6672( 7) - 08/02/96 13:23 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6672 Version: 7 Check-in Date: 02-Aug-1996 14:20:57 1.Proposal Title: Unveiling the massive star content in the prototypical nuclear starburst NGC7714 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle GALAXIES & CLUSTERS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: MARIA LUISA GARCIA-VARGAS VILSPA CoI: Jeff Goldader Space Telescope Science Institute N CoI: Rosa Gonzalez-Delgado Space Telescope Science Institute N CoI: Ariane Lanccon Observatoire de Strasbourg N CoI: Claus Leitherer Space Telescope Science Institute N CoI: Antonella Nota Space Telescope Science Institute N CoI: Anna Pasquali Space Telescope Science Institute N ------------------------------------------------------------------------------------ 6. Abstract We propose ultraviolet spectroscopy with GHRS+G140L and UV WFPC2(F380W) imaging of the prototypical starburst galaxy NGC 7714. The compact appearance and the brightness of the nuclear starburst, as well as the amount of multiwavelength observational data, lead us to propose this object as a laboratory to study the star formation processes in the violent nuclei of starburst galaxies. The Cycle 6 spectroscopy will allow us to derive the massive star content in the nuclear starburst of NGC 7714 directly from the signatures of the massive stars in the ultraviolet spectrum. These spectra will complete the multiwavelength body of data for this nuclear starburst. A complete evolutionary synthesis code will be used to interpret the data. These two complementary aspects (observational and theoretical) are the master keys to unveil the stellar content of star-forming regions and, in particular, to derive the age of the stellar population present in the nuclear starburst, and to constrain the slope and upper limit for the Initial Mass Function. Finally, the UV WFPC2 imaging will complete the study by permitting us to compare the locations of the UV and H-alpha (HST archive image) sources with the spatial distribution of massive stars in the nucleus and circumnuclear regions, constituting an additional observational constraint for the theoretical models. ------------------------------------------------------------------------------------ 6672( 7) - 08/02/96 13:23 - [ 2] Observations Description ------------------------ GHRS spectroscopy: our goal is to obtain GHRS UV spectra of the starburst nucleus of NGC7714. These will be compared to 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). We need S/N = 30 per diode 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. 1995; Leitherer et al. 1995). Lower S/N makes it difficult to resolve spectral lines which are blends of stellar and interstellar contributions. 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+G140L is the instrument of choice. Above all, the most important strategic lines used to characterize the burst (C IV 1550, Si IV 1400, N V 1240) have wavelengths below 1600 Angstrom, where the GHRS sensitivity is clearly superior. The flux of the nucleus of NGC7714 is ~ 4* 10**-14 erg s**-1 cm**-2 AA**-1 at 1400 AA from the IUE spectrum of Kinney et al. (1993). Note that the IUE flux is entirely dominated by the nucleus despite the much larger aperture size of the IUE. We determined the flux encompassed by the GHRS LSA aperture from pre-Costar HST images in the Archive. About 50 % of the IUE flux falls within the 1.7'' aperture of the GHRS. Moreover, the use of the 1''.7 arc sec aperture for spectroscopy, similar to ground observations will be useful to compare the WR content from ultraviolet fitting and from optical WR bumps). The chosen aperture will encompass the total UV light of the nuclear starburst in NGC 7714. In fact, from the analysis of the circumnuclear regions (Garcia- Vargas et al. 1995c), and on the basis of evolutionary synthesis models (Garcia -Vargas, Bressan & Diaz 1995a,b) we derive that the contribution of the circumnuclear emission to the UV flux is 2 order or magnitudes lower than the nuclear one. This implies that the UV spectrum is dominated by the nuclear starburst. Therefore, the long-slit capabilities of STIS would bring no real advantage. We need two grating positions to cover the essential wavelength region from 1200 AA to 1700 AA. At the shorter wavelengths, S/N ~ 30 is reached in 1 orbit for each grating setting. The target acquisition can be done directly with the GHRS, i.e. the FOS is not needed. However, the acquisition has to be done with Side-2, as NGC7714 is too faint for a direct Side-1 acquisition. This will be done in the first orbit, followed by a side switch partially buried in the occultation. The grand total for the spectroscopy is therefore 3 orbits. WFPC2 imaging: We selected the WFPC2 passband F380W which is optimized for measurements of hot stars and has the advantage with respect to the standard F336W of minimizing the red leak contribution from the redder