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Assigned ID: 6747
----- Attempting Submission 1 (Thu Apr 10 21:52:03 GMT 2025) -----
HST Phase I Proposal 6747 successfully submitted.
Receipt: # 6747-1
----- Attempting Submission 2 (Thu Apr 10 23:53:53 GMT 2025) -----
update STIS ACQ and POS TARG
181121062
----- Attempting Submission 1 (Thu Aug 14 06:51:32 GMT 2025) -----
HST Phase II Proposal 18038 successfully submitted.
Receipt: # 18038-1
----- Attempting Submission 2 (Tue Sep 30 09:50:37 GMT 2025) -----
HST Phase II Proposal 18038 successfully submitted.
Receipt: # 18038-2
----- Attempting Submission 3 (Tue Nov 11 08:29:48 GMT 2025) -----
Unravel the Puzzles of Little Red Dots: HST View on Local Analogs
One of the most impactful early discoveries from JWST is the ubiquity of "little red dots" (LRDs) at z=2-10. LRDs are characterized by broad hydrogen emission lines and v-shaped SEDs (blue UV and red optical continua), interpreted as low-luminosity AGNs. A significant fraction of LRDs also exhibit Balmer absorption and Balmer breaks, which could be produced by a shell of dense gas surrounding the black hole. However, these models leave many degeneracies, particularly regarding the origin of the UV light. Spatially decomposing AGN and stellar light, as well as LyA observations, is the most direct way to test these LRD models. Yet at high redshift, LRDs are too faint and compact for such decomposition, even with JWST. Local analogs of LRDs provide the best opportunities to unravel the physical nature of this population.
We propose HST UVIS imaging and STIS spectroscopy on three LRD analogs at z=0.1-0.3. These analogs, selected from SDSS, exhibit emission line profiles, UV-to-optical SEDs and Balmer absorptions that most closely resemble those of high-redshift LRDs. They establish an effective testbed and benchmark for LRD studies at z>2. By spatially resolving their UV-to-optical continua and mapping their Lya profile, we aim to: (1) unravel the origin of UV light, separate AGN and galaxy contributions and obtain a resolved view of host galaxies, (2) reveal the existence and measure the density and size of the hydrogen gas envelops surrounding the center BH. Fully leveraging the unique UV capabilities of HST in both imaging and spectroscopy, this program will directly test theoretical hypotheses of LRDs and address one of the most debated questions since the launch of JWST.
Imaging: We propose WFC3 UVIS imaging with F225W+F336W for all three targets, and medium-band filters tailored to each: F467M+F621M for J1025+1402; F467M+F689M for J1047+0739; and F547M+F689M for J1022+0841. These medium bands are selected to map the continuum in line-free windows between the Balmer break, [O III]4960,5008, and Ha, tracing the Balmer break strength when combined with UV filters.
Spectroscopy: STIS G140L with 52x0.5 slit. It offers high sensitivity in the FUV range and provides adequate spectral resolution (R~1000) to measure their Lya properties.
Exposure Time (using HST ETC v33.1):
(1) Imaging: We estimate the UV surface brightness from GALEX and SDSS-u photometry to be approximately 3e-18 to 5e-18 erg/s/cm2/A/arcsec2, and derive the optical surface brightness from SDSS photometry. We aim to achieve a signal-to-noise ratio (S/N) of at least 3 per 5x5 pixel region. For each target, we request 2 orbits for the F225W filter, 1 orbit for F336W, and 2 orbits for the medium-band filters. This results in 5 UVIS imaging orbits per object, or a total of 15 orbits for all three targets.
(2) STIS spectroscopy: Based on the spectra of the most similar high-z LRDs to the proposed targets, we estimate their underlying continua around the Lya line to be in the range of 1e-16 to 5e-17 erg/s/cm2/A. We aim to achieve S/N > 3 per resolution element to resolve Lya profiles. We therefore propose 3 orbits of STIS G140L observations for each of the two targets at z = 0.1, and 4 orbits for J1022+0841 due to its fainter flux.
The team has considerable expertise in high-redshift AGN and galaxy physics, as well as HST data reduction required to execute the project.
We have experts on high-redshift AGNs, in particular the JWST-discovered Little Red Dots: Xiaojing Lin, Xiaohui Fan, Junjie Mao, Jenny Greene, Yilun Ma, and David Setton, Jinyi Yang. They have been leading or involving many high-redshift quasar, AGN, and LRD-related projects.
We have experts on high-redshift galaxy, stellar population, and their LyA properties: Xiaojing Lin, Xiaohui Fan, Zheng Cai, Fengwu Sun, Eiichi Egami, and Fuyan Bian. They have been working on galaxy evolution for a long time and have strong expertise in analyzing the gas content of galaxies, including DLA and Balmer break modeling.
Many of the team members have expertise in HST data reduction: Xiaohui Fan, Jenny Greene, Zheng Cai, Fengwu Sun, and Fuyan Bian.
Supermassive Black Holes and Active Galaxies
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GALAXY
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STIS/FUV-MAMA
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WFC3/UVIS
Imaging
No
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F621M
STIS/FUV-MAMA
Spectroscopic
No
No
G140L
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WFC3/UVIS
Imaging
No
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F689M
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WFC3/UVIS
Imaging
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No
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:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 2977
:pcs-gap 0
)
(Tic-Orbit
:proposal-id 18038
:visit-id "10"
:orbit-number 2
:pcs-time 380
:si-vis-utilization 2578
:al-overhead-at-end 0
:internal-dur-at-end 375
:dark-offset NIL
:dark-utilization 0
:max-dark-utilization 1920
:low-sky "N"
:pcs-visibility 2977
:pcs-gap 0
)
true
2
0