9381( 3) - 02/13/02 14:57 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9381 Version: 3 Check-in Date: 13-Feb-2002 14:56:50 1.Proposal Title: The Birth of a Dwarf Galaxy: The Star Formation History of the Tidal Arm near NGC 3077. ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Fabian Walter California Institute of Technology CoI: Dr. Shoko Sakai University of California, Los Angeles N CoI: Dr. Crystal Martin California Institute of Technology N ------------------------------------------------------------------------------------ 5. Abstract The extended tidal arm of neutral gas near NGC 3077 (member of the M 81 triplet, D~3.8 Mpc) is one of the most dramatic features of its kind seen in the local universe; it was created by an interaction with M 81 some 3*10^8 yr ago. It is one of the few tidal systems where atomic (HI) and molecular (CO) gas as well as low-- level star formation (HAlpha) is detected over an area of several kpc^2. This tidal complex is believed to be in the process of forming a tidal dwarf galaxy. Using the unique resolving capability of the Hubble Space Telescope (HST) and the wide--field imaging capabilities of the ACS, we propose to perform a stellar population study of this tidally created system. By combining various methods to recover the star formation history (e.g., using the luminosity function of the core Helium burning stars) we will 1) measure the SF history since closest encounter with M 81, 2) determine whether older stars are present in the tidal feature or not and 3) investigate propagation of star formation across the tidal feature. This can be done with F435W, F555W, and F814W imaging using HST's ACS with only 8 orbits. We will, for the first time, recover the star formation history and the distribution of stars within a tidally created system. This study will also shed light on the creation and evolution of other tidal dwarf galaxies which are typically much further away. ------------------------------------------------------------------------------------ 9381( 3) - 02/13/02 14:57 - [ 2] Observations Description ------------------------ The goal of this proposal is to construct and interpret color-magnitude diagrams (CMDs) for the stellar population in the tidal arm near NGC 3077. Because NGC 3077 is located in the CVZ, deep imaging can be achieved very efficiently. At the distance of NGC 3077, the resolution of the ACS is only 1 pc (0.05''), which is very well suited to perform a stellar population study (this has been nicely shown in a number of studies; see e.g., Sextans A, Dohm-- Palmer et al. 1997a, b; GR 8, Dohm-Palmer et al. 1998 or the M 81 group dwarfs by Caldwell et al. 1998 and Lynds et al. 1998 using the WFPC2 which has 2 times lower resolution in the WF than the ACS). Crowding will not be a problem, and a possible background population, if present at all, will be faint (see Fig. 1). Likewise, contamination by Galactic foreground stars is minimal at this Galactic latitude. Obviously, this study can not be undertaken using ground-- based telescopes. In order to interpret the CMD, three pieces of ancillary data are required: a) determinations of distance, b) metallicity, and c) reddening. a): Sakai & Madore (2000) derived a distance modulus of ~ 28.0 for the halo of NGC 3077 which is situated close to our region of interest. From our data, we will also be able to determine a precise distance from the tip of the red giant branch (TRGB) . b) Based on our spectra of selected HII regions in the tidal arm which we have recently obtained at the Palomar 200'' telescope we derive a metallicity of Z~0.01. c) At high galactic latitude (+45^o) foreground reddening is moderate (E(B-V) = 0.036 from HI column density) and, due to the low metallicity, internal reddening is expected to be small. Our proposed observations will accomplish the following four goals: (1) an MS luminosity function, (2) a core He burning luminosity function, (3) a color-color diagram to check for reddening and (4) an emission line image to check for nebular contamination: In the following we assume a distance of 4 Mpc (m-M = 28, Sakai & Madore, 2001), and estimate the depth of photometry required to study the important features in the CMD. The TRGB at I ~ -4 requires a depth in I to ~26. To get the recent star formation history we need to study the main sequence (MS) back significantly in time. E.g., a turnoff age of 100 Myr translates to roughly V = 27, B = 26. This can be reached in the F435W filter in 3 * 2400 s and in the F555W filter in 4 * 2400 s (S/N ~ 13). To detect the HeB stars up to the ages of ~ 600 Myr, F814W integrations of 8 * 2400 s are required in order to reach I = 27. Our proposed photometric limits are shown in Figure 3, compared with metallicity appropriate isochrones from Bertelli et al. (1994). A short HAlpha (F658N) integration (of order 600 s) will be included to identify regions of nebular contamination and the presence of emission line stars. We will now estimate the total number of Helium burning stars in the tidal feature. This is obviously not straightforward to do since similar features have yet not been studied with HST. Based on the study of Sextans A (Dohm- Palmer) we will estimate the expected number of HeB stars by 1) comparing the total optical magnitudes of Sex A with that of the tidal feature and 2) comparing the current star formation rates: 1) Sharina et al. derived a total magnitude for the stars in the tidal arm (the `Garland') of m_B=16.5 which translates to an absolute blue magnitude of M_B=-11.5. This can be compared to the absolute blue magnitude of Sextans A (Skillman et al. 1989) of M_B=-13.8: Sextans A is about an order of magnitude brighter (in physical units) than the tidal feature. 2) the total SFR in the tidal feature is 2.6x10^-3 Msun yr^-1, Sextans A's SFR is 2x10^-3 Msun yr^-1 (Mateo 1998). In other words, based on today's star formation properties both objects are remarkably similar. In their HST study of Sextans A, Dohm-Palmer et al. detect a total of 19000 stars over about half of the area of Sextans A (i.e., a full study of Sextans A would have yielded some 40000 stars). Out of these, 1500 stars (3000 for the entire galaxy) are HeB stars (Dohm- Palmer et al. 1997b, Fig. 12). Our object is located at about 2.5 times the distance to Sextans A but we will go much deeper than their study in absolute terms and will hence reach more or less the same depth than their study. The resolution will be the same (ACS vs. WF in the WFPC2). Based on 1) the total absolute magnitude of the stars we estimate that we will recover some 4000 stars in the tidal arm, out of which 300 will be HeB stars. Since the magnitude of a HeB star is directly related to its age, this number is sufficient to derive the star formation history along the lines discussed above. If we base our estimate on 2) the current SFR we reach a total number of 3000 HeB stars. We believe that it is more likely that the number of HeB stars scales with the total magnitude of an object and therefore estimate the presence of at least a few hundred HeB stars in the region of interest. This total number of stars will populate the CMD well enough to, e.g., do isochrone fitting (see the discussion in `Aim of this Proposal'). Real Time Justification ----------------------- The target is located in the CVZ with a large number of opportunities (53 avaliable orbits in cycle 11 spread over 2 opportunities). Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9381( 3) - 02/13/02 14:57 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC3077- GALAXY, TIDAL TAIL RA=10H 3M 51.7S +/- 1", DEC=68D J2000 V = 20 V = 27 PHOENIX 41' 25" +/- 1", PLATE-ID=00M9 Coordinate Source: GUIDE_STAR_CATALOG Comments: The aim of the proposal is to construct color magnitude diagrams down to B, V, I=27 mag. The brightest star in the field has a magnitude of V~12.7 9381( 3) - 02/13/02 14:57 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 3.011 Coordinate_Frame POS-TARG Pattern_Orient 85.28 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9381( 3) - 02/13/02 14:57 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ ORIENT 144D TO 194D On Hold Comments: Additional Comments: The roll angle constraint is to 1) exclude the brightest star in the region (V~10.7 mag) and 2) to maximise the areal coverage of the tidal dwarf galaxy. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC3077-PHO ACS/WF ACCUM WFC F435W CR-SPLIT=3 1 3000 S PATTERN 1 10 ENIX C ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC3077-PHO ACS/WF ACCUM WFC F555W CR-SPLIT=3 1 4800 S PATTERN 1 20 ENIX C ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC3077-PHO ACS/WF ACCUM WFC F658N CR-SPLIT=2 1 2000 S PATTERN 1 30 ENIX C ------------------------------------------------------------------------------------------------------------------------------------ 9381( 3) - 02/13/02 14:57 - [ 6] Visit: 02 Visit Priority: Visit Requirements: CVZ SAME ORIENT AS 01 On Hold Comments: Additional Comments: It is essential that the orientation (roll angle) of the observations in visit 2 is exactly the same as in visit 1. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 NGC3077-PHO ACS/WF ACCUM WFC F814W CR-SPLIT=4 1 9600 S PATTERN 1 10 ENIX C ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC3077-PHO ACS/WF ACCUM WFC F625W CR-SPLIT=NO 1 700 S PATTERN 1 20 ENIX C ------------------------------------------------------------------------------------------------------------------------------------ 9381( 3) - 02/13/02 14:57 - [ 7] Summary Form for Proposal 9381 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=3 CR-SPLIT=2 CR-SPLIT=4 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ORIENT 144D TO 194D PATTERN 1 10 PATTERN 1 20 PATTERN 1 30 CVZ SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F435W F555W F658N F814W F625W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC3077-PHOENIX ------------------------------------------------------------------------------------------------------------------------------------