population which we know to be present in NGC7714. A limiting V magnitude of 26 is sufficient at this wavelength to observe the hottest (brighter) stars. We will use the PC, to exploit the highest spatial resolution offered by WFPC2. In order to demonstrate the need for the requested exposure time we will consider the case of a B0 star with a V magnitude of 26. The predicted count rate discussed here has been calculated using Synphot, and the WWW WPC2 exposure time estimator, which are assumed to have the most up to date information on WFPC2's throughput. A B0 star with a V magnitude of 26 yields a source count rate of 0.11 electrons/sec through the F380W filter. A single 800 second observation will produce an integrated flux of 85 counts. WFPC2 has a relatively low pixel modulation transfer function (MTF) which effectively results in significant smoothing of the stellar point spread function. Consequently, the fraction of counts in the central PC pixel is 0.26 of the total count at a wavelength of 4000 Angstrom. The central pixel therefore contains 22 counts (WFPC2 Handbook). In addition to photon counting statistics there are additional noise contributions due to: 1) CCD read noise ~eq 7e-, 2) CCD dark current ~eq 0.005 e-/sec, 3) Sky background ~eq 0.005 e/sec at 4000 Angstrom. For a single exposure, combining all noise contributions, we derive a value for the S/N in the brightest pixel of 2. We require 4 exposures to achieve a S/N of 4 in the central pixel. In addition to the signal to noise considerations, we require a minimum of 4 exposures to obtain 4 CR-SPLIT images at two dither points. Dithering is a strategy essential to remove systematics during analysis of the data. Therefore, we request a 4*800 seconds in order to meet our goal of a limiting U magnitude of 26 in the F380W filter. With the assumption of an orbit lasting 55 minutes, our total request of WFPC2 imaging time is 1 orbit. Relation to theoretical models: The fitting to the HST ultraviolet observations will be made with the use of the models by Leitherer, Robert & Heckman (1995), after a careful deblending to avoid interstellar contamination. The multiwavelengh match will be make with the use of three different synthesis codes which have been developed by members of our team (Garcia Vargas, Lanccon and Leitherer and respective collaborators) independently in Madrid, Strasbourg and Baltimore, offering theoretical results, covering the full wavelength range, as a function of the IMF and SFR, for a complete grid of cluster parameters (mass, age and metallicity). The active collaboration between the three PIs responsible for the codes permits us to test the models, and to be able to synthetize in great detail the whole UV- IR spectrum, as well as the ionizing photon spectrum and therefore the associated emission line spectrum of the gas. Star-forming regions, like the actual case of Starburst galaxies, require a special treatment in theoretical models to study their stellar content. Since the dominant population in these systems has to be a very young stellar cluster capable of ionizing the surrounding gas and producing the observed emission line spectrum, evolutionary synthesis models must include a photoionization code. The properties of the young clusters hidden inside the nebula are controlled by the most massive stars. These properties have to be derived from the emitting gas in an effort to un-mask the stellar population. For a given IMF, the evolution of massive stars (more than 15 solar masses) and their spectral energy distributions, as functions of the metallicity, will be the main inputs for the synthesis codes. The study of how the use of different evolutionary tracks and atmospheres can affect the output results is crucial, and for this reason Garcia Vargas and Leitherer (1995 in preparation) have tested their respective codes to control the effects of the evolutionary tracks and atmospheres models involved. The synthesis+photoinization part is better described by Garcia -Vargas's models. Leitherer's code takes into account the mechanical energy input due to massive stars. It also makes a very specific spectral synthesis to interpret the high resolution profiles of massive stars' lines in HST UV spectra, in terms of IMF parameters and cluster ages. Finally, the extension to the IR will not possible without Lanccon's models. Strong IR constraints like the calcium triplet observations (near-IR) and CO observations (at 2.3 microns will be use to test the presence of red supergiants or giants, whose evolutionary stages would correspond to very different populations in this galaxy. Therefore the disentangle between the existence of a previous burst rich en RSG, or the presence of a dominant old bulge population will be necessary to unveil the star formation history of this galaxy. From a preliminary analysis of the IR data, and according to evolutionary synthesis models, we find that CO is not