! Hubble Space Telescope Cycle 6 (1996) Phase II Proposal Template ! Id: 6639 ! ! Refer to the HST Phase II Proposal Instructions to fill this out ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Mutchler ! Phone: 410-338-1321 , E-mail: mutchler@stsci.edu Proposal_Information ! Section 4 Title: Star Clusters And The Duration Of Starbursts Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_name: Gerhardt R. Meurer PI_Institution: The Johns Hopkins University CoI_Name: Timothy M. Heckman CoI_Institution: The Johns Hopkins University Contact: ! Y or N (designate at most one contact) CoI_Name: Claus Leitherer CoI_Institution: Space Telecope Science Institute Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) Starbursts are a fascinating but poorly understood phenomenon that probably play a key role in galaxy evolution. Here, we propose to determine one of their most basic properties, namely the duration, Delta t, of the burst. This quantity is crucial for understanding how starbursts work (what turns them on and what turns them off), for estimating the fraction of galaxies that undergo starbusts, and for determining how effective starbursts are at heating and enriching the inter- galactic medium (via galactic winds). One thing we do know about starbursts is that they efficiently manufacture star clusters. Each individual cluster is expected to form over a timescale < 1 Myr, and each is thus a ``true'' instantaneous burst. Here we propose using the clusters found in starbursts as a fossil record that will provide a direct measurement of Delta t. This will be done by obtaining broad band photometry of a sample of starbursts already observed in the ultraviolet with the FOC. The new observations, alone, and in conjunction with the previous observations will be used to determine the colors of the clusters. The ages, t, of the clusters will be estimated from two color diagrams, to an accuracy of a factor of ~ 2. The width of the t distribution of the clusters then provides a direct estimate of Delta t for the surrounding starburst in which they reside. Questions ! Free format text (please update) Observing_Description: Broad band photometry is an accurate and cheap (in telescope time) method of determining the luminosity and ages of multiple clusters in a galaxy. The method is simple: we use images obtained through the filters F220W (with the FOC; already obtained), F336W, F439W, and F814W (the last three are the proposed new observations with WFPC2) and compare the colors of the clusters with theoretical models (e.g.\ 21,22). There are three main complications we are likely to face. (1) The color evolution of clusters is neither smooth nor monotonic. This is shown in fig. 2 which shows the age sequence of clusters in the F336W - F439W, versus F439W - F814W plane, and the F220W - F336W versus F439W - F814W plane. The early photometric evolution of star clusters is complicated by the excursion to the red when red supergiants (RSG) stars are produced, and later on when asymptotic giant branch (AGB) stars appear. Although the existence of these phases is well established, the magnitude of the effect is not well known nor well understood 21 (and references therein). Ultimately these excursions limit the precision to which we can determine ages from the two color diagram. Nevertheless it is clear from fig 2a that we should be able to distinguish clusters with t <= 4 Myr, t ~ 10, 40, 100, 300, 600, and 1000 Myr from each other. Thus we should be able to determine logarithmic ages to an accuracy of 0.3 -- 0.4 dex. (2) Reddening. This is insidious because the aging vector is roughly parallel to the reddening vector in Fig 2a. Since the dust distribution may be patchy, a spread in E(B- V)_i can mimic an apparent spread in t. We have two responses to the reddening problem. First, we have limited our sample to galaxies with E(B-V)_i < 0.2. The one exception to this limit, NGC3690, has so many clusters that we can't neglect it from this study. For t > 3 Myr, a scatter sigma_E(B-V)_i = 0.2 amounts to sigma_\log(t) ~ 0.3 dex, thus the uncertainty due to E(B-V)_i is no worse than that from the color evolution of clusters. Second, the previous F220W observations with the FOC will help us distinguish highly reddened clusters. The reddening vector in the F220W - F336W vs. F336W - F439W plane (Fig 2b) will displace highly reddened clusters from the reddening free locus for t < 20 Myr (and t > 400 Myr; although such clusters will probably be difficult to detect in the F220W frames). (3) Fading and metallicity. By t = 200 Myr a 10^5 Msun cluster will be fainter than M = -10 mag in all bands making it hard to distinguish from a star. It may be possible to get rough age estimates for clusters up to a few 100 Myr, but at about 1 Gyr metallicity effects become important; ages older than this can not be estimated from the colors alone. The filters were chosen to give a large color range over the age range of interest. Specifically, the F336W and F439W filters were chosen because they bracket the Balmer discontinuity. Thus F336W - F439W has a strong age dependence without requiring a long Lambda baseline. The F814W filter is included to accurately identify clusters in the RSG phase. These broad band filters allow deep exposures to be obtained in relatively short integrations. For example in a single orbit we should be able to obtain CR split images of duration at least 15, 9 and 5.5 minutes in F336W, F439W and F814W. At the distances involved most clusters will be point sources. With these exposure times they can be detected to limiting magnitudes of 22.1, 22.5, and 24.0 at a S/N = 10 (with optimal weighting as in the WFPC2 handbook) on top of an expected starburst background of 17.0, 17.5, and 18.8 mag arcsec^-2 (all magnitudes in STMAG system) through the F336W, F439W, and F814W filters respectively. We are primarily interested in measuring the clusters we already know exist from our UV observations down to M_220 = -13. A cluster with t = 10 Myr and M_220 = -13 has absolute magnitudes -12.0, -11.7, and -11.3 in F336W, F439W, and F814W, and can thus be measured at S/N = 10 out to an apparent distance modulus 34.1 in all three bands, or D = 66 Mpc for unextincted systems. Thus we should be able to achieve our goal of obtaining S/N = 10 photometry of the previously known clusters in one orbit per galaxy. A 0.1 mag uncertainty in each band will result in a \log t precision of ~0.1 dex (except in the RSG phase). The F336W filter has a significant redleak problem. Since we will also be obtaining F814W images, we can in principle estimate the amount of redleak. However this is unnecessary for two reasons. (1) As in Fig.\ 2, we will make our comparisons in the natural HST system, calculating colors from model spectra that extend beyond 1 micron using SYNPHOT and transmission curves including the red leak. Since our model will incorporate redleak, no adjustment is necessary to the observations. (2) The clusters that will be most affected by redleak will be in the RSG phase, when we can not fine tune the ages anyway because of the limitations of the theory (see above). We plan to use WFPC2 instead of the FOC because of the former's high linearity and dynamic range, and the limited photometric accuracy of the latter. We will place the starburst center on the PC chip to get the highest angular resolution available with WFPC2. The WF chips will be useful to look for clusters outside the central starburst regions. One of the program galaxies in our proposal is in the CVZ, and we forgot to mention this in our phase I proposal. I request that we observe this galaxy, NGC3690, in the CVZ mode if possible. The main scientific reasons for doing this are that NGC3690 is the most distant and highly reddened galaxy in our sample, yet it is the richest in star clusters. Observing this galaxy in the CVZ mode will bring back the highest scientific return at the highest efficiency for the HST. The CVZ mode observations will also allow us to dither the observations which is especially desirable because of the high degree of crowding of the clusters in NGC3690. I realize this request may be rejected. Nevertheless I am planning my observations as if it would be accepted. NGC3690 is visit 4 in my phase II proposal. I am including a commented out version of visit 4 which is what the observation would look like without the CVZ. This version can be used if the CVZ request is turned down. Real_Time_Justification: Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: NGC3310 Alternate_Names: UGC5786,ARP217 Description: GALAXY, STARBURST, SPIRAL Position: RA=10H 38M 45.84S +/-0.08S, DEC=+53D 30' 12.5" +/-0.7" Equinox: J2000 RV_or_Z: V=980 Flux: SURF(V)=16.4+/-1.0, E(B-V)=0.18+/-0.05, SURF(2300)=4.0+/-0.8 E-16 Comments: Fixed_Targets ! Section 5.1 Target_Number: 2 Target_Name: TOLOLO1924-416 Alternate_Names: IRAS19245-4140 Description: GALAXY, STARBURST Position: RA=19H 27M 58.35S +/-0.06S, DEC=-41D 34' 30.0" +/-0.7" Equinox: J2000 RV_or_Z: V=2843 Flux: SURF(V)=16.4+/-1.0, E(B-V)=0.11+/-0.05, SURF(2300)=5.6+/-1.1 E-16 Comments: Fixed_Targets ! Section 5.1 Target_Number: 3 Target_Name: NGC4670 Alternate_Names: UGC7930,HARO9,ARP163 Description: GALAXY, STARBURST, AMORPHOUS IRREGULAR, DWARF COMPACT Position: RA=12H 45M 17.12S +/-0.05S, DEC=+27D 07' 31.1" +/-0.7" Equinox: J2000 RV_or_Z: V=1069 Flux: SURF(V)=16.4+/-1.0, E(B-V)=0.12+/-0.1, SURF(2300)=5.6+/-1.1 E-16 Comments: Fixed_Targets ! Section 5.1 Target_Number: 4 Target_Name: NGC3690 Alternate_Names: UGC6471,MRK171,ARP299,IC694 Description: GALAXY, STARBURST, ULTRALUMINOUS IR GAL, INTERACTING GALAXY Position: RA=11H 28M 31.328S +/- 0.06S, DEC=+58D 33' 43.94" +/- 0.7" Equinox: J2000 RV_or_Z: V=2992 Flux: SURF(V)=15.4+/-1.0, E(B-V)=0.44+/-0.2, SURF(2300)=4.7+/-0.9 E-16 Comments: ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits ! Section 6 Visit_Number: 1 Visit_Requirements: ! Section 7.1 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! DROP TO GYRO IF NECESSARY [NO REACQuisition] ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! SCHEDulability ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN