! Proposal 5900, submission 2 ! PI: Peter S. Conti ! Received Tue Feb 28 16:24:43 EST 1995 ! From: claus@stsci.edu ! Hubble Space Telescope Cycle 5 (1995) Phase II Proposal Template ! $Id: 5900,v 3.1 1995/03/10 19:02:48 pepsa Exp $ ! ! 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: Marc Bremmer ! Phone: 410 338-4458 , E-mail: bremmer@stsci.edu ! ! This partially completed template was generated from a Phase I proposal. ! Date generated: Sat Dec 17 09:54:24 EST 1994 ! Proposal_Information ! Section 4 Title: Spectroscopy and Imaging of Starburst Knots in Wolf-Rayet Galaxies Proposal_Category: GO Scientific_Category: Stellar Populations Cycle: 5 Investigators PI_name: Peter S. Conti PI_Institution: Joint Institute for Laboratory Astrophysics CoI_Name: Claus Leitherer CoI_Institution: Space Telescope Science Institute Contact: ! Y or N (designate at most one contact) CoI_Name: William D. Vacca CoI_Institution: University of California, Berkeley Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) Wolf-Rayet (W-R) galaxies are a subset of starburst galaxies observed at a propitious moment, soon after the onset of the burst of star formation, when a large number of the most massive stars have evolved to the W-R stage. HST UV images of 17 of these galaxies, obtained as part of Cycle 2, revealed that the O, B, and W-R stars in the starbursts are clustered within intense, compact knots. We now propose a detailed investigation, employing both optical images and UV spectra, of the recently discovered starburst knots in 3 such W-R galaxies: He2-10, NGC3125, and NGC1741. WFPC2 optical photometry of individual knots will be used with spectral synthesis models to constrain the ages of the starburst events in the knots. If the initial mass function is ``normal'', and there are no lower mass cut-offs in this kind of star formation, the superior imaging of HST should allow detection of the knots in contrast against the host galaxy continuum. GHRS UV spectra of starburst regions can be used in a spectral synthesis analysis to determine their hot star populations and to constrain the parameters of their initial mass functions. The physical parameters of such regions located in each of two galaxies of an interacting pair will be compared. All three of our proposed target galaxies are known merger remnants or merger candidates. The intense bursts of star formation in the compact knots observed in W-R galaxies may have been triggered by these galaxy - galaxy interactions. Questions ! Free format text (please update) Observing_Description: WFPC2 imaging. We selected four passbands to optimally constrain the stellar content of the starburst regions in the proposed galaxies: F814W, F555W, F439W, and F658N. The choice of filters is a trade-off between exposure times (which become prohibitively long in the ultraviolet), and suitability to constrain the young stellar population (by separating age and extinction effects). The data obtained with these filters will be combined with the previously- obtained FOC UV (F220W) images. The primary filter to be used to detect hot, ionizing stars within the regions is F658N. (F675W will be used for the HAlpha observations of NGC1741, whose redshift is large enough that HAlpha is shifted out of the F658N filter passband). An alternative would be the UV Woods filter, but its low transmission and susceptibility to dust extinction make this filter less suitable. On the other hand, measurement of the HAlpha equivalent width provides an excellent age discriminant for a young starburst (cf. Fig. 2). W(HAlpha) is a normalized quantity, and therefore does not depend on the star-formation rate, the total stellar mass, or the low-mass end of the IMF. (At the same time it is not very useful to constrain the IMF slope. This will be done with our spectroscopy). The continuum points will be taken from the V and I measurements, with the interpolation at HAlpha following from our population synthesis models. This method works only if the starburst regions are resolved. Therefore, only HST is capable of doing these observations. Ground-based measurements of distant H II regions yield equivalent widths which are generally much lower than model predictions (Garc'\ia-Vargas & D'\iaz 1994); this is due to contamination of the continuum by the surrounding field population. Heap (1994) clearly demonstrated the superiority of using HST to observe and measure starburst regions. WFPC1 HAlpha observations of the nuclear H II regions of M83 led to an upward revision of earlier ground-based equivalent widths by up to an order of magnitude, in very good agreement with models. The observed B (F439W), V (F555W), and I (F814W) magnitudes will be used to determine extinctions and detect underlying older stellar populations in the starburst knots. The B and V magnitudes are sensitive to age effects as well (cf. Fig. 2). Having constrained the age with HAlpha, we will use (B - V) to determine the interstellar extinction, which is substantial in the case of He2-10 and NGC3125 (A_V = 1.6 and 1.8, respectively). With images taken through the F814W filter, we can then construct (B - V) versus (V - I) color- color diagrams. If red supergiants are present in the starburst regions, they will be detectable by their red colors (cf. Cervi\ no & Mas Hesse 1994). Although in general we do not expect to observe red supergiants in a singular burst