A new window on the UV SED of star-forming galaxies: direct measurements of ionizing spectra in the Lyman continuum

0 Assigned ID: 452 ----- Attempting Submission 1 (Fri Mar 06 11:49:04 GMT 2020) ----- HST Phase I Proposal 452 successfully submitted. Receipt: # 452-1 ----- Attempting Submission 2 (Fri Mar 06 17:13:19 GMT 2020) ----- 0 ----- Attempting Submission 1 (Fri Jun 12 15:23:29 GMT 2020) ----- A new window on the UV SED of star-forming galaxies: direct measurements of ionizing spectra in the Lyman continuum The Lyman continuum (LyC = hydrogen-ionizing radiation at wavelengths < 912 Ang) of galaxies plays a fundamental role in determining the physical and observational properties of the ISM and IGM, including cosmic reionization. Yet it is basically inaccessible to direct observations, and one therefore heavily relies on theoretical, yet untested predictions from synthesis models. Furthermore the ionizing spectra differ significantly between different models, with important implications on widely used emission line diagnostics and other observables. We here propose to observe for the first time the shape of the ionizing continuum of several star-forming galaxies shortward of (from ~600 to 912 Angstroem) and across the Lyman break. This can be achieved with COS by targeting LyC emitters at z~0.6-0.8. The observations will measure for the first time stellar and nebular emission features in the LyC, and thus provide the first direct constraints for stellar population models in this spectral range. The proposed observations will provide unique insight on the hardness of the ionizing spectra of star-forming galaxies, with numerous possible implications on our understanding of the emission line properties of distant galaxies, sources of cosmic reionization, emission line diagnostics, and related topics. We propose to observe for the first time the shape of the ionizing continuum of several star-forming galaxies shortward of the Lyman break (between ~ 600 - 912 A). This is feasible in z ~ 0.6 - 0.8 galaxies with a significant escape of LyC radiation. To achieve this goal we have selected six z ~ 0.6 - 0.8 galaxies such that their UV spectra from 610 - 680 A to 1100 - 1200 A rest-frame can be observed with a single setting with the G140L grism in a spectral range where COS is efficient. The targets have been selected from the latest SDSS release using the same criteria as Izotov and collaborators, namely (1) compact or unresolved (size < 2.5") star-forming galaxies with (2) strong emission lines (EW(Hbeta) ~ 100 - 200 A), and (3) a high ratio [O iii]/[O ii]> 3 - 4. This method has so far shown a success rate of 100% in finding LyC emitters. We have also verified that the targets show Mg ii 2796,2808 strongly in emission, which is an additional indication that these galaxies should be LyC emitters (Henry et al. 2018). And we have verified that the emission line ratios of our galaxies are compatible with photoionization by stars, not AGN (using different line ratios, line widths etc.). The targets have also GALEX near-UV magnitudes, metallicities, low extinction, and other properties comparable to those of the strong LyC emitters discovered by Izotov et al. (2018ab). The large EW(Hbeta) shows that the UV emission must be dominated by young ( < 6 Myr) massive stars, as also found for the sources of Izotov et al. (2018b). All these findings show that our sources are ideal targets to study for the first time the SED in the Lyman continuum. We propose to target 6 galaxies to gather these unique observations. Observing several galaxies is important since neither their flux in the Lyman continuum nor the exact shape can be predicted precisely. Indeed, variations in the intrinsic SED are expected and measuring them is among the main objectives of the proposal. Spectra of several sources may also be needed to avoid possible contamination by intervening low-z Lyman Limit systems, which could render part of the SED at < 912 A unusable. For all these reasons we propose a small sample of galaxies for this pilot study. We have verified that our targets are sufficiently faint to satisfy safety conditions for observing with the COS (see Target Summary) and that there are no sources brighter than the COS safety limits in proximity (within circle of 43" diameter) of our targets. We will obtain NUV acquisition images of the targets with the standard Mirror A and the AQN/IMAGE mode, with exposures of ~ 1500 seconds to reach a S/N ~ 20 within a 9x9 pixel box centered on the brightest part of the galaxy. We have used the GALEX NUV magnitudes to estimate the S/N. The angular galaxy radius is chosen to be 0.2", inside which most of the