0Assigned ID: 318
----- Attempting Submission 1 (Fri Apr 10 00:42:58 GMT 2015) -----
HST Phase I Proposal 318 (HST23_LyCont.apt) successfully submitted.
Receipt: # 318-10----- Attempting Submission 1 (Wed Jul 22 23:44:51 GMT 2015) -----Does All The Lyman Continuum Emission Escape From Young, Low Mass Starbursts?Searches for escaping Lyman continuum emission from star-forming galaxies have been largely unsuccessful across a wide range of redshifts, seemingly in contradiction with the observed ionized Universe. However, the main focus of these searches has been massive galaxies, despite theoretical studies that predict a higher escape fraction of ionizing radiation in galaxies at the low mass end. We propose to measure the escaping Lyman continuum emission from a sample of 11 low mass, emission line galaxies at 1.2 < z < 1.4, selected from the 3DHST survey, using the ACS SBC camera and its F150LP filter. These low mass emission line galaxies have all been selected to have very high (> 190 Angstrom) equivalent widths, indicating that their stellar populations are both very young and very low metallicity, similar to the primordial star formation believed to have re-ionized the universe. We will be sensitive to an intrinsic escape fraction of 5% for these low mass sources. Efficient searches for escaping Lyman continuum emission require HST ultraviolet imaging, because of the rapid rise in IGM opacity with redshift.We propose deep FUV imaging of 11 sources at 1.2 < z < 1.4 in the GOODS fields, using the ACS/SBC in the F150LP filter. The low sky background and dark count, coupled with the absence of read noise, means that ACS can detect galaxies in the far-UV with a high efficiency.
Our goal is to measure the Lyman continuum from a high equivalent width (EW) H-alpha emitter of U < 24.2, down to an escape fraction of 5%. Our definition of fesc is the fraction of the total intrinsic Lyman continuum radiation which escapes the galaxy. A young stellar population has an SED with rest- frame f700/f1500 ~ 0.1. Combined with the average intergalactic absorption from the Ly alpha forest (~60% at z = 1.3), this makes the expected drop from U band to the FUV about 3.5 magnitudes. Therefore, a 3 sigma sensitivity of FUVAB ~ 29.2 enables us to detect an U=24.2 galaxy from which 25% of the LC escapes relative to the UV continuum at 1500 Angstroms. Much of the UV continuum does not escape, however, due to dust extinction. Assuming an additonal factor of ~5 extinction in FUV flux due to dust reddening (which we infer from the slope of the UV continuum, Beta, see Meurer et al. 1999), implies an intrinsic fesc of 5%.
Our signal to noise estimates are based on our observations of ACS/SBC surveys of the UDF and GOODS, including those presented in Siana et al. (2010). After accounting for minimized dark current and extraction apertures, we can achieve 3 sigma sensitivities of FUVAB = 29.2 in 8000 seconds, or 3 orbits. This assumes a 0.5" diameter aperture, which is appropriate for the highly compact nature of these sources. The target galaxies span a magnitude in U band magnitudes and also include one very bright (U=22.5) source, so we propose to scale the length of each observation to match the brightness of each source. Our total survey request is 26 orbits of F150LP imaging with SBC, targeting 11 UV-bright, high-EW 1.2 < z < 1.4 galaxies.
The dark current is the primary source of noise in the MAMA detector and our primary observing strategy is therefore driven by minimizing the dark current. First, the MAMA detectors warm up while in use, causing a significant dark "glow" which doubles every 2 hours in use. We will therefore work with the Program Coordinator at STScI to schedule each object in visits where the SBC has not been used for long periods of time. The SBC is not used often, so there is little danger of other long SBC observations randomly being scheduled near this program. The dark "glow" is most prominent near the center of the chip. Therefore, we will modify our target positions once the roll angle is known to place our sources in the corner least effected by the glow (x=250,y=250). The dark glow rate in this region is a factor of ~10 less than at the peak. This still allows 6" on all sides of the object for background fitting and subtraction.
We also note that most of our proposed fields include additional Halpha emitters of varying brightness and EWs that did not make our final candidate cuts. Therefore in addition to the robust limits for our 11 strongest candidates, we will also measure significant limits (5-25%) for fesc for an additional 7 UV-bright, low mass star forming galaxies.COSMOLOGY12falsefalsefalsefalseSUPPORTEDGALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000GALAXYEMISSION LINE NEBULAHIGH REDSHIFT GALAXYSTARBURSTJ2000J2000ACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPACS/SBCImagingNoNoF150LPPOS-TARGDITHER0.179LINE0.116falsefalse63.65420.02ACS-SBC-DITHER-BOX1falseThe scheduling data was generated by CASM 23.2.CASM was run in Phase II mode.We request that the SBC not be used 24 hours before this visit in order to mitigate the effects of dark current.(Tic-Visit
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