deep enough to allow for the K band spectrum to be completely dominated by normal red supergiants. Possible explanations are the relatively low metallicity of the nucleus (0.4 x solar), a continuum contribution due to hot dust (L band emission would test this), or a continuum distribution from young stars. In fact, this last approach, including a young burst (3-5 Myr) plus an older episode of star formation (10-15 Myr), agrees with all the observational optical-ir constraints, in particular the Wolf-Rayet WR features and the emission line spectrum of the gas in the optical range (3-5 Myr), and the detection of the calcium triplet in the near-IR, revealing the presence of red supergiant RSG stars. This hyphothesis is more consistent with the observations than the continuous 20Myr long burst suggested by Bernlohr (1993). The HeI 2.06/Br-Gamma ratio of 0.56 +/- 0.08 is consistent with either an IMF upper mass around 50 and an age of a few Myr, or with an IMF upper mass above 70 and an old burst. HST spectroscopy will be able to dissentangle the two possibilities, constraining the IMF parameters with the use of evolutionary models described in section 3 and the deblended ultraviolet CIV and SiIV lines. As we have showed in the descriptions of data from NGC 7714, the GHRS spectrum as well as the WFPC2 image are needed to detect directly the massive stars associated with the nuclear starburst, and therefore to complete our multiwavelength study focused on the star-formation processes in starburst galaxies. Real Time Justification ----------------------- No supporting observations are planned. In addition to the UV spectroscopy and imaging we ask from HST, we have already observed NGC 7714 with several telescopes (4.2 WHT and 1.0 JKT Observatorio Roque de los Muchachos, La Palma Spain; 3.5 NTT, ESO Obs. Chile, and the 2.3 m Siding Spring Observatory Australia), including spectroscopy, and broad and narrow imaging covering the optical and IR ranges. We have very high quality spectroscopical data in the optical --- 3200-10000 AA --- (Gonzalez et al. 1995). Considering the longer IR wavelengths, since the previous observations in the literature (Puxley & Brandt 1994) were not sufficient for our multiwavelength study, we obtained our own IR data in two different campaigns: the first one included J,H,K and 2.3 microns CO images (0.5"/pix) as well as a K band spectrum, corresponding to the central 1.5 x 3 arcsec, and obtained with CASPIR, a cross-dispersed grism spectrograph on the 2.3m telescope of the Siding Spring Observatory (Australia). The second campaign (NTT at ESO, Chile) includes spectroscopic data around the HeI 2.06 microns and Br-Gamma 2.17 microns recombination lines, and part of the CO absorption band, as well as K imaging. Finally, the IUE low resolution spectrum (Kinney et al. 1993) has been used to calculate the exposure times since most of the UV continuum comes from the nuclear starburst. The HST archive images will be also included in our analysis. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6672( 7) - 08/02/96 13:23 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6672( 7) - 08/02/96 13:23 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC7714 GALAXY,STARBURST,NUCLEU RA=23H 36M 14.11S +/- 0.15S, J2000 V=+2808 V = 12.7+/-0.6 S,KNOT,STAR FORMING DEC=+02D 09'18.1" +/- 1", PLATE- SURF(V) = 18+/-2.0 REGION,SPIRAL ID=0515 F-CONT(1400)=30+/-10e-15 B-V = 0.5+/-0.1 E(B-V) = 0.22+/-0.05 6672( 7) - 08/02/96 13:23 - [ 5] Visit: 01 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC7714 HRS ACQ 2.0 MIRROR-N1 BRIGHT=RETURN,SEARCH-SIZE=3 1 90 S ONBOARD ACQUISITION FOR 2-3 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC7714 HRS ACCUM 2.0 G140L 1317 1 1659.2 ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC7714 HRS ACCUM 2.0 G140L 1542 1 4542.4 ------------------------------------------------------------------------------------------------------------------------------------ 6672( 7) - 08/02/96 13:23 - [ 6] Visit: 02 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC7714 WFPC2 IMAGE PC1 F380W 3964 CR-SPLIT=0.5 1 1000.0 ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC7714 WFPC2 IMAGE PC1 F380W 3964 CR-SPLIT=0.5 1 800 S POS TARG 0.25,0.25 ------------------------------------------------------------------------------------------------------------------------------------ 6672( 7) - 08/02/96 13:23 - [ 7] Summary Form for Proposal 6672 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BRIGHT=RETURN SEARCH-SIZE=3 CR-SPLIT=0.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category GALAXIES & CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQUISITION FOR 2-3 POS TARG 0.25,0.25 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N1 G140L F380W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC7714 ------------------------------------------------------------------------------------------------------------------------------------