galaxy light is concentrated. The total time for acquisition is up to 120s+2x1500s = 3120s per object (HST Primer manual), i.e. one orbit. The acquisition image will allow us to analyse the UV morphology of the sources. The COS will be used in combination with the low-resolution G140L grating centered at 800 A to measure the LyC. The required number of orbits for each object is calculated so as to detect the LyC at the level above 3sigma with a 400-pixel binning of the data at the observed wavelength 912x(1+z). For the LyC flux we have assumed the mimimum value corresponding to an escape fraction of 5 %, and the intrinsic LyC flux density is obtained from the relation I(Hbeta)/I(912) ~ 9 A (Izotov et al. 2016b), where I(Hbeta) is the extinction-corrected flux of the Hbeta emission line. We have also done simulations of the expected observations using the synthetic spectra to confirm that a sufficient S/N can be obtained over the entire wavelength range of interest. The extinction of the targets, measured from the Balmer decrement is very low (E(B-V) ~ 0 - 0.07). All the available data (including combination of COS spectra, GALEX photometry and SED) will be used to model and correct for selective extinction. It is difficult to foresee the selective extinction in this spectral range. For each target 1 orbit is needed for acquisition, and 4 orbits for the spectrum with the low-resolution G140L grating, i.e. 5 orbits in total. We therefore request a total of 30 orbits for 6 objects. Our observations do not require constraints on the HST orientation. Therefore, operating HST in reduced gyro mode would likely affect our program a little, slightly increasing the time for acquisition and slightly reducing the time for science observations. The team has all the expertise, tools, and background to successfully carry out the proposed project. Our team has been responsible for the recent discovery of the low-z strong LyC leakers/emitters with HST/COS observations (Izotov et al. 2016ab, 2018ab). These COS observations of 11 z~0.3-0.4 compact star-forming galaxies have led to a series of numerous interesting publications on LyC, Lyman-alpha, UV absorption lines, outflows, ISM etc. and to significant new insight onto analogs of the sources of cosmic reionization. Data reduction, analysis, and interpretation will be done as in our previous GO 13744 and GO 14635 projects and documented in Izotov et al. (2016ab, 2018ab). Our team includes experts on all relevant aspects and disposes of all necessary tools, including a custom pipeline for faint COS spectra (Worseck et al. 2011), which has also been used for simulations of the observations. The team also includes experts of stellar synthesis models and their applications to UV spectra (Schaerer and Chisholm). We can also rely on a large grid of CLOUDY photoionization models, which has recently been computed to examine the behavior of the main emission lines in presence of different ionizing spectra (low metallicities, very hot stars, PopIII, and AGN), and which have been confronted to UV line measurements from the VUDS survey and to most HST observations of low-z galaxies (Nakajima et al. 2018). The model calculations have also been extended to treat the case of density-bounded, i.e. LyC emitting regions (see Schaerer et al. 2018). Finally, we have recently constructed tailored CLOUDY photoionization models of LyC emitters, using one- and two-zone models of the sources, to reproduce/study the observed emission line properties of the LyC emitters (Ramambason, Schaerer et al. 2020). These model suites will be used to analyse the z~0.6-0.8 galaxies of this proposal, taking also into account the direct constraints which will be obtained from the first LyC SED measurements. The available rich ancillary data of the targets including existing imaging (GALEX, SDSS, WISE etc.) and spectroscopic data (SDSS) has already partly been analyzed using methods described in Izotov et al. (2016ab, 2018b). The analysis will be completed once the HST data will be obtained. Galaxies Intergalactic Medium and the Circumgalactic Medium 6 false false true false false false false SUPPORTED GALAXY DWARF COMPACT STARBURST NO J2000 J2000 FUV=22.40+/-0.14, NUV=20.94+/-0.05, F(900)=8.3e-17 erg/s/cm2/A - expected intrinsic flux of the Lyman continuum. GALAXY DWARF COMPACT STARBURST NO J2000 J2000 FUV=24.24+/-0.25, NUV=21.75+/-0.07, F(900)=9.0e-17 erg/s/cm2/A - expected intrinsic flux of the Lyman continuum. GALAXY DWARF COMPACT STARBURST NO J2000 J2000 FUV=22.15+/-0.53, NUV=22.16+/-0.35, F(900)=8.2e-17 erg/s/cm2/A - expected intrinsic flux of the Lyman continuum. 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