0Assigned ID: 7646
----- Attempting Submission 1 (Thu Apr 16 21:09:07 GMT 2026) -----
HST Phase I Proposal 7646 successfully submitted.
Receipt: # 7646-1
----- Attempting Submission 2 (Thu Apr 16 23:09:54 GMT 2026) -----For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser.
For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser.
For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser.
For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser.0----- Attempting Submission 1 (Thu Aug 06 18:00:03 GMT 2026) -----
HST Phase II Proposal 18352 successfully submitted.
Receipt: # 18352-1
----- Attempting Submission 2 (Mon Aug 17 15:22:17 GMT 2026) -----Minor warnings only...Constraining the ionizing and FUV output of Little Red Dots with HST: a decisive test of their role in Cosmic ReionizationWe propose 23 orbits of HST COS spectroscopy to obtain far-ultraviolet (FUV) and Lyman continuum (LyC) observations of seven recently discovered Little Red Dots (LRDs) in the nearby universe (z = 0.3). LRDs, a population of compact, red sources at z > 3 revealed by JWST, exhibit unusual spectral energy distributions and broad Balmer emission lines, suggesting rapid black hole growth, yet the physical origin of their ultraviolet continuum emission remains unresolved. Current models invoke either stellar-dominated emission from young populations surrounding a gas-enshrouded black hole, or accretion-dominated emission from a structured disk, but these scenarios make fundamentally different predictions for the ionizing spectrum.
At high redshift, direct constraints on LyC emission are precluded by intergalactic medium absorption. The proposed low-redshift LRD sample provide a unique opportunity to directly measure both the ionizing output and rest-frame FUV spectral features of LRD systems. Using COS/G140L, we will measure the LyC-to-UV flux ratio and search for diagnostic stellar wind and photospheric features, enabling a decisive test of the dominant emission mechanism.
These observations will deliver the first empirical constraints on the ionizing efficiency of LRDs. Given their high inferred number densities at high-z, LRDs may represent a significant, previously unaccounted-for contributor to the ionizing photon budget during cosmic reionization. This program will therefore establish whether accreting black holes in LRDs can rival star-forming galaxies as drivers of reionization.Here we propose 23 orbits of HST COS spectroscopy to obtain far-ultraviolet (FUV) and Lyman continuum (LyC) observations of seven recently discovered Little Red Dots (LRDs) in the nearby universe (z = 0.3), enabling the search for diagnostic stellar features and the dominant emission mechanism at these (so-far unexpolored) wavelengths.
Science observations will employ the medium resolution COS G140L grating at CENWAVE = 800 in the Primary Science Aperture (PSA), to cover both the ionizing and non-ionizing rest-FUV continuum. Exposure Time Calculator (ETC, v34.1.1) estimates, assuming point-source profiles and nominal background levels, and including acquisition overheads, require between 3 and 4 orbits per target, for a total of 23 orbits across the seven-object sample. As such, 3-orbit integrations will be done in the same visit, while 4-orbit exposures will be split in two 2-orbit visits. FP-POS 1 to 4 are used consecutively within the 3-orbit visits, but FP-POS are split into FP-POS = (1,2) and (3,4) in the case of 4-orbit visits of the same target. According to the ETC (see provided ETC IDs), these observations require BUFFER-TIMEs of ~12,000s per FP-POS. These observations will use FLASH illumination.
To ensure accurate centering, target acquisition will be performed through standard NUV imaging in the ACQ/IMAGE mode, using the MIRRIRA spectral element and the PSA aperturte. These acquisitions will require between 300s and 450s exposures to achieve S/N > 50, based on the GALEX NUV magnitudes of the objects (21-22 AB). All targets satisfy COS safety constraints, with no contaminating sources exceeding limits within the 43'' x 43'' aperture. Predicted count-rates remain well below thresholds relevant for lifetime position (LP) constraints, and we will use the LP11 as recommended by the cycle 34 COS policies for the G140L grating.Our team has ample experience in observational studies of star-forming galaxies, with a significant focus on the UV regime. Over the past decade we have led a large number of HST campaigns concerning UV imaging and spectrsopcy of starburst galaxies.
In particular, A. Saldana-Lopez, M. J. Hayes, C. Scarlata, A. Le Reste, A. Henry and M. Giavalisco are co-Is of the Cycle 26 Low Redshift Lyman Continuum Survey (LzLCS), the largest HST program probing the LyC of nearby galaxies using HST/COS G140L. A. Henry is PI of other HST/COS programs studying the UV absoprtion/emission and Ly-alpha properties of Green Pea galaxies. R. Lin and J. Zavala are experts on BL-AGN searches using SDSS+GALEX, and on the characterization of these galaxies.
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