when W-R stars are present, the starburst regions may have a very complex star formation history with multiple previous bursts. We will determine absolute magnitudes M_B and M_V and determine a relationship between continuum magnitude and linear size. Most of the starburst regions will be resolved, as suggested by our previous FOC images. With the ages of the starburst events in the knots estimated from the H Alpha fluxes, we can then determine the masses of the knots by comparing the observed luminosities with those predicted from spectral synthesis models. We will construct luminosity and mass functions for the various knots and examine any variations among the starburst regions. We will compare the derived parameters to those of the recently formed globular clusters in the elliptical/merging galaxies NGC1275 (Holtzman et al. 1992) and NGC7252 (Whitmore et al. 1993). This will serve as a first step towards determining a physical relationship between starburst regions in W-R galaxies, recently formed clusters in merging galaxies, and globular clusters in general. Exposures times for the proposed images were calculated based on the fluxes measured on our pre-COSTAR FOC images at 2200 Angstrom\ and ground- based optical spectra. The total integration times per galaxy are 50 min (F439W), 30 min (F555W), 30 min (F814W), and 60 min (F656N). The exposures will be split into four sub- exposures to allow two dithered positions, and each CR-SPLIT will be used to identify the cosmic rays. Taking into account the overhead, we can accomplish the imaging in four orbits per galaxy. GHRS spectroscopy. Even with a relatively flat IMF, stars with masses above 10 M_\odot contribute little to the optical continuum and their spectral features in this wavelength range are correspondingly weak. The situation improves dramatically in the UV at wavelengths below 2000 Angstrom\ where massive stars display a variety of strong stellar absorption lines. Our main goal is to obtain HST GHRS UV spectra of starburst knots in He2-10 and NGC3125. (We expect to acquire GHRS UV spectra of NGC 1741 as part of a Cycle 2 carry-over.) These will be compared with our spectral synthesis models and used to determine the parameters of the massive star population (e.g., burst age, slope of the upper end of the IMF, upper cut-off mass) within the knots. We need S/N = 15 in the continuum to detect and model age- and IMF- sensitive stellar absorption lines. This value is based on our experience with previous modeling of IUE and HST data (Robert et al. 1993; Vacca et al. 1994). Lower S/N makes it difficult to resolve spectral lines which are blends of stellar and interstellar contributions. An example of our modeling technique is shown in Fig. 3. The FOS spectrum of the W-R galaxy NGC4241 is well reproduced by a young (t ~ 2.5 Myr) starburst with a Salpeter-type IMF. We compared the capabilities of the FOS and G130H grating and of the GHRS and G140L grating. After taking into account instrumental overheads, sensitivity, scattered light in the far-UV, wavelength coverage, aperture sizes, and spectral resolution, we find that the GHRS is the instrument of choice. Above all, the most important strategic lines used to characterize the burst (He II 1640, C IV 1550, Si IV 1400, N V 1240) have wavelengths near or below 1600 Angstrom where the GHRS sensitivity is clearly superior. Starburst regions containing W-R stars are younger than 10 Myr. Strong N V is unique to such regions and constitutes an invaluable age and IMF indicator (Figure 3). Only the GHRS has the required sensitivity, scattered light behavior, and spectral resolution (needed to separate the emission line from nearby geocoronal LyAlpha emission) to provide high-quality observations of N V. The fluxes of the starburst regions are around ~ 3* 10^-15 erg s^-1 cm^-2 AA^-1 at 1400 Angstrom. This estimate is based on our FOC images at 2200 Angstrom\ and the integrated spectral energy distributions of the galaxies in IUE spectra. We need two grating positions to cover the essential wavelength region from 1200 Angstrom\ to 1700 Angstrom. At the shorter wavelengths, S/N ~ 15 is reached in 1.5 hours. 2.5 hours are required at the longer wavelength setting. We propose to observe two starburst regions in He2-10 and in NGC3125, i.e. a total of four. The two regions in each galaxy are sufficiently close that the guide star and target acquisition has to be performed only once. Therefore the first objects requires 6 orbits, and the second only 5. The grand total for the spectroscopy is 22 orbits for all four starburst regions. Real_Time_Justification: none While we have no coordinated observations, we have on-going ground-based programs to study W-R galaxy spectra at optical, infrared, mm (CO) and cm (HI) wavelengths. We also are proposing observations on the ISO satellite for IR imaging and spectroscopy of these objects. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: NGC1741 Alternate_Names: Description: GALAXY,DWARF COMPACT,STARBURST Position: RA=4H 59M 9.1S +/- 0.5S, DEC=-4D 19' 44.3" +/- 3" ! Most common specification format is ! RA=0H 0M 0.00S +/- 0S, ! DEC=0D 0' 0.0" +/- 0", ! PLATE-ID=0000 Equinox: 1950 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux: V=14.0 +/- 1.0 ! Include at least V and B-V 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 ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN