parallel pointing above the bright disk of the galaxy, thus probing the lower halo, and to avoid having bright stars in the STIS aperture. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC55-HII3 WFPC2 IMAGE PC1-FIX F555W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 21 WITH 10-20 ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC55-HII3 WFPC2 IMAGE PC1-FIX F555W ATD-GAIN=7,CR-SPLIT=NO 1 800 S ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC55-HII3 STIS/C ACCUM 50CCD MIRROR GAIN=1,CR-SPLIT=2 1 2768 S CD ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC55-HII3 WFPC2 IMAGE PC1-FIX F255W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 41 WITH 30-40 ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC55-HII3 WFPC2 IMAGE PC1-FIX F255W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC55-HII3 STIS/C ACCUM 50CCD MIRROR GAIN=1,CR-SPLIT=2 1 2850 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 NGC55-HII3 WFPC2 IMAGE PC1-FIX F814W ATD-GAIN=7,CR-SPLIT=0.5 1 1600 S PAR 61 WITH 50-60 ------------------------------------------------------------------------------------------------------------------------------------ 60 NGC55-HII3 WFPC2 IMAGE PC1-FIX F814W ATD-GAIN=7,CR-SPLIT=NO 1 900 S ------------------------------------------------------------------------------------------------------------------------------------ 61 NGC55-HII3 STIS/C ACCUM F28X50L MIRROR GAIN=1,CR-SPLIT=3 1 2787 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 8697( 4) - 05/10/00 14:02 - [ 7] Summary Form for Proposal 8697 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX 50CCD F28X50LP WF3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN=7 CR-SPLIT=0.5 CR-SPLIT=NO GAIN=1 CR-SPLIT=2 CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 70D TO 215D PAR 21 WITH 10-20 PAR 41 WITH 30-40 PAR 61 WITH 50-60 ORIENT 75D TO 115D ORIENT 155D TO 245D ORIENT 150D TO 160D ORIENT 175D TO 240D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W MIRROR F255W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC55-HII1 NGC55-HII2 NGC55-HII3 ------------------------------------------------------------------------------------------------------------------------------------