He II Emission from Wolf-Rayet Stars: a New Dust Attenuation Measure in Star-forming Galaxies

and another minor tweak 1269866587 Assigned ID: 440 ----- Attempting Submission 1 (Fri Apr 05 14:05:35 GMT 2019) ----- HST Phase I Proposal 440 (WR-Bump-C27.apt) successfully submitted. Receipt: # 440-1 ----- Attempting Submission 2 (Fri Apr 05 19:09:53 GMT 2019) ----- HST Phase I Proposal 440 (WR-Bump-C27.apt) successfully submitted. Receipt: # 440-2 ----- Attempting Submission 3 (Fri Apr 05 20:17:59 GMT 2019) ----- HST Phase I Proposal 440 (WR-Bump-C27.apt) successfully submitted. Receipt: # 440-3 STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS STIS EXPOSURE TIME ROUNDED DOWN TO NEAREST 0.1 SECONDS Tweaked positions of targets 3 and 4 by about 1 arcsec. 971081730 ----- Attempting Submission 1 (Tue Jul 09 16:40:20 GMT 2019) ----- HST Phase II Proposal 15846 (WR-Bump-C27.apt) successfully submitted. Receipt: # 15846-1 ----- Attempting Submission 2 (Thu Jan 16 13:12:01 GMT 2020) ----- these warnings may be ignored He II Emission from Wolf-Rayet Stars: a New Dust Attenuation Measure in Star-forming Galaxies We developed and calibrated a novel technique to measure dust attenuation in star-forming galaxies. The technique utilizes the strong stellar-wind emission lines in Wolf-Rayet stars, which are routinely observed in galaxy spectra locally and up to redshift 3. The He II 1640 and 4686 features are recombination lines whose intrinsic ratio is almost exclusively determined by atomic physics. Therefore it can serve as a stellar dust probe in the same way as the nebular hydrogen-line ratio can be used to measure the reddening of the gas phase. The new technique allows us to study stellar and nebular attenuation in galaxies separately and to test its effects at different stellar age and mass regimes, independently of assumptions on the stellar initial mass function. In this proposal we request co-spatial optical STIS spectroscopy of eight star-forming galaxies with existing archival ultraviolet spectra but lacking co-spatial optical data through matching apertures.The need for high spatial resolution and matching apertures precludes the use of ground-based data. We will use the line ratios of the broad (non-nebular) Wolf-Rayet lines at 1640 and 4686 A to determine the dust attenuation of the most massive stars and compare it to the reddening of the less massive stars, as indicated by the ultraviolet continuum slope, as well as to the gas reddening. This allows us to disentangle the effects of stellar mass, age, and stellar versus nebular reddening in star-forming galaxies. The eight targeted galaxies have been successfully observed before as part of programs 7513 and 9036 using STIS with the G140L and G230L gratings and the 52x0.2 and 52x0.5 apertures. Therefore the coordinates and acquisition parameters are well known, and the requested observations in G430M and G430L are risk-free. As has been done in both previous programs, we will use an imaging acquisition with the optical long-pass filter. No peak-up will be performed. Typical exposure times are between 20 and 60 s. In Table 1, column 5 we list one archival UV dataset for each galaxy. In all cases, there are additional G140L and (in six cases) G230L spectra. The available UV spectra permit a safe estimate of the expected fluxes around 4686. The derived values are in column 6 of Table 1. We will collect spectra with the G430M (4706) and G430L (4300) gratings. While the broad W-R 4686 line itself will be easily detected with G430L, we expect some contribution by nebular lines of [Fe III] 4658, [Ar IV] 4711 and [Ar IV] 4740 (Schaerer et al. 1999). This is not an issue at the wavelength of the UV 1640 line, where there are no such lines, but requires more care in the case of the optical 4686 line. These effects can be mitigated by the higher resolution of the G430M grating. Column 8 of Table 1 gives the required exposure times in order to reach S/N = 20 with the G430M grating. This S/N is based on our prior extensive experience with HST spectra of star-forming galaxies. We will also take short G430L spectra to measure the continuum in combination with the UV spectra. The G430L spectra will also be used to determine the nebular reddening from Ha and Hb. The exposure times for G430L are negligible (300 s or less). All spectra will be cr-split for cosmic-ray removal. The total overhead consists of the guide-star acquisition (6 min), target acquisition (7 min), instrument set-up (5 min), and guide-star re-acquisition (4 min), plus a few minutes for data managements. The estimated orbit duration for the eight galaxies is about 54 min. Therefore we can fit the observations for the two gratings into one or two orbits per galaxy. Andreas Faisst is an Assistant Astronomer (former SNSF post-doctoral fellow) at Caltech/IPAC. His main interests include early galaxy formation at very high redshifts and the physics of star-formation quenching in massive galaxies at intermediate redshifts. Janice Lee is an Associate Astronomer at Caltech/IPAC and experienced HST user. Her research focuses on the physics that drive, regulate, and extinguish massive star formation in galaxies through observation of the process at the extremes: in low mass dwarf galaxies, in low metallicity environments, and in starbursts. Claus Leitherer is an astronomer at STScI. His main scientific interests include the properties of individual massive stars and of young stellar populations, population synthesis modeling, spectral energy distributions of galaxies, and ultraviolet to near-infrared observations of galaxies. Galaxies and the IGM Stellar Populations 6 false false false false false false false SUPPORTED GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the NED database. J2000 J2000 F(4700) = 1e-15 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 F(4700) = 4e-15 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the NED database. J2000 J2000 F(4700) = 6e-16 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 F(4700) = 8e-16 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the NED database. J2000 J2000 F(4700) = 1e-15 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 F(4700) = 9e-16 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 F(4700) = 2e-15 GALAXY STAR FORMING REGION NO This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 F(4700) = 8e-16 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G430M 4706 STIS/CCD Spectroscopic No No G430L 4300 1579180069854 The scheduling data was generated by CASM 27.2. 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